GPS Signal RequirementsMethod (code) to identify each satelliteThe location of the satellite or some information on how to determine itInformation regarding the amount of time elapsed since the signal left the satelliteDetails on the satellite clock status

Complicated SignalMULTI-USER SYSTEMOne-way measurements --a listen-only system REAL-TIME POSITIONINGSimultaneous measurements to many satellites --need to identify different signalsUnambiguous range measurements --need to determine signal delaySatellite positions needed --broadcast ephemerides HIGH ACCURACY POSITIONINGHigh frequency modulation --P code at 10 MHzDual-frequency --for ionospheric delayMicrowave carrier frequency --1.2 to 1.6 GHz ANTI-JAMMING REQUIREMENTSpread spectrum technique MILITARY AND CIVILIAN USERSNeed two codes and restriction on dual-frequency use --P and C/A codes

Overview of Satellite TransmissionsAll transmissions derive from a fundamental frequency of 10.23 MhzL1 = 154 10.23 = 1575.42 MhzL2 = 120 10.23 = 1227.60 MhzAll codes initialized once per GPS week at midnight from Saturday to SundayChipping rate for C/A is 1.023 MhzChipping rate for P(Y) is 10.23 Mhz

Code SignalTwo ranging codes: Coarse/Acquisition(C/A) code, which is freely available to the public, and the restrictedPrecision(P) code, usually reserved for military applications.

Coarse/Acquisition codeis a 1,023bitdeterministic sequence calledpseudorandom noise(alsopseudorandom binary sequence) (PN or PRN code) which, when transmitted at 1.023megabits per second(Mbit/s), repeats everymillisecond. Each satellite transmits a unique PRN code, which does not correlate well with any other satellite's PRN code. This is a form ofcode division multiple access(CDMA), which allows the receiver to recognize multiple satellites on the same frequency.

Code Signal contd..Precision codeThe P-code is also a PRN; however, each satellite's P-code PRN code is 6.1871 1012bits long (~720.213 gigabytes) and only repeats once a week (it is transmitted at 10.23 Mbit/s). The extreme length of the P-code increases its correlation gain and eliminates any range ambiguity within theSolar System. However, the code is so long and complex it was believed that a receiver could not directly acquire and synchronize with this signal alone. It was expected that the receiver would first lock onto the relatively simple C/A code and then, after obtaining the current time and approximate position, synchronize with the P-code.

To prevent unauthorized users from using or potentially interfering with the military signal through a process calledspoofing, it was decided to encrypt the P-code. To that end the P-code was modulated with theW-code, a special encryption sequence, to generate theY-code. The Y-code is what the satellites have been transmitting since theanti-spoofing modulewas set to the "on" state. The encrypted signal is referred to as theP(Y)-code.The navigation message is made up of three major components. The first part contains the GPS date and time, plus the satellite's status and an indication of its health. The second part contains orbital information calledephemerisdata and allows the receiver to calculate the position of the satellite. The third part, called the almanac, contains information and status concerning all the satellites; their locations and PRN numbers.

Navigation Message In order for a GPS navigator to derive real-time position (and to make the task of the GPS surveyor easier when he comes to reduce his data), a Navigation Message is transmitted on both L-band frequencies, containing the following information predictedsatellite ephemerides. predictedsatellite clock correction modelcoefficients. GPSsystem statusinformation. the GPS systemionospheric model.

Schematic of GPS codes and carrier phase

GPS Signal Characteristics

Digital Modulation MethodsAmplitude Modulation (AM) also known as amplitude-shift keying. This method requires changing the amplitude of the carrier phase between 0 and 1 to encode the digital signal.Frequency Modulation (FM) also known as frequency-shift keying. Must alter the frequency of the carrier to correspond to 0 or 1.Phase Modulation (PM) also known as phase-shift keying. At each phase shift, the bit is flipped from 0 to 1 or vice versa. This is the method used in GPS.

Modulation Schematics

Modulo-2 recovery of GPS codeModulo-2 arithmetic: 0 + 0 = 0; 0 + 1 = 1; 1 + 0 = 1; 1 + 1 = 0Bit shifts alignedMUST MOD-2 ADD RECEIVER-GENERATED CODE TO RECOVER

Schematic of C/A-code acquisitionSince C/A-code is 1023 chips long and repeats every 1/1000 s, it is inherently ambiguous by 1 msec or ~300 km. Must modulo-2 add the transmitted and received codes after correlation to increase SNR and narrow bandwidth.

Methods to Cope with Anti-spoofingAnti-spoofing: Implemented in 1994 to make P-code unavailable to non-military users. Encrypted P-code is referred to as Y-code.Squaring: Yields half-wavelength carrier and greatly reduces SNR. Old technology.

Code-aided squaring: Uses mathematical similarity of the Y-code to P-code. L1 carrier is down-converted and multiplied with a local replica of the P-code, then squared. Results in less reduction of SNR than simple squaring.

Anti-spoofing Methods, cont.Cross-correlation: Takes advantage of the fact that both L1 and L2 are modulated with the same P(Y)-code, despite lack of knowledge of the actual P-code. Yields the difference in pseudoranges, P1(Y) - P2(Y), and the phase difference of L1 and L2. Again less SNR loss compared with squaring. Can be difficult to track at low elevation angles. Technique employed in Trimble 4000SSi/SSE.

Z-tracking: Takes advantage of the fact that Y-code is the modulo-2 sum of the P-code with a lower encryption rate. Yields L1 and L2 Y-code pseudoranges and the full carrier phases of L1 & L2. This method yields the best SNR. Multipath performance is better than other methods. Technique employed in Ashtech Z-12 and micro-Z.

AS Technologies Summary TableTrimble 4000SSiAshtech Z-12 & ZFrom Ashjaee & Lorenz, 1992

BandFrequency (MHz)PhaseOriginal usageModernized usageL11575.42 (10.23154)In-phase (I)Encrypted Precision P(Y) codeQuadrature- phase (Q)Coarse-acquisition (C/A) codeC/A, L1 Civilian (L1C), and Military (M) codeL21227.60 (10.23120)In-phase (I)Encrypted Precision P(Y) codeQuadrature- phase (Q)Unmodulated carrierL2 Civilian (L2C) code and Military (M) codeL31381.05 (10.23135)Used by Nuclear Detonation (NUDET) Detection System Payload (NDS); signals nuclear detonations/high-energy infrared events. Used to enforce nuclear test ban treaties.L41379.913 (10.231214/9)(No transmission)Being studied for additional ionospheric correctionL51176.45 (10.23115)In-phase (I)(No transmission)Safety-of-Life (SoL) Data signalQuadrature- phase (Q)Safety-of-Life (SoL) Pilot signal

L2COne of the first announcements was the addition of a new civilian-use signal, to be transmitted on a frequency other than the L1 frequency used for the coarse/acquisition (C/A) signal. Ultimately, this became the L2C signal, so called because it is broadcast on the L2 frequency. It is only transmitted by the so-called Block IIR-M and later design satellites. Used for improving accuracy of navigation, providing an easy to track signal, and acting as a redundant signal in case of localized interference.Military (M-code)A major component of the modernization process is a new military signal. Called the Military code, or M-code, it was designed to further improve the anti-jamming and secure access of the military GPS signals. It contains a PRN code of unknown length transmitted at 5.115MHz. Unlike the P(Y)-code, the M-code is designed to be autonomous, meaning that a user can calculate their position using only the M-code signal

L5, Safety of LifeA civiliansafety of lifesignal (broadcast in a frequency band protected by theITUforaeronautical radionavigation service), first broadcast for demonstration purposes on satelliteUSA-203(a Block IIR-M series satellite), and available on allGPS IIFsatellites (and beyond).Two PRN ranging codes are transmitted on L5: the in-phase code (denoted as the I5-code); and the quadrature-phase code (denoted as the Q5-code). Both codes are 10,230 bits long and transmitted at 10.23MHz (1ms repetition).

Higher transmitted power than L1/L2 signal (~3dB, or 2 as powerful)Wider bandwidth provides a 10 processing gainLonger spreading codes (10 longer than C/A)Uses the Aeronautical Radionavigation Services band