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Vaughan Wesson, September 2000, revised February 2003
Introduction
The GPS receivers used in all SRC field equipment are various models of Trimble Core Modules. The Kelunji recorder (Classic or D series) communicates with the receiver using a binary protocol over a serial link. The receiver responds to a number of predefined commands including some which indicate the status of the receiver. These status messages are used to display messages on the screen. The sections below describe these messages.
Further general information about the GPS system can be found in our technical notes TN199812A, TN199812B and TN199901A.
Clock Check Screen
In the clock check screen, when using a GPS two boxes are displayed. The right hand box shows information from the GPS receiver. The bottom two lines of this box are used to display status and error messages.
Status Messages
Any one of the following status messages may be displayed.
| Message |
Meaning |
| Have obtained GPS time |
The receiver has locked on to a sufficient number of satellites to determine its position and the absolute time. Also, the offset between GPS time and UTC has been received. |
| No satellite found yet |
The receiver has not yet locked on to the signal from any satellite. |
| PDOP is too high |
The receiver has signal from a number of satellites, but their orientation is such that an accurate position cannot be determined. |
| No usable satellites |
The receiver has a signal from one or more satellites but has not yet locked on the signal from any one of them. |
| Only
1 usable satellite |
The receiver has only locked on to the signal from the indicated number of satellites. |
Error Messages
If there is an "error" condition detected by the GPS receiver, this will be indicated in one of the following messages, These messages are listed in priority order. If an error condition is indicated by one of these messages, an error indicated by a lower priority message will not be displayed.
The error messages which begin with a '*' are "sticky". That is, the message will be displayed until the GPS is power cycled, even if the error condition is removed.
| Message |
Meaning |
| * Battery backup failed |
The backup battery power to the GPS receiver has been removed and all settings etc. lost. This usually indicates that the module containing the GPS receiver (KGB, KG1 or DK1/DI1) has been removed. |
| Antenna open or shorted |
Indicates that the antenna is disconnected or damaged. This message does not appear with earlier model receivers. |
| * Signal processor error |
The signal processor within the GPS receiver has failed |
| * Alignment error |
|
| Excessive ref freq error |
|
| UTC offset unknown |
The receiver has not yet obtained the offset between GPS time and UTC. This is discussed in TN199901A. |
| Synthesiser fault |
|
| Still collecting almanac |
The receiver is still obtaining almanac information from the satellites. This can take up to 12.5 minutes. |
| <Blank> |
No error |
Scheduled Clock Checks
The GPS receiver may also be used to perform scheduled clock checks. There are a number of error messages which may be logged in the journal to indicate problems with a GPS receiver in this case. These differ for a Kelunji Classic and a Kelunji D Series.
Kelunji Classic
| Message |
Meaning |
| Could not communicate with GPS receiver |
A command was sent to the GPS receiver, but it did not respond within a reasonable period of time |
| GPS antenna opened or shorted |
The GPS receiver is indicating that it has a problem with the antenna |
| GPS has not obtained time |
The allowable GPS wait time has expired and the GPS receiver has still not locked on to enough satellites to obtain a time fix |
| No pulse per second from GPS receiver | The GPS receiver is not putting out its PPS. This may be because it has lost all satellites it was previously using or due to a hardware fault |
| GPS sync of (ss).mmm, UTC offset unknown | We have obtained a consistent sync value but do not yet have the UTC offset. The true sync can be obtained by correcting the value displayed by the known UTC offset |
| GPS sync of ??.mmm, sync second unresolved | A good sync value was obtained, but it was not possible to determine the correct second |
| GPS sync of ss.mmm (+/-0.eee). Satellites used m | A good, consistent clock sync has been obtained |
Kelunji D Series
| Message |
Meaning |
| No satellites found for automatic clock check |
Either (i) there was a problem opening the communication link to the GPS receiver or (ii) the GPS receiver is working OK, but we could not obtain a consistent sync value |
| GPS antenna is opened or shorted |
The GPS receiver is indicating that it has a problem with the antenna |
| GPS status xx, error code xx, extra status xx |
Each of the xx
values is a hex value. Main Status : 01 Don't have GPS time yet |
| Scheduled clock sync of ss.mmmmm | A good, consistent clock sync has been obtained |
| GPS Pos: Long nn.nnnnn, Lat nn.nnnnn, Alt nnn | The most recently determined position calculation |
| Pos status m, nD POs with PDOP of nn.nn | Status information about the position determined |