gagan - the indian satellite based augmentation...

Indian Journal of Radio & Space Physics

Vol. 36, August 2007, pp. 293-302

GAGAN - The Indian satellite based augmentation system

K N Suryanarayana Rao

ISRO Satellite Centre, Airport Road, Bangalore 560 017, India

Received 2 April 2007; accepted 14 May 2007

Global Positioning System (GPS) from the USA, Global Navigation Satellite System (GLONASS) from the Russian

Federation and the proposed GALILEO satellite navigation system from Europe are meant for providing position and timing

information for a variety of applications. However, for Safety Critical applications the basic constellations cannot meet the

requirements in terms of accuracy, integrity and availability. For this purpose, the basic constellations are augmented by an

overlay system. Indian Space Research Organization (ISRO), along with Airport Authority of India (AAI) is implementing

the Satellite Based Augmentation System (SBAS) for the Indian region. The project called GAGAN (GPS Aided Geo

Augmented Navigation) has a full complement of the SBAS inclusive of ground and onboard segment. The first phase of

GAGAN is nearing completion. This paper deals with the basic SBAS concept, GAGAN configuration, implementation and

the challenges involved. The roadmap towards the final operational phase is also indicated.

Keywords: GAGAN, Global Navigation Satellite System (GNSS), Satellite based augmentation system (SBAS), Indian

Master Control Station (INMCC), Indian Reference Station (INRES)

PACS No.: 84.40.Ua

1 Introduction Navigation experts worldwide have been discussing

for many years about the concept of one navigational

system that is available everywhere on the globe, at

all the time with extreme accuracy, trusted and easy to

use, which overcomes the limitations of the existing

conventional navigational aids. The concept is Global

Navigation Satellite System (GNSS). Such a system

could be used as the sole means of a navigation

system and could eventually replace most, if not all,

of the costly ground based infrastructures. Satellite

navigation and positioning systems represent the most

important technological breakthrough in civil aviation

navigation, surveillance, and air traffic management

since radar was introduced over half a century ago.

The GPS, developed by the United States is currently

approved for supplemental use in all weather

conditions during en-route, terminal air navigation

and for non-precision approaches. For the civil

aviation community whose requirements are stringent,

GPS/GLONASS constellations alone fail to meet such

requirements. Thus, the need for augmenting these

constellations arises to meet the required navigation

performance for aviation use as navigational system

covering various phases of the flight.

Augmentation—This term refers to enhancements to

a navigation system. For a navigation system to be

declared as usable for civil aviation purposes, it must

fulfill the requirements in a given phase of flight.

However, a given navigation system considered alone

cannot meet all the desirable requirements. It is thus

necessary to hybridize two or more systems of

navigation in order to obtain suitable performance.

The various augmentation options are as follows:

(i) Receiver algorithms (RAIM)

(ii) Additional sensors

(iii) Extra systems

(a) GLONASS; (b) GNSS2-Galileo

(i) GPS Modernization

(ii) Local Area Augmentation Systems (LAAS)

(iii) Wide Area Augmentation Systems (WAAS)

(a) EGNOS, US WAAS, MSAS

The GPS/GLONASS can be augmented in various

ways, but the end results vary accordingly.

Current GPS and GLONASS constellation does not

satisfy the integrity, accuracy and availability

requirements for all phases of flight, particularly for

the more stringent precision approaches.

INDIAN J RADIO & SPACE PHYS, AUGUST 2007

294

Integrity—It is the ability to protect the user from

inaccurate information in a timely manner. Integrity is

not guaranteed as all GPS satellites are not monitored

at all times. In case of any fault, the time-to-alarm is

from minutes to hours. The quality of the service is

not indicated.

Accuracy—It stands for the difference between

measured and true positions of a vehicle at any given

time. Accuracy is not sufficient even with SA off.

Vertical accuracy is greater than 10 m.

Continuity—It stands for the ability to complete an

operation without triggering an alarm.

Availability—It stands for the ability of the system

to be used by the user whenever it is required.

Thus the basic constellation needs to be augmented

for this purpose.

2 Satellite Based Augmentation System (SBAS) The basic functions of an SBAS system are as

follows:

Ranging—It provides additional ranging signals to

improve availability, typically via geo-stationary

satellites.

Integrity channel—It provides transmission of GPS

and integrity data to navigators.

2.1 Wide area differential (WAD)

Wide area differential1 (WAD) provides the

following:

(i) Differential correction data to users to improve

accuracy

(ii) Satellite orbit and clock errors

(iii) Differential range corrections

(iv) Ionospheric grid computation

An SBAS must provide augmentation services with

adequate reliability and continuity.

2.2 SBAS concept

As shown in Fig. 1 SBAS employs a ranging function

to generate GPS-like signals and enable users to use

the concerned geo-stationary satellite as one more

GPS satellite for ranging purposes. Information of the

GPS constellation is transmitted to each user in the

real-time via the integrity function of SBAS, while the

differential correction function provides ranging error

data to each user. The space-based augmentation

systems (SBAS) will provide en-route through

precision approach navigation services for all aircraft

within the covered airspace.

2.3 Major SBAS segments

The major segments of an SBAS are shown in

Fig. 2.

Ground segment—It consists of reference stations

located at precisely surveyed locations for ranging

and integrity monitoring.

Master Control Centre—This centre collects,

estimates and processes the data to generate wide area

correction messages and integrity information to the

user.

Navigation Land Earth Station—It up-links the

messages to the geo-stationary satellites (GEO) for

Fig. 1—SBAS concept

RAO: GAGAN - THE INDIAN SATELLITE BASED AUGMENTATION SYSTEM

295

further broadcast and communication links to transfer

data collected from the reference station to the master

control station.

Space segment—Space segments are the following:

(i) GPS satellites, GLONASS satellites

(ii) GEO satellites for data transmission and ranging

function (GEO)

User segment—Similarly, user segments are the

following:

(i) Signal in Space (SIS)

(ii) Receiver capable of receiving and decoding the

GPS/GLONASS/GEO broadcast message

Figure 3 shows the signal flow between the various

SBAS elements.

2.4 SBAS concept of operation

The concept of operations of SBAS is described

briefly in five steps as follows:

The SBAS reference stations are deployed

throughout the region of service at pre-surveyed

locations to measure pseudoranges and carrier phases

on L1 and L2 frequencies from all visible satellites.

(A semi-codeless technique is used to derive a code

measurement on L2).

The reference stations send these measurements to

SBAS Master Station, which calculate clock and

ephemeris corrections for each GPS satellite

monitored, ephemeris information for each GEO, and

ionospheric vertical delays on a grid. The grid

consists of fixed ionospheric grid points (IGPs) at an

Fig. 2—Major SBAS segments

Fig. 3—SBAS data flow


296

altitude of 350 km above the Earth’s surface. Grid

spacings are 5 deg × 5 deg between 55°S and 55°N

and are larger beyond this region.

In addition to the corrections, the Master Station

calculates error bounds for ionospheric corrections

called grid ionospheric vertical errors (GIVEs) at each

IGP, and also combined error bounds for clock and

ephemeris corrections for each visible satellite, called

user differential range errors (UDREs).

The Master Station sends these corrections and

error bounds to the users through GEO

communication satellites with a data rate of 250 bits/s.

User avionics apply these corrections to their

pseudoranges obtained from GPS measurements, in

order to improve the accuracy of their position

estimates. They also use the UDREs and GIVEs and

other information to calculate error bounds on

position error called the Vertical Protection Level

(VPL) and Horizontal Protection Level (HPL). For

the integrity of the system, these protection levels

must bound the position errors with probability

greater or equal to 0.9999999 in one hour for en-route

through NPA operations and for PA in 150 s.

Thus the SBAS signal received at the user looks

just like a GPS signal with the exception that the

transmitted message modulation rate is 250 bps

instead of 50 bps for GPS data stream and the data are

FEC error coded.

2.5 Ionospheric corrections in SBAS

The ionospheric errors are the most dominant

source of errors for GPS users (see Table 1,

Appendix 1). The major effects the ionosphere can

have on the GPS signal are (i) group delay of the

signal modulation, or absolute range error; (ii) carrier

phase advance, or relative range error; (iii) Doppler

shift, or range rate errors; (iv) Faraday rotation of

linearly polarized signals; (v) refraction or bending of

the radio wave; (vi) distortion of pulse waveforms;

(vii) signal amplitude fading or amplitude

scintillation; and (viii) phase scintillation. Due to the

above the delays range from a few meters at night to a

maximum of 10 or 20 m at about 1400 hrs.

The ionosphere is a region of ionized plasma that

extends from roughly 50 km to 2000 km above the

surface of the earth. Generally, the ionosphere can be

divided into several layers in altitude according to

electron density, which reaches its peak value at about

350 km in altitude. For 2D ionospheric modeling, the

ionosphere is assumed to be concentrated on a

spherical shell of infinitesimal thickness located at the

altitude of about 350 km above the earth’s surface.

The ionosphere introduces frequency dependent

delays in the signal, which is a function of total

electron content (TEC). This dependence can be

exploited by dual-frequency receivers to get an

accurate measure of the ionospheric activity along

that signal path.

The master station collects all the ionospheric data

collected from all the reference stations. The

implementation of the single-layer grid model

requires computation of the intersection of the line-of-

sight between the GPS receiver and the observed

satellite on the ionosphere shell as illustrated in Fig. 4.

The intersection point of the GPS signal with the

ionospheric shell is defined as ionospheric pierce

point (IPP), at which the slant ionospheric delay has

an elevation angle of E. The slant delays are

converted to equivalent vertical delays at the point

Table 1—Standalone GPS error budget3

Error source Single

frequency

receiver

(C/A), m

Dual frequency

receiver

(P/Y), m

Ephemeris data 2.1 2.1

Satellite clock 2.1 2.1

Ionosphere 4.0 1.2

Troposphere 0.7 0.7

Multipath 1.4 1.4

Receiver

measurements

0.5 0.5

Vertical error (1-σ) 12.8 8.3

Horizontal error (1-σ) 10.2 6.6

Fig. 4—Ionospheric shell


297

where the LOS pierces the shell. This results in a

model which is invariant in the vertical direction and

varies only with latitude, longitude and time.

Using these samples of equivalent vertical delays, a

deterministic trend is fit to the ionosphere. This trend

is used to predict the delays on a grid of points, called

the ionospheric grid points (IGP’s). As shown in

Fig. 5 the world is divided into imaginary fixed grid

of 5 deg × 5 deg at a height of 350 km from the earth

surface.

The master station converts the slant delay

observations to vertical delay estimate to the

surrounding grid node. The grid points for

interpolation, the ionospheric delay value and the

GIVE indicator are sent to the user. The GIVE

provides a bound on the accuracy of broadcast

ionospheric delay. The user determines the

ionospheric pierce point (IPP) for a satellite and

interpolates using appropriate surrounding grid points

to derive the satellite specific vertical ionospheric

delay.

3 GPS Aided Geo Augmented Navigation

(GAGAN) overview Indian Space Research Organisation (ISRO) along

with Airports Authority of India (AAI) has worked

out a joint programme to implement the Satellite

Based Augmentation System using GPS/GLONASS

over Indian airspace. Although meant for civil

aviation, the system can be used by a vast majority of

users like personal and public vehicles, railways,

shipping, surveys, etc.

GAGAN Technology Demonstration System

(TDS) is a forerunner for the operation of SBAS over

the Indian region. The TDS phase of the project

implements a minimum set of elements for

demonstrating the SBAS proof of concept over the

Indian region. The minimum set includes the

following:

(i) 8 Indian Reference Stations (INRES)

(ii) 1 Indian Master Control Centre (INMCC)

(iii) 1 Indian Land Up Link Station

(iv) Navigation Transponder with L1 and L5

functionality

(v) Navigation Software

(vi) Associated communication links between

INRES, INLUS and INMCC

(vii) Total Electron Content (TEC) measurement

network and associated ionospheric studies

The system configuration of GAGAN is shown in

Fig. 6. The following paragraphs provide an insight

into the implementation of the various major

elements.

3.1 Indian Reference Stations (INRES)

The INRES collect measurement data and

broadcast messages from all GPS and GEO satellites

in view and forward to Indian Mission Control Centre

(INMCC). Eight INRES stations are established

during the TDS phase at Delhi, Bangalore,

Ahmedabad, Calcutta, Jammu, Port Blair, Guwahati

and Trivandrum as shown in Fig. 7. The sites are

identified after carrying out preliminary site survey.

Fig. 5—SBAS ionospheric grid points


298

The suitability of the sites was established after

carrying out the multipath study, noise survey and

elevation profile pattern. Figure 8 shows one of the

Typical INRES stations.

3.2 Indian Master Control Station (INMCC)

An Indian Master Control Centre (INMCC) is

established at Kundalahalli, Bangalore. The

measurement data collected every second from each

of the INRES receiver chains are transmitted in

realtime to the INMCC for correction and integrity

processing and generation of SBAS messages with the

aid of the navigation software resident. The INMCC

comprises of various subsystems like Data

Communication Subsystem (DCSS), Correction and

Verification Subsystem (C&VS), Operation and

Maintenance Subsystem (OMSS) and Service

Monitoring Subsystem (SMS).

Figure 9 shows the Indian Master Control Centre at

GAGAN Project site at Kundalahalli, Bangalore.

3.3 Indian Navigation Land Earth Uplink Station (INLUS)

The INLUS receives correction messages from the

INMCC, format those messages for GPS

compatibility and transmit them to the GEO satellites

for broadcast to user platforms. The INLUS is

collocated with INMCC at Bangalore. The INLUS

also provides GEO satellite ranging information and

corrections to the GEO satellite clocks. Message

formats and timing will be according to the functional

and performance specifications, which are derived

from MOPS (Minimum Operation Performance

Standard). Figure 10 shows 11-meter dish antenna for

up-linking the SBAS messages to GSAT 4.

Fig. 6—System configuration of GAGAN

Fig. 7Location of the eight Indian reference stations

Fig. 8—GAGAN INRES station at Bangalore

Fig. 9—GAGAN INMCC station at Bangalore


299

3.4 Navigation payload

Geo-stationary satellite component consisting of a

GPS L1 and L5 compatible navigation payload on an

Indian satellite positioned at 83°E is part of the

GAGAN configuration in the TDS phase. The

navigational payload will be flown on GSAT 4, which

is scheduled to be launched by April/May 2008. The

navigational payload is of indigenous design and is

being built in ISRO as per specifications meeting

international requirements for signal-in-space.

Figure 11 shows the block diagram of the GAGAN

navigation payload.

The major functions of the geo-stationary payload

are to provide the following:

(i) C × L path using the C-Band uplink for relaying

the geo-stationary overlay signal for use by

modified GPS receivers and ionospheric

correction

(ii) A long loop involving INLUS and the C × L

payload to correct code and carrier phase errors

and achieve coherence between them at satellite

output point

(iii) Adequate short term stability of the transponder

signal to ensure accurate operation of the user

receivers

The navigation transponder being designed and

built at Space Application Centre (SAC) is having one

of the best features in its class and meets the latest

FAA specifications. Figure 12 shows the coverage of

GAGAN.

3.5 Communication links

The communication links play a vital role in the

GAGAN system. Availability of the system will be

affected by the poor performance of communication

links. All the INRES stations but for Port Blair are

linked to INMCC by optical fibre cables. The INRES

at Port Blair is linked to INMCC by VSAT link. The

data rate is 128 kbits per second.

4 Ionospheric studies over Indian region

An essential and very important component of the

SBAS is the ionospheric correction generated and

Fig 10—GAGAN INLUS 11-metre dish antenna at Bangalore

Fig. 11—GAGAN payload block diagram


300

broadcasted to the SBAS users through geo-stationary

satellite.

The low and mid-latitude region is characterised by

large temporal and spatial gradients in the ionospheric

delay. This coupled with large amplitude scintillation

in these regions presents unique problems for the grid

based scheme presently used in SBAS. In addition,

the SBAS system may also have to deal with large

total electron content (TEC) depletion in small

localised areas. The above effects result in increased

errors when the existing SBAS grid based ionospheric

algorithms are used.

The successful implementation of the SBAS

depends on the ionospheric model over the region.

For this study TEC receivers are installed over the

region as shown in Fig. 13 below at various airports

for data collection.

A number of academic and R&D institutions are

involved in the process of studying the ionospheric

behaviour over Indian region2 in the context of SBAS

in general and GAGAN in particular.

4.1 Initial experimental phase

After successful completion of the TDS,

redundancies will be provided to the space segment,

INMCC, INLUS and the system validation carried out

over the entire Indian airspace. Based on the

experience of the TDS, additional augmentation will

be worked out.

4.2 Final operational phase

During this phase, it is expected that the SATNAV

programme will become operational. INMCC will be

augmented to meet additional requirements.

Additional redundancies will be built-in wherever

necessary.

Acknowledgements Thanks are due to Shri K Anbarasu, GAGAN

Project, who has helped in the preparation of the

Fig. 12—Typical GAGAN coverage

Fig. 13—TEC stations


301

manuscript. The encouragement received from Dr S

Pal, PMB, GAGAN and Dr. Shankara, Director,

ISAC is gratefully acknowledged.

References 1 Mishra Pratap & Enge Per, Global Positioning System,

Signals, Measurements and Performance (Ganga-Jamuna

Press, Lincoln, Mass, USA), 2001, pp.-123-173.

2 Klobuchar J A, P H Doherty, M B El-Arini, Lejeune R,

Dehel T, de Paula E R & Rodrigues F S, Ionospheric Issues

for a SBAS in the Equatorial Region, Ionospheric Effects

Symposium, Alexandria, Virginia, 7-9 May 2002.

3 Parkinson Bradford W & Spilker James J, Global

Positioning System: Theory and Applications, Volume I, by

(Jr. American Institute of Aeronautics and Astronautics Inc,

USA), 1996, pp. 10-17, 478-483, 485-513.

Appendix 1

A Factors affecting GPS signal Global Positioning System (GPS) is a complex

system based on a constellation of satellites

transmitting navigational information. There is a

potential for failure at any stage of the system, which

may cause error in the broadcast navigational

information.

The pseudorange from the user receiver u to the kth

satellite (ρuk), is given by

ρuk = (ru

k. lu

k) + bu – B

k + Iu

k + Tu

k + νu

k …(1)

Also the continuous carrier phase from the user

receiver u to the kth satellite (ρuk), is given by

ϕuk = (ru

k. lu

k) + bu – B

k + Iu

k + Nu

kλL1 + ξuk …(2)

where

ρuk – the pseudorange from the user receiver u to the

kth satellite

ϕuk

– the continuous carrier phase from the user

receiver u to the kth satellite

l uk – the line of sight from the user receiver u to the

kth satellite

ruk. lu

k – the calculated range from the user receiver u

to the kth satellite

bu – the user receiver clock offset from GPS time

Bk – the kth satellite clock offset from GPS time

Iuk – the ionospheric delay along the line-of-sight from

the user receiver u to the kth satellite

Tuk – the tropospheric delay along the line-of-sight

from the user receiver u to the kth satellite

Nuk – the continuous phase cycle ambiguity from the

user receiver u to the kth satellite

λL1 – the L1 carrier phase wavelength, 0.1903 m

νuk – the pseudorange measurement error

ξuk – the carrier phase measurement error

As implied by Eqs (1) and (2), a number of factors

conspire to corrupt the pseudorange and carrier phase

measurements for GPS. The GPS ranging errors are

grouped into the following six classes.

(i) Ephemeris data – Errors in the transmitted

location of the satellite

(ii) Satellite clock – Errors in the transmitted clock,

including satellite augmentation (SA)

(iii) Ionosphere – Errors in the corrections of

pseudorange caused by ionospheric effects

(iv) Troposphere – Errors in the corrections of

pseudorange caused by tropospheric effects

(v) Multipath – Errors caused by reflected signals

entering the receiver antenna

(vi) Receiver – Errors in the receiver's measurement

of range caused by thermal noise, software

accuracy, and inter-channel biases

A.1 Ephemeris errors

Ephemeris errors result when the GPS message

does not transmit the correct satellite location.

Because satellite errors reflect a position prediction,

they tend to grow with time from the last control

station upload. These errors were closely correlated

with the satellite clock, as one would expect. Note

that these errors are the same for both the P- and C/A-

codes. Each satellite has a unique Precision (P) and

Coarse Acquisition (CA) codes that distinguish

between the different satellites comprising the GPS.

A.2 Satellite clock errors

Fundamental to GPS is the one-way ranging that

ultimately depends on satellite clock predictability.

These satellite clock errors affect both the C/A- and

P-code users in the same way.

This effect is also independent of satellite direction,

which is important when the technique of differential

corrections is used. All differential stations and users

measure an identical satellite clock error. The ability

to predict clock behaviour is a measure of clock

quality. The GPS uses atomic clocks (cesium and

rubidium oscillators), which have stability of about 1

part in 10E13 over a day. If a clock can be predicted


302

to this accuracy, its error in a day (~10E5 s) will be

about 10E- 8 s or about 3.5 m.

A.3 Ionosphere errors

Because of free electrons in the ionosphere, GPS

signals do not travel at the vacuum speed of light as

they transit this region. The modulation on the signal

is delayed in proportion to the number of free

electrons encountered and is also (to first order)

proportional to the inverse of the carrier frequency

squared (1/f2). The phase of the radio frequency

carrier is advanced by the same amount because of

these effects. Carrier-smoothed receivers should take

this into account in the design of their filters. The

ionosphere is usually reasonably well-behaved and

stable in the temperate zones; near the equator or

magnetic poles it can fluctuate considerably.

A.4 Troposphere errors

Another deviation from the vacuum speed of light

is caused by the troposphere. Variations in

temperature, pressure, and humidity all contribute to

variations in the speed of light and radio waves. Both

the code and carrier will have the same delays.

A.5 Multipath errors

Multipath is the error caused by reflected signals

entering the front end of the receiver and masking the

real correlation peak. These effects tend to be more

pronounced in a static receiver near large reflecting

surfaces. Monitor or reference stations require special

care in siting to avoid unacceptable errors. The first

line of defense is to use the combination of antenna

cut-off angle and antenna location that minimizes this

problem. A second approach is to use so-called

"narrow correlator” receivers, which tend to minimize

the impact of multipath on range tracking accuracy.

A.6 Receiver errors

Initially most GPS commercial receivers were

sequential, in that one or two tracking channels shared

the burden of locking on to four or more satellites.

With modem chip technology, it is common to place

three or more tracking channels on a single

inexpensive chip. As the size and cost have shrunk,

techniques have improved and 10- or 12-channel

receivers are common. Most modem receivers use

reconstructed carrier to aid the code tracking loops.

Inter-channel bias is minimized with digital sampling

and all-digital designs.

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