Altimeters and high altitude recording

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Chris_H

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For flights over 30,000', I understand that barometric altitude measurement becomes less accurate?

And, that for records, Tripoli has a list of approved, GPS based altimeters.


How do these GPS altimeters work, given that the GPS unit may lose lock with it's satellites?
Is the altitude somehow still recorded via GPS when this happens?


Is this a situation where someone could place a properly wired and oriented GPS unit like a TeleGPS, or AIM Xtra, in the rocket, and there would almost certainly be a GPS based altitude measure?

If someone builds a rocket that is headed for the thin air, is redundency in GPS based data recording also a 'must?


Cheers.
 
GPS lock is usually lost during the burn. This can be because the unit can not calculate a solution under highly dynamic situations, also most units have limits imposed to prevent military use so will not report coordates at high speed even though the unit has a lock. As the rocket slows downs near apogee the GPS is usually about to reaquire lock so location and altitude can be obtained.

Even during almost all extreme flights in our hobby a good baro altimiter will reliably detect when apogee occurs despite not having an accurate reading on the actual altitude, so this often satisfies the safety and redundancy side of things. Getting the rocket back is where the GPS is required. I’ve not flown redundant GPS, but obviously for more extreme flights / expensive the rockets this will become more important. I don’t think it is a requirement though.
 
I was more specifically asking about higher and faster flights where GPS lock loss is certain, and wondering about the cases where it is not regained, and specifically for altitude data recording. If altitude data is important, then that single GPS based altimeter becomes pretty important? If GPS lock is not regained through apogee and well into descent, if at all, and GPS lock is lost early on in ascent, then there is no GPS based altitude data?
 
I was more specifically asking about higher and faster flights where GPS lock loss is certain, and wondering about the cases where it is not regained, and specifically for altitude data recording. If altitude data is important, then that single GPS based altimeter becomes pretty important? If GPS lock is not regained through apogee and well into descent, if at all, and GPS lock is lost early on in ascent, then there is no GPS based altitude data?

Depends on what your goal is. If you're looking for documentation of a Tripoli record or more detailed analysis of the flight profile, redundant GPS units might be a good idea, though there may be a weight tradeoff depending on where the rocket's mass is compared to optimum mass for altitude on that particular motor. If you only need to get the pieces back, the lost GPS lock is very likely to be regained as it drifts down. There was a recent thread about redundant trackers--might also be good to look that up.
 
30,000' is well within the range of most modern baro altimeters. The ORIGINAL Eggtimer was limited to 30K because that was the limit of the stated accuracy of the sensor. It didn't necessarily mean that it couldn't go higher. Most sensors now extrapolate that in their data sheet and give you an "extended range" accuracy figure, which is typically a bit less than at lower altitudes/higher pressures. Our current altimeter crop (except for the Eggtimer Classic) is rated to 60,000'... a few feet here or there isn't going to make much difference at that altitude.
 
GPS receivers have gotten better in the last few years, and with the latest U-Blox receivers we have seen lock maintained throughout some pretty high performance flights. In the occasions where lock was lost on a high-performance flight, it came back within a few seconds after the end of the boost, long before apogee.

The accuracy of Baro altimeters is limited by the standard atmosphere model equations that all altimeter manufacturers use. That model assumes an air temperature of around 60F at sea level. On hot days, baro altimeters using that model can under-report the altitude significantly, sometimes 10% or more. On a 30,000 foot flight that can be an error of 3000 feet. Modern GPS units have altitude repeatability that's typically around 30 feet or better, and that's true for all altitudes where they will provide data. So for flights that are 300 feet or higher, a GPS will generally have better absolute accuracy than a baro altimeter. But since all baro altimeters are subject to the same systematic errors, you should be able to fly 2 different baro altimeters, even from 2 different makers, and have around 1% or better agreement comparing one baro altimeter to the other if you fly them on the same day, so that's why they have been considered acceptable for altitude records.
 
There are also GPS units that are "unlocked" and capable of tracking the entire flight. They are in the region of $5k to purchase if you want one.

I have read of a way of post-processing the GPS data to gain accurate information, but it is calculation intensive thus not realtime. Non-trivial.

Alternatively, if you are really keen, you can program an FPGA to correlate the GPS signals yourself, and have the data during flight. Definitely non-trivial.
 
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Thank you for the replies.

I have been trying to talk myself into picking up another transmitter for my Aim Xtra base.

The Garmin dog collars are awesome, but all they do is track.
 
Thank you for the replies.

I have been trying to talk myself into picking up another transmitter for my Aim Xtra base.

The Garmin dog collars are awesome, but all they do is track.

Be careful because some of the earlier altimeters had what was called a floating ground and were susceptible to high energy Rf interference. Some Adept altimeters were extremely prone to failure with a dog tracker riding in the same bay as the altimeters. The dog trackers put out 2 watts Rf which is a lot of power for rocket tracking.
If you’ve had successful flights with a given set of hardware, you should be good. The modern devices with opto-isolation are pretty resistant to Rf interference.
If uncertain about compatibility, put contained ematches on the altimeter channels, set the rocket upright and turn on everything. Let sit for an hour and if the matches don’t blow, the altimeter(s) don’t shut down or recycle you should be good to go. The other thing is the dog trackers might not be good for above 60k but I forgot the altitude limits so anyone may correct me if they know. Kurt
 
Be careful because some of the earlier altimeters had what was called a floating ground and were susceptible to high energy Rf interference. Some Adept altimeters were extremely prone to failure with a dog tracker riding in the same bay as the altimeters. The dog trackers put out 2 watts Rf which is a lot of power for rocket tracking.
If you’ve had successful flights with a given set of hardware, you should be good. The modern devices with opto-isolation are pretty resistant to Rf interference.
If uncertain about compatibility, put contained ematches on the altimeter channels, set the rocket upright and turn on everything. Let sit for an hour and if the matches don’t blow, the altimeter(s) don’t shut down or recycle you should be good to go. The other thing is the dog trackers might not be good for above 60k but I forgot the altitude limits so anyone may correct me if they know. Kurt


I have tested and flown the dog collar with the Aim Xtra, and also a Marsa 54 with no detectable issues from interference between the units.
 
I have tested and flown the dog collar with the Aim Xtra, and also a Marsa 54 with no detectable issues from interference between the units.

For the Marsa: With a high power (>1W) ~150Mhz you just need to keep the antenna or antenna transmission line away from the Baro sensor. A few to 3 inches is necessary. Baro sensors can double as crystal radio's sometime.
 
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