sotamat, how the ft8 audio is synchronized

Hello!

The software sotamat by Brian, @AB6D looks to me very elaborated and promising.

I just do not understand, how a ft8 audio containing a one way command can be sychronized by the app, when there is no cell service, no internet and no gps time available. That could be the case in a remote area with a phone without built in gps.

Probably the information exists already somewhere and I could not find it.

Thank you in advance and 73, Johannes

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I believe it uses the device’s internal clock. Normally, when connected to cellular service or Wi-Fi, this clock is synchronized. However, when you’re out of service, the clock can drift after a few hours or days. That’s why the app includes a synchronization correction feature. You manually tap the button when you hear the start of an FT8 frame to correct the drift.

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@OE6JBG,

It works as @F4LEK says. FT8 needs a good time sync, but not a crazy-good time sync. In practice I find you have to be within +/- about 1.5 seconds for at least one of the monitoring stations to pick you up. That is a 3-second window out of the 15 second period, or put another way 20%. In other words, if you randomly pick a time you might be successful 20% of the time.

After years of working and backpacking with SOTAMAT, I find that iPhone’s have very stable clocks that maintain +/- 1.5 seconds for at least several days off grid. I don’t know how they do this. It could be that they use the GPS time signal to correct. It could be that they have better clocks. I don’t know why but I have experience.

Android phones, however, tend to time drift very quickly when off grid. They can drift multiple seconds in a day. There are apps that allow you to get a GPS time signal and set your Android system clock. I had looked into adding such functionality to SOTAMAT but found that the approach is very complex and somewhat manufacturer specific. Since I am making an app that works for both iPhone and Android, I simply don’t have the free time to make auto GPS time work. Instead there are three work-around approaches for Android (two of which also work for iPhones):

  1. The SOTAMAT app (under the “Setup” tab) has a button called “Sync to time source”. If you have any time source you simply click that button at the top of the minute and it will compute the offset between when you clicked the button and your phone’s internal second hand. It will program that offset (some value between -7.5 seconds to +7.5 seconds within the 15-second window) into a time offset for SOTAMAT: a calibration. It will not change your phone’s clock, but will change SOTAMAT’s clock relative to the phone’s clock. The next question is where do I get a time source? Well you can:
    A. Listen to the other FT8 stations on an FT8 frequency. ~80% of those stations will be GPS time locked and are a very good time reference. For example on14.074MHz you can hear all the other stations transmit FT8 packets for 12.5 seconds and then go quiet for 2.5 seconds and then when you hear the majority of stations starting their next 15-second window (the beginning of transmissions) you click the SOTAMAT “Sync to time source” button. You are now sync’ed and ready to send your own FT8 packet +/-1 second of accuracy.
    B. The SOTAMAT app has a menu tab “… (more)” where there are additional SOTAMAT reference documentation pages. One of those reference pages is called “Frequencies Reference”. Inside the “Frequencies Reference” is a list of “Time References” which is a table of world-wide atomic clock broadcasts from various governments. In the USA we have an atomic clock broadcaster called “WWV” (Colorado, USA), and “WWVH” (Hawaii, USA) which transmit time information on 10.0, 15.0, and 20.0 MHz. You tune your radio to a beacon, and click the “Sync to time source” button when you hear the top of the minute.
  2. You can use an app that shows you GPS time and find your system offset and manually correct for it.

My recommendation is to get your time to be +/- 1 second, even though some skimmers might be a bit more forgiving. The easiest of the methods above is (1)(A), and it means you’ll already be on the right frequency to send your self-spot immediately after sync’ing your time.

But in practice I think you’ll end up using method #0 which is:

  1. You’ll find your phone keeps good enough time for half a day (or more) and just works without any time correction.

Good luck,
73 de AB6D - Brian

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There is a YouTube video that shows clock sync over the air: https://youtu.be/eBgVnuC_bSg?si=umMjYX_oS0BJefNx&t=1000

(…and if you play the video from the beginning it is an entire tutorial).

-Brian

Doesn’t sound unreasonable for a <2 secs drift in a few days.

OT: I have a Citizen EcoDrive watch, it was pricey, c. ÂŁ450 in 2011. It was last set against ntp controlled PC time in 2023. Currently it is 9 seconds fast. I find this quite impressive.

Anyway, if your phone is on the network when you park up, walking up a summit or two with no network isn’t going to lead to massive loss of sync.

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Hi Brian,

thank you for the comprehensive and detailed information to my questions. Now I clearly see the limits of and prerequisites to the procedure. That helps a lot.

Good luck and keep the good work going, 73, Johannes