As more and more satellite constellations are being launched to provide data services, the more complex the decision-making process in selecting an aircraft satellite communication system. Regulatory requirements, aircraft type, area of operation, data usage needs, band of operation, hardware availability and budget are key considerations.

There are many satellite systems, service providers, and avionics manufacturers indicating that they are offering, or planning to offer, an airborne satellite service. These satellite systems cover L-band, Ku-band and Ka-band frequencies, which are the dominant commercial satellite bands. Almost without exception, these satellite services are for fixed (stationary) terminals. The exception is Iridium and Inmarsat which from the outset designed their systems for mobile users in mind. The other systems have adapted their network and terminals to support maritime, portable land mobile, and aeronautical mobile terminals.

The following discussion considers only those satellite systems that provide coverage over Australia and capable of 24/7/365 services. There are numerous microsatellite operators (satellites that are about the size of a shoebox) that provide a communications service from a low earth orbit (LEO), but not continuously. Consequently, they are not considered in this review.

Frequency band of operation

While there are a number of commercial satellite systems available for aeronautical applications, each needs to be assessed in terms of quality of service (ability to support a communications link) in all operational scenarios which the end user may encounter. One fundamental parameter to consider is the frequency band of operation. The constellations being considered operate in one or more of the following three frequency bands.

Where satellite data is required for safety of flight datalink applications, L-band is the only option approved for Air Traffic Management and Air Traffic Services due to its high quality of service. However, when datalink applications are not an operational consideration the selection process becomes more involved.

High data usage demanded by cabin entertainment or mission systems would suggest use of a Ka/Ku-band system with higher data throughput capability. However, this option is not available for all aircraft types, is more susceptible to reduced quality of service and subject to higher data costs. Depending on data requirements, these factors may determine an L-band system a more suitable solution.

Rain Fade

The most significant degradation in quality of a satellite transmission is caused by precipitation. This is known as rain fade which impacts the Ka/Ku-band significantly when compared to the lower frequency L-band. Whilst rain fade margins are built into most networks, design trade-offs are made between the probability of a fade event, duration of a signal loss and decrease in data rate. The higher the elevation to a satellite, the lower the likelihood the signal will pass through adverse rain and cloud. Conversely, low elevation look angles will mean there is a greater probability of the signal passing through rain and cloud, as it would when communicating with a Low Earth Orbit (LEO) satellite.

It is also the case that the heavier the rain event, the greater the signal attenuation, so a deeper signal fade is more likely in a tropical region when compared to a more temperate zone.

Signal latency

Signal latency is another consideration depending on end-user application. The geostationary satellite (GEO) orbit is approximately 36,000 km from the Earth’s surface. In comparison, the LEO and medium earth orbits (MEO) are 200 to 2000 kms and 8000kms retrospectively. The geostationary orbit latency (return signal delay is approximately 250 msec) whereas the LEO and MEO orbit satellite systems signal latency is approximately 10 – 150 msec. For some end user applications where there is a high degree of interactivity between the user on the aircraft and the server on the ground (for example gaming and some interactive cloud-based services), latency needs to be considered. Applications that are not interactive, like file and image transfer and/or status information transfer, higher latency of a GEO service can be used effectively.

If previous considerations determined Ka/Ku band most suitable, availability of aircraft hardware must be assessed. Many Ka/Ku band solutions are not offered to aircraft with a smaller diameter fuselage. Table 1 indicates what solutions are available now and into the foreseeable future. If satellite system hardware is available for an aircraft type, the final consideration must assess cost of equipment and ongoing data costs.

Table 1: Summary of the key aspects of aeronautical satellite terminals available on the market now and into the future.

Summary

In summary, Ka/Ku-band services offer a higher throughput of data but quality of service is compromised when operating in precipitation. Consequently, the Ka/Ku-band services are best suited to jet aircraft operating at higher flight levels. This prioritises Ka/Ku-band antenna solutions for wide-bodied aircraft with limited consideration of aircraft with a smaller diameter fuselage.

Inversely, L-band offers a better quality of service for aircraft operating at flight levels more susceptible to precipitation; typically, small jet and turbo-prop aircraft. A greater number of L-band antenna solutions are available for aircraft such as the PC12, PC24 and KingAir B200/350/360.

For assistance navigating the satellite system selection process, contact us.