4 ms·

When I was tracking flights some years ago (use case was determining whether speed monitoring aircraft were in the air, and a prediction model of their schedule

When I was tracking flights some years ago (use case was determining whether speed monitoring aircraft were in the air, and a prediction model of their schedule) I learned that the ADS-B transmitters didn't need to broadcast GPS co-ordinates below a certain ceiling. You could get the signal that a given tail number was in the air, but if it stayed below a certain altitude, you couldn't always receive its location. The FAA has guidelines on what kind of data you can receive: https://www.faa.gov/nextgen/equipadsb/research/airspace/ https://www.faa.gov/nextgen/equipadsb/research/airspace/ I lost interest in it because I just don't speed that much and I don't have a sport bike fast enough for it to matter, but useful to know that ADS-B doesn't cover all aircraft.

amatecha 6y ago
Oh that's interesting, I actually thought about that recently when I saw a "speed monitored by aircraft" sign on a highway. I wondered if an ADS-B receiver would pick it up. But... indeed, it's quite often that I see an aircraft and observe that it is not being picked up by my ADS-B receiver. Not sure whether due to low altitude , or that they are simply not transmitting..
Scoundreller 6y ago
> You could get the signal that a given tail number was in the air, but if it stayed below a certain altitude, you couldn't always receive its location. I believe trackers (like ADS-B exchange) will estimate location using mutlilateration. IE: enough receivers with GPS-sync’d time and knowing where they are can estimate origin with the differences in receipt timing. With good enough equipment, even one ground station might be able to tell a lot with Doppler shifts. Unsure if that’s don’t in practice though.
- Scoundreller 6y ago
  Looks like there’s much info here: http://www.multilateration.com/downloads/MLAT-ADS-B-Reference-Guide.pdf http://www.multilateration.com/downloads/MLAT-ADS-B-Referenc... (Big PDF)
- sorenjan 6y ago
  They use multilateration, but there's no GPS synced clocks needed. They use ADS-B messages with position as reference and uses the relative arrival times of those messages to model the clock characteristics of each receiver. That way all you need is an inexpensive receiver (like a RTL-SDR) and with enough receivers in an area you'll get pretty good position estimates of aircraft only transmitting Mode S messages. This is the most popular mlat server software, used by ADS-B Exchange among others: https://github.com/adsbxchange/mlat-server https://github.com/adsbxchange/mlat-server
jcrawfordor 6y ago
The regulations around ADS-B are pretty weird, basically because the FAA wants to require it of all aircraft but all the aircraft owners object because it's pretty expensive to install it. So the FAA has kind of been taking a "death by a thousand cuts" approach where they never quite "mandate" ADS-B but they require it in more and more situations... So aircraft are perfectly allowed to not transmit ADS-B as long as they stay out of certain types of airspaces and outside of a certain distance of other types of airspaces and etc. So the effective result is pretty much that you need to have ADS-B if you ever want to land at a controlled airport but the details get a little weird. These rules have also been changing reasonably quickly in federal terms. The current set of ADS-B requirements only took effect the first of this year. Among other things, these rules require that any aircraft equipped with ADS-B out have it turned on at all times... but that's only been the rule as of recently (not sure if this revision or the previous one), so not that long ago it was acceptable to turn it on and off if you were outside of areas where it was required.
- sokoloff 6y ago
  Not all controlled airports, for example most class D airports are outside ADS-B required airspace. Basically, you need ADS-B in airspace where you already required a mode-C transponder previously. Further nit: the rule went into effect on Jan 2 this year, because of the wording of the actual law. It seems likely that the intent was to be on the 1st, but that’s not what was written and enacted.
zymhan 6y ago
That's changed since 2020, you have to be far away from controlled airspace to just broadcast Mode-S anymore.
- sokoloff 6y ago
  Not true. Most of the US is controlled airspace where you aren’t required to be ADS-B equipped. It’s a small minority of airspace under 10K MSL where it’s required (just class B veil and class C airspace and above class C rings). Most of class D and E airspace is not in the above category and therefore not ADS-B required while still being controlled airspace.

amatecha 6y ago

Oh that's interesting, I actually thought about that recently when I saw a "speed monitored by aircraft" sign on a highway. I wondered if an ADS-B receiver would pick it up. But... indeed, it's quite often that I see an aircraft and observe that it is not being picked up by my ADS-B receiver. Not sure whether due to low altitude , or that they are simply not transmitting..

Scoundreller 6y ago

> You could get the signal that a given tail number was in the air, but if it stayed below a certain altitude, you couldn't always receive its location. I believe trackers (like ADS-B exchange) will estimate location using mutlilateration. IE: enough receivers with GPS-sync’d time and knowing where they are can estimate origin with the differences in receipt timing. With good enough equipment, even one ground station might be able to tell a lot with Doppler shifts. Unsure if that’s don’t in practice though.

Scoundreller 6y ago
Looks like there’s much info here: http://www.multilateration.com/downloads/MLAT-ADS-B-Reference-Guide.pdf http://www.multilateration.com/downloads/MLAT-ADS-B-Referenc... (Big PDF)
sorenjan 6y ago
They use multilateration, but there's no GPS synced clocks needed. They use ADS-B messages with position as reference and uses the relative arrival times of those messages to model the clock characteristics of each receiver. That way all you need is an inexpensive receiver (like a RTL-SDR) and with enough receivers in an area you'll get pretty good position estimates of aircraft only transmitting Mode S messages. This is the most popular mlat server software, used by ADS-B Exchange among others: https://github.com/adsbxchange/mlat-server https://github.com/adsbxchange/mlat-server

jcrawfordor 6y ago

The regulations around ADS-B are pretty weird, basically because the FAA wants to require it of all aircraft but all the aircraft owners object because it's pretty expensive to install it. So the FAA has kind of been taking a "death by a thousand cuts" approach where they never quite "mandate" ADS-B but they require it in more and more situations... So aircraft are perfectly allowed to not transmit ADS-B as long as they stay out of certain types of airspaces and outside of a certain distance of other types of airspaces and etc. So the effective result is pretty much that you need to have ADS-B if you ever want to land at a controlled airport but the details get a little weird. These rules have also been changing reasonably quickly in federal terms. The current set of ADS-B requirements only took effect the first of this year. Among other things, these rules require that any aircraft equipped with ADS-B out have it turned on at all times... but that's only been the rule as of recently (not sure if this revision or the previous one), so not that long ago it was acceptable to turn it on and off if you were outside of areas where it was required.

sokoloff 6y ago
Not all controlled airports, for example most class D airports are outside ADS-B required airspace. Basically, you need ADS-B in airspace where you already required a mode-C transponder previously. Further nit: the rule went into effect on Jan 2 this year, because of the wording of the actual law. It seems likely that the intent was to be on the 1st, but that’s not what was written and enacted.

zymhan 6y ago

That's changed since 2020, you have to be far away from controlled airspace to just broadcast Mode-S anymore.

sokoloff 6y ago
Not true. Most of the US is controlled airspace where you aren’t required to be ADS-B equipped. It’s a small minority of airspace under 10K MSL where it’s required (just class B veil and class C airspace and above class C rings). Most of class D and E airspace is not in the above category and therefore not ADS-B required while still being controlled airspace.