GPS Signals - Frequencies Used By GPS

Frequencies Used By GPS

GPS Frequencies
Band Frequency
(MHz)		 Phase Original Usage Modernized Usage
L1 1575.42
(10.23×154)		 In-Phase (I) Encrypted Precision P(Y) code
Quadrature-
Phase (Q)		 Coarse-acquisition (C/A) code C/A, L1 Civilian (L1C), and
Military (M) code
L2 1227.60
(10.23×120)		 In-Phase (I) Encrypted Precision P(Y) code
Quadrature-
Phase (Q)		 Unmodulated carrier L2 Civilian (L2C) code and
Military (M) code
L3 1381.05
(10.23×135)		 Used by Nuclear Detonation (NUDET)
Detection System Payload (NDS);
signals nuclear detonations/
high-energy infrared events.
Used to enforce nuclear test
ban treaties.
L4 1379.913
(10.23×1214/9)		 (No transmission) Being studied for additional
ionospheric correction
L5 1176.45
(10.23×115)		 In-Phase (I) (No transmission) Safety-of-Life (SoL) Data signal
Quadrature-
Phase (Q)		 Safety-of-Life (SoL) Pilot signal

All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The satellite network uses a CDMA spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver must be aware of the PRN codes for each satellite to reconstruct the actual message data. The C/A code, for civilian use, transmits data at 1.023 million chips per second, whereas the P code, for U.S. military use, transmits at 10.23 million chips per second. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code. The P code can be encrypted as a so-called P(Y) code which is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.

Each composite signal (in-phase and quadrature phase) becomes:

S(t) = \sqrt{P_\text{I}} X_\text{I} (t) \cos (\omega t + \phi_0) \underbrace{{} - \sqrt{P_\text{Q}} X_\text{Q} (t) \sin (\omega t + \phi_0)}_{+ \sqrt{P_\text{Q}} X_\text{Q} (t) \cos\left(\omega t + \phi_0 + \frac{\pi}{2}\right)} ,

where and represent signal powers; and represent codes with/without data .

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