PaulisHome wrote:If the choice of 12 Jun 06 is relevant to the discussion, I note it wasn't a good gliding day, at least in East Anglia. My club just launched its training fleet and didn't do any soaring. No cross country at all. So the GA model may not be particularly good.
More generally, having spent most of my career in a technical consultancy, I know that it is possible for such consultancies, including QinetiQ, to write things that are wrong, not well thought through, or address a different question from the one now being asked. I have no idea whether that happened here, but an appeal to authority ["It's QinetiQ, they are big, therefore they must be right"] isn't particularly compelling. (It's not necessary to assume nefarious practices, though consultancies delivering the answer a client wants are not unknown). Better I think to actually look at what they did, and what the basis of their conclusions are.
It's a fairly complicated problem. It's not as simple as less power = less range. At these frequencies, range can be up to line of sight (we know we can pick up Flarm signals from gliders > 100km away with a suitable antenna, and Flarm is the lowest power of the systems we're discussing). Often the driver of whether you can see a signal at a receiver in a congested network, is the signal to noise - and the noise is the contribution from everything that's emitting in line of sight).
And then what's the question? Is it "Is System X suitable for GA Air to Air traffic detection, at least most of the time" Or is it, "Is System X suitable for CAT separation all of the time?"
So I think we need to be quite specific about the question we're trying to answer, and look closely at the modelling which is used to answer it. Do we have these?
I think the really key piece here is this:
Their model simulated the effects of introducing 828 Low Power ADS-B Transceivers (LPATs) prototypes into the operating environment at any one time.
So they created a model with airliners, biz jets, helicopters and GA aircraft seen on that date in the summer - using their positions for the model. They then added 828 LPATs to their model emitting 20W to see if interference was an issue - according to them, it wasn’t.
Then your point on making errors - yes, I agree they can be made. But there were more than one report and they created several models that we know of (2012, 2014 and 2015). I think it is highly unlikely such an error is made at least 3 times, don’t you?
Also, if this error was so bad, how about all the Class 1 and Class 2 transponders that are now emitting ADS-B on GA aircraft at 10-20 times the power of a CAP1391 device. Do we not think this so-called issue would not have started manifesting itself by now?
Finally, power is most definitely linked to range in the most simplest of equations - free space path loss. Yes, antennae design can also assist
with this so-called problem (which can also be fed into the free space path loss equation if required). Further, receiver design can also improve things. Technology shifts on this all of the time (normally incremental gains) and certainly filters for noise on 1090Mhz have come a long with digital processing. So it may be that in the 6-8 years that this study was completed that matters have improved
over what the study reported. Further, the removal of more of the non-selective Mode A transponders since will most certainly have cleaned up 1090Mhz.
I’m sorry, but I again believe that there are too many competent organisations that have looked at this to poo-poo it. There is also another big country (Australia) that have also agreed that low-power ADS-B on 1090Mhz works. Don’t you think that’s a little bit of an indicator (exaggeration!) of this solution being the way ahead?