uBlox LEA-5T, LEA6-T? SkyTraq Venus5?

On Tue, Oct 16, 2012 at 11:06 PM, xaos@darksmile.net wrote:

On 10/16/2012 05:06 PM, xaos@darksmile.net wrote:

It's not a terrible idea, but the RPI has a USB ethernet transceiver so
in addition to the latency/jitter of the ethernet it also has the
latency/jitter of the USB. I've also heard of stability problems just
keeping it running for weeks to months so you should integrate some kind
of watchdog timer if you can. The actual GPS module doesn't matter much
since NTP will smooth out even the worst GPS jitter. I have heard
second-hand (or third-hand or fourth-...) that some have a significant
persistent delay and that could be more of a problem. If you want to go
for a timing-oriented receiver you can get a used Trimble Resolution T
from ebay but they have a 2mm pitch header.

You will want to house the RPI and GPS receiver in a box where it will
not be subject to wide temperature swings, insulated and shielded from
drafts. It would also be interesting to upgrade the main oscillator to a
temperature-compensated model so NTP doesn't have to work as hard to
keep the frequency locked.

Personally I would recommend getting a more robust single-board
computer, e.g. a PC Engines ALIX or Olimex olinuxino. RPI is cheap but
hard to source, not open-source, and does not have good long-term
prospects due to the microprocessor being used. Most of the attention is
due to deliberate publicity by the manufacturer and not novelty or
merit. If you must use the cheapest board then by all means do so, but
just know there is better available for not much more.

Happy ticking,
-- m. tharp

Michael,

Thank you for your most valuable information.
This is why time-nuts is so amazing!

My goal is actually two-fold.

First, I would like to have a NTP server that I can easily
hook up (well, as easily as possible) without
dedicating a Linux server each time.

My personal use would require 3 separate NTP servers
because I am a time-nut and stratum 2 is not an option.

The idea here is to create a smart piggy-back architecture
where the CPU/Ethernet sits on the bottom
with (perhaps) a few modules on top of that
with GPS right on top (let's call it the penthouse).

For obvious reasons I would put the Power supply
separately.

So we are looking for a very small footprint
NTP server.

Now, I thought the RasberryPi would be ok
even though I did not like the limitations you
mentioned as well. It is very popular.

You mentioned the Olimex olinuxino. Wow!
This is a thing of beauty indeed!

https://www.olimex.com/Products/OLinuXino/A13/A13-OLinuXino/

I really like this board.

My second goal is that if I go to the
trouble of creating a GPS-daughterboard and all that
I'd like to have other people use it as well
and maybe I would make a small batch.

Cost wise this should be in the 10-20 dollar range tops.

I think I will do some research on this board and maybe
order one. It looks very promising.

I wonder if I can replace the crystal with a better
high stability one.

-George

Quoting Michael Tharp gxti@partiallystapled.com:

On Tue, Oct 16, 2012 at 3:26 PM,  xaos@darksmile.net wrote:

One does NOT need a dedicated server for NTP.  NTP can run on a linux
system that is also a web and mail server or on e  linux desktop
system that you use for web surfing and web browse ring, just as long
as the box stays running and you don't turn it off.

What level of accuracy is required?  Don't say "as good as posable"
because that can be quite expensive.  What really do you need?  About
the best you can expect from NTP without exotic hardware is 2 u-sec
level.  But maybe you'd be happy with 2 milli seconds.

Remember that you cn buy a fully integrated Intel Atom mainboard that
is a bout 6" square and uses only a few watts for $90. The Atom will
run NTP and mail and web servers and allow web surfing all at the same
time and costs $90 including the soldered down CPU.  It will have a
real RS232 port and a good built-in Eithernet.  These boards don't
require fan cooling.

Why?  I hate power cubes

It is popular for some applications.  But a statum 1 NTP server needs
the PPS from the GPS to go to a hardware interrupt and it needs a
solid Ethernet controller with low latency.

Look at this one:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813121442

It is directly usable for NTP with no circuit design or soldering and
the NTP server will run at the few u-sec level while also running as a
file seder, web server and runing LLady Heather inside a virtual
Windows under VMware.  Not heat or fan noise.

--

Chris Albertson
Redondo Beach, California

One option would be just a DE-9 connector with power on pin ??? (I forget).
You may need inverters and/or level shifters on Rx and Tx.

That would let you connect up any of the serial GPS devices.

--
These are my opinions.  I hate spam.

On Tue, 16 Oct 2012 15:06:19 -0600
xaos@darksmile.net wrote:

I would go for the LEA modules. For once because you get pretty complete
documentation for them (for free, w/o NDA) and because you can get the
older modules (like LEA4-S) for 20-30CHF on ebay. Yes, they are not
timing receivers, but the datasheet specs the PPS output as better than
60ns, which is for the use of an ntp server more than good enough.
The newer ones are footprint compatible (modulo new function pins)
and with a little care when doing the design you can solder all from
LEA4 to LEA6 onto the same board without changes.

A new LEA6-T single piece cost IIRC 190EUR from the webshop.
A LEA5-T was IIRC half price.

And a little caution: the LEA5 modules use considerably more power during
aquisition than the LEA4 did and the LEA6. IIRC we measured up to 150mA
(compared to <100mA with LEA4). I don't know the Rasberry, but i guess
that should be not a proble, but you should check for it nevertheless.

		Attila Kinali

--
The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
-- Tirin, The Dispossessed, U. Le Guin

Hi George,

Welcome back to the list; you've been quiet for a long time. This sounds like a great project. I hope you keep us informed with progress and also make the results available.

You should be able to find GPS modules with sub-microsecond 1PPS for $25. Check sparkfun.com or the "Skylab" chip used on the www.leapsecond.com/pages/MG1613S board. This and other chip solutions require SMT/PCB mounting, connectors, external antenna and cabling which can add to the net cost of complexity.

A number of people I know who want an inexpensive, turn-key, sub-microsecond, embedded GPS timing solution continue to chose the Garmin 18x/LVC. These are nice because of the high-gain integrated antenna (they work indoors), compact encapsulated design, and extreme ease of use (5V, ground, 1PPS).

/tvb

Attila,

I'd say the u-blox 5T or 6T is rather over-kill for George's application, both in price and performance (and learning curve and integration). Remember that the world of NTP is milli- and microseconds. State-of-the-art GPS timing receivers and GPSDO are in the nano- and picosecond accuracy/jitter range.

/tvb