8.3.2 Ambiguity Resolution: The Key to Modern GPS Surveying

AMBIGUITY RESOLUTION REVISITED

For all modern GPS surveying procedures, with the possible exception of the "reoccupation" technique, ambiguity resolution plays a critical role. In the case of "rapid static" techniques the aim is to consistently resolve ambiguities for very short observation sessions. For the "stop & go" and "kinematic" techniques, ambiguity resolution is a precondition for "carrier-range" positioning. Although in the case of these two techniques the method of resolving ambiguities is not important, obviously if it can be done as quickly as possible then the survey operation (using "carrier-range") can be commenced sooner. Further time savings can be had if the ambiguities could be resolved while the receiver is tracking satellites and moving to the first survey point, in the so-called "on-the-fly" ambiguity method. It is necessary to re-examine the ambiguity resolution process from these perspectives.

The following strategies can be used to decrease the length of the observation session and increase the reliability of the ambiguity resolution process:

• Keep baselines short!

  Chances for ambiguity resolution are much higher for baselines < 20km in length.

   

• Track as many satellites as possible and ensure good satellite geometry.

  Generally a minimum of five visible satellites is recommended, with a low PDOP.

   

• Improve ambiguity search, selection and testing algorithms.

  Better statistical testing, improved search techniques such as Least Squares searching and Ambiguity Function technique, better apriori baseline knowledge, sequential or boot-strapping procedures.

   

• Use dual-frequency observations.

  It is possible to boot-strap from resolved "wide-lane" ambiguities to L1/L2 ambiguities.

   

• Use precise pseudo-range data.

  Replacing the known geometric range quantities in eqn (8.2-2) by measured pseudo-ranges, leading to eqn (8.2-1).

   

• Use apriori information such as known baseline length.

  If baseline components are known then the double-differenced ambiguities can be directly estimated from eqn (8.2-2).

The first two strategies are essential preconditions for reliable ambiguity resolution, but are largely beyond the control of the surveyor. The last strategy is only useful for redetermination of the ambiguities when cycle slips occur as the GPS antenna is moved from one survey point to another, in the "stop & go" technique (section 5.5.4).

The ambiguity search, selection and testing algorithms for the "rapid static" and "kinematic" techniques are now to be re-examined.

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© Chris Rizos, SNAP-UNSW, 1999