IGS Analysis Center Workshop - IGS - NASA

3.4 Format of submitted solutionsWe acknowledge that there is work in progress by several individuals on working towardsa universally acceptable solution format (J.T, Freymueller, T. Herring, R.W, King, and R.Noomen, private comm.). Rather than explicitly define a format, we prefer at this point tolist the information content that we think the file ought to contain. For now, we discussonly GPS solutions, but it should be a relatively small step to allow, for example, VLBI,SLR, LLR, trilateration, and triangulation solutions. For all techniques, our previouscomments on the importance of fiducial-free solutions are relevant (for example, no datumconstraints in the case of traditional geodesy). The following guidelines have been writtenfor representation (a) above, but can be easily adapted for representations (b) and (c). Werecommend that:(i) The format be ASCII, with up to 80 characters per line.(ii) The covariance matrix be represented as an upper triangular correlation matrix wherethe diagonal elements are the standard deviations. The upper triangular array is written outcolumn by column (write column i for all rows 1 to i before moving to column i+ 1 ) sothat the position of the matrix element is independent of the number of parameters. Sinceparameters can be very correlated in free-network solutions, correlation coefficients shouldbe quoted to 15 significant digits.(iii) Each component of the estimate vector be the w estimate, meaning that it is the apriori plus the delta estimate. This approach is attractive since it is likely that differentgroups will use different a priori values, and we only need to know the full estimate whencombining solutions. (assuming they are close enough for validity of linearity), and (q%when a new station comes on line). For the record, a priori values and their constraints (apriori standard deviations) should also be stored in the file. This might be useful, forexample, if it is suspected that the basic assumption of linearity might be violated, or if apriori constraints might have had a significant and undesirable effect.(iv) The estimate refer to the monument, except for those cases where the ARP (antennareference point) is defined to be the monument.(v) The basic unit be the meter for coordinates, radians for X and Y polar motion, andseconds for LODR.(vi) Each file include for every station identifier the eccentricity vector from the monumentto the ARP, and the assumed LC phase center offset, and the starting date for thisinformation (to allow for changes in antennas). This information needs to be givenexplicitly because eccentricity vectors and phase center offsets may be in error, may beinconsistent between groups, or may get updated by new surveys or antennameasurements. In addition, a flag can be optionally set to indicate any removal ofredundant information (as explained towards the end of item vii below).(vii) Standard 6-character station identifiers be used in the station coordinate parameternames. Characters 1-4 should uniquely identify the monument. Characters 5-6 should bean “occupation number,” used to force separate solutions for different epochs. In thecontext of permanent networks, the “occupation number” needs to be changed only if thestation undergoes co-seismic displacement, or if the antenna is moved or changed. In thetraditional context of field campaigns, this number might be used to identify experimentnumber. Note that every 6-character station identifier must have the information specifiedin item (vi) above. Note also, that if the antenna offset is changed in a known way, thenthis constraint can be applied as a last step. In the limit that the offset change is perfectly76