GPS loss of signal explanation

[Jay Honeck]

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"

[Jonathan Goodish]

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

[ron]

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

[Marc CYBW]

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

[[email protected]]

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.

[Jim Burns[_1_]]

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"
>

[Robert M. Gary]

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.

-Robert


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.

[Dan Luke]

"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

[mbremer216]

I have a full Narco stack in my Cherokee and keep the antennae for my Garmin
196 just to the left side of the windshield brace. I've never have seen a
problem with the GPS in the past 5 years.

This spring I upgraded the radios to the 12D+ package with the same results.

Mike

"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"
>

[.Blueskies.]

> 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...

[Jon Kraus]

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...
>
>

[Peter Duniho]

".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

[Bob Noel]

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

[Peter Duniho]

"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

[[email protected]]

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.

[[email protected]]

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

>".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
>

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.

[Robert M. Gary]

mbremer216 wrote:
> I have a full Narco stack in my Cherokee

My deepest sympathies. ;)

-robert

[Casey Wilson[_1_]]

"Robert M. Gary" > wrote in message
ups.com...
>
> mbremer216 wrote:
>> I have a full Narco stack in my Cherokee
>
> My deepest sympathies. ;)
>
> -robert

LOL! For the Narcos or the Cherokee -- or both...
That was cruel, I'll take it back. Hehehehe....

[mbremer216]

I don't know why...I've never had a lick of trouble out of any of it.

Mike
"Robert M. Gary" > wrote in message
ups.com...
>
> mbremer216 wrote:
>> I have a full Narco stack in my Cherokee
>
> My deepest sympathies. ;)
>
> -robert
>

[Jay Honeck]

> I don't know why...I've never had a lick of trouble out of any of it.

Atlas has two Narco coms, a Narco DME, and two Narco GS/VORs. All work
perfectly, all the time.

I've not quite figured out where the anti-Narco sentiment come from,
but it's almost as insidious as the anti-Microsoft attitudes out there.

--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Robert M. Gary]

Jay Honeck wrote:
> > I don't know why...I've never had a lick of trouble out of any of it.
>
> Atlas has two Narco coms, a Narco DME, and two Narco GS/VORs. All work
> perfectly, all the time.
>
> I've not quite figured out where the anti-Narco sentiment come from,
> but it's almost as insidious as the anti-Microsoft attitudes out there.

They probably are good radios. The bad rap against them came because
they were the first to introduce the concept of "non-field serviceable
units". They took away the right for avionics shops to work on them,
requiring they all be returned to the factory (at a price they set).
However, today we have Garmin doing the same. Compare that to a King
radio where any just-out-of-school avionics guy can do a field strip in
an hour.

-Robert

[Morgans[_3_]]

"Jay Honeck" > wrote

> I've not quite figured out where the anti-Narco sentiment come from,
> but it's almost as insidious as the anti-Microsoft attitudes out there.

Perhaps 'cause they are just old, without the new bells and whistles?
--
Jim in NC

[Ray Andraka]

Jay Honeck wrote:

>>I don't know why...I've never had a lick of trouble out of any of it.
>
>
> Atlas has two Narco coms, a Narco DME, and two Narco GS/VORs. All work
> perfectly, all the time.
>
> I've not quite figured out where the anti-Narco sentiment come from,
> but it's almost as insidious as the anti-Microsoft attitudes out there.
>
> --
> Jay Honeck
> Iowa City, IA
> Pathfinder N56993
> www.AlexisParkInn.com
> "Your Aviation Destination"
>

Jay,

It came from avionics shops that got pretty ****ed at Narco when Narco
decided to pull all service into the factory. Narco's reasons for doing
it are understandable, and I think make sense, but the avionics shops
have never forgiven them for taking away a good chunk of their repair
business.

[[email protected]]

On Tue, 08 Aug 2006 00:00:46 -0400, Ray Andraka >
wrote:

>Jay Honeck wrote:
>
>>>I don't know why...I've never had a lick of trouble out of any of it.
>>
>>
>> Atlas has two Narco coms, a Narco DME, and two Narco GS/VORs. All work
>> perfectly, all the time.
>>
>> I've not quite figured out where the anti-Narco sentiment come from,
>> but it's almost as insidious as the anti-Microsoft attitudes out there.
>>
>> --
>> Jay Honeck
>> Iowa City, IA
>> Pathfinder N56993
>> www.AlexisParkInn.com
>> "Your Aviation Destination"
>>
>
>Jay,
>
>It came from avionics shops that got pretty ****ed at Narco when Narco
>decided to pull all service into the factory. Narco's reasons for doing
>it are understandable, and I think make sense, but the avionics shops
>have never forgiven them for taking away a good chunk of their repair
>business.

Jay, I can give you an indication why they have their reputation!

An aircraft owner had Narco in his aircraft and said they were 'now'
ok so we took his advice, but...

Purchased 2x MK12D+, CPM136, IDME891 approx 3 years ago.

The IDME891 (ILS/DME) does not work properly at very low frequencies
(around 108.1MHz) the ILS is ok but the DME doesn't always work. The
first flight showed a problem but since it was subsequently not used
at these frequencies for some time the fault was not discovered for a
considerable time.

Both MK12D+ NAV/COM had Transmitter audio failures. Apparently it is a
known diode failure problem. Now I know what happens when you have a
COM failure when overflying a military airfield :-(

The latest is a severe dimming of the NAV display and the first digit
has failed completely.

The CPM136 AF panel produced smoke when COM 2 button pressed.

The IDME891 saved us panel space and should be ideal but the
intermittent DME reading problem has still to be solved.

David

[B A R R Y[_1_]]

Jay Honeck wrote:
>
> I've not quite figured out where the anti-Narco sentiment come from,
> but it's almost as insidious as the anti-Microsoft attitudes out there.

I've had several $300 Narco auto-dimming EL displays fail. According to
some very reputable local avionics businesses, Narco isn't easy to deal
with.

[Jay Honeck]

> Jay, I can give you an indication why they have their reputation!
>
> An aircraft owner had Narco in his aircraft and said they were 'now'
> ok so we took his advice, but...

<Awful tale of woe snipped>

Okay, I'd be plenty ****ed, if that were my plane. I just haven't had
any of those problems, thankfully!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[[email protected]]

In rec.aviation.piloting Jay Honeck > wrote:
> > I don't know why...I've never had a lick of trouble out of any of it.

You are apparently among the blessed few who have not had problems
(yet). May it ever be thus!

> Atlas has two Narco coms, a Narco DME, and two Narco GS/VORs. All work
> perfectly, all the time.

> I've not quite figured out where the anti-Narco sentiment come from,
> but it's almost as insidious as the anti-Microsoft attitudes out there.

Actually, Jay, it is from those of us who have experienced multiple
failures of Narco equipment, and been unable to get it repaired or
replaced is a timely fashion... because Narco REFUSED to provide
schematics and parts to the local avionics shop and INSISTED that
the equipment be sent to Narco for repair, and then the equipment
would come back MONTHS later with "no problem found", and the
equipment won't even turn ON, so you can tell that they did not even
attempt to fix the problem. It is those of us who have had BAD
experience with Narco "customer service" (read SEVERE lack thereof),
who will NEVER purchase Narco equipment again. It is us who had
Terra equipment that needed service (and Narco bought Terra) and put
it out of business. It is US who tell our FRIENDS to stay away from
Narco, and tell our non-friends to purchase Narco.

For the few of you with Narco equipment that have not had problems,
may it always be well for you. You are the blessed few.

Best regards,

Jer/ "Flight instruction and mountain flying are my vocations!"
--
Jer/ (Slash) Eberhard, Mountain Flying Aviation, LTD, Ft Collins, CO
CELL 970 231-6325 EMAIL jer<at>frii.com http://users.frii.com/jer/
C-206 N9513G, CFII Airplane&Glider FAA-DEN Aviation Safety Counselor
CAP-CO Mission&Aircraft CheckPilot BM218 HAM N0FZD 240 Young Eagles!

[Ray Andraka]

B A R R Y wrote:
> Jay Honeck wrote:
>
>>
>> I've not quite figured out where the anti-Narco sentiment come from,
>> but it's almost as insidious as the anti-Microsoft attitudes out there.
>
>
> I've had several $300 Narco auto-dimming EL displays fail. According to
> some very reputable local avionics businesses, Narco isn't easy to deal
> with.

"Not easy to deal with" because Narco won't supply them parts or
approved repair manuals. For the owner though, it is a pretty good deal
except you aren't going to drop into a shop and have them fix it while
you wait.

Units sent to Narco for factory repair are brought up to the latest mod
level and are 100% checked before they are returned to you. After the
repair, the whole unit is warrantied for 90 days. If anything breaks on
it during the warranty period (anything, not just the repair), they fix
it at no additional cost. As an owner, you can take comfort in knowing
it is fixed correctly, completely calibrated to factory spec, and
brought up to the latest revision. The shop rate is, I believe,
$105/hr, which is a little higher than most avionics shops, but they
also generally fix it in fewer hours. You can also get the unit
evaluated before it is repaired for a flat fee of (IIRC) $105, then if
you decide to repair it, that evaluation fee is applied toward the
repair. You also have the option of trading it in for another radio,
although the trade-in terms are not all that generous. I haven't found
the factory service to be hard to deal with at all as an owner.

On the other hand, I've got some horrendous stories dealing with
avionics shops that applied bandaid fixes to radios, and the warranty
most shops I've dealt with offer covers only the repair that they did,
and expires more or less as soon as your wheels leave the ground. From
the avionics shop's perspective, yeah, Narco gave them a raw deal and is
hard to deal with because they won't let the avionics shops service the
radios. The Narco radios are not any worse than other radios of the
same vintage. Face it, aircraft panels are a harsh environment for
avionics, and after being in there for 20 years you are bound to have
problems. Buying 20-30 year old used equipment, even if the seller
tells you it is like "now" is a crap shoot. You can't blame the
manufacturer for that.

[B A R R Y[_1_]]

Ray Andraka wrote:

> On the other hand, I've got some horrendous stories dealing with
> avionics shops that applied bandaid fixes to radios, and the warranty
> most shops I've dealt with offer covers only the repair that they did,
> and expires more or less as soon as your wheels leave the ground. From
> the avionics shop's perspective, yeah, Narco gave them a raw deal and is
> hard to deal with because they won't let the avionics shops service the
> radios.

FWIW, one of the avionics shops I mention is good enough to have a
popular "Flying" columnist 1300+ miles for a GPS installation. They
don't do band-aids... <G>

Every one of our failed EL displays was replaced by Narco's "factory
service". We also had audio output issues were Narco returned the
radios and blamed the panel and wiring. On both occasions, the radios
WERE screwed up, and we paid Narco AGAIN to fix them.