PA8W Amateur
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| Accuracy of a pseudo doppler Radio Direction Finder | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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The accuracy of a pseudo doppler is generaly stated as 5 degrees for a 4 antenna array and around 2,5 degrees of an 8 antenna array. Of course this depends on a lot of factors: For a stationary RDF: How clean is your site? An elevated array with a free line of sight up to the horizon is a dream that will hardly ever be forfilled. A flat desert or a calm ocean would do the trick. In that case you may achieve up to 2 degrees error for a 4 antenna array and around 1 degree for an 8 antenna version. This is confirmed in real life: NCI-Folkestone uses one of my dopplers on the very best site I know: Looking down on the sea from a 60m high cliff: They achieve around 2 degrees of error with the 4 dipole array. But this is clearly an exception. Even if the antenna is well above surrounding buildings and trees there will still be a lot of scatter from these obstacles. These reflections add up to the direct signal and this will increase the overall error. For an antenna that just peeks over the surrounding roofs you may expect something like the below table, made with my 8-antenna V3 conventional doppler in the 2m band:
For a mobile RDF: First of all the antenna is generally much lower than surrounding buildings and trees. This obviously makes things worse. On the other hand a mobile RDF can move, and if it moves enough in resonable circumstances we can gather enough information to be able to calculate a fair bearing. In the conventional dopplers we achieve a pretty good result using a very high Q of the digital fiilter. This makes the RDF slower but it will generally point more or less into the right direction, because a lot of faulty bearings add up and cancel each other partially. In the microcontroller based RDF's we can check the reliability of incoming bearings and ignore the ones that are obviously corrupted by reflections. This improves the bearing estimate considerably. But there will always be a chance that a reflection from a large building is more powerful than the original signal; in that case every RDF will point to that building... With my RDF40 I drove a route back and forth, tracking a wellknown 424MHz source, in the below picture marked as TARGET. On three straight roads on that route I took 2 good bearings driving in opposite directions. Back at home I used Google Earth to project my bearings on the map, and to see how much they were off. 
As
you can see the overall average error is not bad at all, if you manage
to ignore the total chaos during most of the route.
The average error of these 10 measurements is 2,2 degrees, with a peak error of 4,5 degrees. These figures are pretty close to figures stated by professional manufacturers. Possible causes of severe bearing deviations: In the past years I found that reflections may lead to severe bearing deviations. (multipath signals) For example if the transmitter is behind a big obstruction or below the surrounding landscape. Especially when the transmitter has horizontal polarisation these effects can be severe. In these cases there 's a bad or non-existing direct path between transmitter and RDF. A horizontally polarized signal for example will produce a poor signal on the RDF antenna, however any reflection against a bridge or crane may produce enough vertically polarised signal to be seen as the main signal. Unfortunately, there 's very little we can do to solve this, take a look at the following picture. Just for simplicity, I have drawn an incoming radio "beam" as a fat red line. (In real life, it is a wavefront of course, but this simplification will help to explain.) The signal comes in from the right, where obviously the transmitter is located. For the RDF car, the blue one in the picture, this signal is totally blocked by the buildings on the right. The higher buildings on the opposite side of the road however reflect and scatter the incoming signal, illustrated by the thinner red lines. Repeat this idea a million times and you will have a good idea of the truth; total chaos! For the hunting car, the only signals coming through are false, scattered, multiple reflections. Sometimes even crisp and clear, when the building turns out to be a good reflector, but always coming from a false direction. There's no algorithm that can solve this, only operator skill will help you make the right decisions. A rooky in this situation is tempted to throw the doppler out of the window...  Another phenomenon that often occurs in a city- or industrial environment: Tunneling: A signal that is striking a city with avenues running more or less in the same direction will appear to stick to that avenue. Because that avenue offers a path with reatively low attenuation, signals seem to originate somewhere from the end point of that road. Once the road leads you out
of the city, the true bearing will reveal itself.  Note that this effect will be observed only with long and good averaging settings of your RDF. A fast responding RDF in a city environment will show total chaos. I can't judge you eyeballs, but mine are not at all capable of "eyeball averaging" such a mess. So, as a rooky, pick a nice quiet open road and a loud stable signal to improve your skills, and keep your eyeballs on the road. With the proper settings and a lot of averaging, a quick look at the RDF will tell you all you need to know to take the right turn. 73, Wil. |
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