Plane Plotter operation and received message types

Introduction Mlat Operation – any Mode-S message type Beamfinder Operation – DF11 and DF17 message types Aside - why does Beamfinder show a coverage “hole” around the radar head? Beamfinder Plus Operation – Mode A message responses Aside – why not talk about the PRF of the radar? Aside – are there regional differences? Additional Material Footnote

Introduction

Plane Plotter processes all and any Mode-S or ADS-B messages that it receives and updates the internal database accordingly.  This is the information displayed in View..Aircraft, for example.  The only subtlety is that the time tag is updated by any Mode-S or ADS-B message provided that there is no known position.  If a position is known, then the time tag is only updated by an ADS-B position report, an Mlat (multilateration) fix or a BF/BF+ (Beamfinder/Beamfinder Plus) fix.

Mlat Operation – any Mode-S message type

Mlat operation is described in the Plane Plotter Help file, and can use any received Mode-S message.  The time calibrating reference aircraft message must be an ADS-B position-type report.  Note that not all ADS-B reports are positions.  The target aircraft message can be any Mode-S message from the target.  In practice, since a high proportion of Ground Stations are SBS users and therefore not processing most message types, the Mode-S messages that succeed are most likely to be the subset that the SBS hardware and firmware does handle.

Beamfinder Operation – DF11 and DF17 message types

Each Mode-S radar site has an Interrogation Code (IC, or IID), which is in the range II-1 to II-15 (12 civil codes available) or SI-1 to SI-63 (52 civil codes available).  Beamfinder uses the IID, which according to the documentation, can be overlaid on DF11 and DF17 messages sent by the aircraft.   The beam azimuth calibration requires a response that comes from an aircraft with a known (ADS-B) position.  The beams appear on the Plane Plotter display for either DF11 or DF17 messages from the designated target when they exhibit the IID of a known site.  Those sites are listed in the radar.txt file.

Bev notes: notwithstanding what the documents say, I have never seen an IID overlaid on a DF17 ping.   In the areas where I have tested it, only DF11 messages show this information.  It is possible that other areas use DF17 somehow and if so, Plane Plotter would handle that.  But this is in dispute: Dave Reid writes: I can't see any conflict with the Annex 10 specification, which says (for DF17): "The PI field shall be encoded with II equal to 0".  That makes sense when you think about it, as DF17 is a squitter and not a response to a Mode S interrogation.  David T writes: perhaps Bev's note should be dropped, together with references to DF17 message types here?

Steven Sampson writes: A couple of things about DF17. When an aircraft squitters (DF11 and DF17), then they must overlay a Radar ID of 0.  Thus the only time you have an actual Radar ID, is if the DF11 or DF17 is in response to an uplink from an interrogator (UF11 or UF17/No UF17 are defined that I am aware of).

The other thing, is that DF17 does not send the squawk, so squawk values added to a PP track must come from a Mode-S downlink reply to an interrogator (DF05 or DF21).
Thus, to make use of BF+ you need a Mode-S DF05/21 interrogator, a DF17 position squitter, and a Mode-3A ground radar site that doesn't use short P4 pulses.
Note: Short P4 pulses are used after the normal P1, P2, P3 Mode-3A interrogation pulses, and are used to tell Mode-S transponders to NOT reply. Thus, if the ground site is using P4 pulses, then you can't use it for BF+.
The really bad thing, is if the ground Mode-S radars are clustered and share radar codes, then you can't use BF, and if the old SSR radars have been upgraded to use P4 pulses, then you can't use BF+.

Dave Reid further notes: Yes, as I understand it, you will get the identifier code in the DF11 All-Call Reply if the Interrogator Code is an SI, but not if it's an II.

Aside - why does Beamfinder show a coverage “hole” around the radar head?

The IID is only overlaid on a message in response to a Mode-S who-are-you or “all-call” uplink message.  All uplink messages identify the radar site but the aircraft only overlays the radar site identity on downlinks in response to  “all-call” uplinks.  The reply consists of both the IID of the interrogating radar and the aircraft’s own 24-bit address.  Once the radar station identifies the aircraft as being of interest, i.e. within range, it sends a “silence” control message telling the aircraft not to reply to “all-call” uplinks, which the aircraft will do for the next 18 seconds or until the “silence” control message is repeated.  This silence from a known aircraft increases the chance of responses from new aircraft to be heard.  Of course, the aircraft will respond to other Mode-S uplink messages, but not to the “all-call” message, and hence will appear silent to Plane Plotter’s Beamfinder function as the aircraft responses will no longer include the radar IID.
Thus once the aircraft is within the site’s region of interest, this silence can be seen as a “hole” in the permanent tracks – were you to be using the “b” option to show just the aircraft being illuminated from one particular radar site.

Tony Scott Warren writes: There is at least one situation where the Beamfinder "hole" doesn't completely apply, and that is here in the Channel Islands.  The Jersey Mode-S radar is only interested in traffic within the Jersey zone, and so there is no hole due to silence control messages for higher level aircraft - though of course there would be for those below FL200.

Beamfinder Plus Operation – Mode A message responses

Beamfinder Plus identifies the radar site from the pulse interval of the radar.  The beam azimuth calibration aircraft must have a known position and a known squawk.  A Mode-A ping matching the squawk attached to a Mode-S/ADS-B aircraft is deemed to come from that aircraft (assumes Mode-A is locally unique) and calibrates the beam azimuth accordingly.  The target aircraft can only be "beamed" if it has a known squawk (from a DF17 message, for example).  Any Mode-A ping with that squawk will produce a beam (assumes Mode-A is locally unique).  As regards Mode-C pings, if you were lucky and the altitude was locally unique, then PP could do BF+ on the Mode-C pings but I have not implemented that possibility since it is hard to be sure that there is not a non-Mode-S aircraft in range with the same altitude.

Aside – why not talk about the PRF of the radar?

I lean away from PRF, because a lot of radars use pulse staggering to avoid synchronous conflicts and so there is not really any meaningful repetition frequency involved.  That is why many sites have multiple pulse intervals that are characteristic of that site.  Listening to 1030 MHz on AM, the difference is a tuneful ziiip (no stagger) and a rasping ziiip (stagger).
As Mode-A messages are not subject to the same lockout function as described above for Mode-S messages, there will not be a “hole” in the region of interest for a particular radar head.

Aside – are there regional differences?

It depends what you are asking.  There is no way for a traditional Mode-A/C radar site to select who replies.  The interrogation is just three pulses so all aircraft will respond.  A Mode-S radar can add the extra pulse and then Mode-S aircraft will reply using DF11 (or not depending on the lockout state) but traditional transponders will respond to that unconditionally.
One problem is that we do not know what strategy the sites are using.  In Portugal there are (almost) no Mode-S radars and so the system works fine.  In the UK, it seems more complicated.  In Morocco, the only radars with enough range for me to pick up their “customers” are Mode-S and I get almost no useful BF+ data from them.  Here in South Australia, there is one Mode-A/C and one Mode-S site.  The former works well with BF+ and the latter works reasonably with BF but not with BF+.   Part of the problem is that the jitter is so complicated that there are lots of pulse intervals to choose from and currently (to be fixed), PP does not correlate different pulse interval matches from the same site.  This massively dilutes the opportunity to calibrate the rotation and that, together with the low traffic density, makes it perform rather poorly.

Additional Material

You may find this useful reading: http://www.icao.int/WACAF/Documents/Meetings/2011/asi_ws/pp1_ssr_modes_coordination.pdf

Mode-S Uplink formats: http://www.radartutorial.eu/13.ssr/sr21.en.html

Mode-S downlink formats: http://www.radartutorial.eu/13.ssr/sr24.en.html

Footnote

This document was mostly written by Bev in response to some questions by Tom Wylie and me David Taylor.  I confess to not knowing very much about radar and discovering that Beamfinder and Beamfinder Plus require a little more effort than just sharing data and watching the planes on the display.  It makes you realise just how much thought, sweat and tears have gone into Plane Plotter and the diverse functions offered!