3 NAVIGATION OVERVIEW AND POSITION INFORMATION

3.1 Navigation Overview for NeMO 2000 - Susan Merle, Bill Chadwick, Julia Getsiv, Lisa Crowder

For the NeMO 2000 Cruise we made use of the R/V Ron Brown 's extendable through hull transducer pole. While in Newport prior to the cruise, the NOAA range meter was mounted on the pole so it could be used for transponder calibrations. ROPOS also had a spare range meter that was mounted on the cage. The extendable through hull transducer pole method eliminated the need to use "El Guapo" for calibration of transponder nets. The bad micro-processor in the NOAA PS-8000 that was a problem last year was replaced prior to the 2000 cruise and worked well this year throughout the cruise.

As in previous years, 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 Ron Brown, ROPOS sensor data and the PS8000 data from the range meter. Customized output files were created to save additional ROV sensor information for Imagenex processing (such as depth, altimeter, gyro, pitch and roll).

Transponders were already in place for the South Cleft, ASHES and the North Rift Zone (NRZ) nets. Deployed in 1998, the expendable transponders should have five year lifetimes. The transponders for those three nets only needed to be enabled. A fourth net, at the Southern Pillow Anomaly at 45° 52'N was added with three recoverable transponders.

One issue that was more problematic in 2000 than in the past was "cage-motor noise". When the cage motor was on navigation was sporadic, at best. When the cage motor was turned off nav improved dramatically. This year the ROPOS crew did not want to turn the cage motor off very often, as they were afraid they wouldn't be able to start it back up again because of other changes related to their new cable. There were also problems with deckset errors which made it necessary to quit out of nav and cycle the PS8000, usually resulting in good ranges when the program was restarted. For the last 3 years we have had a persistent problem with the RMS errors being inexplicably much higher (RMS=12-25) on the ROPOS fixes than on the cage fixes (RMS=1-2). The range legs to ROPOS are all consistently long, which is problematic because it causes fixes to jump large distances depending on which transponders are heard. The low cage RMS suggests the transponder calibration is fine, plus we have this problem in all our nets, and we have double checked all the transponder parameters. We do not know if this is a hardware problem (perhaps if the relay on ROPOS actually has a different turn around time than indicated, for example) or if it is a software problem somehow.

Navigation fixes are recorded in UTM x/y (meters) in the log files and were processed by Julia Getsiv using the IDL program navedit2 (written by Bill Chadwick). The processed navigation was brought into GMT (generic mapping tools) by Susan Merle for dive map production. The dive maps are a useful guide to determine navigation gaps, bad navigation, etc. When acoustic nav was not available GPS ship nav was used.