Is the GPS signal long wave or short wave?

GPS signal is a signal generated by an oscillator on a global positioning system (GPS) satellite. All GPS signals are composed of fundamental frequency f0= 10.23Mhz. The signals transmitted by GPS satellites are mainly divided into three parts: carrier, ranging code and navigation message.

A carrier wave

The two carriers used by GPS satellites are located in the L-band of microwave, which are called L 1 carrier and L2 carrier respectively.

Two related carrier signals:

l 1(t)= a 1cos(2πf 1t+φ 1)

L2(t) = A2cos(2πf2t + φ2)

φ 1 and φ2 describe the phase noise. The frequency of the carrier signal is an integer multiple of the fundamental frequency f0.

f 1 = 154 F0 = 1575.42 MHz

F2 = 120 F0 = 1227.60 MHz

\ to \ lambda _ 1 \ about 19 cm, \ lambda _ 2 \ about 24 cm.

Using L-band high-frequency carrier can accurately measure Doppler frequency shift and carrier phase, and improve speed measurement and positioning accuracy. Ionospheric delay can also be measured by using two frequencies.

Two-way code

There are two kinds of ranging codes, both of which belong to pseudo-random noise code (PRN): C/A code and P code.

C/A code is a kind of coarse code, which is used for coarse ranging and acquisition of P code, also known as acquisition code. The period Tu is 1ms, and one period contains symbols, that is, the code length nu = 210-1=1023, and the duration of each symbol is the symbol period tu =1ms1023 = 0. C/A code is an open plaintext code, which can be used by users all over the world for free. However, C/A codes are generally only modulated on L 1 carrier, so ionospheric delay cannot be accurately eliminated. The ranging accuracy is generally (2 ~ 3) meters.

P code is a ranging code, also called fine code, which can accurately measure the distance from GPS satellite to user receiver. The actual period is one week, the code length is 6.1871×1kloc-0/2 symbols, the symbol period is 0.097752 microseconds, and the corresponding symbol width is 29.3 meters. P code is modulated on carrier L 1 and carrier L2 at the same time, and the ranging accuracy is 0.3m Because of its great military value, the United States has implemented the as policy since 1994, so at present, only the military of the United States and its allies and a few users authorized by the United States government can use P code. Ordinary users can capture the C/A code first and calculate the position of the P code in the whole sequence code from the data provided by the navigation message.

Navigation information

GPS signal modulation scheme. svg

Users who only have ranging codes cannot obtain detailed information of each satellite. Therefore, the GPS system modulates the navigation message before the ranging code, and the navigation message contains the information necessary for GPS positioning, such as the spatial position of the satellite, the correction parameters of the satellite clock, the ionospheric delay correction number, etc., so the navigation message is also called data message (D code).

Navigation message is a binary code with a certain format, which is sent to users in units of "frames". Each frame message contains 1500 bits, with a transmission rate of 50 bits/second, and each main frame contains 5 subframes:

Subframe 1 contains satellite clock correction number, GPS cycle number and satellite working status information.

Subframe 2 and subframe 3 mainly provide users with information about calculating the on-orbit position of satellites, including broadcast ephemeris parameters and data offset age (AODO).

Subframe 4 and subframe 5 provide information such as satellite navigation and constellation almanac.