I think the GPS is closer. Baro altimeters using the standard atmosphere model will under-report the actual altitude when the weather warms up. It wasn't that hot this weekend, but it was still warmer than 39 degrees, which is the only temperature for which the standard atmosphere model is accurate at this elevation. The error is about 2 percent for every 10 degrees F.

If GPS is so incredibly accurate, can anyone tell me why every GPS handheld unit I have ever owned has wildly erratic altitude readings - even when it is sitting still?

I seem to recall the altitude coordinate is not NEARLY as accurate as the latitude / longitude, but it has been a while since I read the manual.

My GPS says my current elevation has changed over 300' over the last few minutes. Interesting, as I'm sitting in my study right now 😯 I'd have to believe the Garmin GPS used in the collars is similar to the Garmin GPS sitting on my desk right now?

I think I can answer this question: According to the Garmin Astro 220 Specs available from the specs page of their website the handheld unit contains a barometric pressure sensor to measure the altitude. This is due to the fact that four satellites are required to be in view simultaneously to provide altitude rather than the three which are required to provide a latitude and longitudinal location.

I don't have my Astro in front of me, but I'm pretty sure there is a feature which displays satellite altitude and a separate feature for barometric altitude. In addition, the unit seems to have a preference for displaying the barometric altitude on the graphs even though it reports that it is pretty continuously connected to eight or more satellites.

However, on the DC20 Specs page it does not indicate the presence of a barometric pressure sensor and it does indicate the ability to connect to up to 12 satellites simultaneously. This means that any altitude that is downloaded from a DC20 was obtained from satellite data and is subject to the tolerances displayed on their website which do vary with the velocity of the object.

Ill have to look more into it.
I do know however, that before I sent them in to get the 10,000 ft. bug fixed, they use to read right with the GPS, within 50 to 100 feet or so. Now they are 8 to 9% low.
.......... I STILL WANT MY EXTRA THOUSAND!

Its robbing me of $35.00 worth of propelant! 😡
Multiply that by several launches and you loose 2 or 3 launches.
Never looked at it that way have ya. 😉