4
$\begingroup$

There is a question Why is the speed graph of a survey flight a square wave? with a Piper PA-31-150 accident, and its ground speed graph is part of the question.

This made me questioning - how the gound speed of GA aircrafts is known to the services? I briefly searched for the basic info GA transponder sends, and did not see the gound speed in there. Plus, to know a ground speed, such measurer should be installed on the aircraft - either a sat-nav, or an inertial measurer, or something else - and it needs to be connected to the transponder as well.

It can be that the services calculate it based on the locations sent, but I did not find GA aircrafts send coordinates either.

And another question - if this data is sent to the services, why the information might be that valuable to an aircraft - so they both calculate it and send it? I imagine, the true speed is the one that is important, not the ground speed - so, why sending it at all, even if the aircraft has the data onboard? To measure the fuel we can use time and air speed/RPM, why else we would send it - and have it on the service?

Improve this question
$\endgroup$
0

3 Answers 3

Reset to default
6
$\begingroup$

Since 2020, all aircraft in most controlled airspace are required to have Automatic Dependent Surveillance-Broadcast (ADS-B) out. This means they are required to send, among other information, their GPS coordinates and altitude at least once per second. (See 14 CFR 91.225)

Using these GPS coordinates, which contain time and position data, ground speed can be calculated.

The data GPS data is collected mainly from community "feeders" who build ADS-B recievers and transmit the data to sites like flightaware, adsb-exchange, etc. If you're wondering why someone would do this for free - well some people think it's fun, and some sites offer premium memberships in exchange for feeding data.

GPS data of the accident aircraft seen here is from the ADS-B transmitter. GPS data of accident aircraft mentioned

Regarding your other questions - the data is not even available to the crew on board. It is purely for ATC separation of aircraft in controlled airspace, as well as dinging people for illegal stunts or unauthorized entry into certain airspaces. If you have ADS-B "in" you can also use this to augment your situational awareness by knowing the location of nearby broadcasting aircraft.

Note 0: Heading is calculated simply using a straight or smoothed line between the GPS "hits". This is not entirely accurate since aircraft usually have at least some wind correction angle relative to their ground track. During aerobatic maneuvers and accidents the actual heading (that is, the direction the nose is pointing) of the aircraft is unkonwn and can only be guessed based on external knowledge of the aircraft attitude.

Note 1: ADS-B and most other surveillance methods cannot calculate "air" speed (that is, how fast the air is flowing over the aircraft). This is the speed shown on the airspeed indicator of an aircraft and is dependent on the relative wind. Consider a Cessna 172 flying at 100 knots indicated airspeed into a 50 knot headwind. The cockpit will register 100 knots, but the groundspeed is only 50 knots.

Note 2: GPS is transmitter via an ADS-B transmitter, which is a simply on-off device in the cockpit. It transmits the GPS coordinates to anyone with an ADS-B "in" receiver. This GPS data is used for tracking and surveillance but cannot be used for navigation (as far as I know).

Note 3: The rules governing who needs ADS-B are slightly more nuanced than I stated above, and I believe any aircraft which has ADS-B installed must turn it on during all operation. The "controlled airspace" restriction simply means aircraft without ADS-B are prohibited from many areas in the USA National Airspace System (NAS).

Improve this answer
$\endgroup$
8
  • $\begingroup$ Great answer! Thank you! I was not aware the ADS-B has its own GPS chip to embed the data into the transmittion. However, it makes perfect sense for the ATC to have the coordinates - it is indeed very valuable info to them! $\endgroup$
    – Mykola
    Commented 2 days ago
  • $\begingroup$ @Mykola not so much "has its own GPS chip" as "needs to be plugged into a compatible GPS unit", as I understand it. $\endgroup$
    – hobbs
    Commented yesterday
  • $\begingroup$ @hobbs but if it needs to be plugged in - it means you have to have a sat-nav instrument in your plane - which does not seem to be a requirement? $\endgroup$
    – Mykola
    Commented 21 hours ago
  • $\begingroup$ I'm not sure the answer to what you guys are discussing, but I just want to point out that you can get a "GPS chip" for $18 on amazon (search "GPYes"), so it's not like GPS is some unattainable technology. Obviously the technology used for aircraft surveillance will be higher price and quality but still nothing extraordinary to add to a transponder. $\endgroup$
    – codenoob
    Commented 14 hours ago
  • 1
    $\begingroup$ @codenoob The issue isn't as much the GPS receiver, though that will cost you about $400 extra in a ADS-B out transponder, it's the fact you have to mount a GPS antenna on the top of your plane, which will cost you another $300 to buy, and then maybe $1000 in labor to cut a hole in the roof, install, seal the antenna, route the cable through the interior. $\endgroup$
    – user71659
    Commented 12 hours ago
1
$\begingroup$

These services are based on ground based radar and transponders -- not GPS or SATNAV.

The transponder (Mode C) sends out an ID and pressure altitude. The ground based radar is good at bearing and range -- but not altitude. Combining these gets it a 3D fix on the aircraft. Tracking that over time gives ground speed.

Newer technologies Mode S and ADS-B and GPS all improve on this, but the old-school answer is radar + altitude.

Improve this answer
$\endgroup$
2
  • 1
    $\begingroup$ I did not know FlightAware or FlightRadar24 have radars on the ground - I always thought they rely on ADS-B of their networks plus the networks of volunteers. Do you have more information on the radars positions and types? Those are not cheap and require a license to operate as they are not passive and the radiating power is normally high for non-license equipment. $\endgroup$
    – Mykola
    Commented yesterday
  • 1
    $\begingroup$ Perhaps they are operating off of a network of ADS-B receivers. I thought that the government operated radar stations provided data to sites like those listed. I could well be wrong. Actually, it looks like a bit of all of the above... flightaware.com/about/datasources $\endgroup$
    – Rob McDonald
    Commented yesterday
1
$\begingroup$

Which aircraft can be tracked?

The only aircraft appearing on tracking sites are

  • The ones controlled by ANSP, usually they broadcast ADS-B messages in addition of being tracked by SSR stations. All commercial aircraft are tracked over land, and often over oceans. However in the latter case they must use space-ADS-B to be visible, as SSR stations are mostly on land.

  • The ones not controlled (of which GA aircraft) but having their transponder switched on or broadcasting ADS-B (or FLARM) messages. They must be in sight of a receiving station, aircraft flying low may escape detection by SSR.

How is ground speed determined?

Aircraft positions are collected by tracking sites at regular intervals (a few seconds). These positions are relative to the ground. The distance between two positions can be calculated, the ground speed is obtained by dividing by the time between the two transmissions.

Is ground speed useful for aircraft crews?

It is the only way to determine the time of arrival at the next waypoint or at the airport. Even before GPS, avionics was used to compute the ground speed, usually with inertial systems.

Details follow about:

  • What are ANSP, ADS-B, FLARM, SSR
  • How are positions determined.

How are aircraft positions collected?

Flight tracking sites get they information using two categories of feeds:

  • Their own network of ADS-B receivers. They started with this technology by federating individual ADS-B receivers owned by hobbyists. ADS-B is a voluntary transmission from the aircraft using a transponder or an equivalent system (UAT) containing the aircraft altitude and position and other data. A in ADS-B is for automatic, meaning the transmission is automatic, D is for dependent, meaning ADS-B relies on an external source for the information to be transmitted, e.g. on a GNSS receiver (GPS, Galileo, etc) to get the position. FLARM is a technology somehow similar to ADS-B and collision avoidance systems, used by small aircraft and gliders.

  • Data received by ANSP (air navigation service providers), using a subscription, likely for a price. ANSP can receive aircraft position using multiple technologies:

    • SSR (secondary surveillance radar). The ground station interrogates the transponder aboard the aircraft. The transponder answers with its altitude. The SSR station is able to determine the position by classic radar ranging. That's how most aircraft are still tracked today by ANSP.

    • ADS-B data, ANSP receive ADS-B using SSR stations and UAT receivers. The aircraft must broadcast ADS-B messages spontaneously. This is how aircraft (and UAV) will be tracked in the future by ANSP. ADS-B receiving stations are land-based.

    • Space-based ADS-B, ANSP receive data from Aireon, which manages ADS-B receivers hosted on Iridium NEXT satellites. This is a complement of land ADS-B for aircraft out of range of land receivers, e.g. flying over oceans or deserts.

    • Multilateration, ANSP manage their own network of multilateration receivers (MLAT and WAM). Multilateration consists in simultaneously receiving the aircraft transponder or ADS-B signal at different positions, and determining its position by comparing the different times of arrival.

    • ACARS, aircraft may send messages to ANSP, or to their airline operations management, for the purpose of planning and maintenance. These messages can be sent to receiving stations on land (VHF link) or in space (Satcom link). Tracking sites are interested in ACARS mostly to know the actual departure and arrival times.

    • FLIFO (Flight Information) from flight reservation systems, for general flight information from airlines. FLIFO is used to know future flights.

For all sources, but SSR, the position is determined aboard the aircraft using GNSS and transmitted. For SSR the ground station determines the range, the bearing, and the ground speed, this is less accurate.

Tracking sites also receive flight plans from ANSP, they can associate an aircraft identifier with a flight plan.

Improve this answer
$\endgroup$
3
  • $\begingroup$ Thanks for the answer! So, would a GNSS device be separate to ADS-B? Does it mean the plane has both installed? Would it (the coordinates transmitted over ADS-B) be optional in some regions - and mandatory in some else? $\endgroup$
    – Mykola
    Commented 9 hours ago
  • 1
    $\begingroup$ @Mykola: When a mode C transponder is used for ADS-B Out transmission (usual case), ADS-B Out is a software application receiving the position from an independent GNSS hardware and using the transponder as a transmitter. For details see this. Position is required (§5.1.1.1.1 of Annex 10 volume 4). $\endgroup$
    – mins
    Commented 8 hours ago
  • 1
    $\begingroup$ @Mykola: See also What's required for the ADS-B to function? A GPS or Mode S Transponder?. Also note a GNSS is not required per se, just a mean to get a precise position (but currently it is a GNSS) $\endgroup$
    – mins
    Commented 7 hours ago

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .