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Corrupt Data over 3G/4G


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Vladimir
Vladimir
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I have this problem every time I go flying with SkyDemon:

When I am on the ground with Wifi connected, SkyDemon (version 3.4.4.23 for iPad) downloads and interprets all data successfully and doesn't have any errors.
When I am in the airplane with 3G/4G connection on iPad Air, SkyDemon gives me these error messages:

Corrupt data detected while interpreting downloaded GAFOR information.
Corrupt data detected while interpreting downloaded SIGMET information.
Corrupt data detected while interpreting downloaded TAF/METAR information.

http://forums.skydemon.aero/uploads/images/06575059-d5dd-49a6-9ac4-e0ae.png

They pop out all the time until I lose the connection which is pretty distracting. What causes these errors and how to get fi them?
rikur
rikur
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I've experienced a similar issue - albeit not as severe. 

I do find when using a mobile data connection (in my case EE) that I am much more prone to getting corrupt data warnings than when I'm using fixed broadband.

I speculate this may be due to the content compression that mobile network providers like EE use to compress data being sent over their network as I've seen this cause problems with other data services.
rikur
rikur
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I guess there could be many reasons for this - including occasionally genuine corrupt data in the METAR/TAF themselves.

the actual data files that SkyDemon fetches for these are tiny (often less than 1kB) - so I guess a timeout could be possible, but it feels unlikely

in my case I still see these even when using a 4G 20mbps network connection - but then they work fine on the 2mbps club wifi - so speed doesn't appear to be the issue.  it's also fairly intermittent, generally it's ok - but then some days it just errors repeatedly on 3G/4G.

doing a little bit more digging, I've noticed that the data for meta/taf/sigmet is sent with a content-type header of text/plain (metar/taf) and text/html (sigmet) - so these files will be subject to on-the-fly 'optimisation' by some mobile networks (this optimisation does things like removing double spaces and line breaks that in html serve no purpose).

looking at the content - it isn't plain text though, so 'optimisation' may cause corruption.

may be Tim could change the content-type header to application/octet-stream for these files?



Edited 2/23/2016 8:18:39 AM by rikur
Tim Dawson
Tim Dawson
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The corrupt data message is nearly always because we have asked our server for some data, and the response we got back is from your mobile broadband provider, which is sending a page saying something like "your data is about to run out! click here to purchase more!" etc.
rikur
rikur
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I'll have to pay more attention next time I get one of these (which isn't that often).  
In my case it won't have been the mobile network asking me to buy more data - but perhaps the content filtering didn't like me flying near Scunthorpe or the TAF had too much flesh coloured content ;-).
Whatever it is, I think we're both agreeing it's the mobile operator meddling with the data for some purpose or other.


Vladimir
Vladimir
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Today's connectivity clearly has these problems and it's not surprising that some messages fail. It is the way to handle them that is important. In this case the question is if it is really needed to get such big red messages taking almost half the screen to inform a pilot in flight or on the ground that there was a connectivity problem. Maybe a more subtle message would be more appropriate.
Tim Dawson
Tim Dawson
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It's not a connectivity problem. It's the data being tampered with enroute. This is unusual and should not be happening, so we're definitely happier with the standard error messages than we would be either ignoring it, or showing a new kind of error interface which we'd have to invent.

Next time you see it, quickly switch to your web browser and try going to our website, and see what happens.
rikur
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Hi Tim,
is there any way to see what the corrupt data is?
I've just got a 'Corrupt data detected while interpreting downloaded Forecast Wind information' - and a web browser opens up just fine - and no mention of corrupt METARs/TAFs/etc. 
I wasn't sure if the 'corrupt data' gets written to a temporary file somewhere?
I realise this may be a different corruption cause than the mobile network meddling.



Tim Dawson
Tim Dawson
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It doesn't get written to a file. SkyDemon tries to parse the data returned, and if there's an exception while parsing it, that's what triggers the error being discussed.
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