What are LEO satellites, what can they be used for, and how can they be tracked using RF? In this blog, we will find out.

Low Earth Orbit (LEO) satellites are far from a new phenomenon (indeed, the very first satellite, Sputnik, was an example back in 1957), but they are attracting increasing interest for their current and potential future uses in financial services, communication provision to remote areas, and intelligence gathering. In the context of satellites, “low” means less than 2000km above the surface of the Earth, and may be as low as 160 km (1), (2). This is in contrast to satellites in geostationary (GEO) orbit, which move in sync with the rotation of the Earth, and need to be just under 36,000 km above the surface.

LEO Satellite Uses

LEO satellites are attractive to financial services firms as they provide the potential for faster transactions than are possible via even the best fiber communications networks. As signals effectively travel at the speed of light, they can be sent to a satellite and retransmitted back to Earth with delays measured in milliseconds (3). For these firms, the tiniest fractional improvement in the time it takes to connect to trading exchanges can be worth vast sums of money.

The use of LEO satellites for intelligence gathering is well established, with high resolution photo and video capabilities. The resolution of the images that can be obtained by satellites is directly related to their distance from the target, so the lower their orbit, the better.

LEO satellites also offer the enticing possibility of providing broadband connectivity to inaccessible areas where laying fiber networks is unfeasible. Because of the curvature of the Earth, satellites in geostationary orbits are unable to provide a signal to locations in high latitudes, so a network capable of supplying connectivity over the whole of the Earth must be based on LEO. Each satellite can only provide a connection when there is a line of sight to the receiver on the ground. As one satellite passes over the horizon on its orbital path, another satellite is required to take over. The number of satellites that would be needed to provide uninterrupted internet access (which will certainly be in the 100s, and might well be much higher) is one of the main reasons to believe that low Earth orbit is about to get significantly more congested. It is even possible that satellite broadband might overtake fiber broadband across the whole of the globe, as it becomes progressively less expensive to provide (4).

Tracking LEO Satellites

Given their uses, particularly for imaging purposes, it is desirable to be able to track the movements of LEO satellites. For example, this allows military forces to restrict any sensitive operations while opposing forces’ spy satellites pass overhead. So how can this be done? The vast majority of current systems in operation use radar, with cutting-edge systems such as the USAF Space Fence capable of monitoring almost 100,000 objects in orbit around the Earth (5). However, as satellites become increasingly small, it will be more difficult to differentiate them from the general debris of space junk – an estimated one million pieces (larger than 2mm) of which are thought to be in orbit around the Earth (6)

A potential way to supplement radar provision is the use of RF geolocation (although the geo- prefix is slightly misleading in this context), where satellites are located by tracing the origin of RF signals that they send out. Using a technique known as 3D TDOA, their locations can be pinpointed in three dimensions. The significant advantage of using RF geolocation is that it only detects objects that are emitting RF radiation, allowing satellites to be picked out from the non-RF-emitting majority of space junk. The radio receivers necessary to perform this technique can be deployed at ground level, and will be able to locate satellites whose transmissions reach the ground. However, spy satellites can potentially avoid detection in this way by communicating only with other satellites, using an RF frequency that is absorbed by the atmosphere before reaching the ground (7). To detect this activity, it would be necessary to deploy receivers on satellites, where they would be able to intercept the transmission before it is absorbed. And if this is to be feasible, then the size, weight and power of the receivers needs to be as low as possible. CRFS’ RFeye Nodes provide high performance in a low SWaP product. With the number of satellites in low Earth orbit poised to increase exponentially, anyone needing to track LEO satellites should consider RF geolocation a key part of their detection platform.

Further reading

3D TDOA Whitepaper

Operating principles of 3D TDOA

RFeye AirDefense

3D geolocation and tracking of aircraft & UAS