RE: Questions (Answered)

[Ken.Hendrickson]

I have found more information that answers most (if not all) of my questions.

The gpsd.8 man page indicates that gpsd can use the Zodiac binary data.

Austin's Nerdy Things describes a microsecond accurate solution on a Raspberry 
Pi w/ 1PPS.
https://AustinsNerdyThings.com/2021/04/19/microsecond-accurate-ntp-with-a-raspberry-pi-and-pps-gps/

Finally, I found these IETF documents:
https://DataTracker.IETF.org/doc/html/rfc2783
https://DataTracker.IETF.org/doc/html/rfc1589

So most of my questions have been answered.  I plan to:
* wire up the GPS so that it outputs Zodiac binary messages
* use a Raspberry Pi in the chassis
* wire up the 1PPS to the Raspberry Pi
* read the Zodiac binary messages from the Pi's serial port

This will give me a stand-alone time-frequency standard that will work as long 
as GPS is available.
Differential GPS would hold no value for me, nor be practical if using the 
system portable or mobile.

Original set of questions below:

EXECUTIVE SUMMARY

With the equipment I have, would it be worth while to configure it as a 
stand-alone time server?
(Even if no, it will be useful as a 10 MHz frequency standard.)

DETAILS

About 15 years ago, I bought a kit for a GPS disciplined 10 MHz oscillator.  It 
consists of a Rockwell Jupiter GPS receiver, an Isotemp OCXO (actually a VFO), 
and a phase locked loop.  The GPS receiver puts out 1PPS, and a 10 kHz 
reference signal.  The PLL divides down the 10 MHz oscillator by 1000, compares 
it to the 10 kHz reference signal, and provides an error signal to drive the 10 
MHz OCXO on frequency.  The GPS receiver has two serial ports; one can be used 
for differential GPS input, and the other can be used either for NMEA messages, 
or a proprietary Zodiac binary message format.

I have complete documentation for all of the above.  I will provide this 
documentation to the gpsd project.

This web page shows pictures of some of the components (OCXO, Jupiter GPS, PLL 
board, PLL board schematic):
http://www.DiyPhysics.com/2012/02/19/d-i-y-gps-disciplined-10-mhz-frequency-standard-gps-based-universal-time-clock/

I'm finally getting around to finishing this project, because now I have a need 
for a 10 MHz frequency standard.  I have options in the way I put the 
components together.  That is what this email is about.

Can gpsd read the proprietary Zodiac binary message format?  If not, I could 
write that software.  Apparently, the time mark message which precedes the 1PPS 
pulse is only output in the 9600 bps binary message format, but not 4800 bps 
NMEA.  (The UTC Time Mark pulse output message provides the number of seconds 
since the beginning of the week, about 0.5 second before the 1PPS signal.)

What is currently the best way to utilize the 1PPS signal?  GPIO on something 
like a Raspberry Pi or BeagleBone?  I would be very willing to package a small 
computer along with the other hardware to run gpsd and ntpd.  What are any 
accuracy limitations to using GPIO or similar?  At what rate can a GPIO digital 
input be read?  Is there a better way to read the 1PPS, such as a non-maskable 
interrupt, to operate at essentially the CPU clock rate?

Using the 1PPS signal seems to require being able to read the proprietary 
Zodiac binary message format.  That is configurable by pulling some pins on the 
Jupiter GPS receiver high or low.  You can't have the NMEA standard messages 
and the Zodiac binary messages simultaneously; it's one or the other, but not 
both.  However, if better than 0.5 second clock accuracy could be obtained by 
other means --- so the binary Time Mark message wasn't required to identify 
what second the upcoming pulse was --- then the 1PPS signal could improve its 
accuracy considerably.

One objective I have, if possible, is to have a stand-alone time server with no 
internet connection.  Is this possible?  What would the gpsd requirements be?

Does anybody use differential GPS?  What would be required?  Would differential 
GPS only improve the geolocation solution, or would it also improve a time 
solution?

I look forward to receiving any replies.  I have already done some preliminary 
searching of the gpsd-users email list, but not found the answers I'm looking 
for.

Thanks,
Ken


  

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