Issues with Short Sessions in GAMIT

• Effect of multipath and troposphere is higher because there is less averaging

• Shorter satellite tracks mean less time to separate the signatures in the data,

leading to higher correlations and thus higher uncertainties in coordinates and

ambiguity parameters, particularly the east component

• Less averaging of noise and higher geometric uncertainties make ambiguity

resolution more difficult; when it fails with short tracks, the uncertainties

become much larger

A 2-hr session between 12:00 and 16:00 (left plot) would be much more likely to resolve ambiguities than a 2-hr session between 20:00 an 24:00 (right plot)

Effect of multipath and atmospheric noise

Geometric affect of of limited sky tracks

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C1_07_(m)Theory_(m)C1_28_(m)Theory_(m)C1_26_(m)Theory_(m)C1_11_(m)Theory_(m)C1_02_(m)Theory_(m)

Range (m)

Time_Hrs

The rate of change of the observed phase between19:00 and 21:00 is much less than if the session extended to 24:00, making the coordinates more highly correlated with the ambiguity parameters (a vertical bias in the plots)

Continuous network in Italy used to test the effect of session length and network configuration on coordinate repeatabilities

Test site is PRAT

Firuzabadi and King (2011)

Time series for 2-hr sessions with 4 reference sites.

Note large sigmas on day 61 and outliers on days 62 and 63; with these removed the rms is 2 mm horizontal and 7 mm vertical

Precision vs Session Length for Network Processing

Bars show repeatability in position over 31 days of test site with respect to networks of 2 to 16 sites spanning 180-600 km.

With at least 4 reference stations, outliers were less than 5% for sessions of 2 hrs or more.

Precision vs Session Length for Single Baselines

Bars show repeatability in position over 31 days of a test site with respect to each of 7 sites 26-585 km away in single-baseline processing.

10% of the 1- and 2-hr sessions had large uncertainties and were omitted. 1-hr results degrade significantly for baselines longer than 100 km

GAMIT Settings for Sessions < 3 Hours

• Consider using 15s sampling

• Run sh_gamit with the –sesinfo option specifying the start time, sampling interval, and number of epochs

• If more than one session per day, run sh_gamit with the –netext option, using a different letter for each session

• Don’t decimate the preliminary or final solutions Decimation factor = 1

Quick-pre decimation factor = 1