What makes NISAR unique among the earth observation satellites

The highly anticipated NASA-ISRO Synthetic Aperture Radar (NISAR) was successfully launched recently from Sriharikota aboard GSLV F16, fondly known as ISRO’s ‘Bahubali’. In the coming days, the spacecraft will stabilise in a circular orbit approximately 747 km above earth. Operating like a watchful sentinel in space, NISAR will detect minute tectonic shifts and forest encroachment among other critical earth surface changes, with its advanced radar technology capable of measuring movements as small as a few centimetres. This enables planners to anticipate natural disasters and prevent environmental damage through timely action.

Sun-synchronous orbit

NISAR will be placed in a sun-synchronous orbit (SSO) with an orbital tilt of 98.4 degrees making it orbit in a near-polar path, covering the entire globe as the earth turns beneath it. The satellite will make about 14.7 revolutions every day, completing one orbit every 98 minutes.

NISAR will visit each place again every 12 days, as the earth rotates on its axis. With repeated images, we can track subtle changes in the earth’s surface, including urban development and changes in land use. This orbital arrangement ensures that the satellite crosses the same point at the same local solar time, so the lighting remains constant with the sun behind the spacecraft. This configuration minimises shadows, which makes SSO great for observing the earth, tracking the weather, and taking clear time-series pictures for environmental monitoring.

Orbit significance

Although NISAR does not carry an optical imaging system, making sunlight conditions irrelevant, its sun-synchronous orbit remains essential. The satellite relies on dual-frequency microwaves (L-band, 24 cm wavelength and S-band, 10 cm wavelength) emitted toward earth, with reflected signals captured by its antenna to analyse ground conditions. This capability allows the detection of land-use changes, crop health, water stress, and other parameters. Since microwave imaging works independently of visible light, NISAR can capture data, day and/or night. Additionally, as microwaves penetrate cloud and dust cover, NISAR can provide all-weather monitoring.

Nonetheless, SSO is vital. NISAR’s data will often be combined with optical imagery from satellites like ISRO’s Resourcesat or NASA’s Landsat, many of which share similar SSOs and equatorial crossing times.

Furthermore, while synthetic aperture radar (SAR) imaging uses microwaves, NISAR itself depends on solar power. The SSO ensures that the solar panels receive the optimal amount of sunlight by maintaining a constant angle relative to the sun. The orbit also maintains a stable temperature of the craft by limiting significant changes in light, which is crucial for the performance of its 12-metre antenna and sensitive SAR electronics.

Large antenna advantage

NISAR features a 12-metre-wide, deployable mesh reflector antenna, one of the largest radar antennas deployed for earth observation. For comparison, NovaSAR-1 has a 3m × 1m microstrip patch array antenna, while ICEYE’s antennas measure 3 x 0.4 metres. Commercial satellites, such as TerraSAR-X and PAZ, have smaller antennas, typically measuring 4.8 metres × 0.7 metres.

The big antenna picks up more radar echoes from the earth’s surface, just like a big ear picks up faint sounds. This enhanced signal reception improves data quality and the signal-to-noise ratio, enabling even small ground movements resulting from tectonic shifts or volcanic activity to be detected.

SAR technology

Imagine trying to read small print on a distant wall using a torch. A small torch spreads light weakly, making the text appear blurry and hard to read. However, a larger torch produces a thick beam, illuminating the letters more clearly. Similarly, NISAR uses its massive 12-metre antenna like a powerful torch, concentrating microwave beams precisely on earth’s surface.

What makes NISAR even more remarkable is its synthetic aperture radar (SAR) technology. As the satellite orbits earth at an altitude of 747 km, it continuously emits thousands of microwave pulses per second toward the ground. The 12-metre antenna collects the returning echoes, much like how a photographer takes multiple shots of a moving car from different angles to create a crisp image. By combining echoes from various positions along its path, NISAR effectively mimics an antenna hundreds of times larger than its actual size, which is the ‘synthetic aperture’ advantage. Advanced signal processing then turns these combined echoes into clear images with resolutions of 3 to 10 metres. These images can pick up on-ground movements and changes that typical radar systems cannot see.

SweepSAR Technique

NISAR’s 12-metre antenna illuminates a limited area per pulse, approximately 14 km for L-band and 6 km for S-band, similar to the spot size of a torchlight’s beam, which depends on its distance and aperture. However, the innovative SweepSAR technology electronically steers the beam to cover a 240-km swath by combining multiple pulses.

This provides NISAR with unprecedented coverage, significantly exceeding that of other SAR satellites, such as ICEYE (approx 50 km) or TerraSAR-X (approx 30-100 km). The 240-km swath enables NISAR to map earth’s entire land and ice surfaces every 12 days, impossible for systems with smaller antennas.

The ISRO-NASA collaboration has been highly successful. NASA’s Jet Propulsion Laboratory designed the antenna and supervised its production by Northrop Grumman’s Astro Aerospace. ISRO contributed the spacecraft bus, S-band SAR electronics, solid-state recorder, and the GSLV launch vehicle — combining the expertise of both in advanced earth observation, environmental monitoring, and disaster management.

— The writer is a visiting professor at IISER, Mohali

Features