GENERIC:
between 24 and 32 SV (Space Vehicles, i.e. Satellites) at approx. 20,200km, on 6 orbital planes at 55° with a 12h period
approx. 10 visible (LOS) at any given point

RANGING CODES:
# C/A = Coarse / Acquisition code (freely available), 1023 bits of PRN, unique for each satellite, use correlators, type of CDMA, transmitted at 1.023Mbps (repeats every msec)
# P(Y) = Precision code (restricted), 6.1871×10^12 bits of PRN, transmitted at 10.23Mbps (repeats every week), P-code encrypted with W-code produces the encrypted P(Y) code (for anti-spoofing)

NAVIGATION MESSAGE:
# Navigation Message (NM) = 1500 bits transmitted at 50bps (repeats every 30 seconds), modulated on top of C/A and P(Y), divided into 5 subframes of 300 bits
– NM(a) = GPS date & time, SV’s status & health (subframe 1)
– NM(b) = ephemeris -> calculate the location of a satellite (subframes 2 & 3)
– NM(c) = almanac -> info & status of all satellites: locations and PRN numbers (subframes 4 & 5)
the full almanac = 15kbits => 12.5 minutes required to receive full almanac from a single satellite

FREQUENCIES
# L1 (1575.42 MHz = 10.23 MHz × 154): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code, plus the new L1C on future Block III satellites.
# L2 (1227.60 MHz = 10.23 MHz × 120): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.
# L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2009

C/A transmitted on L1 in BPSK
P(Y) transmitted on both L1 & L2 in BPSK, but orthogonally to C/A ( -> QPSK on L1)

Besides the redundancy and increased resistance to jamming, a critical benefit of having two frequencies transmitted from one satellite is the ability to directly measure, and therefore remove, the ionospheric delay error for that satellite.

Selective Availability (SA) was intentional pseudo-random errors to the NAV messages. It was switched-off on 01.05.2000 by orders of Clinton. DGPS can bypass the effects of SA.

PVT = Position, Velocity, and Time
SPS = Standard Positioning Service (C/A + NAV only, just L1)
PPS = Precise Positioning Service (C/A + P(Y) + NAV, L1 + L2)

References:
http://en.wikipedia.org/wiki/GPS_signals
http://www.navcen.uscg.gov/gps/geninfo/IS-GPS-200D.pdf
http://tycho.usno.navy.mil/gpsinfo.html

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