GNSS Checks & Calibrations

[galindo]

Hey Everyone,

I'm still pretty new with GPS so I was hoping for some advice.

Recently I have gotten a Trimble R10 assigned to me with my new company and I noticed it's been dropped a few times. No obvious errors have happened with it while performing rapid static observations, but how would I know if it's still performing at the same quality?

What kind of checks/calibrations do you prefer to do with your GPS equipment and how often? Also when performing your own static observations are there any additional checks you do before it starts measuring?

Just trying to figure out the best field practices. Would appreciate the knowledge!

[Jim Frame]

GNSS receivers are largely solid-state devices, with no moving parts other than switches, cable/battery connections and port covers. An antenna that's been subjected to enough mechanical abuse *might* get misaligned, so it would be worth checking out. Probably the simplest way is to compare to results returned by a known-good receiver positioning the same point as close in time as is practical. Another way is to take multiple observations of the same point, rotating the antenna by 180° between observations. Length of observations will depend on vector length and suspected extent of damage; with a short vector (<1 km) and high damage probability, I'd probably go for 4 minutes (RTK) or 10 minutes (rapid static). If you're hoping to pick out millimeters, I'd up the rapid-static time to 20 minutes and skip RTK entirely. Naturally, you'd want to pick a wide-open location for the test observations, with no obstructions or multipath reflectors nearby.

I've only had to do this once in 20 years, after an Ashtech Geodetic III ("Whopper") on a 2-meter tripod blew over into a ditch. The overall drop was about 10 feet, but the soil in the ditch was relatively soft. I couldn't detect any change after testing.

[galindo]

Thanks Jim! From the looks of this receiver it seems it fell with a 2-meter tripod in the street!

Quick follow up question... Have you noticed large differences with (rapid) static surveys with an antenna vs. without one? An antenna would seem ideal, but not everyone prefers this method for convenience.

[Jim Frame]

Unless a site is subject to significant multipath reflection, I wouldn't expect any significant difference between using an external antenna and the receiver's internal antenna. Antenna design has come a long way in the last 20 years, as has signal processing firmware. Unless you're trying to wring the last millimeter out of your survey (which means long static sessions in wide-open sites), I wouldn't bother with an external antenna with a modern receiver.