Twinkle, Twinkle, Little SAR

Copyright : NASA/JPL Artist's impression of the Seasat Satellite

Copyright : NASA/JPL
Artist’s impression of the Seasat Satellite

Last week ESA released a new synthetic aperture radar (SAR) dataset from NASA’s Seasat mission; nothing unusual in that you might think, except that this data is over 36 years old. As part of its Long Term Data Preservation Programme, ESA has retrieved, consolidated and reprocessed the Seasat data it holds, and made this available to the Earth observation (EO) community.

Seasat was a landmark satellite in EO terms when it was launched on the 27th June 1978. Not only was it the first satellite specifically designed for remote sensing of the oceans, but it was also the first to carry a SAR instrument. Seasat was only in orbit for 106 days as a problem with the electrical system ended the mission just over three months later on 10th October. Although, there is a conspiracy theory that the electrical fault was just a cover story, and the military actually shut down Seasat once they discovered it could detect submerged submarines wakes!

Synthetic aperture radar (SAR) is so called as it uses a small physical antenna to imitate having a large physical antenna; to detect the long wavelengths would require a physical antenna of thousands of metres, while the same result can be achieved with a synthetic antenna of around 10 metres in length. It is an active sensing radar system which works in the microwave part of the electromagnetic spectrum, and uses pulses of radiation to map the surface of the Earth. Pulses are transmitted with wavelengths of between metres and millimetres, some of these pules are absorbed by the surface, whereas others are reflected back and recorded by the SAR. As the satellite moves, the antenna’s position relative to the area that it is mapping changes over time providing multiple observations. This movement crates a large synthetic antenna aperture, because all the recorded reflections of a particular area are processed together as if they were collected by a single large physical antenna, which gives an improved spatial resolution.

SAR is extremely sensitive to small changes in surface roughness, and can provide both day and night imagery as it works independently of visible light, and is generally unaffected by cloud cover. It is used for assessing changes in waves, sea ice features and ocean topography, and recent research is applying it to other fields such as flood mapping. Seasat blazed the trail for SAR instruments, which has since been followed by many other satellites including ESA’s ERS-1 and ERS-2, ENVISAT’s ASAR, RadarSAT, COSMO-SkyMed, TerraSAR-X; and in 2014 both the Japanese ALOS, and ESA’s Sentinel-1, satellites carried SAR instruments.

The potential value residing in Seasat data is demonstrated not only by ESA reprocessing Seasat, but last year NASA also released a reprocessed Seasat dataset. The use of historic data is one of EO most powerful tools, and it is one the remote sensing community needs to exploit more.

SMAP ready to map!

Artist's rendering of the Soil Moisture Active Passive satellite.  Image credit: NASA/JPL-Caltech

Artist’s rendering of the Soil Moisture Active Passive satellite.
Image credit: NASA/JPL-Caltech

On the 31st January NASA launched their Soil Moisture Active Passive satellite, generally known by the more pronounceable acronym SMAP, aboard the Delta 2 rocket. It will go into a near polar sun-synchronous orbit at an altitude of 685km.

The SMAP mission will measure the amount of water in the top five centimetres of soil, and whether the ground is frozen or not. These two measurements will be combined to produce global maps of soil moisture to improve understanding of the water, carbon and energy cycles. This data will support applications ranging from weather forecasting, monitoring droughts, flood prediction and crop productivity, as well as providing valuable information to climate science.

The satellite carries two instruments; a passive L-Band radiometer and an active L-Band synthetic aperture radar (SAR). Once in space the satellite will deploy a spinning 6m gold-coated mesh antenna which will measure the backscatter of radar pulses, and the naturally occurring microwave emissions, from off the Earth’s surface. Rotating 14.6 times every minute, the antenna will provide overlapping loops of 1000km giving a wide measurement swath. This means that whilst the satellite itself only has an eight day repeat cycle, SMAP will take global measurements every two to three days.

Interestingly, although antennas have previously been used in large communication satellites, this will be the first time a deployable antenna, and the first time a spinning application, have been used for scientific measurement.

The radiometer has a high soil moisture measurement accuracy, but has a spatial resolution of only 40km; whereas the SAR instrument has much higher spatial resolution of 10km, but with lower soil moisture measurement sensitivity. Combining the passive and active observations will give measurements of soil moisture at 10km, and freeze/thaw ground state at 3km. Whilst SMAP is focussed on provided on mapping Earth’s non-water surface, it’s also anticipated to provide valuable data on ocean salinity.

SMAP will provide data about soil moisture content across the world, the variability of which is not currently well understood. However, it’s vital to understanding both the water and carbon cycles that impact our weather and climate.