For sure equipment clocks will drift. The drift is basically similar to that of a quartz crystal controlled watch – probably of the order of seconds per day. It is trivial to make this orders of magnitude better with a TXCO, and you can buy atomic clock modules like this
So long-term GPS accurate time dependence is not necessary.
You can for sure jam GPS but not over a vast area; that would need a lot of technology and be done from a high altitude (not from the ground) and IMHO would result in military action pretty fast.
In Central Europe there is also DCF77 which went into operation in 1959 and is used as the timing source for a lot of mission critical things. A lot of harm could be done by taking out the DFC77 transmitter which is just one single location. The threat is very old and there have been different groups aiming at causing damage to the society for many decades.
The scenario of taking out GPS (and the other systems which all modern receivers should use in parallel) is very obscure in my view. Aviation would be the least concern there and if we really want to take precaution, other areas as Timothy says are much more relevant than aviation which could cope with it.
eLoran is much easier to jam than GPS which is why it provides close to zero value.
This is just one look at the subject. There are lots out there, from scary gutter press journalism to academic papers.
It would seem that the real time clocks installed in comms and internet boxes are nowhere near as accurate as the atomic clocks provided for free by GPS and breakdown in the absence of GNSS signals can vary from a few seconds in the case of mobile phones to a few hours for data systems. When just one satellite failed for 12 hours in 2014, there was widespread chaos and data loss. And that was just one satellite that was fixed (or, rather, I think, taken out of service) quite quickly.
If the whole constellation was jammed or spoofed, or if there were a major natural disaster from, say, sunspot activity, it could be much worse.
As an electronics design engineer I would find it incredible (though obviously possible since human stupidity has few bounds, especially these days when older engineers are either retiring or moving into “management”) that somebody would design a critical system which gets time from the GPS signal without running its own real time clock which merely uses GPS for a periodic sync.
If share dealing breaks for a bit, I won’t be too sorry 
If I was a Ferrari dealer, maybe…
Where would be the PSTN and internet vulnerability? All the boxes that need date/time have their own real time clocks.
DME/DME is for airliner INS fix-ups, not for timing.
E-Loran is not viable for GPS replacement not because it would not work but because no receivers exist and none will exist because the bottom dropped out of the navigation market about 20 years ago, ue to GPS, so nobody is going to develop a box for it – for aviation or any other use.
So I think we will have to live with the risk. It can’t be eliminated anyway. Let’s say a 737 is flying Gatwick to Brussels, and the ILS in Brussels has packed up (and the backup too because somebody has vandalised the antennae). The plane has to divert to another ILS. Same for GPS… you have to have an ILS alternate. Having a GPS alternate for a GPS IAP is dumb.
You will notice on rereading that my principal concern is not for navigation (air, sea or road) but for all the critical industries that use GNSS for timing, without which they would either shut down or fail. Banking is a big worry, but also communications systems (definitely mobile phones and I suspect the PSTN and Internet) and power grids.
I have seen some real doomsday predictions for when GNSS fails or is jammed.
If a technology savvy power (N Korea, Russia, China, possibly Daesh or Al Qaeda) wanted to cause maximum disruption to the Western economy, it could, as I understand it, do so with some pretty lower power equipment, either from international airspace or using satellites.
To me it seems mad to depend so heavily on GNSS, which is the most fragile and weak of technologies.
If system timing from GNSS can be solved with DME/DME then that’s brilliant. I suspect that that is a laughable idea. eLoran is the only backup I know that might keep the world going in the absence of GNSS. My understanding is limited, but I thought that it was harder to jam.
Peter wrote:
OK, but is this “LPV 200” anything technically meaningful, or is it just an administrative acceptance that LPV is good enough for 200ft rather than whatever the current absolute system minimum is (250ft?)?
LPV 200 or any LPV with a DH below 250 requires a lower VAL of 35 meters verses the standard VAL of 50 Meters. The tighter tolerance on the maximum permitted VPL value reduces the availability of the procedure in some parts of the US, in particular California and Alaska. Compare the service are for standard LPV http://www.nstb.tc.faa.gov/24Hr_WaasLPV.htm verses LPV 200 service http://www.nstb.tc.faa.gov/24Hr_WaasLPV200.htm on any given day and you will see a difference. When the sun is active, the impact is mostly on LPV 200.
One the VAL limit is exceeded by VPL, the vertical guidance (LPV) minimums are unavailable and downgrades are to LNAV.
Airliners have INS and, over land, DME/DME. I am sure that should be an adequate backup for GPS which only modern airliners have only just recently begun using anyway…
For sure you could disrupt GPS using some simple techniques and it would create havoc on the roads (satnav etc) but it wouldn’t affect airline traffic. For example GPS was totally jammed all over the Athens area when I overflew it a couple of years ago. No notam, either.
As I mentioned, DME/DME is a viable backup.
We closed down the complete CAT on an imaginary ash cloud (best flying week of my life) and we survived. So we can handle. If we really want to have a sensible backup to GNSS, then please come up with something realistic, not eLoran…
achimha wrote:
22 years of flawless operation and multiple redundant systems operational today.
I don’t know why the worry about N Korea. 65 years of operation and they haven’t launched a nuclear missile against the USA.
achimha wrote:
Loran is much easier to take out than GNSS. There is no value it would add. What’s the fragility of GNSS? 22 years of flawless operation and multiple redundant systems operational today. Just a way to get government money for some obscure project in my view.Just wait for the next Carrington event.
