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
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
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
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
Instrumental Corrections are
already applied to correct the retracked
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
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.