Guralp Systems Limited
MAN-D16-0001

Chapter 3. Digitizer status information

The digitizer outputs various status information to report the system operation such as GPS and time synchronisation status. This status information is in plain ASCII text packaged in the same block structure as the channel data. There are usually 12 lines of information in a block.

To access a Status window right click on the Stream ID ‘****00’, (where **** is the digitizer). In the example below this is 102600

Notice this is the only stream with ‘0’samples per second

During boot-up the units report their module type, firmware revision number, the system-id and serial number. This information is followed by the count of resets that have occurred and the time of this re-boot from the internal back-up clock. The following lines report the configuration of the unit sample rates, output taps selected, and the baud-rates of the serial ports.

Typical digitizer re-boot status message:

System

type

System ID & serial number

Internal clock

Sample rates

DSP filter version

Selected tap, sample

rate and channel

Data port baud rate

GPS port baud rate

The system will produce this status message whenever it is powered up. If this status is reported at other times it indicates that the system has been reset by the built-in ‘watchdog’ monitor, which will occur if the system has suffered a corruption due to external noise or power dips.

If the digitizer does not have a GPS unit connected for time synchronisation no further status reports are produced.

When a GPS unit is fitted its operational status is reported and the behaviour of the time synchronisation software will also be shown.

From a ‘cold’ start GPS will initially report ‘No GPS time’ and its last position from the internally backed up status. All messages from GPS which involve a change of its status are automatically reported, repeated status messages are not shown to avoid unnecessary accumulation of the same information.

Initial GPS status report will be like this :

3-D position requires minimum of 3 SVs (Satellites in View)

4 shown here

Position from GPS

Satellite in View number, elevation, bearing and either obscured or Signal/Noise ratio of signal (30 to 50dB normal)

If GPS is having difficulty in acquiring satellites there can be a delay of several minutes before a new message is displayed, but normally if the system has not been moved from its previous location it should report acquisition of 1 or more satellites and GPS time in a very short time. The report will also show the satellite numbers and their corresponding signal strengths.

The internal time synchronisation and control software will wait for the GPS unit to report a good position fix (requires 3 satellites) before starting operation. The system actually waits 6 consecutive ‘good’ messages, which normally occur every 10 to 20 seconds.

If GPS maintains a good fix from the satellites available, the system will then switch on the control process and set the internal clock as shown by the status messages over the page.

The system jam-sets its internal clock at this point to be synchronised to GPS time and will also re-synchronise the Analog to Digital Converters so that the data is accurately time-stamped to this new reference. The data transmitted upto this point will be stamped with the time from the internal back-up ‘Real-Time Clock’ ; this is also now reset to this accurate time. Re-synchronisation will also result in the received data showing a discontinuity.

The control process will now attempt to keep the internal time-base synchronised to the GPS 1 pulse per second output by adjusting the voltage controlled crystal oscillator. The control algorithm has two stages - initially it compares its internal 1 Hz timebase with the GPS 1pps and adjusts the voltage control to minimise the error. Once this has been achieved it then controls the crystal to minimise both the ‘phase error’ (offset between its internal 1 Hz and GPS) and the drift (frequency error) relative to GPS. During the control process the system reports the measured errors and the control signal applied, as a ‘pwm’ value - Pulse Width Modulation - digital to analog conversion.

Synchronisation process commenced

Internal clock synchronised to GPS

Notice time shift of 2 seconds from

17:10:30 to 17:10:32 in this example

Synchronisation control parameters.(zero drift in the first instance during coarse adjustment. pwm is a saved parameter)

During the initial ‘coarse’ adjustment only the coarse voltage control is used and no ‘drift’ calculation is made. If the system is operating in a similar environment to that when the system was last powered (i.e. same temperature) the saved control parameters will be appropriate and the system should rapidly switch to the ‘fine’ control mode.

The system reports its control status and parameters each minute as shown below:

The offset and drift figures are the total accumulated error measurements during the previous minute in time-base units (nominally 0.5 µsec). To convert the figures to time, divide by 120 (=60 × 2) to give micro-seconds. In a stable temperature environment the system should soon settle down showing an offset error of only a few thousand (average error < 100 µsec) and a drift rate under 100 counts (< 1 in 10 -6).

The screen shot below shows, from the top graph down, the off-set, drift and pwm of a digitizer internal clock tracking and homing-in on a GPS clock pulse over approximately a twelve hour period.

Offset or phase error between digitiser internal 1Hz and GPS 1pps.Closes in to less than 5s

Drift or frequency error between digitiser internal clock and GPS

PWM Pulse Width Modulation control signal applied to a DtoA to control internal clock/GPS error

The above graphical image was printed from a Guralp plot module to demonstrate the effectivenes of digitizer clock sychronisation and subsequently time stamped data.