Anomaly Report - no MLAT


#1

“Anomaly report for PiAware feeder with a MAC address of b8:27:eb:79:e4:eb:
This feeder is not being used for multilateration because its timing information appears to be unreliable. This can be caused by the site location being incorrect, or because your Pi is running out of free CPU.”

It so happens that shortly before this report was generated and MLAT aircraft disappeared from the view, I noticed the time seemed to be about 1 second away from the time of my other receiver. I have their displays on separate windows. I dismissed the discrepancy as possibly a javascript refresh timing issue, but maybe it actually is a timing issue.

Or are the clocks in the view displayed from my computer’s clock?

Anyway, any suggestions on how to remedy this situation? I’ll double check the siting, but I’m pretty sure I entered it correctly.


#2

Problem solved. Apparently my dyslexia kicked in. I had transposed two digits of the latitude when entering the location. MLAT is now working again.

Interestingly, the error involved was only about 1 1/4 miles - enough for the software to notice it must be wrong. :slight_smile:


#3

So doing a bit of quick math and also a bit of Raspberry Pi research, it looks like the Raspberry Pi’s click resolution is 1 microsecond, and light travels 1.25 miles in just over 6 microseconds. Now I’m curious how the accuracy of the clock in Raspberry Pi running MLAT is maintained.

I don’t think a Raspberry Pi has a GPS chip. Someone, please correct me if I’m wrong about that. That means, the time must be updated over the internet, and for that, latency is a significant factor.

I suppose PiAware must not only calculate aircraft positions from receivers but must also calculate receiver positions from aircraft positions, and the aircraft have GPS time available. There must be some algorithm that calculates an offset or something. Correct?


#4

It does not use the rpi system clock or GPS time. It counts samples fron the receiver and uses ADS-B aircraft as a reference beacon to synchronize receivers.


#5

What is “it” here? What sort of samples are being counted? What do you mean by receivers? I cannot believe FA servers are counting time elapsed between messages received from various receivers. Internet delays would would cause overwhelming noise.


#6

The FlightAware MLAT client.
Sampling_(signal_processing)
PiAware systems.
The client, not the servers, is counting the time elapsed between ADS-B and non-ADS-B traffic.


#7

OK. That makes sense. Thanks.