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5 SYSTEM CONFIGURATION AND TIMING 5.1 BACKGROUND In order to create a set of valid depth measurements, results from a number of subsystems need to be brought together. These can include: Bathyswath sonar Attitude system, providing roll, pitch & heave, possibly also heading Compass, providing heading GPS system, providing position Real-time speed of sound Echosounder Some of these subsystems can be integrated into one unit; for example, the CodaOcotpus F180 and Applanix POS/MV provide position, attitude and heading. In order to bring this data together correctly, the relative timing of all the data streams must be known. The most time-critical measurement is roll. For extremes of motion on a small vessel, a timing accuracy of better than 5ms may be needed. On a small vessel in a moderate sea, a timing error of 20ms in the attitude data is just detectable on the depth displays and data output. Pitch, heading and position are required with a timing accuracy of better than 0.5 seconds (500ms). Some survey protocols require that all data is logged with time information that can be traced back to a common time source, usually UTC time derived from GPS signals. This can be achieved using Bathyswath, but may not be supported by some auxiliary sensor systems used with it. 5.1.1 PC clock The clocks in most PC computers are not particularly accurate. PC clocks are reputed to be accurate to 30 to 100ppm (parts per million; e.g. 100ppm is 0.36s in an hour). However, the Windows operating system adds its own timing errors and uncertainties. Tip: the Windows Time Service can cause the PC time data to vary at rates of up to half a second per minute [8000ppm!]. Therefore, it can help to disable it, using ‘Settings > Control Panel > Administrative Tools > Services > Windows Time’. Therefore, a method of synchronising the PC time to GPS time should be considered. Some integrated attitude and position systems provide this as part of the supplied package. Alternatively, an NTP (Network Time Protocol) time server can be integrated into the system. If a distributed computing system is being used, and one of the PCs is acquiring UTC-GPS time, e.g. via PPS, then an NTP time server can be set up on that PC, and NTP clients set up on the other PCs in the system. These NTP servers and clients are software applications, which can be obtained as shareware products for a few tens of dollars on the Internet. Bathyswath surveys have been successfully carried out using such configurations. The information contained on this sheet is subject to restrictions listed on the cover page of the document Version 5.00 - 13/05/15 Page 41 Unclassified document ETD-2004
