Coastal deadspots

Caldbeck VHF coverage

The Solway coast in Scotland is a beautiful stretch of coastline, popular for its estuary wildlife and sailing. Despite seeming sheltered and close to civilisation, the tidal estuary sees more than its share of drama which is why it is served by two small lifeboats at Kippford and the Nith inshore rescue at Glencapel who communicate with Marine VHF and GSM.

VHF

VHF coverage for the area is provided by a Maritime and Coastguard agency repeater located on Caldbeck tower in England which as the third highest tower in England offers substantial coverage for the region. To simulate the uplink from boats to the tower, a maximum path loss value of 125dB was calculated based upon a receiver sensitivity of -100dBm and an EIRP of 30dBm (1 Watt) with 5dB fade. The repeater's power output is unknown and it's range is likely much further than shown but for the focus here is the boat's ability to talk in to the repeater. Marine radios are generally 5W or 25W. After an enthusiastic sailor has daisy chained several tatty co-axial cables together to get the best height on their mast and sea water has corroded the antenna system, the transmission loss of many boat's systems is substantial which is why only 1W was used to simulate a worst case scenario.

The VHF coverage map revealed three areas of concern where a vessel close to land may be out of VHF contact due to terrain shielding. To compound issues, an enormous wind-farm has been built in the middle of the estuary but due to the generous spacing of the turbines this has not been modelled. Ironically the best place to get a signal is in the middle of the estuary away from the land.

GSM

Using OFCOM sitefinder data, 600 towers for EE, Vodafone and O2 were mapped via the CloudRF API to show the uplink path loss. Like the VHF model, the focus was on the weaker uplink which usually gets marginalised when a network provider rolls out a coverage map. focusing on the more generous downlink. A path loss of 125dB was selected based upon EIRP of 23dBm (0.2W), a receiver sensitivity of -110dBm and a fade of 8dB. Like the VHF map, this showed signal deadspots close to the coast, especially where there were cliffs.

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