ILRS RSG (Nice 2003)

Nice, April 2003

1. Recent Activities

1.1 Within the last months a contribution to IERS-Conventions concerning an update of the chapter for tropospheric refraction was furnished and submitted to Denis McCarthy.

1.2 As projected in the last RSG meeting at the Washington SLR workshop, a mapping-function analysis has been performed using 3 years of Lageos-2 normal point data (see appendix A and B). After orbit determination the nornmal point residuals were binned with respect to the elevation angle at which they have been recorded. The results reported by Cynthia Luceri and Erricos Pavlis show a clear improvement using the new mapping function, recently derived by Virgilio Mendes, instead of the Marini-Murray refraction model. Part of the analysis was also a test of the Saastomoinen zenith delay model applied to the Mendes' mapping function in it's original form and a modified form, enhancing the performance for other than 532nm wavelengths.

1.3 Philip Ciddor provided his refractive index subroutines (coded by Jean Rueger) for the study and comparison of zenith delay models.

2. New Zenith Delay Model

Concerning the modelling of the zenith delay the achievable accuracy as well as the limitations with respect to the input of surface meteorological data only remains to be determined. As it is not clear by now up to which extent non ideal gas behaviour of the atmospheric constituents affects the zenith delay, statistics on this quantity have to be gathered out of numerical weather prediction (NWP) or radiosound data. As anomalous and nonlinear dispersion effects may play a role for certain wavelengths, such as 1064nm, at the millimeter level, a seperate meeting with Yuri Galkin is scheduled for the end of July.

3. Horizontal Gradients

As there have been reports on low elevation tracking data taken between 10 and 20 degrees elevation, showing deviations on the order of 10 centimeters with respect to the MARINI-MURRAY refraction model, work remains to be done in the modelling and application of horizontal refractivity gradients. Statistical studies on the site dependent significance of horizontal refractivity gradients and their traceability in actual tracking data as well as surface meteorological data are underway. Global atmospheric models, data of satellite based remote sensing missions such as AIRS and limb sounding techniques are also considered as a data source for the determination of horizontal refractivity gradients, which has to be evaluated.

4. Combination Issues

As presented within the Analysis Working Group meeting and projected in the IERS retreat the data gathered by the services supporting the IERS will be subject to two distinct modes of combination: a) Combination of technique specific solutions (Combination at solution level) b) Combination at observation level. Whereas mode a) is in favour of near real time data processing (rapid availability of earth rotation parameters and station coordinates), which requires refraction data obtained in real time as well, mode b) permits the estimation of atmospheric refraction in the solution, since information of a vast multitude of observing techniques, i.e. different wavelengths, is accessible in the parameter estimation process. However mode a) imposes strict rules on how to obtain the refraction data: Combination at the solution level relies on uncorrelated solutions. Therefore a correlation of the ILRS solutions with the IGS solutions, e.g. by using refraction data obtained from GPS tomography, is to be avoided.

5. Governing Board Recommendations and Encouragements

Once the open questions mentioned in paragraph 2) are completed, the Governing Board will be addressed formally in order to present recommendations for optimum procedures to improve refraction modelling, as foreseen by the study group charter. To improve the assessment of refraction and horizontal refractivity models, the SLR network is encouraged to gather more low elevation tracking data as well as to optimize two colour satellite ranging techniques.