GPS Satellite Question

[Scythe13]

(1 May 2014, 15:03)Skean Dhude Wrote: Think about the question. It is not much good for Sat Nav if it is miles. It is Metres and it is adjustable. They can reduce accuracy when there is tension to reduce the chances of it being used against them and simply shut it off if they want to.

GPS is not often accurate. I was apparently in Kazakstan then in Australia, and a couple of times in China or Japan. Which is much more than 10 miles! So I thought I'd ask.

Thanks for clarifying though SD.

[Devonian]

Yes as SD says, Metres not Miles

[Scythe13]

Well, how I see it, at the moment...

WTSHTF, if the satellites are still going to be up and active, so that a GPS system is able to work, it would make sense to have such a device available.

However, if the network with which it works is compromised or is under surveillance (we won't know if it is, so I'd assume it was), then for the initial time period, it would make more sense to have the device turned off, but available.

Further to this, I'd probably consider turning it on a few months down the line, once major damage and the alike has been stabilised. Having said that, I'd only turn it on momentarily, for few that the network is being monitored.

With this kind of option available, it should be much easier to navigate and the alike.


On a seperate note:
Would it be possible to read other signals in the area? For example, some devices are able to interact via bluetooth and the alike. So would it be possible to relay a signal to align other GPS units?

[River Song]

S13
Interesting analysis but as others have pointed out, your GPS instrument
does not transmit. It only receives coordinates - it isn't nor can it be monitored.

Even smartphones which may have GPS included doesnt transmit its location by this method
but does it using the cellphone part of it. And it the case of a SHTF situation, the policy is NOT to close it down, but to restrict access to those with upwards of class 12 sim cards.

Tally-ho

[BeardyMan]

GPS isn't that hot anyway, get one that does GPS AND GLONASS and you'll be much happier. More satellites at once, more accurate.

[Tartar Horde]

The question is, "do you really need GPS"? It seems to me it's the most complicated of all systems for direction, as it's the most likely to fail due to it's complexity and dependance on modern technology. A map and Compass are far more reliable, and if you don't intend to bug out far you don't even need them if you know your patch well enough.

[Skean Dhude]

Like everything else it has its place. If it is on it gives you a starting point plus a set of maps you can walk with.

The system only needs power and the ability to receive the signals from the Sats, it then does some complicated maths with the more Sats it sees the more accurate it is. Sounds like you had a faulty one there S13 or it could only see one sat.

When you switch them on it tells you the number of sats it can see. It needs a min of three to place you close to where you are. The more the better though.

My view is if it is there, even adjusted to a 50 M window, I'd use it for a starting point. If it isn't there then I have maps and things for fallback.

[Lightspeed]

I remember reading an article about UK military being able to shut down / corrupt GPS signals received in specific geographic areas. In particular I remember an advanced notice of interruption to GPS services around a south coast military establishment.

So, if I remember correctly(?), yes, the military can disable / corrupt GPS data received on civilian equipment.

I cannot find a link to the article but this one is close in cntent and describes shut down of parts of the country:

http://www.equipmentworld.com/gps-shutdo...umstances/

[Skvez]

Each GPS satellite transmits two frequencies the ‘civilian’ and ‘military’ frequencies.

The US military have a number of options to degrade or block civilian receivers.

The simplest is the poorly named “selective availability” which just adds random noise to the lowest significant digits of the time signal making the calculated location less accurate. The US military turned off selective availability during desert storm I (as they couldn’t get enough military receivers and many of their soldiers were using civilian units) and haven’t turned it back on again. However it’s there and could be turned on.

The military receivers (at least the early ones) use both the civilian and military frequencies and use the difference in the time signals to infer information about density of the air in the signal path and thus the time distortion. This allows improved accuracy over using just the civilian frequencies but the military receivers need *both* frequencies to work. Turning off the civilian frequency would also stop military receivers. (I can’t think of any reason why a military receiver couldn’t be designed to work on just the military frequency with similar accuracy to a civilian receiver, however this is not how they were intended and not how the early ones work).

Over a local area a transmitter can broadcast noise on the civilian frequency and essentially block any receivers from getting a lock. This is how GPS jammers work. Jamming GPS
Jamming GPS over a local area (perhaps up to 100 miles without too unreasonably large a transmitter) is potentially available to anyone who knows the frequencies to jam, this option is not restricted to the US military.

The satellites get their clocks updated multiple times per day from ground stations (with more accurate much bulkier clocks). In a major event (global pandemic for instance) it’s probable that the ground stations would stop being manned and these clock updates could stop happening. It’s unclear how autonomous the ground stations are or how quickly the accuracy would degrade once the satellites stop getting clock corrections from the ground. Like most clocks some will probably drift more than others so accuracy may depend on chance depending on which satellites are overhead.

Typically my GPS receiver is rarely out more than 10 meters and newer ones (with high gain antenna) are even better.

GPS is a very useful tool but like all tools its good to have a backup and if it tells you you’re in China when you’re pretty sure you’re in Wales then I’d consider it suspect.