So it’s been pleasing to note the welcome establishment of fresh 162MHz marine monitoring stations on both the NZ and the vast Australian coastlines.

How far can you see (or LOS radio waves travel? Simply use this formula - don't forget that both your height and the height of the ship's AIS transmitter make a considerable difference.

Perhaps the most notable recent installation has been that near Dampier on the north west coast of WA, as its associated marine traffic web feed has dramatically served to show the scale and intensity of ore and gas shipments from the Pilbara region.

“Like bees ’round a honeypot” was one comment arising after viewing the associated AIS activity.

Shore-based observers would only see a handful of ships at a time and hence may never be conscious of the virtual armada lurking over the horizon! At the time of writing (early November 2009) around 100 vessels were often detected, with many in shipping lanes stretching clear to Indonesia, almost 1000km away.

Coastal viewers have a visual horizon at distance D (in km), related to the observer’s altitude H (in m), by the formula D = √(13 x H).

Although downward refraction (“bending”) gives modest VHF coverage a little beyond this range, such signals essentially propagate line of sight (LOS), meaning a radio horizon of perhaps just 20km or so for most ship-mounted AIS antennas.