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Tom Rogers, Ph.D. |
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| About the Author ... |
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Tom Rogers is Avionics Editor for AVweb.
Tom operates
Avionics West, Inc. at Santa
Maria, California, one of the finest radio shops on the West Coast. Tom is an
instrument pilot, an FAA Designated Engineering Representative (DER) for
avionics, and has a Ph.D. in nuclear physics. (We're not sure why he got the
doctorate, but we call him "Dr. Tom," and he seems to like that.)
You can send Tom your avionics questions at
avionics@avweb.com.
Tom's company, Avionics West,
is one of the worlds largest discounters of handheld and
panel-mount avionics for general aviation. They offer all leading brands of
aviation electronics at deep-discount prices. Every item sold by AWI is
covered by a 30-day no-questions-asked return privilege.
Avionics West features handheld GPS receivers from Garmin and Lowrance;
headsets from David Clark,
LightSPEED and Telex; and panel-mount GPS navigators from II Morrow. AWI
stocks these units and generally sells them at prices substantially below the
manufacturers' "Minimum Advertised Price" (MAP). Consequently, AWI is not
permitted to advertise these prices on its web site, but you can obtain them
by sending an email to the AWI auto-responder at
avionics-specials@avweb.com, or
by telephoning AWI at 1-805-928-3601 (M-F, 8-5 Pacific Time). |
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| Ask Tom an Avionics Question |
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| Avionics West |
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Without doubt, GPS has been the greatest advance in aviation
navigation since the invention of railroad tracks. It operates in just about any
weather, uses a small antenna, and doesn't necessarily have to be panel-mounted.
In fact, some of the portable GPSs have more features than the panel mounts
(because they aren't hamstrung by the need to comply with complex FAA TSO
requirements). The new GPS receivers are astonishingly small and low in power
consumption. And the best ones are easy to operate because the avionics
manufacturers have learned how to design user-friendly software. (Those of you
that struggled with first-generation loran receivers know what I mean!)
An incredibly powerful handheld GPS costs between $600 and $1,200 including a
moving map display, while panel mounted boxes run from $1,900 to $3,500. (In
contrast, the first general aviation Loran C receiver I installed cost over
$8,000...in 1982 dollars!)
It appears that the next generation of GPS satellites (or "SVs" if
you want to impress your friends) will operate on the "L5 band" which is a
different frequency than the "L1 band" that current GPS satellites use. This
change should help improve navigation accuracy by reducing errors when the
signal passes through the ionosphere. This new "Block IIF" constellation is
supposed to consist of 33 satellites, and "should" be launched by the first
decade of the 21st century...but you know how that goes. The new satellites will
work in conjunction with the old ones, although many of the satellites in the
present constellation are getting long in the tooth.
The other good news is that the Defense Department has agreed to decommission
Selective Availability (SA), the "wobulation" they intentionally designed into
the signals that civilians use to mess up navigation accuracy. We've know for
years that this was a dumb idea, and finally the military has agreed and pledged
to do away with SA sometime in the next ten years.
GPS just gets better and better, and it's revolutionizing navigation not just
for aircraft, but for boats, cars, hiking, surveying, and all sorts of other
activities as well. The potential uses of GPS seem almost endless.
With all this great news, you're probably ready to yank out all of your
panel-mounted nav radios, velcro a handheld GPS to your control yoke, and fly
away into the sunset. Well, don't be so quick about writing off those VORs and
DMEs! You might be able to get away with GPS as your sole means of navigation if
you fly strictly VFR. But according to my FAA inspector friend, you must have
another means of navigation if you fly IFR and file an alternate on your flight
plan. I'm not sure all FAA inspectors are preaching out of the same hymnal on
this point, but I'm inclined to agree with the thought behind it.
Unfortunately, GPS is not bulletproof. Any receiver that depends on signals
at -160 db (i.e., incredibly weak) will have problems from time to time. GPSs
are susceptible to ionospheric distortion, especially around sunset and sunrise.
Solar flare activity is on the increase again, and I predict that GPS
reliability will decrease as solar activity increases. GPS is also very
susceptible to radio frequency interferance (RFI) from other pieces of
navigation equipment. Some DME frequencies can interfere with GPS receivers. VHF
comm transmitters can shut down the GPS when you transmit on certain
frequencies.
Another big problem with GPS is the satellites. When one goes on the fritz,
it's not easy to get it fixed. You don't just call the Maytag repairman! Rumor
has it that if a satellite goes down, the cost to build a replacement and launch
it into orbit is greater than five years worth of maintenance on all the VOR and
ILS stations in the United States. And get this: the early block of GPS
satellites had a lifespan of only 7.5 years...most have been up for over five
years now, and some have already failed. Let's see a show of hands from those
interested in going up one weekend to do some orbital repair work.
Throughout our aviation training, we are taught to have backups for critical
systems. Our engines have two magnetos and two spark plugs in each cylinder.
Many of our aircraft have dual vacuum pumps and dual alternators...and some have
dual engines. Most have an emergency exit door (or window) in case the main door
jams. Most serious pilots have dual VOR/ILS receivers, plus DME and other
navigation equipment. The basic idea is to make sure we can still fly safely,
even in the soup, if any one critical device or subsystem fails.
I think the same philosophy has to be used with GPS. I'm happy to use GPS as
my primary means of navigation, but never as my sole means. I'll always back it
up with loran and/or VOR/DME, preferably both. I'd suggest you do the same.
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