ADS-B Made Simple

[rmartingt]

Originally Posted by BobTurner

If you never talk to atc then maybe you never go near class B or C; if you stay below 10,000' too then you can just ignore the whole thing. Quite possible for easterners. Out west, it can be tough to safely fly over the mountains and stay out of the mode C required airspace.

My point is that the full 2020 mandate is excessive, because it's written to satisfy the needs of large, fast aircraft operating in crowded airspace in IMC. The technical requirements are overkill for VFR operations in light airplanes, and almost certainly a result of some desk-driver who imagined that nothing smaller than a King Air ever operates in the applicable airspace.

Full disclosure: 90% of my flying has taken place within Atlanta's Mode C veil, underneath the Class B. Yet since my private checkride, I have spoken to ATC on only two occasions--both during BFR's here in Savannah.

[kamikaze]

Separation requirements have nothing to do with whether you're VFR or not, or the type of aircraft you fly ... If you're in controlled airspace, especially class B, you're in busy airspace with lots of targets, and for dependable, accurate positioning, everyone needs to be reporting their position to the same standards ... If there's a 737 reporting really accurately, and an RV-7 reporting far less accurately, you loose the ability to keep separation with close margins ... the aircraft with the least reliable signal needs a bigger buffer around it.

I suppose the technology could allow for this, each aircraft gets a given "buffer" around it based on the quality of the position reporting ... but that could be a big headache for ATC, especially where there are fixed routes, corridors, etc. that depend on a given level of accuracy (i.e. approaches and such).

[Jordan1976]

The tough requirements to meet for an ADS-B position source have to do with the deep analysis of failure cases and proving you are 10^-X reliable. It isn't hard for a cheap GPS to produce very accurate position 99.99% of the time, and to prove that.

Thus, in VFR the argument is that legally, separation IS different in VFR because separation is via eyeball, not via controller vector. It's your fault if you hit that fully visible airliner, not the GPS, no matter what the GPS is saying.

So, in VFR, where it's already up to the pilots to stay apart from one another, why do we require a position source that is just as reliable as it is in IMC, instead of allowing some reduction in the assurance of that device given it is only a backup to the Mark 1 Eyeball, not the primary separation device?

Here's an interesting point: Today's radars only have about 1/4 mile position accuracy when you're 30 miles away from the radar, and that's only with 95% confidence. Additionally, the latency is as high as 12 seconds, which at 250 knots is a full mile of flight.

ADS-B requires that the 95% position accuracy be 0.05 NM (5X better than radar) and that the 99.9999% accuracy be 0.2NM or better (tighter than radar's 95% accuracy). So ADS-B is spec'd at being WAY better than the radar we use today. On top of that, ADS-B must have a latency below 1.6 seconds.

So in the end, even for VFR applications, the ADS-B position source is required to be about an order of magnitude more reliable and accurate than current radar is. Allowing VFR aircraft to give up some of that over-the top reliability seems reasonable.

[RV7A Flyer]

Originally Posted by Jordan1976

Today's radars only have about 1/4 mile position accuracy when you're 30 miles away from the radar, and that's only with 95% confidence. Additionally, the latency is as high as 12 seconds, which at 250 knots is a full mile of flight.

ADS-B requires that the 95% position accuracy be 0.05 NM (5X better than radar) and that the 99.9999% accuracy be 0.2NM or better (tighter than radar's 95% accuracy). So ADS-B is spec'd at being WAY better than the radar we use today. On top of that, ADS-B must have a latency below 1.6 seconds.

So in the end, even for VFR applications, the ADS-B position source is required to be about an order of magnitude more reliable and accurate than current radar is.

I sure hope so. Given the capability of today's technology, it would be foolish to go for less, I'd think, and then be in the same situation again in a decade or so.

The whole NextGen program is extremely comprehensive and multi-faceted. ADS-B is just one part of it.

[shuttle]

I mentioned this on the other ADS-B thread but as this thread has got onto the subject of uncertified GPS I thought I would re-post.
The CAA in the UK are well aware of the issues for GA of insisting on certified GPS and are now investigating the consequences of authorising uncertified GPS data as the position input to ADS-B.


General Aviation ADS-B Trial in Southern England


NATS had envisaged issuing an AIC back in the autumn initiating a nationwide trial of GPS connectivity to Mode S transponders as an enabler to ADSB in/out for the GA fleet. At the request of the CAA we have not issued that open invitation to participate but have written to approximately 180 clubs and flying organisations offering the opportunity to participate in a formal trial. A sample letter is copied below;

Dear Sir/Madam,

Re: General Aviation ADS-B Trial in Southern England

As part of the development of tools to help improve flight safety by enhancing situational awareness, NATS is conducting a trial with general aviation pilots to assess the accuracy of position data from a non-certified GPS. NATS is inviting owners of suitably equipped aircraft to participate in the trial to gather sufficient data to make the study meaningful. The data can be broadcast via a Mode-S transponder with ?Extended Squitter? (ES) functionality otherwise known as Automatic Dependent Surveillance- Broadcast (ADS-B).

The purpose of this trial is to assess;

a) The typical accuracy of ADS-B reported positions from non-certified GPS sources,
b) The impact of low integrity GPS data on ATC surveillance systems, and
c) The interest of GA users in ?situational awareness? applications that assist a pilot to visually acquire nearby traffic.

The trial will be conducted for an 8 month period from 1st January 2015 to 31st August 2015 in airspace in southern England. Further information is included in attachment 1.
Aircraft owners will need to submit a minor modification for approval to the appropriate airworthiness authority to allow the GPS to be physically connected to the aircraft?s transponder. The CAA has agreed to waive their fees for the minor modification to Annex II aircraft during the period of this trial, provided the owner registers for the trial prior to submitting the modification.
Once the modification is carried out, the participant will need to do nothing more other than fly their aircraft with the transponder and GPS turned on. Participants may be required to disconnect the GPS source at the end of the trial.

NATS will not use this data for any purpose other than the trial, such as for the provision of an ATC service. If the non-certified GPS source is used with on-board equipment that detects aircraft via ADS-B, the equipment will not be certified for collision avoidance. Pilots will continue to be responsible for maintaining an effective look out and avoiding other aircraft.

If you are willing to volunteer to modify your aircraft to connect a GPS source to your aircrafts? transponders, please would you provide details of your:

1. Aircraft type,
2. Transponder type,
3. GPS equipment, and,
4. Aircraft engineer who will be conducting the minor modification (if known).

Participants are requested to respond to this invitation either in writing or by email to me at the address below.

Thanks in advance for your interest, and I look forward to working with you.




Attachment1: General Aviation ADS-B Trial in Southern England ? Trial Description

The purpose of this trial is to assess;

a) The typical accuracy of ADS-B reported positions from non-certified GPS sources,
b) The impact of low integrity GPS data on ATC surveillance systems,
c) The interest of GA users in ?situational awareness? applications that assist a pilot to visually acquire nearby traffic.

The trial will be conducted for an 8 month period from 1st January 2015 to 31st August 2015 in airspace in southern England.
NATS low-level ADS-B coverage is centered on southern England. The diagram below is indicative of current coverage at 3500ft.An additional ADS-B receiver will supplement coverage to the north-west of this region in Spring 2015.Higher level coverage extends to most of England.

The Minimum Operational Performance Specifications for ADS-B allow the provision of position information from a non-certified GPS source, provided the Surveillance Integrity Level (SIL) parameter is set to ?unknown? (zero). Therefore, you may participate in this trial with a non-certified GPS source, so long as the SIL parameter in the ADS-B message is set to ?unknown?. The positions reported via ADS-B will be compared with radar track data to establish the impact of low integrity data on surveillance tracking systems.

Not all transponders are suitable for this trial and the method for connecting the devices depends on the transponder and the GPS equipment. NATS and the CAA will work with you, your engineer and the transponder manufacturer to determine whether your aircraft and equipment are suitable for this trial and how to implement the modification. The CAA will also provide guidance for how to submit the minor modification request to EASA, see also http://www.caa.co.uk/default.aspx?catid ... ageid=9259.

Owners of EASA type aircraft will need to consult with a licensed engineer to submit an EASA form 32 to request a minor modification. Under EASA rules, it is permissible to submit one modification for multiple aircraft if they are the same aircraft type and the same equipment is used. Owners of Annex II aircraft will need to request approval of a minor modification from the CAA. The CAA has agreed to waive their fees for Annex II owners during the period of this trial.

In both cases the modification will need to demonstrate:

? The installation of the GPS equipment does not affect the safe operation of the aircraft,
? The SIL is set to zero, and,
? The GPS is not be used as the primary navigational aid
? The connection between the aircraft?s transponder and the GPS unit is non-permanent.

If the non-certified GPS source is used with equipment that detects proximate aircraft via ADS-B, the equipment will not be certified for collision avoidance. Pilots will continue to be responsible for maintaining an effective look out and avoiding other aircraft.

It is clear, from the months of debate within this thread, that a number of individual FLYER Forum members would be interested in participating in such a trial and I would urge those people interested to contact Adrian Price via the details above, and help us progress this exciting development.

[kamikaze]

"Thus, in VFR the argument is that legally, separation IS different in VFR because separation is via eyeball, not via controller vector. It's your fault if you hit that fully visible airliner, not the GPS, no matter what the GPS is saying."

Yes, but it's ATC's fault if the 737 was vectored too close to you because your position reporting is off by x many miles. Even ff you're in class B and doing everything right (other than reporting an inaccurate position), ATC could still vector a 747 on top of your head because oops, he thought you were a couple of miles to the left.

Could be because the GPS itself has issues, but it could also be installation issues for example.

Sounds like the ADS-B messaging can provide the necessary information regarding accuracy for the system and ATC to use it and make decisions accordingly (NAC, NIC, SIL, etc.) ... So non-certified systems could just declare the lowest levels of these things, and ATC could just give you a bit them a bit more room!

[az_gila]

Originally Posted by kamikaze

Separation requirements have nothing to do with whether you're VFR or not, or the type of aircraft you fly ... If you're in controlled airspace, especially class B, you're in busy airspace with lots of targets, and for dependable, accurate positioning, everyone needs to be reporting their position to the same standards ... If there's a 737 reporting really accurately, and an RV-7 reporting far less accurately, you loose the ability to keep separation with close margins ... the aircraft with the least reliable signal needs a bigger buffer around it.

Is that really the case?

Doesn't the ADBS standard output signal include the error margin?

Low accuracy (a relative term in this case) targets could simply be given more spacing. If they are slow this would be easy to do.

[kamikaze]

Gil,

You are correct, and I did mention that same idea later on.

you could simply but a "ring" around your target on the screen, and have the systems understand that "nothing should enter this area". Actually I suppose this works in 3D so it should be a "bubble".

[N941WR]

Not to worry AOPA is on the case.

[Brantel]

I still don't buy it that experimentals have no path to compliance.

I would bet that a large percentage of those 8,800 GA aircraft that are already equipped are experimentals thanks to the amazing experimental avionics vendors we have available to us these days.