The SARs equipping the Sentinel-1A/B satellites transmit on a C-band radio frequency of 5.405 gigahertz. In 2018, geographic information system expert Harel Dan made a startling discovery. He noticed that the Sentinel satellites’ radar suffered interference from radars on the ground transmitting in C-band. As the Sentinel satellites orbit overhead, they build up pictures of the Earth below. Dan realised that C-band radars on the ground caused interference, which would show up on the Sentinel radar pictures. When the radar passes over a stretch of ground where a C-band radar is transmitting it not only receives echoes from its own transmissions, but also powerful signals from the radar below. Each swathe of territorycovered by the radar is 5x250 kilometres – a total of 1,250 square kilometres. The interference from the C-band radar is so powerful that it inundates the radar picture of its surrounding area with interference, which shows up as a bright stripe, almost like someone had used a highlighter pen on the image. As the constellation includes two satellites, each looking at an angle towards the ground, each satellites’ radar depicts the interference slightly differently. By blending the pictures of the two satellites together, one arrives at a distinct X shape, betraying the presence of the radar. Dan says that this discolouring is caused by the higher signal strength of the C-band radar, compared to the satellites’ SAR. Along with echoes from their own radar transmissions, the satellites receive transmissions from the C-band radars. C-band interference gathered by the satellite when in an ascending orbit (south to north) is depicted in red. When the orbit is descending (north to south) the interference is shown in green. Dan’s revelations are significant. Armed forces around the world are enthusiastic C-band radar users. This is a favoured frequency for groundbased air surveillance radars. These radars are used by armies and air forces to detect aircraft, air-to-surface and surface-to-surface weapons. They will provide air defence coverage over a theatre of operations, a deployment of forces or help protect a nation’s airspace. The choice of frequency for any radar is the result of a compromise based on the radar’s task, desired performance and the radar’s physical size. C-band radars detect targets at relatively long ranges, depicting them in some detail. This helps accurately determine the type of target, along with giving a relatively precise indication of the target’s position. Such information is integral to surface-to-air weaponry fire control, or for directing a fighter towards a target. C-band radars have antennas which are of a size practical for providing this performance, while also being deployable. This explains The Radar Interference Tracker is a new open source intelligence tool, providing useful electronic intelligence. On 3 April 2014, the European Space Agency’s (ESA) Sentinel-1A earth observation satellite reached the cosmos. She was joined two years later by Sentinel-1B on 25 April 2016. Both satellites were built by ThalesAleniaSpace and Airbus’ Defence and Space subsidiary. The two spacecraft orbit at 374 nautical miles above Earth and are equipped with Synthetic Aperture Radars (SAR). These devices gather detailed radar pictures of the ground below. ESA makes the satellites’ radar pictures freely available. One benefit of these satellites’ SAR radar is that, unlike spacecraft equipped with cameras, they can gather pictures even if the Earth below is covered with cloud or smoke. They also gather detailed imagery regardless of whether it is night or day. SAR Technology SAR was invented by engineer and mathematician Carl Wiley in June 1952. The first space-based SAR equipped the US National Reconnaissance Office’ (NRO) OPS-3762 satellite, which was launched in 1964 and became the lynchpin of the NRO’s Quill programme. Quill was declassified in 2012 and revealed as an NRO programme to evaluate space-based radar imaging. SAR radars use the movement of the aircraft or satellite carrying the radar to build up a picture. The radar is switched on while the vehicle moves. This motion helps to build a picture of a swathe of territory either underneath or at a slant aspect/range from the aircraft or spacecraft. The radar transmits pulses of RF energy toward the ground below. These hit the ground and objects on the ground and are reflected to the radar. The radar records the relative differences in the time it takes each pulse to be transmitted and reflected. All the pulses travel at the speed of light, but even so very small differences in the time it takes for a pulse to be returned – from the top of a multi-story building or its front door, for example – can be accurately determined. By measuring these differences in reflection times, the radar determines the height of objects below relative to its own position. The radar continuously scans the ground below as the vehicle carrying it moves across the terrain. The radar’s processor will take all these differing ranges of objects and assemble them into an image of the ground below – images that can be stunning in the level of detail they provide. Thomas Withington Open Skies Sentinel-1 is one of the European Space Agency’s two Sentinel earth observation satellites. Both are equipped with a synthetic aperture radar. (Photo: ESA) 68 · MT 3/2022 C4ISR Forum
RkJQdWJsaXNoZXIy MTM5Mjg=