This is only a preview of the January 2010 issue of Silicon Chip. You can view 18 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "A Multi-Function GPS Car Computer, Pt.1":
Items relevant to "A Balanced Output Board for the Stereo DAC":
Items relevant to "Precision Temperature Logger & Controller, Pt.1":
Articles in this series:
Items relevant to "Voltage Interceptor For Cars With ECUs, Pt.2":
Items relevant to "Web Server In a Box, Pt.3":
Purchase a printed copy of this issue for $10.00. |
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
|