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SQWK Mlat - How To Guides

Page history last edited by Danny Gooch 3 years, 3 months ago



Mlatting Mode-A XXnnnn type ICAO codes


The feature that allows Mlats on Mode-A-only targets (XXnnnn) has a few subtle differences from normal Mlats.


Mlat technique


If you start an Mlat on an XXnnnn target, the request will go to the best 20 or so Mode-A-capable Ground Station in the area surrounding the center of your current map or outline.


Unless someone else has already requested that same XXnnnn code, initially there will be no data in the various GS's raw data buffers.  However, once such a request is received, the GSs will start to record raw data for that XXnnnn code.


On the next attempt, provided it is not too soon, there will be data accumulated in the buffer and the request will then yield some raw data.


If you start a manual Mlat with Ctrl-Shift-Left click on an aircraft in the View..Aircraft iist, when the first attempt fails, a second attempt will follow almost immediately.  Since the request asks for data in the last 20 seconds, there will barely have been enough time for any worthwhile data to have been accumulated.  Accordingly, you should expect that the first double try of a manual Mlat on an XXnnnn target will fail and you will need to try again after 20 seconds or so.


Ctrl-f XXnnnn


If you want to try to Mlat an aircraft that you are not receiving locally, you can insert a dummy XXnnnn code using Ctrl-f.


The time tag on that dummy code will be the current time when you created it.  As explained above, if you then do a manual Mlat on that code, it will probably fail the first pair of times.  There is a bit of a race on because the time tag is not updating and it takes a while to do two unsuccessful Mlats before you can do the one that is more likely to succeed. If you are slow in doing that, PP will decide that the 'report' from your XXnnnn was too long ago and it is not worth trying.  If a message to that effect appears, the quickest workaround is to do an Options..Purge..Flights and recreate the wanted XXnnnn and immediately do another Mlat.  The slower solution is to wait for it to time out and then recreate it and immediately initiate the Mlat.


Slight buglet


In the current public release, the dummy code XXnnnn is created by Ctrl-f but the squawk value set for the dummy entry is 0000.  This is fixed in the next release but if you are filtering Mode-A codes in View..Aircraft list display options..Mode-A/C to exclude the squawk 0000, you will be perplexed that you are apparently unable to create any dummy squawks at all; they are all being filtered out by your 0000 rule.  The temporary solution is to enable 0000 in that filter.  After the next release that will not be necessary as the correct squawk code is added to the dummy creation.


GS selection


If, as will usually be the case, there is no known position for the XXnnnn target of interest, the choice of GSs to be asked for data, will be based on the centre of the current (or most recent) chart or outline.  The process takes the best 20 Mode-A-capable GSs within 100 nm of the centre of the chart.  If you have opened a chart of the whole of the country, there is a very good chance that the center of it might be more than 100nm from the target of interest, which will more or less guarantee failure.

Once you get a fix on the target, the GS are then chosen to be within 100nm of the last fix, which is fine, but an initial ab-initio fix will be much more likely to succeed if you have centred your chart or outline on the appoximate area of the target of interest.  This is particularly relevant if you are attempting an out-of-area Ctrl-f Mlat where the target of interest may be a long way from where your current chart or outline is looking.


Signal to noise ratio


Please bear in mind that the Mode-A squawks (and the other weird pings you have discovered) have no checksums so noise and fading are an inevitable source of garbage.  If you have been succeeding with Mode-A squawk Mlats, you will certainly have seen that in addition to the hyperbolae that converge on the fix, there are often a number of delinquent hyperbolae that pass nowhere near the fix.  This is significantly different from the situation with normal Mode-S Mlats and simply reflects the fact that there is a lot of noise (in the information sense) in amongst the signal.  Remember too that the Mode-A capable Ground Stations are only a small subset of the total number of validated Ground Stations for Mode-S Mlats; having fewer contributing GS also degrades the performance of the method.


I hope these explanations and suggestions will help to increase your understanding and your success rate with Mode-A Mlats.





17/7/2020 16:03



An easy guide to Sqwk Mlat across the UK


Notes from John in the main PP forum. Message id 116116 February 2021


I make no apologies for directing this to UK PlanePlotter users.
The simple fact is that the UK is one of the few nations to use a structured squawk allocation system , which from our point of view , makes Sqwk Mlat [c] so much easier to use.

The purpose of this email is to explain a simple process for users who are still not too sure how important this unique facility is.

Key to the process is this document


It lists all the codes in use across UK airspace.

This second document describes the sectors controlled by ATC and the inital contact sqwks in use...


Let's simplify things a bit more purely for the purpose of this exercise....

There are three main categories of flight which use Mode A/C only

A) Light aircraft : GA : Microlights which do not have Mode-S out equipment.

B) Military flights not using Mode S/ADS-B out

C) Flights which wish to remain covert

You will not see any of these flights tracking on conventional flight trackers , with the exeption of those using FLARM.

Flights in category A will usually but not always use locally assigned sqwks . For example here in the north west they might use 5050 which is the Liverpool zone conspicuity code.
At the present time local/pleasure flying is not allowed due to lockdown , consequently there will be very little activity in ths category.

Flights in category C will use codes from a specially designated section and will , in most cases , retain that code all the way through their sortie

Category B therefore is our main source of activity , military flights.
We can subdivide this category further.
Aircraft which rarely use Mode-s/ADS-B for example F15s and aircraft which do have Mode-S /ADS-B and sometimes use it , sometimes not , for example RAF fast jets where the lead a/c will sqwk as requested by ATC the remainder of the flight will work Mode A/C only.

Now let's try and example....
An imaginary flight from an active airfield in East Anglia.
A pre flight briefing will no doubt include the local sqwk in use for the initial climb out. The sortie will be to the North Wales Military Training Area [NWMTA]
As the flight climbs and heads west they will soon encounter the edge of Swanwick Central [ see https://www.aidu.mod.uk/aip/pdf/enr/ENR-6-6.pdf ]
and ATC will advise the lead aircraft to sqwk 6401...or similar depending if 6401 is already in use . The rest of the flight will usually , but not always be assigned consecutive sqwks.

As the flight moves west they may well fly through the Lichfield Radar corridor or simply cross the Midlands towards Wales ....in either case 6401 or nearest will apply.

Nearing the edge of the central sector the lead may well call for descent into the NWMTA at which point ATC will assign a new sqwk 6440 or 644n for the rest of the flight.

This set of sqwks will invariably remain with the flight until they have finsihed in the training area at which point they will head back east and revert to 6401 etc through the central sector.

Its a bit like knowing which number bus you need to catch.
In its basic form ,no need to monitor ATC because most traffic will stick to this sort of pattern.

To find the sqwks in use in your area check you ICAO hex column and look for XXnnnn codes.

Again , for example , if living in the Birmingham area PP starts to show XX6401 , then there is a Mode A/C aircraft in your reception area.

Initiate a manual mlat on the code and within two cycles you should get a fix which will allow you to track the target.

If you don't see the code appear because you are too far away from the Midlands , but suspect traffic is active, for example through the Lichfield corridor , create an entry in your ICAO hex column using the Ctrl f XX6401 /OK procedure , then carry out the manual mlat.

All of this is at present semi automated using a small network of advanced sharers but in the near future will become fully automated for each sector of the UK.

In this guide I have discussed "Mode 3" sqwk tracking techniques available to GS/MU and also Master Users.
Why not give it a try?
I appreciate seasoned mil air enthusiasts will see this as over simplified , but I hope you'll agree , its a good starting point .
If you "are" monitoring Mil Air and do know the identity , type etc of the aircraft in question then by all means complete the missing data in the APR [right click on the fix] , particularly aircraft type and callsign for all to see.



Location by Altitude - Mode-C



Hopefully , by now members are making full use of PlanePlotter's unique ability to locate and track using Mode A [ Squawk ]..... Sqwk Mlat. As explained at the outset of this project Mode A/C , as the name suggests , uses two modes as directed by ATC and only the controller will know what mode he has asked for. Most aircraft however will respond to requests in the format ACACACAC etc or a combination of A and or C

We know that Mode A , sqwk , will in the main be specific to a particular flight at a given time , which is why we can use it to locate and track. Mode Charlie [C] is a different matter as it will represent the altitude of the aircraft and most likely be changing continually as the flight manoeuvres....changes altitude.

If however the aircraft  is in level flight , for example , in the cruise , then Mode C will be constant. Consequently if you know the altitude , then using the attached lookup table it is possible to locate the flight.

For example, 5124 represents 35,000 feet , so by using CTRL f XX5124 , then initiating a manual mlat on that entry in your ICAO hex column , you should find that the fix hits a flight at that level.

That's fine I hear you say , but what if there is more than one flight at that altitude within the area covered by your chart? In this case , each successive fix will probably hit a different aircraft each time. Not very useful, however there are certain applications for this method.

Take for example flights travelling through the Lichfield RVC (Radar Video Corridor) in the UK. They fly though at 14000 feet and consequently , knowing that a flight is entering the corridor , it can be assumed that it will be at 14,000 feet and hence , located using

Those monitoring flights by radio may often miss the sqwk that has been assigned but ,  climb to , or maintain a certain flight level is often more discernable and can be used as an initial  attempt to locate. There are other uses for this method , especially where traffic flies at higher levels and could be more accurately located by virtue of the fact that little else will be flying that high.

Test the theory

  • Go to your aircraft list and find a flight that is obviously in the cruise , note its altitude
  • Go to the attached list and look up the associated Mode C value , for example 41,000 feet....7324
  • Enter that in the format CTRL f XX7324 and initiate a manual mlat
  • The resultant fix will be centred on a flight at that altitude
  • Subsequent fixes will focus on any other a/c at the same level , if there are any

This will only work in areas where there are enough Mode A/C sharing ground stations so I would encourage all those who can , to make sure they are Mode A/C capable.

The method has limited , but very specific aplications. I'll leave the reader to determine what they are.


Mode-C: How it Works and Reference Table Lookup


Use this link to view a mode-C squawk / altitude cross reference table and details on how Mode-A/C works in the air SQWK Mlat - Mode-C Squawk and Altitude Reference Table


If you are interested in an alternate source of information about mode-A/C squawks and how they relate to altitude and ATC assignment you will find a spreadsheet lookup table that allows you to enter specific squawk codes to see what mode-C altitude they refer to and also which mode-A ATC reference the code could releate to (the mode-A reference is biased to the UK region). Use the link above to go to the files section in the Planeplotter Groups.io pages to get this reference doc.

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