# Pseudo Random Noise GPS

## Explain why two frequencies are used?

Pseudo random noise code: Each satellite have a unique code to identify the satellite called a
random noise code. The satellites are identified by the users through this PRN codes. These PRN codes are only pseudo random.

Transmission of signals: The satellite translate the signal into different codes. P- code: Precision code is only received by USA military and its allies. Only their GPS receiver has access and and can decode the P-code. In case of P- code, the chief frequency is 10.23 Mbits/sec. The entire sequence is a 266 days long but only 7 days piece of code is used. during this period there is no repetition of the code.

P code can be transmitted encrypted. During this anti spoofing mode the P code is encrypted into Y
code.

C/A code: It is known as course acquisition Code. All civil uses of GPS decode the CA code And it also
helps the military receivers to access more accurate P-code.

The CA code is weight of a sequence of keeps having a frequency of 1. 023 MHz. This codes are modulated by phase modulation techniques on to carrier frequencies.

L1= 1575.42 megahertz
L2= 1227.60 megahertz

The L1 signal consists of both P-code & C/A code and L2 signal consists only P-code.

## Reason for using 2 frequency:

L1= 1575.42 megahertz
L2= 1227.60 megahertz
The L1 signal consists of both P-code & C/A code and L2 signal consists only P-code.
L1 frequency is accessible by any civil user as well as the military services. But L2 frequency is only
accessible by the US military and its allies for their own security advantages. Therefore there are two
frequencies used in GPS.

Dual frequency receiver are also used for eliminating the error due to ionospheric and tropospheric
delay.