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The serial messages from the GPS are received on serial port 3, /dev/ttyS3. The pulse-per-second square-wave signal (PPS) from the GPS is also connected to the CTS line of that serial port. The PPS patch has been added to the Linux kernel on the data system so that an interrupt function is can be registered to run in response to the CTS interrupts. This interrupt function is then will be called immediately after the rising edge of the PPS signal has been detected by the serial port hardware.
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- GPSdiff: The time difference, in seconds, between the system clock time-tag that was assigned to a RMC message and the date and time that is contained within the message. The time-tag assigned to a sample is the value of the system clock at the moment the first byte of the sample was received. For example, a value of 0.6 secs means that the data system assigned a time-tag to the RMC message that was 0.6 seconds later than the time value contained in the message. GPSdiff will be effected by processing lags within the GPS, DSM data sampling lags, and the drift of the DSM system clock relative to the clock within the GPS receiver. When 5 minute statistics are computed, the maximum and minimum values of GPSdiff for each 5 minute period are written to the output NetCDF files as GPSdiff_max and GPSdiff_min, and then plotted on the daily web plots.
- GPSnsat: number of satellites being tracked by the receiver - , that is, the number of satellites whose signals are used in the time and location solution.
- NTPClockOffset, NTPFreqOffset: These values are computed by the NTP software on the DSM, and written to the NTP "loopstats" file. See http://www.eecis.udel.edu/~mills/ntp/html/monopt.html
for information on the NTP monitoring options. NTP ClockOffset is the estimated offset of the GPS time from the data system time. A negative value indicates that the GPS system clock is indicating an earlier time than the system GPS clock. NTPFreqOffset is the estimated error in the GPS clock frequency in parts-per-million. An positive slope in NTPFreqOffset indicates that NTP estimates that the GPS clock is speeding up relative to the system clock, or conversely, that the system clock is slowing, relative to the GPS. These NTP values have not been recorded consistently since the beginning of the project. Only data from May 3 to August 12, 2010, Oct 14 to November 9, 2010, and April 9, 2011 onward have been archived.
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