TOPEX/POSEIDON SATELLITE - ALTIMETER DATA

PO.DAAC (NASA, USA)

Announced availability: 17 August 1993

Data Access

TOPEX/POSEIDON Merged Geophysical Data Record (MGDR) data are available upon request from the Physical Oceanography Archive Center (PO.DAAC) at the Jet Propulsion Laboratory.

Background

On 10 August 1992, the joint US/French altimetric satellite TOPEX/POSEIDON was launched on an Ariane rocket from Kourou, French Guyana. The satellite was maneuvered to an orbit with a ten-day repeat ground track of 127 revolutions. The equatorial track spacing is approximately 315 km. The height and inclination are 1335 km and 66.04 degrees, respectively. The objective of the mission is to determine ocean topography with a sea surface height measurement precision of 3 cm and a geocentric sea-level measurement accuracy of 13 cm.

The satellite has two nadir-looking altimeters on board. One is the NASA instrument, which is a dual-frequency (Ku and C band) altimeter whose general design is similar to the GEOSAT altimeter. The other is the French (CNES) instrument, Poseidon, which is a proof-of-concept solid-state altimeter which operates only in Ku band. The altimeters share the same antenna and cannot operate simultaneously. Maximum ocean coverage requires data from both altimeters be used. Other instruments onboard are the nadir-looking TOPEX Microwave Radiometer (TMR) which provides radiometer data used to make wet troposphere corrections to the altimeter range and three systems used for precision satellite tracking: Laser retroreflectors - a passive system of corner reflectors used for ground-based laser tracking which is used to produce the NASA precision orbit; DORIS, a tracking beacon using the worldwide system of French ground stations which is used to produce the CNES precision orbit; and a Global Positioning System (GPS) receiver, an experimental instrument using the constellation of GPS satellites.

Range is determined by measuring the round-trip time from the antenna to the sea surface. The altimeters' Ku carrier frequency is 13.6 GHz with a chirp pulse bandwidth of 320 MHz. The pulse limited footprint diameter is 2 km for calm seas and 7 km for 5 m Significant Wave Height (SWH).

Data File Information

MGDR contents
The MGDR contains data from both altimeters in a common format. Each record contains the time-tagged, geolocated range and altitude, instrument and environmental correction and other instrument measurements. Sea surface height is calculated by subtracting the range from the satellite altitude. The sea surface height can then be corrected for instrument and environmental effects by selecting and adding the corrections from the MGDR. The environmental corrections include wet (from both TMR measurements and weather models) and dry tropospheric path delay, ionospheric path delay and electromagnetic bias. The altimeter also measures SWH to within 0.5 m or 10% of SWH and the radar backscatter coefficient to within 0.25 dB precision, 1.0 dB absolute. Estimates of the wind speed (based on radar backscatter), geoid, mean sea surface and various tides are also present. brightness temperatures at nadir from the TMR at 18, 21, 37 GHz are given.

MGDR data format and medium
The PO. DAAC MGDR is distributed on CD-ROMs containing two 10-day repeat cycles of merged data. Each cycle is in its own subdirectory and contains a single ASCII header file which describes the cycle and up to 254 pass files. A pass is one half revolution and extends from lowest to highest latitudes for ascending passes and vice versa for descending passes. Each cycle contains 254 passes, although certain passes may not be present because of maneuvers or data dropouts.

Parameters include:

  • Time
  • Latitude
  • Longitude
  • Altitude
  • Values and RMS of the following:
  • Range
  • SWH
  • Radar backscatter
  • Corrections to instrument, EM bias, wet troposphere, dry troposphere, ionosphere
  • Tides (Cartwright and Ray, and modified Schwiderski)
  • TMR brightness temperatures
  • Geoid
  • Mean sea surface
To order Topex/Poseidon MGDR please contact:

User Services Office
Physical Oceanography Archive Center (PO.DAAC)
Jet Propulsion Laboratory
M/S 300-320
4800 Oak Grove Drive
Pasadena, CA 91109
USA

email: PO.DAAC.JPL.NASA.GOV on INTERNET
Phone: (818) 354-2296 or (818) 354-0906
FAX: (818) 393-6720 (Attention: PO.DAAC User Services Office)
TELEX: 675429 (Attention: PO.DAAC User Services Office)

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