Low level MLAT - what's happening here?


While tracking an aircraft in the pattern, I noticed that the aircraft’s position is good most of the time, it shows down to short final on the southerly runway. But, after a touch & go (roller) on the climb out, the first position that is MLAT calculated is either 2 miles south east or 2 miles south west of it’s actual position, as shown below. I know that 3-4 receivers are required for an accurate position, but I do know of 4 stations locally that are in range here. Does anyone know what might be causing this ‘jumping’ and why the MLAT system is plotting the target in the wrong position?


There could be a number of issues.
Are the receiver laid out in a pattern that make it geometrically easy to calculate positions. (Lookup HDOP)
Are some receiver too close or have their gain too high to receive accurate signals?
Are there too many reflected signals confusing MLAT?
Is the dongle timing in the receivers stable?
Are the receivers in the same MLAT group?
Is the MLAT server too busy?
Is the CPU too busy?
Is the USB bus too busy or noisy?

This looks to me like a loss of Signal or loss of enough receivers issue.


The minimum number of sites to do MLAT is 4. The MLAT solver will use more sites and pick the best sites (based on the clock timing) to run through the MLAT solver if they are available.

It looks like this problem is caused by low altitude. The sites able to see the plane was only 4 and the solver ran based on these 4 sites. One of those 4 probably has a misconfigured location or the clock timing is slightly off that caused the huge errors.

Jonhawkes is right. There are a ton of other errors that creep into the MLAT solver that are usually averaged out when more sites are available.


Fo me it does not matter if plane position is inaccurate or jumps. I am happy that I can at least see the otherwise “invisible” planes.