Jackson Labs RSR GNSS Transcoder, Руководство пользователя

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RSR GNSS Transcoder™ User Manual
26 © 2017 Jackson Labs Technologies, Inc.
totally autonomously either from an internally-stored time-stamped vector file (feature to be released
in late 2017), or a stationary position fix.
3.8.1 Transcoding with External GNSS Receiver
Any GNSS receiver, INS or other device capable of providing GGA and RMC sentence NMEA
output at 1Hz can be used as a PVT source for transcoding. Transcoding with precise time and UTC
phase transfer also requires a 1PPS from the transcoding receiver. When configured in transcoding
mode, the RSR GNSS Transcoder™ will wait and look for NMEA data at 9600 and/or 38400 baud
and the presence of a 1PPS input after power-on. When NMEA data is found, the RSR GNSS
Transcoder™ will wait for a valid position fix and a stable 1PPS output at which time it will enable
the GPS L1 RF output and begin transcoding. The unit may also optionally be configured to
transcode NMEA PVT information without an external 1PPS reference signal being present.
The RSR GNSS Transcoder™ will continue to update the position and synchronize precise timing
with the 1PPS reference input until the external GNSS receiver indicates a loss of lock. The RSR
GNSS Transcoder™ can operate in three different user-selectable holdover modes when the external
receiver indicates a loss of GNSS fix. The holdover mode is controlled with the
SIMulation:HOLDover:MODE <ON|OFF|LIMIT> command as described in Section
4.3.4 .
ON mode: This mode will cause the RF output to remain on by propagating time using either the
internal high stability TCXO or optional CSAC atomic clock as a timing reference, and applying the
currently selected filter mode to the transcoded position. Both clocks can be used for precise time
transfer to the target receiver, with the CSAC being able to maintain precise timing for longer
holdover periods. The ON mode is best used to provide timing holdover to the target GPS receiver
and when guaranteed position accuracy during holdover is not required. When using the ON mode
and the position filter mode is set to INS, the transcoded position continues to be updated based on a
dead reckoning solution from the internal INS. The INS solution accuracy will degrade as time
passes, and is only to be used as an approximate positioning reference. See Section
3.8.8 for more
details on configuring the filtering mode.
JLT has devised a way to communicate fix status of the GPS simulator entirely through the RF
antenna feed by artificially enabling and disabling certain PRN numbers in the simulation output or
attenuating certain PRN numbers. This allows a target GPS receiver to be used without any
modification, while at the same time transferring information about the simulated PVT fix quality.
Thus it is possible to determine if the GNSS Transcoder™ is in holdover mode or receiving valid
external GNSS reference fixes for example.
Two holdover indicate modes configured with the SIMulation:HOLDover:INDicate <SV32 |
HIGHEST | OFF> command are available to communicate to the target GPS receiver that the
transcoder is operating in holdover mode. In SV32 mode, PRN 32 is enabled only when the holdover
state is active. So when PRN 32 is being tracked by the target GPS receiver, the target GPS receiver
can determine if the holdover state is on or off. The target system can use this information to
determine that the RSR GNSS Transcoder™ is itself in CSAC/TCXO/INS holdover mode. For
example, when the target system detects a holdover condition by sensing the presense of PRN32 in
the simulator RF output, it may no longer use the position information as it is known at that time that
the position is just estimated, but the system may continue to use the 1PPS phase reference provided
by the target receiver for a user-determined period of time based on the holdover performance of the
RSR GNSS Transcoder™’s oscillator holdover performance. The SV32 mode indicates the holdover
state by using an extra satellite that is not used in the constellation so the accuracy of the PVT
solution in the target receiver is not affected. However, some target GPS receivers may not