The Transmission of GPS Signals
The Transmission of GPS signals has some interesting characteristics that pertain to it and certain properties that it must follow for it all to work in the transmission part at least. The carrier frequency that will allow us to transport high frequencies has to obey some laws of nature in order for the signal to be transmitted and received successfully. The GPS needs to have a carrier frequency of less than 2 GHz because if the carrier frequency was 2 GHz or more than it would require than it would require the receiver to have a beam antenna, which may not be cost effective. There are time delays in the ionosphere for frequencies below 100 MHz and above 10 GHz. Another factor is that the frequency should be chosen such that the carrier frequency would not be influenced by the weather such as rain or snow. Current research has found that the only frequency range suitable for transmission and reception purposes is 1-2 GHz. Satellites send two frequencies within these bands nowadays commonly referred to as L1 and L2. L1 carries the navigation data and the SPS (Standard Positioning Code) code while L2 primarily carries the P code for transmission. L2 however requires special receivers that can handle the PPS (Precision Positioning Code) code. The GPS signal is transmitted primarily by using phase modulation, which is rarely used for other communication systems. While the carrier signals L1 and L2 are transmitted at the same time, they do not interact with each other or interfere the other signal. The L1 signal is modulated according whatever the digital signal the P-code (Precise Code) and C/A (Course Acquisition) code look like. The L2 signal is modulated according to the digital signals of the navigation message and the P-Code. Each P-code of a satellite is 6,187,100,000,000 bits ~720.213 gigabytes long, but it is only a small part of the master P-Code that is 235,000,000,000,000 bits, ~26.716 terabytes long. In order to keep threats of interference with military use of GPS, the P-Code has been modulated with a W-Code as a special type of encryption. this generates the Y-code. In order for any of the code to work, it is important to note that for the GPS to work, the code necessary must be matched with an atomic clock because the satellite is far enough from earth for there to be a delay. In order to modulate the data onto the carrier signal, GPS uses phase modulations which is a rare technique compared to AM modulation and FM modulation. When the data is to be modulated onto the carrier signal, the carrier signal abruptly shifts phase by 180 Degrees. One of the other reasons phase modulation is used is because it has a longer range. when working with satellites it is key that we use a medium like this because our distance is very great.