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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 -------------------- Doug Vetter, ATP/CFI http://www.dvatp.com -------------------- |
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