Figure-1 Imaging Modes of RISAT-1

RISAT-1: Configuration and Performance Evaluation

Tapan Misra1 and A S Kirankumar2

1Space Applications Centre (ISRO), Ahmedabad-380015, India email:misratapan@sac.isro.gov.in

2Space Applications Centre (ISRO), Ahmedabad-380015, India email:kiran@sac.isro.gov.in

Abstract

Radar Imaging Satellite (RISAT-1) was launched on 26th April, 2012 and is available to international SAR community and researchers as a source of multi resolution/multi-swath/multi-polarisation SAR data. The first satellite, RISAT-1, carries a multi-mode Synthetic Aperture Radar (SAR) in C-band as the sole payload. RISAT-SAR payload supports variety of imaging modes e.g., stripmap, scanSAR and spotlight. All these imaging modes can be operated in Hybrid polarimetry mode also, making it the first of its kind.

The payload is based on active antenna array technology. The active antenna has nominal aperture of 6 mx2m, consisting of 12 tiles which are grouped into 3 panels. The RISAT-1 spacecraft has been built around the SAR payload in order to optimise the spacecraft weight and structure. The satellite is placed in a sun-synchronous orbit with 6 am-6 pm equatorial crossing, to maximize solar power availability.

Initial performance assessment indicates very satisfactory performance of the SAR system of RISAT-1 in terms of image quality, calibration, geometric fidelity and consistent product performance and active antenna pattern synthesis. Already from September 2012, data products are available operationally from National Remote Sensing Centre, Hyderabad.

1. Introduction

As the first of new series of microwave imaging satellites in ISRO’s fleet of remote sensing satellites, RISAT-1 (Radar Imaging Satellite) has been launched on 26th April, 2012. This radar data is now available to international SAR community and researchers. RISAT-1 carries a multi-mode Synthetic Aperture Radar (SAR) in C-band as the sole payload with primary applications targeted for (a) Agriculture monitoring, primarily during monsoon season and (b) Flood mapping, as part of national Disaster Management Programme.

2. Imaging Capability

RISAT-SAR payload is a multimode SAR which operates

from a sun synchronous orbit at a nominal altitude of 536 km. The basic imaging modes, incorporated in the payload, are as follows (Fig.-1):

• Coarse Resolution ScanSAR Mode (CRS): 50 m resolution, 223 km swath

• Medium Resolution ScanSAR Mode (MRS): 25 m resolution, 115 km swath

• Fine Resolution Stripmap Mode-1 (FRS-1): 3 m resolution, 25 km swath

• Fine Resolution Stripmap Mode-2 (FRS-2): 9 m resolution, 25 km swath.

• High Resolution Spotlight Mode (HRS): 10 km swath, 10-100 km azimuth extent, 1 m resolution.

The data from all the above modes, except FRS-2 can be operated with co and cross polarization options. Quad polarization capability is incorporated in FRS-2. As far as

radiometric resolution is concerned, all the images will be available in single look only, except in CRS mode, where the possibility of 2 range looks is provided.

For the first time in spaceborne SAR systems, hybrid polarimetric operation [1,2] is available in seamless fashion in all the above imaging modes. In this mode, pulse is transmitted in circular polarization and received signal is digitized in

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Figure-2 RISAT-1 with Antenna in Deployed and Stowed Condition

two linear polarizations. The hybrid polarimetry mode does not require doubling of PRF as in case of linear polarimetry. Hence it can be operated seamlessly over all incidence angles and all resolution-swath modes as PRF remains unchanged.

HRS mode is configured as a sliding spotlight one and presently is being operated in experimental mode. Detailed performance specifications of the various imaging modes are provided in Table-1.

The SAR can image on either side of the track by roll tilting of the spacecraft by ±36O. However, in one orbit, only one side of the orbit can be imaged. On either side, imaging area is restricted over 550 km distance starting at a stand-off distance of 107

km. 3. RISAT-1 Satellite and Payload Configuration

A detailed description of RISAT-1 satellite and payload is available in [3]. The RISAT-1 spacecraft has been built

around the SAR payload in order to optimise the spacecraft weight and structure (Fig.-2). In-orbit mass of the satellite is around 1858 kg, of which the SAR payload contributes around 950 kg. Two solar panels with high efficiency multi junction solar cell, charges Ni-H2 batteries total having 70 Ah capacity. The power sizing ensures 10 minutes operation

in each orbit. The satellite has a capability of storing up to 240 Gbits of data in on-board solid state recorder (SSR). The on-board data transmitter can transmit with a maximum data rate of 640 Mbits/sec in X-band. In the non-operating condition, the active antenna looks at nadir. Prior to operation, the spacecraft is roll tilted by ±36o to enable viewing either right or left side of the flight track. The satellite is continuously yaw steered on-board to ensure Doppler

centroid within ±100 Hz. RISAT-1 operates in a sun-synchronous dawn to dusk orbit with revisit period of 25 days. Salient features of RISAT-1 satellite are presented in Table-2.

The SAR system consists of two broad segments namely (A) Deployable Active Antenna and (B) RF and Baseband Systems housed on the satellite deck. The active antenna has approximately 6mx2m antenna aperture, distributed in two deployable and one fixed panels. Each of the panels have 4 tiles of aperture area of 1mx1m. Each tile consists of 24 dual polarised linear arrays, aligned along azimuth direction. Each of these linear arrays is fed by functionally two separate TR modules, feeding two separate distribution networks for V and H operation with the same radiating patches. A glass wool blanket on the antenna isolates it earth albedo. TR modules and associated electronics are mounted on the backside of CFRP tile structure.

All the 24 TR modules on the tile are controlled by one Tile Control Unit (TCU). TCU provides synchronisation of TR modules with a master reference.

An extensive on-board calibration facility is provided with the help of a set of CAL switches and dedicated distribution networks,

for calibrating transmit and receive paths of each of the TR modules separately. The peak power from active antenna is 2880 kW and it consumes up to 3.9 kW DC power during SAR operation.

The RF and Baseband subsystems are housed on the satellite deck just behind the static antenna panel. Two separate chains of receiver (Rx) and Data Acquisition and Compression Systems (DACS) cater to simultaneous operation in two polarisations. One feeder SSPA provides pulse signal to all the 576 TR modules with a common phase reference. Two

Table-1 RISAT-1 Image Quality Parameters

Swath Coverage Selectable within 107 – 659 km off-nadir distance on either side

Incidence Angle Coverage 12° – 55°

Image Quality

HRS 1m(Azimuth)x 0.67m (Range) resolution, 10 x 10-100 Km Spot, Min σo= -16 dB

FRS-1 3m(Azimuth)x2m (Range) resolution 25 Km swath,Min σo= -17 dB

FRS-2 3m(Azimuth)x4m (Range) resolution, 25 Km swath

Min σo= -20 dB

(Hybrid Polarimetry)

9m(Azimuth)x4m (Range) resolution, 25Km swath σ

o= -19dB

(Quad Polarisation)

MRS 21-23 m (Azimuth)x8m (Range) resolution,115 Km swath, Min σo= -17 dB

CRS 41-55 m (Azimuth)x8m (Range) resolution,223 Km swath, Min σo= -17 dB

Table-2 Salient Risat-1 Spacecraft Parameters

Orbit Circular Polar Sun Synchronous Orbit Altitude 536 km Orbit Inclination 97.552O Orbit Period 95.49 minutes No. of orbits per day 14 Equator Crossing 6.00 AM/6.00 PM Spacecraft Height 3.85 m Mass 1858 kg Power Solar Array Generating 2200W

and one 70 AH Ni-H2 Battery Max power handling capacity

4.3 KW

Data rate

2x160 Mbps (Total 640 Mbps in 2 chains)

SSR 240 Gbits (EOL) TT&C S-band Payload down link X-band Power 70V bus /42V bus Pointing accuracy 0.05O Drift rate 5.0 x 10-5 O/sec Attitude knowledge 0.02 O

Figure-3 RISAT Active Antenna in PNF and Comparison of a Measured and Simulated Antenna Pattern

different pulse-widths are used for different polarization combinations. For single/dual polarization operation, 20 µsec is used to feed either of H or V TR modules. For hybrid polarimetric operaaatiiion , 10 µsec pulse is fed to both H and V TR modules with 90O phase difference. The chirp pulse is generated by a digital chirp generator (DCG) and all frequency coherence is maintained by a coherent frequency generator (FG)). All the subsystems are configured with 100% redundancy.

On board data rate is minimized with a provision of seam less BAQ (Block Adaptive Quantisation) ranging from 2-6 bits. Total data rate from the SAR payload to Spacecraft Data Handling subsystem ranges from 140-1500 Mbps. Major specifications of the

SAR system are summarised in Table-3.

4. Performance of Integrated Payload

RISAT-1 SAR has total 126 antenna beams. Each beam definition comprises of two transmit beams and two receive beams in H and V polarizations. These 126 beam pointings are divided into 63 pointings for each side of satellite track. Since left and right orientations are achieved in shortest time by roll tilting, the 63 pointings, on either side of the flight track. are distinct. Antenna pattern was measured in the integration laboratory itself in Planar Near Field (PNF) using Time Gating (Fig.-3). Payload was used to measure itself in the PNF and 12 pattern measurements were carried out simultaneously for a particular polarization and transmit/receive configuration. From limited measurements of 24 patterns out of total 504, rest were predicted.

5. Post Launch Performance

RISAT-1 SAR was switched ON on

1st May 2012. After calibration and validation of the image products, they were released for global users from 19th October 2012. National Remote Sensing Centre (NRSC), Hyderabad is responsible for operationally generating and distributing the products.

During In Orbit Test (IOT), azimuth antenna pattern in both transmit and receive mode of the RISAT active antenna was measured through a ground receiver. Similarly combined Tx-Rx elevation pattern was measured from images over Amazon rain forest. Fig.-4 shows close agreement of the measured pattern with the predicted antenna pattern. It is to be noted that, during integrated testing, antenna patterns were predicted based on the limited Near Field measurements. During IOT only, for the first time far field antenna patterns were measured. Further, radiometric correction using predicted elevation pattern could result in excellent radiometric balance and calibration accuracy over required swath within ±0.5 dB (Fig.-5). Results from repeated corner reflector campaigns show excellent image quality performance as per design.

Table-3 Salient Hardware Specification Of RISAT-SAR

Frequency 5.350 GHz Antenna Type Printed Antenna Antenna Size 6m (Along Flight) x 2m (Cross Flight) Side Lobe Level -13 dB (Az.), -13 dB (El.) No. of TR modules 576, each with 10 W peak power Pulse width 20/10 µsec Average DC Input Power

1.8-3.9 kW

HRS FRS-1 FRS-2 MRS/CRS Chirp Bandwidth 225 MHz 75 MHz 37.5 MHz 18.75 MHz Sampling Rate 250 MHz 83.3 MHz 41.67 MHz 20.83 MHz PRF 3500 Hz

± 200 Hz 3000 Hz ± 200 Hz

3000 Hz ± 200 Hz

3000 Hz ± 200 Hz

Quantisation 2/3 BAQ 2/3/4/5/6 bit BAQ MAX. Data Rate @ 3 bit BAQ for HRS @ 6 bit BAQ for rest of the modes

739 Mbits/sec (Single Pol.)

1478 Mbits/sec (Dual Pol.)

556 Mbits/sec (Single Pol.)

1112 Mbits/sec (Dual Pol.)

564 Mbits/sec 142 Mbits/sec (Single Pol.)

284 Mbits/sec (Dual Pol.)

Typical images, obtained by RISAT-1, are shown in Fig.-6. They demonstrate the quality of the RISAT-1 SAR images in nutshell. It is to be noted that for the first time, polarimetric SAR products are available only from RISAT-1 for all imaging modes (including scanSAR and spotlight) and in all incidence angles.

6. References

1. R. K. Raney, “ Hybrid-Polarity

SAR Architecture”, EUSAR 2006 IEEE TGARS Special Issue

2. R. Keith Raney, IEEE, “Hybrid-Polarity SAR Architecture”, IEEE Transactions on Geoscience and Remote Sensing, VOL. 45, NO. 11, NOVEMBER 2007

3. Tapan Misra et al, “Synthetic Aperture Radar payload on-board RISAT-1: configuration, technology and performance”, Current Science, VOL. 104, NO. 4, 25 FEBRUARY 2013, pp 446-461

HRS VV

HRS Hybrid Polarimetry

MRS Hybrid Polarimetry FRS-1 Hybrid Polarimetry

Figure-6 Sample Images in Different Modes as Imaged by RISAT-1

Figure-4 (a) Comparison of Predicted and IOT Measured Transmit Azimuth Pattern (b) Comparison of Predicted Tx-Rx Combined and IOT Measured Elevation Pattern, measured over Amazon Rain Forest

Figure-5 Calibration Stability of FRS-1 Mode as per Measurement σO Over Amazon Rain Forest