Global Positioning System Reference
In-Depth Information
The category with aided information is that a separate station receives the
GPS signals of normal strength, decodes them, and transmits the information,
such as carrier frequency and navigation data, to the users in the neighborhood.
With this information the user can acquire very weak signals. The acquisition
method used in this approach can be very close to the tracking method, and it is
will be discussed briefly in Section 12.13.
In order to achieve high sensitivity of a receiver, the dynamic of the receiver
must be low, which means the receiver cannot have high acceleration. The
receiver is either stationary or moving at relatively low constant velocity. Under
this condition most of the Doppler frequency change is caused by the motion
of the satellites. From Section 3.8 we know that a maximum Doppler frequency
shift of about 1 Hz/sec is caused by the satellite motion.
The actual procedure of acquiring a weak signal contains the following
five steps:
1. Perform coherent integration.
2. Perform noncoherent integration.
3. From the noise distribution, set a threshold to detect a signal. The detected
signal contains the initial phase of the C/A code and the coarse car-
rier frequency.
4. Find
fine
carrier
frequency
for
determining
an
accurate
navigation
data transition.
5. Find the location of the navigation data transition.
Before the presentation of the actual acquisition method the sensitivity of a
receiver will be discussed.
10.2 SIGNAL-TO-NOISE RATIO (S/N) ( 1-3 )
In order to determine the S / N , the strength of the input signal and noise floor
must be known. The GPS signal strength is at a nominal level of
130 dBm, as
discussed in Section 5.2. This noise floor is discussed in Section 6.3, and the noise
figure of the receiver is discussed in Section 6.5. Let us arbitrarily assume that
the antenna temperature is 180 K, and the corresponding noise floor is at about
176 dBm/Hz. Let us also assume that the receiver front end has a noise figure
of 2 dB; thus the overall noise floor is at
174 dBm/Hz or
114 dBm/MHz.
For simplicity, the discussion is based on this noise floor.
For a GPS receiver there is no unique way to present the S / N because a unique
bandwidth is not universally acceptable. Sometimes the signal power level is used
for sensitivity; other times S / N is used. Here four terms will be introduced: the
input power level, the input S / N referenced to the input bandwidth, the S / N
referenced to 1 kHz and the S / N referenced to 1 Hz, which is often expressed
as C/N 0 .
Search WWH ::




Custom Search