FlightAware Discussions

Not many MLAT for me


#1

Looking at my stats, I see “Trailing” and “Last” numbers. How do those terms differ? I thought I knew what “trailing” meant.
Is last the last 24 hours, and trailing the previous 24, prior to last?

My real question is why my MLAT positions are so low, around 100 on many days, while my ADS-B is over 3000.

Is this because all of the flights around me are equipped with ADS-B? :wink:
No, that’s not it, because my SkyView sometimes shows a high number of AC with no position.
Just now, it was 12/4. Should my nearby piaware compatriots be helping me to resolve those with MLAT? I only have one MLAT listed right now, and it’s 3 miles away from me. I actually saw it change color to MLAT, as I was viewing. That one was coming in from the north, and has now stayed MLAT as it goes farther south, into the populated SF Bay Area, and stopped getting records, starting to age, at 37 miles.

Looking at the site closest to mine n112 confuses me. There are three stations, listed with Nearest Airport as three different ones, none mine. He does have MLAT enabled, also with the low numbers.

Multilateration (MLAT): Supported / Enabled (synchronized with 84 nearby receivers)
My position and altitude are exact. (GPS nut for 20 years).
How many stations are needed reading a single aircraft for it to be resolved by MLAT?
It seems like I need some help to the north. Can I see a plot of all of my “nearby”, or all of the MLAT that I sync with? They might all be south, towards the population center.
I had been thinking about giving a piAware station to a friend near 39,-122.7, where I want better coverage for low position-only flights, anyway.
As I look at the generic ADS-B overview map, I see that there are no stations at all where I might like to have some.


#2

Last 24h is today, trailing 24h is yesterday.

you need 4 receivers to calculate mlat positions, right now you are connected to 189, so that is enough

mlat is only necessery for planes that do not uses ads-b, either they are not equipped with it or they choose not to use it.

You can receive mode-s message from a plane without position info.


#3

I think 184 of those 189 are all south of me :frowning:


#4

Looking at 7-Dec from your site and mine south of you, you have a much larger percentage of “Other” flights. The absolute number of flights difference doesn’t matter, since I have line of sight to to the very busy SFO, OAK, and SJC. But you have a much higher percentage of aircraft that can’t be MLAT’ed. I would guess that’s because of the larger percentage of lower altitude and weaker transponder Mode-S GA aircraft, combined with the fewer possible local MLAT peers who are under similar circumstances.

image


#5

If you are in an area with many receivers like you are FA can’t really make use of all stations receiving MLAT aircraft.
So it chooses a limited number of stations to calculate a MLAT position for an aircraft.
Only those stations are fed back MLAT data.
That’s what i have gathered from posts by obj who programs that stuff as far as i know.

I don’t know how that selection works but my guess is that consistenly getting messages from that particular plane is required.
Due do your limited setup and location that may mean other receivers are preferred :confused:


#6

I think that anyone that selects to be part of MLAT, gets the MLAT data back from servers, regardless what stations FA choose to use for their calculations.


#7

I see my position used for very few aircraft, but that is due to my location, up against some hills. There are plenty of stations off in the direction where I have visibility.
I have a view to the southeast down a valley, and I catch some over there.
Skyview gives me a lot more plots than what I see in the “positions from this feeder” list, which is 14 at the moment.

I can hear a small plane flying around overhead, nothing on SkyView… stealth aircraft :wink:


#8

I’m too lazy to look for a quote by obj right now but:

I have two stations and the MLAT tracks of airplanes that are clearly in sight for both my antennas differ quite a bit.
So how would you explain that otherwise?

The post by obj i remember also talks about increasing the grace period to 3 minutes.
So once your station was used to calculate a MLAT position you will get MLAT positions of that plane for the next 3 minutes even if your station is no longer used.


#9

I have four airports located within 50km and I get a good view of traffic flying in and out of all of them.

Since 95% of aircraft I see are flying into or out of one of those airports I ‘see’ almost the same number of planes from a really good antenna mounted high outside or just a relatively poorly mounted dipole.

On the good antenna I will track planes for 200NM and on a lesser antenna I will track them for only about 75NM. I still track almost the same number of planes.

I have spent far too much time looking at the full table in Skyview. When watching the table I sort (by clicking the column heading twice) the Source inversely so that I get Mode S at the top then MLAT then ADS-B.

Sometimes the first packet that arrives identifies the plane and gives it a location so that it appears as an ADS-B aircraft.

Often, the first packet only has identity and it is listed as Mode S. A few seconds later the MLAT process works out where it is and it is changed to MLAT and appears on the map. When an ADS-B packet arrives it is tagged as ADS-B.

What I have noticed with a poor antenna is that more planes appear as Mode S and never elevate to MLAT and ADS-B. Having multiple systems I can watch the same plane with the good antenna and the bad antenna.

With the good antenna I see ADS-B and MLAT from >150NM and track for 30 minutes. On the poor antenna the same plane may only receive a couple of packets and never elevate up the table past Mode S and quickly disappear.

For my location, the longer a plane is in range the more likely it is to transition from Mode S to MLAT and if, so equipped, to ADS-B.

Around here, all commercial and more than half GA are now ADS-B equipped.

Sorry for the long post,

S.


#10

Ok, so just being synchronized does not mean one is actually able to mlat low flying planes if they aren’t enough receivers nearby


#11

Ok, another reason why a station might see fewer mlat planes.


#12

No, it is not.
Obs just said that MLAT, being triangulation, works if the strangulation base is wide. As in more that 10 miles apart. The further away, the better precision.
He didn’t say nothing there about “only the samples used by us in MLAT will get back the results”.


#13

@wiedehopf This was quote you were refering to?


#14

Not just wide, but actually surrounding the plane. So for the calculations, being at the fringe of a mlat network could result in the site being less used for mlat.

If you get the results anyway, you are right it does not matter, but I think it works differently, see the other quote I posted.


#15

Yes it is. Thanks a lot for the archeological work :slight_smile:

So read the quote found by bieke above. How do you interpret it? Anyway you are free to disagree.


#16

I interpret it as in “Server-side MLAT calculation is dependent on triangulation, so in order to work we need…”
It doesn’t say “your MLAT numbers and/or display depends if we used or not your samples”.

IMO once you activated MLAT, you will get those return results from FA servers, to augment the Mode S flights that your receiver already have seen with their calculated positions.

Explanation:

License to use:


#17

I guess you read the wrong quote.
Check this whole thread to get some context:
(Missing MLAT track pieces)

Anyway the mlat track differences between my two receivers are hard to explain differently.


#18

Found another thread with a quote from @obj.


#19

That doesn’t say “your actual samples were used to MLAT calculations in our servers”. I don’t think they can even track that in real time.
IMO that means that if you cut off your RAW “MLAT” data, they will cut off the processed MLAT data they send back.
Probably @obj can clarify this at some point.

PS: MLAT data has more samples per second, and they can discern it from the aggregated data that is sent with MLAT deactivated.


#20

Mmh, only one these statements can be true?

I understand it as just because my feeder is assigned or synchronized to a mlat group it, it does not automatically receives all mlat data generated by feeders in that group, only when it is actually used for a specific calculation.

And whether it is actually used or not is pretty random, even if one decides to let the sites data to be used for mlat, it might not be used every time, not even all the time for the same aircraft, and when it is not used, you won’t get the results, after a grace period, hence the truncated tracks on the map.

Or are we completely misunderstanding each other here?

Edit: tried to clarify, not sure it worked