Tractor GPS puzzler for you

I'm still madly trying to get beans in. The wheat is ready of course. So today I put my partner on the combine and she cut while I planted. Of course the combine broke down twice. Nothing major.

Here's the puzzle. I use a Raven Cruiser 2 GPS to plant. I plant with markers as well. Basically I do my headlands and then snap the line with the GPS. I do this so spraying is easy and I can set that GPS to follow this one. After that straight line is set I just use the markers. I can plant much straighter with markers but leave the GPS running to track acres for seed refill. Both times the combine broke down I idled the tractor down, left the GPS and seed vacuum running, and went to fix the combine. Took a good chunk of time each time. When I left the tractor it was in the neighborhood of .2-.4 off of my snapped line. I have never known whether that's degrees or what - guys with GPS units will know what I mean. It's the deviation from parallel with your straight start line. Anyway, when I came back each time it was suddenly 2.2 - 2.4 off. I ignored it that pass and used my markers and within two passes it was back to normal - the whole numbers had corrected themselves and we're back to partials. It seemed the longer I was gone the more off it was but always in whole numbers.

I was totally puzzled. I'm thinking maybe it's because of the Earth's rotation. Is that realistic? The tractor is in a different position in "space" even though it hasn't moved. Plausible?

The satellites your GPS units were keying off of moved while you were away. That changed how far your GPS unit thought it was off of the line you had it programed for. It corrected itself after you started planting again. Was the time interval of your repairs about the same??? IF it was then the "drift" would have been about the same each time.

This "drift" is why your have to have more accurate GPS systems for many operations where you need sub-inch accuracy. This is why GPS units that need to be real accurate have a set land beacon for the system to calculate the drift out of its readings.

The GPS satellites don't "drift" over the short term. Atmospheric conditions mess with the signal, causing error.

Subscription $$$ GPS monitors the signal from the ground, from known locations, and generates a correction signal in real time.

"This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere."
WAAS

Bob it all depends on which satellite the GPS unit is working off of. Some do drift a lot in a days time. Their orbit is not perfect and it shows up in the signal they send. My son has a cheaper GPS unit in the fertilize spreader. If you turn it on and leave it set in one spot with the mapping feature turned on, it will slowly draw a circle over a 24 hour period. The units is not moving but the satellite is not perfectly above it.

Rtk uses the cell network- the tower is a known fixed location.