Andrew Gideon wrote:
On Tue, 29 Aug 2006 23:34:27 -0400, Doug Vetter wrote:

The FAA is continuing to invest in Mode-S radars (the ASR-11 is replacing
the ASR 7, 8, and 9 units that currently support TIS), so an investment in
the airborne component is still a wise move and will be for MANY years.

Aside from TIS, what is the benefit (either to the pilot or ATC) of mode S?

- Andrew

Here are a few advantages of Mode-S over Mode-A:

1) Selective interrogations. Because the mode-s airborne component has
a unique address, any ground or airborne interrogator may request your
particular transponder (and only your transponder) to reply. It learns
your unique ID by using a broadcast request called "all call".

This is helpful on a technical level where the radar is attempting to
acquire the position of many aircraft that may be on or near the same
radial from the radar site (say, for example, three aircraft on the 237
radial from the radar antenna at 5, 10, and 40 miles away). The result
is less of a chance for ghosting and other radar anomalies. For
example, I used to have a problem when talking to Phili approach near
the Yardley VOR where they used to report a ghost image of my aircraft
about a mile away. They haven't reported the problem since I upgraded
to Mode-S.

This increases the distance at which a transponder may be identified
(thus increasing the effective range of the radar) because the site can
keep the aircraft at 5 and 10 miles quiet while it listens for a reply
from the aircraft 40 miles out. There are some practical limitations to
this benefit, mostly caused by the continued use of mode-a transponders,
but hopefully that won't be the case forever.

2) Support for Ground mode, which replies only with the unique ID
(Mode-S ID or Tail Number) and a software flag that tells the
interrogator the unit is on the ground. It does NOT respond with the
currently-entered squawk code (1200 or otherwise).

This is a nice feature simply because it helps identify your aircraft to
ground radar (like that deployed for testing at Providence, RI), and
because it prevents the need to turn the transponder off to prevent
squawking an old code after landing (most important to IFR pilots).

The great thing about the 330 in particular is that the unit switches
modes automatically based on either a landing gear position switch or
(in the case of our fixed gear 172) the speed of the aircraft as derived
from a GPS' serial data interface. It even knows whether to switch from
ALT to GND or from ALT to STBY, based on recent activity, or so said a
Garmin tech rep.

3) Although this is not unique to the 330 or Mode-S, I mention it
because I've seen the test results with my own eyes on my 330 --
specifically, the ability to respond to a interrogation rate at 100% of
the theoretical maximum, while most older "analog" transponders can only
manage 40% of that rate before becoming saturated.

The ability of a transponder to handle a high reply rate will become
more important as more aircraft are equipped with systems such as TCAS
and Skywatch that actively interrogate targets much in the same way
ground facilities do.

For more info, check out the 330 review on my site. Click through:

Aviation-Articles-Reviews-Garmin 396

-Doug

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Doug Vetter, ATP/CFI

http://www.dvatp.com
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