[ntp:questions] Too much offset from GPS?

Steve Kostecke kostecke at ntp.org
Tue Mar 11 13:43:42 UTC 2008

On 2008-03-11, chowalit.lab Chowalit Lab Linux <chowalit.lab at gmail.com> wrote:

> So It's wrong on my GPS (GPSmap60).

The problem with the GPSMAP 60 is that it is the wrong type of GPS
receiver for an application which needs highly stable time.

There are two types of GPS receivers: mapping and timing.

The key difference between these two type of receivers is that the
timing receivers provide a Pulse Per Second (PPS) output which is
typically aligned to within 1 microsecond (0.000001 sec) of UTC time.
Higher quality timing receivers may be capable of closer (e.g.
nanosecond) alignment.

Both types of receivers are able to emit NMEA sentences via a serial or
USB interface. These NMEA sentences are emitted when the receiver is
not busy doing other things such as processing satellite data, updating
the display, etc., and can occur at _any_ _time_ during second they are
valid for. Time sources utilizing only NMEA sentences are not suitable
for a Stratum 1 time server.

It is up to you to decide if the GPSMAP 60 is good enough for your

The USD249.99 GPSMAP 60 is a mapping receiver.

The USD74.50 GPS18 LVC is a timing receiver.



> Oh... So If I still use this GPS I should add offset with fudge
> option.

You may correct (i.e. fudge) the approximate offset but not the massive
jitter inherent in the NMEA sentence(s).

You also should enable only one of the NMEA sentences to reduce the
receiver's workload.

>Can I get other source from this GPS?

No. The GPSMAP 60 is not a timing receiver and does not provide a PPS

Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/

More information about the questions mailing list