GNSS Signal Simulation - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 157

U.S. Naval Observatory (USNO). The offset between TAI and UTC is an
integer number of seconds.
Satellite Constellation The GPS baseline system is specified for 24 satellites;
however, the system currently consists of more than 24 satellites. The con-
stellation contains 6 orbital planes inclined 55 ◦ to the equator. Each plane
contains 4-5 active satellites. The satellite orbit altitude is 20,183 km from
mean surface of the Earth and the satellites have an orbital period of 11
hours 58 minutes.
The Galileo space segment will comprise 30 satellites in a Walker constel-
lation with 3 orbital planes inclined 56 ◦ to the equator. Each plane will con-
tain 9 operational satellites (for a total of 27 active satellites) equally spaced
40 ◦ apart plus 1 inactive spare. The satellite orbit altitude is 23,222 km.
This corresponds to a constellation repeat cycle of 10 days during which
each satellite has completed 17 revolutions.

B.5 Differences in Signal Processing

The following is an outline of the receiver signal processing differences.

Signal at IF The Galileo OS signals require two times wider signal bandwidth for
the main lobes, i.e., 2 046 MHz for GPS. In

practical applications it might be prudent to use at least twice this minimum
bandwidth to ensure less distortion of the ACF of the received signals. Thus,
a GPS front end with narrow bandwidth might need an update to be ideal
for Galileo reception.
If modifying a GPS front end to increase reception bandwidth, care should
be taken not to let mixer spectrum mirrors overlap the original signal spec-
trum. Thus, the IF frequency might need adjustments. This in turn means
that the ranges of possible sampling frequencies might be affected since
care should be taken to avoid aliasing occuring at f alias =

× 2 . 046 MHz, compared to 1 × 2 .

f signal .
Otherwise there should be no need for changes at the radio/IF receiver part
since both GPS and Galileo signals share the L1 carrier frequency and are
transmitted at comparable signal power levels. Front ends for Galileo sig-
nals will not need any modifications to receive GPS signals.
Differences Common to Acquisition and Tracking The main difference between
GPS and Galileo, which is common to acquisition and tracking, is the dif-
ferent spreading codes. It is hard to outline the precise differences, because
the Galileo PRN generator details are not published yet. But in any case
there will have to be two different PRN generators for GPS and Galileo.
For Galileo the output of the PRN generators must be modulated onto a
square wave to make the BOC signal. The Galileo codes are 4 times longer
than the GPS codes. Since the chipping rate is the same, this specifies a

f sampling −