[gpsd-users] Time precision of gpsd with u-blox GPS receiver

[Benedikt Kleinmeier]

Hi,

I use a u-blox NEO-7 GPS receiver as stratum 1 source and gpsd/ntpd to set the 
time of a connected PC very precisely.

I used the following HOWTO:

  http://www.catb.org/gpsd/gpsd-time-service-howto.html

Now, I'm interested in the accuracy of this solution.

Default, u-blox GPS receiver only sends NMEA 0183 Version 2.3 sentences. 
According to NMEA specification, in NMEA messages, time is always reported 
rounded to the nearest hundredth of a second (i.e. at least 10 ms). This is not 
very accurate for a stratum 1 source.

On the other hand, u-blox GPS receiver supports the binary UBX protocol. 
According to protocol specification [1], some UBX messages provide a field, 
called "nano", to allow nano precision. When analyzing logs, I only can 
recognize second precision:

  gpsd:DATA: TIMEGPS: time=1410420630.000000 mask={TIME}
  gpsd:DATA: TIMEGPS: time=1410420631.000000 mask={TIME}
  gpsd:DATA: TIMEGPS: time=1410420632.000000 mask={TIME}
  gpsd:DATA: TIMEGPS: time=1410420633.000000 mask={TIME}
  gpsd:DATA: TIMEGPS: time=1410420634.000000 mask={TIME}

Where are the fractions of a seconds?

Note: logs where generated by

  gpsctl -b /dev/ttyACM0          # Switch to binary UBX protocol.
  gpsd -b -N -n -D 6 /dev/ttyACM0 # Use log level 6 to make "gpsd:DATA" 
messages visible.

Looking at the code in "drivers_ubx.c", reveals that only "tow" field is read 
(time of GPS week in milliseconds) but the millisecond digits are always zero 
and "ftow" field (which provides "nano" precision" is completely ignored. See 
the following log of a slightly modified version of "drivers_ubx.c". Look at 
the "tow=" strings:

  gpsd:DATA: TIMEGPS: gw=1809 tow=372408000 ftow=-262153 
tow_with_fractional_part=372407.999738
  gpsd:DATA: TIMEGPS: time=1410420391.999738 mask={TIME}
  gpsd:DATA: TIMEGPS: gw=1809 tow=372409000 ftow=-261526 
tow_with_fractional_part=372408.999738
  gpsd:DATA: TIMEGPS: time=1410420392.999738 mask={TIME}
  gpsd:DATA: TIMEGPS: gw=1809 tow=372410000 ftow=-260899 
tow_with_fractional_part=372409.999739
  gpsd:DATA: TIMEGPS: time=1410420393.999739 mask={TIME}
  gpsd:DATA: TIMEGPS: gw=1809 tow=372411000 ftow=-260276 
tow_with_fractional_part=372410.999740
  gpsd:DATA: TIMEGPS: time=1410420394.999740 mask={TIME}
  gpsd:DATA: TIMEGPS: gw=1809 tow=372412000 ftow=-259652 
tow_with_fractional_part=372411.999740
  gpsd:DATA: TIMEGPS: time=1410420395.999740 mask={TIME}

Is it only possible to get second precision with this setup (u-blox 
UBX/gpsd/ntpd)?

Thanks for your help,
Benedikt

  [1] 
http://www.u-blox.com/images/downloads/Product_Docs/u-blox7-V14_ReceiverDescriptionProtocolSpec_Public_%28GPS.G7-SW-12001%29.pdf
  [2] http://git.savannah.gnu.org/cgit/gpsd.git/tree/driver_ubx.c#n213