GPS loss of signal explanation

DME operates by transmitting to and receiving paired pulses from the
ground station. The transmitter in the aircraft sends out very narrow
pulses at a frequency of about 1,000 MHz. These signals are received at
the ground station and trigger a second transmission on a different
frequency. These reply pulses are sensed by timing circuits in the
aircraft's receiver that measure the elapsed time between transmission
and reception. Electronic circuits within the radio convert this
measurement to electrical signals that operate the distance and ground
speed indicators.

Jon Kraus
'79 Mooney 201
443H @ UMP

..Blueskies. wrote:
wrote in message ...
: On Fri, 04 Aug 2006 09:37:44 -0400, Jonathan Goodish
: wrote:
:
: In article .com,
: "Jay Honeck" wrote:
: http://www.scn.org/~bk269/gps.html
:
: This is an interesting site that explains why some pilots (seemingly
: mostly running Garmin-brand portable GPS units) are having trouble with
: losing satellite lock.
:
:
: We had a problem in our PA28 with a Skymap II (panel mounted GPS) when
: using an antenna on the top of the instrument panel. The culprit was
: the Narco IDME 825 and depended which frequency the DME was tuned. The
: DME transmitter blocked the GPS receiver. There was a considerable
: improvement when we fitted an external roof antenna. We have now
: fitted a Skymap IIIc GPS which is understand has a better receiver
: and so far appears ok.


Didn't know DME transmitted...

".Blueskies." wrote in message
...
Excellent, thanks!

In addition to Jon's nice explanation of DME, I'd like to point out that
because DME is an active receive-and-reply system, it can get saturated when
too many aircraft are using the same DME site. A pilot using DME in busy
airspace should keep this in mind, in case some funny numbers start getting
spit out, or the DME just stops giving any indications.

I admit, I've never actually seen this happen, but it is theoretically
possible.

Pete

In article ,
"Peter Duniho" wrote:

In addition to Jon's nice explanation of DME, I'd like to point out that
because DME is an active receive-and-reply system, it can get saturated when
too many aircraft are using the same DME site. A pilot using DME in busy
airspace should keep this in mind, in case some funny numbers start getting
spit out, or the DME just stops giving any indications.

I admit, I've never actually seen this happen, but it is theoretically
possible.

Interesting. I hadn't thought about the interrogator before. But It's more
likely that the ground station will reduce the sensitivity to the point that
it doesn't transmit that many replies - thereby reducing the probability
the DME will be listening to a reply intended for someone else.

For a DME interrogator that already acquired the ground station and
is tracking, it's extremely unlikely that its tracking algorithm will drift
off. Remember that the interrogations are psuedorandom, there are
only 30 interrogations per second (IIRC), and they are short.

--
Bob Noel
Looking for a sig the
lawyers will hate

Robert M. Gary wrote:
I think its usually a software problem. I've had various issues with
losing signal in different scenarios with my Garmin handhelds and
Garmin has always provided me a ROM update which appears to address the
issue.

I agree. The most recent rash of reception problems is very likely a
firmware issue because it's happening to too many people with units of
varying age. One guy in this group reported the problem on his (then)
new unit, while mine was about 6 months old at the time. No way those
units are of the same manufacturing batch.

As further evidence, I reset the almanac to no avail, and when the
receiver went totally deaf in the airplane a couple months back, I hit
the avionics power off for 30 seconds. No difference. While I admit
the IF radio interference issue is real, this particular problem doesn't
have anything to do with that.

I have an email into Garmin tech support asking whether I'll wedge the
box by downgrading to 2.4 (the version it came with), 2.6, or 2.8 in an
effort to determine if firmware is indeed at fault. Can't wait to hear
their answer. It better be a good one.

-Doug

--------------------
Doug Vetter, ATP/CFI

http://www.dvatp.com
--------------------

"Bob Noel" wrote in message
...
Interesting. I hadn't thought about the interrogator before. But It's
more
likely that the ground station will reduce the sensitivity to the point
that
it doesn't transmit that many replies - thereby reducing the probability
the DME will be listening to a reply intended for someone else.

That would cause the DME in the airplane to stop providing DME information,
which is one of the consequences I mentioned. I admit, I wasn't precise
about which "DME" would stop giving nay indications...it's the one in the
airplane I was talking about.

For a DME interrogator that already acquired the ground station and
is tracking, it's extremely unlikely that its tracking algorithm will
drift
off. Remember that the interrogations are psuedorandom, there are
only 30 interrogations per second (IIRC), and they are short.

I agree it's less likely you'll get erroneous data from the DME. It'd be a
pretty rare situation in which that happens, assuming it has ever happened
at all anywhere. My main point is just that pilots should be aware of the
limitations of DME (I guess we could mention slant-range here too, but I
think we were just talking about the radio signal itself? I've lost track
).

Pete

On Fri, 04 Aug 2006 22:14:39 GMT, ".Blueskies."
wrote:


wrote in message ...
: On Fri, 04 Aug 2006 09:37:44 -0400, Jonathan Goodish
: wrote:
:
: In article .com,
: "Jay Honeck" wrote:
: http://www.scn.org/~bk269/gps.html
:
: This is an interesting site that explains why some pilots (seemingly
: mostly running Garmin-brand portable GPS units) are having trouble with
: losing satellite lock.
:
:
: We had a problem in our PA28 with a Skymap II (panel mounted GPS) when
: using an antenna on the top of the instrument panel. The culprit was
: the Narco IDME 825 and depended which frequency the DME was tuned. The
: DME transmitter blocked the GPS receiver. There was a considerable
: improvement when we fitted an external roof antenna. We have now
: fitted a Skymap IIIc GPS which is understand has a better receiver
: and so far appears ok.


Didn't know DME transmitted...


Yes it does as nicely explained by Jon.
I should have pur IDME 891 (not 825) and it transmitts with 25 watts.

On Fri, 4 Aug 2006 16:29:56 -0700, "Peter Duniho"
wrote:

".Blueskies." wrote in message
m...
Excellent, thanks!

In addition to Jon's nice explanation of DME, I'd like to point out that
because DME is an active receive-and-reply system, it can get saturated when
too many aircraft are using the same DME site. A pilot using DME in busy
airspace should keep this in mind, in case some funny numbers start getting
spit out, or the DME just stops giving any indications.

I admit, I've never actually seen this happen, but it is theoretically
possible.

Pete


I can't remember the numbers but a VOR/DME will accept a large number
of aircraft before it saturates. Some Terminal DME's are only capable
of supporting about 5 or 10 aircraft (only needed for an instrument
approach anyway). Since they are short range it's not usually a
problem.

I have more, than once, lost DME but not usually for long (less than a
minute). Mostly at longer range where other aircraft are probably
nearer the DME and a stronger signal than me.

"Doug Vetter" wrote:

I agree. The most recent rash of reception problems is very likely a
firmware issue because it's happening to too many people with units of
varying age. One guy in this group reported the problem on his (then) new
unit, while mine was about 6 months old at the time. No way those units
are of the same manufacturing batch.

If it's a firmware issue, wouldn't you expect *everyone* to be having
problems?

Through almost a year of revisions, my 396 was always solid. Others have
reported the same.


--
Dan
C172RG at BFM

Dan Luke wrote:
If it's a firmware issue, wouldn't you expect *everyone* to be having
problems?

Not necessarily. Firmware is often used to handle discrepancies between
various revs of the hardware so the end-user experience is identical
across those revs, and a fix for one version may cause problems on
another (or only a few of a given batch if they have poor parts tolerances).

I just find it interesting that my system was functioning normally for a
good six+ months in all sorts of conditions (hot and cold) and then it
just started freaking out. Could it be hardware? Certainly. But based
on what I've seen in my time as am embedded systems developer, nine
times out of ten, problems like this can be traced to a software error
(never mine, of course) :-)

-Doug

--------------------
Doug Vetter, ATP/CFI

http://www.dvatp.com
--------------------

Dan Luke wrote:
"Doug Vetter" wrote:
If it's a firmware issue, wouldn't you expect *everyone* to be having
problems?

No, in fact one of my problems didn't happen until after 200 hours of
use. It was just a specific scenario in the software that happened to
be tickled. Its just like when you PC locks up, you'd think everyone's
PC should be locking up but your scenario is different. Perhaps a
couple sat's came in at just the right (or wrong) strength and the unit
hit a part of the software that was coded wrong.
-Robert