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GPS jamming and spoofing and relying on GPS, and GPS backup plan ?

Why would you be concerned about GPS jamming? When it's NOTAMed, you ought to know about it.

Indeed, I do see it on the NOTAM's, but I do fly VFR (or that UK IFR/IMC OCAS in class G phenomenon) and GPS is my primary means of navigation. Its not a major concern as there are other ways of navigation, but just a nuisance, and I am still not sure why they do it. Some tests have been in the Cambridge area, which is not remote - though I appreciate the tests in Caithness, and West Wales are.

OK let's assume that for some unforeseen reason all the satellite systems are hit by some meteor phenomena and are knocked out, could some other navigation system be developed using an earthbound technique such mobile phone or television transmitters. Is there simpler way of implementing ILS?

jxk
EGHI, United Kingdom

Yes, apart from similar satellite based systems (Russian GLONASS -- every modern GPS can use it, European Galileo in a few years, Chinese, etc.), there are plans to build a DME based infrastructure as a backup. It works exactly like GPS, you determine your distance from 2 or more stations and thereby get your 2D/3D position. This requires a lot more DME stations though but they tend to be cheaper than satellite and they're already there.

Whether eLORAN will see widespread use, I don't know. Its main advantage is range. You can't easily have stations on the ocean so DME is not suitable.

OK let's assume that for some unforeseen reason all the satellite systems are hit by some meteor phenomena and are knocked out, could some other navigation system be developed using an earthbound technique such mobile phone or television transmitters. Is there simpler way of implementing ILS?

ILS does not use satellites. It uses a signal radiated from transmitters near the runway.

Administrator
Shoreham EGKA, United Kingdom

It takes very little power to knock out a GPS receiver, particularly if it is WAAS. I have a KY197A as my number 2 Com. As it has gotten older, it leaks RF out thru the front of the display. If I transmit on any frequency close to 121.2, my WAAS GPS will instantly loose all satellites and if the transmission is of any length, the GNS530W will be unusable for 5 to 10 minutes. The RF is the 13th harmonic, which in spite of being very low power, kills the GPS reception. The transmitter has a notch filter and the antenna is on the belly. I substituted another KY197A and it exhibited the same issue. On the advice of an avionics technician, I put an anti-static bag (the ones with the metallic material inside) over the front face of the KY197A as a test and it eliminates the interference. I took the aircraft to the avionics shop and we did some investigating on both units using a frequency analyzer set to look at the GPS spectrum area. We used a probe with a small loop of wire as an antenna to see where the RF harmonic was coming from. Un-attenuated, as soon as the transmitter was keyed, a large pulse immediately was visible in the GPS band. Attenuated sufficiently so that you had to get close to the source of the RF energy, we found a shielded cable that came from the transmitter to the modulator to be the hot spot. We cut, wrapped, and tapped some of the antistatic bag around the cable, and this greatly reduced the leaking RF energy, to the point that it no longer interferes with my GPS. Note that the energy was not leaking out thru the rear connectors or wire bundle, just thru the front of the unit thru the display.

KUZA, United States

My King KDM705 ancient DME does the same. If I tune it to 108.05MHz (DME channel 17Y, 1041MHz transmit, 1104MHz receive), all GPS receivers go blank. I haven't figured out why and I don't understand how these frequencies relate to the GPS signal. A couple of other DME channels do the same, some attenuate the signal, others kill it.

The KX155A/165A radios do the same - here.

However the problem is not made easier by most installers using cheap coax. They should use RG400 which is double shielded stuff but it costs about €8/metre.

I also think that a lot of these VHF-GPS interference cases can be solved by installing the dual VHF+GPS antenna such as I used here. This antenna contains a resonant notch filter at its base which takes out the 1575MHz harmonic from the VHF. It's not a cheap solution but probably cheaper than having an avionics shop spend some hours poking around.

On a bit of a tangent, the Socata TB oil pressure gauge falls to zero when transmitting on some (quite a number) VHF frequencies. It is not often noticed because one is rarely watching the oil pressure while doing the radio, and when it is noticed, the connection is not readily made. There is a simple ferrite DB25 connector filter one can install.

It takes very little power to knock out a GPS receiver, particularly if it is WAAS

However, would a WAAS GPS detect the interference more readily?

The military have the capability to spoof GPS i.e. the receiver sees nothing other than a position fix which is some distance off, but that is quite complicated to achieve.

Administrator
Shoreham EGKA, United Kingdom

The local avionics shop has got a GPS simulator. You can stand next to the plane put in latitude and longitude. The GPS will show you in Greenland or wherever you like. The signal is not very strong but it made me aware not to neglect the other instruments that are in the panel just to cross check...

www.ing-golze.de
EDAZ

ILS does not use satellites. It uses a signal radiated from transmitters near the runway.

Sorry Peter, I know ILS doesn't use GPS but as you and others are doing GPS approaches I got to wondering if GPS was knocked out whether it would possible to develop a 'cheap' form of ILS using some new technology for smaller airfields.

I'm also fascinated that GPS receivers can lose the satellites when other transmissions are made. I remember that the Garmin 150 did the same thing if the comms box was set to 121.5 (the default frequency).

jxk
EGHI, United Kingdom

The 121.5 (exactly 121.5) issue is slightly different. That one is caused by the PI-tank output filter (example) in the old 121.5-only ELTs getting wildly excited by a VHF transmission on/near 121.5, and re-radiating the stuff at a sufficient power to swamp the GPS receiver even though there was no 1575MHz component at all.

The 13th harmonic of 121.15 (practically 121.10 to 121.20) is what also affects GPS - see my original KLN94 stuff. I guess that is more common than the 121.50 issue since not many people (in the UK) have the old ELTs (the 121.5+406MHz ELTs which are now de facto mandatory don't have that issue). This 13th harmonic problem is normally not manifested at 121.5.

My take on this, and also Sebastian's post, is that the best thing you can do is to sort out your plane as best as you can, with really well installed GPS antennae, really good quality wiring, and then it will be very hard to jam it by some ground based idiot.

Of course there are fairly obvious ways to do an airborne jammer but fortunately not many idiots have yet thought of it, and it involves a bit more work.

I don't know of any way to do an "ILS" other than a ground based transmitter. One could do a real ILS for far less that the cost of the proper ones (the electronics costs probably $1000 or so) but it could never be approved officially.

Administrator
Shoreham EGKA, United Kingdom
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