R2Sonic 2022, Operation Manual

Page: 73 / 133

Page 73 of 133
Version 3.0 REV r000
Date 25-08-2010
APPENDIX I: Multibeam Survey Suite
Components
8 Auxiliary Sensors and Components
A multibeam survey system is comprised of more components than just the Sonic 2024/2022
Multibeam Echosounder. These components are the auxiliary sensors, which are required to
provide the necessary information for a multibeam survey. This does not mean that these sensors
are a minor part of the survey system; each auxiliary sensor is required for any multibeam survey
operation. The required sensor data:
•
Position: Differential Global Positioning System Receiver
•
Heading: Gyrocompass
•
Attitude: Motion Sensor
•
Refraction correction: Sound Velocity Probe
Each of the individual sensors requires their own setup and operation procedures. The details,
discussed here, concerning the installation and calibration of the auxiliary sensors, is supplemental
to any and all manufacturer’s documentation.
8.1 Differential Global Positioning System
The Global Positioning System (GPS) is well known to all surveyors. There was a period of time
when the GPS position was intentionally made less accurate; this was Selective Availability (SA).
When SA was enacted, the GPS position became too inaccurate for survey use. It was during this
period that the concept of differential corrections was established. Differential corrections were
derived from users monitoring the GPS position at a known survey point and computing the
corrections required to adjust the various pseudo ranges to make the GPS position agree with the
known survey position. If a vessel was operating within the local area and observing the same
satellite constellation, the derived pseudo range corrections could be applied on board to make for
a more accurate and consistent position. The corrections are normally transmitted over a radio link
and applied within the GPS receiver.
8.1.1 Installation
The first and foremost consideration when installing the DGPS system is the location of the
respective antennae. Both the GPS antenna and the differential antenna (if they are two separate
antennae) need to be mounted on the vessel in such a way so as to have a totally unobstructed view
of the sky.
When installing the GPS antenna, the surveyor should be aware of the position of the stacks and
masts; in particular are davits or cranes that may be currently in a stored position, but will be in use