Conventional space observation usually involved electro optical equipment such as CCD, CMOS sensors, IR and UV sensor. CMOS stands for ‘complementary metal-oxide semiconductor. A CCD sensor is a charged coupled device. like a CMOS sensor, it converts light into electrons. Unlike a CMOS sensor, it is an analog device. Being an analog device, the output image is converted to a digital signal. CCD sensor was the prevalent technology for capturing high-quality, low-noise images. But CCD sensors are expensive to manufacture and hence they often come with a higher price tag. CCD sensors also consume more power than CMOS sensors, sometimes a hundred times more.
CMOS sensor capability is almost similar to the quality and capabilities of CCD technology, and with a significantly lower price tag, smaller size, and power consumption. IR sensors are used for applications such as detecting missile launches and tracking the missile thought its course. IR sensors are also used to detect explosions and troop movement. The main drawback of electro optical sensors is their inability to see through clouds. Cloud cover prevents the continuous observation of the terrain. Electro optical sensors also require a source of light and hence restrict earth observation during the night.
Synthetic aperture radars have been previously used in the Magellan spacecraft to map the surface of Venus. Venus never had clear skies. SAR was able to penetrate the cloud cover and reveal the surface of the planet. SAR uses a small antenna and moving that small antenna over a large distance. SAR flies over the terrain in a satellite and creates detailed images. The satellite carrying SAR is pointed sideways and downwards at the earth’s surface. The radar pulses bounce back from the earth’s surface and is picked up by the antenna. SAR also requires significant processing to achieve quality images. Due to the advancements in computer technology, large quantity of data can be processed quickly and hence has aided in proliferation of SAR systems.
The LEO SAR is considered to be most favorable as it economical to put micro satellites into LEO than other orbits such as MEO and GEO. Numerous private players such as Capella Space, ICEYE, PAZ and TerraSAR-X. The operational capabilities that SAR satellites offer are very critical for reconnaissance specific missions, where real time tracking is vital. In 2018 Capella started off as an internal demonstrator with just two satellites in orbit. Capella achieved full operational capability in 2019 with 6 satellites in orbit and number of revisits to a location of interest was 3-6 hours. As of 2021, Capella had a total of 18 satellites in orbit and maximum revisit time was hourly. Capella satellites operate at an altitude of 500kms and operated at the X-brand frequency. The Argentinian SAOCOM SAR mission operates in the L band frequency and focuses on data to support agriculture, hydrology, soil moisture maps and interferometric capability testing. PredaSAR and Capella SAR have been selected by the National Reconnaissance Office for collection of strategic data. PredaSAR also won a $2 million contract from the US Space Force to demonstrate interoperability between PredaSAR satellites and DARPA’s Blackjack constellation. PredaSAR plans to have 96 satellites in orbit.
MDA, the Canadian technology company is planning its constellation of satellites for commercial earth observations called CHORUS. CHORUS focuses on providing critical time sensitive data and intelligence on maritime and land activities for the purpose of national security, minoring of crops, critical infrastructure and climate change. Information on disasters such as forest fire, hurricanes, volcanoes and floods. MDA has selected ICEYE for providing X-Band satellites for the CHORUS constellation. Spacety SAR constellation provides real time imagery and can operate in C-band and X- band frequency. The constellation plans to have 56 satellites in orbit and has very short revisit time.
With increasing geo political tensions, Gthe need for constant real time surveillance keeps increasing. Constellations of commercially available SAR satellites will continue to provide critical reconnaissance data. The evolution of computer technology will also help in processing of huge amounts of data obtained from SAR satellites. The cost benefit and operational capability of SARs over conventional electro optical earth observation systems will also drive the SAR satellites market in the years to come. Companies such as Capella offer accessibility of data through console and APIs. This ensures ease of use and for rapid imagery exploration and assures mission success with the aid of best in class cloud services. The system assures quality of images and no processing delays.