The SCATSAT-1 Satellite Explained

SCATSAT-1 is an Indian satellite launched by ISRO in September 2016 to measure wind speed and direction over the world’s oceans using a Ku-band scatterometer, and it is India’s dedicated scatterometer mission for cyclone tracking and weather and ocean forecasting. It was launched on the Polar Satellite Launch Vehicle PSLV-C35 and placed in a polar sun-synchronous orbit at an altitude of roughly 720 km.

SCATSAT-1 was built quickly to restore an observing capability that India lost when the scatterometer on Oceansat-2 stopped working. This article explains what a scatterometer is, how SCATSAT-1 measures ocean winds, why the mission was created, what makes its PSLV-C35 launch notable, and how forecasters in India and abroad use its data.

Key Takeaways

• SCATSAT-1 is an ISRO satellite launched in September 2016 on PSLV-C35 to measure ocean-surface wind speed and direction with a Ku-band scatterometer.
• SCATSAT-1 was built using spare hardware and heritage from Oceansat-2 to replace the scatterometer capability lost when the OSCAT instrument stopped working.
• A scatterometer such as the one on SCATSAT-1 infers wind by measuring how much radar energy the ocean surface scatters back to the satellite.
• SCATSAT-1 supports tracking of tropical cyclones, weather forecasting and ocean-state prediction for India and the wider region.
• The PSLV-C35 mission that launched SCATSAT-1 was the first PSLV flight to deliver payloads into two different orbits in a single launch.
• SCATSAT-1 data is distributed through ISRO’s MOSDAC portal and is used by IMD and by weather agencies around the world.

What Is SCATSAT-1?

SCATSAT-1 is an ISRO ocean and weather satellite whose single main job is to measure winds blowing over the sea surface. Its name comes from “scatterometer satellite”, and the “1” marks it as the first satellite of that name.

The satellite carries one primary instrument, a Ku-band scatterometer designed and built at ISRO’s Space Applications Centre (SAC) in Ahmedabad. The satellite platform itself is based on the standard small Indian satellite bus developed at ISRO Satellite Centre in Bengaluru, which has been used in earlier missions. Reusing proven designs and spares allowed the mission to move from approval to launch in a short time.

Winds over the sea are difficult to measure directly across large and remote ocean areas, where ships and buoys are sparse. A satellite scatterometer can cover most of the global ocean in a day or two, so it fills a major gap in observation.

What Is a Scatterometer and How Does It Work?

A scatterometer is a radar instrument that sends microwave pulses toward the Earth’s surface and measures the echo that scatters back, which allows scientists to calculate wind speed and direction at the sea surface. It does not measure wind directly. Instead, it measures the roughness of the ocean, which is closely related to the wind.

The principle works in four steps:

• Wind blowing over the sea creates tiny ripples, called capillary waves, on the surface.
• The radar pulse is scattered by these ripples, and stronger winds produce rougher surfaces and stronger backscatter.
• The strength of the backscatter also depends on the direction of the wind relative to the radar beam.
• By viewing the same patch of sea from different angles, the instrument collects several measurements, and a mathematical relationship called a geophysical model function converts these into wind speed and direction.

Because the measurement relies on the satellite’s own microwave signal, a scatterometer works day and night. It is also much less affected by clouds than optical or infrared sensors, although heavy rain can disturb the signal at Ku-band frequencies.

What Instrument Does SCATSAT-1 Carry?

SCATSAT-1 carries a Ku-band scatterometer that operates near 13.5 GHz and uses a rotating dish antenna to produce two pencil-like beams. This is called a conically scanning design because the beams trace circles on the ocean surface as the antenna spins.

The two beams are set at different incidence angles and use different polarisations: an inner beam with horizontal polarisation and an outer beam with vertical polarisation. As the antenna rotates and the satellite moves along its orbit, each point on the ocean is seen from several directions, which is what makes the wind direction retrieval possible. The combined swath is very wide, in the region of about 1,800 km, so a single satellite sees a large fraction of the globe each day.

Why Was SCATSAT-1 Built?

SCATSAT-1 was built to continue the supply of ocean-wind data after the scatterometer on Oceansat-2 stopped functioning in the middle of its mission life. Oceansat-2, launched in 2009, carried a scatterometer called OSCAT, which became an important source of wind data for forecasters and researchers until it failed.

The loss created a gap in Indian ocean-wind data just as users were relying on it for cyclone forecasting and numerical weather prediction. ISRO therefore decided to build a dedicated replacement mission, reusing the design of OSCAT and the spare hardware that had been prepared for it. The approach shortened development time significantly compared with a new design.

The decision reflects a broader principle in ISRO’s planning: services such as weather forecasting and cyclone warning depend on continuous data, so key observations are replaced quickly when instruments are lost.

What Was Special About the PSLV-C35 Launch?

The PSLV-C35 launch was special because it was the first PSLV mission to place satellites in two different orbits in a single flight, using a restart of the rocket’s upper stage. SCATSAT-1 was injected into its polar orbit at about 720 km first, and the upper stage then coasted before firing again to deliver the remaining satellites to a lower orbit.

This demonstrated a technique that makes the PSLV more flexible and economical, since one rocket can serve customers who require different orbits. It also made use of a restartable, multi-burn capability of the PSLV’s fourth stage, which has been used in later missions.

In addition to SCATSAT-1, PSLV-C35 carried several smaller satellites. They included small university-built satellites from India and satellites from other countries, including Algeria, Canada and the United States. The mission lasted several hours from lift-off to final separation, one of the longer PSLV flights up to that time.

How Is SCATSAT-1 Used for Cyclone Tracking?

SCATSAT-1 is used for cyclone tracking by showing the pattern of surface winds around a developing storm, which helps forecasters locate its centre, judge its size and estimate its strength. Cyclones over the Bay of Bengal and the Arabian Sea are a regular and serious hazard for India’s long coastline.

Scatterometer winds are valuable at several stages of a cyclone’s life:

• Early detection: Wind patterns can reveal a closed circulation before a storm becomes obvious in cloud images.
• Position fixing: The wind field helps pinpoint the storm centre, which improves track forecasts.
• Intensity and size: The extent of strong winds at the surface helps define the storm’s radius of gale-force winds.
• Landfall planning: Better knowledge of the wind field helps authorities prepare coastal warnings and evacuations.

The India Meteorological Department (IMD) uses scatterometer data along with satellite imagery, buoys, radar and numerical models to issue cyclone advisories. Because Ku-band signals can be affected by heavy rain, forecasters interpret the winds in the strongest part of a storm with caution and combine them with other observations.

What Other Uses Does SCATSAT-1 Data Have?

SCATSAT-1 data has many uses beyond cyclone tracking, including numerical weather prediction, ocean-state forecasting, monsoon research and polar ice monitoring. The ocean-wind field is a key input to models that predict both the atmosphere and the sea.

• Weather forecasting: Wind vectors are assimilated into global and regional weather models to improve the starting conditions of forecasts.
• Ocean forecasting: Winds drive waves, currents and storm surge, so they are used in ocean-state and wave forecasts for shipping, fishing and offshore operations.
• Monsoon research: Changes in low-level winds over the Arabian Sea and Bay of Bengal are tied to monsoon onset and strength.
• Sea-ice and polar studies: Radar backscatter differs between open water, sea ice and ice sheets, so scatterometers can help map ice extent.
• Land studies: Backscatter over land can be related to vegetation and soil moisture, and has been explored for such research.

Fisheries advisories, port operations and marine safety services also benefit when reliable wind and wave forecasts are available.

Who Uses SCATSAT-1 Data?

SCATSAT-1 data is used by Indian agencies such as the India Meteorological Department, the Indian National Centre for Ocean Information Services (INCOIS), and academic and research institutions, as well as by weather agencies in other countries. The data is processed and distributed by ISRO through the Meteorological and Oceanographic Satellite Data Archival Centre (MOSDAC) at the Space Applications Centre.

Several international forecasting centres have assimilated Indian scatterometer data into their models. This reflects the global nature of weather prediction, where observations from any country’s satellite can improve forecasts everywhere. Researchers also use the archive to build long records of ocean winds for climate studies.

How Does SCATSAT-1 Compare with Other Wind-Measuring Satellites?

SCATSAT-1 is comparable in purpose to other scatterometer missions flown by space agencies around the world, such as the European ASCAT instruments on the MetOp satellites and the instruments on Chinese ocean satellites. All of them measure ocean-surface wind vectors, but they use different radar bands, swaths and designs.

Scatterometers fall into two main design types. Fan-beam instruments use several fixed antennas looking at the surface from different angles and usually operate in C-band. Pencil-beam instruments, like the one on SCATSAT-1, use a spinning dish and operate in Ku-band. Ku-band instruments are more sensitive to light winds and give a wide swath, though they are more affected by rain than C-band.

Combining several scatterometers improves coverage, because each satellite sees a given place only at certain times. That is why data from the Indian mission was useful to the global community as one piece of a larger observing network.

How Are SCATSAT-1 Wind Measurements Processed and Checked?

SCATSAT-1 measurements are processed on the ground into wind vector products by converting the raw radar backscatter into estimates of wind speed and direction, and the results are then checked against independent observations. Processing is carried out by ISRO’s data centres, and the finished products are released in standard formats that forecasting centres can read.

The processing chain has several stages. First, the received radar echoes are calibrated so that the measured backscatter is comparable from one orbit to the next. Second, the backscatter values from the different beams and look directions are combined for each patch of ocean. Third, the geophysical model function is applied to compute the most likely wind vectors. Because the same set of measurements can fit more than one wind direction, a step called ambiguity removal selects the most consistent solution, often with the help of a reference wind field from a weather model.

To keep the data trustworthy, ISRO scientists compare the satellite winds with measurements from moored ocean buoys, ships and other scatterometers. Where differences appear, the retrieval is adjusted. Quality flags are attached to each product to mark cells that may be affected by rain, land nearby or sea ice, so users can decide how much weight to give them.

What Is the Legacy of SCATSAT-1?

The legacy of SCATSAT-1 is that it kept India’s capability to measure ocean winds alive after the loss of OSCAT and demonstrated how quickly ISRO can replace a failed observing system. It also strengthened the Indian ocean and weather services with dependable satellite wind data.

ISRO has continued to develop its ocean-observation programme through later Oceansat missions, and further scatterometer missions have been discussed as part of the plan for continuity. For students of space and meteorology, SCATSAT-1 is a good example of how a modest, quickly built satellite can serve a large public purpose such as cyclone preparedness.

Bottom Line

SCATSAT-1 is ISRO’s dedicated scatterometer satellite, launched on PSLV-C35 in 2016 to measure ocean-surface winds with a Ku-band radar. Built rapidly after the loss of Oceansat-2’s scatterometer, it supports cyclone tracking, weather and ocean forecasting and climate research for India and many partners worldwide.

Frequently Asked Questions

What is SCATSAT-1?

SCATSAT-1 is an Indian satellite launched by ISRO in September 2016 to measure wind speed and direction over the ocean surface using a Ku-band scatterometer. It supports cyclone tracking, weather forecasting and ocean-state prediction.

When and how was SCATSAT-1 launched?

SCATSAT-1 was launched in September 2016 by ISRO’s Polar Satellite Launch Vehicle on the PSLV-C35 mission from Sriharikota. It was placed in a polar sun-synchronous orbit at roughly 720 km altitude, and the flight was the first PSLV mission to deliver satellites to two different orbits.

What is a scatterometer?

A scatterometer is a radar instrument that sends microwave pulses to the Earth’s surface and measures the echo scattered back. Because stronger winds create rougher sea surfaces that scatter more energy, the instrument can calculate ocean wind speed and direction from the strength and angle of the echo.

Why was SCATSAT-1 built?

SCATSAT-1 was built to replace the ocean-wind observing capability lost when the OSCAT scatterometer on Oceansat-2 stopped working. ISRO reused the OSCAT design and spare hardware so that the new satellite could be developed and launched in a short time.

How does SCATSAT-1 help in cyclone forecasting?

SCATSAT-1 helps in cyclone forecasting by showing the surface wind pattern around a storm. Forecasters use it to locate the storm centre, judge its size and strength, and improve track forecasts, and the India Meteorological Department uses such data alongside other observations.

Who uses SCATSAT-1 data?

SCATSAT-1 data is used by the India Meteorological Department, ocean information services, research institutions and international weather agencies. ISRO distributes the data through its MOSDAC portal at the Space Applications Centre in Ahmedabad.

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