Estimation of Satellite-User Ranges Through GNSS Code Phase Measurements - Global Navigation Satellite Systems: Signal,Theory and Applications - page 115

Fig. 5. Structure of the navigation message included in the GPS civil signal, transmitted on
the L1 frequency
The rate of the navigation data bits is 50 bits per second. The whole message is 12.5 minutes
long and is divided in 25 frames. Each frame lasts 30 seconds and is further divided in 5
subframes, six seconds long. Each subframe of the navigation message always starts with
two special words, the Telemetry (TLM) and the Handover word (HOW).
In case of the Galileo E1 signal, the complete navigation message is transmitted on the data
channel (E1-B) as a sequence of frames. A frame is composed of several sub-frames, and a
sub-frame, in turn, is composed of several pages. The page is the basic structure to build the
navigation message. Fig. 6 shows the structure of the Galileo data and an example of page
for the E1-B message.
Prior to the navigation data decoding, the receives seeks for the preamble, a defined
sequence of n bits, that marks the beginning of a subframe for the GPS L1 C/A, a page for
the Galileo E1-B. A simple, but efficient, way to detect the preamble is to correlate the
navigation data stream with a local binary sequence equal to the preamble. A maximum is
detected when such a local sequence is aligned with the preamble. Naturally, the bit pattern
used for the preamble can occur anywhere in the received data stream, thus an additional
check must be carried out to authenticate the real preamble (e.g. in case of GPS, only when
the maximum of correlation is found exactly every 6 seconds). When the beginning of the
subframe is identified, the content of the subframe can be decoded. The receiver retrieves all
the orbital parameters (i.e. ephemeris) necessary to compute the satellite position