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Re: [gpsd-users] Trouble Receiving Time from Delorme EarthMate USB (LT-2

From: Gary E. Miller
Subject: Re: [gpsd-users] Trouble Receiving Time from Delorme EarthMate USB (LT-20)
Date: Wed, 13 Mar 2019 12:39:18 -0700

Yo Jeff!

On Wed, 13 Mar 2019 08:47:20 -0700
Jeff Morris <address@hidden> wrote:

> I recently came across a Delorme EarthMate USB (LT-20) at a thrift
> shop for a couple of bucks, and picked it up with the hope of being
> able to use it as an NTP stratum-0 time source for an NTP server on
> my home network.

Ever wonder why it was so cheap?  You can buy real good new GPS
for around $10.

> I've managed to get gpsd installed and running, and it sees the 
> EarthMate. The EarthMate acquires satellites and gets a fix and shows
> my correct position, so it seems to be working properly. However gpsd
> is reporting a date from the EarthMate far in the future. For example
> for today, 2019-03-13, it's reporting the date as 2099-07-28. The
> time is correct, but the date is way off. If I look at the raw time
> in seconds-since-the-epoch format, I see numbers like 134439.000.
> (These numbers are both from the output of gpsmon.)

Surprise, your unit has the GPS week rollover bug...

> I've added the following lines to /etc/ntp.conf (and removed all
> other servers):
> server minpoll 4 maxpoll 4 prefer
> fudge time1 0.0 refid GPS

NTP Classic or NTPsec?

> However, ntpq -p reports that it has never successfully polled this
> source:

Before you look there, does the cgps output look good?  What does
"ntpshmmon" show?

My guess would be that the bad date has the time stamp thrown away.

> I'm aware of the GPS week rollover issue, and given that this is an
> old receiver that I picked up at a thrift shop it may very well be 
> susceptible to that.

You already confirmed that with the bad date.

 However, the numbers I'm seeing don't make any 
> sense to me. The 10-bit week rollover issue should result in a
> rollover every 20 years? The date I'm seeing is 80 years in the
> future

Yup.  A century minus 20.

The Delorme were not very good when they were new, and now they are
really old...

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin

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