Andy

On Mar 17, 11:32 pm, Andy wrote:
On Mar 17, 9:22 am, Darryl Ramm wrote:



I worry about how well we will have ADS-B ground station coverage
along busy low-level routs like major ridge soaring locations, the
white mountains etc. Without that, gliders with UATs and 1090ES will
not see each other.

Darryl,

Was I mistaken in taking from the survey that ADS-B supports aircraft-
to-aircraft position reporting without a ground station repeater? I
figured that would allow two gliders in any remote location to see
each other if they both had ADS-B transceivers (like the Mitre unit).
With ground stations you then should get position reporting between
transponder and ADS-B.

That is TIS-B and that only happens if the transponder equipped
aircraft is within range of ATC radar. The ATC radar data is processed
by back-end systems and then relayed through an ADS-B ground station.
Just being within range of an ADS-B groudn station does *not* mean you
see nearby transponder equipped aircraft. The ADS-B ground statiosn
are going to provide coverage over vast areas where there is no ATC
radar coverage. I think some people might be confused about this,
thinking maybe the ADS-B ground stations have radar or multilateration
etc. can can locate transponder equipped aircraft. They cannot. That
data that is delivered as TIS-B (SSR radar/transponder derived data)
comes from existing ATC radar systems.

This is one reason it's nice to have say PCAS in the PowerFLARM so you
can get some warning of nearby transponder equipped aircraft even
outside radar coverage (in the case of PCAS outside of ATC radar
coverage this assumes some other interrogator like TCAS in overflying
aircraft).

BTW one of the interesting problems with the whole alphabet soup of
ADS-B/TIS-B/ADS-R/FLARM/PCAS/TCAD/TCAS etc. is threat deduplication.
There are many problems that can cause a threat to appear duplicated
if detected via different technologies or you can even see a ghost of
yourself. I expect this to be an interesting problem with many of
these systems, and we'll need to see how well they work in practice.


ATC radar coverage is not

I figured any collision avoidance advice would be a function of on-
board software capability, not constrained by the means of position
reporting - as long as you know your position and track and a target's
position and track, shouldn't you be able to figure out the collision
avoidance part?

It seems to me that you should be able to design ADS-B such that you
no longer need transponders. What did I miss?

9B


TCAS uses transponders, that's how it works. We can't change that just
becasue it seems antiquated (which it does). TCAS "sees" other
aircraft based on Mode C or Mode S interrogations. TCAS-II is deeply
integrated with Mode S and uses Mode S data links between different
TCAS-II equipped aircraft to coordinate resolution advisories.

If you even want to think about using ADS-B for collision avoidance
you need everybody carrying ADS-B data-out. That won't happen in the
USA for another 10 years. The only technology designed to be used in
cockpits that issues an RA that pilots are supposed to follow is TCAS.
Today in areas like around Reno/Minden (where Andy flys), TCAS is the
last fallback for avoiding a collision between a glider and a fast jet
or airliner. For other folks flying out in the boondocks, I don't
care, but around dense airline and jet traffic we need to make sure we
have compatibility with TCAS. Collision avoidance (vs. traffic
awareness) is complex, TCAS is a RTCA standard, there is no equivalent
standard developed for collision avoidance using ADS-B. Without that
manufactures cannot make something, regulatory agencies cannot require
it, etc. It just does not exist.

As ADS-B technologies roll out we also need a back-up in the national
airspace system, and ATC radar and transponders will be that backup so
I just don't see them going away in our lifetimes.

The FAA has been struggling to just get mandatory ADS-B data-out
carriage by ~2020. The motivations for getting that adopted in GA has
been muddled, which is a pitty I think this could have been marketed
much better to owner/pilots. But without wide option of ADS-B data-
out, let alone data-in doing TCAS style collision avoidance based on
ADS-B makes no sense. But I don't really blame the FAA, they've been
trying to work all this and Nextgen out, with a awful lack of overall
funding.

Long long terms (==decades) ADS-B based systems could offer more than
TCAS or could work better with it. Today TCAS-II version 7 can use
ADS-B to reduce the transponder pinging between aircraft but it never
issues an RA based on ADS-B position data. It would be great to see a
system that offers the capabilities of TCAS-III like horizontal RA
maneuvers, and maybe better range than TCAS in crowded airspace
environments. TCAS-III became so complex it never got to market.

And again just be careful exactly what you are getting in any
scenario. ADS-B data-out just sends your position, and you have to add
a GPS source (which today if you wanted to fully comply with future
carriage requirements (which don't apply to gliders) is expensive).
You can have a box that does ADS-B data-in but how is traffic
displayed? Just having an "ADS-B receiver" does not mean you will get
traffic warnings. The box might well happily watch the treat aircraft
as you both collide. That traffic display/warning happens at an
application layer above ADS-B. There are standards for symbology etc.
to display in aircraft for ADS-B data but much of that is not that
relevant to us. Some new traffic awareness system from vendors like
Garmin will mix active transponder interrogation (TCAS-I like) with
ADS-B 1090ES data-in derived traffic. But those systems never issues
RAs anyhow.

---

Like Mike says. UAT will see UAT directly, 1090ES will see 1090ES
directly. We are going to have both in the USA glider fleet and need
to start realizing that. The FAA thought that UAT would be popular in
the GA/low-end market etc. but they never really produced a convincing
driver for adoption of ADS-B in general or of UATs over 1090ES. Way
way back when this started transponders looked expensive and complex
but market dynamics produce things like the Garmin GTX 330 in GA and
the Trig TT21 well suited for gliders. If you are engineering a modern
Mode S transponder it is not much to add the capability to do 1090ES
data-out and add a small box to do 1090ES data-in. In the GA market
that makes sense since most aircraft are required to have a
transponder anyhow so you might as well do ADS-B over 1090ES.

Europe has mandated Mode S transponder usage so we are seeing lots of
Mode S innovation driven by that, that's why we have the Trig TT21.
And that is why European glider related avionics companies like
Butterfly/PowerFLARM and Garrecht have 1090ES based products. UAT
should have benefits, like lower power consumption (but the TT-21 has
pretty low power consumption), maybe FIS-B (needs a way to display the
data), maybe lower cost - but we've got to see actual products in
market to be sure about the cost. Current UAT transceivers are not
really low cost/suitable for gliders, hopefully this will change. The
marketing guy in me thinks the time to market will have a big impact
on adoption of 1090ES vs UAT in gliders, and right now 1090ES based
devices from Europe like the TT21 and PowerFLARM are becoming
available. For several $k you will soon be able to set up a pretty
impressive Trig TT21+1090ES data-out+PowerFLARM system, not bad for
early adopter costs. For folks with only Mode C transponders or who
fly in areas where there are no need for transponders I would hope low-
cost/low-power UAT transceivers do become available. It is going to be
interesting to see what happens.

How is that for a rambling reply?

Darryl