Off-Grid GPS Time Sync

K7JLJ.com
02.18.19

One reason why digital is (IMO) never going to be a true SHTF mode of operation is the elaborate equipment, required operating knowledge and need for time synchronization.

From the WSJT-X Manual

System Requirements:

  • SSB transceiver and antenna
  • Computer running Windows (XP or later), Linux, or OS X
  • 1.5 GHz or faster CPU and 100 MB of available memory
  • Monitor with at least 1024 x 780 resolution (more is better)
  • Computer-to-radio interface using a (virtual) serial port for T/R switching or CAT control or VOX, as is needed for your radio to computer setup
  • Audio input and output devices supported by the operating system, capable of 48 kHz sample rate
  • Audio or equivalent USB connections between transceiver and computer
  • A means for synchronizing the computer clock to UTC within ±1 second

While there are those trying to bring digital modes for SHTF to the masses like OH8STN and groups practicing off-grid comms like RADAR... When you look at the small number of stations operating digital (compared to SSB / CW) then subtract all operators that require grid power to do so, the numbers are not there yet.

I'm trying to setup my own EmComms ability to operate "digital off-grid", but while I don't consider it a primary SHTF mode, there is no denying that digital modes like JS8Call will reach out so much farther than SSB / CW in crappy propagation conditions.

It would be unwise to ignore digital modes entirely though, as they may be the only way to "make the trip" when you need to the most.

Like all things HAM related though, unless you are building your digital mode proficiency now, you will not own it when you need it the most..

For instance; if you can not copy even 5WPM CW now, you won't learn it in a crisis. Same goes for setting up digital modes.

GET TO IT ALREADY!

Anyway, this post is about how to keep your computer synced via a cheap GPS dongle for these modes when the Internet is down.

Programs like JS8Call have solved this problem to an extent by allowing one to sync their computer to the reception of a signal start or stop time and with practice you can do so pretty easily, as you only need to be within +/- 2 seconds for it to work! For a great video on the matter, checkout KM4ACK's YouTube channel.

But for less than $15.00, there is a more elegant way.
The device is a U-Blox USB dongle that you can purchase of Amazon. It's advertised as a Windows device, but Linux is able to use it with a generic driver as shown below.

I first saw this dongle GPS on Julian's (OH8STN) channel being used on a Raspberry Pi, but not knowing a thing about them (Raspberry's) other than I read it uses a form of Linux, I bought one for my Linux laptop hoping I could figure out how to get it going after the fact.

Somehow I actually managed to get it working! Not that I knew what I was doing, but after a day of Googling, I found someone that did and simply followed their instruction.

The guide I'm referring to is at:
https://blog.rapid7.com/2015/07/27/adding-a-gps-time-source-to-ntpd/

In case that page disappears as well as a saved PDF of the website page at my DropBox: HERE


DOWN AND DIRTY TERMINAL COMMANDS

sudo apt-get install gpsd gpsd-clients python-gps chrony  

sudo nano /etc/default/gpsd  

    START_DAEMON=”true”   
    USBAUTO=”true”   
    DEVICES=”/dev/ttyACM0″   
    GPSD_OPTIONS=”-n”  

sudo nano /etc/chrony/chrony.conf  
add to EOF  
    refclock SHM 0 offset 0.5 delay 0.2 refid NMEA

Check that both services are active

systemctl is-active gpsd  
systemctl is-active chronyd

See the data: Be patient, it can take up to 5 min to find satellites. Don't forget you might need to unplug GPS and reinsert on initial setup.

gpsmon -n

Shows number of SATs and strength

cgps

Shows Maidenhead Location

timedatectl set-ntp true

Sets clock to GPS.


Below are images of testing the sync offline

right click and open in another tab for larger image

First thing is to offset the clock and disconnect from Internet update mode. I have disabled WIFI and rebooted after this snapshot.

After a reboot of the computer. I then plugged in the USB dongle and confirmed that both services were running. Notice that the clock has updated now and is showing the correct 15:29 instead of 10:28

Screen grab of data windows using chronyc services to see where timesync is coming from the GPS, cgps on left which gives your Maidenhead locator, and gpsmon -n which give number of sats and strength.

Using this setup, and leaving the clock set to get time from the network (see first image) you will have accurate time from either source automatically.

The end.

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