Some GPS receivers offer a 1PPS (one pulse per second) output that
GPSD is capable of passing along to ntpd for use as an
extremely accurate clock source. The "Claimed Accuracy" figures are taken
from manufacturers' data sheets, and have not been independently
verified.
Where specified, NTP time offsets have been measured by
comparison with the NTP time service and reported by
gpsd.
Note that either a direct RS-232 connection or the "Magic Macx mod" (1PPS on the GPS chip connected to DCD on the serial-to-USB adaptor) is required for PPS. Typical gpsd/PPS users report ntp clock stability on the order of 1μs (microsecond) for RS232, 1ms (millisecond) for USB. From GPSes that are native USB but not PPS-capable, GPSD can extract time information from the serial data accurate to about 250ms.
| Receiver | Claimed Accuracy |
NTP time offset | Notes |
|---|---|---|---|
| SigNav TM3-01 | 10ns | - | - |
| Trimble Thunderbolt E | 15ns | - | - |
| Trimble Resolution-T | 15ns | - | - |
| Motorola iLotus Synergy-GPS M12M |
20ns 6ns corrected |
- | - |
| Navisys GR601W (Macx-1) | 1ms | - | - |
| NavSync CW25-TIM | 25ns | - | - |
| NavSync CW12-TIM | 30ns | - | - |
| Motorola OnCore UT+ | 50ns | - | - |
| Trimble Lassen iQ | 50ns | 0.675sec | - |
| Trimble Copernicus | 50ns | - | - |
| ublox Antaris | 50ns | - | - |
| ublox Antaris4 | 50ns | - | - |
| ublox Antaris4T | 50ns 15ns compensated |
- | - |
| ublox Antaris 5 | 50ns 50ns |
- | - |
| Novatel SuperStarII | 200ns | - | - |
| Thales AC12 | 250ns | - | - |
| Garmin GPS18/LVC | 1usec | 0.125sec | Must be outside with a clear sky view. At 19200bps; drops to 0.525 at 4800bps |
| Rockwell Jupiter | 1μsec | - | - |
| SiRFstar2 | 1usec | - | - |
| SiRFstar3 (GlobalSat MR-350P) |
1usec | - | May work inside wood frame buildings. |
| Jackson Labs Firefly | 50ns | - | GPS-disciplined oscillator, so the 1PPS output stays accurate even if GPS receiption is lost. |
All gpses require "raw" measurements internally to compute a navigation solution. The ability for users to postprocess these measurements is then a function of firmware and protocol support.
| Receiver | Raw | Notes |
|---|---|---|
| NavSync | No | I asked them about raw measurements and they told me that a custom firmware build ($$$!) would be required, and that I probably wasn't authorized to get that data. |
| SiRF | Partial | sirfstar1 has raw data in message 5, sirfstar2 has raw data in message 28, sirfstar3 has some raw data in message 28 but carrier phase is not available. rumor has it that carrier phase is available as a premium feature in superstar3. |
| ublox | Partial | The RXM-RAW message contains raw data. This is implemented in all Antaris chips. This is only implemented in the timing versions of Antaris4. In ublox5 it is "only available with premium feature raw data". |
| Novatel SuperStarII | Partial | Message #23 contains raw measurements in carrier-phase-enabled firmwares. |
| Fastrax | Yes | The "PSEUDO" message contains raw measurements |
| Trimble | Yes | Packet 0x5a contains raw measurements |
| Thales AC12 | Yes | The PASHR,MCA message contains raw measurements. |