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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
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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.
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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
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[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.,
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[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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