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the satellites is stored or the information is outdated.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).
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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.
