1. INTRODUCTION TO MGDR-B


INTRODUCTION TO MGDR-B

This new TOPEX/POSEIDON product provides the full mission data set in a new format with many data quality improvements brought about by the work of many scientists during the mission. It is a revised form of the MGDR-A CD-ROM set which covered data from the beginning of the mission, September 22, 1992 to April 23, 1996. All data from the begining of the mission will be released in MGDR-B. It incorporates the improved orbits, new tide models and some improved algorithms which have been developed during the mission. Section 1.3 contains a list of the significant changes.

Handbook Purpose

The purpose of this document is to provide a comprehensive description of contents and formats for users of the Physical Oceanography Distributed Active Archive Center (PO.DAAC) new Generation B Merged TOPEX/POSEIDON Geophysical Data Record (MGDR-B). This document also provides an overview of the TOPEX/POSEIDON mission and an introduction to measurements, corrections and errors. More information on data algorithms and sensors can be found in the POSEIDON and TOPEX project documents (see Appendix B).

PO.DAAC, which distributes TOPEX/POSEIDON data, is one element of the Earth Observing System Data and Information System (EOSDIS). The goal of the PO.DAAC is to serve the needs of the oceanographic, geophysical and interdisciplinary science communities which require physical information about the oceans. This goal is primarily accomplished through the acquisition, processing, archiving, and distribution of data obtained through remote sensing or conventional instruments.

TOPEX/POSEIDON data is also distributed by Archivage, Validation et Interpretation des donnes des Satellites Ocanographiques (AVISO). The French AVISO is a multi-satellite databank dedicated to space oceanography. AVISO is being developed in stages by the French space agency, Centre National d'Etudes Spatiales (CNES). They are developing a product similar to the MGDR distributed by PO.DAAC. The geographical arrangement for distributing the TOPEX/POSEIDON data products to the international scientific community are covered by a CNES-NASA agreement. Both centers will disseminate all MGDR data.

Handbook Overview

This is a combination of a guide to data use and a reference handbook, so not all of it will be needed by all readers. The list of the sections are marked with READ! or REFERENCE based on how critical it is for the reader to learn the contents. All users should read the sections marked READ!, even those familiar with TOPEX GDR and the AVISO produced Merged IGDR.

Section 1 provides background information about the MGDR and this document. It also contains a summary of significant changes in the new MGDR-B. READ!

Section 2 gives an overview of the TOPEX/POSEIDON mission. READ! if you are not familiar with the project, otherwise REFERENCE.

Section 3 introduces the reader to how to use the data. READ!

Section 4 is an introduction to altimeter algorithms and accuracy. READ!

Section 5 gives a description of TOPEX/POSEIDON MGDR contents in the PO.DAAC CD-ROM. It gives the content and format of all files. REFERENCE

Section 6 is a detailed description of each field of the header files on the CD-ROM. REFERENCE

Section 7 is a detailed description of each field of the MGDR files on the CD-ROM. REFERENCE

Section 8 is a detailed description of each field of the crossover files on the CD-ROM. REFERENCE

Appendix A contains acronyms. REFERENCE

Appendix B contains references. REFERENCE

Appendix C contains a list of Instrument Anomalies. REFERENCE

Appendix D contains a description of the VAX/VMS format. READ! if you are not familiar with the difference between VAX/VMS byte order and other byte orders such as UNIX, otherwise REFERENCE

Appendix E tells how to order information or data from PO.DAAC and lists related Web sites. REFERENCE

New Features in the MGDR-B

1. New Orbits - Both NASA and CNES orbit ephemerides have been substantially improved. (Sec 4)

2. Tide Models - Two new elastic ocean tide models are used in the MGDR-B (Sec 4) They are the University of Texas Center for Space Research model CSR 3.0.1 and the Grenoble Ocean Modelling Group model FES95.2.1. The loading tide is from the CSR 3.0.1 model. The flag assignments in Geo_Bad_2 have been changed (Sec 7).

3. The Precision Orbit Determination Team (POD) discovered a discrepancy in the pole tide on the TOPEX values which is corrected in the MGDR-B (Sec 4)

4. New Electro-Magnetic Bias Algorithm - (Sec 4)

5. Range - The altimeter range as found on the MGDR-B now has bias corrections included (Sec 3). The range has been corrected for the oscillator drift error (detected Summer '96).

6. Sigma-0 - the TOPEX sigma-0 for cycles 1-132 have been corrected for instrumental drift with the table from Wallops dated October 8, 1996.

7. Sigma0_C - NOTE: anyone using the TOPEX C band sigma0 must read Section 4.9 for a discussion of how to correct the MGDR value for atmospheric attenuation!

8. MSS & Geoid - New values of the Mean Sea Surface and the Geoid have been calculated by R. Rapp's group at Ohio State University. (Secs 4.3, 4.4)

9. Windspeed - The windspeed value on the MGDR-B is calculated for 10m whereas the MGDR-A value was calculated for 19.5 m. The windspeed algorithm has not changed. (Sec 4.10)

10. Geo_Bad_2 - Bit 0 is still the rain flag, but it now indicates a liquid water estimate greater than 600 microns rather than the previous limit of 1000 microns (Sec 4.7).

11. SWH_K and C are now given in centimeters rather than 0.1 meters and is now stored as a 2 byte unsigned integer rather than a byte integer.

12. Ind_Rtk - A new flag field indicating that the POSEIDON range is based on retracking. (Sec 7)

13. H_Ocs - The H_Ocs (ocean depth in meters) found in the MGDR-B corrects errors found in the values on the MGDR-A. Not very many values are affected.

14. Shorter MGDR-B record - spare fields have been removed (Sec 5).

15. Three cycles/CD - Three cycles now fit on a CD thanks to the reduced MGDR-B record size. (Sec 5)

16. Orbit files eliminated - The Precision Orbit Files on the MGDR-A have not been included on MGDR-B. If you need orbit ephemerides other that those found in the MGDR-B record, contact PO.DAAC (See Appendix E).

17. Paperless distribution - Coverage maps are on the CD in GIF and PostScript format. Documentation is also on the CD in ASCII text, HTML, Microsoft Word or Postscript format as appropriate. However, a hardcopy of this handbook will be sent with the first CD shipment to a user.

18. Web Browser - the documentation on the CDs is now accessable by using your web browser.

Conventions

Vocabulary

In order to reduce confusion in discussing altimeter measurements and corrections, the following terms are used in this document as defined below.

DISTANCE or LENGTH are generic terms with no special meaning in this document.

RANGE is the distance from the satellite to the surface of the Earth, as measured by the altimeters. Thus, the altimeter measurement is referred to as "range" or "altimeter range," not height.

ALTITUDE is the distance of a satellite or altimeter above a reference point. The reference point will usually be either the reference ellipsoid (as in the MGDR) or the center of the Earth. This distance is computed from the satellite ephemeris data.

HEIGHT is the distance of the sea surface above the reference ellipsoid. The sea surface height is the difference of altimeter range and satellite altitude above the reference ellipsoid.

Orbits, Revs and Passes

An ORBIT is one circuit of the earth by the satellite as measured from one ascending node crossing to the next. An ascending node occurs when the subsatellite point crosses the earth's equator going from south to north. A REVOLUTION or REV is synonymous with orbit.

The merged GDR data is organized into pass files in order to avoid having data boundaries in the middle of the oceans, as would happen if the data were organized by orbit. A PASS is half a revolution of the earth by the satellite from extreme latitude to the opposite extreme latitude. Thus an ASCENDING PASS begins at -66.04deg. and ends at +66.04deg.. A DESCENDING PASS is the opposite. The passes are numbered from 1 to 254 representing a full repeat cycle of TOPEX/POSEIDON data. Ascending passes are odd numbered and descending passes are even numbered.

See also Section 2.3.2.

Reference Ellipsoid

The "reference ellipsoid" is the first-order definition of the non-spherical shape of the Earth as an ellipsoid of revolution with equatorial radius of 6378.1363 kilometers and a flattening coefficient of 1/298.257.

Correction Conventions

All environmental and instrument corrections are computed so that they should be added to the quantity which they correct. That is, a correction is applied to a measured value by

Corrected Quantity = Measured Value + Correction

This means that a correction to the altimeter range for an effect which lengthens the apparent signal path (e.g. wet troposphere correction) will be computed as a negative number. Adding this negative number to the uncorrected (measured) range will reduce the range from its original value toward the correct value.

Examples:

Therefore,

Time Convention

Times are UTC and referenced to January 1, 1958 00:00:00.00, sometimes abbreviated UTC58.

A UTC leap second can occur on June 30 or December 31 of any year. The leap second is a sixty-first second introduced in the last minute of the day. Thus, the UTC values (minutes:seconds) appear as: 59:58 ; 59:59 ; 59:60 ; 00:00 ; 00:01 (In fact, these values will never be encountered on the MGDR because the input data has a gap of a few minutes around each leap second.)

In Section 5, reference will be made to UTC1 and UTC2. These are ASCII expressions of UTC times expressed using the following format:

Unit Convention

All distances and distance corrections are reported in millimeters.

Flagging and Editing

In general, flagging consists of three parts: instrument flags (on/off), telemetry flags (preliminary flagging and editing) and data quality flags (geophysical processing flags).

Instrument flags specify which instruments are on or which frequency is used.

Telemetry flags are first based on altimeter modes and checking of telemetry data quality. Only severely corrupted data are not processed. Flag setting is designed to get a maximum amount of data into the Sensor Data Records. Science data are processed only when the altimeter is in a tracking mode.

Quality flags involve residuals from smoothing or fits through the data themselves. Flag setting checks for gaps, exceeding limits and excessive changes.

Default Values

Data elements are recorded as integers in a limited number of bytes. Default values are defined as the maximum possible value for the storage type (e.g. signed 2 byte integer). They are given when the parameter value is unavailable (missing data, flagged data).

Default values are also given when the field has no meaning for the altimeter which is currently on. For example, all fields dealing with C-band data (e.g. Sigma0_C) contain default values when the POSEIDON Solid State Altimeter is on because it uses Ku-band only.

The following are the default values used for the different storage types and sizes:

signed integer 1 byte 127
unsigned integer 1 byte 255
bitfield 1 byte 255
signed integer 2 bytes 32767
unsigned integer 2 bytes 65535
bitfield 2 bytes 65535
signed integer 4 bytes 2147483647
unsigned integer 4 bytes not used