Receiver Hardware Considerations - Fundamentals of Global Positioning System Receivers: A Software Approach - page 107

is in the opposite direction. In general it is difficult to suppress the multipath
because it can come from any direction. If the direction of the reflected signal
is known, the antenna can be designed to suppress it. One common multipath is
the reflection from the ground below the antenna. This multipath can be reduced
because the direction of the incoming signal is known. Therefore, a GPS antenna
should have a low back lobe. Some techniques such as a specially designed
ground plane can be used to minimize the multipath from the ground below.
The multipath requirement usually complicates the antenna design and increases
its size.
Since the GPS receivers are getting smaller as a result of the advance of inte-
grated circuit technology, it is desirable to have a small antenna. If an antenna
is used for airborne applications, its profile is very important because it will be
installed on the surface of an aircraft. One common antenna design to receive a
circular polarized signal is a spiral antenna, which inherently has a wide band-
width. Another type of popular design is a microstrip antenna, sometimes also
referred to as the patch antenna. If the shape is properly designed and the feed
point properly selected, a patch antenna can produce a circular polarized wave.
The advantage of the patch antenna is its simplicity and small size.
In some commercial GPS receivers the antenna is an integral part of the
receiver unit. Other antennas are integrated with an amplifier. These antennas
can be connected to the receiver through a long cable because the amplifier gain
can compensate the cable loss. A patch antenna (M/A COM ANP-C-114-5) with
an integrated amplifier is used in the data collection system discussed in this
chapter. The internal amplifier has a gain of 26 dB with a noise figure of 2.5 dB.
The overall size of the antenna including the amplifier is diameter of 3 and
thickness about 0.75 . The antenna pattern is measured in an anechoic chamber
and the result is shown in Figure 6.1a. Figure 6.1b shows the frequency response
of the antenna. The beam of this antenna is rather broad. The gain in the zenith
direction is about + 3.5 dBic where ic stands for isotropic circular polarization.
The gain at 10 degrees is about − 3dBic.

6.3 AMPLIFICATION CONSIDERATION ( 5-7,10 )

In this section the signal level and the required amplification will be discussed.
The C/A code signal level at the receiver set should be at least

130 dBm ( 5 ) as
discussed in Section 5.2. The available thermal noise power N i at the input of a
receiver is: ( 6 )

−

N i = kT B watts ( 6 . 1 )

where k is the Boltzmann's constant ( = 1 . 38 × 10 − 23 J / ◦ K) T is the temperature
of resistor R ( R is not included in the above equation) in Kelvin, B is the
bandwidth of the receiver in hertz, N i is the noise power in watts. The thermal
noise at room temperature where T

= 290 ◦ K expressed in dBm is

N i ( dBm ) =− 174 dBm / Hz or N i ( dBm ) =− 114 dBm / MHz ( 6 . 2 )