Qs re ADS-B technology

[Andrew Gideon]

Someone gave an ADS-B presentation at our club's meeting last night. It
was terrific, but some interesting questions were asked. I'm wondering if
anyone happens to know the answers or if someone can point me to a
reference.

ADS-B ground stations appear to have a range of about 150 miles, from
what was said. In the area in which we fly (the KEWR class B), that's a
*lot* of aircraft. Assuming all are ADS-B equiped, how does each aircraft
broadcast its information to the ground station?

Is it collision detection like Ethernet? I'd expect a lot of collisions
in this area, thereby slowing the accumulation of information. But a
token-based solution seems a little unwieldy to me given that an aircraft
might be in view of multiple ground stations at any moment in time. More,
aircraft are supposed to broadcast to each other even outside the view of
a ground station. There'd be no master in that case.

So how does that work?

Another question is the broadcasting of the TIS-B and FIS-B information.
Does the ground station just broadcast it all, with the aircraft UATs
determining which (for example) targets are relevant via its GPS location?
How often is a completely broadcast cycle completed? Is this dependent
upon factors like the number of targets visible to the ground station
(ie. the amount of TIS-B information to transmit)?

Thanks...

Andrew

[[email protected]]

Andrew Gideon wrote:
[...]
> I've found:
>
> http://spacecom.grc.nasa.gov/icnsconf/docs/2002/04/Session_B1-3_Castle.pdf
>
> which discussed the trade-offs between these mechanisms in terms of
> receipt likelihood.
>
> - Andrew

Probably more than most will ever want to know about UAT, but some good
background on how the issues raised in the thread are addressed in the
design:

<http://www.alaska.faa.gov/capstone/docs/UAT%20GBT%20Spec.pdf>

HTH.

Regards,
Jon

[jmk]

Andrew Gideon wrote:
> > It's somewhat like Ethernet. Listen before talk, but you still can have
> > collisions. Collision detection is problematical (listen after talk).
>
> If I recall correctly, Ethernet uses "listen while talking". If what is
> sent isn't what is heard, collision is presumed and the sender backs off.
> There's also a problem, again: if I recall correctly, in the back-off. I
> think that the problem was potential starvation as a sender could be
> repeatedly forced to back-off.

That's correct. "Listen while talking" doesn't work for ADS-B. In
fact, the only part that is really of value is the "Listen before
talking". The backoff problem you refer to is the "knee" in Ethernet
performance. One big advantage of Ethernet (over the other types) is
that the performance for a lightly loaded line is GREAT. It then tends
to drop off just a little bit worse than would be forced simply by the
amount of time each additional sender requires... until you hit a
"knee" where the performance starts to degrade very very badly with
each new load placed on the line. Basically, as you state, almost
every packet sent gets a collision with someone.

Fortunately, ADS-B does have the "burn through" phenomena - something
that wired Ethernet doesn't have.

> It's why one should select non-overlapping channels for neighboring clouds.

I wish the FAA would show the same sense. But they are bound and
determined to fit the entire ADS-B thing into MODE-S, in spite of
existing MODE-S congestion in many areas already. And in spite of the
known fact that MODE-S can't handle the MANY other potential benefits
of ADS-B, like weather uplink, etc. I hope they don't force it, but
having forced MODE-S onto the airspace system over great opposition,
they are politically pushed to find a use for it. [TCAS doesn't count,
since potentially ADS-B surplants much of that function, especially
when full IN/OUT modes are implemented.]

> But what about the ground stations' transmissions (ie. the TIS-B and FIS-B
> streams)? Are the ground stations' transmitters sufficiently powerful so
> as to "punch through" aircraft-aircraft traffic? Is there some other way
> to give the ground stations' broadcast at least a decent chance of being
> received?

Hey, this is a government program. You want perfection? <G>

Seriously, this is why some suggested that the real solution was to
bypass the transmitting part of the ground stations. Instead, take the
estimated $1 Billion savings from eliminating the present VOR system
(except for a small backup infrastructure) and just BUY every plane out
there an ADS-B system.

> No, I'd thought that this was all already defined. How else could these
> services be running in Alaska and part of the US East Coast?

You'd think so, wouldn't you. But no, it's not already defined. Even
the frequency (MODE-S vs. VHF) are still up for grabs. All those
systems currently running are "test systems" only.

BTW, the latest edition of AVIONICS Magazine had some pretty good
articles in it on ADS-B.

[Andrew Gideon]

On Tue, 11 Jul 2006 06:55:58 -0700, jmk wrote:

>> It's why one should select non-overlapping channels for neighboring
>> clouds.
>
> I wish the FAA would show the same sense. But they are bound and
> determined to fit the entire ADS-B thing into MODE-S, in spite of existing
> MODE-S congestion in many areas already. And in spite of the known fact
> that MODE-S can't handle the MANY other potential benefits of ADS-B, like
> weather uplink, etc. I hope they don't force it,

I don't quite follow your point here. But that exposes another aspect of
ADS-B that's confusing to me.

I don't understand the relationship between VDL, Mode S squitter, and
UAT. My cursory reading tells me that these are three different
communication technologies, any of which can be used to carry ADS-B data.
However, that would appear to defeat a major benefit of ADS-B: if two
aircraft are out of RADAR coverage, they're still supposed to "talk" to
each other. But if one is carrying a UAT and another a Mode S squitter,
how could that communication occur?

[...]

> Instead, take the estimated
> $1 Billion savings from eliminating the present VOR system (except for a
> small backup infrastructure) and just BUY every plane out there an ADS-B
> system.

I like having a good mix of ground and satellite stations. I'd like to
see, in fact, GPS units which include NAV radios (ie. the Garmin 430/530
line) feed the VOR NAV data into the location computation as well. It
would provide an additional check against satellite signal loss. It would
be less precise (esp. when compared to WAAS), but there's no reason that
satellite "backup" needs to be twisting needles and catching CDIs.

>
>> No, I'd thought that this was all already defined. How else could
>> these services be running in Alaska and part of the US East Coast?
>
> You'd think so, wouldn't you.

Well, sure, before this conversation.

> But no, it's not already defined.
> Even
> the frequency (MODE-S vs. VHF) are still up for grabs. All those
> systems currently running are "test systems" only.

Which means I could buy the Garmin GDL-90 and find it useless? That's not
something Garmin is advertising.

>
> BTW, the latest edition of AVIONICS Magazine had some pretty good
> articles in it on ADS-B.

Is that:

http://www.avionicsmagazine.com/

? Is the article there? I'm not seeing it (at least in the June edition,
which is the latest I see online).

Thanks...

- Andrew

[jmk]

Andrew Gideon wrote:
> I don't understand the relationship between VDL, Mode S squitter, and
> UAT. My cursory reading tells me that these are three different
> communication technologies, any of which can be used to carry ADS-B data.
> However, that would appear to defeat a major benefit of ADS-B: if two
> aircraft are out of RADAR coverage, they're still supposed to "talk" to
> each other. But if one is carrying a UAT and another a Mode S squitter,
> how could that communication occur?

Nope, you understand just fine.

Right now you have three major companies, all lobbying for a different
communications means. The FAA is under political pressure from the
airlines to fit it into Mode-S because they had Mode-S datalink rammed
down their throats already. Personally, I think VDL is the right
choice, but I have to admit tht it seems to be the underdog.
Regardless, I hope they DO make a real decision (instead of relying on
TIS-B to make up the interconnect and letting them all compete
nationwide - that's just dumb).

> Which means I could buy the Garmin GDL-90 and find it useless? That's not
> something Garmin is advertising.

Look how many folks have bought the Garmin 330 for TIS, and now the
stations are already starting to go away.
[i]
> > BTW, the latest edition of AVIONICS Magazine had some pretty good
> > articles in it on ADS-B.
> Is that:
> http://www.avionicsmagazine.com/
> ? Is the article there? I'm not seeing it (at least in the June edition,
> which is the latest I see online).

Right magazine, wrong month. The July edition landed on my desk in the
last couple of weeks.