at least iOS can be taught to not do things in the background.
I don't think so...
There is no config for stuff like app version updates.
In general terms, if you disable Wifi and disable Mobile Data, you are safe. This is true for all mobile products. But then you can't do anything with it 
If you have Wifi in the cockpit, passengers will try to use it, with smartphones etc of all kinds.
OK, so one Iridium Axcess point going cheap......
As a trial I bought one as my friend has the Iridium Sat phone so worth a punt, I managed to get it all connected with maximum compression, with a full signal it took a couple of minutes to down load the metars, so reasonable. This was on the IPAD
After 7 minutes + it would not download the radar overlay which takes just a second or two on 3G
I knew the Iridium data was going to be slow but it was an easy test to rule it out.
Also now understand the problem of the ipad likely allways trying to download something
So back to the drawing board, using the ipad is damn convenient but I am agreeing not very practical now.
Funnily on the MLX770, radar data downloads quickly as do all metars and ir SATs. Must be using the right compression.
radar data downloads quickly
How quickly is a completely fresh image downloaded, and what is the geographical area it represents?
Anyway, I don't think compression is necessarily the answer here.
The problem with the "internet" is that it uses the IP protocol which doesn't like long latencies (packet delays). It "works" in as much as the error correction will take care of it, but it is sloooooow.
The worst possible thing is running an "internet" IP-protocol connection, at an Iridium-like low speed, with the long latencies inherent in satellite comms.
I have no knowledge of what Avidyne are going but if I was doing this, I would just transmit everything in UDP packets and to hell with losing a few. There is a CRC on each one so you know if the taf or metar you just got is duff, so you request a retransmission and a few seconds later you get a good one, and nobody will care about the delay. With weather images, a lost or corrupted packet doesn't matter. The server can just squirt out the whole image, about 10-20k of data, and maybe fill in the bad ones at the end.
Also the MLX770 has ample opportunity to hide delays because it can download speculatively. Avidyne have an OEM deal with Iridium; they are not paying the eye-watering retail price.
Just as an aside sodding around with the Iridium axcess point trying to get the wx to download last sunday cost $89.00 for our efforts in airtime, onwards and upwards however , going to try globalstar at 0.60 euro per minute and claim to have 4 x faster data download than Iridium, will let you know the outcome.
In terms of a tablet to display the radar image, any recommendations for:
Must have a USB Must have a built in GPS Must have a reasonable battery life of say 4 hours Must have a screen readable in sunlight I think windows is ok for OS
Anything tick all those boxes?
Not many windows tablets around, apart from the vertical-market industrial ones, most of which are quite bulky.
If I was going to replace my LS800 (which I've had since 2005 and which just keeps working perfectly) I would for for the ST312 (but remove the rubber corners).
But if you can get a WIFI network connection, why do you need a windows machine? The only reason I can think of is if you need some "extras" e.g. Dial Up Networking (the traditional way of getting internet over a satphone).
I do not know that I do need a windows machine, I just thought easier to stop unwanted downloads and keep it dedicated for the weather radar. I am more familiar with windows than other OS's. The st312 looks robust and has all the features.
WiFi is not essential now that I have sadly ruled out the ipad, but it would make sense purely from a point of view trying to reduce spaghetti.
I just thought easier to stop unwanted downloads
I don't think that will be easy.
I know nothing about Android and "hidden" comms on that (but presumably anybody who wants to make money on an app shop will be doing hidden comms, for update checks etc) and I know that hidden comms can be stopped only on a jailbroken IOS device (but you need to be a very good unix hacker).
The satphone to wifi products for the yachting business include configurable firewalls to deal with this issue.
Otherwise, the only way I know of is to use dial up networking (DUN) and then you are online only when you want to be online, and that needs a windows machine. It can be done on Android in theory but last time I dug around I did not find a solution.
NEXRAD is delayed, certainly, but there is no NEXRAD in Europe, and never will be, so the best we get is some version of Meteox, with a delay which depends on how much money the agency reselling the data is willing to pay
The two main commercial players are Avidyne MLX770 and Moving Terrain.
