6.2.6 Measurement Biases and Errors

REFERENCE STATION BIAS

Differential GPS positioning (essential for overcoming the satellite dependent biases described above) requires that the coordinates of one of the stations upon which a GPS receiver is located be held fixed during data processing. This station is the base or reference receiver. Hence, in effect, only the relative position (or baseline components) relating the second receiver to the first (base or reference) receiver are estimated. Any error in these fixed reference station coordinates therefore will cause a bias in the solution.

How well must the fixed base station's coordinates be known? The impact of the bias on relative positioning can be considered in a similar manner as the satellite orbit bias. The same "rule-of-thumb" may be applied (see Figure below). The more accurate the relative position is required, the more precisely the base station's coordinates must be known in the coordinate reference of the satellite ephemerides. Hence, the accuracy of the base station coordinates must be commensurate with the accuracy of the GPS satellite orbits.

Approximate relationship between baseline length, accuracy and GPS satellite orbit error.

REFERENCE STATION BIAS   EFFECT: Analogous to an orbit bias, hence "rule-of-thumb" implies that 20m station coordinate uncertainty is acceptable for 1ppm relative accuracies.   OPTIONS: ADJUST all station coordinates -- not feasible with most commercial GPS software, leads to weak datum. AVERAGING of pseudo-range point position solution at reference station is usually satisfactory -- even under SA. TRANSFORM local coordinates to WGS84 using published transformation parameters -- adequate for 1ppm accuracy GPS surveys.

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© Chris Rizos, SNAP-UNSW, 1999