Characteristics of c/A and P Code I C/A-code Generation(p. 75) Parameter/A-code P-code 2. P-code Generation(p. 75) Chipping rate 1.023.10 Bits/s 10 23.10 bits per second 30 ode type 37 unique codes 37 one-week segment More accurate ince the y-code is the modulo two sum of the p-code and the encrypting W-code, access to P-code is only possible when the secret conversion algorithm is known Navigation Message Each subframe The navigation message contains Starts with the telemetry word (TLM)containing a message containing bits with five 2. The second word is hand-over word (HOW) Various correction data subframe identification Time-of-week (Tow), which count for the epoch at the start (leading edge)of the next subframe subframe 2 The Tow count (sometimes called Z-count) is a multiple number of 1. 5-second intervals since the beginning of the One subframe is transmitted in 6 seconds and contains 10 words with 30 bits (a word/0. 6 seconds) A receiver requires at least 30 seconds to lock on a satellite in order to receive the complete navigation message F iThe gps a ame contains week number. The Fifth Subframe has A prediction of the user rang Almanac data Indicators of the satellite health and The age of the dat Health status for the first 24 satellites in orbit Three coefficients for a qu polynomial to model The Fourth Fifth Subframe commonly the satellite clock correction rd Subframe transmit the broadcast Are broadcast by each satellite. Therefore, by tracking phemerides of the satellite only one satellite, the almanac data of all the other satellites in orbit are obtained The Fourth Subframe are reserved for military use and contain The contents are changed in every message and have a Various flags The total information is packed into 25 pages and requires Almanac data(i.e 12.5 minutes for transmission satellites beyond the nominal 24 constellation3 1. C/A-code Generation (p. 75) 2. P-code Generation (p.75) Characteristics of C/A and P Code • In order to protect the P-code against spoofing, the P-code is encrypted to the Y-cod by A-S. • Since the Y-code is the modulo two sum of the P-code and the encrypting W-code, access to P-code is only possible when the secret conversion algorithm is known. Properties Easy to acquire More accurate Code type 37 unique codes 37 one-week segments Repetition rate Millisecond One week Chip length ~ 300 m ~ 30 m 10.23•106 1.023•10 bits per second Chipping rate 6 Bits/s Parameter C/ A-code P-code Navigation Message The navigation message contains • Satellite health status, • Satellite clock, • Satellite orbit, and • Various correction data. Total message containing 1500 bits with five subframes. 1. Starts with the telemetry word (TLM) containing a synchronization pattern and some diagnostic messages. 2. The second word is hand-over word (HOW). • A subframe identification • Some flags, • Time-of-week (TOW), which count for the epoch at the start (leading edge) of the next subframe. Each subframe • One subframe is transmitted in 6 seconds and contains 10 words with 30 bits (a word/0.6 seconds). • A receiver requires at least 30 seconds to lock on a satellite in order to receive the complete navigation message. The TOW count (sometimes called Z-count) is a multiple number of 1.5-second intervals since the beginning of the current GPS week. The First Subframe contains • The GPS week number, • A prediction of the user range accuracy, • Indicators of the satellite health and • The age of the data, • An estimation of the signal group delay, and • Three coefficients for a quadratic polynomial to model the satellite clock correction. The Second & Third Subframe transmit the broadcast ephemerides of the satellite The Fourth Subframe are reserved for military use and contain • Ionosphere • UTC data • Various flags • Almanac data (i.e., low-accuracy orbital data) for satellites beyond the nominal 24 constellation. The Fifth Subframe has – Almanac data – Health status for the first 24 satellites in orbit. • Are broadcast by each satellite. Therefore, by tracking only one satellite, the almanac data of all the other satellites in orbit are obtained. • The contents are changed in every message and have a repetition rate of 25. • The total information is packed into 25 pages and requires 12.5 minutes for transmission. The Fourth + Fifth Subframe commonly