GPS was developed by the government of the United States, more specifically by the Department of Defense (DoD). In the very first beginning, it was designed for military purposes, but later for a civilian use also. The name GPS means Global Positioning System and as it can be inferred it provide your location and height anywhere in Earth. This is possible thanks to the 24 satellites form the network called Navstar situated in the Medium Earth Orbit (MEO) at 11,500miles, 18,500km. They are synchronized by atomic clocks (very high accuracy clocks) and every region in Earth is always covered at least by 4 of them. To access to this coverage you only need a GPS device, a passive electronic device that can receive and interpret all the data from the satellites within its LOS (Line Of Sight). The signals received in the GPS receiver differ in time, ones arrive before others (the satellites distances are different). This difference of time permits the receiver to calculate the distance between the device and the satellite. Moreover, thanks to a special kind of data called Ephemeris we can calculate the position in the 3 dimensions.
Typically GPS use two frequencies L1 = 1575,42MHz and L2=1227,60MHz which belong to the radiofrequency wavelengths. L1 is for civilian and military applications whereas, L2 is only for military purposes. Moreover, newest GPS satellites also implement L5=1176,45MHz but it is not very used yet. GPS satellites emit the signal only with 50 Watt of power, the same power used by a domestic light bulb. This is possible with CDMA (Code Division Multiple Access) which uses a form of transmission called DSSS (Direct Sequence Spread Spectrum). This means, the original signal is multiplied by a code. This operation decreases the signal power while it increases its bandwidth (spreading) creating a new signal very similar to noise. Using this method we can send multiple signals from different satellites in the same frequency with almost the certainty that the data will not be lost or modified and with a very low power rate.
In the reception we can rebuild the original signal only with the right code, any other code used will rebuild nothing. In the case of GPS there exist two different codes, Coarse/acquisition codes (C/A – codes) which are public, and Precision (P) codes, which are only used for military applications.
The C/A –code is a pseudo random code (PRN) different for each satellite. The codes are orthogonal which means that they correlate well with their satellite code not with the rest. They are deterministic sequences of 1023 bits long transmitted at 1.023Mbits/s.
The P code is also a PRN, however, it is much longer, 720 Gigabytes transmitted at 10.23 Mbits/s. Its correlation is even better than in C/A-code because of the size of its PRN. Nevertheless, it is firstly synchronized with the C/A-code and later with the P one. It is important to remember that this P code is used only for military purposes and because of that it is important to prevent unauthorized access users, this is called anti-spoofing. The anti-spoofing is done by modulating the P-code with a W-code, thus, the result is a Y-code which is what really the satellite transmits.
We already know how they codify the signal and how they send it but, which information do they send to the receivers? They send a L1 C/A Navigation Message (NAV). The NAV message is modulated in both carriers L1 and L2 at 50bps in 25 frames which last 30 seconds each, 12.5 min in total. Each frame of those 25 is divided into 5 sub-frames. The sub-frame 1 contains the GPS week, the parameters to correct the atomic clock and the satellite status and health. The sub-frames 2 and 3 contain the satellite ephemeris data. The sub-frame 4 the parameters for the ionosferic model, the UTC (universal hour) info, the *almanac and the information about whether the anti-spoofing is activated or not. Last, the sub-frame 5 contains the rest of the almanac data and the constellation status. Sub-frames 4 and 5 are always the same for every satellite.
However, there also exist other kinds of messages more modern than L1 C/A Navigation Message, they are: L2-CNAV, L5-CNAV (the only one modulated in L5 frequency), MNAV and CNAV-2. The 3 first ones have a similar structure than L1 C/A Navigation Message but they use a modernized format providing more flexibility, better control and improved content. Moreover, MNAV is used for military purposes so it has improved the security and robustness of the military message. Lastly CNAV-2 is modulated in L1 frequency so it shares the same band as the L1 C/A Navigation message.
*The almanac is simply a list of visible satellites based on position and time, it also relates the GPS time to UTC and allows single frequency to correct ionosfereic delay error by using the ionosferic model.
Ramón,
Founder of Tech For Space
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