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 Only 2 usable satellites Only 3 usable satellites	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 03 PDOP is too high 08 No usable satellites 09 Only 1 usable satellite 0A Only 2 usable satellites 0B Only 3 usable satellites Error code : 01 Battery backup failed 20 Antenna shorted Extra status: 08 Almanac not up to date
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