Gps Odometer Watch

Global positioning system

INTRODUCTION 1. Navigation is defined as the science of getting a craft or person from one place to another. Each one of us conducts some form of navigation in our daily lives. Driving to work or walking to a store requires that we employ fundamental navigation skills. For most of us these skills require utilizing our eyes, common sense, and landmarks. However, in some cases where a more accurate knowledge of our position, intended course, or transit time to a desired destination is required, navigation aids other than landmarks are used. These may be in the form of simple clock to determine the velocity over a known distance or the odometer in our car to keep track of the distance traveled. Some other navigation skills are more complex and transmit electronic signals. These are referred to as radionavigation techniques.2. Signals from one or more radionavigation techniques enable a person (herein referred to as the user) to compute their position. It is important to note that it is the user’s radionavigation receiver that processes these signals and computes the position fix. Various types of radionavigation aids exist, and for the purpose of this text they can be categorized as either ground-based or space-based. For the most part the accuracy of ground-based radionavigation aids is proportional to their operating frequency. Highly accurate systems transmit at relatively short wavelengths and the user must remain in line-of-sight range, whereas the systems broadcasting at lower frequencies (larger wavelengths) are not limited to line-of-sight but are less accurate. 
GPS OVERVIEW
3. In the early 1960s, several U.S. government organizations including the military, the National Aeronautics and Space Administration (NASA), and the Department of the Transportation were interested in developing satellite system for position determination. The optimum system was viewed as having the following attributes:
a. Global Coverageb. Continuous/all weather Operationc. Ability to serve high Dynamic Platformsd. High Accuracy
4. To achieve above-mentioned objectives the Global positioning system was developed. A short history and overview is given below.
a. Before the GPS
For thousands of years, speed was limited to a walking pace and landmarks were used to find location. At sea, early navigators limited their voyages to coastal routes to avoid becoming lost. New methods for determining position arose as trade between distant ports increased. Polaris, the North Star, was used to determine north-south distance (latitude) in the Northern Hemisphere. But mariners also had to find latitude when sailing in the Southern Hemisphere, and they lacked a method for determining east-west position (longitude). The solution, celestial navigation, required accurate time. In the late 18th century this led to the development of the marine chronometer, an accurate sea-going timepiece.Electronic navigation introduced all-weather capability, ease of use, and eventually, increased accuracy. These land-based electronic navigation systems were accurate to within several miles, equivalent to celestial navigation. In the mid-1960′s the U.S. Navy’s NAVigation SATellite System (NAVSAT), also known as TRANSIT, was developed to provide more accurate positions for ships and submarines.TRANSIT was the first operational satellite positioning system. Six satellites gave worldwide coverage every 90 minutes and provided positions that were accurate to within 200 meters (660 feet). Positions were obtained by measuring the Doppler shift of the satellite signal. TRANSIT was effective, but it was limited by low accuracy and lack of 24-hour availability. The TRANSIT system operated until 1996.
b. The GPS Revolution
Throughout the 1960s the U.S. Navy and Air Force worked on a number of systems that would provide navigation capability for a variety of applications. Many of these systems were incompatible with one another. In 1973 the Department of Defense directed the services to unify their systems. The basis for the new system would be atomic clocks carried on satellites, a concept successfully tested in an earlier Navy program called TIMATION. The Air Force would operate the new system, which it called the Navstar Global Positioning System. It has since come to be known simply as GPS. The new system called for three components: ground stations that controlled the system, a “constellation” of satellites in Earth orbit, and receivers carried by users. The system was designed so that receivers did not require atomic clocks, and so could be made small and inexpensively. The Soviet Union also developed a satellite-based navigation system, called GLONASS, which is in operation today.
How Does GPS Work
5. Global Positioning System satellites transmit signals to equipment on the ground. GPS receivers passively receive satellite signals; they do not transmit. GPS receivers require an unobstructed view of the sky, so they are used only outdoors and they often do not perform well within forested areas or near tall buildings. GPS operations depend on a very accurate time reference, which is provided by atomic clocks, each GPS satellite has atomic clocks on board. 6. Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions.
GPS Applications
7. GPS has almost revolutionized every field. The applications of GPS in Land, Air, Sea Navigation are obvious. GPS is also very helpful in weather forecasts, surveying, land mapping, and geographical mapping. All these applications are discussed in detail in the next chapters. 
FUNDAMENTALS OF SATELLITE COMMUNICATION
Ranging Using Time-of-Interval Measurements           1. TOA determines user position. This concept entails measuring the time it takes for a signal transmitted by an emitter satellite at a known location to reach a user receiver. This time is referred as the” Signal propagation time” is then multiplied with the speed of the signal to obtain emitter to receiver distance. By measuring the propagation time of the signal broadcast from multiple satellites the receiver can determine it’s position.
Determination of Position via Satellite-Generated Ranging Signals         2. GPS employs TOA for user position determination. The satellite ranging signals ranging signals travel at the speed of light, which is about 340 m/s. It is assumed that satellite locations are accurately known. Assume that there is a single satellite transmitting a ranging signal. A clock on board the satellite controls the timing of the ranging signal broadcast. This clock and others on board each of the satellites within the constellation are effectively synchronized to an interval system time scale denoted as GPS system time. The user receiver also contains clock, which is synchronized to system time. Timing information is embedded within the satellite ranging signal that enables the receiver to calculate when the signal left the satellite. By noting the time when the receiver received the signal, the satellite-to-user time of propagation is found and as a result of calculations, the user would be located on the surface of a sphere centered about the satellite, as shown in figure at right.            If the signal of two satellites were processed simultaneously, the user would be also located on the surface of 2nd sphere that is concentric about the 2nd satellite. Thus, the user would then be somewhere on the surface of both the spheres, which could be either on the perimeter of the shaded circle in figure at left or on the intersection of the two spheres. The plane of intersection is perpendicular to a line connecting the satellites.3, Repeating the measurement process using 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 points. However only one of the points is the correct user position. It can be observed that candidate locations are mirror image of one another with respect to the plane of satellites. For a user on the earth, the lower point of the two will be the true location.

Satellite Orbits
4. GPS user needs accurate information about the GPS satellite to compute its position on earth. So it is important to know how GPS orbits are characterized. The most important forces acting on the satellite is the force of gravity. The force of gravity is not equal due to irregular shape of earth (not exactly spherical). To solve this any point on the earth’s surface is described as in terms of it’s spherical co-ordinates (,/,). Additional forces acting on the satellite are the so-called “third-body” forces of gravity from the sun and moon. Another force acting on the satellite acting on the satellite is the solar radiation pressure, which results from momentum transfer from solar photons to a satellite. Outgassing is also an additional force. To model a satellite’s orbit all these perturbations to earth’s gravitational field must be modeled.

Position Determination Using Pseudorandom Noise (PRN) Codes 
5. GPS satellite transmission utilizes direct sequence spread spectrum (DSS) modulation. The ranging signals are PRN codes that binary phase shift key modulate the satellite carrier frequencies. These codes look like and have spectral properties similar to random binary sequences, but are actually deterministic.6. Each GPS satellite broadcast two types of PRN ranging codes: A “short” coarse/acquisition (C/A) code and a “long” precision (P) code. The (C/A) code has a 1msec period and repeats constantly, whereas the P-code satellite transmission is a 7-day sequence that repeats every midnight Saturday/Sunday.
Determining Satellite to User Range
7. Two-dimensional & three dimensional position determining requires that user clock is synchronized with the system time. Most of the time this is not the case. There are number of error sources which affect range accuracy i.e., noise and propagation delays. But more errors are due to asynchronous clock. So first objective in error is clock-offset determination 8.         We wish to determine the vector u, which represents receiver position in ECEF co-ordinates. Vector  represents the vector offset from the user to satellite. The satellite is located at the XS, YS, ZS while user at Xu, Yu, Z u in ECEF co-ordinates. Vector S represents position of satellite relative to the co-ordinate origin, which is the center of earth.                               
9.          is calculated by measuring the propagation time required for a satellite generated ranging code to transit from the satellite to the user receiver antenna. Satellite use highly accurate cesium or rubidium atomic clock. All frequency generation and timing is based on this clock. Replica codes are used in the receiver to determine the satellite code transmission line.10. For determination of user position in three dimensions, pseudorange measurements are made by four satellites give a set of equations, which is solved by user receiver to find the position in that co-ordinate system.  
Calculation of User Velocity        
11. GPS also provides three dimensional user velocity which can be denoted by u. Several methods can be used to determine user velocity. In some receivers, velocity is estimated by forming an approximate derivative of the user position.12.       This calculation is only valid if the velocity of user is constant in that time period. User is not subjected to any acceleration or jerks. Many GPS receivers calculate the velocity of the user by “Doppler Shift Effect”. The Doppler Shift is produced relative motion of the satellite with respect to the receiver. Received Doppler Frequency increases as the satellite approaches user and decreases as the satellite recedes from the user.

GPS SYSTEM SEGMENTS
Overview of GPS system 
1. The global positioning system consists of three essential segments: Satellite constellation, Ground control/monitoring network and User receiving equipment. Satellite constellation consists of satellites, which provide ranging signals and data messages to user equipment. The operational control system (OCS) tracks and maintains satellites in space. The OCS monitors satellites health and signal integrity. The user equipment performs navigation, timing or other related functions and calculations.
GPS Satellite Constellation
2. The satellite constellation consists of nominal 24 satellites. The satellites are positioned in six earth-centered orbital planes with four satellites in each plane. The nominal orbital period of GPS satellite is one half of a sidereal day or 11 hours 58 minutes. The orbits are nearly circular and equally spaced about the equator at 600 separations with an inclination relative to the equator of nominally 550. The orbit radius is approximately 26,600 km. This satellite constellation provides 24 hrs global user navigation and time determination capability.3. Several different notations are used to refer to the satellites in their orbits. One nomenclature assigns a letter to each orbital plane (A, B, C, D, E, F) with each satellite within a plane assigned a number from 1 to 4. Thus a satellite referenced as B3 refers to the satellite number 3 in orbital plane B. A satellite can also be identified by PRN codes that it generates.Operational Control Segment
4. The OCS has responsibility of maintaining the satellites and their proper functioning. This includes maintaining the satellites in their proper orbital positions (called station keeping) and monitoring satellite subsystem & health and status. OCS also monitors the satellite solar arrays, battery power levels and propellant levels used for maneuvers and activates spare satellites (if available). The OCS updates satellites clock almanac and other indicators once a day or as needed.5. To accomplish the above functions, the control segment is comprised of three different physical components: The master control station, monitor stations and the ground antennas. Each of these is described here.    
a. Monitor Stations Description
The monitor stations form the data collection component of the control segment. A monitor station contains a dual frequency GPS receiver that continuously makes psuedorange and delta range measurements to each satellite in view. The monitor station also contains two cesium clocks referenced to GPS system time. The monitor station receiver dual frequency measurements enable the MCS to determine the ionospheric and tropospheric delays for satellite signals. All the data, after processing is transmitted to Master Control Station (MCS).
b. Ground Uplink Antenna Description
This facility provides the means of commanding and controlling the satellites and uploading the navigation messages and other data. The ground station stores and uploads telemetry tracking and command (TT&C) data. This data is prepared by MCS for a specific satellite and stored in the ground station till that satellite is in view of the ground station. Once in view, 5-band data communication uplink is used to transmit data to the satellite for forwarding to the satellite navigation processor.
c. Master Control Station Description
The MCS performs a multitude of functions to support the operation of GPS as a system. One principal activity of the MCS is to process the data at the remote monitor station to form estimate of the GPS satellite clock, ephemeris, and almanac data. With the data collection from remote monitor stations. The processing starts with the correction of psuedorange measurements for tropospheric and ionospheric delays. Another important element of the MCS processing is monitoring the reliability of the system. The control segment must take meticulous care to ensure that all clock and ephemeris data uploads and other signal transmissions are correct. This monitoring is done principally through the MCS data processing. MCS computes and uploads navigation messages, maintains an image of satellite message for comparison, monitors the uploading of the data and verifies the correct transmission by the satellite. The OCS also monitors satellite’s L-band signal behavior and issues an alarm to MCS personnel within 60 sec of a detected failure.          6. The user receiving equipment, typically referred to as a “GPS Receiver”, processes the signals transmitted from the satellite. There has been a significant evolution, almost revolution, in the technology of GPS receiving sets, paralleling that of the electronics industry in general. The move has been from analogue to digital solid state devices. With today’s technology the GPS receiver typically weighs a few pounds (or ounces) and occupies a small volume. The smallest sets today are those of “wrist watch” size. The GPS set consists of five principal components: antenna, receiver, processor, input/output (I/O) devices such as control display units (CDU), and the power supply. A block diagram of the GPS Receiver set is shown.7. The power supply can be integral, external or combination of the both. Typically alkaline or lithium batteries are used for integral or self contained implementations, such as hand held receivers, where as an existing power supply is used in the integral units such as board mounted receivers installed inside personal computers. Airborne, automotive and shipboard units use platform power. There is usually an internal battery to maintain data stored in volatile RAM integrated circuits and to operate a built-in timepiece in the event platform power is disconnected. 

GPS SATELLITE SIGNAL CHARACTERISTICS, ACQUISITION AND TRACKING
GPS Signal Characteristics  
1. GPS space satellites transmit two carrier frequencies called L1, the primary frequency and L2, the secondary frequency. The carrier frequencies are modulated by spread spectrum codes with a unique PRN sequence associated with satellite and by the navigation data message. All satellites transmit at the same two frequencies, but their signals don’t interfere significantly with each other because of the PRN code modulation. 2. The satellite signals can be separated and detected by a technique called code division multiple access (CDMA). Two carrier frequencies are provided to permit the two-frequency user to measure the ionospheric delay since this delay is related by a scale factor to the difference in signal time of arrival (TOA) for the two carrier frequencies.

GPS Signal Acquisition and Tracking
3. In practice, a GPS receiver must replicate the {RN code that is transmitted by the satellite that is being acquired by the user receiver, then it must shift the phase of the replica code until it correlates with the satellite PRN code. When the phase of the GPS receiver replica PRN code matches the phase of the incoming satellite code, there is maximum correlation. When the phase of the replica code is offset by more than one bit on either side of the incoming satellite signal, the correlation is minimum. This is indeed the manner in which GPS receiver detects the satellite signal when acquiring or tracking the satellite signal in the code phase dimension. It is important to understand that the GPS receiver must also detect the satellite in the carrier phase dimensions by replicating the carrier frequency plus Doppler. Thus, the GPS signal acquisition and tracking process is two dimensional signal replication process.4. The two-dimensional acquisition and tracking process can best be explained and understood in progressive steps. The clearest explanation is in the reverse sequence from the events that actually take place in the real world GPS receiver. The two dimensional search and acquisition process is easier to understand if the two-dimensional steady state tracking process is explained first. The two-dimensional code and carrier tracking process is easier to understand if the carrier tracking process is explained first.  This is the explanation sequence that will be used. The explanations will be given in the context of a generic GPS receiver architecture with minimum use of equations.  

EFFECTS OF RF INTERFERENCE ON GPS SATELLITE SIGNAL RECEIVER TRACKING
Effects of RF interference on tracking
1. Because GPS receiver relies on external radio frequency signals, they are vulnerable to RF interference. RF interference can result in degraded navigation accuracy or complete loss of receiver tracking.
Types of Interference
2. The RF interference may be friendly or intentional. There is certain level of interference to C/A code receivers from the C/A codes of other GPS satellites. If pseudolites are used, operation at close range to these ground transmitters will almost certainly result in jamming of the remaining GPS satellite signals. In fact, the most efficient wide band jamming technique is the use of any 
Type Typical SourcesWideband-Gaussian International noise jammers
Wideband phase/frequency modulation Television transmitter’s harmonics or near-band microwave link transmitters overcoming front-end filter of GPS receiver
Wideband Spread Spectrum International spread spectrum jammers or near-field of pseudolites
Wideband pulse Radar Transmitters
Narrowband phase/frequency modulation AM stations transmitter’s harmonics or CB transmitter’s harmonics
Narrowband-swept continuous wave International CW jammers or FM stations transmitter’s harmonics
Narrowband-continuous wave International CW jammers or near-band unmodulated transmitter’s carriers
Types of RF interference and Typical Sources
of the spread spectrum GPS codes and the GPS code chipping rate to map the power spectrum of the jammer on to the power spectrum of the GPS signal.3. Intentional jamming must be anticipated for military receivers. Hence, all classes of inband jammers must be considered in the design. Also, GPS receivers are vulnerable to spoofing; that is the intentional transmission of a false, but stronger version of the GPS signal so that it captures the receiver tracking loops and fools the navigation process. The encrypted anti spoofing (AS) Y-Code is used to replace a public P-Code for military applications to minimize the potential for spoofing military GPS receivers. However, since the cost of a jammer is so much less than for a spoofer, the principal military GPS receiver threat will most likely be jamming. Y-code operation provides no advantage over P-code against enemy jamming.4. RF interference is expected to originate from friendly (Unintentional), out of band, RF interference sources for commercial GPS receivers. Unfortunately, nonlinear effects may occur in high power transmitters causing lower power harmonics, which become inband RF interference 
RF Interference Effects 5. In order to minimize the effects on the tracking loops of a GPS receiver, RF interference monitoring and mitigation features must be implemented. These features may become an important design requirement for commercial GPS receivers because RF interference is unpredictable and, when it occurs, it degrades the GPS signal integrity regardless of the health of the GPS Satellite that transmit the signals. Fortunately, there is an opportunity for dual use of anti jam techniques that have been successfully developed for military GPS receivers. We will describe some of the most effective design techniques that have been used in military GPS receiver design.
a. Implementation of RF Interference Detector
The first technique is RF interference detection using a jamming to noise power ratio (J/N) meter. A distinguishing feature of a GPS receiver that has been enhanced to operate in the presence of RF interference is a built-in J/N meter. Detecting the presence of RF interference is given first priority because this provides a instant warning of potential loss GPS Satellite signal integrity if RF interference is present. It is a very reliable indicator because it is a measure of the composite RF interference level actually being passed through the GPS receiver antenna and front end. b. Antenna Enhancements
Another technique is implemented at the antenna area.This technique involves the use of an adaptive antenna array. One type is called a beamsteered array, which points a narrow beam of antenna gain toward each satellite tracked. This type of array contains many antenna elements and is extremely expensive. The beamsteered antenna array is impractical for most GPS applications.  Factors that Reduce RF Interference Effects
6. There are a number of factors that help to reduce the RF interference effects. The RF interference can only have the full effect of this analysis on the GPS receiver if it is in the line of sight of the GPS antenna and unobstructed. For commercial aviation applications the RF interference sources will typically be at ground level while the GPS antenna will be elevated during en route navigation. This increases the line-of-sight range, but because the source of the interference will in general, be from below the aircraft’s horizon, the body of aircraft will help to block the interference. However, as the aircraft approaches for landing or for ground based operation of GPS receiver in general, the RF interference signals can be attenuated due to earth curvature, foliage, buildings, and so forth. 
DIFFERENTIAL GPS

1. The GPS SPS provides accuracy of approximately 100 meters horizontally and 126 meters vertically. Many civil applications require greater accuracy. Wide area DGPS (WADGPS) services are also available. Offshore oil exploration and seismic survey industries aid WADGPS systems and remain principle users. Further, the land surveying community is a prime DGPS user. Both the offshore industry and the land surveying community pioneered and perfected many high accuracy measurement techniques. In addition, geographical information system (GIS) utilize DGPS and data a logger to establish a database of items and their corresponding locations. GIS is in use throughout industry and sectors such as forestry for inventory control.2. The use of DGPS enhances standalone GPS accuracy and removes common (i.e., correlated) errors from two or more receivers viewing the same satellites. In the basic form of DGPS, one of these receivers is called the monitoring or reference receiver and is surveyed in; that is, its precise position is known. The other receivers are denoted as “rovers” or users and are in line-of-sight of the reference station. The reference station makes code based GPS pseudorange measurements, just as any standard GPS receiver, but because the monitoring station knows its precise position, it can determine the “biases” in the measurements. For each satellite in view of the monitoring station, these biases are computed by differencing the pseudorange measurement and the satellite to reference station geometric range. These biases contain errors incurred in the pseudorange measurement process (e.g., ionospheric and tropospheric delay, receiver noise, etc.) and the receiver clock offset from GPS system time, tm. For real time applications the reference stations transmits these biases, which are called differential corrections to all users in the coverage area. The users incorporate these corrections to improve the accuracy of their position solution, which is obtained in ECEF coordinates. DGPS achieves enhanced accuracy since the reference and user receivers experience common errors that can be removed by the user. Position errors less than 10 meter are typically realized.

Code Based Techniques
3. Several code-based techniques have been proposed to increase standalone GPS accuracy. These techniques vary in sophistication and complexity from a single reference station that calculates the errors at its position for use with nearby GPS receivers to worldwide networks that provide data for estimating errors from detailed error models at any position near the earth’s surface. They are usually sorted in two categories, LADGPS and WADGPS. 
Local Area DGPS
4. A LADGPS station typically serves receivers within close proximity. For aircraft using very high frequency (VHF) datalinks, the range is limited by the distance that the station can communicate with the receivers through direct line-of-sight data links. For the maritime services, which use medium frequency (MF), LADGPS is extended upto 400 km or more. As stated above, LADGPS accuracy depends on the fact that some of the pseudorange error components are common to al receivers within the local geographical area. If the receiver is close to the reference station, the error component attributed to the space and control segments may be entirely removed while the overall error contributed by the user segment may be reduced significantly. It is for this reason the offshore industry utilizes the wide area DGPS services; LADGPS is limited by spatial decorrelation of errors. . 

Wide Area DGPS
5. WADGPS attempts to attain meter-level accuracy over a large region by using a fraction of the number of reference station required by LADGPS to attain the same accuracy within the same coverage region. The general approach in contrast to that of LADGPS is to breakout the total pseudorange error into its components and to estimate each component for the entire region, rather than just at the station positions. The accuracy, then, does not depend on the closeness of the user to a single reference station. The WADGPS concept includes a network of reference stations that aid in the accurate determination of satellite ephemerids, atmospheric delay, and discrepancies between GPS system time and satellite time tags (i.e., Z-count). Thoughts vary on the architecture of the intercommunications among these networks. The simplest concept perhaps is a system having a master control station that accepts the measurements from all the reference stations distributed throughout the region of coverage (which could include a continent or even the entire world) and updates the satellite ephemeris predictions, estimates the uncorrected satellite clock drift and SA dither, and keeps track of the temporally and spatially varying atmospheric delays.6. Large or far ranging systems, this architecture places a severe burden on one master control station and on the communication system to produce updates and distribute them to the reference stations reliably and in a timely manner. Regional control stations (RCS) being closer to the reference stations and less burdened with processing, can more readily provide timely correction updates and provide active or standby redundancy to take over the functions of a neighboring regional control station, if necessary. These RCSs work in conjunction with one master control station. In this case, the master control station synchronizes the RCS clocks and performs various duties such as coordinating measurements on the satellite by different regional stations and monitoring the health of the RCS. This less centralized architecture does have the problem of synchronizing the clock in all RCSs and the master control station to one system time. Time synchronization amongst elements of the ground control network is essential (just as with GPS itself) to ensure correction and measurement time tagging. For example, the smooth switching to a redistribution of communications due to scheduled maintenance or unscheduled failure of a regional station is dependent on network time synchronization.
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