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Packet Format and Status Byte for GPS Time Stamping
The 6 bytes at the end of the data packet report GPS timing and synchronization data. For every packet, the last 6 bytes are formatted as follows:
Timestamp Bytes in Reverse Order in Microseconds
Bytes | Description | Notes |
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4 | GPS timestamp | 32 bit unsigned integer timestamp. This value represents microseconds from the top |
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| of the hour to the first laser firing in the packet. |
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1 | Status Type | 8 bit ASCII status character as described in Appendix E. The status byte rotates |
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1 | Status Value | 8 bit data as described in Appendix E. |
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Within the GPS status byte, there are 4 GPS status indicators:
•0: no GPS connection.
•A: both PPS and GPS command have signal.
•V: only GPS command signal, no PPS.
•P: only PPS signal, no GPS time command.
Time Stamping Accuracy Rules
The following rules and subsequent accuracy apply for GPS timestamps:
GPS Connection | Timestamp Info | Accuracy | Notes |
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GPS isn’t connected | The sensor starts running on | Expect a drift of about 5 | The sensor clock does not correct |
(GPS Status 0) | its own clock starting at midnight | seconds/day | for leap years. See Appendix E for |
| Jan 1 2000. This date and time data |
| more information. |
| is reflected in the H, M, S, D, N, |
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| and Y data values. |
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GPS is connected | The H, M, S, D, N, and Y data values | GPS time synching runs in |
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| are obtained from the $GPRMC | one of two modes: |
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| NMEA record. | • The GPS has an internal clock |
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| that runs for several weeks that |
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| is used first. The accuracy is that |
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| of the GPS device employed. |
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| • When the GPS achieves lock, |
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| the sensor clock is then within |
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| all times. |
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GPS is disconnected | The sensor continues to run on | Expect drift of about 5 seconds/day |
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after being connected | its own clock. |
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Laser Firing Sequence and Timing
If the GPS timestamp feature is used, it can be useful to determine the exact firing time for each laser so as to properly
The upper block and lower block collect distance points simultaneously, with each block issuing one laser pulse at a time. That is, each upper block laser fires in sequence and in unison with a laser from the lower block.
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