On this page from the Corp of Engineers, rainfall and lake level use “GOES”, which I assume is an acronym for something. Any ideas? “GOES” is not Google friendly.
http://www.swt-wc.usace.army.mil/webdata/gagedata/FTCO2.current.html
I was going to suggest it might have something to do with the Geostationary Operational Environmental Satellite, but your link doesn’t seems to support that idea. (e.g. Hard to see how a satellite measures voltage).
This link suggests that it’s for the transmitter battery, i.e. it’s not part of the environmental data per se but auxiliary info to indicate that the electronics are operating normally.