I do the same thing in my yard. Even with pre-existing snow cover you can usually find an ice crust on its top under the new snow cover in order to obtain a measurement.
Model outputs that I have been seeing show the snow arriving as light in the predawn hours Sunday and then ramping up. Regarding the GPS do you really think it will verify?
Check out the precip types on that model (RRFS). The totals shown do not match that or reflect the temperatures aloft. The snow totals are way overstated. They are basically another NAM - maybe worse.