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1RUAG Space
Demonstrator Test Results for the GEO Atmospheric Sounder (GAS)
Jacob Christensen1, Anders Carlström1, Johan Embretsén2, Andreas Colliander 3,4,
and Peter de Maagt4
1RUAG Space AB, Göteborg, Sweden2Omnisys Instruments AB, Göteborg, Sweden
3Jet Propulsion Laboratory, Pasadena, California, USA 4European Space Agency, Noordwijk, The Netherlands
IGARSS 2011
Vancouver
28 July 2011
2RUAG Space
Project Overview
• The overall study objective (Phase 1&2) has been to develop an imaging microwave sounder concept for Geostationary Earth Orbit (GEO)
• Phase 1 was a feasibility study where the concept was developed and analysed
• Phase 2 has demonstrated the concept by developing and testing a fully operational interferometer system
Study Team:European Space Agency (ESA)
– Specifications and coordination of the projectRUAG Space (Göteborg, Sweden)
Focusing on:– System design– Image retrieval processing and calibration– Antenna design– Mechanical/thermal design
Omnisys Instruments (Göteborg, Sweden)Focusing on:– System design– Front-End Electronics design– Back-End Electronics design
The ESA GEO Atmospheric Sounder Technology Project
3RUAG Space
Background and objectives
Meteorological needs for nowcasting & short range forecasting in the 2015 – 2020 time frame• 15-30 minute revisit time• 30 km resolution
380 GHz
Four frequency bands of interest centred around: 53, 118, 183, 380 GHz
Temperature (AMSU-A)Most important for NWP !
High altitude temperature
High altitude humidity
Humidity (AMSU-B)
166
346
4RUAG Space
Driving requirements
Requirements 2015 – 2020
15 - 30min Revisit Time
=> Geostationary Orbit
All Weather Capability
=> Requires the 53 GHz band
30 km Resolution
=> 8 m Aperture
Solution: Foldable InterferometerCan be LaunchedCan Operate on GEO S/C
Rotating InterferometerProvides images with finer resolution as compared to a stationary interferometer (for a given number of mm-wave receivers)
5RUAG Space
u
v
Interferometer element layout
x
y
λx
u∆=
λy
v∆=
• Minimum spacing of 3.5λ to avoid aliasing• Redundant baselines improves the instrument calibration• Good signal strength for 6, 9 ,12, 16, 19, 22λ due to size of Earth disc• Large spacing near the centre to enable interlacing several frequency bands
u-v sampling after rotation
ξ
η
Without rotation
-0.15 -0.1 -0.05 0 0.05 0.1 0.15
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
ξ
η
-0.15 -0.1 -0.05 0 0.05 0.1 0.15
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
instantaneous u-v sampling
6RUAG Space
Earth
x3
passivelumpedMMIC
thin film MCMLO cable
Cross-correlator
ASIC
Cross-correlator
ASIC
Cross-correlator
ASIC
Cross-correlator
ASICx140
IF cable
x47 x3
x140
x140Dual pol. Front-end.
Front-end Module
Correlator ASIC
Central Electronics
Instrument electronics
The polarisation vector rotates during measurement⇒ All four Stokes parameters are measured⇒ Improved sensitivity
7RUAG Space
Mechanical design
Deployment hinge
Attachment structure
Rotational drive
Boom structure
Radiator
Mounting support
53 GHz
380 GHz
118 GHz
183 GHz
8RUAG Space
Instrument budgets
53 GHz Interferometer S /S118 GHz Interferometer S /S
183 GHz Interferometer S /S380 GHz Interferometer S /S
Boom structure
Thermal HW
Mounting supports
HRMs
GAS Instrument
Boom Harness
I /O & Mode
Control
Cross-Correlation Processor
LO
PWR
Front End Units
Boom Harness
I /O & Mode Control
Cross-Correlation Processor
LO
PWR
Front End Units
Boom Harness
I/O & Mode
Control
Cross-Correlation Processor
LO
PWR
Front End Units
Central Electronics Unit
RotatingControl
Unit
StationarySensors
RotatingSensors
Instrument Management S/S
Calibration S/S
Attachment Structure
Thermo-Mechanical S/S
380 GHz Interferometer S /S 183 GHz Interferometer S /S
118 GHz Interferometer S /S
Ground Segment
Ground processing
Ground control
Spacecraft
CounterReaction
Wheel
Boom Harness
I/O & Mode
Control
Cross-Correlation Processor
LO
Power Distribution
Front End Units
Central Electronics Unit
IF
53 GHz Interferometer S /S
Instr.Contr.Unit
Rotational Drive
TM/TCPWR
Data Link Unit
Four frequency bands with polarimetric capabilityPower: 406 W Mass: 375 kg
The concept is scalable !
9RUAG Space
53 GHz demonstrator
• Includes the central part of the instrument: 21 dual-pol interferometer elements (53 GHz band)• Objective: to demonstrate the imaging concept with rotation, calibration, and post-processing
Parameter Value Remark
Frequency band 49-53 GHz Single 90 MHz channel
Number of elements 21 Dual polarisation
Longest baseline 75 cm ~140 λ
Image Resolution <10 mrad ~300 km on earth
Relative accuracy < 2K
Demonstrator characteristics
10RUAG Space
Antenna design
-80 -60 -40 -20 0 20 40 60 80-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
Angle (deg)
Am
plitu
de (dB
)
GAS 53 GHz
meas Co E-pol
meas Co H-planemeas Cr E-plane
meas Cr H-plane
sim Co E-plane
sim Co H-planesim Cr E-plane
sim Cr H-plane
Coupling between neighbouring elements: < -67 dB
11RUAG Space
Front-end design
LO input
Antenna RF inputs(2 pol)
IF outputs
MMIC designed in collaboration with Chalmers
12RUAG Space
Demonstrator integration
21 front-ends with antennas
Cross-correlator core (42 inputs)Assembled on rotational drive
13RUAG Space
Demonstrator test campaign
Parameters to verify•Image angular resolution•Image beam efficiency•Image polarisation isolation•Image relative accuracy
SourcesCalibration sources: Hot & Cold Loads
Imaging sources: Noise point source
CW point sources
Distributed source
Distributed source
Noise CWCW
14RUAG Space
Demonstrator test results
Relative calibration of all elements using point source in boresight:
Gain stability over 2 hours: 0.03 dB RMS @ 51 GHzPhase stability over 2 hours: 0.2 deg RMS @ 51 GHz
15RUAG Space
Demonstrator test results
Point source imaging:
Demonstrator configuration
1 deg/s
Polarization
XX-pol
YY-pol
Measured pol. isolation is dominated by co-polar sidelobes !
PS1 PS2
16RUAG Space
Demonstrator test results
11:30
12:30
12:00
Solar imaging – complete transition (boresight elevation is 47.4 deg):
17RUAG Space
Outdoor imaging of distributed source (about 15 minutes of integration):
Demonstrator Test Results
The resulting image is freefrom ambiguities!
18RUAG Space
Demonstrator Test Results
Distributed source imaging:
Linearity: 1.1 K RMSVariation over image: 0.7 K
Note that both linearity error and variation across image are systematic effects that can be calibrated!
19RUAG Space
Distributed source imaging – difference between images -> noise error:
Demonstrator Test Results
Noise error (Ne∆T) : 0.8 K RMS at 30 minutes of integration
The theoretical model assumes a Gaussian density distribution of the baselines !
Theory:
20RUAG Space
Compliance status:• Demonstrator requirements are met• The concept is demonstrated
Test Results Summary
Image brightness temp. rel. accuracy:
21RUAG Space
Related activities
Parallel study: Ultra-stable structure for interferometric instrument
Parallel study: MMIC for 118 and 183 GHz
Deployment test
Results show that the design is viable
90 & 118 GHz 166 & 183 GHzNF = 3.1 dB NF = 6.0 dB
Packaging
Expected with 50 nm mHEMT (flight design):NF = 2.5 dB NF = 4.8 dB
Receivers produced using 100 nm mHEMT:
22RUAG Space
Conclusion
• The rotating sparse array interferometer concept allows optimization of sensitivity for a reduced number of elements
• We have developed a demonstrator consisting of 21 dual-polarised receivers in a rotating sparse array
• We have achieved the performance success criteria defined at the start of the activity:
Image resolution: <10 mrad
Image beam efficiency: >95%
Relative accuracy of brightness temperature: <2K
Image polarisation isolation: >10 dB (goal: 20 dB)
• The obtained performance parameters agree well with model predictions
• The breadboarding and demonstration activities have increased the know-how on all levels: from components to systems