gaznav wrote:Err, where is the evidence for this:
A low power ADS-B transmitter (e.g. SE2) could conceivably be in range of enough MLAT receivers to give a good position, but only hit one or two WAM receivers which would therefore be unable to verify its location.
Roughly a 20W SE2 could have the same received power at 40miles as a 250W transponder at 130 miles or a 130W transponder at 95 miles (all approximate).
Those figures make my case perfectly, thank you Gaz. To locate a 20W ADS-B source, there need to be a minimum of 3 receivers within 40 miles of the transmitter. To locate a 250W transponder, that needs three receivers within 130 miles. Spreading that out over the UK, using 20W transmitters requires roughly ten times the number of WAM receivers to be installed. [(130/40)^2 = 10.5625, or (95/40)^2 = 5.64 for more GA-friendly devices] Who is going to pay for five to ten times more WAM receivers? Or are we (GA pilots) just going to be required to fit transponders? I know where I'll place my bets - which is why my aircraft already has an ADS-B transponder and not a low power device.
gaznav wrote:Also, ADS-B can be used as a substitute for SSR in military regulations - see MAA RAs 3222, 3223, 3228, etc...
Throughout this RA, any reference to SSR is equally applicable to Wide Area Multilateration (WAM) and Automatic Dependant Surveillance Broadcast (ADS-B).
Automatic Dependant Surveillance Broadcast (ADS-B) and Wide Area Multilateration (WAM) are acceptable alternatives to SSR.
These quotes are slightly ambiguously worded, but one interpretation is that ADS-B alone is not sufficient and that you need both ADS-B and WAM to equal/surpass the performance of SSR. I would agree with this: without WAM, ADS-B is too easily spoofed, as we have discussed ad nauseam before, both here on Flyer and privately. For non-critical applications, ADS-B alone may be enough - e.g. to "coast" through a temporary loss of three-way WAM on a previously validated ADS-B transmitter - but ADS-B alone is not enough to validate positional information due to the trivial way in which it can be spoofed.
ATS Surveillance System: Primary Surveillance Radar (PSR), Secondary Surveillance Radar (SSR), Automatic Dependant Surveillance Broadcast (ADS-B) or any comparable system (Wide Area Multilateration (WAM)) that is used to determine the position of an AS in range and azimuth. However, units who provide Radar Control Service inside CAS where only SSR, WAM or ADS-B is available should ensure local orders define procedures to cover the eventuality of an AS whose transponder is unserviceable while operating in CAS.
This quote is not equating the performance of PSR, SSR, ADS-B and WAM - it is a reminder that any system of control must cater for aircraft with no or faulty transponder, and nothing further should be read into it.
gaznav wrote:Now I can’t quote from the MATS or whatever the civvy controllers use, but to me it appears that ADS-B is an acceptable alternative to SSR as laid out in the Regulatory Articles (RAs). I’m also not sure where the 3-position fix for ADS-B that you mention comes from - any chance of a reference to that?
As explained above, ADS-B is not a suitable alternative to SSR, it is ADS-B+WAM which is suitable.
As for the 3-position fix, this is simple hyperbolic trigonometry, exactly the same as with e-LORAN, and as covered in any good secondary school mathematics text book - I would link to the excellent Wikipedia page, but I've been roasted on these forums before for daring to believe anything Wikipedia.
Hopefully it is obvious that only thing a single WAM receivers can determine is the presence of an aircraft, so I won't explain that. With two WAM receivers, you measure the time of arrival and this allows you to determine the difference in distance from each receiver to the transmitter, which puts the aircraft somewhere on a hyperbolic curve. You may have direction finding antennas which will improve the situation slightly, but in practice this doesn't make a huge difference as the angles along the hyperbolic curve are largely constant unless the transmitter is quite close to both receivers.[Note: non-mathematicians may wish to skip the next paragraph!]
To visualise this, imagine the two WAM receivers (A and B) are due East-West of each other and 300 metres due east-west of an origin point "O" (0,0), such that A=(+300,0) and B=(-300,0) - in practice they'd be tens or hundreds of miles apart, but 600m separation makes the maths easy for this example. The signal arrives at the A exactly 1us before B, so we know the transmitter is 300m closer to A than B (taking the speed of radio waves as 3e8m/s). The transmitter could therefore be at (+150,0), (+300,+450), (+300,-450), (+600, +1006.23), (+600, -1006.23), (+1000,+1712.45), (+1000, -1712.45), or indeed anywhere else on the hyperbolic curve described by the equation y^2 = 67500*((x^2 /22500)-1) where x >= 150.[Non-mathematicians may wish to start reading again!]
If you add a third receiver (C), you get three hyperbolic curves, each based on a pair of receivers (A-B, B-C, A-C), and thus you can locate the transmitter at the singular intersection of these three curves. If the signal at any of the three stations is lost (e.g. the aircraft moves out of range, of the signal is blocked by terrain), the transmitter can no longer be precisely located, although its position can be predicted for a while based on its observed velocity before the signal was lost, and/or raw ADS-B position reports. To be safe, however, a fourth WAM receiver is highly desirable to provide some resilience.