gps and ground speed
Join Date: Sep 1998
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Great Thread.
The points about Differential GPS, WAAS are very interesting as this disscusion is moving towards RNP/EPU and RNAV operations. Inorder to fly approaches with RNP < 0.3 the aircraft requires and Augmented GNSS system as raw GNSS cannot guarantee accuracy. in some places RNAV procedures are published with (GNSS) as the stated nav aid required to acheive the accuracy in others it is now being removed. The bottom line is the Aeroplane needs to be able to evaluate the nav accuracy and alert the crew if its not able to meet the RNP.
Getting back to Hand helds or in car systems. I have a unit that knows where the speed cameras are and I use the distance coundown to judge the accuracy of the GPS position. Often as I pass the camera there is still some meters to go or the position is already passed before I get to the camera. The shift from too late to too early can happen in a short distance so that from being too early by 20 meters it can bacome too late by the same distance after just 500 meters travelled. If this same position uncertanty is used to calculate the speed without any smoothing then I wouldn't trust it over the tacho.
Getting back to Hand helds or in car systems. I have a unit that knows where the speed cameras are and I use the distance coundown to judge the accuracy of the GPS position. Often as I pass the camera there is still some meters to go or the position is already passed before I get to the camera. The shift from too late to too early can happen in a short distance so that from being too early by 20 meters it can bacome too late by the same distance after just 500 meters travelled. If this same position uncertanty is used to calculate the speed without any smoothing then I wouldn't trust it over the tacho.
ENTREPPRUNEUR
Join Date: Jun 2001
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You can check this for certain. Al you need to do is observe whether the GPS knows where you are. If it does then it must give you an exact ground speed as its clock is extraodinarily accurate. Its true it is using a simplified model of the Earth's surface but that is unlikely to make any noticeable diiference under normal circumstances.
"I have a unit that knows where the speed cameras are and I use the distance coundown to judge the accuracy of the GPS position."
FE Hoppy
Your position indicated by the GPS unit is accurate to within a few tens of metres.
It is the location of the speed camera that is in doubt, because the lists of locations are compiled from a variety of sources including amateur users and are frequently inaccurate.
If you repeat the same journey you will find that the indicated error at each camera is consistent - always the same every day.
FE Hoppy
Your position indicated by the GPS unit is accurate to within a few tens of metres.
It is the location of the speed camera that is in doubt, because the lists of locations are compiled from a variety of sources including amateur users and are frequently inaccurate.
If you repeat the same journey you will find that the indicated error at each camera is consistent - always the same every day.
Doppler
Two points that seem to me missing in the above responses:
1. Consumer GPS generally gives Doppler a considerable weight in the displayed speed--so it is not just a matter of position error deltas for speed error. Generally Doppler does better, which is why they use it. It also responds quite quickly, though not instantaneously, depending on what sort of smoothing the manufacturer puts in for post-processing.
2. Confidence from displayed zero velocity is a bit misplaced. Consumer units generally clamp the displayed velocity to zero when the initially computed value is below some threshold. I noticed they lowered the threshold on the ones I use when SA got turned off. I think I recall it used to be about 1 mph. A few minutes walking about extremely slowly with my Garmin GPS V just now with WAAS turned on and eleven well-positioned satellites in view never once displayed 0.1 km/h, though 0.3 and 0.0 appeared many times, and 0.2 often. I surmise the clipping limit in the current firmware in that condition was probably a little over 0.2 km/h.
Despite those two points, I agree with those who suggest that a _consistent_ difference between an automobile speedometer and even a very cheap consumer GPS is almost certain to find the GPS in the right of things.
Disclaimer: I'm not a pilot, but am an electrical engineer, and have used GPS a lot since 1991.
1. Consumer GPS generally gives Doppler a considerable weight in the displayed speed--so it is not just a matter of position error deltas for speed error. Generally Doppler does better, which is why they use it. It also responds quite quickly, though not instantaneously, depending on what sort of smoothing the manufacturer puts in for post-processing.
2. Confidence from displayed zero velocity is a bit misplaced. Consumer units generally clamp the displayed velocity to zero when the initially computed value is below some threshold. I noticed they lowered the threshold on the ones I use when SA got turned off. I think I recall it used to be about 1 mph. A few minutes walking about extremely slowly with my Garmin GPS V just now with WAAS turned on and eleven well-positioned satellites in view never once displayed 0.1 km/h, though 0.3 and 0.0 appeared many times, and 0.2 often. I surmise the clipping limit in the current firmware in that condition was probably a little over 0.2 km/h.
Despite those two points, I agree with those who suggest that a _consistent_ difference between an automobile speedometer and even a very cheap consumer GPS is almost certain to find the GPS in the right of things.
Disclaimer: I'm not a pilot, but am an electrical engineer, and have used GPS a lot since 1991.
Join Date: Feb 2007
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Maxter, I hope you're still reading this.
The professional GPS equipment you're using. I've always assumed that such equipment, used for mapping purposes, is mounted on a tripod or something and has a button or something that allows you, as the operator, to tell it that its position is now fixed. From that point on, the receiver can start collecting GPS positions and average these positions out over a longer period of time (minutes, hours, perhaps weeks)? This, combined with a differential GPS broadcast device, set up locally, in a known position, will then give you 2cm accuracy, not?
But an average consumer GPS (up to and including the G-1000) will not have the benefit of a "steady state" button, so it has to assume that it's moving all the time, making averaging over a periode more than a few seconds impossible, and will not have the benefit of a locally produced DGPS signal so has to rely on WAAS, if WAAS is available for that area at all. Hence the 6-35 meter accuracy for these devices?
The professional GPS equipment you're using. I've always assumed that such equipment, used for mapping purposes, is mounted on a tripod or something and has a button or something that allows you, as the operator, to tell it that its position is now fixed. From that point on, the receiver can start collecting GPS positions and average these positions out over a longer period of time (minutes, hours, perhaps weeks)? This, combined with a differential GPS broadcast device, set up locally, in a known position, will then give you 2cm accuracy, not?
But an average consumer GPS (up to and including the G-1000) will not have the benefit of a "steady state" button, so it has to assume that it's moving all the time, making averaging over a periode more than a few seconds impossible, and will not have the benefit of a locally produced DGPS signal so has to rely on WAAS, if WAAS is available for that area at all. Hence the 6-35 meter accuracy for these devices?
