[time-nuts] Suggestion for a timing GPS receiver (Trimble / Ublox / other?)

[Attila Kinali]

On Mon, 22 Jan 2018 16:38:01 +0100
Paride Legovini via time-nuts <time-nuts at febo.com> wrote:

> I plan to build a decent GPS/GNSS-based Stratum 1 NTP server, and I'm
> looking for a good and possibly affordable timing GPS receiver.
> 
> As far as I can tell there are a few common options. One is the Trimble
> Resolution T (or SMT), that should be good and proven. The major
> annoyance is converting the 3.3V TTL serial logic-level to RS232, but
> should be doable with a MAX3232 and a few capacitors.
> 
> Another option is this UBLOX LEA-5T:
> 
> https://www.ebay.it/itm/291769145465
> 
> The seller advertises an RS232 output pin, but I'm not sure it's
> actually available, and I'd need at least lines, one for the NMEA
> strings and one for the 1PPS signal. So, even if a RS232 TX pin is
> actually there, it is probably not enough to build a good reference
> clock, and I'd still need to convert some TTL levels to RS232...
> 
> Does anybody have experience with this receiver?
> How does it compare to the Resolution T/SMT?

For raw GPS PPS output, your performance is limited by multi-path
and how much of the sky you see. If you have perfect conditions,
I expect both to perform at the same level. Under not so perfect
conditions, go for the LEA. I have here a Trimble UCCM and 
Oscilloquartz Star4 GPSDO (using a LEA-5T). Compared to the Star4,
the UCCM is basically deaf. And mind you, the LEA-5 family came out
somewhen around 2006, ie is already a more than 10 year old design
and probably of the same age as the UCCM. The modern LEA's got a few
dB better yet.

For an NTP server, where you wont need anything better than 1µs
resolution, I think one of the many 10-bucks ublox LEA/NEO boards
that you can find on ebay are good enough (standard navigation
receivers give you better than 100ns accuracy). If you still want
to have a timing receiver, then go for one of the NEO-M8T, LEA-M8T or
LEA-6T based boards. Be aware that the NEO modules do not have the
circuit to provide power to the antenna, so it must be either
passive or you need an external bias-T. The LEA's can provide up
to 50mA (so no Zephyr!). 

If you want to step up your game and get even better stability,
take one of the LEA based GPSDOs (like the Star4 mentioned above).


			Attila Kinali


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