RISAT-1. Radar Imaging Satellite

Radar Imaging Satellite 

Indian Space Research Organisation


IRS satellites so far have been 

designed to provide data in the 

visible and near infrared region of 

electro-magnetic spectrum. The 

collection of data in these bands by 

the electro-optic imaging sensors 

has a limitation in the presence of 

clouds and fog, as the 

electromagnetic radiation in these 

wavelengths cannot penetrate the 

clouds/fog. India being a tropical 

country has a perennial cloud 

presence during kharif season (July 

October), which is the prime 

monsoon period in the country, 

limiting seriously the visible and 

near IR data from satellites for any 

meaningful applications. This 

season is very important from 

agricultural production point of 

view and the country is also prone 

during this season for disasters 

such as flood, cyclones and storm 

surges. Besides, the visible & near 

IR data can only be obtained during 

daytime as it depends on sun's 

reflected radiation for data 

collection. It is where the use of 

microwave remote sensing data 

becomes essential. Microwave data 

such as Synthetic Aperture radar 

(SAR) has the innate capability of 

collecting data in day/night and all 

weather conditions as it operates in 

microwave bands for which the 

atmosphere is transparent. With 

the known radar backscatter 

sensitivity to soil moisture and 

surface roughness, SAR has become 

an important component in many 

applications, either in a stand-alone 

mode or in complementary/ 

supplementary to electro-optical 

sensors, and has been extensively 

used in areas such as agriculture, 

oceanography, forestry, geology and 

hydrology. 


(RISAT-1), a new class of remote 

sensing satellite distinct from the 

established IRS class, is being 

developed by Indian Space Research 

Organisation (ISRO) as its first 

satellite imaging mission using an 

active radar sensor system. RISAT-1 

carries a multi-mode C-band 

Synthetic Aperture Radar (SAR) as 

the sole payload. The RISAT Mission 

is envisaged to augment the 

operational remote sensing 

programme in the country mainly 

enhancing agriculture and disaster 

support related applications. RISAT 

is slated for launch using PSLV by 

during the second half of 2008.

Mission 

The primary mission objectives of RISAT-1: 

• To design, develop, launch and operate a space based SAR mission 

operating in multi-polarisation, multi-imaging modes with the mission life 

of atleast 5 years. 

• To establish ground segment to receive, process and provide SAR data 

products and services in an operational manner to the user community 

• To enable integrate with the existing applications and help develop newer 

applications. 

RISAT-1 will be launched into a polar sun synchronous orbit of 610 kms with 

the local time of equatorial crossing at 6.00 AM in descending node, as SAR 

does not need sun illumination for the target. The choice of this orbit gives 

advantage in terms of maximizing the power generation, with lesser 

complexities in solar panel tracking arrangements and power system 

management, besides simplifying the thermal management. This orbit also 

meets the basic repetitivity cycle requirement of around 25 days for the 

medium resolution (25m) mode of SAR. 

EARTH SENSOR 

STAR SENSOR 

SOLAR PANEL 

THRUSTERS 

SAR ANTENNA 

The major orbit parameters 

Altitude – 610 Km, polar, circular 

Inclination – 97.844° 

Local time of pass – 6.00 AM

Modes 

Coarse Resolution ScanSAR 

Mode (CRS) 

Medium Resolution ScanSAR 

Mode (MRS) 

Fine Resolution Stripmap 

Mode-2 (FRS-2) 

Fine Resolution Stripmap 

Mode-1 (FRS-1) 

SAR Payload 

RISAT-1 will carry a C-band 

Synthetic Aperture Radar (SAR) catering 

to the needs of various applications 

stated elsewhere. It is well known that 

radar backscattering depends on sensor 

parameters such as frequency, 

polarisation, and incidence angle as well 

as on the target parameters such as 

dielectric constant, roughness and 

geometry. The RISAT-1 SAR sensor will 

be based on active phased array 

antenna technology, which will provide 

required electronic agility for achieving 

multimode capability. This state-of-theart 

technology is expected to survive 

technological obsolescence for long 

time to come and is upgradeable to 

other frequency bands in succeeding 

missions. 

RISAT SAR will operate with basic 

0 0 

elevation beam-width of 2.9 to 1.6 

over a total ground distance of 400 kms 

starting from a stand-off distance of 

200 kms. Elevation beamwidth will be 

Resolution 

made to vary with pointing angles in 

order to achieve pointing independent 

swath of 30 kms and near constant 

optimum radar cross section 

performance. Fast beam pointing and 

beam width control in elevation 

direction is achieved electronically 

with the help of an active phased 

array antenna. 

The active phased array antenna is 

the heart of SAR payload. Essentially, 

active antenna consists of a printed 

dual polarized radiating aperture; dual 

channel Transmit-Receive (TR) 

modules comprising TR-RF, TR control 

and miniaturized EPCs; the control 

unit; and a signal distribution and 

calibration network. The RF and baseband 

system consists of feeder Solid 

State Power Amplifiers (SSPA); 

frequency generator; receiver; digital 

chirp generator; data acquisition unit 

comprising of high speed digitiser, 

BAQ and formatter; calibration switch 

matrix; and payload controller. 

2-4 look (depending upon pointing) 50 m 

resolution over a swath of 240 km in either 

single or dual polarisation imaging 

1-2 looks (depending upon pointing) 25 m 

resolution over a swath of 120 km in either 

single or dual polarisation imaging 

Single look 9-12 m resolution image over 

30 km swath in quad polarisation 

Single look 3-6 m resolution image over 30 km 

swath in either single or dual polarization

Modes of Operation 

100 km 

(UNQUALIFIED) 

100 km 

(UNQUALIFIED) 

608 km 

FRS1&2 MODE 

200 

km 

400 km 

(QUALIFIED) 

CRS MODE 

MRS MODE 


The active antenna consists of three 

panels each of 2m x 2m size, totaling 

6x2minall,andisdividedinto12tiles 

of 1m x 1m each. Front side of each tile 

has a grid of 24 x 24 radiation patches. 

Backside of each tile carries 24 T/R 

modules, their power supply modules 

and controllers. With the help of phase 

shifters in the T/R modules, the antenna 

beam can be steered in range direction. 

Output RF peak power of each T/R 

module is 10 watts and the noise figure 

of its receiver is 2 dB. The antenna has 

its own calibration network to cross 

check the performance of its various 

elements on ground and in orbit. Input 

signals to and output signals from TR 

modules are processed using RF and 

baseband subsystems. A chirped signal 

at 5.35 GHz, having a pulse width upto 

20 µ sec is fed to the T/R module for 

further amplification and transmission. 

The output of the receiver is filtered, 

digitised using block adaptive 

quantisation and sent to data formatter. 

The data from SAR is formatted and 

fed to RF transmitter. A solid-state 

recorder (SSR) is part of the baseband 

data handling subsystem. The payload 

data can be transmitted in real time 

mode or in playback mode. 

It is planned to transmit the SAR 

data, with 640 MBPS data rate using X- 

band down-link in QPSK mode. A phased 

array antenna with dual polarization is 

used to overcome the limitation of 

available bandwidth. The data rate for 

each polarisation is, thus, planned to be 

2 x160 Mbps. Data transmitter antenna 

will have onboard beam 

steering towards the 

ground station during 

data transmission.

Hardware Specification of RISAT-SAR 

Frequency 

Antenna Type 

Antenna Size 

Side Lobe Level 

Number of TR modules 

Pulse width 

Average DC Input Power 

5.350 GHz 

Microstrip Active Antenna 

6m (Along Flight Direction) x 2m (Cross Flight Direction) 

-15 dB (Azimuth), -18 dB (Elevation) 

576, each with 10 W peak power 

20 µ sec 

3.5 kW 

Imaging Modes FRS-1 FRS-2 MRS/CRS 

Chirp Bandwidth 75 MHz 37.5 MHz 18.75 MHz 

Sampling Rate 83.3 MHz 41.67 MHz 20.83 MHz 

PRF 3000 Hz ± 200 Hz 3000 Hz ± 200 Hz 3000 Hz ± 200 Hz 

Quantisation 

2/3/4/5/6 bit BAQ 

MAX. Data Rate 556 Mbits/sec 564 Mbits/sec 142 Mbits/sec 

@ 6 bit BAQ (Single Pol.) 1112 (Single Pol.) 284 

Mbits/sec (Dual Pol. 

Mbits/sec (Dual Pol.) 

Specification of RISAT Antenna 

Size 

6m x 2m 

Frequency 

5.35 GHz ± 112.5 MHz 

Antenna Beamwidth 0.5° x 1.5° 

Beam Scanning Elevation ±15° 

Gain at center frequency 

Gain Bandwidth 

Antenna Bandwidth 

44.5 dB 

1.0 dB 

225 MHz 

3 Panels with each panel size 2m x 2m 

Cross polarization 

Polarisation 

-23 dB 

Dual linear polarisation

Spacecraft 

Characteristics 


RISAT-1 spacecraft mainframe 

bus is configured by deriving 

heritage from previous IRS missions 

as well as mission specific new subsystem 

designs. The prism shape of 

the satellite allows stowing of the 

active antenna in three folds around 

the prism structure. The prism 

structure is built around a central 

cylinder. Most of the spacecraft 

subsystems and the complete 

payload are integrated in the prism 

structure and the central cylinder. 

The solar panel and rest of the 

spacecraft subsystems are mounted 

on the cuboid portion of the 

satellite. Two solar panels with high 

efficiency multi-junction solar cell 

charge Ni-H2 

battery of 70 AH 

capacity. The satellite has an onboard 

recorder with storage 

capability of 240 Gbits of data. The 

on-board data transmitter can 

transmit with maximum data rate of 

640 Mbits/sec in X-band on two 

polarizations (RHC and LHC) by 

reuse of X-band carrier. In the nonoperating 

condition, the active 

antenna looks at nadir. Prior to 

operation, the spacecraft will be roll 

0 

tilted by ±34 to enable viewing 

either right or left side of the flight 

track. The satellite will also have 

yaw steering capability to minimize 

earth rotation effects. 

Attitude and Orbit Control 

System (AOCS) caters to various 

modes of SAR payload operation. 

The pointing accuracy requirement 

of RISAT is 0.05deg (3 σ); and 

-5 

attitude rate specification is 5x10 

deg/sec (3 σ) for frequencies 

-4 

0 - 0.2 Hz and 1x10 deg/sec (3 σ) 

for frequencies ≥ 2 Hz; and the 

post-facto attitude knowledge 

requirement is 0.02deg. One of the 

payload operating modes, namely 

'spot tracking' calls for imparting a 

large rate (around 0.42deg/sec) 

about pitch axis and imaging for a 

few seconds in the step and stare 

mode. The basic attitude reference 

during spot, strip, and ScanSAR 

modes of payload operation will be 

from gyros. For strip and ScanSAR 

modes, the updates will be from 

star sensors. 

The attitude sensors will include 

star sensors (18 deg.x18 deg. 

FOV), 4π 

sun sensors, 

magnetometers, Inertial Reference 

Unit (IRU) with DTGs, and conical 

earth sensors. The actuators will 

include RCS thrusters (eight 11N 

thrusters for attitude control and 

one 11N thruster for orbit control), 

eight 1N canted thrusters to 

produce control torques around all 

axes, and four 11N thrusters for 

orbit control with off-modulation 

capability); reaction wheels (0.3 NM 

and momentum storage capacity of 

50 NMs); and magnetic torquers of 

2 

60 Ampm capacity. A bus 

management unit (BMU) is 

designed around the MA31750 

processor which implements the 

MIL-STD –1750 architecture. The 

BMU interfaces with TTC (RF), 

sensors, power, thrusters, MTCs, 

DTG, WDE and payload for 

command and house keeping 

telemetry functions. It has MIL- 

STD-1553 interface with star sensor 

and SAR payload.

Disassembled View of RISAT 

SOLAR PANEL 

RCS TANK 

RW DECK 

+R 

+Y 

+P 


PAA DECK 

PAYLOAD DECK 

SAR ANTENNA

Mechanical System 

Structure 

Thermal System 

CFRP - Aluminium prism honeycomb sandwich cylinder with 

Aluminium honeycomb panels 

Passive / Semi-active thermal control with paints, blankets, 

OSRs and closed looped auto temperature controllers 

Thermal Control Battery -5 ± 5° C 

Electronics 

0 to 40° C 

Mechanisms 

Power System 

Solar panels 

Solar array power 

Chemical Battery 

Solar Array Deployment 

SAR Antenna Deployment 

3 on either side, sun-tracking, 10 m 

1500 W, normal to sun 

1 x 70 AH Ni-H Battery 

Power Electronics Two regulated raw buses (42 & 70 V) 

TTC System 

Telecommand 

Telemetry 

PCM/PSK/PM Modulation, 4 Kbps Time tag command facility 

PCM/PSK/PM Modulation 4 Kbps (real time); 16 Kbps (playback) 

Transponder Uplink frequency : S-band 

Downlink frequency : S-band 

AOCS 

Sensors 

Actuators 

AOCE 

Earth sensors, Star sensors, 4Pi Sun sensors, Magnetometer, 

IRU with 3-DTGs, DSS 

Reaction wheels, Magnetic Torquers, Reaction control thrusters 

Microprocessor based system 

2 

2 

Pointing accuracy 

Positional accuracy 

Drift rate 

Data Handling System 

Data rate 

RF System 

X-band Frequency 

Satellite Mass 

0 

± 0.05 about all axes 

20 – 25 m (using SPS in autonomous mode) 

-5 

< 5.0 x 10 deg /sec 

2 x 160 Mbps (each polarization) 

QPSK Modulated Transmitter 

8025 – 8400 MHz 

1750 Kg

Ground 

Segment 

RISAT-1 ground segment 

comprises of various elements to 

support the space segment, data 

reception, data processing and 

dissemination. It includes planning 

and development of various 

software and hardware systems for 

mission management by 

Spacecraft Control Centre (SCC) at 

ISTRAC, Bangalore, including a 

network of international ground 

stations. Data reception, 

processing, archival and 

dissemination at National Remote 

Sensing Agency (NRSA), 

Hyderabad. ISRO Satellite Centre 

(ISAC), Bangalore and Space 

Applications Centre (SAC), 

Ahmedabad provides support for 

developing and operationalizing 

the mission and data product 

software. 

Spacecraft Control Centre (SCC): 

SCC is responsible for tracking 

the satellite, and carrying out 

mission operations. Using the 

existing ISTRAC Telemetry, Tracking 

and Command Network Stations 

at Lucknow, Bangalore, Bearslake, 

Mauritius and Biak mission 

operations, satellite health 

monitoring, health analysis, 

payload operations scheduling and 

payload programming are planned 

to be carried out with necessary 

interfaces with NRSA centre to 

cater to the user-driven payload 

programming requests. Being in 

dawn/dusk sun synchronous orbit, 

RISAT will have no visibility clash 

with other IRS satellites over a 

ground station. The other IRS 

satellites have 10.30 AM to 12 

Noon equatorial crossing time for 

descending passes. 

Data Reception Station (DRS) 

Payload Data Reception is at 

NRSA Data Reception Station 

(DRS) at Shadnagar, Hyderabad 

with necessary augmentation for 

handling the high data rate of 320 

Mbps on each of the reception 

chains. It is planned to have a 

7.5 m dual shaped cassegranian 

antenna system with G/T of 32 dB/ 

deg.K in X-band. Auto diversity of 

X-band tracking receiver is planned 

to facilitate tracking on either of 

two channels namely RHC or LHC. 

The Data reception station has the 

capability to receive the RT data as 

well as the data stored onboard 

satellite, which will be played back 

in the visibility. 

The data reception and the 

first level of processing (Level-0) 

systems are collocated in the DRS 

itself. A H/W quick look/Near Real 

Time processor system facilitates 

a quick look viewing of the image 

to assess the health of the 

Payload system in real time. The 

first level of products are 

generated at the DRS itself after 

the payload pass using the 

ancillary information. Later the 

data is transported to the dataprocessing 

centre at Balanagar 

through storage media like DLT 

and also through network for 

further processing. 

Data Processing and Product 

Generation 

The Data Processing Systems 

are implemented at Balanagar 

NRSA. The SAR data processing, 

unlike the optical sensors, has to 

undergo signal processing, both in 

the range and in azimuth 

directions, before images are 

formed. The data products 

envisaged for RISAT mission could 

be generally categorized as 

browse, standard (geo-tagged and 

geocoded), value added and 

application specific products 

(precision geocoded, terrain 

geocoded, Pol-SAR products, large 

area mosaics etc.).

RISAT 

& 

Beyond 

RISAT-1 will be followed with improved versions of microwave payloads for 

the various applications as part of the land and water thematic series of IRS 

satellites planned under the Indian EO programme. As part of the strategy 

the coming years will see the development of X-band and L-band SAR system 

as well as L-band Synthetic Aperture Radiometer. These satellites will also 

provide necessary support for the disaster management applications. 


Simulated images from Airborne SAR

For more details about IRS programme & satellite 

Dr V Jayaraman 

Director, Earth Observations System 

Indian Space Research Organisation Hq. 

Antariksh Bhavan, New BEL Road 

Bangalore-560094, India 

Tel: +91-80-23416358 / 23415474 

Fax: +91-80-23415298 / 23413806 

Email: vjay@isro.gov.in 

Mr M Krishnaswamy 

Programme Director, IRS 

ISRO Satellite Centre (ISAC) 

Airport Road, Bangalore-560 017, India 

Tel: +91-80-25205268 

Fax: +91-80-25206263 

E-mail: krishnas@isac.gov.in 

Mr R N Tyagi 

Project Director, RISAT 

ISRO Satellite Centre (ISAC) 

Airport Road, Bangalore-560 017, India 

Tel: +91-80-25082428 

Fax: +91-80-25205271 

E-mail: rntyagi@isac.gov.in 

Data Supply 

NRSA Data Centre 

National Remote Sensing Agency 

Balanagar 

Hyderabad-500037, India 

Tel: +91-40-23078560 / 23079572 

Fax: +91-40-23078664 / 23078158 

September 2007 

Published by Publications and Public Relations, ISRO Headquarters, Bangalore 

Designed by Imagic Creatives, Bangalore and Printed at Pragati Offset Pvt. Ltd., Hyderabad

RISAT-1. Radar Imaging Satellite

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