Global Positioning System (GPS)

Remark:

• Modulo 2 Sum means that sums are formed according to arithmetic rules. If the result is larger than 2, only the rest is kept which cannot be divided by 2. The four possibilities are 0+0=0; 0+1=1; 1+0=1; 1+1=0.

• Navigation / system data is data that contain almanac data and ephemeris data.

Firstly, the navigation signal (50 Hz) will either modulate with C/A code or P code (both are the pseudo random code) depending on the type of application.

The navigation signal modulated with C/A code will mix with L1 carrier producing a L1 signal. Whereas the navigation signal modulated with P code will mix with both the L1 and L2 carrier produce a L2 signal.

These signals contain data like satellite orbits, clock corrections and other system parameters (information about the status of the satellites such as almanac data and ephemeris data).

The complete data signal consists of 37500 bit and at a transmission rate of 50 bit/s. A total of 12.5 minutes is necessary to receive the complete signal. This is the require time for a GPS receiver to determine the first possible position.

The data signal is divided into 25 frames, each having a length of 1500 bit (meaning an interval of 30 seconds for transmission). The 25 frames are divided into subframes (300 bit, 6 sec.), which divided again into 10 words each (30 bit, 0.6 sec).

The first word of each subframe is the TLM (telemetry word). It contains information about the age of the ephemeris data. The second word is HOW (hand over word), which contains the number of counted z-epochs. These two data contain the time since last “restart” of the GPS time (on the previous Sunday 0:00 o’clock). HOW is suitable used for P-code (as it is seven days long).

Other than that, the rest of the subframes are data about status and accuracy of the transmitting satellite as well as clock correction data. Subframe 2 and 3 contain ephemeris data while subframe 4 and 5 contain almanac data. In detail, subframe 4 contains data for the satellites number 25 to 32, ionospheric correction data, special information and Coordinated Universal Time (UTC) information; subframe 5 contains almanac data for the satellites 1 to 24 as well as time and the number of the GPS week.

Apart from that, there are subframe design for the clock correction data. Why do we need data correction for the clock since atomic clock in GPS is very accurate? Although each satellite carries several atomic clocks that have very accurate time, but the atomic clocks of those individual satellites are not synchronized to the GPS reference time, in other word, they run on their own. Therefore correction data for the clocks of each satellite are necessary. Moreover, the GPS reference time is different from UTC (synchronized with the rotation of the earth by means of leap seconds).

The first three subframes are identical for all 25 frames. Every 30 seconds the most important data for the position determination are transmitted with these three subframes. From the almanac data, the GPS receiver identifies the satellites that are likely to be received from the actual position. Therefore, it help accelerated the position determination as the receiver has limit its search for the satellites in view.

In case if a satellite transmits its data wrongly or its orbit is unstable, it will be marked as unhealthy by the control station. This information is transmitted in the satellite’s signal too. Hence, receivers will not take the data from this satellite for the position determination.

When ephemeris and almanac data are stored in the GPS receiver, the time for GPS first position determination is depend on the condition of the receiver itself. If the receiver has lost contact to the satellites for a long period, the first position determination will take longer time. On the other hand, if the contact has only been interrupted for a short period (e.g. when driving through a tunnel), the position determination is restarted instantly. This is often called reacquisition.