risat-1

Indian Space Research Organisation

Radar Imaging Satellite

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.


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.

Altitude – 610 Km, polar, circular

Inclination – 97.844°

Local time of pass – 6.00 AM

The major orbit parameters

MissionMission

STAR SENSOR

SOLAR PANEL

SAR ANTENNA

EARTH SENSOR

THRUSTERS

Rad

Coarse Resolution ScanSAR 2-4 look (depending upon pointing)

single or dual polarisation imaging

Medium Resolution ScanSAR 1-2 looks (depending upon pointing) 25 m

Fine Resolution Stripmap Single look 9-12 m resolution image over

Fine Resolution Stripmap Single look 3-6 m resolution image over

50 m

Mode (CRS) resolution over a swath of 240 km in either

Mode (MRS) resolution over a swath of 120 km in either

single or dual polarisation imaging

Mode-2 (FRS-2) 30 km swath in quad polarisation

30 km

Mode-1 (FRS-1) swath in either single or dual polarization

Modes Resolution

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-the-

art 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

elevation beam-width of 2.9 to 1.6

over a total ground distance of 400 kms

starting from a stand-off distance of

0 0

200 kms. Elevation beamwidth will be

SAR Payloadmade 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 base-

band 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.

The active antenna consists of three

panels each of 2m x 2m size, totaling

6 x 2 m in all, and is divided into 12 tiles

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.


Modes of Operation

400km

(QUALIFIED

)

200km

608 km

100km

(UNQUALIFIED

)

100km

(UNQUALIFIED

)

CRS MODE

MRS MODE

FRS1&2 MODE

Frequency 5.350 GHz

Antenna Type Microstrip Active Antenna

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

Side Lobe Level -15 dB (Azimuth), -18 dB (Elevation)

Number of TR modules 576, each with 10 W peak power

Pulse width 20 µ sec

Average DC Input Power 3.5 kW

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

Imaging Modes FRS-1 FRS-2 MRS/CRS

564 Mbits/sec 142 Mbits/sec

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

Mbits/sec (Dual Pol. Mbits/sec (Dual Pol.)

Hardware Specification of RISAT-SAR

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 44.5 dB

Gain Bandwidth 1.0 dB

Antenna Bandwidth 225 MHz

3 Panels with each panel size 2m x 2m

Cross polarization -23 dB

Polarisation Dual linear polarisation

°

RR

RISAT-1 spacecraft mainframe

bus is configured by deriving

heritage from previous IRS missions

as well as mission specific new sub-

system 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-H battery of 70 AH

capacity. The satellite has an on-

board 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 non-

operating condition, the active

antenna looks at nadir. Prior to

operation, the spacecraft will be roll

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

2

0

of RISAT is 0.05deg (3 ); and

attitude rate specification is 5x10

deg/sec (3 ) for frequencies

0 - 0.2 Hz and 1x10 deg/sec (3 )

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), 4 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

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.

σ

σσ

≥

π

-5

-4

2

SpacecraftSpacecraftCharacteristicsCharacteristics

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Structure CFRP - Aluminium prism honeycomb sandwich cylinder with

Aluminium honeycomb panels

Thermal System 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 Solar Array Deployment

SAR Antenna Deployment

Solar panels 3 on either side, sun-tracking, 10 m

Solar array power 1500 W, normal to sun

Chemical Battery 1 x 70 AH Ni-H Battery

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

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

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

Transponder Uplink frequency : S-band

Downlink frequency : S-band

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

IRU with 3-DTGs, DSS

Actuators Reaction wheels, Magnetic Torquers, Reaction control thrusters

AOCE Microprocessor based system

Pointing accuracy ± 0.05 about all axes

Positional accuracy 20 – 25 m (using SPS in autonomous mode)

Drift rate < 5.0 x 10 deg /sec

Data rate 2 x 160 Mbps (each polarization)

RF System QPSK Modulated Transmitter

X-band Frequency 8025 – 8400 MHz

2

0

-5

2

Mechanical System

Power System

TTC System

AOCS

Data Handling System

Satellite Mass 1750 Kg

R

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 data-

processing centre at Balanagar

through storage media like DLT

and also through network for

further processing.

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.).

Data Processing and Product

Generation

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.

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.

Data Reception Station (DRS)

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.

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,

Spacecraft Control Centre (SCC):

GroundSegment

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.

RISAT&Beyond


Simulated images from Airborne SAR

S

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For more details about IRS programme & satellite

Data Supply

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:

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:

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:

NRSA Data Centre

National Remote Sensing Agency

Balanagar

Hyderabad-500037, India

Tel: +91-40-23078560 / 23079572

Fax: +91-40-23078664 / 23078158

[email protected]

[email protected]

[email protected]

Published by Publications and Public Relations, ISRO Headquarters, Bangalore

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

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