SARAL to boost remote sensing ability of India

November 8, 2012

SARAL (Satellite with ARgos and ALtiKa)
With the launch of SARAL satellite carrying AltiKa and an Argos-3 instrument2 is scheduled to be launched on December 2012 and with this the remote sensing ability of the country will get a boost up.
The SARAL Mission:
The SARAL-Altika mission is a combined effort of ISRO and the French Space Agency (CLS).

CLS, a subsidiary of CNES (French Space Agency ‘Centre National d’Etudes Spatiales’).
The satellite will map the variability of the sea level.
It will also provide data to support operational research in marine meteorology and sea state forecast as well as seasonal forecast.
The satellite will be carrying the ARGOS system for CLS and that will help in the transmission of data from Argo floats.
The mission is complementary to the Jason-2 mission of NASA/NOAA and CNES/EUMETSAT.
It will fill the gap b/w Envisat and the Sentinel 3 mission of the European GMES program.

Firstly lets see some terms:
What is an Altimeter?

Altimeter (altitude meter)
Instrument to measure the altitude of an object above a fixed level.

Altimetry: The measurement of altitude is called altimetry.
Bathymetry: The measurement of depth underwater.
Now, about AltiKa:

Altika is an the altimeter and it is the prime payload of the SARAL mission.
AltiKa will provide accurate measurements of ocean topography, particularly near coastlines.

DORIS (Doppler Orbitography and Radio-positioning Integrated by Satellite): It will enable precise determination of the orbit.
LRA (Laser Retroreflector Array): It will help to calibrate the precise orbit determination system and the altimeter system several times throughout the mission.
Argos:

Established in 1978 under a MoU b/w CNES (France), NASA (USA) and NOAA(USA).
NOAA (National Oceanic and Atmospheric Administration).
Argos is a satellite-based system which collects, processes and disseminates environmental data from fixed and mobile platforms worldwide. What makes Argos unique is the ability to geographically locate the source of the data anywhere on the Earth utilizing the Doppler effect.

What is so special about Altika?

Altika will be the first spaceborne altimeter to operate at Ka band, operating at 35.75.
Signal frequencies in the Ka-band will enable better observation of oceans, ice, rain, coastal zones, land masses, and wave heights.
Dual frequency total power type microwave radiometer (23.8 and 37 GHz) is embedded in the altimeter to correct tropospheric effects on the altimeter measurement.

How is Altika different from other existing altimeters?
The present satellite-borne altimeters determine sea level by bouncing a radar signal off the surface and measuring the return-trip time.
However, AltiKa operates at a high frequency in Ka band. Now, this has a two fold advantage:

When we use other altimeter’s, Earth’s atmosphere slows down the radar signal, so the measurements are skewed and we have to carry additional equipment to correct for this error. But as AltiKa uses high frequency in Ka band, it does not have to carry an instrument to correct for atmospheric effects as current-generation altimeters do.
Operating at higher frequencies (Ka) gives higher accuracy. (AltiKa will measure ocean surface topography with an accuracy of 8 mm, against 2.5 cm on average using current-generation altimeters).

What is the disadvantage of using Altika over present other altimeters?

High-frequency waves in the Ka band are extremely sensitive to rain, even drizzle. So, statistically, it is expected that there will be a 10% loss of data.

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