Peter wrote:
It is the 11th and 13th sub-harmonic of the GPS L1 ~1575MHz carrier frequency – see e.g. here.
172driver wrote:
Every GPS receiver I know has a status page. That’s where you look.
Great tips – thanks a lot – I was completely oblivious to this test. Time for another donation to EuroGA!
The extent to which this is a problem depends mostly on whether the installer used crappy cable. There has been a tendency to do that, because good coax is expensive – €5-10 per metre. Search for RG400 – example. I have always free issued RG400 to installers, to make sure it gets used. There are similar cables now which are lighter and probably cheaper, but as per my link above I had a bad problem with this in the TB20 because Socata used poor cable.
Some radios also radiate these harmonics a lot more than others. The 1970s designs are worse; no GPS in those days.
Then you also get the ELT antenna impedance matching network getting excited by VHF transmissions, and the resulting emission from the ELT antenna wipes out GPS. I think this issue is not common; not actually heard of a real case, IIRC.
Peter wrote:
Then you also get the ELT antenna impedance matching network getting excited by VHF transmissions, and the resulting emission from the ELT antenna wipes out GPS. I think this issue is not common; not actually heard of a real case, IIRC.
When we upgraded the avionics of a PA28 last year, the installer insisted in removing the quite old ELT for that reason. As the aircraft had a PLB anyway, there was no legal need for an ELT so we let him. Also, that saved some weight…
The comm GPS test is a Transport Canada requirement when I approve (by STC) a wiring change to an aircraft. I routinely do the test, as it is not difficult (though can be time consuming with dual comms and GPS). The quality of the antenna wiring is one thing, but, what I’m more looking for is the possible effect of other non avionics (so not shielded) wiring which simply runs close to both a comm and GPS antenna. The comm transmission can be carried from the comm antenna back into the “wire”, which then conducts the signal near enough the GPS antenna to obliterate the GPS signal.
It is a courtesy to be confident that you’re not blatting away on a locally used frequency, so depending upon locale, I might choose to not check one of the frequencies. Amusingly, I took a new Transport Canada Aircraft Certification Engineer on a test flight for buffet and vibration for a camera hole I was approving in the floor of a 182, there had not been any wiring changes. Want wanting to impress the TC Engineer about my thoroughness, I did the test after we landed, and the GPS signal crashed on the second comm frequency. I reported to the airplane owner that he had a wiring problem to be fixed, and the TC Engineer saw a test fail, which is a good thing for validating why we do the tests!
If you set the GPS on the page which shows the satellite signal levels, and just press PTT on the radio, without speaking, you get the same result. You see the signal levels start to fall within seconds.
Peter wrote:
If you set the GPS on the page which shows the satellite signal levels, and just press PTT on the radio, without speaking, you get the same result. You see the signal levels start to fall within seconds.
I’ve done the test and as you say, the levels start falling but interestingly enough they then recover somewhat. In any case you only have a problem if levels drop enough to cause a loss of integrity.
Peter wrote:
If you set the GPS on the page which shows the satellite signal levels, and just press PTT on the radio, without speaking, you get the same result. You see the signal levels start to fall within seconds.
What’s the practical implication of this? If I’m transmitting at most a couple seconds I wouldn’t even notice. The only time I can imagine any problem would be on a coupled RNP approach where the CDI momentarily deflects or flags and the AP handles it poorly. I’ve certainly never seen anything in my or any other airplane that would indicate GPS problems on transmit.
dutch_flyer wrote:
What’s the practical implication of this?
A former TC Engineer who I used to work with was a first hand observer to this inadequacy when he witnessed the flight testing of a Dash 8 converted to maritime reconnaissance in Toronto. The GPS approach kicked off when the pilot transmitted on the comm. Of course, the GPS will recover, but depending upon how long it takes, it could affect an approach. In any case, not acceptable, so test….
Peter wrote:
There are similar cables now which are lighter and probably cheaper
[than RG400] – what cable would that be? My interior is still out and I might be tempted to check what’s there and possibly replace.
I don’t know but recall reading about it. A google on rg400 should find the lighter alternatives. Whether they are as good I don’t know. Usually the attenuation suffers (there is very little “new under the sun” when it comes to cables; it’s been known since well before WW2) but it may not matter in your application if the frequency is low or the length is short.
Personally I would just use RG400. The weight is immaterial in GA applications. The drive to lighter cables has been in airliners where you have potentially hundreds of kg of coax.
