This is really strange… Apparently, GPS is going to be either inaccurate or down in Florida because of some DoD ‘tests.’
Interesting information about GPS (mainly technical information):
- Some common misconceptions about GPS that people seem to have (the stuff that I have heard amazes me):
- GPS signals come from satellites… that are in the sky… in space. Yes, that’s right folks, spaaace. (This undoubtedly makes it on the list of the dumbest things that I have had the ‘pleasure’ of enlightening people about…)
- GPS itself is a one-way system. Your GPS receiver does not talk to the satellites. The satellites are constantly blabbering, and your GPS receiver or phone just listens in and does some math to find out how close they are. The satellites don’t know where you are – there is no way for the satellites to track you. You’re the one tracking the satellites. (However, with some forms of AGPS, or assisted GPS, a GPS receiver’s data is combined with data that may have been obtained through such sources as WiFi or cellular networks, which are two-way systems. Therefore, it might be possible to track devices using AGPS given enough resources.)
- If you cannot see the sky, or there are large microwave-reflecting objects (i.e metal, concrete) in your vicinity, GPS accuracy will be reduced because the signal will be reflected, absorbed and distorted. This means you cannot usually use GPS underground, and especially that you should not rely solely on GPS to navigate structures below ground…
- GPS signals are transmitted using BPSK from the satellite constellation at approximately 1.2 GHz, which is in the 23-cm microwave band. There are two frequencies used for the main signals, the L1 carrier at 1575.42 MHz, and the L2 carrier at 1227.60 MHz. Basically, all of the satellites transmit on the same frequencies simultaneously, but each one uses a different spread-spectrum code, which forms a CDMA system that GPS receivers can ‘pick through’ to identify and trilaterate each individual satellite.
- There are multiple codes that are multiplexed and modulated on the GPS carriers using several techniques. Some are for military use only and are encrypted, for both anti-spoofing and tactical advantage purposes. The chip rate of the military GPS code (the P(Y) code) is 10.23 Mbps, which is 10 times faster than the civilian GPS code (the C/A code,) which is 1.023 Mbps. The pseudorandom PRN code for the P code repeats every week and is approximately 700 gigabytes long, while the PRN code for the C/A code repeats every millisecond and is much shorter. The PRN codes are different for each satellite. However, the P code PRN is a small chunk assigned from a master code that is approximately 67 terabytes long. The sequences are obviously not stored on the satellites – a pseudorandom number generator is used to generate the bits when they are needed.
- GPS signals are extremely weak by the time they reach Earth’s surface – usually around -125 dBm. This is below the thermal noise floor for a GPS channel. Thermal noise is the ‘hissing’ created by the electrons spazzing randomly as they move around, which is usually insignificant to the design of many electronic devices. The GPS signal can still be recovered accurately due to the large bandwidth and the receiver’s knowledge of the pseudorandom sequence.
- GPS receiver manufacturers have found methods to increase the accuracy of GPS using land-based WAAS, different types of AGPS, and the other codes that are transmitted from the satellites.
Thoughts and theories:
- It seems rather strange for GPS reception to be unavailable in a specific area, since there are so many satellites.
- This GPS outage could be intentional or unintentional (as some kind of side effect or natural occurrence)
- If interference is the cause of the outage, a massive amount of interference would have to be transmitted since GPS is designed to work below the noise floor. However, if the interference was near the satellites, it would make it more difficult to receive the signal using a phased array or directional antenna.
- It is also possible that GPS satellites are being disabled for the length of time that they are above Florida.
- Maybe a signal is being transmitted either from space or on the Earth’s surface?
- If a space weapon or EMP is being tested, the GPS satellites may go down or have to be turned off for the duration of the test.
- A predicted collision between a GPS satellite and another object could be covered up by saying the satellite will go down for a test.
- Maybe a satellite’s security is being compromised?
- If all characteristics of future GPS signals are known, I think it might be possible to jam GPS signals more effectively by modulating the jamming signal with portions of the known signal.
It would be worth monitoring the measured accuracy of GPS signals in a stationary location using a large antenna. People should work together and look for places with unusually inaccurate GPS (i.e. not explainable by interference, multipath, bad receiver design, etc.)
January 22nd, 2011 | 
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FAA warns of ongoing GPS issues in southeastern US due to Defense Department 'tests' http://t.co/oO5toGp