Terje Mathisen <terje.mathisen@tmsw.no> wrote:
> Jim Pennino wrote:
>> Ted Heise <theise@panix.com> wrote:
>>> On Thu, 5 Aug 2021 17:50:16 +0200,
>>> Terje Mathisen <terje.mathisen@tmsw.no> wrote:
>>>> Jim Pennino wrote:
>>>>> David Woolley <david@ex.djwhome.demon.invalid> wrote: So, in
>>>>> what today we call a gps, there are at least three separate
>>>>> devices, a radio receiver to obtain the the GPS data, a device
>>>>> to generate PPS, and a device to convert GPS data to NMEA
>>>>> sentences.
>>>>
>>>> Except that in a modern chipset as used in a cell phone, all of
>>>> those functions, plus wifi and possibly even more, is a single
>>>> chip.
>>>>>
>>>>> BTW, NMEA stands for National Marine Electronics Association.
>>>>> Guess why commercial GPS devices output data in a marine
>>>>> standard format.
>>>>>
>>>>> In circa 1996 those three devices would likely be in three 19
>>>>> inch rack mount boxes containing mostly discrete components.
>>>>
>>>> Your timing is off by a substantial number of years: Garmin
>>>> sold (portable) GPSs for $105M already in 1995, I have track
>>>> logs which are older than this.
>>>>
>>>> https://en.wikipedia.org/wiki/Garmin#Handheld_GPS
>>>
>>> I for one have been fascinated by this discussion. I ran my own
>>> linux server (Slackware) for the better part of two decades, and
>>> spent quite a lot of time getting ntp up and running to my
>>> satisfaction.
>>>
>>> The Garmin page reminds me I have an old etrex Vista HCx sitting
>>> in a closet at home. I don't suppose there's any way to hack it
>>> into use as a GPS clock? Guessing it may not even generate a PPS
>>> signal, let alone have any way to get it out of the unit.
>>>
>>> It was a terrific device for its time. It rode on the handlebars
>>> of my bike for tracking speed, mileage, and routes.
>>
>> I doubt it even has a PPS signal anywhere as a general purpose
>> navigation device would have no need for it. If it outputs serial NMEA
>> sentences, which I would doubt, you could use it without PPS assuming
>> that an NTP accuracy in the millseconds is good enough for you.
>
> The etrex was a late relative to the Garmin GPS II, II+, III etc, the
> very first devices Garmin made used a two -channel receiver which they
> had to multiplex across the tracked channels, but then they very early
> made their own 12-channel chipset which was used all over the their many
> offerings.
>
> One of those was the model 18 (afair), a puck antenna/receiver which had
> a serial interface, and which included a PPS signal (although at TTL
> levels, but that usually worked as is, otherwise the needed RS232
> buffering chip was dirt cheap to add.
>
> I used both this and the Motorola Oncore UT+ as the sole internal
> reference for a multi-national corporation with 70-80K employees and
> offices and/or factories in 130 countries.

The 18 series is still alive, updated, and targeted for mostly niche
markets, such as aviation.

>> Assuming you want to use something Linux PC based, then your options
>> are for minimal investment:
>>
>> Finding something old on ebay or in a surplus store.
>>
>> A DYI project using inexpensive board sets. This will cost in the $50
>> range and requires soldering, building cables, etc.
>
> Right, except it is in fact easy to get even lower these days.

That depends on how good a shopper you are and how big your junk box of
miscellaneous stuff is as just a DB-9 connector costs about $10 on
mouser. Given the size of my junk box I could probably do it for about
$10, but not everyone has decades of stuffed stached in their garage.

>>
>> A Commercial Off The Shelf device you can just plug in. The cheapest
>> COTS devices I have found that provide both NMEA sentences and PPS are
>> about $180 new on ebay. They also contain a disciplined oven crystal
>
> Almost certainly not: OCXOs were almost completely replaced by TCXOs due
> to both faster startup times and much lower power usage, while getting
> more or less exactly the same performance.
>
> Have this changed back over the last 5+ years?

You bet it has.

Go on ebay and search for GNSSDO GNSS Disciplined Oscillator.

Do that search again and add rubidium to the search.

It appears someone is making a general purpose circuit board where the
final assembler adds their oscillator module of choice and boxes the
whole thing up. It all may be one company, but who knows?

BTW, the price I quoted includes the shipping costs from China to the
USA and the specified settling time is 24 hours for full accuracy, which
is not an issue for something you will run 24/7.

>
>> oscillator with both 10 MHz and 1 PPS singnals on BNC connectors. These
>> are handy if you also do RF or test equipment stuff.
>>
>> There does not seem to be any current market for simple GPS receiver
>> devices with a serial output and PPS. There is a huge market for USB
>> GPS, actually GNSS, devices and if NTP accuracy in the millseconds is
>> good enough for you, they cost about $15 on amazon. Look for the VK-162
>> G-Mouse available from several vendors.
>>
>> If you have a SBC such as a Arduino or Raspberry Pi, you have LOTS of
>> cheap options for building a NTP server.
>>
>> And finally, if you have more money than common sense and want to be the
>> envy of your geek friends, you can get a new rubidium standard box on ebay
>> for about $800 complete with NMEA and PPS on a DB-9 connector and 10 MHz
>> and PPS on BNC's.
>
> What's funny is that you can in fact do far better even using USB, if
> you use the same type of protocol as some of the early (non-GPS)
> reference clocks did, i.e. a two-way protocol where the computer would
> tell the reference clock what it thought the current time was, while the
> receiver measured the round-trip delay, and then sent back a timestamp
> adjusted to be as close as possible when the computer read it.

The only problem with that is that there is no such USB device that I
know of.

> On RS232 this allowed sub-ms, with USB3 speeds you should be able to get
> into the low microseconds range.

The only problem with that is that there are currently no USB 3 GPS devices
at all that I know of.

Since USB 3 has more than a 4 wire interface, it should be possible to
have a USB 3 GNSS device with a real PPS, but again, no such device yet.