GPS upended?

[Drew-]

Myself and 3 other family members had GPS issues over the weekend. I couldnt understand it and neither could they because we all had never had issues with it before. I thought it was a strange coincidence.

[StuM]

Originally Posted by barnakiel

Interference.

What interference? US agency FCC clearly was of a different opinion, eh?

barnakiel

The potential interference that the link in the original post was expressing concern about. i.e. the topic of this thread.

[StuM]

Originally Posted by Drew-

Myself and 3 other family members had GPS issues over the weekend. I couldnt understand it and neither could they because we all had never had issues with it before. I thought it was a strange coincidence.

Location(s) ?
Potential proximity to GPS jamming devices?

[barnakiel]

Originally Posted by StuM

The potential interference that the link in the original post was expressing concern about. i.e. the topic of this thread.

OK.


Now I 100% am with you.


The word 'potential' makes a world of difference.



Do we by any chance know how high this potential is?


Clearly, that American agency (FCC) that gave a go-ahead estimated it to be very very low.


Any reason not to trust FCC? Have they had many bloopers before?


barnakiel

[Auspicious]

Originally Posted by alex_sauvage

It probably has something to do with being in Norfolk: one of the biggest naval bases.

The Navy and other elements of DoD sometimes conduct GPS spoofing tests in the lower Chesapeake. The effects are usually pretty localized. Check the LNTM Tests are always published.

[Pete7]

Originally Posted by Auspicious

No. You're confusing selective availability (SA) with mainstream GPS. Same satellites, same frequencies, same power. SA has been turned off for a long time and just adds jitter to the timing that military GPS receivers can back out. It all become moot with differential GPS and WAAS.

That isn't to say military GPS receivers don't have higher sensitivity and better selectivity. It was embarrassing during Desert Shield to have US military using civilian GPS receivers for higher reliability. *sigh*

In the early days I thought there were two signals, called S and P. Civies got the S signal whilst the military got an encrypted P signal for some equipment. SA only degraded the S signal.

However, there was such a shortage that all production of S equipments were bought up by the coalition forces, so during DS there was a mix of equipment. Even so only two S receivers were available at company level, normally one for the OC and one for the ambulance, for obvious reasons. Trimble and another make Trak something? with two lines of display and a replacement cost of £4000.

The rollout of GPS civilian sets was delayed due to the demand and we didn't see civilian sets available to buy in Europe until early Jan 92 in any quantity. I did have my name down for a Lowrance but none available in Mar 92 so went with the Interphase which we broke. Replaced with the first Lowrance versions.

No idea were we are up to now, but we are back teaching the use of a sextant. Pretty difficult to jam a sextant.

Originally Posted by Auspicious

The Navy and other elements of DoD sometimes conduct GPS spoofing tests in the lower Chesapeake. The effects are usually pretty localized. Check the LNTM Tests are always published.

NATO exercises do the same, normally out of the way places like Northern Scotland.

[Auspicious]

Originally Posted by Pete7

In the early days I thought there were two signals, called S and P. Civies got the S signal whilst the military got an encrypted P signal for some equipment. SA only degraded the S signal.

What I think I remember is L1 and L2 as the two frequencies. C/A code is on L1 and P code is on both L1 and L2. The two frequencies allows determination of atmospheric delay (latency) by the client with only one piece of external information: time. The P code is what allowed backing out the imposed time jitter of selective availability.

ETA: See GPS signals (L1, L2, L5) - Geoconnect and scroll down to Frequency Information.

[Drew-]

Originally Posted by StuM

Location(s) ?
Potential proximity to GPS jamming devices?

DC Beltway into Frederick/Hagerstown area and from Baltimore beltway.
I'll take off my foil hat now. LOL. Just found it strange.

[redneckrob]

Originally Posted by Auspicious

What I think I remember is L1 and L2 as the two frequencies. C/A code is on L1 and P code is on both L1 and L2. The two frequencies allows determination of atmospheric delay (latency) by the client with only one piece of external information: time. The P code is what allowed backing out the imposed time jitter of selective availability.

ETA: See GPS signals (L1, L2, L5) - Geoconnect and scroll down to Frequency Information.

Well you're almost there! There are two frequencies. The second requires the codes to use, and it allows the receiver to back out propagation delays. It has been encoded since day 1 and still is today, it was always a pain to have to ensure cores were loaded in the helicopter and reload them when it inexplicably dumped them in my CG pilot days.

Selective availability (SA) has nothing to do with the second frequency. It was a purposeful distortion of the primary frequency that required a second code to remove. It was turned off years ago and is no longer in use.

Just to muddy the water some more, there was a marine and is an aviation differential GPS system that uses ground stations and satellites to transmit corrections to GPS that are independent from the GPS system itself. The marine version has been shutting down over the past 3 years and is supposed to be gone by this year.

[StuM]

Originally Posted by redneckrob

Just to muddy the water some more, there was a marine and is an aviation differential GPS system that uses ground stations and satellites to transmit corrections to GPS that are independent from the GPS system itself. The marine version has been shutting down over the past 3 years and is supposed to be gone by this year.

Are you taking about WAAS? If so, do you have a reference to it being shut down? And what's the difference beteen marine and aviation in that context?
If not, what satellite correction transmitting system are you talking about?

[redneckrob]

Originally Posted by StuM

Are you taking about WAAS? If so, do you have a reference to it being shut down? And what's the difference beteen marine and aviation in that context?
If not, what satellite correction transmitting system are you talking about?

The Coast Guard DGPS system is being shut down (https://www.navcen.uscg.gov/?pageName=dgpsMain) WAAS is alive and well but not typically used (or needed) by cruisers.

[a64pilot]

Originally Posted by Auspicious

No. You're confusing selective availability (SA) with mainstream GPS. Same satellites, same frequencies, same power. SA has been turned off for a long time and just adds jitter to the timing that military GPS receivers can back out. It all become moot with differential GPS and WAAS.

That isn't to say military GPS receivers don't have higher sensitivity and better selectivity. It was embarrassing during Desert Shield to have US military using civilian GPS receivers for higher reliability. *sigh*

No, I’m not, used to be civilians used C code and military P (y) Y code if you had a secure fill, but now I believe the military uses M code.
Any area that military operations are being carried out, a directional signal, is sent to cover that area, increasing the signal strength tremendously.
https://en.wikipedia.org/wiki/GPS_signals

GPS is and was from the beginning a US Military system, the use of which was freely given away to the world, but don’t think your GPS is the same as a Military one.

Military GPS has never been a victim of selected availability, Bill Clinton had SA disabled, by doing so it made the US GPS more accurate than the Russian system at the time.

[a64pilot]

Originally Posted by StuM

Are you taking about WAAS? If so, do you have a reference to it being shut down? And what's the difference beteen marine and aviation in that context?
If not, what satellite correction transmitting system are you talking about?

No, he’s not WAAS is good, very good for boat navigation, but for instance if you want to use GPS to steer your farm tractor and need dependable accuracy to a couple of inches or less, then you will have to have a ground station to augment the Satellite system.

GPS steering of farm tractors has increased field yields significantly because the rows are exactly, precisely spaced.

The ground station is known as DGPS or Differential
https://en.wikipedia.org/wiki/Differential_GPS

Instrument approach GPS receivers self monitor somehow and will illuminate a light if the accuracy isn’t within spec, I don’t know how they self monitor, but is a requirement for an approach certified GPS.
Initially GPS was to end used for precision approaches, and at that time that required DGPS.
I’ve since lost track of what goes on in Military GPS as I Retired in 2002, so almost 20 years ago, and 20 years in the electronics world is a long time.

However the accuracy of two ring laser INU’s, each having their own military GPS’s, with a secure fill is astonishing.

[a64pilot]

Originally Posted by Auspicious

It was embarrassing during Desert Shield to have US military using civilian GPS receivers for higher reliability. *sigh*

That never happened either, many, many Commercial GPS receivers were used due to the lack of funding, there were very few Military GPS receivers, none for example in Army Aviation at the time.
The opening shots of the war were when AH-64 helicopters destroyed radar sites, but what wasn’t widely publicized was that they followed Air Force Pave Lows for part of the route because they had GPS, and the Army didn’t.

On edit, I believe you are speaking about Desert Storm, not Shield.

[Jammer]

I work in the industry.


The concern about the satellite broadband allocations is that they:
1) May hypothetically interfere with some (many) badly designed GPS receivers and
2) This interference, if it occurs, will be global in scope and difficult to mitigate.


All receivers (GPS or otherwise) have a limited amount of selectivity, that is, they are susceptible to interference from strong signals on adjacent frequencies. Well designed receivers are more selective. Well designed receivers also cost more.


The problem for GPS users is that there are millions of receivers out there that were designed to be low cost and that were designed when adjacent channels weren't in use, so selectivity was not important. There is no way of knowing which receivers are affected or the extent of the degradation, in advance, without gathering up a (large) representative sample of GPS receivers and running tests in a lab.


The FCC deals with these sort of complaints -- from incumbent users of spectrum who now have to deal with new users on adjacent (or nearby) channels/frequencies -- all the time. It is perhaps the most common source of complaints that the FCC receives. It is the FCC's position that spectrum is shared and receivers have to be designed to be selective and to allow for both present and future uses of adjacent frequencies.


There is going to be increasing use of spectrum for communications to vehicles in space and incumbent spectrum users, on the whole, are just going to have to be prepared.


I don't believe that well designed GPS receivers will be affected in a measurable way.