How and Why do we have to calibrate a GPS instrument?

Question

The GPS mentioned are L2 Dual Frequency Receivers in RealTime Kinematic mode, with accuracy of 2cm horizontal and 3cm vertical at 20km baseline distance. The instrument is used to established government Primary Control Stations. At the moment, a Validation Network with 60k baseline is utilized to ensure that it is within its accuracy tolerance, although this procedure is NOT a CALIBRATION system.

Answer 1

The calibration that is necessary for this type of instrument is related to internal processing error, which is considered insignificant for most GPS receivers, especially consumer models which commonly have errors of 10 meters or more. For what you are describing it is necessary to use two or more receivers, a base station and rovers (the kinematic unit). Because these instruments are resolving to centimeter scales, the processing error can be significant.

A zero baseline calibration ensures that the receivers are collecting data correctly and that the vector processing software is arriving at the proper and equal solutions. The basic methodology of a zero baseline consists of multiple receivers collecting data through a single antenna. With two GPS receivers connected to the same antenna, biases that are external such as differences in the GPS clock calculations and the satellite clock, and calculations of the atmospheric path through the troposphere and ionosphere will cancel out during the data processing. The result is a zero baseline that assures the two units are processing the same and handling random error and noise in a similar manner. Without this calibration it is more likely that a systematic error will exist between the base station instrument and the rover instrument.

See zero baseline:
http://www.gmat.unsw.edu.au/snap/gps/glossary_r-z.htm

Consumer models are rarely used in combination and do not communicate with each other as the units you are describing, so it never occurs to most GPS users to calibrate. The consumer units aren't precise or accurate enough for it to matter. The units you are describing use two satellite frequencies from each satellite in order to improve processing while consumer units are single frequency. However, when working in the field with other users who have consumer grade GPS units, I find it is worthwhile to at least take a common reading to determine our range of error between units.

Answer 2

Normal consumer-grade GPS receivers require no calibration, the GPS signals themselves provide all the information required. However, the receivers in the question are evidently something different: the stated precision is considerably higher than consumer-grade gear. It may be that the calibration is to refine the frequency of an internal crystal oscillator (that being the only component which isn't digital in nature). The operating manual for the receiver in question should give an answer to this, and I'd be interested to see one.

Answer 3

A GPS, when started, will start searching for satelites. After a bare minimum of three (more often 10 or more) satelites are found, they can triangulate your position. The reason this is done is every satelite has a distance from your reciever to a known point. This creates a circle, and you can be anywhere on that circle. The 2nd satelite has you on a different circle, and those two circles intersect twice, and you can be at either of those two points. The third circle should, in theory, place you exactly. However, due to error and finding the third dimention (elevation), more circles are required, and more circles gets more accuracy.

Answer 4

Most GPS receivers do not require calibration. Receivers intended for extremely precise use conceivably might require adjustments to account for things like antenna position, local anomalies in reception, and so on. In general, though, GPS receivers are self-calibrating: after obtaining an initial fix, they will improve upon it as time passes (depending on their design).

Answer 5

well if you want it to work right read the manual. it should be explained there or call their customer number and they'll walk you though it