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    24-Jul-2014
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2.3 Definitions and Conventions

The Radar Altimetry user community has developed a vocabulary of common terms which have a specific meaning. While these are (mostly) clear to experienced users the terms can be confusing to new-comers. This section explains common terms and conventions as used within this Product Handbook.

 

Range is the one-way distance from the satellite to the mean surface below. It is referenced to the satellite centre of gravity. It is the principal measurement of the radar altimeter. Range is estimated from the echo waveforms as part of the processing so-called retracking. This measurement is not the altitude, it is still only a measurement of distance.

 

Altitude is the distance of a satellite centre of mass above a reference point on the earth. The reference point will usually be on a geodetic reference frame or the centre of the Earth. The altitude is given by the orbit computation.

 

Height is the elevation of the mean surface observed at nadir above the reference ellipsoid. As a first approximation it is calculated from range and altitude (height = altitude - range).

 

Time delay is the basic on-board instrument measurement converted to standard physical units. It is the 2-way travel time of the radar pulse from the satellite to the surface and back. It is uncalibrated. The measurement is referenced to the centre of the range window: that is bin 63 (in the range 0 - 127) for the Ku band window, and bin 31 (in the range 0 - 63) for the S band window.

 

AGC, automatic gain control, is the setting of the on-board receiver attenuator as transmitted in the telemetry.

 

Sigma0 is the backscatter estimate calculated from AGC and the power level of the radar echo. The signal path attenuation, as calculated from the in-flight Calibration records, is applied. To compute an accurate sigma0 geophysical corrections such as liquid water attenuation correction must be applied.

 

Slope refers to the gradient of the leading edge of the radar echo, so called the leading-edge slope.

 

SWH is the significant waveheight calculated from the radar echo leading edge Slope.

 

Footprint is the area on the surface illuminated by the Radar pulse. The Altimeter boresite is pointed at Nadir and the antenna half-power beamwidth is 1.3 degrees. At a height of 800km this corresponds to a circular area 18km across. However the short duration of the radar pulse normally means that a much smaller area of illumination is seen by the instrument. This is often referred to as the Pulse Limited Footprint.

 

Array numbering convention: Arrays of parameters are numbered from 0 to n_elements -1. For example this means high rate measurements are numbered 0 to 19, and waveform bins numbered 0 to 127 for Ku-band and 0 to 63 for S-band.

 

Timing: The convention for the ENVISAT mission is to use a Modified Julian Day which is referenced to Universal Time from a datum of 1st January 2000.

 

Instrument source packet is a group of 20 elementary measurements packaged on-board and downlinked in the telemetry. It holds the basic science data.

 

Elementary measurements are the 20 measurements in the source packet.

 

Individual echoes, or individual waveforms, are the 1800 Hz un-averaged waveforms. No other Radar Altimeter provide individual echoes until RA-2.

 

Pass is one half revolution around the earth. Usually from minimum (resp., maximum) latitude to maximum (resp., minimum) latitude, in other words, from pole to pole.

 

Orbit is one revolution around the earth, when referring to amount of data. Else it refers to the positioning of the satellite; orbital altitude.

 

Sea level is synonymous with sea surface height (SSH).

 

Sea surface height is synonymous to height.

 

Sea surface topography, or dynamic topography, is the departure of the sea surface from an equipotential surface, the marine geoid.

 

Instrumental Corrections are already applied to correct the retracked range.

Range_Instr_Corr = Doppler_Comp_Update + Td_Flight_Cal + Td_Ground_Cal + Ant_COG

Using the Doppler Compensation correction, the In flight Calibration, Ground Calibration, and Centre of Gravity corrections.

 

Geophysical corrections are to correct the measurement for environmental effects (e.g. tropospheric, ionospheric) or remove geophysical signal of no interest (or even detrimental) to the application pursued (e.g. tides). These corrections are external to the measurement and come from other sources of data and models. As a rule the geophysical correction given in the Level 2 Products already hold the appropriate sign and are to be added to the range.

 

Flags are used to convey quality information or operating modes. They are usually set to zero to mean OK and 1 for not OK. The spare flag are set to zero. There may be exceptions, then a particular description of the usage of the flag is provided.

 

Default value: when a physically meaningful value cannot be computed a default value is provided. It is in most cases the maximum value of the field. There may be exceptions, then a particular description of the default value is provided.


Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry