In the UK Mode S downlinked parameters are currently available to th London and Manchester TMA although it will be rolled out further over he nxt few years.
There are many really quite significant benefits to it's introduction and IMHO the single most important safety benefit is the reduction in undetected level busts before the bust has actually taken place.
I'll no doubt miss off something but according to the settings onboard the a/c, the following can be downlinked and viewed at suitably equipped units:
1 - The serial number of the transponder (this is an improvement over just the squawk as it also enables us to identify a/c that at on A7000 to improve detection and reduction on CAS infringements and tracing action)
2 - IAS
3 - Current Heading
4 - Selected level (SFL)
5 - Actual level
In total there are many many more that have the potential to be downlinked, but these are the main ones. Without doubt the most frequently of benefit is the SFL. It has singularly prevented many many incidents from developing and I wouldn't like to be without it. The primary method of prevention is, and always will be, issuing a safe clearance and receiving an accurate readback, followed by correct action in the cockpit. However we all know that mistakes happen! And although the Mode S data is not always available and at the moment it is a question of whether the controller sees the SFL, it's reassuring how many are caught before they develop.
Further down the line there is the possibility of developing an automated SFL/cleared level check but this would be a few years away. Electronics strips are maybe a stepping stone along the route but also a future hope is that improvements in voice recognition will allow a direct comparison between the clearance issued by the controller with the SFL dialled in the cockpit. But I'd like to reiterate that this is still some way away. For the moment the data is available from most a/c and the manual voluntary check goes a long way to improve safety on a daily basis.
A couple of caveats - this is only the UK perspective, I don't know how far advanced other ANSPs plans are and also I'd like to highlight that QNH is NOT available at any time. It was originally requested but I understand that there were objections from certain countries to this being available - quite why I can not begin to imagine. So this means that we can not detect, and prevent, any incident through incorrect QNH being selected or failure to select 1013 until the level bust has already occurred - what a shame!
Maybe this will be resolved in the future as we see several incidents each year through this......
Anyway, that's my 14p's worth, hope it helps - safe trips....