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Author Topic: MLAT Time Reference  (Read 1038 times)

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ajheatwole

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MLAT Time Reference
« on: August 26, 2023, 06:32:16 PM »
My station is listed as synchronized for MLAT, but my Raspberry Pi doesn't have a GPS receiver. Where does my Pi get its time reference? Does it receive GPS signals through the ADS-B antenna?

abcd567

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Re: MLAT Time Reference
« Reply #1 on: August 27, 2023, 01:23:02 AM »
My station is listed as synchronized for MLAT, but my Raspberry Pi doesn't have a GPS receiver. Where does my Pi get its time reference? Does it receive GPS signals through the ADS-B antenna?

The ADS-B message wavefront arriving at the dongle’s ADC are time-stampped using your RPi's clock. MLAT uses this time-stamp.

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« Last Edit: August 27, 2023, 01:27:12 AM by abcd567 »

ajheatwole

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Re: MLAT Time Reference
« Reply #2 on: August 27, 2023, 01:54:35 AM »
The Rpi's clock has to be accurate to a sub-microsecond level of accuracy for MLAT to work. I'd assume that this has to be a GPS time reference to be sufficiently accurate. NTP is good down to milliseconds, at best.

abcd567

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Re: MLAT Time Reference
« Reply #3 on: August 27, 2023, 03:26:04 AM »
The Rpi's clock has to be accurate to a sub-microsecond level of accuracy for MLAT to work. I'd assume that this has to be a GPS time reference to be sufficiently accurate. NTP is good down to milliseconds, at best.

I do NOT use any GPS with any of my RPis, and my aerial also do NOT have any builtin GPS, as I use a DIY aerial made by myself. Still mlat works perfectly well on all 3 of my stations.Please see screenshots attahed below.

https://www.radarbox.com/stations/EXTRPI000008

https://www.radarbox.com/stations/EXTRPI000035

https://www.radarbox.com/stations/EXTRPI000036



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ajheatwole

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Re: MLAT Time Reference
« Reply #4 on: August 27, 2023, 03:33:10 AM »
Thanks! I don't doubt that it works, I'm just trying to figure out *how* 🙂. Your antenna may be picking up GPS signals without you knowing it. Some GPS frequencies are close to ADS-B frequencies, so the RadarBox code may be picking up GPS signals, as well.

abcd567

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Re: MLAT Time Reference
« Reply #5 on: August 27, 2023, 03:43:37 AM »
Thanks! I don't doubt that it works, I'm just trying to figure out *how* 🙂. Your antenna may be picking up GPS signals without you knowing it. Some GPS frequencies are close to ADS-B frequencies, so the RadarBox code may be picking up GPS signals, as well.

I do NOT use Radarbox24's decoder dump1090-rb. I use Flightaware's decoder dump1090-fa, and the infotmation about time-stamping ofsignals at arrival at dongle using RPi's clock was given in Flightaware forums by Oliver B. Jowett (obj), who is author of dump1090-fa. Also Radarbox's dump1090-rb is forked from Flightaware's dump1090-fa, and is identical to it except for its name.

Please raise your querry in Flightaware forum, where you will get a detailed answer.

ajheatwole

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Re: MLAT Time Reference
« Reply #6 on: August 27, 2023, 04:45:43 AM »
Thanks!

abcd567

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« Last Edit: August 27, 2023, 11:29:41 AM by abcd567 »

abcd567

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Re: MLAT Time Reference
« Reply #8 on: August 27, 2023, 10:38:56 PM »
@Runway31
@ajheatwole

Please note that the links in my above posts are to answers by Oliver B. Jowett (username obj).

Oliver is not only Flightaware staff, but also author of source-codes for following software
(1) dump1090-mutability
(2) dump1090-fa
(3) dump978-fa
(4) mlat-client
(5) mlat-server

Items 2,3,4 & 5 are forked from his source-code by Radarbox24 developers and these forked codes are in use by Radarbox24 users (packages dump1090-rb, dump978-rb, & mlat-client) and by Radarbox24 mlat servers (package mlat-server).


[email protected]

https://github.com/mutability

https://github.com/flightaware

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« Last Edit: August 27, 2023, 10:48:03 PM by abcd567 »

ajheatwole

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Re: MLAT Time Reference
« Reply #9 on: August 27, 2023, 10:52:06 PM »
Thanks. I think that I've figured this out without bothering Oliver.

It isn't necessary for the MLAT stations to have accurate absolute time, only that they agree on a common time reference, accurately. When multiple stations at known locations receive the same signal from a known source - say an ADS-B broadcast by an aircraft with an accurate GPS location - they can confer to establish a common time reference. This time reference can then be used to triangulate signals that aren't broadcasting position information.

abcd567

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Re: MLAT Time Reference
« Reply #10 on: August 28, 2023, 02:16:32 AM »
Thanks. I think that I've figured this out without bothering Oliver.

It isn't necessary for the MLAT stations to have accurate absolute time, only that they agree on a common time reference, accurately. When multiple stations at known locations receive the same signal from a known source - say an ADS-B broadcast by an aircraft with an accurate GPS location - they can confer to establish a common time reference. This time reference can then be used to triangulate signals that aren't broadcasting position information.

Glad to know that your misunderstanding that GPS is a must for MLAT, is clarified now.

You are right that there is no need to bother Oliver. The links to two posts of Oliver I provided are enough to make the conclusion which you have made.


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