[ntp:questions] Too much offset from GPS?

Unruh unruh-spam at physics.ubc.ca
Tue Mar 11 15:18:55 UTC 2008


Steve Kostecke <kostecke at ntp.org> writes:

>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
>requirements.

>The USD249.99 GPSMAP 60 is a mapping receiver.

>The USD74.50 GPS18 LVC is a timing receiver.

68.50 You are quoting the USB which is useless for timing.


>GPSMAP 60
>https://buy.garmin.com/shop/shop.do?pID=224&tab=gpsmap60#specsTab

>GPS 18 OEM
>https://buy.garmin.com/shop/shop.do?cID=158&pID=223

MAKE SURE you get the LVC, not the PC or USB version. Only the LVC is useful
for timing. 






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