silly question about gps

[ifmdalvey]

The AIS problem sounds similar to a problem i had on my radar. The MARPA facility did not take account of my heading and speed, even though there was a GPS plugged into the chart plotter . So cans and nuns would always show as coming towards me and my velocity.
I think the solution is an accurate heading source, Have just fitted an EVO 1 system but until i hit the water in 3 months don't know if that will cure my problem.

 

[Jackdaw]

jackdaw, timing is also an integral part of GPS operations. Without it, the ground base GPS will not be in sync with the space based vehicle. There just isn't one source for determining VTG. The part I was not 100% is agreeing was the part where you left out the clocks. As I said before, they all work together. If one fails, the cascading affect will bring down the entire system (not falling out of the sky).

Since you failed to mention it I assumed you had left it out on purpose.

Indeed, it was not my intent to write a thesis on how the GPS system works, but just to explain how Doppler comes into play, and is used for speed calculations.

 

[heritage]

For the record, I was wrong stating that GPS speed is determined only by fix to fix calculations. This site has a good explanation for laymen of how it works: http://gpssystems.net/accurate-gpsbased-speedometer/

 

[Jackdaw]

For the record, I was wrong stating that GPS speed is determined only by fix to fix calculations. This site has a good explanation for laymen of how it works: http://gpssystems.net/accurate-gpsbased-speedometer/

To make this clear:

GPS hardware (the chip) uses ONLY Doppler to calculate speed.

GPS devices (like plotters, handhelds, and speedometers) that report speed will often have smoothing algorithms that attempt remove jitter between readings. Often they will do an average of the last N readings. In theory they could do their own TSD calculations based on the lat/long, but I have never known a device that does that.