Geoida Help

Network of GPS baselines won't converge

Question: To coordinate two points I measured fast static GPS baselines from four fixed points and measured three distances and an angle to tighten it up. However the least squares adjustment will not converge - why?

Answer:

• Fast-static observations may not be the best choice for quality GPS baselines
• There are too many fixed control points compared to unfixed points. Always check internal consistency in a network by first setting only ONE point fixed - this will remove any strain created in the network due to poor control points. When all observation anomalies have been resolved, additional fixed points may then be set provided a reliable adjustment can be obtained
• Check for inconsistency between separate GPS baseline observations - if you turn on Loop Checks prior to Compute, a larger than expected maximum PPM may indicate problems with one or more baseline observations. Examination of the corrections made to the observations in the summary may indicate which baselines are at fault - experimenting by disabling observations in turn may derive a better adjustment result
• The variance-covariances of the baseline observations may be too tight and may do well to be relaxed by scaling - this will not make any difference to the adjustment if they are all rescaled by the same amount. By selectively rescaling observations according to their reliability an improved result may be obtained
• If the observations are of poor quality, then relaxing the Convergence Criteria in LS Options might help to get a workable convergence
• Disproportionate error ellipses on some points may suggest poor adjacent observations - try making a different point fixed to help isolate problems
• Including terrestrial levelling, horizontal angle and distance observations should improve the strength of the network and further help to isolate any problems