IGS Analysis Center Workshop - IGS - NASA

acksubstitution of the orbital parameters has not taken place for the global solution.Therefore, in order to ensure that the two free-network sister solutions have the sameorientation, it is necessary to impose minimal rotational constraints on the global networkcovariance matrix (not the estimates!) prior to combining orbital parameters. This willensure that the orbits will not rotate as a result of the combination. Perhaps this should bean option, and another flag could indicate whether or not reference frame orientation isconsistent between the sister solutions. If not, the NAC’s can still use the 3 commonstations to ensure orientation consistency.3.6 Global calibrations for clocks and tropospheric delayClock Calibration Clock solutions produced by GAC’S have several uses for RAC’S. (i)Some software packages initialize satellite clocks as the first step, and it would bepreferable to use the most precise solutions available. (ii) Some packages (e.g., GIPSY)use a reference clock in order to realize coordinate time, and in this case it would be mostuseful if a good nominal clock solution were available for one of the “common stations” inthe regional network. This will ensure consistency between coordinate time for theregional solution and the coordinate time for the global solution (i.e., the orbits will appearin the right place at the right time!). We therefore recommend that both satellite clocks andstation clock solutions be available to RAC’s, so that both philosophies of data processingcan be easily accommodated. Minor modifications of the major software packages maybenecessary to read in these precise clock solutions and perform interpolation as necessary.. ,T oposphe~c call- It would also be beneficial for RAC’S to calibrate troposphericd~lay at one of the co~on stations, based on GAC solutions. This is especially importantfor smaller regional networks, where only a weak solution for absolute tropospheric delaycan be obtained from the regional data alone. Two possible approaches to troposphericcalibration are given. (i) GAC’s provide a line of sight tropospheric delay for every datapoint from a given station, These calibrations could be provided along with the data recordin the RINEX file, or alternatively calibrated RINEX files could be provided; (ii) the zenithtropospheric delay is given (both wet and dry components) at fixed time intervals (say,every 30 rein) which can then be interpolated by the RAC, on the understanding that theRAC applies the appropriate wet and dry mapping fimctions.We favor option (ii) since it becomes much easier to compare tropospheric calibrationsbetween GAC’S. We note that tropospheric estimation is done very differently by differentGAC’S, for example; some have a new constant zenith troposphere parameter over a fixedinterval of hours, some estimate the zenith troposphere as a random walk process, and it isallowed to vary every data epoch. It is unlikely that differences in mapping functions willcause a problem for data above 15 degrees elevation. These differences should be muchsmaller than the ability of the regional GPS data themselves to resolve the absolutetropospheric delay.3.7 “Noise-Calibrated Pseuabrange”The following discussion illustrates that much research can be done with regard tointegrating regional and global solutions. We should acknowledge that the technical issuesare quite complicated and techniques are likely to evolve and improve over the next fewyears, so I~S must be careful not to fix its mode of operation prematurely.The concept of GAC’S producing “clean” data has been around for a while, but “clean” inthis context has meant free of cycle slips and outliers. We can extend this concept bynoting that post-fit residuals from GAC solutions are very good estimates of the actual data,78