Primarily for general aviation discussion, but other aviation topics are also welcome.
#1271808
Coming off the back of the drop zone thread, one side said that they had a GPS track, the other mentioned radar tracks.

Which of those would be considered more accurate. My guess is that gps should be but the source is probably uncertified. What's the real answer?
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By Irv Lee
FLYER Club Member  FLYER Club Member
#1271815
As part of the GPS articles back in 2001 in Flyer we did a flight with a simple hand held GPS under the watchful eye of Bournemouth Radar, to keep entering, exiting, and flying along their boundary at various points, with them reporting to us when they thought we were just in, just out, or exactly on, and I couldn't have argued with anything they said.
By EdwardG
FLYER Club Member  FLYER Club Member
#1271906
Radar track accuracy depends on a lot of things --- principally on the received signal-to-noise ratio (SNR) and whether you are interested in accuracy along the range radial or perpendicular to it --- radars measure in polar coordinates (i.e. distance from the radar (or time) and azimuth angle) not cartesian position so the translation of angular measurement accuracy to positional error depends on the range. From memory:
- The range accuracy of ATC type en route radars is of the order of at least several tens of metres and usually into the hundreds.

- Cross-range accuracy depends on distance from the radar and can become quite a significant at longer ranges. Angle accuracy is also more dependent on SNR, so tracks on smaller aircraft (and especially composite ones) may be rather uncertain.

- Elevation angle (or height) accuracy of primary ATC radar tracks is either poor or (most usually) totally absent. Mode C height is, ISTR expected to be within +/-100 feet. Again, GPS height/altitude will usually be somewhat better than this.

It is fair to assume that a GPS trace will, on average, be more accurate than a radar track --- by up to an order of magnitude or more. However, this is on average, and it doesn't mean that any particular radar track isn't very accurate; it just means that most won't be or won't remain so for long.

On average (again) tracks will tend to be better for slower aircraft (conditions apply) since the tracking filters can do more averaging.
By Carl B
#1272125
I seem to recall seeing an article on the BBC that said that GPS accuracy varies throughout the year, as the atomic clocks on the GPS satellites get out of sync with the earth (since its rotation isn't always exactly 24 hours). Every so often, the atomic clocks are adjusted by a leap second to bring them back in sync.

Carl B
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By Talkdownman
#1272147
GPS. PPI Radar position is at Slant Range projected to horizontal.
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By T67M
FLYER Club Member  FLYER Club Member
#1272308
Comparing the radar track of a flight where I had an airprox with my GPS trace of the and flight revealed a difference of nearly a mile. The GPS track aligned perfectly with my routing over a very prominent ground feature (a large lake) while the radar trace, needless to say, did not.
#1272343
Radar will show the two aircraft relatively accurately assuming they are close together as in an airprox but for absolute accuracy the GPS will be far better.
In James' link to the Citation crash three radars give three positions with errors of up to 0.15 of a mile or 300 metres.