ImRich
Veteran
Location: Derry, NH USA
| It has been some time since I've worked with nav equipment so I may be a bit rusty.
But one thing to keep in mind, especially with regard to GPS (or any other measurement device) is that there are many parameters which can affect the accuracy of the readings you are getting.
A GPS is a combination hardware/software device. It has to track radio signals from a satelite in space and measure subtle differences in these signals as received.
The on board computer and software algorithms must average many readings to filter out noise and such. The averaging algorithms have all sorts of software PLLs in them. There is reaction time and other things to think about.
To give the MOST accurate positioning measurements they will use long PLL times. This will make a device lousy (inaccurate) for use while moving. For moving results, they need shorter PLL periods. This will make it more accurate for movement, yet less accurate for fixed position accuracy.
Most these days use adaptive algorithms. That is they may dynamically change the PLL loops to try to determing if the GPS receiver is moving or if it is stationary.
As you can imagine, accelleration can really make it even more complicated. GPS receivers tend to use accelleration prediction to determine the optimum PLL settings.
All of this is a compromise.
The gist of what I am saying is that GPS is not 100% accurate, 100% of the time! You have better results if you are stationary (for position or alititude), or moving at a constant rate (no accelleration/deaccelleration).
The big question for heli measurements would be, how long do I have to fly at a constant speed to be able to get a reasonably accurate speed reading? How much error is acceptable?
This goes for most speed measurement devices too, even police Radar!  Devices have certain parameters they are designed to work with. If you go outside of this design performance envelope, then your results will vary.
Often there are GPS receivers which are designed for different markets. One designed for a land based use (handheld, or use in an automobile) and designed for use at ground speeds from zero to (maybe) 200 MPH maximum will react totally different than one designed for aircraft use (60 MPH minimum to 600 MPH or more). Remember, the faster you go, the less accurate you have to be for position, because at speed you are not in one location for very long!
I think most people think that GPS is hi-tech, so therefore it is all knowing and all accurate! This may not be the case!
Just take into account the design parameters. Longer straight path runs at constant speeds will give you the most accuracy. The question is how long do you have to go to get a reasonable accuracy. Simply getting repeatability in your results may not give you that answer.
Also, as someone else mentioned, you are reading calculated ground speed, there is no adjustment for windspeed, etc.
I hope this helps a bit, and doesn't confuse too many people. I'm just trying to help.
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Rich |
