# How to determine altitude where altimeters fail?

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#### Lucas Thelen

##### Member
So above the majority of the atmosphere, altimeters that use barometric pressure fail, so barring regulation GPS modules, that generally don't work on extremely high altitude flights, how do you determine the altitude of your flights?

#### jderimig

Up to 50km, gps. Above 50km, space rated gps.

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#### Lucas Thelen

##### Member
Up to 50km, gps. Above 50km, space rated gps.
Thanks, and apparently.

#### Wallace

##### Well-Known Member
If I remember correctly, if, it's around 5K over the hardware cost to "attempt" to have a gps chip unlocked for anything over 60,000'. Not me, just life...

#### Lucas Thelen

##### Member
If I remember correctly, if, it's around 5K over the hardware cost to "attempt" to have a gps chip unlocked for anything over 60,000'. Not me, just life...
Though I have found an interesting way to pick up GPS satellite signals using an SDR radio. Maybe there's a way to decode it with python...

#### jderimig

Yes, you can fly an sdr and record the GPS carrier. You need a large flash drive. Then extract the position data on a PC running gnss-sdr.

#### JimJarvis50

##### Well-Known Member
It won't be long before moderately priced gps units are available that don't lock up at 50K meters.

Jim

#### jderimig

It won't be long before moderately priced gps units are available that don't lock up at 50K meters.

Jim
Yup. ;-)

#### Lucas Thelen

##### Member
Yes, you can fly an sdr and record the GPS carrier. You need a large flash drive. Then extract the position data on a PC running gnss-sdr.
If one can pump the data into a Raspberry Pi, you could probably calculate the data live, if you keep updates to once a second or so. I know GNSS-SDR can be compiled to embedded platforms.

#### jderimig

If one can pump the data into a Raspberry Pi, you could probably calculate the data live, if you keep updates to once a second or so. I know GNSS-SDR can be compiled to embedded platforms.
Conceivably yes but there is alot of optimization tuning required to get gnss-sdr to work. Its one thing to acquire and lock a stationary gps receiver and another to do the same with a moving receiver and antenna.

If you have a recorded signal you can try all the options on the ground until you find a way to decode the signal.

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