4 NAVIGATION

4.1 Navigation Overview - Susan Merle and Bill Chadwick

All ROPOS dives were navigated using long-baseline transponder nets with the Seascape navigation software. The navigation computer had three main inputs into Seascape to aid in ROPOS navigation: P-code GPS from the R/V Thomas Thompson, ROV depth data provided by the ROPOS sensor and the PS8000 data from the range meter. Transponders were already in place for the ASHES net and the North Rift Zone (NRZ) net. Deployed last year during NeMO'98, the expendable transponders should have five year lifetimes. The transponders for those two nets only needed to be enabled. A third net, on the South Rift Zone (SRZ), was added with three recoverable transponders. Unfortunately, due to hardware problems with the NOAA PS-8000, the calibration of the SRZ Net was poor and this net was only marginally useful.

Once the cage reached its final depth and ROPOS drove to the seafloor, the cage depth was manually entered into the Seascape program and was held constant, unless the wire out for the cage changed during the dive. The range meter was attached to the top of the cage, was hard-wired to the hydro lab and triggered by Seascape on the navigation computer. Cage and ROV fixes were generally scattered with RMS errors of about 30 meters. Navigation fixes are recorded in latitude/longitude and UTM x/y (in meters) in the log files and were processed by Susan Merle in the IDL programs navedit2 and navedit3 (written by Bill Chadwick).

In 1999 navigation was somewhat less reliable than in 1998. During several dives the 11.0 transponder in the ASHES net was accidentally disabled and this caused the 1999 navigation fixes to be shifted to the west or south of the 1998. Since '98 navigation was more robust, no old vent or marker positions were changed, even if the '99 positions did not agree with those of '98. After the '99 cruise a few of the mysterious navigation problems we had were identified. The NOAA PS-8000 on El Guapo was determined to have a bad microprocessor, which created problems with the SRZ net calibration. Another mystery solved concerned the fact that it seemed that transponder 11.0 in the ASHES net would shut itself off. It was discovered on Bill Chadwick's Cleft cruise in September '99 that the 11.0 had a disable code of C, not B like all the other transponders. This would not have been a problem, if the ROPOS transponder we tried to use, unsuccessfully, had not had an enable code of C. So, we would try to enable the ROPOS transponder, unwittingly disabling the 11.0 transponder in the ASHES net.

The dive plots will be a useful guide to determine navigation gaps, bad navigation, etc. When acoustic nav was not available GPS ship nav was used. Plotted positions are for the reference point, about 37 meters forward of the stern at the GPS antennae.

4.2 FINAL CALIBRATED TRANSPONDER POSITIONS

North Rift Net

ASHES Net

South Rift Net