Using Beamfinder and Beamfinder Plus

For Beamfinder, you will need Plane Plotter with a suitable receiver.  Currently the SBS1, PGR, AVR/Beast, miniADSB, microADSB, SSRx allow use of the Beamfinder facility.

For Beamfinder Plus, you need a receiver also capable of capturing the mode A/C transponder pings.  Currently, only The ADS-B Beast allows this.

Using Beamfinder

Having run a log of the radar identifier pings, you can then use the Tools, Radar site analysis, Analyse radar site ping log to get an estimate of the location of a radar site.  The analysis does not give you the answer on a plate - but it does provide one source of information about the possible whereabouts of the sites in question.  You need to draw on other information as well.

The analysis result is more likely to be a good approximation to the actual position if your receiver coverage and that of the site in question, overlap to a large extent.  If your coverage only covers a small arc of the radar site's beam, then the uncertainties in the analysis become very large and the results will be less useful.
In any case, depending on the distribution of ADS-B aircraft in that overlap area during your log recording, the results may be more or less representative of the actual position.  You should not be surprised that the results vary from run to run - especially if the calibrating aircraft are relatively few and especially if they are not well distributed over the whole of the mutual coverage area.

Beamfinder FAQ

I get very inconsistent results all the time

It is quite possible that some pulse rates or IIDs cannot be solved at all, because they come from more than one site.  If you get persistently meaningless results, then give up on that pulse rate or IID.  There is no point in battling on if there is no solution.

I've spent a lot of time analysing data, but I get different results each time

It occurs to me that in all the time that you might spend in trying again and again to use the analysis tool to locate the sites, you could have gone through your list of the most active pulse rates or IIDs one at a time, made a radar.txt file with only that one value in it and tried inserting the coordinates of each of the known radar sites within 250 nm of your location, one at a time, until you find the location that makes the beams match the target.  With half a dozen rates or IIDs and half a dozen radar sites, it would only take 36 trials, each one only taking a minute or two, to solve them.

What does the Display Ping Log file show for identifiers?

Use the Display Ping Log file after capturing the identifier pings to see whether a particular II or SI file shows a clear "hole" centred on a radar site.  There's an example here showing the "hole" around Inverness, Scotland radar.

Beamfinder Plus FAQ

I see quite a list of PRFs when the "z" key is pressed, but some are quite similar?

The acceptance of a matching PRF (pulse repetition frequency) has a tolerance of 2 microseconds.  Similar values may be from different radars or they be members of the family of intervals created by the jitter pattern that some radars use to avoid synchronous confusion with other sites.
For example, here in Adelaide, one of the transmitters has a nominal PRF of 3323 us and the other transmitter has a nominal PRF of 3325 µs.  The transmitters are used on alternate days, I think.  Both of them have a 5-fold jitter pattern of +10, +20, -20, -10, 0 µs.  This leads to the following observed pulse intervals for the first transmission:
  6656, 6676, 6646, 6616, 6636 µs
and these for the second transmission:
  6660, 6680, 6650, 6620, 6640 µs

Do radars from the same manufacturer have the same PRF?

The manufacturer does not define the PRF.  The systems analysts who design the overall system will specify PRFs such that synchronous interference between overlapping sites is avoided or controlled.

Why are the entries in the "z" list apparently in a random order?

They are not random, they are sorted in descending order of frequency of occurrence, with the most common interval at the top of the list.  Sorted in this way, analysing them in the order given gives you the best results first and when the analysis starts to fail, you can usually ignore the rest.

Why do I see multiple PRFs from the same site?

Radars can do all sorts of interesting things. They can introduce a systematic jitter in the interval between pulses and they can use various strategies for separately interrogating Mode-A and Mode-C responses from aircraft.

Imagine a radar that is alternately polling Mode-A and Mode-C. Suppose that it also has an interval jitter that contains a number of steps that is not a multiple of two. You can easily imagine that there are several apparent intervals, measured from A-ping to A-ping and from C-ping to C-ping, manifested by the same site.

In the case where the polls go C,C,A,C,C,A then it would not be surprising if the interval between pings from the same site was seen to be both "t" (C-ping to adjacent C-ping) and also "2t" (C-ping to non-adjacent C-ping) and also "3t" (A-ping to A ping).

The only thing that matters is what intervals PP sees coming out of the receiver and whether or not they turn out to be unique to a site and therefore capable of uniquely identifying that site.

(from a message from Bev in this thread on the Plane Plotter Yahoo group)

Should I include entries in Radar.txt for the multiple PRFs I see?

Multiple PRFs are only to be expected.  Given the number of message collisions, it is going to be fairly likely that one ping is not received, either at the aircraft, so the aircraft responds after 2t or at the ground even if the aircraft responds after t.  The observed pulse interval will then be twice the nominal value.  It is worth including these so that a "beam" results even when one ping is lost.

What does the Display Ping Log file show for pulse rates?

Use the Display Ping Log file after capturing the pulse rate pings to see whether there are lines pointing towards a radar site.  There's an example here showing the lines pointing at Dublin.  Not very clear!  Anyone have a better example?

How are the different message types used in the Beamfinder functions?

Please see this page.