GNSS Antennas and Front Ends - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 52

−80

GPS C/A
Noise floor, 2 MHz BW

−90

−100

−110

−120

−130

−140

−150

−160

−170

−180

−190

−5 −1 0 1 5

Frequency [MHz]

FIGURE 4.1. Frequency domain depiction of the GPS signal and thermal noise power.
Remember that 30 dBm = 1 dBW. Center frequency 1575 . 42 MHz.

10 − 23 J/ ◦ K, the absolute
temperature by t in ◦ K, and the equivalent noise bandwidth by B in Hz, then

Let the Boltzmann's constant be denoted by k = 1 . 38 ·

P Thermal Noise = kt B . (4.1)

10 − 23

For the GPS C/A code signal P Thermal Noise can be approximated by 1 . 38 · ×

10 6 10 − 15 290 × 2 · = 8 . 004 · or more conveniently expressed in dB as 10 ×

10 − 15 log 10 ( 97 dBm.

This is quite unique in the field of radio transmission. For example, if you
connected a traditional GPS antenna to a spectrum analyzer and searched for the
presence of the GPS signal, then any such characteristics of the signal would
be hidden as the observation would be dominated by the thermal noise. This is
a feature of the code division multiple access (CDMA) spread spectrum signal
and requires the appropriate signal processing to acquire and process the signal.
This also implies that the design of the front end is based more on the level of
the thermal noise rather than the received L1 band navigation signal. Thus, the
voltage induced within the GNSS antenna element results from the thermal noise,
which dominates, as well as the GNSS signals from the satellites in view. Given
that that L1 GNSS band is a designated Aeronautical Radio Navigation Service
frequency band, no other signals should be present within the frequency span.
The analog voltage that results from the incident GPS signal and thermal noise
remains much too weak and at too high a frequency for most analog-to-digital
converters (ADCs) to operate. In order to overcome this, the front end will utilize a
combination of amplifier(s), mixer(s), filter(s), and its own oscillator to condition
the incident voltage on the antenna to the resulting digital samples.
A fully functional GNSS L1 front end is depicted in Figure 4.2. In the coming
sections, the function of each of the elements within the figure will be discussed
using this implementation as a case study, Gromov et al. (2000), pages 447-457.

8 . 004 · ) =− 140 . 97 dBW =− 110 .