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Going “tropo” in the tropics . . . AIS in the Pilbara by Stan Swan Regular SILICON CHIP readers will recall Stan Swan’s AIS article in August 2009. It’s not stretching the truth to say it has stirred an enormous amount of interest. But we bemoaned the fact that there appeared to be very little coverage in Australia’s north west. S o it’s been pleasing to note the welcome establishment of fresh 162MHz marine monitoring stations on both the NZ and the vast Australian coastlines. 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. 12  Silicon Chip 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 h d horizon! At the time of writing (early November 2009) around 100 vessels were often detected, with many in shipping lanes stretching clear to In- EARTH’S HORIZON D ONSHORE RECEIVER H Distance to visual horizon: D = 13 H (where H = height in metres, D = distance in km) 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. siliconchip.com.au The Dampier AIS receiving station is hidden amongst the giant communications antenna farm. It was originally set up to house the local amateur radio repeater. The AIS antenna is that tiny 1/4-wave whip (highlighted) on the roof of the building in the foreground. Normally only line-of-sight, under seasonal “tropo ducting” this often receives VHF marine radio signals out to 1000km away! donesia, 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. Going tropo So how then can these numerous Dampier vessels, many “hidden” well below the earth’s curvature, be so readily detected in this region using such simple technology? The answer relates to seasonal tropospheric propagation enhancement, well known to arise about now in that region. The troposphere forms the earth’s Tropospheric propagation allows reception many, many times that suggested by the diagram at left. In fact, signals have travelled 1000km and more, sometimes resulting in confusion with VHF radio networks! siliconchip.com.au January 2010  13 Web-sourced maps showing Hepburn Tropospheric propagation possibilities give invaluable insights into tropo enhancement. It’s indeed fortunate that the Pilbara region experiences both intense seasonal tropo ducting and massive off shore AIS activity, as productive “data mining” insights (technical and commercial) may result. lowest atmospheric layer and under suitable conditions higher frequency electromagnetic waves can become trapped near ground level. Such “tropo” propagation typically occurs during periods of stable, anticyclonic weather and usually relates to a thin layer of warm dry air blanketing cooler surface air. Temperature inversions of this type are often encountered when hot summer air (perhaps from deserts) overlays cooler ocean water – fogs are often a visible sign. Unlike near-space ionospheric reflections (greatly influencing medium and short wave reception), it’s the very-much-higher frequencies (VHF, UHF and microwaves) that are usually more favourably tropo propagated. Diverse tropo enhancement effects are known but wave-guide style refractive ducting is the most pronounced. Ducted signals may travel more than 1000 km, leading to unexpectedly long distance reception (under these conditions Australian eastern state FM radio stations may become audible across the Tasman in NZ. Such ducted propagation is usually blocked by mountains. Ground 14  Silicon Chip level receivers may be below a tropoenhanced layer, yet signals could be readily received when on perhaps a 300m hilltop. Long distance (“DX”) tropo reception is often associated with powerful FM and TV broadcasters. In some parts of the world, notably the Mediterranean Sea, Carribean and Persian Gulf, tropospheric ducting conditions can become so established during summer months that viewers regularly receive good quality VHF/ UHF terrestrial TV signals over great distances. Prior to satellite TV, such “tropo season” reception greatly annoyed totalitarian authorities in countries (such as Cuba) attempting to restrict external information! Radio amateurs have long strived for long-distance (DX) contacts via weak VHF/UHF signals. Thanks to the global tropo forecasts of Canadian (and professional meteorologist) William Hepburn – hosted at www. dxinfocentre.com/tropo–aus.html – anticipating suitable conditions is now much easier. Hepburn’s maps graphically indi- If you thought the AIS plot on page 14 was impressive, what about this one! Over 200 vessels have been identified stretching all the way up to and through the Indonesian Archipelago. Comparison of the Hepburn map with the Pilbara’s AIS reception coverage shows HTI propagation predictions well confirmed. As vessel movements naturally tend to follow defined routes, the region’s shipping lanes and movement intensity can be clearly established. siliconchip.com.au As we explained in the article in our August issue, you don’t need a lot of equipment to receive AIS signals. FM RADIO & NOW DIGITAL TV TRANSMITTERS from cate the likely strength of a region’s tropospheric ducting on a ten-point HTI (Hepburn Tropo Index) scale – zero being negligible while ten becomes near “armchair quality”. Dampier’s intense late Octoberearly November 10+ HTI predictions indeed correlated closely with the region’s noted AIS enhancement. Given the well-defined tropo AIS coverage, considerable scope for marine-sourced investigations (both commercial and technical) may now arise. As tropo propagation of higher frequency microwaves can occur, it’s even tempting to consider a crack at the 2.4GHz WiFi record – presently a “mere” 382km! Although FSPL (Free Space Path Losses) increase significantly at such microwave frequencies, it’s feasible tropo-enhanced WiFi links could be made using high gain 2.4GHz dishes to give decent EIRP (Effective Isotropic Radiated Power). After all, AIS signals (originating from ~12W shipboard VHF transmitters) were received over 1000 km away in Dampier with just a simple ¼-wave whip! POWER References: Hosted at: www.manuka.orcon.net. SC nz/ais.htm Looking out over the port of Dampier belies the enormous amount of shipping just over the horizon. It also says nothing about the huge economic benefit to the region, the state and to Australia. Here’s a new record just set (30 November) by the Dampier AIS station: the “Limousin Express” was detected near Christmas Island, nearly 1500km out in the Indian Ocean. It’s just another example of AIS tropo reception. siliconchip.com.au January 2010  15