Range

Range

The range is the distance from the satellite to the surface, based on the two-way time, under the assumption that the radar pulse travels at the vacuum speed of light (the slowing down of the pulse due to the atmosphere is handled by the different atmospheric corrections discussed next). The range is determined from two components: the (corrected) tracker range and the epoch obtained through retracking.

The tracker range is the distance equivalent to the two-way time between when the pulse was transmitted by the altimeter and when the window was opened to receive the returned pulse. The retracker epoch relates to the two-way time within the returned waveform that corresponds to the desired distance. For LRM-mode operations, this is the midpoint of the rising slope of the waveform. (See Overview of retracking for more information). 

In post-processing, the tracker range is corrected (more details here) to account for:

• The external group (that is the time taken for the signal to pass from the antenna to the receiver circuit, which may be temperature dependent)

• Ageing of the internal group (aspects of the receiver circuit that can be calibrated inflight, and any changes to the outgoing pulse).

• The difference in position between the satellite centre of mass and the antenna reference position (which changes in flight as fuel is used up)

• The doppler shift due to the satellite's motion

• The 'FAI' correction used to align the individual pulses within a 20 Hz receiving period.