Product 068
TOPEX/Poseidon Altimeter MGDR Generation B (NASA/PO.DAAC)

Source/sensor: NASA TOPEX altimeter and CNES Poseidon solid-state altimeter

Coverage: 22 September 1992 - 8 October 2005, global

Abstract: The TOPEX/POSEIDON MGDR (Merged Geophysical Data Record) contains global coverage altimeter data. This data set (MGDR-B) contains generation B data which replaces the generation A data set (JPL PO.DAAC Product 028). The objective of the TOPEX/POSEIDON mission, launched in August 1992 and currently operating, is to determine ocean topography with a sea surface height measurement precision of 3 cm and a sealevel measurement accuracy of 13 cm. The data product combines measurements from two altimeters, a NASA dual frequency (Ku and C band) instrument similar to the Geosat altimeter, and a French (CNES) instrument which is a proof-of-concept solid-state altimeter (Ku band). The MGDR data are distributed via the PO.DAAC public ftp site: /net/ftp/pub/sea_surface_height/topex_poseidon/mgdrb in an integer format usable on VAX, LINUX, PC and Macs. The data from both altimeters are in a common format.

Up until July, 2003, CD-ROMs were produced and available for ordering and also distributed to users. Each of these CD-ROMs contained three 10-day cycles of data including read software. The CD-ROM production was discontinued at volume 123, cycle 369. These CD-ROMs are no longer available.

It is emphasized that this product is considered research data because of the form and content of the data. The data consist entirely of files comprising headers and data records which contain over a hundred parameters for each second. It is swath data and there are no images. Analysis software is the responsibility of the user. Calculation of sea surface height from the altimeter range and environmental corrections is the responsibility of the user. The User's manual (Ref. 3) can be accessed here.

Comments: Waveform data are not included in the MGDR. Waveform data are provided in Product 035, the TOPEX altimeter sensor data record. This is a product that can be provided on tape by the User Services Office at the JPL PO.DAAC. It is not listed in this handbook because for most purposes it is not a recommended product. The handbook is available through the ftp podaac.jpl.nasa.gov/pub site as are some data files; this is provided to enable people to prepare for the reprocessed data.

Data Set Volume: Each Cycle on the PO.DAAC ftp site is gzipped and the volume is approximately 16 MB per cycle. Each passfile is approximately 350 KB.

Smallest order: 1 pass file via ftp

Std. product: ftp

Formats: Linux byte order, binary data with ASCII header on passfiles.