Re: [gpsd-users] Trouble Receiving Time from Delorme EarthMate USB (LT-20)

[Jeff Morris]

On 3/13/2019 12:39 PM, Gary E. Miller wrote:
> Yo Jeff!

Hi!

> 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?

Well as I said, "thrift shop", lol. Everything there is cheap.  :-) 
(I've actually gotten quite a few good deals on older gear there. Lots 
of fun projects. I even found a working Raspberry Pi with a case and SD 
card for $5 one day.) So no, I didn't really question it.


> You can buy real good new GPS for around $10.

Good to know. The last time I thought about doing something like this 
and looked into it (probably 5-6 years ago), they were nowhere near that 
inexpensive. More like $100+ and I just couldn't justify it for a toy. I 
also tend to shy away from the dirt cheap "straight from China" stuff 
that the market seems so flooded with these days, since I've found so 
much of it to be just plain bug-infested, unsupported junk. I figured 
buying the older Delorme would be a better/more reliable route to take. 
Silly me. :-( Do you have any specific recommendations for a good unit 
for the $10 you quoted?


> > 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...

Yup, I kind of suspected that. The numbers just weren't making sense to 
me though. I was also hoping there might be a workaround.


> > I've added the following lines to /etc/ntp.conf (and removed all
> > other servers):
> >
> > server 127.127.28.0 minpoll 4 maxpoll 4 prefer
> > fudge 127.127.28.0 time1 0.0 refid GPS
> NTP Classic or NTPsec?

Classic (the one distributed by ntp.org.) I'm running CentOS, so it's 
just the off-the shelf ntp package included in the EPEL repo


> > However, ntpq -p reports that it has never successfully polled this
> > source:
> Before you look there, does the cgps output look good?

Cgps output looks good, in the sense that I can see it has acquired 
satellites and is giving me valid location data (disclaimer: I'm new to 
gpsd, so I may not be qualified to determine if it's output is really 
"good" or not.) :-) Here are a few lines of output from it. (I have no 
idea why that second line seems truncated. Some sort of ncurses issue 
possibly):

{"class":"VERSION","release":"3.11~dev","rev":"3.10-5.20140524gitd6b65b.el7","proto_major":3,"proto_minor":9}

87Z","flags":1,"native":0,"bps":115200,"parity":"N","stopbits":1,"cycle":1.00}]}
{"class":"WATCH","enable":true,"json":true,"nmea":false,"raw":0,"scaled":false,"timing":false,"split24":false,"pps":false}
{"class":"TPV","tag":"RMC","device":"/dev/ttyUSB0","mode":2,"time":"2099-07-28T20:19:51.000Z","ept":0.005,"lat":47.751133333,"lon":-122.480016667,"epx":17.607,"epy":21.771,"track":0.0000,"speed":0.051,"eps":43.54}
{"class":"TPV","tag":"GGA","device":"/dev/ttyUSB0","mode":3,"time":"2099-07-28T20:19:51.000Z","ept":0.005,"lat":47.751132500,"lon":-122.480024833,"alt":50.780,"epx":17.607,"epy":21.771,"epv":59.800,"track":0.0000,"speed":0.051,"eps":43.54}
{"class":"SKY","tag":"GSV","device":"/dev/ttyUSB0","xdop":1.17,"ydop":1.45,"vdop":2.60,"tdop":1.94,"hdop":1.70,"gdop":4.74,"pdop":3.10,"satellites":[{"PRN":5,"el":25,"az":283,"ss":0,"used":false},{"PRN":7,"el":72,"az":67,"ss":26,"used":true},{"PRN":8,"el":40,"az":83,"ss":0,"used":false},{"PRN":9,"el":39,"az":158,"ss":35,"used":true},{"PRN":11,"el":6,"az":125,"ss":0,"used":false},{"PRN":13,"el":15,"az":313,"ss":0,"used":false},{"PRN":23,"el":9,"az":147,"ss":0,"used":false},{"PRN":27,"el":25,"az":48,"ss":30,"used":true},{"PRN":28,"el":39,"az":219,"ss":33,"used":true},{"PRN":30,"el":67,"az":295,"ss":37,"used":true}]}
{"class":"TPV","tag":"RMC","device":"/dev/ttyUSB0","mode":2,"time":"2099-07-28T20:19:52.000Z","ept":0.005,"lat":47.751133333,"lon":-122.480016667,"epx":17.607,"epy":21.771,"track":0.0000,"speed":0.051,"eps":43.54}
{"class":"TPV","tag":"GGA","device":"/dev/ttyUSB0","mode":3,"time":"2099-07-28T20:19:52.000Z","ept":0.005,"lat":47.751132333,"lon":-122.480024833,"alt":50.760,"epx":17.607,"epy":21.771,"epv":59.800,"track":0.0000,"speed":0.051,"eps":43.54}
{"class":"SKY","tag":"GSV","device":"/dev/ttyUSB0","xdop":1.17,"ydop":1.45,"vdop":2.60,"tdop":1.94,"hdop":1.70,"gdop":4.74,"pdop":3.10,"satellites":[{"PRN":5,"el":25,"az":283,"ss":0,"used":false},{"PRN":7,"el":72,"az":67,"ss":25,"used":true},{"PRN":8,"el":40,"az":83,"ss":0,"used":false},{"PRN":9,"el":39,"az":158,"ss":35,"used":true},{"PRN":11,"el":6,"az":125,"ss":0,"used":false},{"PRN":13,"el":15,"az":313,"ss":0,"used":false},{"PRN":23,"el":9,"az":147,"ss":0,"used":false},{"PRN":27,"el":25,"az":48,"ss":29,"used":true},{"PRN":28,"el":39,"az":219,"ss":34,"used":true},{"PRN":30,"el":67,"az":295,"ss":37,"used":true}]}
{"class":"TPV","tag":"RMC","device":"/dev/ttyUSB0","mode":2,"time":"2099-07-28T20:19:53.000Z","ept":0.005,"lat":47.751133333,"lon":-122.480033333,"epx":17.607,"epy":21.771,"track":0.0000,"speed":0.051,"eps":43.54}
{"class":"TPV","tag":"GGA","device":"/dev/ttyUSB0","mode":3,"time":"2099-07-28T20:19:53.000Z","ept":0.005,"lat":47.751132500,"lon":-122.480025167,"alt":50.750,"epx":17.607,"epy":21.771,"epv":59.800,"track":0.0000,"speed":0.051,"eps":43.54}
{"class":"SKY","tag":"GSV","device":"/dev/ttyUSB0","xdop":1.17,"ydop":1.45,"vdop":2.60,"tdop":1.94,"hdop":1.70,"gdop":4.74,"pdop":3.10,"satellites":[{"PRN":5,"el":25,"az":283,"ss":0,"used":false},{"PRN":7,"el":72,"az":67,"ss":24,"used":true},{"PRN":8,"el":40,"az":83,"ss":0,"used":false},{"PRN":9,"el":39,"az":158,"ss":35,"used":true},{"PRN":11,"el":6,"az":125,"ss":0,"used":false},{"PRN":13,"el":15,"az":313,"ss":0,"used":false},{"PRN":23,"el":9,"az":147,"ss":0,"used":false},{"PRN":27,"el":25,"az":48,"ss":30,"used":true},{"PRN":28,"el":39,"az":219,"ss":33,"used":true},{"PRN":30,"el":67,"az":295,"ss":37,"used":true}]}


> What does "ntpshmmon" show?

Hmm. I don't seem to have an ntpshmmon program in my distro. I don't see 
it in any other packages in the standard repo either. Is it perhaps only 
included with NTPsec?


> 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.

Ah! Ok, that makes more sense. I was trying to figure out the scenario 
under which I could end up with a date 80 years off.


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

Well, I'm old myself and still ticking ;-) and I've always gone with the 
philosophy that if it isn't broken, no need to fix it, so I figured an 
old unit would work fine for such a basic need as just pulling the 
timestamp from GPS. Le sigh.

While searching for a solution to this, before I posted to the list, I 
came across an old thread where someone was discussing the possibility 
of adding a fix for the rollover bug to gpsd. I think the general 
consensus was that since it isn't a gpsd bug, that gpsd shouldn't be the 
place to fix it, plus there are challenges in coming up with a 
workaround that could automatically and reliably determine what offset 
to use. I understand the philosophy, but I do wish there was a way to 
just manually set the epoch date or something like that, for cheap old 
farts like me that just want to use the hardware they already have, and 
who enjoy the challenge of getting obscure, broken stuff to work. :-)

Thanks for your help! Again, if you have a recommendation for a $10 unit 
that will work reliably, I'll probably end up going that route.