# GPSD JSON client driver

## 1. Synopsis

    Name: gpsd
    Reference ID: GPSD
    Serial Port: /dev/gpsu

## 2. Description

This driver is a client driver to the GPSD daemon, which over time has become increasingly popular for UN*Xish platforms. GPSD can manage several devices in parallel, aggregate information, and acts as a data hub for client applications. GPSD can also auto-detect and handle PPS hardware signals on serial ports. Have a look at the GPSD project page.

It is important to understand that this driver works best using a GPS device with PPS support.

The GPSD protocol is text based, using JSON notation to transfer records in the form of JSON objects. The driver uses a TCP/IP connection to `localhost:gpsd` to connect to the daemon and then requests the GPS device `/dev/gpsu` to be watched. (Different clock units use different devices, and GPSD is able to give only the relevant information to a clock instance.)

This driver does not expect GPSD to be running or the clock device to be present a priori; it will try to re-establish a lost or hitherto unsuccessful connection and will wait for the device to come up in GPSD. There is an initial 10 seconds delay between a connection loss or failed attempt and the next reconnect attempt; this ensures that there is no thrashing on the network layer. If the connection fails again, an exponential back off is used with an upper limit of approximately 10 minutes.

The overall accuracy depends on the receiver used. The driver uses the error estimations (95% probability limits) provided by GPSD to set the clock precision dynamically according to these readings.

The driver needs the VERSION, TPV, PPS, WATCH and TOFF objects of the GPSD protocol. (Others are quietly ignored.) The driver can operate without the TOFF objects, which are available with the protocol version 3.10 and above. (Not to be confused with the release version of GPSD!) Running without TOFF objects has a negative impact on the jitter and offset of the serial timing information; if possible, a version of GPSD with support for TOFF objects should be used.

The acronym IBT is used here as a synonym for "in-band time" from the data channel of the receiver, no matter what objects were used to obtain it.

## 3. Naming a Device

The GPSD driver uses the same device name as the NMEA driver, namely `/dev/gpsu`. There is a simple reason for that: While the NMEA driver and the GPSD driver can be active at the same time for different devices, they cannot access the same device at the same time. Having the same name helps prevent that collision. It also eases migration from using NMEA directly to using GPSD, as no new links etc. need to be created.

GPSD is normally started with the device name to access; it can also be instructed by hot-plug scripts to add or remove devices from its device pool. Luckily, the symlinks used by the NMEA driver are happily accepted and used by GPSD; this makes it possible to use the symlink names as device identification. This makes the migration from the built-in NMEA driver a bit easier.

Note: GPSD (as of version 3.10) cannot use kernel mode PPS on devices that are hot-plugged. This would require to attach the PPS line discipline to the character special file, which is not possible when running with root privileges already dropped. This is not likely to change in the future.

## 4. The 'mode' word

A few operational modes can be selected with the mode word.

The Mode Word
Bits | Value | Description

0..1

0

IBT only operation. This mode is affected by the timing stability of whatever protocol is used between the GPS and GPSD

Running on IBT only is not recommended in general. Possible use cases include:

- The receiver does not provide a PPS signal.

- The receiver does provide a PPS signal and the secondary PPS unit is used.

- The receiver has a stable serial timing and a proper fudge can be established.

- You have other time sources available and want to establish a useful fudge value for `time2`.

1

Strict operation. This mode needs a valid PPS and a valid IBT to combine the absolute time from the IBT with the time stamp from the PPS record. Does not feed clock samples if no valid PPS+IBT pair is available.

This type of operation results in an ordinary clock with a very low jitter as long as the PPS data is available, but the clock fails once PPS drops out. This mode is a possible choice for receivers that provide a PPS signal most of the time but have an unstable serial timing that cannot be fudge-compensated.

2

Automatic mode. Tries to operate in strict mode unless it fails to process valid samples for some time, currently 120 s. Then it reverts to IBT-only operation until the PPS is stable again for 40 s, when strict mode is engaged again.

Important Notice: This is an experimental feature!

Switching between strict and IBT-only mode will cause changes in offset and jitter. Use this mode only if IBT-only works fairly well with your setup, or if you expect longer dropouts of the PPS signal and prefer to use IBT alone over not getting synchronised at all.

3 | (reserved for future extension, do not use)
2..31 | (reserved for future extension, do not use)

## 5. Syslog flood throttle

This driver can create a lot of syslog messages when things go wrong, and cluttering the log files is frowned upon. So we attempt to log persistent or recurring errors only once per hour. On the other hand, when tracking a problem, the syslog flood throttle can get in the way.

Therefore, option flag3 can be used to disable the flood throttle at any time; the throttle is engaged by default. Running with the syslog flood throttle disabled for extended periods is not recommended unless the log files are closely monitored.

## 6. PPS secondary clock unit

Units with numbers ≥128 act as secondary clock unit for the primary clock unit (u mod 128). A secondary unit processes only the PPS data from GPSD and needs the corresponding master unit to work1. Use the noselect keyword on the primary unit if you are not interested in its data.

The way to use a secondary clock unit is to pair it with a primary in "tandem mode": see the configuration examples below.

The secondary unit employs the usual precautions before feeding clock samples:

- The system must be already in a synchronised state.

- The system offset must be less than 400 ms absolute.

- The phase adjustment from the PPS signal must also be less than 400 ms absolute.

If option `flag1` is set for the secondary unit, the unit asserts the PPS flag on the clock as long as PPS data is available. This makes the unit eligible as PPS peer and should only be used if the GPS receiver can be trusted for the quality of its PPS signal2. The PPS flag gets cleared if no PPS records can be acquired for some time. The unit also flushes the sample buffer at this point to avoid the use of stale PPS data.

Attention: This unit uses its own PPS fudge value, which must be set as fudge `time1`. Only the fudge values `time1` and the `flag1` option have an impact on secondary units.

## 7. Clockstats

If flag4 is set when the driver is polled, a clockstats record is written for the primary clock unit. (The secondary PPS unit does not provide clock stats on its own.) The first 3 fields are the normal date, time, and IP address common to all clockstats records.

The Clockstats Line
field | Description
1 | Date as day number since NTP epoch.
2 | Time as seconds since midnight.
3 | (Pseudo-) IP address of clock unit.
4 | Number of received known JSON records since last poll. The driver knows about TPV, PPS, TOFF, VERSION and WATCH records; others are silently ignored.
5 | Bad replies since last poll. A record is considered malformed or a bad reply when it is missing vital fields or the fields contain malformed data that cannot be parsed.
6 | Number of sample cycles since last poll that were discarded because there was no GPS fix. This is effectively the number of TPV records with a fix value < 2 or without a time stamp.
7 | Number of serial time information records (TPV or TOFF, depending on the GPSD version) received since last poll.
8 | Number of serial time information records used for clock samples since the last poll.
9 | Number of PPS records received since the last poll.
10 | Number of PPS records used for clock samples on the secondary channel since the last poll.

## 8. 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 PPS time offset calibration factor, in seconds and fraction, with default 0.0.

`time2` time

  [Primary Unit] Specifies the TPV/TIME time offset calibration factor, in seconds and fraction, with default 0.0.

`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 `GPSD`.

`flag1 {0 | 1}`

  [Secondary Unit] When set, flags the secondary clock unit as a potential PPS peer as long as good PPS data is available.

`flag2 {0 | 1}`

  [Primary Unit] When set, disables the processing of incoming PPS records. Intended as an aide to test the effects of a PPS dropout when using automatic mode (mode 2).

`flag3 {0 | 1}`

  [Primary Unit] If set, disables the log throttle. Useful when tracking problems in the interaction between GPSD and ntpd, since now all error events are logged. Persistent/recurrent errors can easily fill up the log, so this should only be enabled during bug hunts.

`flag4 {0 | 1}`

  [Primary Unit] If set, write a clock stats line on every poll cycle.

`mode`

  Control IBT and strict operating modes.

`path` filename

  Overrides the default device path.

`ppspath` filename

  Not used by this driver.

`baud` number

  Not used by this driver.

1)Data transmission and decoding is done only once by the primary unit. The decoded data is then processed independently in both clock units. This avoids double transmission over two sockets and decoding the same data twice, but the primary unit is always needed as a downside of this approach.

2)The clock driver suppresses the processing of PPS records when the TPV/TIME data indicates the receiver has no fix. It can also deal with situations where the PPS signal is not delivered to GPSD. But once it is available, it is also processed and used to create samples. If a receiver cannot be trusted for the precision of its PPS signal, it should not be used to create a possible PPS peer: These get extra clout and can effectively become the sole source of input for the control loop. You do not want to use sloppy data for that.

## 9. Configuration Example

    # Conentional mode 2 on unit 0, IBD only, experimental automatic mode
    refclock gpsd mode 2 path /dev/tty.usbserial

    # Tandem mode, IBD and PPS from unit 0
    server gpsd unit 0 minpoll 4 maxpoll 4 time1 0.142
    server gpsd unit 128 minpoll 4 maxpoll 4 time1 0.001500

## 10. Known bugs

If your GPS has firmware made more than 1024 weeks (19 years and 36 weeks) in the past, its internal date counter may well wrap around and generate spurious timestamps. Some newer GPSes may have a longer wraparound (8192 weeks, or 157 years and 28 weeks) but this is almost never documented and not safe to bet on. GPSD cannot tell what the device’s wraparound time is, nor what any base-date assumption in its firmware might be.

This problem is fundamental and cannot be compensated for in code without relying on the accuracy of the local system clock, which is what GPSD (primarily designed for geospatial location) does. This choice creates a risk of perverse GIGO failure modes (especially at startup time). The only remedy is (a) not to use ancient GPS hardware, and (b) not to rely on GPSD to manage a time source if there is any serious risk that your NTP host might boot with a system clock time that is wildly off.

Even with these constraints there is some risk of flaky failures in a time window around GPS defined by your host system’s worst-case clock skew from UTC at boot time.

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