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How Does GPS Find Your Position?
User Segment
The user segment represents a GPS unit somewhere on the Earth that is receiving the signals from the satellite. The receiver can then use this information to compute its position on the Earth. A key component of a GPS receiver
is how many channels the receiver has—the number of channels reflects the
number of satellites the receiver can obtain signals from at one time. Thus, a
12-channel receiver can potentially pick up signals from up to 12 satellites.
Also, the type of unit affects which of the frequencies it can receive—it can
either be a single frequency receiver (which can pick up the L1 frequency) or
a dual frequency model (which can read both L1 and L2). One thing to keep
in mind about the receiver is that no matter what size, capabilities, or number
of channels it has, a receiver (like the name implies) can only receive satellite
data, not transmit data back up to the satellites. As such, GPS is referred to
as a one-way ranging system since the user on the ground cannot send any
information to a satellite.
How Does GPS Find Your Position?
At this point, the three segments of GPS have satellites broadcasting signals,
ground stations monitoring and correcting those signals, and users receiving
those signals—and all three of these segments work together to determine
your position on the Earth’s surface. The signals being sent from the satellites
are the key to the whole process—the signals contain information about the
satellite’s status, the orbit, and location (referred to as the almanac) that’s
sending them, as well as more precise data about satellite location (this information is referred to as the ephemeris).
The information is sent in one of two digital pseudo-random codes—the
C/A code (coarse acquisition code) and the P code (precise code). The C/A
code is broadcast on the L1 carrier frequency and is the information that
civilian receivers can pick up. The P code is broadcast on the L1 and L2 carrier
frequencies and contains more precise information, but a military receiver is
required to pick up this signal. The Y code is an encrypted version of the P code
and is used to prevent false P code information from being sent to a receiver by
hostile forces. This “anti-spoofing” technique is commonly used to make sure
that only the correct data is being received. Basically, the satellites are transmitting information in the codes about the location of the satellite, and they
can also be used to determine the time when the signal was sent. By using this
data, the receiver can find its position relative to that one satellite.
The signals are transmitted from space using high-frequency radio
waves (the L1 and L2 carrier frequencies). These radio waves are forms of
electromagnetic energy (to be discussed in Chapter 10) and will thus be
moving at the speed of light. Your receiver can compute the time it took for
the signal to arrive from the satellite. If you know how long the transmission
time (t) was, and also that the radio waves are moving at the speed of light
(c), then the distance between you and that one satellite can be calculated
user segment one of
the three segments of
GPS, consisting of the
GPS receivers on the
ground that pick up
the signals from the
satellites.
channels the number
of satellite signals a
GPS unit can receive.
single frequency a
GPS receiver that can
pick up only the L1
frequency.
dual frequency a GPS
receiver that can pick
up both the L1 and L2
frequency.
almanac data
concerning the status
of a GPS satellite,
which is included in
the information being
transmitted by the
satellite.
ephemeris data
referring to the GPS
satellite’s position in
orbit.
C/A code the digital
code broadcast on the
L1 frequency, which is
accessible by all GPS
receivers.
P code the digital
code broadcast on the
L1 and L2 frequencies,
which is accessible by
the military.
Y code an encrypted
version of the P code.
How Does GPS Find Your Position?
User Segment
The user segment represents a GPS unit somewhere on the Earth that is receiving the signals from the satellite. The receiver can then use this information to compute its position on the Earth. A key component of a GPS receiver
is how many channels the receiver has—the number of channels reflects the
number of satellites the receiver can obtain signals from at one time. Thus, a
12-channel receiver can potentially pick up signals from up to 12 satellites.
Also, the type of unit affects which of the frequencies it can receive—it can
either be a single frequency receiver (which can pick up the L1 frequency) or
a dual frequency model (which can read both L1 and L2). One thing to keep
in mind about the receiver is that no matter what size, capabilities, or number
of channels it has, a receiver (like the name implies) can only receive satellite
data, not transmit data back up to the satellites. As such, GPS is referred to
as a one-way ranging system since the user on the ground cannot send any
information to a satellite.
How Does GPS Find Your Position?
At this point, the three segments of GPS have satellites broadcasting signals,
ground stations monitoring and correcting those signals, and users receiving
those signals—and all three of these segments work together to determine
your position on the Earth’s surface. The signals being sent from the satellites
are the key to the whole process—the signals contain information about the
satellite’s status, the orbit, and location (referred to as the almanac) that’s
sending them, as well as more precise data about satellite location (this information is referred to as the ephemeris).
The information is sent in one of two digital pseudo-random codes—the
C/A code (coarse acquisition code) and the P code (precise code). The C/A
code is broadcast on the L1 carrier frequency and is the information that
civilian receivers can pick up. The P code is broadcast on the L1 and L2 carrier
frequencies and contains more precise information, but a military receiver is
required to pick up this signal. The Y code is an encrypted version of the P code
and is used to prevent false P code information from being sent to a receiver by
hostile forces. This “anti-spoofing” technique is commonly used to make sure
that only the correct data is being received. Basically, the satellites are transmitting information in the codes about the location of the satellite, and they
can also be used to determine the time when the signal was sent. By using this
data, the receiver can find its position relative to that one satellite.
The signals are transmitted from space using high-frequency radio
waves (the L1 and L2 carrier frequencies). These radio waves are forms of
electromagnetic energy (to be discussed in Chapter 10) and will thus be
moving at the speed of light. Your receiver can compute the time it took for
the signal to arrive from the satellite. If you know how long the transmission
time (t) was, and also that the radio waves are moving at the speed of light
(c), then the distance between you and that one satellite can be calculated
user segment one of
the three segments of
GPS, consisting of the
GPS receivers on the
ground that pick up
the signals from the
satellites.
channels the number
of satellite signals a
GPS unit can receive.
single frequency a
GPS receiver that can
pick up only the L1
frequency.
dual frequency a GPS
receiver that can pick
up both the L1 and L2
frequency.
almanac data
concerning the status
of a GPS satellite,
which is included in
the information being
transmitted by the
satellite.
ephemeris data
referring to the GPS
satellite’s position in
orbit.
C/A code the digital
code broadcast on the
L1 frequency, which is
accessible by all GPS
receivers.
P code the digital
code broadcast on the
L1 and L2 frequencies,
which is accessible by
the military.
Y code an encrypted
version of the P code.