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RISAT-1 (Radar Imaging Satellite-1)

RISAT is the first indigenous satellite imaging mission of ISRO (India n Space Res earch Organization) using an

active radar sensor system, namely a C-band SAR (Synthetic Apertur e Radar) imager, an important microwave

complement to its optical IRS (Indian Remote Sensing Satellite) series observation missions. The overall

objective of the RISAT miss ion is to us e the all-weather as well as the day-and-night SAR observation capability in

applications s uch as agriculture, forestry, soil m oisture, geology, sea ice, coastal monitoring, object identification,

and flood m onitoring. The RISAT specifications have been drawn with the national requirements in mind. 1) 2) 3) 4)

5)

RISAT is a newly developed agile spacecraft, featuring a multi-mode and multi-polarization SAR system in C-

band, providing spatial res olutions in the range of 1-50 m on swath widths ranging from of 10-225 km.

Figure 1: Illustration of the deployed RISAT-1 spacecraft (image credit: ISRO)

RISAT-1 comprises around 1400 subsystems, including 300 processors. The active array subsystems are large

in number and less on design variety. Each of the subsystems requires rigorous space grade fabrication and

qualification. Fabrication and characterization of each of these subs ystems are typically spread over 5–6 weeks.

Industrial production and space qual ification of the subs ystems were carried out by the Indian indus try based on

in-house des igns of ISRO. These indus tries had limited exposure to space-grade electronics and therefore in the

spirit of partnership, they had to undergo a rigorous regime of training in space-grade fabrication processes,

qualification methods and documentation processes. This also helped in the development of indigenous source

of RF MMICs (Monolithic Microwave Integrated Circuits), TR modules, ASICs (Application Specific Integrated

Circuits), miniaturized power supply and printed antenna array. RISAT-1 effectively acted as a catalyst in expanding

the indigenous industrial base for production of space-grade SAR subs ystems. 6)

ISRO used its in-hous e pool of ingenuity in conceptualizing, engineering and realizing the SAR system of RISAT-1,which is a vastly complex payload with significant level of flexibility in reconfiguration to meet different imaging

requirements and ease of operability. This was possible because of large on-board software spread over 300

processors. The characterization of the system itself was unique, where all the 126 beams have been

characterized with precision. This resulted in calibration and quick operationalization of the s ystem.

Realization of the RISAT-1 state-of-the-art radar imaging satellite needed significant developments in the

spacecraft capabilities to accommodate large mass, power and transmission data rates. For example, the data

transmission rate was increased six fold from 110 to 640 Mbit/s. With a mass of 1858 kg, RISAT-1 is heaviest

among ISRO’s remote sensing satellites, it is the lightest satellite compared to those belonging to the same

class.

Spacecraft:

The spacecraft structure is designed to meet the stiffness, strength and pointing requirements of the payload,

sens ors and als o confining the overall bus volume within the launch vehicle envelope. It is bas ed on a s ingle bus

concept built around a central cylinder. A truncated triangular structure is built around the cylinder to hold the SAR

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antenna and major bus service elements. A cuboid structure is built on top of the cylinder to accommodate the

solar a rrays, maj ority of the sens ors and antennae. The primary structure consists of a central cylinder, interface

rings and shear webs. The central cylinder is of sandwich construction with aluminum core and CFRP (Carbon

Fiber Reinforced Polymer) face skin. It has an aluminum alloy interface ring at the bottom to interface with the

launch vehicle. The cylinder also p rovides interface for the propellant tank and reaction wheel deck. The secondary

structures cons ist of equipment panels/decks of the payload module and the cuboid module (Ref. 5).

Figure 2: Illustration of the RISAT-1 main structure (image credit: ISRO)

Figure 3: Block diagram of the RISAT-1 spacecraft ( image credit: ISRO, Ref. 5)

The payload m odule structure consists of three equipm ent panels, three corner panels and top and bottom deck.

All the equipment panels and corner panel s of the payload module are made of sandwich cons truction with

aluminum core and aluminum face skin, whereas the shear webs are made of sandwich construction with CFRP

face skin. The triangular decks carry the hold-down brackets to hold the SAR antenna in launch configuration.

The SAR antenna is comprised of three panels, of which one is fixed and the other two are stowed onto either

sides of the triangular structure during launch and are deployed in the orbit. Tile substrate and panel frame are

two basic structures over which the SAR payload is built. The radiation patch antennae are bonded on one side of

the tile substrate and the tile electronics mounted on the other side o f the substrate. Four tiles form a pane l for the

SAR antenna. To support these four tiles, a framed structure is evolved. Most of the sensors, antennae, solar


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arrays and their associated electronics are mounted in the cuboid module. RISAT-1 main structure is shown in

Figure 2.

The subsystem layout has been evolved considering various factors like electrical requirements, interfaces

among various subsystems, physical size and location feasibility, look angle and FOV (Field of View)

requirements of various elements (payloads, sensors, antennae), thermal requirements, mechanical loads,

transmissibility factors, physical parameters and balancing, ease of assembly/dis-assembly and accessibility

during AIT (Assembly, Integration and Testing) and pre-launch operations. All the subsystem electronics

packages are accommodated on the equipmen t decks/panels.

The payload module (triangular structure) accommodates most of the mainframe systems and the payload

electronics. The cuboid m odule accomm odates so lar arrays, most of the sens ors and antennae, viz. DSS (Digital

Sun Sensor), SPSS (Solar Panel Sun Sensor), ES (Earth Sensor), 4π sun sens ors, PAA (Phase Array Antenna),

TTC antennae and SPS (Satellite Positioning Subsystem). All the RCS (Reaction Control Subsystem)

components are accommodated on one of the shear webs and the exterior surface of the triangular bottom deck.

The propellant tank is mounted inside the main cylinder. The reaction wheels are m ounted on a circular deck in a

tetrahedral configuration. The circular deck is accommodated inside the main cylinder below the tank and is

connected to the cylinder through a ring.

TCS (Thermal Control Subsystem): The configuration and equatorial cross ing time of RISAT-1 are different from

other satellites in the IRS series of ISRO. Though it is an Earth-oriented satellite, during payload operation the

satellite will be rotated by ± 36° about the roll axis. This new configuration, orientation and equatorial crossing

time result in new external load patterns and extreme load conditions which are d ifferent from other IRS satellites.

Moreover, a number of heat dissipating packages are accomm odated inside the structure.

Thermal control is provided using space-proven thermal control elements such as an OSR (Optical Solar

Reflector), MLI (Multilayer Insulation), paints, thermal control tapes, quartz wool blanket, sink plates and heat

pipes. In addition, heaters will be provided to maintain temperatures during cold conditions .

Mechanisms: The RISAT-1 spacecraft employs a SAR antenna deployment mechanism and a solar array

deployment mechanism . The SAR antenna and the s olar array are stowed during the launch and are deployed in

the orbit in order to m eet the constraints impos ed by the launch vehicle. In order to perform deployment in orbit, a

hold-down and release mechanism is employed. The solar array deployment mechanism is identical to earlier

IRS miss ions.

The deployed SAR antenna has dimens ions o f 6.29 m x 2.09 m x 0.220 m. It consists of three panels out of which

one is rigidly attached to the triangular structure. In the launch configuration, the deployable panels are folded over

the triangular structure and are held by using a hold-down mechanism. In orbit, both deployable panels are

released s equentially and deployed. The mass of each panel is about 290 kg.

EPS (Electrical Power Subsystem): The EPS consists of solar arrays for power generation, chemical battery for

power storage and power electronics for power conditioning and distribution. It is des igned to meet the 6 hour and

18 hour orbit illumination conditions, the large power requirement of the SAR payload and the solar eclipse

conditions during the summer solstice.

The solar array consists of six panels arranged in two wings with three panels in each wing in the positive roll and

the negative roll axes. The array consists of multi-junction cells connected in series and parallel for optimumperformance. The solar array drive assembly helps in compensating the roll bias (± 36°) given during payload

operation and also aids in obtaining more generation near pole transit. The energy storage system for RISAT-1

employs a single NiH2 battery of 70 AH capacity to meet the peak load requirement and also the eclipse

requirement.

The EPS uses a single-bus system operating at 70 V, and the configuration is arrived at to meet all the

requirements of users and interfaces. During the sunli t period, the array is regulated to 70 V and the battery gets

charged. A BDR (Battery Discharge Regulator) supports power to the bus when the load demand exceeds the

array generation during payload operation and eclips e conditions by regulating the bus to 70 V. The bus voltage

selection is mainly driven by the payload requirement. The single bus of 70 V is fully protected against over

voltage, over current and is single-point failure proof. The bus is distributed to all users through fuses, centrally

located in fuse-distribution packages. Software logic (software resident in the on-board controller) enhances the

safety of the power system.

OBC (On-board Computer): To minimize power, weight and volume, the spacecraft functions like commanding,

housekeeping (telemetry), attitude and orbit control, thermal management, sensor data processing, etc., havebeen integrated into a single package called OBC, which also implements the MIL STD 1553B protocol for

interfacing with other s ubsystems of the spacecraft (Figure 4).


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Figure 4: Block diagram of the OBC (image credit: ISRO)

The use of MIL STD 1553B interfaces between the OBC and the other subs ystems greatly decreases the volume

and mass of cabling, and the associated connectors. The OBC system is realized with the functions of sensor

electronics, command processing, telemetry and housekeeping, attitude and orbit control and thermalmanagem ent. Besides, the OBC interfaces with power, telemetry–telecommand (TM–TC; RF) for comm and and

telemetry, sensors , heaters, thrusters and reaction wheels through s pecial control logic.

AOCS (Attitude and Orbit Control Subsystem): The integrated AOCS specifications during imaging are as follows :

pointing: ± 0.05° (3σ ); drift rate: ± 3.0 x 10 -4 º/s. The AOCS for RISAT-1 is configured with 4π sun sensor,

magnetometer, IRU (Inertial Reference Unit), star sensor, earth sensor, DSS (Digital Sun Sensor) and SPSS

(Solar Panel Sun Sens or). Actuation is provided by eight 11 N canted thrusters (m ono propellant hydrazine system

operating in blow-down mode) with two-axis canting from + pitch axis for acquisition and OM operation, one (1)

central 11 N thruster for OM operation, 4 reaction wheels (of capacity 0.3 Nm torque and 50.0 Nm s) m ounted in

tetrahedral configuration about the – pitch axis, and magnetic torquers of 60.0 Am 2 capacity for momentum

dumping. The sun sensors, star sensors and magnetometer provide attitude data in the form of absolute attitude

errors. The magnetometer, 4π sun sensor and temperature sensor data are processed in the OBC. All AOCS

software modules are imp lemented in the OBC.

RCS (Reaction Control Subsystem): The RCS compris es a propellant tank, thrusters (9 of 11 N), latch valves, fill

and drain/vent valves, pressure transducers, system filters, thermocouples , flow control valves and titanium tubes

to connect all the reaction control elements. A block schematic of the RCS is given in Figure 5. One central 11 N

thruster is mean t for orbit control and the remaining eight 11 N thrusters for attitude control.


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Figure 5: Block diagram of the RCS (image credit: ISRO)

TT&C subsystem: The RF communications for RISAT-1 consists of two chains of PLL (Phase Locked Loop)

coherent S-band transponder connected to a comm on antenna s ystem. The bas ic configuration is identical to the

ones employed in earlier IRS mis sions . The TC demodulation schem e is PSK (Phase Shift Keying)/PCM (Pulse

Code Modulation) with a date rate of 4 kbit/s. The transponder cons ists of a receiving and transm itting system and

can operate in either coherent or non-coherent mode. The range and two-way Doppler data from the transponder

are useful for orbit determination.

PDHS (Payload Data Handling Subsystem): The RISAT-1 payload data need to be transmitted either in real-time

or in playback mode depending upon the data rates at different modes . The data-handling system of RISAT-1 is

configured with two formatters for each of the SAR payload receivers respectively (Figure 7).

These are high data rate formatters for different data rates of payload with memories for burst data formatting. Thesystems have been reali zed with FPGAs (Field-Programm able Gate Arrays) and the design is optimized for mas s,

power and volume. Whenever the data rate of the SAR payload and BDH overhead together is greater than 640

Mbit/s, real-time transm ission is no t possible and the data is recorded in SSR. The recorded data can be played

back later. The PDHS can operate in real-time, real-time stretch mode, record, and pl ayback modes .

Figure 6: Photo of the payload data formatters (image credit: ISRO)

SSR (Solid State Recorder): The SSR has a capacity of 300 Gbit, realized with six memory boards of 50 Gbit

capacity each. The memory boards, by default are configured into two partitions each of 150 Gbit with three

mem ory boards per partition. The SSR has two control units for configuring and controlling the internal operations.

The controller has two s eparate 32-bit parallel interface with mem ory boards. The default configuration is for two

partitions; however, the system can be configured for single partition with allocation of all the memory boards to

the selected partition. The SSR is able to manage up to 32 different files for each input port. The memory

management guarantees the usage of all good devices by automatic configuration after the diagnostics

command is issued.

X-band subsystem: The X-band RF is required to accept the payload data from the baseband data handling

system, modulate the above data on two X-band carriers and transmit the same to the ground after suitable

amplification and filtering.

The SAR payload of RISAT-1, when operated in dual polarization imaging mode, generates data at the rate of 640

Mbit/s and this needs to be transm itted to the ground s tations. Data rates up to 170 Mbit/s have been transmitted

in X-band using a shaped beam antenna in earlier m issions like IRS-1C/1D and PAA (Phased Array Antenna) in

Technology experiment satellite. In order to meet the high data rate transmission requirement in X-band, QPSK

(Quadrature Phase Shift Keying) m odulation with frequency reuse by polarization discrimination is implem ented.

In the data transmission for RISAT-1, half the data, i.e. 320 Mbit/s will be transmitted in RHCP (Right-Hand

Circular Polarization) and the remaining 320 Mbit/s in the LHCP (Left-Hand Circular Polarization); two identical

chains ope rating at X-band are us ed to transmit 640 Mbit/s of payload data. The carrier generation s ection, QPSK

modulator section, filter units, and the selection of the main and redundant chain units are identical in all the

chains, as the frequency of operation and modulation schemes is identical. Both the chains have end-to-end

redundancy.

PAA (Phased Array Antenna): The spherical PAA has radiating elements distributed almost uniformly on a

hemis pherical surface. It generates a beam in the required direction by switching ‘ON’ only those elem ents which

can contribute significantly towards the beam direction. It is propos ed to us e the 64 element array.

Operationally, PAA consists of two identical phased arrays, one operating in RHCP and the other operating in

LHCP, and located in the same hardware. On the spherical dome, an element is located at a defined location. A

waveguide radiating elem ent fed by a septum polarizer is planned and this has two ports, one for RHCP and the

other for LHCP. The radiating element is optimized to provide the required isolation (better than –25 dB) between

the two polarizations to min imize the interference.

The RHCP and LHCP po rts of the phas ed array are connected to two separate s ets of power d ividers and MMIC

(Monolithic Microwave Integrated Circuit) amplifiers. A common beam steering electronics controls the switch

position and phase s etting for all the MMIC amplifiers. The data transmiss ion chain is given in Figure 7.


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Figure 7: Schematic view of the data flow fr om SAR payload to the PAA (Phased Array Antenna), image credit:

ISRO

SPS (Satellite Pointing Subsystem): The SPS for RISAT-1 comprises a 10-channel C/A code GPS receiver at L1

(1575.42 MHz) frequency. SPS is designed for computing the state vector of the high-dynamic platform. The SPS

will have a full-chain (end-to-end) redundancy. Each chain consists of a receiving antenna, low-noise am plifier, RF

amplifier and power divider in L-band followed by a 10-channel and 8-channel GPS receiver with a MIL 1553B

interface. Each GPS receiver consists of two highly dynamic GPS RCE (Receiver Core Engine) modules to

compute the state vectors, one receiver chain will be active at a time.

The SPS is placed on the RISAT-1 spacecraft to track the GPS signals continuously. It requires an an tenna system

with hemispherical radiation coverage to receive the circularly polarized GPS signal from the navigational

satellites. A micro-strip patch antenna is used for this application.

Spacecraft launch mas s 1858 kg, including ~ 950 kg of SAR payload mas s

AOCS (Attitude Orbit Control Subsystem) - Pointing accuracy: 0.05º

- Drift rate: 5.0 x 10 –5 º/s

- Attitude knowledge: 0.02º

Power - Solar array generating 2.2 kW,

- 1 NiH2 battery set with a capacity of 70 Ah

- Max power handling capacity: 4.3 kW

- 70 V bus / 42 V bus

Design life 5 years

RF communications TT&C data in S-bandPayload data transm ission in X-band at 640 Mbit/s

SSR (Solid State Recorder) of 300 Gbit

Orbit SSO, altitude = 536 km, inclination = 97.55º, LTAN = 6:00 hr and 18 hr

Payload operations duration (duty cycle) 12 minutes/orbit

Table 1: Summary of mission parameters

In the non-observation support m ode, the active antenna is pointed in the nadir direction. Prior to each obs ervation

sequence, the spacecraft is roll-tilted to an angle of ± 34º. This means observations can be performed on either

side of the ground track (an advantage for event monitoring support). In addition, the spacecraft offers the

capability of pitch steering of up to ± 13º in s upport of high-resolution im aging (HRS mode). RISAT features also

the capability of yaw-steering to minimize the Earth rotation effects.

The new technologies in RISAT include: 160 x 4 Mbit/s data handling system, 50 Nms reaction wheels (with a


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torquing capability of 0.3 Nm), a SAR antenna deployment mechanism, and a phased array communication

antenna with dual polarization.

Figure 8: Blow-up illustration of the RISAT-1 spacecraf t (image credit: ISRO)

Figure 9: Alternate view of the deployed spacecraft (image credit: ISRO)

Figure 10: Photo of RISAT-1 with one of its solar panel wings deployed (image credit: ISRO)


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Figure 11: Photo of RISAT-1 in stowed launch configuration (image credit: ISRO)

Launch: RISAT-1 was launched on April 26, 2012 from SDSC (Satish Dhawan Space Center) at SHAR (Sriharikota

in Andhra Pradesh, on the east cost of India) on the PSLV-C19 vehicle. On this flight, the PSLV-XL version is used

with six extended strap-on m otors (PSOM-XL), each carrying 12 tons of solid propellant. 7) 8) - PSLV-C19 was the

first PSLV-XL to be launched from the FLP (First Launch Pad) of SDSC .

PSLV has three variants, namely, PSLV – the generic version with six regular strap-on motors (S9), PSLV–CA – the

core alone version without strap-on m otors and the more powerful PSLV-XL with S12 s trap-on motors (S12 is the

extended version of the regular S9 strap-ons in terms of length and propellant loading). The current payload

capability of the PSLV-XL vehicle is 1750 kg in 600 km SSPO (Sun-Synchronous Polar Orbit), and 1425 kg for the

Sub-GTO (Sub Geosynchronous Transfer Orbit) of 284 km x 21,000 km. The PSLV-C19/RISAT-1 mission

employed the PSLV-XL configuration of the launch vehicle with its uppe r stage (PS4) loaded with 2.5 tons of liquid

propellant to carry the heaviest satellite (1858 kg) ever entrusted to PSLV. 9)

Orbit planning: The initial mass budget for RISAT-1 was 1725 kg aiming a SSPO, 627 km above the Earth. Later,

the satellite mass was respecified to 1858 kg. The corresponding capability of the PSLV-XL was assessed for

various feasible orbits and it was decided to inject the satellite in 480 km circular orbit with an inclination

corresponding to 536 km SSPO mission, so that the orbit could be raised to 536 km using the spacecraft

propulsion system (Ref. 9).

Orbit: Sun-synchronous near-circular dawn-dusk orbit, altitude = 536 km, inclination = 97.552º, period = 95.49

minutes, LTAN (Local Time on Ascending Node) at 6 hours and 18 hours. The revisit period is 25 days with an

advantage of a12 day inner cycle in the CRS (Coarse Res olution ScanSAR) mode. Global coverage is achieved

twice in the revisit cycle, once by a set of descending pass es and next by a set of as cending pas ses, as SAR is a

microwave payload with no illum ination constraints.

RF communications: An onboard da ta storage capability of 300 Gbit is provided. The data downlink is in X-band

with a maximum data rate of 640 Mbit/s on two polarizations (320 Mbit/s RHCP and 320 Mbit/s LHCP) modula ted

on the sam e carrier (QPSK modulation).

The ISTRAC (ISRO Telemetry, Tracking and Com mand Network) is providing data acquis ition and TT&C services

through an integrated network of ground stations at Bangalore, Lucknow, Sriharikota, Port Blair,

Thiruvananthapuram, Mauritius, Bearslake (Russia), Brunei and Biak (Indonesia) with a multimission SCC

(Spacecraft Control Center) at Bangalore, India.


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Figure 12: Alternate view of the deployed RISAT-1 spacecraft ( image credit: ISRO) 10)

Status of mission:

• The RISAT-1 spacecraft and its payload are operating nominally in 2014. Data can be ordered through NRSC or

Antrix Corpora tion. 11)

- On April 2, 2013, RISAT was operated in s potlight mode for the first time.

- The antenna pattern was updated in Sept-Oct 2013.

- For the first time, RISAT-1 could provide polarimetric data in Spotlight and in ScanSAR mode. In addition, RISAT

-1 demons trated polarime tric observations at multiple incidence angles .


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Figure 13: RISAT-1 Spotlight image of Salt Lake Township, Kolkatta (Calcutta), India (image credit: ISRO/SAC)

Figure 14: Point target response of RISAT-1 Spotlight image of Figure 13 (image credit: ISRO/SAC)


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Figure 15: RISAT-1 hybrid polarimetric image of Jaipur, India, in ScanSAR mode (image credit: ISRO/SAC)

• The RISAT-1 spacecraft and its payload are operating nominally in 2013. 12)

Figure 16: RISAT-1 coverage in the per iod July 1, 2012 to Feb. 8, 2013 (image credit: ISRO, Ref. 12)

• According to Figure 16, the RISAT-1 SAR imagery was collected since July 1, 2012 (also during the calibration

phase until October 2012).

- Initial characterization comp leted including coverage over the Amazon region for calibration

- Data being regularly received at Shadnagar GS (Ground Station) of NRSC (National Remote Sensing Center)

and success ful test download at Tromsø, Norway

- Early valuation studies completed, users have demons trated pola rimetry applications with RISAT-1

- FRS, MRS & CRS mode data releas ed to users .


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Figure 17: Sivaganga area of Tamil Nadu, India, showing Samba rice area (cyan color), image credit: ISRO (Ref.

12)

Legend to Figure 17: The image is a two date compos ite of Oct. 25 and Nov. 19, 2012 with the SAR instrument of

RISAT-1 observing in MRS (Medium Res olution ScanSAR) operating mode.

• Operational status of the mission in October 2012: After calibration and validation of the image products, the

RISAT-1 image p roducts were releas ed for global users from October 19, 2012 onwards. They are available from

NRSC (National Remote Sens ing Center), Hyderabad. Typical images, obtained by RISAT-1, are shown in Figure

18. They demonstrate the quality of the RISAT-1 SAR images in a nutshell (Ref. 19).


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Figure 18: Typical images obtained by RISAT-1 (image credit: ISRO, Ref. 19)

Figure 19: NRSC Ground Station in Antartica: captured by RISAT-1 in Dual Pol (HH+HV), image credit: ISRO

• During IOT (In-Orbit Test), azimuth antenna patterns of the RISAT active antenna were measured through a

ground receiver. Figure 20 shows close agreem ent of the measured pattern with the predicted antenna pattern. It

is to be noted that during integrated testing, the antenna patterns were predicted based on the limited NF (Near

Field) meas urements. During IOT only, for the first time far-field antenna patterns were m easured. Furthermore,

radiometric correction using predicted elevation pattern could result in excellent radiometric balance over the

required swath within ± 0.5 dB. Both the above observations led to confidence in achieving calibration of the

RISAT–SAR system using a single corner reflector (Ref. 19).


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Figure 20: Close match of measured active antenna patterns during IOT with predicted ones (image credit:

ISRO)

The performance of calibration is shown in Figure 21 for an FRS-1 m ode im age over the Amazon rainforest. The

reported average sigma naught (σο ) over the Amazon rainforest is –7.5 dB. The calibrated average es timated σο

from RISAT-1 is clos e to the reported num ber.

Figure 21: Estimated average σο over the Amazon rainforest in FRS-1 mode (image credit: ISRO, Ref. 19)

• Eclipse operations: Due to the dawn/dus k orbit of RISAT-1, the orbital eclipse phas e is only seasonal (May 2 – August 12, 2012) with a maximum eclips e duration of about 22 minutes (around June 23, when the sun

declination was 23.5°); this is unlike the other IRS missions of ISRO where an eclipse is encountered in every

orbit. Regular monitoring and managem ent of the solar a rray, the battery resources, and the thermal control of the

on-board systems were carried out to match with the variable eclipse periods and seasonal variations. Daily

uploads of the eclipse start time and duration to the on-board systems was necessary for the initiation of the

SADA (Solar Array Drive Assem bly) auto capture in case of panel non-tracking during the sunlit pe riod. During an

orbital eclipse period, the SAR payload operation is avoided as the full load of the payload along with the

mainframe is requ ired to be s upported only by the battery (Ref. 29).

• In early May 2012, the RISAT-1 SAR instrument observed the first imagery over India (Figure 22). 13) 14)


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Figure 22: RISAT-1 image showing part of Mumbai as observed on May 4, 2012 (image credit: ISRO)

• The RISAT-1 SAR payload commissioning-related exercises started from 29 April 2012 onwards after the

miss ion orbit of 536 km was reached. Unlike other s atellites, for the first time in RISAT-1, a single X-band carrier

was being used to transmit V and H polarization data in RHCP and LHCP modes. Thus, systematic

characterization of the ground reception systems was carried out with data handling tests using RHCP mode

alone in one pass, LHCP mode alone in another pass and then both together in yet another pass. The SAR

payload commissioning started in a planned manner by operating the payload in FRS-1 (Fine Resolution

Stripmap-1) mode with single beam operation and then MRS/CRS (Medium and Coarse Resolution ScanSAR)

modes with multiple beam operations (Ref. 29).

The near beam(s) and far beam(s) energizing exercises were conducted for various modes and their power

profiles were characterized in-orbit. About 27 test cases , including on-board calibration operations were exercised

during the in-orbit commis sioning of SAR payload. The SSR (Solid State Recorder) was als o comm issioned with

recording and downloading o f the PN sequence, followed by imaging ses sions that required SSR recording.

• On April 27-28, 2012 the spacecraft's propuls ion system was used in 4 orbital m aneuvers to raise the altitude to

its nominal 536 km near-circular orbit. 15) 16) - The launch of PSLV-C19 had placed RISAT-1 into a required

intermittent polar orbit of 470 km x 480 km (in view of the large spacecraft mass of 1858 kg). The planned 4 orbital

maneuvers used 37 kg of on-board fuel to reach the nom inal orbit of 536.6 km

• Both the star trackers were s witched ON and norm alized to get star updates. The GOODS (GPS-based On-board

Orbit Determination System) was initialized a fter confirmation o f the SPS (Satellite Positioning System) tracking

the satellite. Thermal heaters and auto temperature control limits were fine-tuned with respect to on-orbit

configuration. After confirming star sensor updates, the spacecraft was put in normal mode with star sensor in

loop followed later by star Kalman filter mode. The safety features on-board the spacecraft – hardware s afe mode,

wheel over-speed logic, spurious speed logic, auto reconfiguration logic for wheels , failure detection logic of the

solar array drive, battery temperature control, etc. were enabled.

• In orbit 2, and with the corresponding network visibilities from Svalbard, Lucknow, Bangalore, Mauritius and

Trolls, further activities for norm alization of the spacecraft were carried out. Wheels were switched ON and run at

nominal control speed (3500 rpm ) to get a better dynamic friction estimation, which was subs equently changed to

the recommended nominal 1500 rpm in orbit 3.


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• Immedia tely after the spacecraft injection into its polar Sun-Synchronous Orbit, the automatic deployment of s olar

panels and SAR antenna deployments were carried out by the on-board timers triggered by the launcher and the

initial acquisition was initiated over the Troll ground station near the South Pole using the pre-loaded attitude

quaternions, followed by three-axis attitude acquisition usi ng ground com mands (Ref. 29).

Sensor complement:

The payload consists of the s ingle SAR instrument. The principal motivation behind this development is to provide

SAR imagery which will complement and supplement the optical imagery of ISRO spacecraft. During monsoon

periods and also for the regions which are perennially under cloud cover, RISAT will be the sole source of datawhen operational. The choice of the C-band frequency of operation and the RISAT-1 SAR capability of

simultaneous imaging in both co- and cross-polarization, will enable monitoring in a wide field of applications

such as: vegetation, agriculture, forestry, soil moisture, geology, sea ice, coastal processes, and man-made

object identification. In addition, RISAT will als o be used for disaster m onitoring services. 17) 18)

The RISAT-SAR ins trument suppo rts a variety of resolution and swath requirem ents. Both conventional s tripmap

and ScanSAR modes are supported, with dual polarization mode of operation. Additionally a quad polarization

stripmap m ode is p rovided for availing additional res ource classification. In all these m odes, resolu tions from 3 to

50 m can be achieved with swath ranging from 25 to 223 km. On experimental bas is, a slidi ng spotlight mode is

also available. In all the imaging modes, a novel polarimetry mode called circular or hybrid polarimetry can be

exercised seamlessly. The system is capable of imaging on either side of the flight track depending upon prior

programm ing of the satellite.

The payload is based on active antenna array technology. Crucial technology elements like C-band MMICs, TR

module and miniaturized power supplies have already been developed in India. A pulsed mode near-field test

facility has also been developed in-hous e in o rder to characterize the payload in the in tegration laboratory itself. 19)

RISAT-SAR:

The RISAT-SAR instrument, designed and developed at ISRO/SAC (ISRO/Space Applications Center),

Ahmedabad, India . The RISAT-SAR is configured on a dual receiver concept providing identical res olution in both

simultaneous co- and cross-polarization operation support modes (Table 3). The RISAT-SAR instrument consists

of two broad segments, namely:

- The deployable SAR antenna subsystem

- The RF and baseband subsystems m ounted on the satellite deck.

Figure 23: Illustration of the RISAT deployed spacecraft, antenna and detailed view of a tile (image credit:

ISRO)

SAR antenna subsystem: The Earth-facing side o f the active phas ed array antenna is a broadband dual pola rized

microstrip radiating aperture. The antenna consists of three deployable panels, each panel of 2 m x 2 m in size.

Each panel in turn cons ists four tiles o f size 1 m x 1 m with 24 x 24 radiating elements. In each tile, all the 24 x 24

radiating elements are grouped into 24 groups, each group consisting of 24 elements spread along azimuth

directions which are fed by two stripline distribution networks feeding for V and H polarization. Each of these

groups of 24 radiating elements are catered to by two functionally separate T/R (Transmit/Receive) modules,

feeding two separate distribution networks for V and H operation with the same radiating patches.

The peak RF power, fed by each T/R module, is 10 W at a duty cycle of ~7-8%. The two functionally separate T/R

modules are mounted in the same physical enclosure, sharing the same power supply and T/R control

electronics. This sort of grouping also enables phase steering in the elevation direction. All the 24 T/R modules

on one tile are controlled by one TCU (Tile Control Unit). The T/R modules and TCU are mounted on the backside

of the antenna. The mechanical configuration of the complete antenna, grouped into three panels pe r twelve tiles,

and a detailed view of the basic tile structure, are shown in Figure 23.


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An extensive onboard calibra tion facility is provided with the help of a set of CAL switches and dedicated

distribution networks for calibrating transmit and receive paths of each of the T/R modules separately.

Swa th coverage Se lectab le wi thin 107 – 659 km off-nadir di stance on either si de

Incidence angle

coverage

12º – 55º

Operating

mode

Polarization mode

Single Pol

HH/HV/VV/VH

Dual Pol

HH+HV/VV+VH

Circular (Hybrid) Polarimetry

TX: CP

Rx: V and H (Experimental)

Quad Pol

HH+HV+VV+VH

HRS 1 m(Azimuth) x 0.67 m (Range) reso lution, 10 km x 10 km (10 x

100 km Experimental) Spot

Min σo= -16 dB

FRS-1 3 m(Azimuth) x 2 m (Range) res olution, 25 km swath

Min σo= -17 dB

FRS-2 3 m(Azimuth) x 4 m (Range)

resolution, 25 km swath

Min σo= -20 dB

9 m(Azimuth) x 4 m (Range)

resolution, 25 km swath

σo= -19dB

MRS 21-23 m (Azimuth) x 8 m (Range) res olution, 115 km swath

Min σo= -17 dB

CRS 41-55 m (Azimuth) x 8 m (Range) res olution, 223 km swath

Min σo= -17 dB

Table 2: RISAT-1 image quality parameters

Frequency 5.350 GHz (C-band)

SAR antenna type Printed antenna

Antenna size 6 m (along-track) x 2 m (cros s-track)

Side lobe level -13 dB (azimuth), -13 dB (elevation)

No. of TR modules 288, each with 10 W peak power

Pulse width 20/10 µs

Average DC Input power 1.8-3.9 kW (average DC input power)

Operating mode HRS FRS-1 FRS-2 MRS/CRS

Chirp bandwidth 225 MHz 75 MHz 37.5 MHz 18.75 MHz

Sampling rate 250 MHz 83.3 MHz 41.67 MHz 20.83 MHz

PRF (Pulse Repetition

Frequency)

3500 Hz ± 200 Hz 3000 Hz ± 200 Hz 3000 Hz ± 200

Hz

3000 Hz ± 200 Hz

Data quantization 2/3 BAQ 2/3/4/5/6 bit BAQ

Data rate (max)

@ 3 bit BAQ for HRS

@ 6 bit BAQ for rest of modes

739 Mbit/s (single

pol.)

1478 Mbit/s (dua l

pol.)

556 Mbit/s (si ngle

pol.)

1112 Mbit/s (dual

pol.)

564 Mbit/s 142 Mbit/s (sing le

pol.)

284 Mbit/s (dual

pol.)

Table 3: Hardware and imaging paramete rs of the SAR instrument

RF and baseband subsystems: Two separate chains of receiver and data acquisition and compression system

cater to simultaneous operation in two polarizations. However, the feeder SSPA, the frequency generator, and the

digital chirp generator are common to both of the polarization chains. All subsystems are configured with 100%

redundancy. The feeder SSPA transmits a chirped pulse of 20 µs to the active antenna during the transmit period.

The flexible digital chirp generator provides the expanded pulses of four different bandwidths of 225 , 75, 37.5 and

18.75 MHz for operation in the various im aging m odes. The analog base band output is fed to frequency generator

unit, which in turn generates chirped carrier at 5.35 GHz. The chirped carrier is amplified through feeder SSPA and

fed to TR modules. The frequency generator generates IF, LO and frequency reference for data acquisition

subsystems.

Its configuration of dedicating separate set of power amplifiers for V and H polarization transm ission, has made it

a unique spaceborne Hybrid Polarimetric Sensor. The other operational spaceborne SAR instruments like on

Radarsat-2, TerraSAR-X or COSMO Skymed, are equipped with specific linear po larimetric mode which is usually

operated within the restricted coverage of 20° to 30° incidence angle, because of doubling of PRF (Pulse

Repetition Frequency) and usually a specific imaging mode is dedicated for linear polarimetric operation.


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However, in the hybrid polarimetric operation of RISAT-1, signal is transmitted in circular polarization and the

received signal is digitized in two orthogonal polarization chains. This ensures conventional PRF of operation

without any increase in data rate. Hybrid polarization in RISAT-1 can be activated for any imaging mode

(spotlight/stripmap/ScanSAR) and can be operated over any incidence angle ranging from 12° to 55°.

Figure 24: Configuration of RF and baseband system (image credit: ISRO)

The combined signal from active antenna is down-converted to IF which is subsequently I-Q detected prior to

digitization. No provision of onboard range compression is kept and range compression needs to be carried out

on ground. The bas eband I-Q detected receive signal is suitably band limited to m aximize the SNR by a set of four

selectable I-Q filters. The first stage of the data acquisition unit is an 8 bit digitizer. RISAT-SAR provides the unique

option of user choice o f a seam less BAQ (Block Adaptive Quantization) option from 2-6 bits depending upon theapplication requirements. Each of the I and Q channels are sepa rately digitized, compress ed and formatted with

identical repetition of common auxiliary parameters and data is taken out from each I-Q channel as 16 bit parallel

stream (quantized data) at a constant rate of 31.25 MHz. The choice of antenna size put a constraint in the

selection of the PRF (Pulse within a range of 2800-3700 Hz, a consequence from both the Doppler sampling

requirement and range am biguity considerations. 20) 21)

Figure 25: Illustration of RISAT-SAR constituent subsystems (image credit: ISRO) 22)

Operating modes of RISAT-SAR:

The RISAT-SAR system is designed to provide a constant swath for all elevation pointing and almost near-

constant minimum rada r cross section performance. The following operational modes are defined:

• FRS-1 (Fine Resolution Stripmap-1): 3 m x 2 m (Az x Range) resolution, 25 km swath, co- and/or cross

polarization and hybrid polarimetry.

• FRS-2 (Fine Reso lution Stripmap-2): 6 m x 4 m (Az x Range) resolution, 25 km swath, Quad polarization;

3 m x 4 m (Az x Range) res olution, 25 km s wath in hybrid polarimetry.

• MRS (Medium Resolution ScanSAR): 25 m x 8 m (Az x Range) resolution 115 km swath, co- and/or cross

polarization or in hybrid polarimetry. The MRS mode is configured with 6 antenna beam pointings in elevation.

• CRS (Coarse Resolution ScanSAR): 50 m x 8 m (Az x Range) resolution, 223 km swath, co and/or cross

polarization or in hybrid polarime try. The CRS mode uses 12 beams in the elevation.

• HRS (High Resolu tion Spotlight): < 2 m resolution, a spotlight target of 10 km (azimuth) x 10 km (ground range ),

in co- and/or cross polarization or i n hybrid polarimetry.


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HRS features an experimental capability to increase the azimuth extent up to 100 km. However, all the images are

available in single-look on ly except in CRS mode which offers up to 2 range looks.

In the ScanSAR mode, the antenna beam pointing switches electronically in the elevation direction to cover

adjacent sub-swaths (swath covered by individual beam s) in regul ar intervals of time, referred to as burs t time.

RISAT-SAR obs ervations m ay be performed on either s ide of the ground track (left or right looking capability) by

roll-tilting of the antenna by ±36º. However, this tilting feature is lim ited to one s ide per orbit.

RISAT-SAR operates with basic elevation beam width of 2.2º -1.5º over a total ground distance of 550 km starting

from off nadir distance of 107 km. Within this 550 km operating ground range, the image products will be fully

qualified. The off-nadir look angle is 11.5-49.5º.

Figure 26: Illustration of the RISAT-SAR imaging modes (image credit: ISRO, Ref. 19)

Figure 27: Schematic configuration of the RISAT-SAR instrument (image credit: ISRO)

Integrated SAR antenna:

The antenna is a ma jor RISAT element s upporting a total of 126 beam s. The accuracy of pointing and knowledge

of the pattern has a definite bearing on the radiometric performance of the RISAT-SAR ins trument and the overall

mapping requirements. Hence, an extensive laboratory antenna pattern measurement program is undertaken

prior to integration. The measurement concept is based on the PNF (Planar Near-Field) concept as shown in

Figure 32. This involves the transm it and receive pattern meas urement in both polarizations using the RISAT-SAR

pulse.

The measurement will be carried out under zero-G conditions. The proposed measurement scheme will ensure

the mechanical references are kept the same for the 4 different pattern (Tx-V, Tx-H, Rx-V, Rx-H) measurements.

The scanner is basically capable of scanning a probe in x-y plane of the clean scan area of 8 m x 4 m. However,

only a limited z-axis s can capability of 20 cm is provided. In realistic terms, the s can plane has to be made parallel

to the actual antenna plane. A laser tracking instrument is being used to provide the plane information of the

antenna plane.

The Earth-facing side of the active antenna is a broadband dual pola rized m icrostrip radiating aperture. The active


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antenna system consists of three deployable panels, each of 2 m x 2 m s ize. Each of the panels is subdivided into

four tiles of 1 m x 1 m in s ize (Figure 28). Each tile consists of 24 dual polarized linear arrays, aligned along the

azimuth direction. Each of the linear arrays, of length 1 m, is basically composed of 20 equi-spaced microstrip

patches, EM coupled by two orthogonal strip line networks (Figure 29). Each of these linear arrays is fed by

functionally two separate TR modules feeding two separate distribution networks for V and H operation with the

same radiating patches. The outer duroid layer also doubles up as a radome and the patches are printed on the

inner side of the outer duroid layer. A glass-wool blanket on the antenna isolates it from heating by the Earth as

well as s olar radiation or from cooling in the absence of so lar radiation, when the antenna points away from solar

illumination.

Figure 28: Organization of RISAT-1 antenna with detailed view of a tile 8image credit: ISRO)

Figure 29: Typical configuration of microstrip patch used in RISAT-1 (image credit: ISRO)

The printed antenna is grown on one s ide of a CFRP (Carbon Fiber Reinforced Plastic) honeycomb plate. The rest

of the active antenna electronics is mounted on the other s ide of this plate. Fast beam switching and beamwidth

control is achieved by electronic elevation beam control in the active antenna. Sixty-one (61) beam-pointing

positions have been identified to enable imaging anywhere over 550 km region on one side of the subsatellite

track, with the best pos sible performance. Each beam is centered at off-nadir intervals o f ~ 9 km. Two addi tional

beams with no pointing (0º with respect to antenna orientation angle, i.e. ± 36º) are defined for two halves of the

antenna, 6 m x 1 m each. Therefore, there are 63 beam positions defined for imaging on each side of the

subs atellite track. As a result, a total of 126 beam s are used for imaging on either side of the track. An option of

yaw rotation for left–right imaging would have reduced the requirement of the number of beams by half. But

operationally, this option would have an im plication on the time for switching to imaging on either side of the track.

The active beam-width in elevation direction is controlled such that for each beam a 25 km swath with near

identical σο performance is achieved irres pective of the elevation poin ting. Typical σο

performance over different

off-nadir distances is shown in Figure 30. The TR-modules a re switched off in the width direction, equally from the

outer edges of the two adjacent tiles to control elevation beam width between 2.48º and 1.67º. Such a s trategy has


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been adopted for elevation beam control for easing out thermal m anagem ent.

The peak RF power, fed by each TR module, is 10 W at a duty cycle of ~ 7%. These two functionally separate TR

modules are housed in two different physical enclosures, sharing the same power supply and TR control

electronics (TRC). The basic architecture of a TR module is s hown in Figure 31. Phase and amplitude control of

the TR module is achieved by 6 bit phase shifter and 6 bit attenuator, which in turn are shared by both transmit

and receive paths. Each of the TR modu les is extensively characterized over ambient temperature from –10ºC to

60ºC. The LNA (Low Noise Amplifier) of the TR module is protected by a PIN diode switch. At the circulator output a

coupler provides the required calibration stimuli. On the tile, two rows of TR modules, each consisting of 12

modules , feed alternate antenna arrays.

Figure 30: Minimum σο performance over the swath for FRS-1 mode operation (image credit: ISRO)

Figure 31: Block diagram of a TR module for RISAT-1 (image credit: ISRO)

Both the TR modules (H and V) and TRC are powered by a miniaturized pulsed EPC (Electronic Power

Conditioner) called PCDU (Power Conditioning and Processing Unit). An ASIC (Application Specific Integrated

Circuit)-based TRC controls both H and V TR modules and the PCPU. Power sequencing is such that both

transmit and receive paths are switched on by power pulsing only, for the required duration in every PRI (Pulse

Repetition Interval), in order to conserve power. It not only sequences the smooth operation of TR modules, but

also provides requis ite temperature compensation of phase and amp litude variation from stored characterization

table. A thermis tor voltage from the TR m odules provides the requisite i nput for appropriate reading of LUT (Look-

Up Table).

All 24 TR m odules on a til e are controlled by one TCU (Tile Control Unit). It provides synchronization of the TR

modules with a master reference. It also provides requisite ampli tude and phase correction required on each TR

module for appropriate collimation for a particular beam pointing and pattern weighting. No weighting is pos sible

to be provided during transmis sion as all the TR m odules operate in saturation condition. Only on reception, is theweighting applied.


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Figure 32: Mechanical configuration of the planar near-field measurement setup (image credit: ISRO)

Payload control and management:

The RISAT-SAR payload is controlled by an array of controllers organized in a three-tier hierarchy as depicted in

Figure 33. At the top level of the hierarchy, the complete payload is controlled by a central computer, referred to as

PLC (Payload Controller), which interfaces with the RF and baseband subsystems, nam ely the DCG (Digital Chirp

Generator), the V and H receivers, the FG (Frequency Generator), Feeder SSPA (Solid¿State Power Amplifier), CAL

(Calibration Switch Matrix) and four DACSs (Data Acquisition and Compression Subsystems). PLC is an

autonomous controller with only spacecraft interface being DC Power and a 1553 interface with the BMU (Bus

Management Unit) of the spacecraft.

The bit parallel data at the DACU output is directly interfaced with spacecraft's BDH (Baseband Data Handling)

unit for further formatting, recording, encryption and transmiss ion. The payload controller in turn controls the active

antenna via the tile control units residi ng in each TCU (Tile Control Unit). The PLC es sentially transm its the beam

definition command and switching sequence definitions to the active antenna. The TCU controls the beam

pointing and the beam setting in a tile via the T/R controller. It also sequences the TRM (T/R Module) power on/off

command. The TCU transmits T/R module specific beam shifting, beam weighting and residual corporate feed

mismatch compensation related phase and the amplitude coefficients to specific T/R modules. Each of the T/R

modules is controlled by corresponding TRC (T/R Controller). Each TRC controls two independent T/R modules

where each is dedicated for a polarization and one PCPU (Power Conditioning and Process ing Unit) powering the

TRC and two T/R modules. The TRC contains in its memory all the temperature related phase and amplitude

calibration data for each T/R module and imparts the corresponding corrections from instantaneous

measurement of ambient temperature.

Figure 33: Block diagram of the three-tier control of RISAT (image credit: ISRO)


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Advanced Digital Subsystems (technology introduction):

The introduction of advanced and new digital technology has contributed significantly in the optimization of the

various units of the RISAT-SAR (in size, mass, and power). Most of these digital subsystems, except for the active

antenna tile digital electronics, have already been test flown for performance evaluation (on the airbo rne SAR of

ISRO; test flight in November/December 2005).

Apart from design and developm ent of an active antenna based SAR system, the major achievement of the RISAT

project has been extensive industrial participation in developing critical technological elements needed for the

payload. The TR modules, along with all the MMICs (Microwave Monolithic Integrated Circuit) associated with it

have been designed and p roduced in India. 23) 24)

The miniaturized pulsed EPC (Electric Power Conditioner) for powering the TRMs (T/R module), featuring three

different HMCs and planar transformer, has been a feat of Indian indus try. The printed antenna aperture has been

designed in ISRO and produced by the industry of India. The ASIC, meant for controlling active array antenna

elements, has been des igned in-hous e. The notable achievement of RISAT project has been es tablishment of a

novel near field antenna facility, based on time segregation of requisite signal from unwanted echoes, has been

designed in-house and built in cooperation of Indian indus try, for characterization of the active antenna.

The SAR antenna em ploys an active phased array radar because of it's capability in terms of beam s teering, multi-

beam operations, large bandwidth and high efficiency. Electronic beam steering requires the loading of digital

amplitude and phase values to the array of TRMs (Transmit/Receive Modules). For a large array of elements,

distributed digital controllers are being used to load the beam characterization, timing control and serial

communication. The hie rarchy of the dis tributed controllers for the active phased array antenna of RISAT is shown

in Figure 38 with three levels of hierarchy. The PLC (Payload Controller) is at the top of the hierarchy. The PLC

controls 12 dis tributed TCUs (2nd level). Each TCU controls 24 TRCs (T/R Controllers), representing the 3rd level

of the hierarchical structure. 25) 26)

Figure 34: Electronic beam steering using the active phased a rray antenna (image credit: ISRO, Ref. 10)


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Figure 35: Imaging geometry of RISAT-1 (image credit: ISRO)

Figure 36: Photo of a TRM (T/R Module) of size 170 mm x 70 mm x 20 mm (image c redit: ISRO)

Figure 37: Photo of a TRC (T/R Controller) ASIC (image credit: ISRO)


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Figure 38: Distributed controller hierar chy (image credit: ISRO)

1) Baseband Digital Subsystems

• DCG (Digital Chirp Generator). The DCG hardware consi sts of 2 identical modules located in the power s upply. A

DCG device is based on Xilinx Virtex XQVR-600 FPGA synthesizers which transmit the I/Q chirp signal of 0-75 MHz

bandwidth using either a PROM look-up table or a direct digital chirp synthesis approach. The output signal is low-

pass filtered, vector modulated, multiplied by an appropriate factor and subsequently up-converted in the RF

segm ent to the desired carrier frequency.

Figure 39: Standard configuration of the DCG (image credit: ISRO/SAC)

• DACS (Data Acquisition and Compression Subsystem). The objective of DACS is to support the following

functions:

- High-speed signal (8 bit I/Q) quantization of the complex radar echo, including calibration

- Demultiplexing of the signal into multiple channels

- Data compres sion is performed with a real-time flexible BAQ (Block Adaptive Quantization) algorithm o f 2-6 bit

length

- Variable data rate of 64-1562 Mbit/s

- Formatting of demultiplexed data channels and auxiliary information.

The DACS module consists of Atmel's high speed ADC (Analog Digital Converter) & demultiplexer and a Xilinx

XQVR-600 FPGA-based BAQ and forma tter. These are 2 iden tical DACS units along with their power s upplies.


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Figure 40: Standard configuration of the DACS (image credit: ISRO/SAC)

• PLC (Payload Controller). The objective is to provide em bedded control. command and coordination of a ll RISAT-

SAR activities, including the active antenna electronics and instrument configuration. PLC interfaces with the BMU

(Bus Management Unit) and also generates all required control and timing signals in support of coherent SAR

operations. The PLC hardware consists of a motherboard-daughterboard configuration using a single 80C32

microcontroller, a Xilinx XQVR-600 FPGA, a Mil-STD-1553B based control processor and 3 I/O modules along with

its power supply. There are 2 PLCs, primary and redundan t unit.

Figure 41: Standard configuration of the PLC (image credit: ISRO/SAC)

2) Tile Digital Electronics

The active antenna subsystem features a distributed embedded control subsystem which supports digital beam-

forming and beam switching in the elevation direction. A three-stage hierarchy is implemented consisting of the

PLC, TCU (Tile Control Unit) and TRC (Transmit Receive Controller).

• The TCU controls and coordinates the activities of a individual tile; it consists of m ultiple T/R modules with their

electronics. The TCU utilizes the beam configuration related information received from the PLC to translate and

compute the phase and amplitude param eters for the relevant T/R modules (as well as monitoring). The TCU is a

radiation-hardened ASIC (Application Specific Integrated Circuit) module containing the prime and redundant

logics. The antenna consists of 12 TCUs.

• A single TRC un it controls the H and V T/R-RF modules . The TRC stores the entire characterization (like phas e

and gain correction factors for a given T/R module pair (H&V) and controls the phas e shi fter and attenuator (bothat 6 bit) in the T/R RF modules. The TRC is a lso a radiation-hardened ASIC module housed along with the T/R RF

modules . A total of 288 TRC units are used for the active antenna.

3) Ground segment processing and support systems

• QLP/NRTP (Quicklook Processor/Near Real-time SAR Processor). This device consists of a cPCI computer

system, and a COTS-based DSP (Digital Signal Processor). The multiprocessor DSP board uses 8 to 16 DSP

modules with multiple link interfaces. The peripheral boards include SCSI, SVGA, and cus tom FPGA interfaces.

Two-dimensional complex SAR processing, involving range and azimuth compress ion and m otion compensation

tasks are performed by the DSP boards. The processed imagery is displayed on a monitor and stored on a

recorder. A cPCI (Pentium P$ s ingle board) computer performs the control and coordination tasks for the various

DSP devices and interfaces. 27)

DCG (Digital Chirp Generator)

Chirp parameters 2-75 MHz; 20 µs


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Res olution 10 bit am plitude; 12 bit frequency/phas e (m in)

Device size, mass, power 260 mm x 210 mm x 150 mm, 4 kg, 4.5 W

DACS (Data Acquisition and Compression Subsystem)

ADC 8 I+8Q @ 20.8 - 250 MHz sam pling rate

Onboard compression 2-6 bit BAQ (Block Adaptive Quantization) with bypass facility

Sensor data rate (max) 326 Mbit/s (6 bit BAQ), 1562 Mbit/s (3 bit BAQ)

Device size, mass, power 260 mm x 210 mm x 150 mm, 4.6 kg, 27 W

PLC (Payload Controller)

Tim ing 2800-3700 Hz PRF (Puls e Repetition Frequency); 58-184 µs of data collection

window

Interfaces MIL-STD-1553B @ 1 Mbit/s with bus BMU

Device size, mass, power 285 mm x 235 mm x 165 mm, 5 kg, 15 W

TCU/TRC (Tile Digital Electronics)

No of elevation beams 126 (over 100-700 km swath geometry on either side of the ground track)

TRM phase/amp update

rate

18 µs @ PRF rate, HC TTL interface

Device size, mass, power 380 mm x 60 mm x 90 mm, 2 kg, 3 W (TCU)

173 m m x 18.5 mm x 81 mm, 0.2 kg, 1.5 W, (TRC)

QLP/NRTP (Quicklook Processor/Near Real-time SAR Processor)

SAR image parameters Resolution: 1-12 m (ground range) x 1-48 m (azimuth)

Limited swath (4096 range gates, 15-240 km in width) real-time

Full swath (20 k range gates, 30-240 km in width) near real-time

Throughput Rea l-ti me (QLP) to max 45 m inutes (NRTP) pe r s cene , 10 -15 GFLOPS

Device parameters 8-14 slot cPCI chassis, PE SBC, 96 Tiger SHARC DSP @ 250 MHz, 700 W, 20 kg

Table 4: Overview of Advanced Digital Subsystem paramete rs

Figure 42: RISAT-1 data processing levels & products (image credit: ISRO/NRSC) 28)

Standard products Projection UTM/POLYCONIC

Datum WGS84

Resampling CC

Format CEOS/GEOTIFF

Media DVD/DISK

Delivery Courier / FTP


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Value added products Pol-SAR products

Multi-date/sensor regi stered products

Large area mos aic products

Local incidence angle corrected products

Precision geocoded products

Terrain geocoded products

InSAR products

Speckle reduced products

Table 5: Overview of SAR products

Ground segment:

The RISAT-1 satellite health maintenance and SAR payload operations are carried out from the MOX (Mission

Operations Com plex) of ISTRAC (ISRO Telemetry, Tracking and Command Network), Bangalore, us ing various

mission computers and associated mission software and communication links. 29) 30)

The TTC (Telemetry, Tracking and Comm and) functions of the sa tellite in S-band are a lso s upported by a network

of ground stations. The recently operationalized IMGEOS (Integrated Multi-mission Ground segment for Earth

Observation Satellites) facility at NRSC (National Remote Sensing Center), Shadnagar, Hyderabad Complex

carries out the automated execution of entire ground-processing tasks for RISAT-1 mission beginning with SAR

payload programming, data acquisition and SAR signal and image DP (Data Processing) to SAR raw data and

data product dissemination with fast turn-around times (TATs).

Unlike optical sens ors, SAR image or data product generation involves e laborate pre-processing of SAR raw dataas well as compl ex, two-dimensional radar-matched filtering or focusing apart from other motion correction tasks,

all of which have been implemented and operationalized by the SIPA/SAC team. A HWQLP (Hardware Quick Look

SAR Processor/NRTP (Near Real Time SAR Processor) has also been built by the MRSA/SAC team at

Ahmedabad and installed at IMGEOS, NRSC, Shadnagar.

The complexity of RISAT-1 mis sion entailed proper planning, execution and critical mon itoring of on-board payload

subsystems. During LEOP (Launch and Early Orbit Phase), initial and normal phases were provided from the

MOX/ISTRAC of Bangalore. Figure 43 illustrates the functional organization of the RISAT-1 ground segment

operations.

Figure 43: Organization of ground segment operations (image credit: ISRO)

Figure 44 illustrates the TTC ground station configuration. The ISTRAC TTC stations are equipped with an

antenna subsystem with T/R (Transmit/Receive) feed, TTCP (TTC Processor), STC (Station Computer), and

Monitor and Control System. The station is configured to support S-band carrier reception with polarization

diversity mode for auto track and ranging functions. It receives both RHCP and LHCP signals simu ltaneously and

combines them optimally before data detection. The ISTRAC communication network provides the real-timevoice/data/fax connectivity for the mission operations between the MOX, the Vehicle Control Center, TTC stations,

payload data acquisition, and the DP centers. Comm unication is es tablished us ing satellite links, terrestrial links

and dedicated fiber links.

The pre-launch and LEOP operations were supported from the MCR (Miss ion Control Room ) and MAR (Mission

Analysis Room ). The regular norm al phas e opera tions are being supported from a DMCR (Ded icated Mission

Control Room).


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Figure 44: Configuration of the TTC ground station (image credit: ISRO)

Roll bias and attitude steering for zero Doppler: Since the SAR payload is a side-looking radar, there is a roll bias

requirement of ± 36° for the left and right-looking configuration. Also, to nullify the Doppler due to Earth’s rotation

and the Doppl er variations due to the eccentricity of the orbit and Earth's obl ateness, yaw and pitch s teering of the

spacecraft have to be performed. The necessary steering coefficients and bias commands are uploaded to the

satellite through ground commanding. Both yaw and pitch steering coefficients are computed on ground and

uplinked to the satellite. The residual Doppler (50–150 Hz) can be estimated either from the SAR raw data, or

using the s pacecraft attitude data, and is being corrected during processing.

SAR payload operations: RISAT-1 SAR payload operations are carried out using the on-board payload sequencer

by transmitting the commands gene rated by the CSG (Command Sequence Generator) based on the request file

received from NDC (NRSC Data Center), Hyderabad, using the PPS (Payload Programming System). PPS is a

ground-based operational software system to efficiently plan user image acquisition requests and generate

spacecraft payload sequencing commands for imaging the area of user request. It also helps to image the

maximum number of user requested AOI (Areas of Interest) in a pass-wise sequence, by arranging the user

requests in one orbit and optimally using the spacecraft resources.

The PPS is utilized to generate the P/L operations on a given day, including the SSR recording operations

elsewhere in the world. The consolidated P/L plan is sent to the SCC (Spacecraft Control Center) of ISTRAC,

Bangalore for command generation through the CSG system. The CSG is responsible for the generation of

configuration and timing information and the beam parameters for conducting the SAR payload operations. The

SAR P/L operation comm ands are uplinked one day in advance. Thus, the PPS and CSG activities are im portant

components of the MMS (Mission Management System).

DRS (Data Reception System): The DRS compris es four 7.5 m antenna systems with dual polarization configured

in multi-mis sion m ode to track and receive data from any remote sens ing satellite. It is equipped with the state-of-

the-art bore-site facility for validating the data reception chain, both in local loop and radiation mode. Figure 45

shows one of the RISAT-1 DRS chains configured under IMGEOS architecture. It consists of the antenna and

tracking pedestal, dual polarized feed and RF systems, digital servo and automation system, IF and baseband

system and data ingest system. The composite S/X feed is dual circularly polarized in both S-band and X-band

with the capability to receive LHC and RHC polarized signals simultaneously using the frequency reuse

technique. The S-band telemetry data and tracking signals are down-converted to 70 MHz IF. The down-converted

X- and S-band tracking IF signals are fed to a three-channel ITS (Integrated Tracking System). The IF outputs from

first data down-converter (two carriers) and the S-band data IF are transferred to the control room through a m ulti-

core optical fiber cable and fed through a program mable IF matrix to the second down-converter and then to high

data rate digital demodulator. The data and clock signals from high rate digital demodulators are driven through

LVDS interface to the data ingest system for further processing and product generation.

The salient features of the RISAT-1 DRS are as follows:

• 7.5 m Cass egrain antenna s ystem with G/T of 32 dB/°K @ 5° EL.

• Simultaneous RHC and LHC polarized signal reception @ 8212 .5 MHz with dual polarized S/X-band composite

feed using the frequency reuse technique.

• The feed and front-end system are realized with a single channel mono pulse tracking.

• Two data reception chains at 720 MHz IF, each with 320 MHz bandwidth.

• X-band auto track either through RHCP or LHCP carrier.

• QPSK modulated RF carrier with 160 Mbit/s da ta rate each in I and Q channels.

• Synthesized up/down converter with additional channels.

• IF link for transfer of high data rate modulated IF spectra.

• High data rate dem odulators at 320 Mbit/s (I + Q) data rate.


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Figure 45: Schematic view of the RISAT-1 data reception chain (image credit: ISRO)

SAR off-line DP and product generation: The off-line operational DP for RISAT-1 SAR is carried out at NRSC,

Shadnagar in an IMGEOS environment on six SMP nodes with each node having four 8 core-machines. The basic

steps of SAR DP can be summ arized as follows:

• Block adaptive quan tization decompres sion

• Correction for I and Q imbalance

• Doppler centroid estimation

• Range compression

• Range cell m igration correction

• Azimuth compres sion

• Single-look complex or m ulti-look data generation

• Slant range to ground range conversion

• Geocoding.

Figure 46 shows the basic data flow diagram for SAR processor. The request for data product generation is

ingested through Data Product work flow managers. Master and slave schedulers execute on separate hosts.

Once a work-order arrives, the software automatically routes it to a free slave node and generates the outputs. The

status of work-orders, viz. running, sus pended, aborted, scheduled, error or completed for a particular scheduler

sess ion can be known from GUI. The data products generation facility caters to Stripmap, ScanSAR and Spotlight

imaging modes of the RISAT-1 satellite with the following product level specifications.

• Raw Signal Products (level-0)

• Geo-Tagged Products (level-1)

• Terrain-corrected Geocoded Products (level-2).

Figure 46: RISAT data flow for the SAR processor (image credit: ISRO, Ref. 29)


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The data products from RISAT-1 have already been released to us ers from 19 October 2012 onwards. The RISAT-

1 imaging products are expected to enhance the application potential of SAR data not only in India, but also

globally in many important resource applications and di saster m anagemen t situations. RADAR (Radio Detection

and Ranging) data from space platforms have already made a significant mark the world over because of the

ability of the radars to m ake observations during the day or night, look through cloud cover and achieve resolution

and observe details that are difficult to obtain for optical and infrared sensors. Many operational modes and the

hybrid polarimetric capabilities of RISAT-1 are expected to open up newer avenues, as it provides many more

observable parameters like amplitude, phase and state of polarization, enabling many new scientific studies

leading to diverse and novel applications us ing mi crowave data (Ref. 6).

1) T. Misra, S. S. Rana, V. H. Bora, N. M. Desai, C. V. N. Rao, Rajeevjyothi, “SAR Payload of Radar

Imaging Satellite (RISAT) of ISRO,” Proceedings of EUSAR 2006, Dresden, Germany, May 16-18, 2006

2) T. Misra, S. S. Rana, R. N. Tyagi, K. Thyagarajan, ”RISAT: first planned SAR mission of ISRO,” Asia-

Pacific Remote Sensing, Conference 6407, `GEOSS and Next-Generation Sensors and Missions,' Stephen

A. Mango, Ranganath R. Navalgund, Yoshifumi Yasuoka, Editors, Proceedings of SPIE, Vol. 6407, Goa,

India, Nov. 13-17, 2006

3) B. Asha Rani, P. Deepak, T. Misra, “Development of the ScanSAR Processing Algorithm for Spaceborne

SAR,” Proceedings of IRSI (International Radar Symposium India), Bangalore, India, Dec. 20-22, 2005

4) “RISAT-1 Radar Imaging Satellite,” Sept. 2007, URL:

http://www.scanex.ru/en/publications/pdf/publication50.pdf

5) N. Valarmathi, R. N. Tyagi, S. M. Kamath, B. Trinatha Reddy, M. VenkataRamana, V. V. Srinivasan,

Chayan Dutta, N. Veena, K. Venketesh, G. N. Raveendranath, G. Ravi Chandra Babu, K. Sreenivasa

Prasad, Rajeev R. Badagandi, P. Natarajan, S. Sudhakar, J. Subhalakshmi, Sreenivasa Rao, M. Krishna

Reddy, “RISAT-1 spacecraft configuration: architecture, technology and performance,” Current Science, Vol.

104, No 4, February 25, 2013, Special Section: Radar Imaging Satellite-1, pp. 462-471, URL:http://www.currentscience.ac.in/Volumes/104/04/0462.pdf

6) A. S. Kiran Kumar, “Significance of RISAT-1 in ISRO’s Earth Observation Program,” Current Science,

Vol. 104, No 4, February 25, 2013, Special Section: Radar Imaging Satellite-1,Foreword, pp. 444-445, URL:

http://www.currentscience.ac.in/Volumes/104/04/0444.pdf

7) “RISAT-1,” ISRO, URL: http://www.isro.org/satellites/risat-1.aspx

8) “RISAT-1 brochure,” ISRO, URL: http://www.isro.org/pslv-c19/pdf/pslv-c19-brochure.pdf

9) P. Kunhikrishnan, L. Sowmianarayanan, M. Vishnu Nampoothiri, “PSLV-C19/RISAT-1 mission: the

launcher aspects,” Current Science, Vol. 104, No 4, February 25, 2013, Special Section: Radar Imaging

Satellite-1, pp. 472-476, URL: http://www.currentscience.ac.in/Volumes/104/04/0472.pdf

10) “Radar Imaging Satellite-1,” Proceedings of UN COPUOS (Committee on the Peaceful Uses of Outer

Space), UNOOSA, Vienna, Austria, June 6-15, 2012, URL:

http://www.oosa.unvienna.org/pdf/pres/copuos2012/tech-21.pdf

11) Information provided by Tapan Misra, Deputy Director of MRSA (Microwave Remote Sensors Area),

ISRO/SAC (Space Application Center), Ahmedabad, India

12) Vinay K Dadhwal, “RADAR Imaging Satellite ((RISAT)) 1 - SAR Mission of ISRO,” Proceedings of the

50th Session of Scientific & Technical Subcommittee of UNCOPUOS, Vienna, Austria, Feb. 11-22, 2013,

URL: http://www.oosa.unvienna.org/pdf/pres/stsc2013/tech-25E.pdf

13) http://www.isro.org/pslv-c19/Imagegallery/satelliteimages.aspx#3

14) “Radar Imaging Satellite (RISAT-1) launched,” NNRMS Bulletin 36, June 2012, p. 12, URL:

http://www.isro.org/newsletters/contents/nnrms/NNRMS%20Bulletin-%20June%202012.pdf

15) “RISAT-1 placed in final Polar Sun-synchronous orbit,” April 29, 2012, URL: http://www.gktoday.in/risat-

1-placed-in-final-polar-sun-synchronous-orbit/

16) “RISAT-1 successfully placed in its final orbit,” WebIndia, April 28, 2012, URL:

http://news.webindia123.com/news/Articles/India/20120428/1974316.html

17) T. Misra, S. S. Rana, K. N. Shankara, “Synthetic Aperture Radar Payload of Radar Imaging Satellite

(RISAT) of ISRO,” URSI-GA (Union Radio Scientifique Internationale-General Assembly) ,New Delhi, India,

Oct. 21-29, 2005, URL: http://www.ursi.org/Proceedings/ProcGA05/pdf/F08.6(01643).pdf

18) A. S. Kiran Kumar, “Significance of RISAT-1 in ISRO’s Earth Observation Program,” Current Science,

Vol. 104, No 4, February 25, 2013, Special Section: Radar Imaging Satellite-1, pp. 444-445, URL:

http://www.currentscience.ac.in/Volumes/104/04/0444.pdf

19) Tapan Misra, S. S. Rana, N. M. Desai, D. B. Dave, Rajeevjyoti, R. K. Arora, C. V. N. Rao, B. V. Bakori,

R. Neelakantan, J. G. Vachchani, “Synthetic Aperture Radar payload on-board RISAT-1: configuration,

technology and performance,” Current Science, Vol. 104, No 4, February 25, 2013, Special Section: Radar

Imaging Satellite-1, pp. 446-461, URL: http://www.currentscience.ac.in/Volumes/104/04/0446.pdf

20) N. M. Desai, C. V. N. Rao, R. Neelakantan, B. V. Bakori, D. B. Dave, Tapan Misra, R. K . Arora, V. R.

http://www.currentscience.ac.in/Volumes/104/04/0446.pdfhttp://www.currentscience.ac.in/Volumes/104/04/0444.pdfhttp://www.ursi.org/Proceedings/ProcGA05/pdf/F08.6%2801643%29.pdfhttp://news.webindia123.com/news/Articles/India/20120428/1974316.htmlhttp://www.gktoday.in/risat-1-placed-in-final-polar-sun-synchronous-orbit/http://www.isro.org/newsletters/contents/nnrms/NNRMS%20Bulletin%20June%202012.pdfhttp://www.isro.org/pslv-c19/Imagegallery/satelliteimages.aspx#3http://www.oosa.unvienna.org/pdf/pres/stsc2013/tech-25E.pdfhttp://www.oosa.unvienna.org/pdf/pres/copuos2012/tech-21.pdfhttp://www.currentscience.ac.in/Volumes/104/04/0472.pdfhttp://www.isro.org/pslv-c19/pdf/pslv-c19-brochure.pdfhttp://www.isro.org/satellites/risat-1.aspxhttp://www.currentscience.ac.in/Volumes/104/04/0444.pdfhttp://www.currentscience.ac.in/Volumes/104/04/0462.pdfhttp://www.scanex.ru/en/publications/pdf/publication50.pdf


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© EOPortal 2000 - 2014

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Kunal, J. G. Vachhani, “Onboard Signal Processors for ISRO's Microwave Radars,” Proceedings of the 60th

IAC (International Astronautical Congress), Daejeon, Korea, Oct. 12-16, 2009, paper: IAC-09-B1.4.12

22) A. S. Kiran Kumar, “Earth Observation Systems,” User Interaction Meeting, NRSC (National Remote

Sensing Center), Hyderabad, India, Feb. 16-17, 2012, URL: http://www.nrsc.gov.in/assets/pdf/kr.pdf

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