Description:

"Chapter 1 Introduction to GPS The NAVSTAR Global Positioning System (GPS) is a satellite-based radio-positioning and time-transfer system designed, financed, deployed, and operated by the U.S. Department of Defense. GPS has also demonstrated significant benefits for the civilian community who apply it to an expanding number of applications. What attracts us to GPS are: - High positioning accuracies from tens of meters down to the millimeter level. - Capability to determine velocity and time with accuracy commensurate with position. - Signals available anywhere on Earth: in air, on ground, or at sea. - A positioning system with no user charges that requires only relatively low-cost hardware. - An all-weather system available 24/7. - Three-dimensional positioning information provided. The number of civilian users is already significantly greater than military ones. However, the U.S. military still operates several "levers" to control GPS performance (Section 1.2.3). Despite this, there continues to be tremendous product innovation in the civilian sector, partly directed at overcoming constraints applied by military operators. 1.1 Introduction to System Components 1.1.1 System Design Considerations Development work on GPS commenced within the U.S. Department of Defense in 1973 with the goal of creating an all-weather, 24/7 global positioning system for armed forces and allies' needs (see reference _1_). The system was designed to replace existing navigational systems, emphasizing reliability and survivability. Stringent conditions included: - Suitable for all classes of platforms: aircraft, ships, land vehicles, handheld devices, space. - Ability to handle various dynamics. - Real-time positioning, velocity, and time determination with appropriate accuracy. - Positioning results available on a single global geodetic datum. - Highest accuracy restricted to certain users. - Resistance to jamming (intentional and unintentional). - Redundancy provisions for system survivability. - Passive positioning without user-to-satellite signal transmission. - Service provided to an unlimited number of users. - Low cost, low power, with complexity built into the satellite segment. This led to a design based on: - One-way ranging system where satellites transmit signals unaware of who uses them (no receiving function). - Use of latest atomic clock and microwave technology including spread spectrum techniques. - System making range-like measurements using pseudo-random binary codes modulated on carrier signals. - Satellite signals unaffected by cloud or rain. - Multiple satellite system ensuring sufficient visibility anywhere, anytime. - Graceful degradation in positioning accuracy. Unforeseen by designers was the power of product innovation significantly enhancing GPS's versatility for precise positioning and navigation. For example, it supports various modes from general navigational accuracies to very high relative positioning accuracies (sub-centimeters). Key technological advancements include: - Space System Reliability: U.S. space program demonstrated reliability by 1973 with Transit system lessons. - Atomic Clock Technology: Development of reliable, stable, compact atomic frequency oscillators (rubidium and cesium). - Quartz Crystal Oscillator Technology: Low-cost quartz crystal oscillators for user equipment. - Precise Satellite Tracking and Orbit Determination: Successful operation depends on precise knowledge and prediction of satellite positions. - Spread-Spectrum Technology: Ability to track GPS signals in noisy environments using spread-spectrum coding. - Large-Scale Integrated Circuit Technology: VLSI circuits for low cost, power, and size. The GPS system consists of three segments (Figure 1.1): - Space Segment: Satellites and broadcast signals. - Control Segment: Ground facilities tracking satellites, computing orbits, telemetry, and supervision. - User Segment: Applications equipment and computational techniques available to users. Figure 1.1: GPS System Elements. 1.1.2 The Space Segment The Space Segment includes the constellation of spacecraft and their signals allowing position, velocity, and time determination. Basic satellite functions are: - Receive and store data from Control Segment stations. - Maintain accurate time using onboard atomic clocks. - Transmit information and signals on two L-band frequencies to users. - Provide a stable platform for L-band transmitters. Several constellations of GPS satellites have been deployed with more planned. Experimental "Block I" satellites were built by Rockwell, launched from 1978 to 1985 (one exploded). Operational "Block II" and "Block IIA" satellites were also built by Rockwell. Replacement "Block IIR" series are built by General Electric (now Lockheed Martin), first launched in 1997. The "Block IIF" series, still in design, may include additional civilian transmission frequencies. Operational satellite IDs are separated into three space vehicle numbering series: SVN 13-21 for Block II, SVN 22-40 for Block IIA, and SVN 41+ for Block IIR. The current status of the GPS constellation and launch details can be found in references _2_ and _3_. 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