test and calibration for mwhts payload onboard chinese fy-3d satellite

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Remote Sensing Science July 2016, Volume 4, Issue 2, PP.14-22

Test and Calibration for MWHTS Payload

onboard Chinese FY-3D Satellite Jieying HE, Shengwei ZHANG

Key Laboratory of Microwave Remote Sensing, National Space Science Center, Chinese Academy of Sciences, Beijing 100190,

China

Email: [email protected]

Abstract

The Microwave Humidity and Temperature Sounder (MWHTS) is the main payload of FengYun 3D (FY-3D), designed for

atmospheric humidity and temperature sounding, and also for monitoring severe weather systems such as typhoons and

rainstorms which will be launched in 2016. Before the launch of MWHTS, a series of experiments will be conducted in normal

environment and a thermal/vacuum (T/V) chamber to determine radiometric characteristics of each channel, which are of very

importance before the launch. In this paper, design and component description, as well as technical specifications and test results

for RF and IF, will be provided. Then T/V calibration results, such as bandwidth correction, nonlinear error, calibration accuracy

and sensitivity for all channels.

Keywords: Index Terms MWHTS, FY-3D, calibration

1 INTRODUCTION OF MWHTS ONBOARD FY-3D

FengYun 3 (FY-3) is the second generation sun synchronous satellite from China [1]. Microwave Humidity and

Temperature Sounder (MWHTS) onboard the FY-3C/D satellite is a direct descendant of Microwave Humidity

Sounder (MWHS) onboard FY-3 A/B satellite[2], which has been successfully launched on May 27, 2008 and

November 9, 2011, respectively. It has 8 temperature sounding channels grouped around the 118.75GHz oxygen

absorption lines and 5 moister sounding channels grouped around 183.31GHz water vapor absorption line. Two so-

called window channels (at 89 GHz and 150GHz, respectively) measure a part of water vapor spectral continuum.

Therefore, it is primarily a humidity and temperature sounder. Also it can monitor the severe weather systems such

as typhoons and rainstorms.

2 DESIGN AND IMPLEMENT OF MWHTS ONBOARD FY-3D

FIG. 1 CAD VIEW OF THE MWHTS OPTICS AND RECEIVER BOXES

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MWHTS onboard FY-3D is a 15-channel microwave sounder implemented as two operated modules: the first

module has 9 channels in the 89/118GHz which provide surface and temperature information and the second module

has 6 channels in the 150/183GHz, which provide surface and moisture information, as shown in Fig.1.

The setups of the two receiver frontends are shown in Figure 2 and Figure 3. The incoming microwave radiation is

split into its vertical and horizontal polarization via a polarizer grid. The two signals are feed into the receiver feeds

and into the mixers. The 89 GHz channel has an additional low-noise RF-LNA between feed and mixer. All mixers

work on the 2nd harmonic except the 89 GHz mixer working on the 3rd harmonic. The mixers are pumped by

frequency multiplied DROs with fundamental frequencies between 12.5 and 15.2758 GHz. The IF-output signals of

the mixers are fully amplified and form the two IF-output signals of each receiver frontend.

FIG.2: SETUP OF THE COMBINED 89/119 GHZ RECEIVER FRONTEND

FIG. 3 SETUP OF THE COMBINED 150/183 GHZ RECEIVER FRONTEND

Like AMSU-B, MWHTS is a cross-track scanner. It should be emphasized that these two modules are tightly

coupled and share main system resources and scanned independently [3].

The characteristics of each channel are listed in Table 1. The table lists six specifications, including Center

Frequency, Polarization, Bandwidth, NEΔT, Calibration resolution and dynamic range. For each channel, the RF

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feed selects a linear polarization which is fixed relative to the feedhorn. However, due to the rotating scan reflector

the selected polarization is not fixed relative to the scan plane (and therefore relative to the earth). Rather, it rotates

as the antenna reflector rotates. Thus the polarization vector for channels labeled "V" forms an angle φ with the scan

plane, while the "H"-polarization direction forms an angle 90o-φ with the scan plane. At nadir the two directions are

in the scan plane and perpendicular to the scan plane, respectively. This is why the authors design a rotating platform

to achieve the radiations at different incident angles.

In the normal operational scan mode, it has a scanning period of 2.667s. Main beams of the antenna scan over the

observing swath (±53.35º from nadir) in the cross-track direction at a constant time of 1.71s. There are 98 scenes per

scanning line during 1.71s and each sample has the same integration period. For in-flight calibration, 3 samples are

taken for warm and cold target per scan, time and velocity distribution for a complete scan cycle is shown in Fig.4.

The calibration process is implemented by observing a built-in hot target and the background emission of the cold

sky.

TABLE1 CHANNEL CHARACTERISTICS OF MWHTS RECEIVERS

No.

Center

frequency

(GHz)

P Bandwidt

h

(MHz)

NEΔT

(K)

Calibration

accuracy

(K)

No.

Center

frequency

(GHz)

P Bandwidth

(MHz)

NEΔT

(K)

Calibration

accuracy (K)

1 89.0 V 1500 1.0 1.3 10

150.0 V 1500 1.0 1.3

2 118.750.08 H 20 3.6 2.0 11

183.31±1 H 500 1.0 1.3

3 118.750.2 H 100 2.0 2.0 12

183.31±1.8 H 700 1.0 1.3

4 118.750.3 H 165 1.6 2.0 13

183.31±3 H 1000 1.0 1.3

5 118.750.8 H 200 1.6 2.0 14

183.31±4.5 H 2000 1.0 1.3

6 118.751.1 H 200 1.6 2.0 15

183.31±7 H 2000 1.0 1.3

7 118.752.5 H 200 1.6 2.0

8 118.753.0 H 1000 1.0 2.0

9 118.755.0 H 2000 1.0 2.0

FIG.4 TIME AND VELOCITY DISTRIBUTION IN DIFFERENT SCANNING ANGLES IN A CALIBRATION PERIOD

3 DESIGN REPORT AND COMPONENT DESCRIPTION

3.1 Feed Horn Antenna

The receivers are equipped with corrugated feed horn antennas. This type of antenna offers an exceptionally good

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beam pattern with low side-lobes, very close to a Gaussian profile together with low cross-polarization and good

impedance matching. The feed horns are manufactured in one piece, using space-qualified copper electroform

technology.

3.2 RF-LNA 89 GHz

The RF-LNA of the 89 GHz receiver is similar to previous MHS 89 GHz receivers. The Noise Figure (NF) of the

frontend is mainly defined by the NF of the LNA and the losses of the preceding polarizer grid and the feed.

3.3 Mixer Blocks

The mixer blocks are state-of-the-art and have been built by RPG in a way that provides a very robust solution,

suitable for space use. All devices at this high frequency are machined to very high tolerances with very high speed

cutters and the waveguides are hand-polished prior to gold plating to ensure the best possible transmission properties.

3.4 DRO

The mixers work at the 2nd and 3rd harmonic and require a Local Oscillator provided by frequency stable Dielectric

Resonator Oscillators (DROs) together with active frequency multipliers. The fundamental DRO frequencies are

12.5 GHz to 15.2758 GHz. Free-running DROs have been chosen to minimize cost and space requirements. All

DROs have been screened by RPG prior to the installation and frequency monitored during the TV-test to check for

temperature variations and vacuum / non-vacuum frequency dependencies. However, the DRO frequency stability is

close to the requirements. For further space projects RPG recommends to use/investigate hermetically sealed DROs

with a higher level of qualification and better frequency stability.

3.5 Active Frequency Multipliers

All active frequency multipliers are designed and manufactured by RPG using Schottky diode technology. The

multipliers have been integrated into metal blocks with SMA input connector and waveguide output connector.

3.6 IF-Amplifiers

The IF-amplifiers amplify the IF-signal of the mixer up to the required IF output power level. For the 89 GHz

receiver the noise figure is determined by the used RF-LNA. For the 150 GHz, 118.75 GHz and 183.31 GHz receiver

the noise figure is determined by the mixer conversion loss and the noise figure of the used IF-amplifier, so special

low noise amplifiers are needed. The IF-amplifiers of the 89 GHz channel and the 150 GHz channel are commonly

used by RPG and well known from previous RPG space project.

3.7 Power Splitters

FIG.5 POWER SPLITTER AND MATCHING STRATEGY OF THE 118.75 GHZ RECEIVER

Each receiver has one single IF-output connector providing the full IF-bandwidth. The 118.75 and the 183.31 GHz

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receiver require power dividers to split the IF-output signal into the different IF-frequency sub-channels.

According to the receiver setup described in Figure 2 and Figure 3, an eight-way power splitter should be used to

produce the required five different frequency channels for the 183.31 GHz receiver. Unfortunately, this is not

possible for the 118.75 GHz receiver, as no power splitter is available on the market covering the full frequency span

from 70MHz to 6.0 GHz. A combination of resistive power splitters and Wilkinson splitters should be used instead.

However, all of the splitters do only properly operate if all ports are fully matched to 50 Ohm within the frequency

range of both splitters. The only way how this can be done is to apply resistive attenuators between the power

splitters to provide a very broad-band match. This satisfies the matching requirements for both frequency ranges of

the splitters at the attenuator. A possible solution is proposed in Fig.5.

FIG.6 PHOTOGRAPH OF THE OPENED VACUUM TEST CHAMBER

4 TEST FOR MWHTS

4.1 TV-Test for front-end

The TV-Test Setup has been used to measure the receiver performance at -20°C, +25° and +45°C under vacuum

conditions. The setup has additionally been used to perform the TV-test.

The receiver is installed inside a high vacuum chamber (Fig.6) mounted on a temperature stabilized table. The

chamber can be evacuated to pressures down to 4e-5 mbar (4e-3 Pa). The table is connected to the cold-head of a

stirling cooler and with heaters installed at the bottom of the table to be able to vary the receiver temperature. The

thermal cycling process is computer controlled and data is logged every 10 seconds. The IF power has been

continuously monitored to ensure there are no special temperatures which causes problems. Power-off and power-on

tests have been performed at both hot and cold temperatures as outlined in the environmental test document supplied.

The whole system is installed inside a class 100,000 clean-room for minimum dust contamination. The

measurements of IF-total power, receiver noise temperature, Cold-start test / Hot-start test, IF-power level at 50MHz

bandwidth and IF-flatness, DRO frequency at -20°C, +25°C and +45°C under vacuum conditions have been taken

during the TV-test.

4.2 Normal Test for IF

The amplitude and frequency characteristics before the signal detection have been tested, including Output frequency

range, bandwidth, flatness, standing and so on. Fig.7 shows the amplitude and frequency characteristics of IF for

channel 1, 2, 10 and 15.

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(A) (B)

(C) (D)

FIG.7 AMPLITUDE AND FREQUENCY CHARACTERISTICS FOR IF TESTS

4.3 Bandwidth Correction

Because the bands are too broad to meet the criterion of monochromatic light, band-correction is of very importance

and directly affects the calibration accuracy, which has been described in detail in reference, the correction

coefficients of MWHTS channels are listed in table 2.

TABLE 2. CORRECTION COEFFICIENTS OF MWHTS CHANNELS

Channel a b Channel a b

89GHz 1.0001971825 0.0012396881 150GHz 1.0000693834 0.0007260458

118GHz-1 1.0000004536 0.0000037817 183GHz-1 1.0000297221 0.0003775382

118GHz-2 1.0000028351 0.0000236358 183GHz-2 1.0000962992 0.0012232220

118GHz-3 1.0000063789 0.0000531805 183GHz-3 1.0002674960 0.0033978262

118GHz-4 1.0000453608 0.0003781721 183GHz-4 1.0006018505 0.0076450047

118GHz-5 1.0000857603 0.0007149817 183GHz-5 1.0014563245 0.0184989711

118GHz-6 1.0004429765 0.0036930864

118GHz-7 1.0006378707 0.0053179779

118GHz-8 1.0017717396 0.0147716278

5 T/V CALIBRATION TEST

4.1 General Description

Prelaunch calibration is aiming to ensure that the sensor meets the performance specification requirements and to

derive the calibration parameters, particularly the nonlinearity parameter , which is needed for accurate on-orbit

data processing [4]. Before the FY-3D satellite is launched, a series of tests will be performed on MWHTS in a T/V

chamber of 2-m diameter in following days.

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Different from the design scheme of FY-3C MWHTS, it not only has the controllable mechanism to achieve the

rising and falling function, but also has the rotation mechanism to realize the azimuth rotating function, which is

mounted on a rotatable platform. By rotating and lifting the platform, channel capability and radiometric

characteristics can be achieved at a plurality of azimuths, which is the improvement of the new design and will be

further demonstrated during the data processing of the T/V calibration experiment.

Fig. 8 shows the schematic diagram of T/V calibration in vacuum chamber. Two 500-mm-diameter blackbody

targets, namely, Earth-target blackbody and cold-target blackbody, are installed on the same platform.

FIG. 8 CONFIGURATION OF THE FY-3A MWHS T/V TEST

The blue target in Fig. 8 is the cold target located at 53.35º from nadir, which is used as a cold reference view when

it is filled with liquid nitrogen and the bottom temperature is kept at about 90–95 K, in lieu of a real space view in

the T/V test since it is very difficult to build a 2.7-K blackbody on the ground to simulate cold space at present.

Therefore, it will result in uncertainties in the calibration of MWHS because the range of brightness temperatures on

orbit cannot be fully covered in the T/V test. The pink blackbody is the earth target located at 0º from nadir that can

be controlled at a given temperature in the range of 95–320 K, which is used to simulate the Earth’s radiation. The

hot target, located within MWHS, is passively kept at the same temperature as the instrument. Between each target

and instrument, there is a shield to avoid electromagnetic interface in other directions.

4.2 Needed Equipments

The equipments needed in the T/V calibration tests are listed in table 3, mainly including the MWHTS instrument

and T/V calibration targets and temperature control devices.

The instrument of MWHS and two 500-mm-diameter blackbody targets (Earth-target blackbody and cold-target

blackbody) are installed on the same platform, which includes different viewing angles.

TABLE 3 THE EQUIPMENTS NEEDED IN THE T/V CALIBRATION TESTS

No. Name of equipments

1 MWHTS

2 Cold calibration target

3 Controllable calibration target

4 Controllable rising, falling and rotating platform

5 Temperature control device

4.3 Calibration Procedure

According to the design and implement of MWHTS instrument, there are two modules, 89/118GHz receiver as one

module, 150/183GHz as the other module. The two modules have antennas in different size and cannot share the

earth-target blackbody and cold-target blackbody. Therefore, they must be calibrated in different incident angles

followed one by one[5].

MWHS stably operated at instrument temperatures (the receiver’s bottom panel) of 278.15K, 288.15K, 298.15K, and

which are controlled by a heater. At each instrument temperature, the temperature of the Earth target is varied from

MWHTS

Temperature-controllable

target

Cold target

Rotatable platform Raising and falling platform

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95 to 320 K with a step size of 15 K or 10K and is kept stable to within 0.15 K.

4.4 Calibration Results and Analysis

The absolute calibration accuracy is defined as the difference between the "measured" brightness temperature and

the actual calculated brightness temperature of a target determined from PRT's on the target and a knowledge of the

target emissivity.

The bias can only be estimated with confidence for the internal target calibration point. The mean of all the

uncorrupted 400 scan line averages of nadir view brightness temperatures, when the Earth target is at the same

temperature as the internal target, allows the warm bias to be determined. The standard deviation of the 400 scan line

averages, which is the randomly varying component of the bias, is also computed.

The sources of errors and uncertainties are summarized in the calibration process. A detailed analysis can be found in

table 4. Errors can be classified as bias errors, which are uncertainties in the bias corrections applied, and random

errors, which are uncertainties due to random fluctuations of the instrument characteristics. We will in general

correct for all known biases, so that only their uncertainties remain. The accuracy of such a radiance is termed the

calibration accuracy, which is strictly defined as the difference between the inferred radiance and the actual radiance

when a blackbody calibration target is placed directly in front of the antenna. It can be expressed as:

1 2

222 221 4CAL W C NL SYST X T X T X X T T

where, X=(Ts-Tc)/( Tw-Tc) and ΔTW denote the uncertainty in the blackbody radiance, ΔTC denotes the uncertainty

in the space view radiance, ΔTNL denotes the uncertainty in the transfer function peak nonlinearity term,ΔTSYS is the

uncertainty due to random instrument fluctuations, TS is the scene radiance. So there are no biases included with the

uncertainty only.

TABLE 4. ABSOLUTE CALIBRATION ACCURACY OF MWHTS

No. ΔTW ΔTC ΔTNL ΔTSYS ΔTCAL No. ΔTW ΔTC ΔTNL ΔTSYS ΔTCAL

89 0.05 0.05 0.45 0.21 0.50 150 0.05 0.05 0.25 0.25 0.36

118-1 0.05 0.05 0.4 1.36 1.42 183-1 0.05 0.05 0.2 0.39 0.44

118-2 0.05 0.05 0.25 0.54 0.60 183-2 0.05 0.05 0.2 0.29 0.36

118-3 0.05 0.05 0.35 0.43 0.56 183-3 0.05 0.05 0.2 0.24 0.32

118-4 0.05 0.05 0.25 0.42 0.49 183-4 0.05 0.05 0.4 0.25 0.48

118-5 0.05 0.05 0.2 0.40 0.45 183-5 0.05 0.05 0.4 0.21 0.46

118-6 0.05 0.05 0.2 0.40 0.45

118-7 0.05 0.05 0.2 0.20 0.29

118-8 0.05 0.05 0.2 0.16 0.27

Compared with the payload on FY-3C, the FY-3D MWHTS has considerable improvements for most technical

indicators, and for channel 2, there is an improvement of 0.25K according to the calibration analysis, as Fig.9 shown,

where, from left to right, the channel number is 1 to 15, y label denotes the improvements of T/V calibration.

6 CONCLUSIONS AND SUMMARY

According to the theory and previous experience of MWHS onboard FY-3A/B, MWHTS onboard FY-3C [6-9], as

well as AMSU-A/B and so on, prelaunch Tests and T/V calibration of FY-3D MWHTS will provide a thorough

investigation of the instrument performance.

The main work is to realize the design and test of front-end and IF, and improve the T/V calibration procedure and

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then calculate the calibration bias, nonlinearity parameter, sensitivity, calibration accuracy and so on. Therefore,

FIG.9 THE IMPROVEMENTS OF T/V CALIBRATION FOR FY-3D MWHTS

REFERENCES

[1] J. Li, S W. Zhang, J S Jiang, X L Dong, “In-orbit performance of Microwave Humidity Sounder (MWHS) of the Chinese FY-3

meteorological satellite,” Geoscience and Remote Sensing Symposium (IGARSS), 2010 IEEE International, ISSN: 2153-6996, pp:

574 – 577, Honolulu, HI, 25-30 July 2010.

[2] S W. Zhang, J. Li, Z Z Wang, “Design of the second generation microwave humidity sounder (MWHS-II) for Chinese

meteorological satellite FY-3,” Geosciences and Remote Sensing Symposium (IGARSS), 2012 IEEE International, ISSN: 2153-

6996, pp: 4672 – 4675, Munich, 22-27 July 2012.

[3] S W. Zhang, J. Li, J. Jiang, Y. Zhang, Z. Wang, X. Dong, “Microwave Humidity Sounder (MWHS) of Chinese Meteorological

Satellite FY-3,” Proceedings of the Microwave Technology and Techniques Workshop -Enabling Future Space Systems,

ESA/ESTEC, Noordwijk, The Netherlands, May 15-16, 2006, ESA SP-632.

[4] S Y. Gu, Y. Guo, Z Z. Wang, et al, “ Calibration analysis for sounding channels of MWHS onboard FY-3A”, IEEE transactions on

geosciences and remote sensing, 2012,50(12): 4885-4891.

[5] Z Z Wang, Jing Li, Shengwei Zhang, and Yun Li. “Prelaunch Calibration of Microwave Humidity Sounder on China’s FY-3A

Meteorological Satellite”, IEEE transactions on geosciences and remote sensing letters, 8(1),2011:29-33.

[6] J Y. He,, ZHANG Shengwei, WANG Zhenzhan, “Advanced Microwave Atmospheric Sounder (AMAS) Channel Specifications

and T/V Calibration Results on FY-3C satellite”, IEEE transactions on geosciences and remote sensing, 2015,53(1):481-493.

[7] R. W. Saunders, T. J. Hewison, S. J. Stringer, and N. C. Atkinson, “The radiometric characterization of AMSU-B,” IEEE Trans.

Microw. Theory Tech., vol. 43, no. 4, pp. 760–771, Apr. 1995.

[8] T. Mo, “Prelaunch calibration of the Advanced Microwave Sounding Unit-A for NOAA-K,” IEEE Trans. Microw. Theory Tech.,

vol. 44, no. 8, pp. 1460–1469, Aug. 1996.

[9] AIRS Project Algorithm Theoretical Basis Document, level 1b, Part 3: Microwave instruments, Nov. 2000. [Online]. Available:

http://eospso.gsfc.nasa.gov/eos_homepage/for_scientists/atbd/docs/AIRS/atbd-airs-L1B_microwave.pd

AUTHORS 1何杰颖，1984 年生，民族汉，中科院博士学位，副研究员。

中国科学院空间科学与应用研究中心微波遥感技术重点实验室，副研究员，研究方向为星载微波湿度计定标与数据处理。

2张升伟，1963 年生，民族汉，硕士学位，中国科学院空间科学与应用研究中心微波遥感技术重点实验室，研究员，研究方向

为星载微波辐射计设计与研制。

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test and calibration for mwhts payload onboard chinese fy-3d satellite

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