Page 413 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 4. 1 Global positioning technology

measurements to the satellites, enabling the user to with improved capabilities, longer service life and at
compute their position in two dimensions. Since all a lower cost. The next-generation models, known as
Transit satellites broadcast their signals at the same fre- Block IIF, are now being designed for launch in 2002.
quencies, the potential for interference allowed for only This system, which currently consists of 28 fully
a small number of satellites. It was this limited number of operational satellites, cost an estimated $10 billion to
satellites that necessitated the long periods of data col- deploy. The constellation is maintained and managed by
lection, reducing the overall effectiveness of the system. the US Air Force Space Command from five monitoring
This system was finally decommissioned in 1996. sites around the world, at an annual cost of between $300
In order to overcome the limitations of signal in- million and $500 million.
terference inherent in Transit and thereby increase the
availability and effectiveness of satellite observation, an 14.1.2.1 GPS system characteristics
alternate technique for signal broadcast was necessary.
US Air Force Project 621B, also begun in the late 1960s, The 28 satellites in the GPS are deployed in six orbital

demonstrated the use of pseudorandom noise (PRN) to planes, each spaced 60 apart and inclined 55 relative to
encode a useful satellite ranging signal. PRN code the equatorial plane. The orbit of each satellite (space
sequences are relatively easy to generate, and by carefully vehicle, or SV) has an approximate radius of 20 200 km,
choosing PRN codes which are nearly orthogonal to one resulting in an orbital period of slightly less than
another, multiple satellites can broadcast ranging signals 12 hours. The system design ensures users worldwide
on the same frequency simultaneously without should be able to observe a minimum of five satellites,
interfering with one another. This simple concept forms and more likely six to eight satellites, at any given time,
the fundamental basis for GPS satellite ranging, and for provided they have an unobstructed view of the sky. This
the future implementation of the Wide Area Augmen- is important because users with no knowledge of their
tation System (WAAS), which will be discussed later in position or accurate time require a minimum of four
this chapter. satellites to determine what is commonly known as
The US Navy’s Timation satellite system, initially a position, velocity and time solution, or PVT. The PVT
launched in 1967, was also in full swing by the early data consists of latitude, longitude, altitude, velocity, and
1970s. Timation satellites carried payloads with atomic corrections to the GPS receiver clock.
time standards used for time keeping and time transfer The GPS satellites continuously broadcast infor-
applications. This enabled a receiver to use the signal mation on two frequencies, referred to as L 1 and L 2 ,at
broadcast by each Timation satellite to measure the 1575.42 MHz and 1227.6 MHz, respectively. The L 1
distance to that satellite by measuring the time it took frequency is used to broadcast the navigation signal for
the signal to reach the receiver. Timation provided a key non-military applications, called the Standard Position-
proof of concept and a foundation building block for the ing Service (SPS). Because the original design called for
GPS, because without accurate time standards, the the SPS signal to be a lower resolution signal, it is
current GPS would not be possible. modulated with a PRN code referred to as the Coarse
In 1973, building on the success and knowledge gained Acquisition (C/A) code. For the purposes of reserving
from Transit, Timation and Project 621B, and with inputs the highest accuracy potential for military users, the
and support from multiple branches of the military, the DoD may also impose intentional satellite clock and
US Department of Defense (DoD) launched the Joint orbital errors to degrade achievable civilian positioning
Program for GPS. Thus, the NAVSTAR GPS project was capabilities. This intentional performance degradation is
born. commonly known as Selective Availability (S/A). For
US military and other DoD-approved applications,
a more accurate navigation signal known as the Precise
14.1.2 The NAVSTAR GPS system Positioning Service (PPS) is broadcast on both the L 1
and L 2 frequencies. The PPS, in addition to the C/A
The NAVSTAR project was conceived as an excellent code available on L 1 , includes a more accurate signal-
way to provide satellite navigation capabilities for a wide modulated with a code known as the Precise code (P-
variety of military and civilian applications, and it has code) if unencrypted, and as the P(Y)-code if
been doing so quite effectively since full operational encrypted. Authorized users who have access to the
capability (FOC) was declared in 1995. Building on PPS can derive more accurate positioning information
previous satellite technology, the initial GPS satellites from the L 1 and L 2 signals. Refer to Table 14.1-1 for
were launched between 1978 and 1985. These so-called a list of the original positioning and timing accuracy
Block I satellites were used to demonstrate the feasibility goals of the SPS and PPS services.
of the GPS concepts. Subsequent production models On 1 May, 2000, US President Bill Clinton announced
included Block II, Block IIA and Block IIR, each designed the cessation of the S/A, which immediately resulted in

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