Global Positioning System Reference
In-Depth Information
where
x
0
,
y
0
,and
z
0
are the user position in ECEF system;
a
e
and
e
e
are the
earth semimajor axis and eccentricity as discussed in Section 2.10. The satellite
position referenced to the user position can be found as
e
0
=−
sin
l(x
−
x
0
)
+
cos
l(y
−
y
0
)
n
0
=−
sin
L
c
cos
l(x
−
x
0
)
−
sin
L
c
sin
l(y
−
y
0
)
+
cos
L
c
(z
−
z
0
)
(12.26)
u
0
=
cos
L
c
cos
l(x
−
x
0
)
+
cos
L
c
sin
l(y
−
y
0
)
+
sin
L
c
(z
−
z
0
)
where
e
0
,
n
0
,and
u
0
are the east, north, and up directions of the satellite with
respect to the user position. One can select all the satellites above a certain
elevation angle, such as 5
◦
, as the observable satellites in the sky.
The equations above provide the position of a satellite. If the input time is
changed by, say, one second, the position of the satellite can be calculated again.
The position difference between these two positions divided by the given time
is the satellite velocity. Let us consider a unit vector
u
us
, which is from the user
position to a certain satellite. The velocity component of that satellite along
u
us
can be found by projecting the satellite velocity on this unit vector. This velocity
component can be used to determine the Doppler frequency of the satellite, which
can be used for acquisition.
12.13 EMERGENCY GEOLOCATION WITH CELL PHONE
OPERATIONS
(
13,14
)
When a cell phone operator calls 911 to report emergency situation, it is desirable
to locate the caller automatically. Often the caller is inside a building. If the
building is equipped with a geolocation system, the caller's location inside the
building can be identified. The location of the building must also be identified.
This combination of information is sent to the emergency unit receiving the
call. Another approach is to use the GPS signal to locate the caller. However,
inside a building the GPS signal strength may be very weak. An autonomy
receiver usually does not have the sensitivity to acquire and process the signals,
as discussed in Chapter 10.
One of the approaches is to use a GPS base station in the neighborhood
to receive signals at normal strength. The GPS signals of normal strength are
acquired and tracked. In other words, the initial phase of the C/A code, the
carrier frequency, and the navigation data can all be obtained from the signals
of normal strength. This information can be transmitted through the cell system
to the cell phone users in the vicinity. The cell phone unit will include a GPS
receiver. This GPS receiver can perform acquisition based on the received GPS
information. Since the carrier frequency of the GPS is known only a very narrow
frequency range need to be searched. Since the navigation data are known, the
sensitivity of the receiver is not limited by the navigation data length, as discussed
in Section 10.3. The navigation data also provide the opportunity to perform
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