Re: Will GPS ever really replace ADF/DME?

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Roger Halstead wrote:

> > Fine, WHAT ERROR ? How far off ? I did it create a situtation where the
> GPS
> > thought it was on the wrong side of the world ?
>
> They (DUATs) didn't say. The briefing said only that GPS should not be
> relied on to be accurate, or data could be misleading in that area. The
> area, as I recall was centered near Denver and became larger the higher you
> went.
>
> >
> > If the net error was LESS than the HUGE error (by comparision) of a
> standard
> > VOR at long range from the station, then this is a NON-ISSUE.
>
> But what if the error is the max of the VOR when you are *close* to the
> waypoint?
> Remember a GPS error amounts to an offset, not an angle like the VOR.
> Remember too, FSS was telling us NOT to rely on GPS in that area at that
> time.
>
> If you are headed into a small, uncontrolled airport with visibility down to
> just above minimums
> and the thing has you 3 or 4 miles off, where the VOR, or RNAV might be 1/4
> mile, then it is a problem. A big problem.
>

Since "they" didn't say what the error was, there does not seem to be much
point in speculation. Most likely, since the GPS solution is not going to be
"off" because a signal was delayed, its a reliability issue, and "they" were
in CYA mode. There is not really much that can happen to a GPS signal that
would lengthen its path and cause a position off error, since GPS signals
are too high frequency to bounce or multipath. Most "off" errors in GPS occur
because of satellite receivability compromise, which causes the number
of satellites to go down, which causes the precision to be reduced.

> Is it likely to happen? Highly unlikely. We know only that GPS can, given
> the proper conditions, provide missleading information that is deemed
> unreliable and possibly dangerous in some locations for enroute. To me that

I wholy disagree. If your GPS is off, then the overriding cause is the number
of satellites being received, which you can check (you do check the sat
receiption page ?).

> > NO. There are many redundant satellites, and they transmit spread spectrum
> > over a wide frequency band.
>
> Spread spectrum is not infallable either. In many instances a discrete
> carrier can wipe out the coherrence of the spread spectrum signal. Digital
> signals are worse and I can think of several more possibilities.
> > The only "single point of failure" is if you
> > only have one GPS unit. I have THREE, in addition to 2 VORs as backup.
>

By having a transmision that wipes out the whole band. Now here is where I
think most GPS bashers don't read what they themselves write.

GPS verses WHAT ? VOR occupies a band, so does loran. Given the same type
of interference, not only would both of those systems fair far worse, but
both are also subject to LONG RANGE interference of their signal. Finally,
since both are fairly simple signals, especially VOR, this hypothetical
situation with a radio transmitter equipped terroist would be far worse
with VOR or Loran, since it would be within todays technical skills
to FAKE a VOR or loran signal that threw off the location by a known
amount.

Saying that GPS is bad because it would not survive a technical onslaught
is pointless, since neither would any other system, and in fact, GPS does
better at that as well. Shall we return to compass only flights ?

> I think you are over reacting a bit. The post was not against GPS, but
> rather pointing out that even the government is realizing it's not a good
> idea to have all your eggs in one basket.
>

Again I disagree. There are many GPS satellites, and in the years to come, we
will see quality solutions such as LAAS that completely back up and check
the GPS system. It is not one satellite, it is not one frequency, and increasingly
it is not even one method of position finding. ALL the arguments I have heard
against GPS involve possible (and completely man made) "attacks" on the system
that are not compared equally with other, existing systems. So people have
an axe to grind against the new technology on the block. I have lived long
enough to remember the day when VOR was a fairly new, high priced technology
and DME was a ridiculously expensive rare option. GPS is going to shovel
VOR into its grave just as VOR shoved four course ranges into theirs.

> I have flown direct, using my hand held in actual IFR. I have the RNAV to
> back it up and stay honest, so it's quite legal. Flying the course on the
> GPS (295) is much easier than setting up the offsets for RNAV. If I have
> to deviate, I can easily enter the new course in the GPS, THEN check the
> RNAV settings against the map and GPS.

When on approaches, VOR/LOC and perhaps GS are how it is done. However,
enroute VOR is already obsolete to me. Oh, yes, I got plenty of lectures
about using it for backup. But that advice does not work in the real world,
because with GPS direct routes, there is no VOR radial to use as a backup,
and its accuracy is untrustable anyways.

Would I revert to VOR if Saddam managed to knock out the GPS constellation ?
Sure. I don't know what that has to do with the quality of GPS.

> This additional point is spread spectrum does not give the GPS signal an
> advantage that can not be over come accidentally, or on purpose.

Again, so does VOR or loran. It is a pointless comparision.

> --
> Roger Halstead (K8RI EN73)
> www.rogerhalstead.com
> N833R World's oldest Debonair? (S# CD-2)