GPS loss of signal explanation

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.

So far I've not had any trouble with our new 496 (and I've never had
any trouble with the Lowrance or AvMap units) -- but it's nice to find
an actual explanation for a problem that has struck several folks on
this group.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

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.

So far I've not had any trouble with our new 496 (and I've never had
any trouble with the Lowrance or AvMap units) -- but it's nice to find
an actual explanation for a problem that has struck several folks on
this group.


I'm not an EE so I can't comment on this guy's conclusions, other than
to say that the Garmin installation manuals for panel-mount GPS units
specifically warn against running the GPS antenna cable near nav/com
radios. My avionics shop told me that modern radios are sufficiently
shielded, but obviously, some older radios are not. Garmin uses the
same active antenna system for panel-mount as they do for portable.

I can also say that I have KX170B nav/coms and have never had a problem
with interference on the 396.



JKG

I can tell you with 100% certainty that when I put the suction cup antenna
in the center of the windshield above the Narco stack in our 172 and
transmit on 119.4, I lose all satellites. This has happened to me with my
old III Pilot and also my Garmin 196. If I put the antenna in the very
lower left corner away from the radio stack it seems ok. It is just that
plane.

In my specific instance it happens not departing, but when I switch to tower
frequency and transmit when landing. Usually right after I call the field
in site and they switch me to tower. Two separate Garmin units, two
different antenna's. It requires a power on and off.

I can also count on losing all satellites when flying right over the
approach control radar antenna on the KLN90B in the panel on another
airplane and it requires a power cycle. It only has happened at my home
airport.



I'm not an EE so I can't comment on this guy's conclusions, other than
to say that the Garmin installation manuals for panel-mount GPS units
specifically warn against running the GPS antenna cable near nav/com
radios. My avionics shop told me that modern radios are sufficiently
shielded, but obviously, some older radios are not. Garmin uses the
same active antenna system for panel-mount as they do for portable.

I can also say that I have KX170B nav/coms and have never had a problem
with interference on the 396.



JKG

I have a Garmin GPS III Pilot portable and have never had a problem. I have
used it in 172s, 182s, and various Piper aircraft both sitting on the
glareshield with its little stubby antenna, as well as sitting on the right
seat using a portable "full-size" antenna with the suction cup unit on the
bottom right corner of the windscreen.

Marc


"Jonathan Goodish" wrote in message
...
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.

So far I've not had any trouble with our new 496 (and I've never had
any trouble with the Lowrance or AvMap units) -- but it's nice to find
an actual explanation for a problem that has struck several folks on
this group.


I'm not an EE so I can't comment on this guy's conclusions, other than
to say that the Garmin installation manuals for panel-mount GPS units
specifically warn against running the GPS antenna cable near nav/com
radios. My avionics shop told me that modern radios are sufficiently
shielded, but obviously, some older radios are not. Garmin uses the
same active antenna system for panel-mount as they do for portable.

I can also say that I have KX170B nav/coms and have never had a problem
with interference on the 396.



JKG

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.

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...

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...

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.

"Jonathan Goodish" wrote:
My avionics shop told me that modern radios are sufficiently
shielded, but obviously, some older radios are not. Garmin uses the
same active antenna system for panel-mount as they do for portable.

I've had two radios that would flat-line GPS reception in the airplane. One
was a Sporty's handheld NAV/COM, the other was a Cessna ARC 385 panel
NAV/COM. The Sporty's would squash my portable GPS reception just by being
on; the ARC 385 would kill panel and portable GPS when transmitting.

--
Dan
C-172RG at BFM

A friend of mine has used http://www.mumetal.com/ on the top of his radio
stacks to shield compasses from magnetic and electro magnetic interference.
He's had wonderful results with this material.

Possibly a combination of mumetal and an RF shield, say from fotofab, would
provide positive results.

Jim


"Jay Honeck" wrote in message
oups.com...
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.

So far I've not had any trouble with our new 496 (and I've never had
any trouble with the Lowrance or AvMap units) -- but it's nice to find
an actual explanation for a problem that has struck several folks on
this group.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"