1. Synopsis

Name: arbiter
Reference ID: GPS
Driver ID: GPS_ARBITER
Serial Port: /dev/gpsu; 9600 bps 8N1
Features: tty_clk

2. Deprecation warning

This refclock is deprecated and obsolete. 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 the Arbiter 1088A/B Satellite Controlled Clock. The claimed accuracy of this clock is 100 ns relative to the PPS output when receiving four or more satellites.

The receiver should be configured before starting the NTP daemon, in order to establish reliable position and operating conditions. It does not initiate surveying or hold mode. For use with NTP, the daylight savings time feature should be disables (D0 command) and the broadcast mode set to operate in UTC (BU command).

The timecode format supported by this driver is selected by the poll sequence B5, which initiates a line in the following format to be repeated once per second until turned off by the B0 command.

Format B5 (24 ASCII printing characters):

<cr><lf>i yy ddd hh:mm:ss.000bbb

on-time = <cr>
i = synchronization flag (' ' = locked, '?' = unlocked)
yy = year of century
ddd = day of year
hh:mm:ss = hours, minutes, seconds
.000 = fraction of second (not used)
bbb = tailing spaces for fill

The alarm condition is indicated by a '?' at i, which indicates the receiver is not synchronized. In normal operation, a line consisting of the timecode followed by the time quality character (TQ) followed by the receiver status string (SR) is written to the clockstats file.

The time quality character is encoded in IEEE P1344 standard:

Format TQ (IEEE P1344 estimated worst-case time quality)

0       clock locked, maximum accuracy
F       clock failure, time not reliable
4       clock unlocked, accuracy < 1 us
5       clock unlocked, accuracy < 10 us
6       clock unlocked, accuracy < 100 us
7       clock unlocked, accuracy < 1 ms
8       clock unlocked, accuracy < 10 ms
9       clock unlocked, accuracy < 100 ms
A       clock unlocked, accuracy < 1 s
B       clock unlocked, accuracy < 10 s

The status string is encoded as follows:

Format SR (25 ASCII printing characters)

V=vv S=ss T=t P=pdop E=ee

vv = satellites visible
ss = relative signal strength
t = satellites tracked
pdop = position dilution of precision (meters)
ee = hardware errors

A three-stage median filter is used to reduce jitter and provide a dispersion measure. The driver makes no attempt to correct for the intrinsic jitter of the radio itself.

4. Monitor Data

When enabled by the flag4 option, an additional line containing the latitude, longitude, elevation and optional deviation data is written to the clockstats file. The deviation data operates with an external pulse-per-second (PPS) input, such as a cesium oscillator or another radio clock. The PPS input should be connected to the B event channel and the radio initialized for deviation data on that channel. The deviation data consists of the mean offset and standard deviation of the external PPS signal relative the GPS signal, both in microseconds over the last 16 seconds.

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}

Enable verbose clockstats recording if set.

subtype

Not used by this driver.

mode

Not used by this driver.

path filename

Overrides the default device path.

ppspath filename

Not used by this driver.

baud number

Overrides the default baud rate.

6. Configuration Example

refclock arbiter

7. Author

David L. Mills <mills@udel.edu>

8. Additional Information

Reference Clock Drivers

9. Known bugs

If your Arbiter has firmware made more than 1024 weeks (19 years and 36 weeks) in the past, its internal date counter may wrap around and generate spurious timestamps.

This problem is fundamental and cannot be compensated for in code without relying on the accuracy of the local system clock, which is exactly what an NTP implementation may not do without risking perverse failure modes (especially at startup time).

The only sure remedy is to be sure the Arbiter’s firmware has been updated within the current GPS era.

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