Composition of the Data Signal

In addition to the C/A code navigational information is modulated into the L1 signal. The information consists of a 50 Hz signal and contains data like satellite orbits, clock corrections and other system parameters (information about the status of the satellites). These data are constantly transmitted by each satellite. From these data receiver gets it’s date, the approximate time and the position of the satellites.
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 time is required by a GPS receiver until the first determination of a position is possible, if no information about the satellites is stored or the information is outdated.
The data signal is divided into 25 frames, each having a length of 1500 bit (meaning an interval of 30 seconds for transmission).

Structure of the GPS data of one “frame”

The 25 frames are divided into subframes (300 bit, 6 sec.), which are again divided 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 next word is the HOW (hand over word), which contains the number of counted z-epoches. These data contain the time since last “restart” of the GPS time on the previous Sunday 0:00 o’clock. As the P-code is 7 days long, the HOW is used by military receivers to locate their access to the P-code.
The rest of the first subframe contains data about status and accuracy of the transmitting satellite as well as clock correction data. The second and third subframes contain ephemeris parameters. Subframes 4 and 5 contain the so-called almanac data which include information about orbit parameters of all satellites, their technical status and actual configuration, identification number and so on. Subframe 4 contains data for the satellites number 25 – 32, ionospheric correction data, special information and UTC time information; subframe 5 contains almanac data for the satellites 1 – 24 as well as time and the number of the GPS week.
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. The receiver limits its search to these previously defined satellites and hence this accelerates the position determination.
As mentioned earlier, the data signal contains correction parameter for the satellite clocks. Why is this necessary, if the atomic clocks are absolutely precise?
Each satellite carries several atomic clocks and has a very accurate time. However the atomic clocks of the individual satellites are not synchronized to the GPS reference time, but run on their own. Therefore correction data for the clocks of each satellite are required. Furthermore, the GPS reference time is different from UTC time (world time) which is synchronized with the rotation of the earth by means of leap seconds.
If a satellite does not transmit its data correctly or its orbit is unstable, it can be marked as inhealthy by the control station. This information is transmitted by the satellite in its signal. Receivers then do not take the data from this satellite into account for the position determination. At least if their firmware is properly programmed.
A typical reason why satellites are marked as defective is the necessity of an orbit correction. In this case the thrusters of the satellite are ignited and the defective marking is removed as soon as the satellite has stabilized in its new orbit.When ephemeris and almanac data are stored in the GPS receiver, it depends on their actuality how long the GPS needs for the first position determination. If the receiver has not had any contact to the satellites for long time, the first position determination will take longer. If the contact has only been interrupted for a short time (e.g. when driving through a tunnel), the position determination is restarted instantly and we speak of reacquisition.
If position and time are known and the almanac and ephemeris data are up-to-date, we speak of a hot start. This is the case when the receiver is turned on at approximately the same position within 2 – 6 hours after the last position determination. In this case a position fix can be obtained within approximately 15 seconds.
If the almanac data are available and the time of the receiver is correct but the ephemeris data are outdated, this is called a warm start. In this case it takes about 45 seconds to actualize the ephemeris data and obtain a position fix. Ephemeris data are outdated when more than 2 – 6 hours have elapsed since the last data reception from the satellites in view. The more new satellites have come into view since the last position determination, the longer the warm start takes.
If neither ephemeris nor almanac data and the last position are known, we talk of a cold start. Then in the first step all almanac data have to be collected from the satellites, this procedure takes up to 12.5 minutes. This happens when the receiver was switched off for several weeks, was stored without batteries or has travelled approximately 300 km or more since the last position fix.
In the last case no almanac data have to be collected, but as the “wrong” satellites are in view, the receiver has to screen all satellites till it finds the ones in view. For a lot of receivers the duration of a cold start can be shortened when the date and approximate position are entered manually. If you want even more detailed information, please have a look here.