The EASA text above refers to Geometric Altitude. No idea what that is.
When you say ADS-B uses Pressure altitude, you mean the raw pressure mesures by the static probe, not the altitude showed on your altimeter, right ?
EGNS certainly sees it all (and they’ve installed recently a multilateration system so they can see the parts of the island they never used to be).
When my GTX335 goes in it’ll be capable of SIL=3, but the LAA forces us to configure it to emit SIL=0 even though it’s TSO’d out the wazoo (the explanation I was given: ‘we don’t yet have the expertise to evaluate an installation for SIL=3, but we’re researching it, more soon’), however I’m not particularly bothered about it because anyone with high-end TCAS which doesn’t display ADS-B with SIL < 3 will see the Mode-S replies anyway, and the inexpensive traffic awareness things (which can’t see Mode A/C/S) that are becoming quite common even in things like microlights will display SIL=0.
I think it’s only a question of time until European ATC starts to make use of satellite-based ADS-B. The system is in final testing and should go live quite shortly. The first applications will be SAR and airline tracking of their own aircraft. The question is likely less one of technical capability vs the various European ATC units’ readiness to take advantage of the available data. The biggest technical question for GA is whether satellite coverage/reception will be acceptable for the normal single-antenna underneath installation in GA aircraft. Since the system is new, live experience and data will tell if other applications such as this are practical.
And a November press release included this:
Aireon is deploying a space-based air traffic surveillance system for Automatic Dependent Surveillance-Broadcast (ADS-B) equipped aircraft throughout the entire globe. Aireon will harness next-generation aviation surveillance technologies that are currently ground-based and, for the first time ever, extend their reach globally to significantly improve efficiency, enhance safety, reduce emissions and provide cost savings benefits to all stakeholders. Real-time ADS-B surveillance will cover oceanic, polar and remote regions, as well as augment existing ground-based systems that are limited to terrestrial airspace. In partnership with leading ANSPs from around the world, like NAV CANADA, the Irish Aviation Authority (IAA), Enav, NATS and Naviair, as well as Iridium Communications, Aireon will provide a global, real-time, space-based air traffic surveillance system to all aviation stakeholders. For more information about Aireon, please visit www.aireon.com.
Having said all that, given the pace at which regulation and implementation moves in Europe and all the rather diverse current implementations in GA aircraft mentioned above, it will likely be 5-10 years before we see something consistent across EASA-land. Some countries might start mandating something in the five year timeframe if they have their own ground-based capabilities, but I don’t see anything sooner on a broader scale, regulation-wise.
When you say ADS-B uses Pressure altitude, you mean the raw pressure mesures by the static probe, not the altitude showed on your altimeter, right ?
Certified ADS-B OUT (SIL=3) uses pressure altitude, from an altitude encoder. That is thus as accurate (or not…) as your Mode C/S altitude return and can be used as a basis for surveillance, separation, and prosecution. These three things are integrated. ATC are not the police but they prepare reports for the CAA to take further action.
Uncertified ADS-B OUT (SIL=0/1) uses GPS altitude, because there is no connection to the aircraft. They could pick up own-transponder Mode C/S data, but do they? I thus cannot see how ATC could use it, for anything. But airports could use it, informally, as a “much improved FR24” (more accurate, and without the time delay) – example.
I think it’s only a question of time until European ATC starts to make use of satellite-based ADS-B.
The NSA can already pick up phone calls that way, reportedly… So I wonder whether the funding is the main challenge. The transponder on the satellite needs to be paid for somehow, and it isn’t going to be cheap.
I thought that MH370 started the idea of space monitoring of ADS-B…
ADS-B Out provides both pressure altitude and geometric altitude. SIL only deals with the trustworthiness of the lateral position and there are other parameters that must be within specified tolerances in order for them to be used for surveillance.
Peter wrote:
The transponder on the satellite needs to be paid for somehow, and it isn’t going to be cheap.I thought that MH370 started the idea of space monitoring of ADS-B…
Perhaps. In any case, there is apparently a new ICAO standard called GADSS that airlines need to meet and the GlobalBeacon product (combined Aireon and FlightAware) is meant to address that requirement. Aireon certainly would have had a business case and plan before they invested in getting all those ADS-B modules up on the Iridium satellites. The Aireon web site mentions three applications: traffic surveillance, alert (read SAR) operated by the Irish Aviation Authority, and meeting the ICAO GADSS requirement for airlines. The revenue streams for the three may well not all be in the same ballpark, and I’m sure it won’t be cheap for the airlines. How much they’ll want for ATS surveillance implementations is anyone’s guess.
Peter wrote:
The NSA can already pick up phone calls that way, reportedly… So I wonder whether the funding is the main challenge. The transponder on the satellite needs to be paid for somehow, and it isn’t going to be cheap.
It is already paid for on the Iridium NEXT constellation. Transport Canada and NATS are part of the consortium.
I wonder about the reason behind totally disregarding SIL=0/1 squitters. After all, even a very unreliable signal has some useful value. It can even be simulated on the display by showing blips with SIL=0/1 as balloons of certain size (and ATC may correlate them with radar returns, too).
Probably it is a legal thing. If you can’t trust the data, especially the vertical bit, then you could decide to separate somebody in a way which makes the situation worse.
I agree with you though… but ATC already deals with bizzare concepts e.g. any primary-only contact in a region where the CAS does not go all the way down to the ground is assumed to be below CAS, even if the CAS base is say 1500ft, yet the same ATCO is required to apply 3000ft (5000ft?) vertical separation against any contact for which he has an altitude readout, verified or not, even though most of the time a Mode C/S altitude will be fairly good and probably better than the GPS-derived value from a SIL=0/1 emitter which has had no temperature etc compensation applied to it.
I also think that if somebody is Mode A, and given there are almost no Mode A boxes, so it is a Mode C with A turned off or broken encoder, they are not likely to be wanting to emit their vertical value using ADS-B OUT 
And probably same comment for non-TXP traffic, in most cases where a TXP could be installed.
The interesting thing is that with SIL=3 (certified) ADS-B OUT, ATC in Europe will always also have the Mode C/S altitude. Usually the source will be the same however so the two values should be identical.
SIL has absolutely nothing to do with the vertical. Why is it constantly referred to with respect to the vertical? It is one parameter that must be compliant to be usable for surveillance in the lateral plane. For the traffic aid to acquisition, SIL only needs to be not zero. With ADS-B traffic in the cockpit, pilots are not authorized to take evasive action unless there is visual contact with the target. But that is not surveillance used by ATC.
With a mode C/S transponder, there isn’t a check on the vertical. At least with ADS-B, there are multiple potential checks including the geometric altitude and an indication that there are multiple pressure altitude sources on board. SIL has nothing to do with either of these parameters.
