The file is a FORTRAN 77 unformatted direct access file. Therefore, there are no FORTRAN control words contained within the file. The file was created on a HP 735 and is IEEE binary. Each record contains information for a single SLA observation. Each record is 56 bytes in length containing seven double precision real words.
| WORD | |
| Mission Elapsed Seconds From 96/01/11 09:41:00.015. | |
| Geodetic latitude of the return point in degrees. | |
| East longitude of the return point in degrees. | |
| Height above the TOPEX ellipsoid in meters. | |
| Earth fixed X coordinate of return point in meters. | |
| Earth fixed Y coordinate of return point in meters. | |
| Earth fixed Z coordinate of return point in meters. |
The TOPEX/Poseidon (T/P) terrestrial reference system has been used for processing the Shuttle orbits and, therefore, geolocating the SLA data. The TOPEX/Poseidon standards are primarily based on the IERS 1992 standards outlined in Technical Note 13 [McCarthy, 1992]. The precise SLR station positioning from LAGEOS ranging provides the realization of the reference frame [Tapley et al., 1994]. This realization is at centimeter level of accuracy and comparisons with VLBI indicate subcentimeter horizontal agreement and centimeter agreement in height [Tapley et al., 1994]. Positioning with respect to the Earth's center of mass is estimated to be at the few millimeter level [Tapley et al., 1994]. This frame is in agreement with the ITRF at the centimeter level.
Both Cartesian and curvilinear geodetic coordinates are provided in the SLA distribution data set. The reference ellipsoid used is the TOPEX/POSEIDON standard reference ellipsoid [Tapley et al., 1994]:
The SLA distribution data sets include Cartesian Earth Centered Fixed (ECF) coordinates to allow the user to transform to any particular datum. However, this must be done with care since each particular reference system may have many different realizations within its accuracy [McCarthy, 1996]. For instance, a seven parameter tranformation from ITRF90 to WGS84 is provided in [McCarthy, 1992]. This transformation is at the meter level while the latest GPS based WGS84 realizations are coincident with ITRF at the less than 10 centimeter level [McCarthy, 1996]. In accordance with the 1996 IERS conventions, the transformation parameters should be obtained by the user for their particular application.
Assuming one is satisfied with the fact that their current WGS84 data is referenced to a GPS realization and the less than 10 cm agreement is sufficient, WGS84 geodetic coordinates may be obtained by simply referencing the SLA T/P ECF coordinates to the WGS84 ellipsoid. A program that computes WGS84 ellipsoid referenced geodetic coordinates from the SLA distribution file ECF coordinates has been provided (wgs84elp.f).
Finally, a program to read the SLA distribution data set and simply write the data in an ASCII format is also provided. The program is limited to the first 100 records, but can obviously be modified to select any amount of data (sladmp.f).
REFERENCES
McCarthy, D.D. (Ed.), IERS Standards 1992, IERS Tech. Note 13, Obs. de Paris, November, 1992.
Tapley, B.D, Ries, J.C., Davis, G.W., Eanes, R.J., Schutz, B.E., Shum, C.K., Watkins, M.M., Marshall, J.A., Nerem, R.S., Putney, B.H., Klosko, S.M., Luthcke, S.B., Pavlis, D.E., Williamson, R.G. and Zelensky, N.P. "Precision Orbit Determination for TOPEX/POSEIDON," Journal of Geophysical Research, Vol. 99, no. 12, December, 1994, pp. 24383-24404.
McCarthy, D.D. (Ed.), IERS Standards 1996, IERS Tech. Note 21, Obs. de Paris, July, 1996.
Responsible NASA official: David Harding