(BHelman) wrote in
om:

I did fly with one and had one. The Monroy would give traffic alerts
of almost every airliner flying thousands of feet above me, and others
that passed well below me. It was clear to me very quickly that the
claim of "within 1500 feet" was just not the case. It sounds like you
like that unit, but my opinion is that it was just more of an
annoyance than useful because aircraft well above me or below me (or
even some that never existed at all!) would set it off, ...

There is a lot of confusion about the Monroy unit, part of the confusion
frankly sponsored by the Monroy web site and advertising.

What's in it: Not bloody much. Just a simple RF power detector (a
simple receiver with RSSI output). The rest is literally just bells and
whistles - a simple processor to work the lights and voice and make some
fairly clever decisions.

How it works: The data stream itself isn't even available, much less
looked at. The processor just looks at the received power level and
guesses at the range. What it *does do* that is really kind of clever
is look at the LENGTH of the power burst to determine what the
transmission is. Short is a Mode-A/C. Longer is Mode-S. Too short or
too long is noise.

That's it. Nothing else. The "cylinder or protection" that is shown in
advertising is nothing but the limits of the sensitivity of the
receiver, coupled with the simple physics for the radiation pattern of a
monopole antenna. [Which, as you observed, is highly distorted by many
factors.]

Having said all that, I do fly with mine. Yes, some days it is a royal
pain with almost constant false alarms. But where it *is* useful is in
those parts of Texas where you haven't been within 100 nm of another
plane for the last 3 hours. It's real hard to keep as active an outside
scan under those conditions as you think you do. The Monroy is a good
"wake up" backup system.

False alarms: There are many places within short range of my home
airport that will ALWAYS set the Monroy off. Rows of chicken coops seem
to do it. Lots of little microwave telemetry sites on the ground will
do it - the bigger higher power ones don't seem to. Areas of poor radar
coverage will often cause your own transponder to set it off - not sure
why.

And then sometimes it is just a cause best left to Mulder and Scully.
No known cause. [With practice, you *do* seem to get a little better at
noticing certain false alarm "patterns" that help keep you from getting
so concerned about that "traffic" that you just can't see.]

For the money, it is an amusing little device, and just might actually
save someone some day. It is just sad that the FAA has had ADS-B
capability available for almost 20 years and not promoting it any
harder.

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James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
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