Weather Forecasts

The United Kingdom is moving ever closer to the introduction of its Skynet-6A military communications satellite with ongoing propagation surveys to support the new spacecraft. Several British military communications satellites support the UK’s armed forces and the country’s Ministry of Defence (MOD). These spacecraft include Skynet-4C which carries X-band (7.9-8.4 gigahertz/GHz uplink/7.25-7.75GHz downlink) and Ultra High Frequency (UHF: 305-315 Megahertz/MHz uplink/250-260MHz downlink) traffic. Skynet-4C is supplemented by three additional satellites in the Skynet-4 constellation: Skynet-E/F carry C-band (5.925-6.425GHz uplink/3.7-4.2GHz downlink) traffic in addition to X-band and UHF links. The Skynet-5 constellation, which superseded Skynet-4, comprises four satellites (Skynet-5A/B/C/D) all handling X-band and UHF traffic. The Skynet-5 constellation also carries Ku-band (14GHz uplink/10.9-12.75GHz downlink) traffic.

Skynet-6A

Airbus received a contract from the MOD to provide the next member of the Skynet family, Skynet-6A, in 2020 with the satellite scheduled for launch in 2025. Alongside the new satellite, the contract covers extensive improvements to the ground infrastructure the Skynet constellations depend on. A key element of introducing Skynet-6A into service is ensuring that existing SATCOM ground stations can support the satellite. This work includes ensuring that rain fade disruption to the satellite’s Ka-band (26.5-40GHz uplink/18-20GHz downlink) provision is minimised. Rain fade is a phenomenon that can affect radio signals above frequencies of circa eleven gigahertz. Rain, snow, sleet, ice and other physical contaminants in Earth’s atmosphere can absorb some of the energy of radio transmissions at these frequencies and above. This absorption can cause potential losses in signal power. The phenomena occurs because some precipitation particles are a similar size to these frequencies’ wavelengths. It is imperative that Skynet-6A ground infrastructure is managed in such a way as to reduce rain fade disruption. Anyone familiar with Britain’s weather will know that rain is a regular occurrence.

Rain Fade

In late August, Atheras Analytics won a contract to provide Ka-band propagation analysis to the UK MOD to help the ministry manage the ground stations that will support Skynet-6A. According to a press release the company will use software to analyse existing ground station sites to ascertain the extent rain fade may cause problems there. The outcome of this analysis will let the ministry plan how it can ensure continued links between ground stations and the satellites at times when rain fade may be a significant problem. The press release continued that the company is using its Design Tool which employs a proprietary artificial intelligence-based Outage Prediction Algorithm (OPA). The OPA is applied to historic rainfall measurements at the ground stations’ location to help determine historic link availability at these sites. Through the analysis of the ground stations’ locations, and their susceptibility to rain fade, ground network design can be optimised to maximise network and service availability. John Yates, Atheras Analytics’ managing director, told Armada that “(t)he next-generation Skynet 6 satellites will incorporate a Ka-band capability which has not existed in previous Skynet generations.” He added that this survey work commenced in July and will conclude in October. “The initial work is to assess the suitability of the existing gateway locations for supporting the planned Ka-band services. If there is any question or uncertainty about the suitability of the existing sites, the MOD has an option to extend the work to assess the suitability of other additional sites.” Mr. Yates observed that the SATCOM sector “has entered a phase of major change in the delivery of consumer and enterprise broadband using high/very high throughput satellites … which can deliver data rates of the order of between 500 gigabits-per-second and one terabit-per-second.” One solution to these high data throughput demands is to use Ka-band frequencies despite rain fade susceptibility. Mr. Yates says that ground station architectures can be managed in such a way so that if one station is affected by rain fade, uplink and downlink services are automatically transferred to another ground station unaffected by the weather. In a country with a climate as wet as the UK managing rain fade to ensure smooth SATCOM is paramount. by Dr. Thomas Withington