Speed Limit of GPS

[BayouRat]

I was always under the impression that there was a 400 MPH speed limit on GPS chips. My Beeline 70cm GPS looses lock on boost as well as my Eggfinder. But I'm told the telagps does not. Any input on this subject would be greatly appreciated.

 

[djkingsley]

Its all governed by the ITAR regulation, it changed a couple years ago. The last change I see was to remove a 60000 ft and 1000 knot category.

Please do not quote me as an expert on ITAR issues, you really need a lawyer who lives in the ITAR world to make an accurate determination of whats allowed.

 

[MWC]

The GPS chip set, patch antenna tuning, and patch antenna orientation will also play a part of this equation... dependent on your GPS chip set, you may also have solution rates and high-dynamics compensation that will help to mitigate transonic effects. Inertial effects on crystals or resonators will also skew timing and potentially disrupt solution calcs. There may also be doppler effects at play here too, both receiving and dowlinking.

 

[ChrisAttebery]

I have a TeleMetrum V2 and it also loses solution at high speed. Here's a Google Earth plot of my City Slicker 54mm minimum diameter rocket from Aeronaut last summer. Notice the kinks in the flight path. I haven't gone through the data to see what the speed was when the GPS lost and regained position, but it was probably in the 400-500 mph range.

 

[keithp]

There are two separate things in play here.

The first is a simple matter of physics -- under high acceleration, the GPS receiver loses track of the satellite signal because of doppler shift. The uBlox chips that many of us sell in tracking devices are good to at least 4Gs, but that varies based on how strong the satellite signal is and where the satellites are in space. Once the acceleration stops, the GPS chip should re-acquire the signal and start reporting accurate positions again in a few seconds.

The second is a matter of law. COCOM (sometimes called ITAR) requires that GPS receivers not report data when the speed is greater than 1000 knots and the height is above 60000 feet. Some receivers mistakenly implement that as 'or' instead of 'and', which means they'll shut down even below 60000 feet at high speed. Again, the uBlox chips that many of us use do the right thing in this case, so if you're below 60000 feet or below 1000 knots, you should get data.

 

[ChrisAttebery]

Keith,

I think your first explanation covers 99% of the flights in HPR as most of us use the 5:1 thrust ratio as a lower limit. I had seen some explanations on the TM email list, but none of them we as concise as that.

Thanks,


Chris