EW0008 GPS Setup

John E. Chance and Associates (Fugro) installed dual SpotBeam
differential GPS systems which provided RTCM (differential) corrections
to their Trimble 4000DS GPS receiver and several Ewing GPS receivers.
The Trimble 4000 NMEA output was used as the primary navigation system
(GP01) for the Ewing steering and navigation.  The only failure of this
system ocurred on day 259 (in the 3-D shoot) and was rectified by
switching from the AM-SAT satellite to the AMSC-E satellite (AM-SAT was
set for Port Newark and AMSC-E was set for the work area).  The switch
was reset to AM-SAT on day 276.

The SpotBeam was selected over the past Inmarsat setup because of the
well known problem of the Ewing mast blocking the Ewing Inmarsat dome.
The two SpotBeam antennae and the Trimble 4000 antenna were installed on
the roof of the A-booth (winch booth on the A-deck), well away from the
Ewing's mast.  The antenna was 6.2 meters starboard of the ship's
centerline and 13.8 meters forward of the transom.

The RTCM differential corrections were supposed to be sent to the
tailbuoy, so that the tailbuoy's Ashtech G-8 GPS receiver would send
back corrected positions (NMEA strings).  This objective was NEVER
successful.  When the ship could receive tailbuoy positions, the
receiver did not send back differential NMEA strings.