GPS

'Mark here' remembers where the model is now. From then on, 'Yards to mark', 'Heading to mark' and 'Max distance (yards)' are measured from there, and the maximum starts again from zero.
Mark the take-off spot before you fly: the page then tells you how far the model went, and which way is back.

'Zero alt' makes the present altitude zero.
Press it with the model on the ground: the altitudes then show the height above that spot. 'Max altitude (feet)' starts again.
Until you press it, they are above mean sea level.

'Telemetry ...' goes back to the Telemetry page.

The sensor

A PA1010D GNSS sensor on an I2C breakout board (address 0x10), wired to the I2C port of a Version 1 receiver.
The V2 receiver does not send GPS data.
The first fix can take a few minutes, so be patient.
'GPS fix' says Yes once it has one.
Accuracy improves with the number of satellites found.
Location is within about 2 metres in good conditions.
In poor conditions it is less accurate.

Units

Distances are in yards, speeds in mph, altitudes in feet.
Headings are in degrees, clockwise from due north.
'Heading' is the way the model is moving.
'Heading to mark' is the way from the model back to the mark.

Clock

The satellites carry very accurate atomic clocks, so the transmitter's clock is set from them automatically.