Take the 2-minute tour ×
Aviation Stack Exchange is a question and answer site for aircraft pilots, mechanics, and enthusiasts. It's 100% free, no registration required.

I use Flight Radar 24 on my iPad to track friends/families flights when they are traveling (not paranoia, just curious). I've noticed that, quite often, their plane will jump from one location to another, or jump backwards down the flight path, or zoom forward, or any other number of motions that I assume the plane isn't actually making. (My personal favorite being when they miss the runway on touchdown and the plane disappears. If that actually happened as often as Flight Radar 24 made it look like it was happening...)

What makes the planes do this on Flight Radar 24? Why can't it just track the planes correctly?

share|improve this question

3 Answers 3

up vote 20 down vote accepted

FR24 relies for most of the tracks on ADS-B data. Part of the ADS-B transmissions is the position. There are three causes of the behaviour that you observe:

1). Aircraft landing next to the runway

The position source onboard the aircraft is not always GPS. Mainly older aircraft (e.g. Fokker 100) have their Inertial Navigation System (INS) coupled to the ADS-B transmitter. The INS relies not on external radio measurements but only on measured acceleration and rotation to track its position. It is usually correct at take-off, but it develops biases over time and therefore the landing appears quite often next to the runway. I can assure you that this is mostly misleading.

2). Short time track offset

In most aircraft the ADS-B position is from GPS. When the GPS loses its position fix, the system falls back to INS. As described above, the INS position is slowing drifting from the truth during the flight, so the ADS-B track will shortly jump aside to the INS position until the GPS comes back. When short (e.g. 1 position update) the offsets appear as spikes.

INS jump

3). Long distance jumps (180 NM, 360 NM)

Due to the limited number of bits available in the ADS-B messages, position is encoded in a complex format called Compact Position Reporting. It requires at least two position messages to decode the first position of a track, from that point onward, every message will constitute a position update. When no proper verification algorithms are used in the software, it may occur that a receiver station initializes the track wrongly, usually causing a jump of 180 or 360 NM.

Also note: When INS is used a the position source, the ADS-B data contains an indication that the data is not to be trusted, but that is silently ignored by FR24.

share|improve this answer
    
Could you possibly edit the question to explain what an Inertial Navigation System actually is? I'm not sure why it would be so different from GPS. –  Jay Carr Mar 14 at 14:26
    
@JayCarr INS is topic on itself, quite complicated. Instead of relying on radio signals like GPS, it relies on measuring accelerations and rotations to keep track of position. Feel free to ask another question and I am sure someone can provide a good explanation. (or else I'll try) –  DeltaLima Mar 14 at 14:30
    
Oh, I probably will. I was more hoping for that one line answer that you put in your reply right there. Just a primer for people (like me) who have no clue what-so-ever and want a one line clarifier. –  Jay Carr Mar 14 at 14:32
    
@DeltaLima - Excellent, exactly what I was looking for on INS (for this answer). –  Jay Carr Mar 14 at 14:38
    
@JayCarr your welcome, thanks for helping me to improve my answer. –  DeltaLima Mar 14 at 14:39

Not to take away from already great answer provided. In my experience the jumping behavior you are describing in Flightradar24 is due to MLAT triangulation that is used to establish approximate location rather than absolute (using GPS).

Older planes transmit a SUBSET of ads-b data, which does not include aircraft location. So FR24 knows that there is an aircraft in the air, but doesn't know its location. This is where multilateration (triangulation really) comes into play.

If you have a few (minimum of 4) receivers available AROUND the aircraft that are tuned precisely to the same clock, you can measure how long that same ads-b signal reaches all 4 receivers from which approximate location can be determined.

In FR24 you can verify which of the "jumpy" planes are being tracked by MLAT setup by looking at "radar" that is tracking it. If MLAT is used you will see "T-MLAT" as the source.

If you indeed see jumpy behavior on the planes that are tracked by "T-###" or "F-###" sources then I would agree with already posted answer by DeltaLima

share|improve this answer
    
Sometimes I wish we could combine answers on this site, just so that if two people have parts of the answer I could mark both as correct... –  Jay Carr Mar 18 at 13:36
    
No worries Jay, it's all about learning, we are all learning from all these different answers, that's what this community is about! –  KORD4me Mar 18 at 13:58

Re: INS - Since there are no ground based radio transmission navigation stations in the ocean (VOR's or ADF's), the INS (inertial navigation system) was designed to help jets navigate safely over the ocean (before GPS). INS uses a series of gyroscopes and accelerometers to measure how far a plane has traveled from a specific point. With multiple accelerometers it's able to figure if the plane turned to a specific heading and how far it's gone (it can do this through multiple turns and speed changes). To set the airplanes INS, a pilot pulls up to a specific INS set point at an airport and calibrates it (before the trip). INS is so accurate that it typically holds a planes position to within a few miles after traveling 2500 miles. The INS system can also be set from a GPS. The major advantage to INS is it doesn't rely on any external signal to work.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.