[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: Questions (Answered)
RE: Questions (Answered)
Wed, 22 Jun 2022 21:17:30 +0000
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.
Finally, I found these IETF documents:
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:
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.)
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):
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
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
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
CONFIDENTIALITY NOTICE: This email and any attachments are for the sole use of
the intended recipient and may contain material that is proprietary,
confidential, privileged or otherwise legally protected or restricted under
applicable government laws. Any review, disclosure, distributing or other use
without expressed permission of the sender is strictly prohibited. If you are
not the intended recipient, please contact the sender and delete all copies
without reading, printing, or saving.
- RE: Questions (Answered),