1. Synopsis

Name: oncore
Reference ID: GPS
Serial Port: /dev/oncore.serial.u; 9600 bps 8N1.
PPS Port: /dev/oncore.pps.u

2. Deprecation warning

This refclock is deprecated and obsolete; the entire Oncore line has been end-of-lifed. The NTPsec maintainers plan to remove it in a future release. If you have a requirement for it, please make this known to us.

This driver reports only two-digit years, and is thus reliant on the system clock to be near correct before samples will be processed properly. You will not be able to use it to run autonomously, nor will it reliably recover from a trashed or zeroed system clock.

It is likely any surving instances of this hardware will have era-rollover issues when reporting dates. One or more "g" suffixes on your time1 option may be useful as a workaround.

3. Description

This driver supports most models of the Motorola Oncore GPS receivers (Basic, PVT6, VP, UT, UT+, GT, GT+, SL, M12, M12+T), as long as they support the Motorola Binary Protocol. All are long end-of-lifed as of 2019.

The (formerly) interesting versions of the Oncore were the VP, the UT+, the "Remote" which is a prepackaged UT+, and the M12 Timing variant.

However, there is one current hardware product that is reported good with this driver; the CNS Clock II. It ships 1PPS on the DCD line of its RS232 port or as a DCD priority packet on USB. Older revisions of the product used M12 hardware, newer ones use a u-blox engine but emulate OnCore behavior. Future versions will drop OnCore reporting entirely, at which time the possibility of removing this driver will be revisited.

See the CNS Clock vendor product page for specifications.

We expect this should also work with the Furuno line of M12 compatibles, including the GT8736 and (now discontinued) GT8536, but we don’t yet have confirmation from the field.

The driver can use use the "position hold" mode with user provided coordinates, the receiver’s built-in site-survey, or a similar algorithm implemented in this driver to determine the antenna position.

4. Monitor Data

The driver always puts a lot of useful information in the clockstats file, and when run with debugging can be quite chatty on stdout. When first starting to use the driver you should definitely review the information written to the clockstats file to verify that the driver is running correctly.

In addition, on platforms supporting Shared Memory, all of the messages received from the Oncore receiver are made available in shared memory for use by other programs. See the Oncore-SHMEM manual page for information on how to use this option. For either debugging or using the SHMEM option, an Oncore Reference Manual for the specific receiver in use will be required.

5. Driver Options

unit number

The driver unit number, defaulting to 0. Used as a distinguishing suffix in the driver device name.

time1 time

Specifies the time offset calibration factor, in seconds and fraction, with default 0.0.

time2 time

Not used by this driver.

stratum number

Specifies the driver stratum, in decimal from 0 to 15, with default 0.

refid string

Specifies the driver reference identifier, an ASCII string from one to four characters, with default GPS.

flag1 {0 | 1}

Not used by this driver.

flag2 {0 | 1}

Not used by this driver.

flag3 {0 | 1}

Not used by this driver.

flag4 {0 | 1}

Not used by this driver.


Not used by this driver.


Not used by this driver.


Not used by this driver.


Not used by this driver.

baud number

Not used by this driver.

6. Configuration Example

refclock oncore

7. Additional Information

The driver was initially developed on FreeBSD, and has since been tested on Linux, SunOS and Solaris.

7.1. Configuration

There is a driver specific configuration file ntp.oncore (or ntp.oncore.u or ntp.oncoreu if you must distinguish between more than one Oncore receiver unit) that contains information on the startup mode, the location of the GPS receiver, an offset of the PPS signal from zero, and the cable delay. The offset shifts the PPS signal to avoid interrupt pileups ‘on’ the second, and adjusts the timestamp accordingly. See the driver source for information on this file. The default with no file is: no delay, no offset, and a site survey is done to get the location of the gps receiver.

The following three options can be set in the driver specific configuration file only if the driver is using the PPSAPI. The edge of the PPS signal that is ‘on-time’ can be set with the keywords [ASSERT/CLEAR] and the word HARDPPS will cause the PPS signal to control the kernel PLL.

7.2. Performance

Even the newest of the Motorola variants, the M12+T with firmware dated 9 Jun 2004 now reports bad dates due to era rollover.

Performance is really good, other than the rollover issue. With the VP/UT+, the generated PPS pulse is referenced to UTC(GPS) with better than 50 ns (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.

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