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Notes on the Altimeter Performance

Two events during our cruise led to some work on the performance of the altimeter systems installed on ALVIN. These were 1) an inquiry from Bill Chadwick of NOAA PMEL to Ken Feldman of SSSG regarding differences between the altimeter readings and the center line of the Mesotech data from the NOAA cruise in July 1994, and 2) some differences we noticed during the descent down the transponder line during Dive 2930 between the altimeters and the Mesotech display.

Comparison of Alitmetry During Mesotech Survey

During Dive 2930 from approximately 1155L to 1250L we ran a Mesotech survey above the Bastille complex at a nominal altitude of 25 meters above the volcanic basaltic surface. Data from the two altimeters and from the center line of the Mesotech sonar were put onto a common time base and plotted:

[Comparison of Altimetry from Different Sensors]

In this plot, the x-axis is decimal time, the y-axis is uncorrected (for position of the instrument on ALVIN) height off of bottom, and the colored curves are for the Benthos altimeter (yellow), Datasonics altimeter (green) and Mesotech center line reading (blue). There appears to be a good correspondence between the newer Datasonics altimeter and the Mesotech sonar, with the older Benthos altimeter reading low.

Altimeter Comparison

A more extensive comparison of the two altimeters was made, using all data from Dive 2930:

[Comparison of Two Altimeters]

Over much of the range the Datasonics altimeter reads higher; based on the comparison of the three instruments above, this is the correct reading. The discrepancy is approximately 20%. However at lower values there appears to be a discontinuity:

[Comparison of Two Altimeters Near Origin]

with both instruments in agreement at altitudes below 13 meters.

The files related to this comparison can be found in $ZEPHYR/data/altimetercomp.

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