TIME

S

SkippyDiesel

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Joined
Nov 24, 2024
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Not a big deal, something I am curious about;

My Dynon is about 3 minutes different, to my local time, as supplied by 1. My phone and 2. Automated Airdrome Weather Information Service (AWIS) .

Obviously I can accommodate a 3 minute variation but wonder why it is so?

😈
 
Skyview gets the time from the GPS satellites. Your phone gets the time from your cellular provider. AWOS frequently gets time from a local clock set at the station.
 
If you've got two Dynon screens, then they should agree to the second - because they use the same GPS time hack. Cell companies set their local time based on where your phone is physically located (pinging the local towers) but their time feed is not guaranteed accurate to the GPS or any other standard.

Other than that - Confucious say "Man with two watches never know what time it is."
 
A GPS time discrepancy of 3 minutes would be remarkable. Given that a *millisecond* error in your receiver (a cycle slip, in GPS solution terms) causes a 300 nm position error, and that GPS satellites use atomic clocks, then your other time sources are untrustworthy.
 
I suspect this is an/another error within SkyView HDX. Flying around yesterday, I compared the times from my SkyView HDX system and my Avidyne IFD540. The SkyView system was off by a little over two minutes. Notably, both the IFD and SkyView showed the same GPS position. My Casio G-Shock, which gets it's time via the NIST time radio broadcast, agreed with the IFD.
 
Wow. That would be a really a basic error in time conversion for display purposes. I have an SV Touch, not HDX, but I'll definitely check this out next time I'm at the hangar.

You can still get NIST time on the phone (WWV broadcast) at 303-499-7111 (and some others for WWVH in Hawaii) if you want the time straight from the source :).
 
swatson999 said:
A GPS time discrepancy of 3 minutes would be remarkable. Given that a *millisecond* error in your receiver (a cycle slip, in GPS solution terms) causes a 300 nm position error, and that GPS satellites use atomic clocks, then your other time sources are untrustworthy.
Click to expand...
Hah! that would be an error of 33,480,000 miles. That could put you somewhere around the orbit of Venus...
 
I'll have to check on mine, I never noticed it. I have two airplanes currently running both Dynon and IFD440 (one certified and one experimental), so I should be able to get two datapoints on this.
 
swatson999 said:
Wow. That would be a really a basic error in time conversion for display purposes. I have an SV Touch, not HDX, but I'll definitely check this out next time I'm at the hangar.
.
Click to expand...
Or possibly clock drift within the system. The usual process for dedicated/embedded systems is to sync the system clock to the external source (GPS, in this case) intermittently then reference the system clock everywhere else. If the system clock drifts, that will show up as a slight difference. No idea if this is actually what is happening, but I'd be kind of surprised if that wasn't the case since the system clock is much more convenient to access than the raw data stream coming from the GPS.
 
True from the engineering standpoint, but still pretty basic. Even for the cheapest of GPS receivers or other systems, letting the displayed time (or internal clock) drift that far from GPS or UTC time would be a grossly poor piece of design.

And you're probably right because the GPS solution itself is correct (GPS only computes X, Y, Z and T).
 
Over a 3-hour flight yesterday my two-screen Dynon HDX and IFD440 stayed within 1 second of each other. I'm running 16.7.2 (certified) and 10.3.1.2 respectively.
 
You're right - there is something here - and I have an inkling of whats going on, working with Dynon tech to flesh it out.

I've confirmed that Dynon does grab the time signal from the satellites - but apparently does not immediately update the system clock to that time, but rather slowly "drifts" the system time to the satellite hack time. The system time will drift over days/weeks of inactivity on the ground due to the inherent issues with quartz oscillators, so then when you power up again and there is a "true time" from the satellite feed, the system time apparently slowly corrects itself to approach that "true time". This morning I started up with 8 seconds discrepancy between Dynon and Avidyne, and when I landed almost 3 hours later there was only 1 second of discrepancy.
 
Thank you for all the very informative technical chit chat and the feedback on out of sync times experienced by a few others.

The short answer, for me, is - the time signal/system is not adjustable by the pilot but may correct itself "over time".

😈
 
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