garmin foretrax: barometric altimeter versus what?

[ljwobker]

I'm considering getting one of the foretrax units to do some data logging during skydiving. (yes, seriously...)

 

I've messed around with the CS60x a little bit, which has a barometric altimeter -- this is actually NOT what I want, since things like exiting the aircraft and where the receiver is in relationship to the relative wind can have pretty significant impact on the short-term barometric pressure, and that introduces a lot of error into my data.

 

So two questions:

 

1) if I get a unit that does NOT have a barometric altimeter, does the unit still log elevation data?

 

2) IF the answer to #1 is yes, does it use the GPS elevation reading as the source data... and if the answer to THAT question is yes, do we know how accurate that reading is?

 

maybe asked another way... is the GPS elevation data the same accuracy as the latitude/longitude data (which I can get down to about +/- 20 feet or so, which is good enough for what I need.

 

--lj

[+GClouse]

I have a triton, and I disabled the barometer. It still logs elevation data.

However, for skydiving I suspect you will have a problem with the logged readings lagging due to the time it takes to calculate the position. If you are simply wanting an idea of accelleration and terminal speed, it might work, but the position log will probably read considerablky higher that the true altitude.

[MtnHermit]

maybe asked another way... is the GPS elevation data the same accuracy as the latitude/longitude data (which I can get down to about +/- 20 feet or so, which is good enough for what I need.

No, figure on Z being 2X of X & Y. It's a geometry issue, you need a satellite overhead for a good Z reading, that rarely happens. For X & Y you need a satellite on the horizon, infinite chance of that.

 

The barometer works great for altitude at walking speeds, but in free fall who knows. Their must be specialized data loggers for your needs, just that a consumer GPS isn't it.