Due to the limitations of the Meridian satellite system, in 1973 the United
States began to plan to develop a new navigation and positioning system, the GPS
positioning system. After 20 years of hard work, the system was initially built
and put into use in 1993. At this time, the system has a total of It consists of
24 satellites, including Block I experimental satellites and Block II, Block IIA
working satellites. After the completion of the GPS navigation constellation,
the use of C/A codes for navigation and positioning can also achieve high
positioning accuracy. Therefore, considering the huge use of GPS in the military
field, in order to prevent the enemy from using GPS to endanger the national
security of the United States, the United States defense The Ministry stipulates
that the SA technology should be implemented on all working satellites, that is,
it is considered to reduce the accuracy of the navigation system. There are two
specific technical means:
1. A technology that artificially adds errors to the broadcast ephemeris of
the satellite to reduce the accuracy of the satellite ephemeris.
2. Technology, consciously adding a pseudo-random process rapid change to
the satellite clock frequency.
On the other hand, in order to prevent the enemy from electronically
spoofing and electronically jamming GPS satellite signals, the US Department of
Defense has implemented the AS policy. The specific method is to add a strictly
confidential W code to the GPS P code. After modulo two addition, a completely
secret Y code is generated. Although the AS policy is a defensive measure, it
also limits the possibility of unlicensed users to use the Y code, so it is
impossible to obtain high-precision ranging codes, which greatly reduces the
navigation accuracy.
Later, the GLONASS satellite navigation system managed by Russia announced
that it would not implement the SA policy, so the positioning accuracy of its
navigation system was much higher than that of the GPS system using the SA
policy. Announced in 2009 to suspend the implementation of SA.
As satellite navigation systems play an increasingly important role in
various fields of military and civilian use. However, the original GPS system
has many shortcomings, such as: 1. It is difficult to receive satellite signals
in hidden areas; 2. The energy of C/A code and P code is concentrated near the
center frequency and is prone to human interference; 3. L2 carrier Ranging code
is not modulated on the device, which makes it impossible for ordinary users to
perform dual-frequency ionospheric delay correction, which affects positioning
accuracy. Therefore, the C/A code was modulated on the L2 carrier of the
navigation satellites of the subsequent Block IIR-M and Block IIF models, and
later changed to the L2C code, so that civilian users can also use
dual-frequency observation to improve positioning accuracy. Later, the L5 signal
was added to the GPS navigation satellite after Block IIF, so that tri-frequency
positioning can be used for non-military users.
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