gaznav wrote:...“Why aren't we raving about this?”. Because it hasn’t quite hit the nail on the head for many of us - the use of another proprietary signal standard, like FLARM, means that it just adds to another standard to detect by extant receivers like other ADS-B In, FLARM or PowerFLARM receivers. The internationally recognised standard of ADS-B is the way ahead and when, or if, PAW starts pushing out ADS-B we really will have something to “rave about”. :thumleft:
I may now really upset the apple cart :?
Firstly lets not confuse an international standard with CAP1391
Can devices which conform to CAP1391 be used outside of UK airspace - No, so how is this international ?
More importantly regarding ADSB on 1090Mhz using OOK (On/Off Keying), how much capacity is available in this spectrum some simple - back of a napkin analysis
ADSB/DF17 emits 4 packets per second
2 x Airborne Velocity
2 x Airborne Position
Each packet consists of a preamble (8uS) and the Data Block (112uS)
that is a total of (8 + 112) x 4 = 480uS every second
I am not great at Math so lets round that to 500uS for simplicity
In the most perfect of worlds, if all ADSB emitters could be synchronised then 2000 emitters could all run without ever corrupting each others data.
If that was a motorway, it would mean every car driving bumper to bumper without touching one another
The world is not ideal, these are not synchronised and so this will never be the case - which means they will stamp on each others broadcast time slot in a random and undefined manner.
This is further complicated, ADSB is not the only emitter on 1090Mhz, we also havethe following transponder transmissions :-
Mode-A (20uS)
Mode-C (20uS)
Mode-S (64uS)
Each of these transponders are interrogated by multiple ground based radar systems and air based TCAS systems, so these are NOT N-times per second, they are as many times per second that they are interrogated - now you start to see how congested the spectrum is.
Now consider the range of these transmissions, I can receive ADS-B transmissions from 200km away using PilotAware on the coaming. So imagine a geographical area with a radius of 200km all broadcasting simultaneously where any transmission can overlay any other transmission.
Usually we think that in RF transmission, the strongest signal wins over, which is great, I will only receive the strongest transmissions, which are closest in proximity right ?
Wrong, On/Off Keying does not work like this, it is a CARRIER-ON / CARRIER-OFF method so data corruption occurs very easily with overlapping transmissions, this is not the same as FSK which is a method that uses a 2 symbol carrier FREQ-1 / FREQ-2, importantly, a FREQ is always present, and never silent
Nobody has done any detailed analysis to know what the system break point is - but the FAA did envisage this was a problem and have encouraged ADS-B/1090(OOK) for aircraft flying above FL180 and UAT/978(FSK) for aircraft flying below FL180. proactively addressing any possible congestion issue.
So to the question
gaznav wrote:...or if, PAW starts pushing out ADS-B we really will have something to “rave about”. :thumleft:
Hmm, well how about, if those encouraging usage of this spectrum, in the manner specified were to indemnify the manufacturers and end users, that should the system fail and need to be retracted - all costs and investment would be compensated.
If the capacity of the system is as robust as those who promote it, then this is perfectly reasonable, as this is a zero cost sum - right ?
Who wants to volunteer first ?
Thx
Lee