VHF-Band Interference Avoidance for Next-Generation Small Satellites

The past decade has seen a dramatic increase in the number of satellites operating or proposing to operate within the VHF frequency band. Antennas, receivers and transmitters for both the ground and the space segment are readily available and inexpensive, whilst propagation conditions are very favourable. This allows communications to and from the satellite using low-cost omni-directional antenna based ground terminals. It is because of these attractions that many amateur and commercial satellites choose to operate within this band. Unfortunately, the attractions of the VHF band has also lead to one of its greatest disadvantages: overcrowding. Low Earth Orbit satellite systems compete for spectrum, often having to coexist with other users. On any but the clearest channels, interference levels frequently make the link unusable-despite promising link budgets. Next generation small satellites making use of the VHF band must therefore be designed to operate within this congested environment. This paper examines measurements of the long-term global interference environment obtained using the HealthSAT -II microsatellite. Analysis of the results reveals that levels vary with geographical location, frequency and time. An autonomous interference avoidance technique is then examined, which exploits the diverse nature of the communications environment, allowing efficient utilisation of the band, improving channel throughput