The two most interesting version of the Oncore are the "UT+"
and the "Remote" which is a prepackaged "UT+". The evaluation kit
can also be recommended, it interfaces to a PC straightaway, using the
parallel port for PPS input (supported under FreeBSD), and packs the
receiver in a nice and sturdy box.
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The driver requires a standard PPS interface for the pulse- per-second output from the receiver. The serial data stream alone does not provide precision time stamps (0-50msec variance, according to the manual), whereas the PPS output is precise down to 50 nsec (1 sigma) for the UT models.
The driver will use the "position hold" mode if available, with either the receivers built-in site-survey or a similar algorithm implemented in this driver.
The driver has been developed under FreeBSD, and may still be pretty FreeBSD centric. Patches are most welcome.
Performance
Really good. With the UT+, the generated PPS pulse is referenced to UTC(GPS) with better than 50 nsec (1 sigma) accuracy. The limiting factor will be the timebase of the computer and the precision with which you can timestamp the rising flank of the PPS signal. Using FreeBSD, a FPGA based Timecounter/PPS interface and an ovenized quartz oscillator, that performance has been reproduced. For more details on this aspect: Sub-Microsecond timekeeping under FreeBSD