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