Sect_1.4.8

1.4.8 Positioning Strategies

SOLUTION STRATEGIES FOR BIASED RANGES: SUMMARY REMARKS

Solutio n strategies for various classes of biased ranges.

Receiver
Bias ( A ) Satellite
Bias ( B ) Receiver-Satellite
Ambiguity ( C )

Change with Time? YES YES NO

Solution Strategy:
1. Multi-satellite?
2. Multi-Receiver?
3. Multi-Epoch?

YES
NO
NO

NO
YES
NO

NO
NO
YES

	Receiver Bias ( A )	Satellite Bias ( B )	Receiver-Satellite Ambiguity ( C )
Change with Time?	YES	YES	NO
Solution Strategy: 1. Multi-satellite? 2. Multi-Receiver? 3. Multi-Epoch?	YES NO NO	NO YES NO	NO NO YES

< BR> The following conclusions can be drawn from the above Table:

If only bias "A" is present in the measurements, then the strategy is to observe more than one satellite at the same time. In the case of 3-D positioning problems, the minimum number of satellites to be observed is four. This ensures real-time positioning capability, and is the basis for POINT-POSITIONING (or absolute positioning) using GPS in the NAVIGATION MODE. This mode is the one commonly used for positioning a moving receiver to moderate accuracy levels. The solution for the 3-D coordinates of a single receiver, and its receiver (clock) bias is known as the "Navigation Solution" (or navigation "fix").
If the biases affecting the range measurements are of type "A + B", then the strategy is to observe multiple satellites simultaneously from two or more receivers and derive the DIFFERENTIAL POSITION. When applied to GPS navigation this is referred to as the DIFFERENTIAL NAVIGATION MODE as it is essentially an instantaneous "fix", and can be implemented in real-time if a communication link exists between the receivers.
If the range biases are of the variety "A + B + C", an extended observation "session" is required (as well as multiple satellites tracked simultaneously from two or more GPS receivers) to "resolve" the ambiguity bias. This is the normal GPS SURVEYING MODE associated with carrier phase measurements (section 3.2.3).

For GPS, a discussion of the relative magnitudes of these biases and the precisions associated with the range-like measurements possible with GPS receivers is given in section 6.2.2.

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