Running Head: GPS Navigation System

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GPS Navigation System

Running Head: GPS Navigation System

GPS Navigation System

Sein Myint and Kim Kaur

IndianaUniversityPurdueUniversity at Fort Wayne

Abstract

Initiallyinstalled and used by the USDepartment of Defense in early 1960’s, Global Positioning System (GPS) is a satellite-based radio positioning navigation, and time-transfer system. GPS provides two levels of accuracy: The Standard Positioning Service (SPS) and the Precise Positioning Service (PPS). While PPS is encrypted and only available for authorized (military) users, SPS has been made available for the general public now. The GPS Navigation System is one of the services (SPS) that is dedicated to the general public. The SPS is available to all users worldwide. There are no restrictions on SPS usage. Presently, GPS is fully operational and meets the criteria established in the 1960s for an optimal positioning system. The system provides accurate, continuous, worldwide, three dimensional position and velocity information to user with the appropriate receiving equipment. GPS can provide service to unlimited number of users since the user receivers operate passively (i.e., receive only).

How GPS Works

The Global Navigation System is a group of 27 Earth-Orbiting Satellites (24 Satellites in operation and 3 extra in case one fails to operate). Each of these 3,000- to 4,000-pound solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky. (Marshall, Bryan and Harris, Tom, n.d.)

The GPS utilizes the time of concept of time-of-arrival (TOA) ranging to determine user position. This concept entails measuring the time it takes for a signal transmitted by an emitter such as foghorn, radio beacon, satellite ( satellite in this case) at a known location to reach a user receiver. This time interval, referred to as the signal propagation time, is then multiplied by the speed of the signal (e.g., speed of sound, speed of light) to obtain the emitter-to-receiver distance. By measuring the propagation time of signals broadcast from multiple emitters (i.e., navigation aids) at known locations, the receiver can determine its position. (Kaplan, Elliot D, 1996).

Position Determination via Satellite-Generated Ranging Signals

GPS employs TOA ranging for user position determination. By making TOA measurement to multiple satellites, three-dimensional positioning is achieved. Though it is similar to the foghorn transmission, GPS signals travels at the light speed approximately 3 × 10^8 m/sec. It is assumed and important that all the satellite calendars are accurate.

GPS system time is the time synchronized on each satellite within the group of satellites. The user receiver also has a clock on it that is synchronized to the GPS system time. The receiver calculates the propagation time using ranging signals from the satellites in which timing information is embedded. Then, that time is multiplied by the speed of light to compute the distance from the user receiver to the satellite. As a result, the user can be located somewhere on the surface of a sphere centered about the satellite. “If a measurement was simultaneously made using the ranging the ranging signal of a second satellite, the user would also be located on the surface of second sphere that is concentric about the second satellite. Thus, the user would be somewhere on both spheres. Repeating the measurement process using the third satellite collocates the user on the perimeter of the circle and the surface of the third sphere. This third sphere intersects the perimeter at two pointswith other two spheres but only one is the correct user position. The candidate locations are mirror images of one another with respect to the plane of the satellites. (Kaplan, Elliot D, 1996).

Position Determination using Pseudo-Random Noise (RPN) codes

GPS satellite transmissions utilize direct sequence spread spectrum (DSSS) modulation. DSSS provides the structure for ranging signals and essential navigation data such as satellite ephemeredes and satellite health. The ranging signals are PRN codes that binary phase shift key (BPSK) modulate the satellite carrier frequencies. These codes look like and have spectral properties similar to random binary sequences, but are actually deterministic. These codes have predictable pattern, which is periodic and replicated by a suitably equipped receiver. Each GPS satellite broadcast two types of PRN ranging codes: a short course/acquisition (C/A) code and a long precision (P) code. The C/A code has a 1-msec period and repeats constantly. P code transmission is a 7 day sequence that repeats every midnight on Saturday.

GPS System Segments

GPS consists of 3 segments: the space segment, the control segment, and the user segment. The space segment consists of the satellites in orbit providing the ranging signals and data messages to the user equipment.

The space segment consists of 24 satellite constellation. Each satellite transmits a signal, which has a number of components: 2 Sine waves (also known as carrier frequencies), 2 digital codes, and a navigation message. The codes and the navigation message are added to the carriers as binary bi-phase modulations. The carriers and codes are mainly used to determine the distance from the user receiver to the satellites. The navigation message contains, along with other information, the location of the satellites as a function of time. The transmitted signals are controlled by the highly accurate atomic clocks onboard the satellites.

The operational control segment consists of worldwide network of tracking stations, with the Master Control Station, located in the United States at Colorado Springs, Colorado. The primary duty of the operational control segment is tracking the GPS satellites in order to determine and predict the satellite locations, system integrity, behavior of the satellite atomic clocks, atmospheric data, the satellite almanac, and other considerations. This information is packed and uploaded into the GPS satellites through the S-Band link.

The user segment includes all military and civilian users. With a GPS receiver connected to a GPS antenna, a user can receive the GPS signals, which can be used to determine his or her position anywhere in the world. GPS is currently available to all users worldwide at no direct charge. (El-Rabbany, Ahmed, 2002).

GPS Signal Structure

GPS Satellites transmit two 2 carrier frequencies (Sine Waves) modulated by 2 digital codes and a navigation message. The 2 carrier frequencies are generated at 1,575.42 MHz (referred to as L1 carrier) and 1,227.60 MHz (referred to as L2 carrier). The corresponding carrier wavelengths are approximately 19 cm and 24.4 cm respectively, which result from the relation between the carrier frequency and the speed of light in space. The availability of the 2 carrier frequencies allows for correcting the GPS error, known as the ionospheric delay. All of the GPS satellites transmit same L1 and L2 carrier frequencies. The code modulation, however is different for each satellite which significantly minimized the signal interference.

The 2 GPS codes are called a short coarse acquisition (C/A code) and a long precision (P) code. Each code consists a stream of binary digits, zeros and ones, known as bits and chips. The codes are commonly known as PRN because they look like random signals. But, actually the codes are generated using mathematical algorithm. Presently, the C/A code is modulated onto the L1 carriers only, while the P code is modulated onto both the L1 and L2 carriers. This modulation is called bi-phase modulation because the carrier phase is shifted 180ºwhen the code value changes from 0 to 1 or from 1 to 0.

The C/A code is a stream of 1,023 binary digits that repeats itself every millisecond. That means that the chipping rate of the C/A code is 1,023 Mbps. Each satellite is assigned a unique C/A code, which enables the GPS receivers to identify which satellite is transmitting a particular code. The C/A code range measurement is relatively less precise compared to that of the P code. It is, though, less complex and available to all users worldwide.(El-Rabbany, Ahmed, 2002).

The P code is a very long sequence of binary digits that repeat itself after 266 days. It is also 10 times faster than C/A code (10.23 Mbps). The P code is a stream of approximately 2.35 × 10^14 chips by multiplying the rate and time it take to repeat itself. The 266 day long is divided into 38 7-day-long segments. 32 segments of these are assigned to various GPS satellites. Then, each satellite transmits a unique 7 day long segment of P code, which is initialized every Saturday/Sunday midnight crossing. The remaining 6 segments are reserved for other uses. A GPS satellite can be identified by its unique 1 week long segment of P code. For example, a GPS satellite with an ID of PRN 20 refers to a GPS satellite that is assigned the 20th week segment of PRN P code. The P code is designed primarily for military purposes. It was available to all users until January 31, 1994. At that time, the P code was encrypted by adding to it an unknown W code. The resulting encrypted code is called the Y code which has the same chipping rate as the P code. This encryption is known as the anti-spoofing. (El-Rabbany, Ahmed, 2002).

The GPS navigation message is a data stream added to both L1 and L2 carriers as binary bi-phase modulation at a low rate of 50 Kbps. It consists of 25 frames of 1,500 bits each or 37,500 bits in total. This means that transmission of the complete navigation message takes 750 seconds or 12.5 minutes. The navigation message contains, along with other information, the coordinates of the GPS satellites as the function of time, the satellite health status, the correction of the satellite clock, the satellite almanac, and atmospheric data. Each satellite transmits its own navigation message with information on the other satellites, such as approximate location and health status. (El-Rabbany, Ahmed, 2002).

GPS Errors

If we want to take GPS navigation system to another level or to make it more precise than it is as of now, we have to remove or avoid the errors that can occur in the system. Basically, GPS errors can be divided into 6 categories:

• Receiver Clock Error
• Satellite Clock Error
• Satellite Orbit Error
• Ionospheric Delay
• Neutral atmosphere delay
• Multipath

Receiver Clock Error

In order for the system to be precise, the receiver clock must be synchronized with the satellite clock even if it is a one1000th of a second different it must be removed or fixed so that the accuracy of the system can be precise.

Satellite Clock Error

Even though the atomic clocks are used in the GPS navigation satellites that control all onboard timing operations including broadcast signal generation, they tend to deviate up to 1 m-sec from GPS system time. An offset of 1 m-sec translates to a 300 Kilometers pseudo-range error. The master control station (MCS) determines and transmits clock correction parameters to the satellites for rebroadcast in the navigation message.

Satellite Orbit Error

Satellite positions as a function of time, which are included in the broadcast navigation message, are predicted from the previous GPS observations at the ground control stations. Typically, overlapping 4 hour GPS data are used by the operational control system to predict fresh satellite orbital elements for each 1 hour period. Modeling the forces action on the GPS will not, in general, be perfect, which causes some errors in the estimated satellite positions, known as ephemeris errors. An ephemeris error is usually in the order of 2 m to 5 m, and can reach up to 50 m under selective availability. An ephemeris error for a particular GPS satellite is identical to all GPS users worldwide. As different users see the same satellite at the different angles, the effect of the ephemeris error on the range measurement, and consequently on the computed position, is different. Combining (differencing) the measurements of 2 receivers simultaneously tracking a particular satellite cannot totally remove the ephemeris error. Users of short separations, however, will have an almost identical range error due to the ephemeris error, which can essentially be removed thorough differencing the observations. (El-Rabbany, Ahmed, 2002).

Ionospheric Delay

The ionospheric delay is dispersive (frequency dependent) and can be determined by observing both of the frequencies transmitted by the GPS satellites (L1 & L2) using a dual-band GPS receiver. These ionospheric delays can be eliminated without reference to observations recorded by other GPS receivers.

Neutral Atmosphere Delay

The neutral atmosphere consists of the stratosphere and the troposphere. Because the troposphere constitutes most of the neutral atmosphere, the neutral atmosphere is often referred to as the troposphere. The tropospheric delay consists of two components. The hydrostatic (or "dry") component that is dependent on the dry air gasses in the atmosphere and accounts for approximately 90% of the delay. And the "wet" component that depends on the moisture content of the atmosphere and accounts for the remaining effect of the delay. Although the dry component has the larger effect, the errors in the models for the wet component are larger than the errors in the models for the dry because the wet component is more spatially and temporally varying.

The dry component is not actually the true dry correction because it depends on the total pressure of the atmosphere which includes the partial pressure of the dry gasses and the partial pressure of the water vapor. Assuming static equilibrium and the ideal gas law, the hydrostatic delay is a linear function of total surface barometric pressure and can be modeled to millimeter level accuracy. (University of Texas at Austin)

Multipath

Multipath is the corruption of the direct GPS signal by one or more signals reflected from the local surroundings. These reflections affect both code and carrier based measurements in a GPS receiver. Code multipath is typically on the order of several meters and is thus quite important for Differential GPS reference stations. It can be characterized through a combination of code and carrier measurements.

GPS Markets and Applications

Initially, GPS was designed to use for military positioning, weapon aiming, and navigation system. However, factors like downing of Korean Airlines Flight 007 by Soviet interceptors forced the GPS system to be made available for public use to avoid navigational errors from happening again. The US government made a commitment on providing some of the GPS signals without a fee resulting industry to make investments in development of hardware, software, and the system. The long term availability of the GPS signals and inexpensiveness of the receivers allow users all over the world to available themselves of technology no matter where they are on the planet.

Uses of GPS for Marine, Air, and Land Navigation

Marine and air navigation are the most obvious applications of globally available positioning technology using satellite signals. The inherent accuracy of unaided, single C/A code GPS, as set by SA (Selective Availability), of 100m 2 drms is adequate for nearly all airborne or seaborne en route requirements.

Approaches to airports or harbors are another matter and for these situations, DGPS techniques have been developed. This brings another factor into the market dimension, as there is a demand for equipment to generate and transmit differential corrections, for equipment to receive and apply them, and for services that generate and broadcast correction signal.

Integrity is another issue to consider when dealing with marine harbor and aircraft navigation, particularly in approach mode. Various enhancements for integrity monitoring and reporting are being studied and implemented. These too have an impact on the rate of acceptance of the technology as well as on the ultimate size of these markets. (Kaplan, Elliot D, 1996).

Marine Navigation

There are about 17 million boats in North America, 46 million worldwide. 98% of these are pleasure boats. Commercial coastal and inland vessels comprise about 970,000 potential platforms for GPS, and there are more than 80,000 registered merchant vessels worldwide, most of which are involved in fishing.

Air Navigation

There are essentially tow kinds of markets and two regimes of operations to consider in the airborne area. There are 224,000 general aviation (GA) aircraft registered in North America and 67,000 in the rest of the world. These aircrafts are privately owned by individuals or companies for personal or corporate transportation, or recreational flying. The second category is the air carrier industry, which employs just over 5,000 aircraft in North America and a similar number worldwide. Both categories demand the high amount of GPS use.

Land Navigation

The most promising market for GPS navigation in terms of sheer size is for land navigation products. There are more than 420 million cars and 130 million trucks in the world, with 150 million and 40 million in the US and Canada. Automatic Vehicle Location systems (AVLS) are being developed or installed in many of North America’s 10.3 million trucking and emergency fleets, currently involving about a million vehicles in North America. Urban transit busses are finding applications of GPS for schedule maintenance and safety enhancement. There are about 750,000 urban transit busses in North America and as of early 1992, at least 30 North America transit authorities were working on AVLS or had them installed, involving over 18,000 vehicles.