GPS Signals & Positioning PDF

GPS satellites transmit three signals: 1. The L1 signal is the oldest GPS signal. The GPS L1 band (1575.42 MHz)frequency with Wave lengths L1~190 mm. Since the L1 is the oldest and most established signal, even the cheapest GPS units are capable of receiving it. However, because its frequen...

GPS satellites transmit three signals: 1. The L1 signal is the oldest GPS signal. The GPS L1 band (1575.42 MHz)frequency with Wave lengths L1~190 mm. Since the L1 is the oldest and most established signal, even the cheapest GPS units are capable of receiving it. However, because its frequency is relatively slow it is not very effective at traveling through obstacles. 2. The L2 frequency was implemented after the L1.The L2 band (1227.60 MHz) frequency with wave length L2~244 mm. This allows the signal to better travel through obstacles such as cloud cover, trees, and buildings. 3. L5 is the third GPS signal band with (1176.45 MHz) It is the most advanced GNSS signal yet. 18 Code types of GPS ⦁ Each GPS satellite transmits unique ranging code signals on two frequencies: 1575.42 MHz (L1) and 1227.60 MHz (L2). ⦁ The Coarse Acquisition (C/A) code is transmitted on L1 and can be received by any type of GPS receiver. C/A code, for civilian use, transmits data at 1.023 million chips per second. ⦁ The Precision (P-code) code is transmitted on L1 and L2. P-code is encrypted and available only to users with appropriate decryption equipment provided by the USA Department of Defence. the P code, for U.S. military use, transmits at 10.23 million chips per second. ⦁ Both codes are synchronized to the satellite’s atomic clocks. 19 GPS Navigation Data Message ⦁ Satellites transmit a navigation message that contain: 1-Ephemeris Data ⦁ contains information on week number, satellite accuracy and health, age of data, satellite clock correction coefficients, orbital parameters ⦁ Used for real time satellite coordinate computation which is required in position computation 20 2-Almanac Data ⦁ The satellites also transmit almanac data, which contains an indicator of the health of all the satellites, satellite clock corrections and coarse orbital data, atmospheric delay parameters, and the current GPS time and offset from UTC time. 21 Computation of Position: ⦁ GPS receiver calculates its distance from a satellite by measuring how long a signal from the satellite takes to reach it. It is implied that the receiver is located somewhere on the surface of an imaginary sphere centered at the satellite (about 20000km) ⦁ The distance to the other satellite will also be calculated by the receiver. Similarly a sphere centered at B (satellite2) with a radius R2 can be imagined on whose surface lies the receiver. Since the receiver is R1 distance from A (satellite1) and R2 distance from B (satellite2), it is clear that the receiver will be on either of the points of intersection of the two spheres 22 ⦁ The distance calculated from the third satellite will add one more sphere to be imagined on whose surface lies the receiver. This gives rise to only one valid intersection i.e. the point where the three spheres intersect is the position of the receiver in a two dimensional space. ⦁ A GPS receiver determines its position by using the signals that it receives from different satellites. Since the receiver must solve for its position (X,Y,Z) and the clock error (d). 23