Receiver Hardware Considerations - Fundamentals of Global Positioning System Receivers: A Software Approach - page 106

real data will be discussed in detail as examples. The impact of the number of
digitized bits will also be discussed.
A digital band folding technique will be discussed that can alias two or more
narrow frequency bands into the baseband. This technique can be used to alias the
L1 and L2 bands of the GPS into the baseband, or to alias the GPS L1 frequency
and the Russian Global Navigation Satellite System (GLONASS) signals into
the baseband. If one desires, all three bands, L1, L2, and the GLONASS, can be
aliased into the baseband. With this arrangement the digitized signal will contain
the information from all three input bands.
One of the advantages of a software receiver is that the receiver can process
data collected with various hardware. For example, the data can be real or com-
plex with various sampling frequencies. A simple program modification in the
receiver should be able to use the data. Or the data can be changed from real to
complex and complex to real such that the receiver can process them.
6.2 ANTENNA ( 2-4 )
A GPS antenna should cover a wide spatial angle to receive the maximum number
of signals. The common requirement is to receive signals from all satellites
about 5 degrees above the horizon. Combining satellites at low elevation angles
and high elevation angles can produce a low value of geometric dilution of
precision (GDOP) as discussed in Section 2.15. A jamming or interfering signal
usually comes from a low elevation angle. In order to minimize the interference,
sometimes an antenna will have a relatively narrow spatial angle to avoid signals
from a low elevation angle. Therefore, in selecting a GPS antenna a trade-off
between the maximum number of receiving satellites and interference must be
carefully evaluated.
If an antenna has small gain variation from zenith to azimuth, the strength of
the received signals will not separate far apart. In a code division multiple access
(CDMA) system it is desirable to have comparable signal strength from all the
received signals. Otherwise, the strong signals may interfere with the weak ones
and make them difficult to detect. Therefore, the antenna should have uniform
gain over a very wide spatial angle.
If an antenna is used to receive both the L1 (1575.42 MHz) and the L2
(1227.6 MHz), the antenna can either have a wide bandwidth to cover the entire
frequency range or have two narrow bands covering the desired frequency ranges.
An antenna with two narrow bands can avoid interference from the signals in
between the two bands.
The antenna should also reject or minimize multipath effect. Multipath effect
is the GPS signal reflections from some objects that reach the antenna indirectly.
Multipath can cause error in the user position calculation. The reflection of a
right-handed circular polarized signal is a left-handed polarized signal. A right-
handed polarized receiving antenna has higher gain for the signals from the
satellites. It has a lower gain for the reflected signals because the polarization