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Ringway Flyer
11th Aug 2007, 20:24
Been to Duxford today and not having our usual machine, took two handheld GPSs with us. As always, we put the lines on the map & being as it was a Good Day for Flying, the GPSs were not really necessary... But, around Daventry, both of them lost all signals. Mine has an active antenna and when working normally shows between 8 and 10 satellites with good strong signals. They were both off for about 10 minutes. Same thing happened coming into MAN over Macclesfield. The Garmin 430s in our usual mount have not (so far) had a problem. Now I know that there is the radio 4 transmitter at Daventry putting out about 400kW on 198 kcs, but would that block the signals? :confused:

Btw - Duxford's new hangar is Very Impressive - the standard of the aircraft there is 1st class. And there were LOTS of Pipers!

Mark 1
11th Aug 2007, 20:37
The front end of the GPS receiver can get saturated by other RF signals, and I have had similar drop-outs very close to that location when using a portable with attached ariel, albeit a few years ago. I know of other similar cases also in the Daventry/Rugby areas. BBC world service also beams out from this area on the HF bands.

So far, I've never had a drop-out using an installed GPS, which significantly shields out ground based signals.

Ringway Flyer
12th Aug 2007, 10:56
Thanks to Mark 1 for his comments. firstly, I want to correct the obvious error in my original post - the R4 tx is of course at Droitwich, not Daventry! and pushes out 500kW these days... The only activity at Rugby now is Loran C, the time signal is now at Anthorn in Cumbria. However as both are VLF, I don't think that they would affect the satellites which are around 1600 MHz?

But I'm sure Mark 1's comment is valid, as the signal is very weak to a GPS. The reason for starting the thread in the 1st place was twofold, one just curiosity as to what's causing the problem and two, to remind people of the vulnerability of hand held GPSs, which seems to be rather more of a problem now than when I used one regularly.

BRL
12th Aug 2007, 11:06
The military often do 'blocking exercises'.

Was there anything in your notams' for this?

neutron
12th Aug 2007, 11:09
Is it possible that you lost GPS reception just after you changed COM frequency in the aircraft? There is a know problem with the local receiver oscillators producing rf harmonics which swamp the GPS receiver when certain COM frequencies are set. One such know problem frequency is 119.875 (Scottish FIR) but there are many others. Note that it is the receiver section of the COM box that causes the problem and the problem is worst if using an aerial close to the radio stack.

WorkingHard
12th Aug 2007, 11:18
As BRL says the military often do blocking excercises which may well be necessary for a number of reasons. That being so can the charge by the RCA be legally enforced when another government department negates the use of a radio installation?

Ringway Flyer
12th Aug 2007, 11:21
I did check the Notams and didn't see anything about the military playing - but I could have missed it. Neutron - that might be the answer, as the radio stack is vintage, an old King and a newer one with 'flip flop' facility. This raises another issue. There are no static discharge wicks on the aircraft and there is significant random noise on both radios and the intercom. And the alternator whines a lot too! Methinks a bit of work needs to be done electrically...:hmm:

PH-UKU
12th Aug 2007, 15:10
Neutron There is a know problem with the local receiver oscillators producing rf harmonics which swamp the GPS receiver when certain COM frequencies are set. One such know problem frequency is 119.875 (Scottish FIR) but there are many others.

Hey, that is exactly what happened to me last week. Was getting interference on 119.875 (like a discharging capacitor breakthrough every 2 secs), but had GPS lose the signal at the same time. Didn't put 2 and 2 together until you mentioned it ...

So, no problems with the GPS if I don't have 119.875 selected.

Bad area for me seems to be around Drymen/Thornhill/Lake of Menteith.

Anyone else had probs with 119.875 and GPS ?

Ringway Flyer
12th Aug 2007, 18:30
So is Coventry approach an 'iffy' frequency? (119.250) That would tie in with our frequency changes/loss of signal...

IO540
12th Aug 2007, 20:08
Part of the certification of an IFR GPS (always panel mounted) is that certain harmonic-frequency tests are done (by setting the DME etc to certain values) to ensure there isn't interference.

I've had signal loss only twice: once near Italy in 2004, when both GPSs lost reception for about 2 minutes, and once in Italy in 2006 when the KLN94 took ~ 1 hour to acquire the signal following power-up on the ground (while a handheld worked fine).

The latter instance was weird and was followed up with Honeywell, with the help of various setup screen photos I took at the time, and the best opinion was that it got its ephemeris (or whatever it's called) data corrupted in a weird way. It came back at FL160 over the Alps - I told ATC I had lost BRNAV capability and got VOR-VOR routes and vectors. The handheld GPS had no usable DCT feature for airways intersections; something I have since fixed...

rotorfossil
12th Aug 2007, 20:14
I've had same problem with Lossiemouth frequency 119.35 on two GPS's simultaneously, one panel mount and one handheld.

wet wet wet
12th Aug 2007, 20:15
I too have experienced loss of GPS signal on several occasions in the DTY area, including in aircraft with "proper" installations. I assume that there is something thereabouts that is jamming the signal (easily done, the GPS signal strength is extremely weak, I was told it is the equivalent to the energy that you get from a candle that is positioned a hundred miles away!).

Unlikely to be the Rugby masts though, as has been said they transmiitted on very low frequencies and, in any case, the last ones were demolished a week or so back!

IO540
12th Aug 2007, 22:16
I was told it is the equivalent to the energy that you get from a candle that is positioned a hundred miles away!).

Probably true, but irrelevant, because the signal has predictable properties which enable its recovery from below the noise floor (Shannon (http://www.exploratorium.edu/complexity/CompLexicon/Shannon.html), 1949 or something like that).

The GPS signal recovery is no less reliable than any other radio link. Look at mobile phones - much greater field strength and a lot of calls are still unworkable. My experience of GPS is that ~ 99.999% of the time it is rock solid. The altitude solution (KLN94 or Garmin 496) is usually within 20ft of a known elevation, or altimeter reading at a low level and with a known accurate QNH.

I would guess that the regular signal losses described here are due to poorly installed units. Most avionics shops do not do a decent job, and inter-equipment interference is very common.

Flap40
12th Aug 2007, 23:06
I've not had any problems with Scottish on 119.875 (my local area) but I did have a problem with two gps's when on a Coventry freq.
IO540, Were you anywhere near Bolzano when you had your problem? It is a known probem area in my airline and has something to do with the Italian mil'.

mm_flynn
13th Aug 2007, 09:53
[I]
would guess that the regular signal losses described here are due to poorly installed units. Most avionics shops do not do a decent job, and inter-equipment interference is very common.

I had a GNS430 in a G-reg plane I acquired. One I moved it over to N the avionics guy needed to do all of the tests that the FAA require for IFR certification - and they found that the number 2 com on several frequencies 'blanked' the GPS signal. I needed some new cabling and a repositioned antenna to fix it.

wsmempson
16th Sep 2007, 20:40
I always assumed the massive "Zub-Zub" noise every 3 seconds around cherbourg (119.625) was caused by the large radar dish going around, but reading this makes me doubt that.

In addition, my 296 has frozen twice here (and nowhere else) so it does make the harmonic problem ring true. The 430 seems to soldier on regardless, I'm pleased to say.