IEEE Geoscience and Remote Sensing Magazine - December 2014 - 49

taBle 2. leo BaSed oceanoGraphic and
atmoSpheric oBSerVation proGramme.

Satellite

SenSor
Spectral
complement Band

Oceansat-1 OCM
MSMR

(a)
HRS Hybrid Polarimetry

Oceansat-2 OCM

HRS VV

MRS Hybrid Polarimetry

FRS-1 Hybrid Polarimetry

Megha
Tropiques
(ISRO/
CNES)

SARAL
(ISRO/
CNES)

(b)

Swath
Spatial
width
reSolution (km)

0.4-0.9 nm
8 bands

360 m #
236 m

1420

6.6 (V&H),
10.65 (V&H),
18(V&H), 21
(V&H) GHz

150, 75, 50,
50 km (for
frequency
sequence)

1360

0.40-0.90 nm 360 m #
8 bands
236 m

1420

OSCAT

13.515 GHz

25 km #
25 km

1400

ROSA

GPS
occultation

MADRAS

5 chan. radiometer

40 km #
60 km

1700

SAPHIR

Humidity sounder

ScaRaB

Radiation budget instrument

GPS-ROS

GPS occultation

AltiKa

35.75 GHz Ka-band altimeter

DORIS

S/C tracking for POD services

Argos-3

Data collection system

LRA

Satellite laser ranging

FIGuRE 4. (a) RISAT-1 in clean room and (b) sample RISAT-1 images

in different modes.
◗ Medium Resolution ScanSAR Mode (MRS): 25 m resolu-

tion, 115 km swath
◗ Fine Resolution Strip map Mode-1 (FRS-1): 3 m resolution, 25 km swath
◗ Fine Resolution Strip map 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. FRS-2 has
quad polarization capability.
For the first time in space borne SAR systems, a hybrid
polarimetric operation mode is available in a seamless fashion, and in all the above imaging modes. In hybrid mode,
a radar pulse is transmitted in circular polarization and the
signal is received in two linear polarizations. Unlike linear
quad-pol polarimetry, hybrid polarimetry does require an
increase in the pulse-repetition frequency, as it is essentially
a dual-polarisation mode, hence there is no increase in
data rate. However because the transmitted signal is circularly polarized, considerable benefits of quad pol operation
are realized, limited up to complete description of Stoke's
parameters [6].
The SAR can image on either side of the track by rolling
the spacecraft by !36°. On either side, the imaging area
is restricted over a 550-km distance beginning at a standoff distance of 107 km. Typical images from RISAT-1 are
shown in Fig. 4.
december 2014

ieee Geoscience and remote sensing magazine

III. LEO BASED OCEANOGRAPHIC AND
ATMOSPHERIC SENSORS
Till now, four LEO based satellites were dedicated for
oceanographic and atmospheric observations, as listed in
Table 2. The suite of instruments covered both the visibleNIR and microwave bands, and are generally characterized
by low resolution and broad swath coverage.
A. OCM-1 And -2
The Ocean Colour Monitor (OCM-1), one of the payloads
onboard Oceansat-1, provided multi-spectral imagery
with narrow spectral width, a large field of view of !43°,
1420 km swath, and a ground resolution of 360 m. OCM
is a solid-state radiometer providing observations in eight
spectral bands in the VNIR region (nm): B1: 402-422, B2:
433-453, B3: 480-500, B4: 500-520, B5: 545-565, B6:
660-680, B7: 745-785, and B8: 845-885. An along-track
instrument tilt capability of !20° was used to avoid sun
glint. OCM-1 was designed to cater to land and ocean
applications globally and had high radiometric sensitivity, with a large dynamic range. It was a major design
and development challenge to meet the OCM-1 requirements: (a) 2 day repetivity (b) narrow spectral bandwidth
(c) large number of bands (d) precision measurement of
ocean radiance (NESR < 1.08 nW.cm-2sr-1µm-1 at 1 # 1
km 2 resolution) in the presence of large atmospheric path
radiance (e) avoidance of radiance contamination due to
sun-glint and (f) imaging of both ocean and land. Each
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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - December 2014
