Improvement in PWV estimation from GPS due to the absolute calibration of antenna phase center variations

Abstract

Climatology of column-integrated atmospheric water vapor over Spain has been carried out by means of three techniques: soundings, sun photometers and GPS receivers. Comparing data from stations equipped with more than one of these instruments, we found that a large discontinuity occurred on November 6, 2006, in the differences between the data series from GPS receivers and those from the other two techniques. Prior to that date, the GPS data indicate a wet bias of 2–3 mm for all stations when compared with sounding or photometer data, whereas after that date this bias practically reduces to zero. The root mean square error also decreases about half of its value. On November 6, 2006, the International GNSS Service adopted an absolute calibration model for the antennas of the GPS satellites and receivers instead of the relative one. This change is expected to be an improvement, increasing the accuracy of station position determination and consequently benefiting post-processing products such as zenith total delay from which the atmospheric water vapor content is calculated.

Appendix

Definitions of statistics

$$ {\text{BIAS}} = {\frac{{\sum\nolimits_{i = 1}^{N} {{\text{PWV}}_{i}^{{({\text{GPS}})}} - {\text{PWV}}_{i}^{{({\text{Sound}}/{\text{Photo}})}} } }}{{N_{\text{data}} }}} $$

$$ {\text{RelativeBIAS}} = {\frac{{\sum\nolimits_{i = 1}^{N} {2 \times {\frac{{{\text{PWV}}_{i}^{{({\text{GPS}})}} - {\text{PWV}}_{i}^{{({\text{Sound}}/{\text{Photo}})}} }}{{{\text{PWV}}_{i}^{{({\text{GPS}})}} + {\text{PWV}}_{i}^{{({\text{Sound}}/{\text{Photo}})}} }}}} }}{{N_{\text{data}} }}} \times 100 $$

$$ {\text{RMDA}} = {\frac{{\sum\nolimits_{i = 1}^{N} {2 \times {\frac{{\left| {{\text{PWV}}_{i}^{{({\text{GPS}})}} - {\text{PWV}}_{i}^{{({\text{Sound}}/{\text{Photo}})}} } \right|}}{{{\text{PWV}}_{i}^{{({\text{GPS}})}} + {\text{PWV}}_{i}^{{({\text{Sound}}/{\text{Photo}})}} }}}} }}{{N_{\text{data}} }}} \times 100 $$

$$ {\text{RMSE}} = \sqrt {{\frac{{\sum\nolimits_{i = 1}^{N} {\left( {{\text{PWV}}_{i}^{{({\text{GPS}})}} - {\text{PWV}}_{i}^{{({\text{Sound}}/{\text{Photo}})}} } \right)^{2} } }}{{N_{\text{data}} }}}} $$