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Land Vehicle Navigation Systems

Saurabh GodhaResearch EngineerPositioning Location and Navigation (PLAN) GroupDepartment of Geomatics Engineering,University of Calgary, [email protected]

INTRODUCTIONA navigation system is a special technology that provides location awareness and enhances the vehicular control, safety and navigation performance significantly. Traditionally, the navigation systems have been associated primarily to the field of marine and air navigation, simply because their routes are virtual, and the position of ships and aircrafts needs to be identified on the maps. Additionally, the navigation systems used to be bulky and expensive which limited their applicability to general consumer applications, such as land vehicle. However, this scenario is rapidly changing with navigation systems now finding their way in to the realms of a land vehicle. The primary reason behind such development is the increasing driving complexity, due mainly to the increasing road density and intensive road traffic, which has accentuated the genuine need for navigation of land vehicles. This need is further boosted by availability of appropriate technologies, like Global Navigation Satellite System (GNSS) such as Global Positioning System (GPS) that can efficiently address to the cost, size and performance constraints of a navigation system for consumer applications. A navigation system used in a land vehicle is called in-car or land vehicle navigation system (LVNS). The purpose of this article is to introduce readers to basic aspects of a LVNS, while emphasizing on one technical and the most important aspects of LVNS, i.e. the positioning technology. Today, LVNS have not only become an attractive product in general consumer navigation market but also critical in several aspects of daily life, such as safety-on-road and economic issues. The market for LVNS has expanded significantly, where LVNS can be readily found in cars, trucks, golf-carts, farming and mining equipment, and a wide-variety of other land vehicle applications. The car manufacturers are now installing the navigation system not as an accessory, but an essential part of multifunctional telematics systems used for providing an assortment of information to the driver, to make driving a safe, comfortable and enjoyable experience. Examples of such systems include Honda Navigation system, Mercedess Teleaid system and GMs Onstar system. Commercial aftermarket navigation systems that can be fitted to any car are now readily found in Europe, Japan and North America. In Japan it is anticipated that 40 % of the cars will be equipped with navigation systems by the year 2007. The current share of the land vehicle applications in the navigation market is estimated over $8 billion US which is expected to grow over $15 billion by the year 2008.RoleA LVNS is typically used to perform two kinds of role, as a navigation system for individual vehicles and other as a tracking device in fleet management systems. Looking from an individual vehicle perspective, a navigation system assists drivers in navigating through unfamiliar geographic space, by selecting the shortest route to the desired destination, and providing turn by turn driving instructions to it. The most obvious advantages of this, from an individual view point, is the efficient time and energy management, fuel savings and economic efficiency, since they dont have to roam around in search of destination point. This also helps reducing the driving stress and road rage (people often get angry and frustrated when they are lost or misses a turn), thus helps in improving the overall driving performance. From the society viewpoint, this is beneficial in the sense that it decreases individual on-road time and facilitates efficient use of the road network, which ultimately translates to enhanced traffic capacity, superior traffic flow, and thereby reduced traffic congestions. Over the last decade, the navigation systems have achieved significant technological advancements, and the functions of todays navigation systems have now expanded beyond just providing the route guidance. This is primarily due to the convergence of growing navigation and communication technologies, which have created an exciting new market known as location-based services (LBS). The modern navigation systems provides access to LBS and exploits the knowledge of location of a vehicle to provides value added services to the user, for example, connects them to nearby points of interest (retail businesses, banks or restaurants), provide weather updates or advise them of current traffic conditions. For instance, Nissan Motor Co. offers a navigation system on some of its top models (mostly in Japan), with a capability of receiving traffic updates in real-time through a radio link. Honda and General Motors (GM) offer similar systems in North America. This is a significant feature, since if a traffic congestion or incidence is reported on a particular route, the navigation system can determine an alternative faster and convenient route, and thus help user to reach destination point in a shortest time possible. This further prevents the development of congestions into a serious traffic jams and helps in their rapid mitigation. An LVNS also helps significantly to enhance the vehicle/driver safety and security. By using the vehicle navigation information, an LVNS can analyze the driving performance of the user and issue timely alert on detecting abnormal driving behavior, for instance behavior pertaining to drunk driving or fatigue. Other important application is in emergency situations. A navigation system can relay its location to service providers (through the cellular phone interface) in case of emergency, for instance when a vehicle crash is detected through crash sensors or when an air bag is deployed. The service provider then in turn can inform appropriate authorities to enable quick help. Furthermore, an LVNS forms centerpiece equipment for current/future advanced vehicle safety systems, such as collision avoidance system (CAS) that alerts the drivers to dangerous situations and impending collisions. Primary role of an LVNS in such safety system is to provide information such as position, speed, heading of the vehicle, geometry of the road, and the predicted path of the vehicle, which is deemed critical for their operation.

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From the fleet management perspective, a navigation system helps fleet owners and managers to track the vehicle, by transmitting the current location of the vehicle to the monitoring/base station digitally through a wireless communication link, where it is subsequently displayed on a local map. Location awareness of each vehicle in a fleet provides owners with wide range of benefits including ability to control and monitor the mobile assets (car/trucks), ability to direct the closest vehicle (ambulance/police car/fire brigades/taxi cabs) to a desired destination in real-time, and also ability to have better coordination and communications with customer based on the current location of the vehicle (transit vehicles/delivery vans). These advantages ultimately lead to superior business decisions, increased productivity, and significant time and cost savings.

A fleet management system can

enable monitoring mobile assets.

enhance vehicle safety and help locate stolen vehicle promptly.

optimize vehicle routes.

enable proper management of employees.

ensure that employees follow the pre-decided path to the destination point.

maximize fuel efficiency.

enable better coordination with customers.

help direct closest vehicle to a desired destination in real time.

help generate daily tracking reports detailing trip information such as time, mileage etc.

help provide prompt assistance to employees in case of vehicle breakdown.

prevent unauthorized vehicle use.

Advantages of Fleet Management Systems

OperationThe basic function of a LVNS is to accurately locate a vehicle on a road network, which can be achieved through integration of a positioning technology and spatial information. A typical LVNS consists of an on-board computer, which collects data from a variety of positioning sensors, and determines vehicles location through an active or passive combination of digital map database (typically stored on CDs/DVDs/HDD). Once the vehicle location is identified, LVNS uses the map information and several algorithms to determine shortest route to a particular destination and directions to it. The information gathered is then conveyed to the user by means of an audio/visual electronic display.

Different Components of a Basic LVNS

Positioning TechnologyAccurate vehicle positioning is fundamental to the operation of LVNS. Positioning defines the method of determining the coordinates of a particular point in a well-defined reference frame. Many types of positioning sensors/technologies can be considered for use in land navigation applications, which can be grouped under two broad categories, dead reckoning (DR) systems (odometers, wheel speed sensors, rate-gyros, accelerometers) and radio navigation system (e.g. LORAN, TRANSIT, GPS, GLONASS, Galileo). The advantages and disadvantages of one sensor/technology compliment the other and thus, a third category can be created through loose/tight integration of technologies listed above.

Characteristics of an ideal positioning technology

Continuity in operation

It is the ability of a system to provide navigation solution without interruption, over a certain time period. In the context of LVNS, navigation solution should be available through out the period of its operation at some preset rate.

Accuracy of positioning

It is the closeness of the computed solution to the truth solution. In general, the accuracy requirement for a land vehicle varies with the application. For instance, the acceptable accuracy level for an emergency service like an ambulance is 15-20 m (2D, 95 %), whereas the corresponding requirement for individual vehicle navigation application may be 2-5 m (2D, 95 %).

Reliability of system

It is the measure of confidence in the computed navigation solution from a particular positioning system.

Low Cost

The cost and performance of a positioning technology shares a direct relationship. The ideal technology for LVNS should be a balance between a good performance and low cost.

Ideal Positioning Technology for LVNSHistoryMost of the early navigation systems developed briefly around World War II and in 1980-1990s operated on the DR principle to determine the vehicle position. DR systems are stand-alone, relative positioning systems and uses a simple idea that, the present position of a vehicle can be computed using the knowledge of departure point of the vehicle and its subsequent speeds and direction. The oldest of all DR sensors is the odometer (and differential odometer), which was developed even before the development of automobiles. An odometer is a device that measures the distance traveled by the vehicle. The odometer formed an essential part of early navigation systems for land vehicles, where it was typically combined with a direction sensing device such as magnetic compass or rate-gyros to track the change in position of the vehicle. Example of one such navigation system is a vehicular odograph, which was developed around World War II for US army vehicles. First ever navigation system for consumer vehicles was Hondas Electro Gyro-Cator, developed in 1981 [1], which used a helium gas rate-gyro and odometer and maps to determine the vehicles path.

Honda Electro Gyro-Cator [1]

Another kind of DR system is an inertial navigation system (INS) which provides dynamic information of position and velocity (and attitude) through direct measurements from an inertial measurement unit (IMU). An IMU consists of three accelerometers and three gyros mounted on an orthogonal triad. The basic operating principle of inertial navigation is based on Newtons law of motion, which says that an object continues to be in a state of rest or uniform motion, unless acted upon by an external force. The application of external force generates an acceleration, which is sensed by the accelerometers contained in an IMU. The sensed acceleration is rotated to appropriate navigation frame through angular measurements obtained from gyros, which when integrated twice, provides the change in the navigation state of the object with respect to the initial conditions. An INS has formed a prominent component of aircraft, ship and submarine navigation since World War II. However, because of high cost of the good quality IMUs and the regulations by the government for their restricted use, their applicability in general land navigation system has been limited. The primary advantages of DR/inertial systems include its robustness, stand-alone operation, and ability to operate in all environments. This allows it to provide a continuous navigation solution, with excellent short-term accuracy. However, the DR systems suffer from time and distance dependent error growth, driven primarily by the errors in sensor measurements, which compromises its long-term accuracy. The data provided by the sensors contains errors, which leads to error accumulation over time due to the integration process. This causes a drift in navigation computation to a point where the position output may become useless after sometime. Thus, the DR systems require frequent updates from external sources, to be able to provide navigation solution accurately and reliably over time.

DR System Operation

One of the early methods to control the DR error growth was map-matching (MM), which in-fact still forms a prominent part of the modern navigation systems. Map-matching uses the fact that the motion of a land vehicle is essentially confined to a finite network of roads, and thus some special constraints can be derived from map information to aid in position computation through DR systems. Map matching basically tries to identify the road segment on which the vehicle is traveling, typically done by applying artificial intelligence pattern recognition concepts to the position output of DR system. Once the road segment is identified, the positioning information from digitized map database is extracted and fused with position output of DR system (using a Kalman filter) to calibrate the DR sensor errors. This enhances the ability of DR sensors to provide navigation solution with acceptable accuracies for longer periods of time. However, the accuracy clearly is a function of the accuracy of the digital maps and accuracy in identification of the road segment (specifically in the high road density areas). One of the first commercially available navigation systems using map-matching technology was the Etak Navigator [2], which employed magnetic compass, differential odometer and digital map database to determine vehicle location. Satellite based radio navigation systems have received considerable attention over the past few decades. The TRANSIT system (1960-1996), also known as NAVSAT, developed by US Navy was the first satellite-based global navigation system, targeting marine applications (ships and submarines). The system was realized with 5-7 satellites, placed in nearly circular orbits at a distance of about 1100 km, broadcasting signals at 150 MHz and 400 MHz. Positions were obtained from this system using the Doppler shift of the satellite signal. At a given time, only one satellite was in view and the user has to wait for about 90-100 minutes between successive satellites passes to determine position. TRANSITs intended role was to provide position updates to the US ships and submarines so as to reset the on-board DR inertial navigation devices. The system was very effective for its intended purpose, but was impractical for land navigation application because of its intermittent coverage. However, the mid 1980s saw some system developed utilizing the sporadic position fixes from TRANSIT to update the DR system based on odometer and magnetic compass. However, due to high receiver prices and degraded accuracy between satellite position fixes, such systems did not receive much commercial attention. Modern positioning technologiesModern day positioning technologies revolve around one name, the Global Positioning System (GPS). GPS is a global navigation satellite system (GNSS) that allows the user to determine range from a known signal transmitting station (i.e. satellites) by measuring the differential time of travel of the signal. GPS receivers take this information and use trilateration to calculate the user's position. Currently (as of August 2006) there are 29 operational GPS satellites in orbits [3]. These satellites broadcast bi-phase modulated signals at two carrier frequencies, L1 (1575.42 MHz) and L2 (1227.60 MHz), of which civil community have access to L1 signal only. Using this signal, a GPS receiver can construct three types of measurements, namely code phase (pseudorange) measurement, carrier phase measurement, and Doppler/incremental phase measurement. The carrier phase measurement is the most precise measurements available to GPS users, and is typically used in high-accuracy (cm-level) application such as geodetic surveying. Pseudorange and Doppler measurements are used more commonly for commercial applications requiring positioning accuracies on the order of few meters.

GPS Segments

Originally designed for the military users, GPS is now being used extensively in applications intended for civilian users, where the estimated number of civilian users is around few million and is growing every year. Specific advantages of GPS include, an all-weather absolute positioning, world-wide availability and free-of-cost service that provides fairly consistent accuracy. Furthermore, ever since the selective availability (SA) of GPS is turned off (on May 1, 2000), the accuracy of GPS positioning available to civil community has improved significantly (stand-alone GPS accuracy within 5-8 meters), which match well with the accuracy requirements of a land navigation application. This accuracy can further be improved, within few meters, through the use of differential GPS (DGPS) technology, where differential corrections are now available from various ground and satellite based augmentation systems (GBAS/SBAS), e.g. WAAS in North America, EGNOS in Europe, MSAS in Japan and GAGAN in India. These advantages combined with increasingly falling cost of GPS receivers, have made GPS an affordable attractive option for design of LVNS. Pioneer Corporation developed one of the first GPS based car navigation system in 1990 [4].Although GPS satisfies the characteristics of an ideal positioning technology under favorable operating conditions (open area with a clear line-of-sight (LOS) to satellites), its performance is inadequate in certain environments, such as urban areas and dense foliage environments. This is because GPS signal is a low-power (-160 dbW), high frequency (L1-1575.42 MHz) signal, which frequently gets blocked/attenuated by tall buildings, tunnels, underpasses and trees, found typically in urban areas. This reduces GPS receivers ability to lock on four or more satellites, which is essential for its operation, thereby causing discontinuous positioning. This problem coupled with the problem of tracking multipath signals, created due to signal reflection from glassy buildings; limits the ability of GPS to deliver the required level of availability, accuracy and reliability for design of LVNS. Having said that, technology advances have now made possible to track degraded power signals, through a technique called high sensitivity GPS (HSGPS). HSGPS uses longer pre-detection integration times (PIT) and navigation data wipe-off methods to track weak attenuated signals with power as low as -186 dbW. Thus, by tracking signals from more satellites, a HSGPS receiver addresses the issue of continuity to some extent. However, the reliability of this method is questionable as a HSGPS receiver is specifically susceptible to measurement faults due to tracking of echo-only signal and signal cross-correlation, which could result in positioning errors of 100s of meters. Thus, in general GPS cannot be solely used for navigation, especially in urban environments.

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Both GPS and DR systems have complementary properties, and thus a GPS/DR integrated system is one of the most viable options to use today for vehicle positioning. GPS, when combined with DR sensors, can restrict their error growth over time, and allows for online estimation of their errors, while the DR sensors can bridge the position estimates when there is no GPS signal reception. Also, the use of DR components allows the GPS measurements to be compared against statistical limits and reject those measurements that are beyond the limits, thereby enhancing the reliability and integrity of the system. The navigation solution derived from the GPS/DR integrated system is, therefore, better than the either stand-alone solutions. Most navigation systems available in the market today combine a low-cost GPS receiver with on-board vehicle sensors such as odometers or wheel speed sensors (for vehicle speed) and gyros (for heading changes), while making use of map database alongside to overcome GPS limitations. The combination of these three systems (GPS/DR/map) provides a positioning system that is tolerant to momentary failure of each individual system.

Traditional and MEMS Inertial Systems

Recent advances in MEMS (Micro Electro-Mechanical Systems) inertial sensor technology have rendered the integrated GPS/MEMS INS system as another low-cost design option for the LVNS. MEMS inertial sensors take advantage of high volume manufacturing methods and flexible packaging options, thus resulting in small, low-cost, low power consuming, and robust inertial sensors. These features of MEMS sensors have opened the door for inertial technology for applications such as LVNS, which were not practical earlier due to the size and cost constraints. Due to relative immaturity of this technology, the performance of the current available sensors is limited, which has discouraged their wide-spread use as positioning device in LVNS yet. However, with steady growth in this technology and improving sensor performance, GPS/MEMS INS system is likely to become the preferred positioning technology for future navigation systems.

INS

GPS

GPS/INS

Advantages

Continuous positioningGood short-term accuracyAutonomousHigh output rateAttitude information

Absolute positioningLong-term consistent accuracies

Continuous positioningConsistent accuraciesAttitude informationHigh output rateJamming resistantEnhanced reliability and integrity Feasibility of using low cost sensors

Disadvantages

Relative positioning system - require accurate initializationAccuracy degradation with timeHigh cost of good quality sensors

Signal blockage discontinuous positioningLow output rateNo attitude informationSusceptible to jamming

INS, GPS and GPS/INSLORAN-C (LOng RAnge Navigation system) is another positioning system which has recently gained much attention for use as a backup system to GPS for land navigation application. LORAN-C is a high power, low frequency (90-100 KHz) terrestrial navigation system that uses a time-difference-of-arrival (TDOA) between radio signals from three or more transmitting stations to determine positions. It has recently undergone significant upgrade and modernization to enhance its accuracy, availability and continuity, and the enhanced version is called eLORAN. The high power and low frequency of eLORAN signals complements GPS well as it is possible to receive them deep in urban areas. However, it remains to be seen how well it can support vehicle navigation applications, during GPS failures. Integration of GPS with other satellite-based radio navigation system is envisioned as another positioning option for future LVNS. At present, Russian GNSS, named GLONASS is the other system available. Although, GPS/GLONASS integration till date has received little attention for commercial applications owing to uncertainty in availability of GLONASS, which currently have only 14 operational satellites (as of June 2006) as oppose to minimum 24 satellites initial design [5]. GLONASS though is being modernized and replenished and is likely to have over 18 satellites by year 2007-2008. Over the course of next few years, another GNSS named Galileo will become available. Galileo is a GNSS being developed by European community, which when fully operational, will feature a minimum of 27 satellites transmitting over ten signals for various services characterized as open service (OS), commercial service (CS) and safety-of-life (SoL) services. The three systems are designed to be interoperable with each other, and together will constitute a constellation of around 70-75 satellites, thus enhancing the signal availability considerably. Furthermore, GPS itself is set to undergo modernization whereby new signal on L2 and L5 frequency will be available to civilian community. This access to additional signals from different systems, in future, will allow instantaneous precise location determination through the use of high quality carrier phase measurements from each system, and thus expanding the feasibility of centimeter level GNSS positioning from high-accuracy applications to general consumer land navigation applications. However, even the combination of these three systems may be inadequate at times in dense urban areas, and thus it is necessary to keep developing DR inertial sensor technology as a backup option to GNSS.

Current and Future GNSSs

SUMMARYA land vehicle navigation system is a special technology that enhances the vehicle control, safety and navigation performance significantly. The fundamental requirement of a navigation system is a positioning technology, which when combined with map information, helps to locate the vehicle on the road network. Different technological options exist for vehicle positioning; however, the practical systems require combination of two or more technologies for continuous, accurate and reliable positioning. GPS in conjunction with DR technologies and map-matching systems is the most viable vehicle positioning option available today. In future, the systems designers will have significantly expanded technological options for vehicle positioning, thanks to modernization and development of new GNSS and the steady growth in MEMS inertial sensor technology. Access to different positioning systems will allow designers to select the most optimal systems combination that maximize the positioning performance and minimize the design costs, for a particular land navigation application. LINKS:

http://world.honda.com/history/challenge/1988navigationsystem/text/01.html http://www.teleatlas.com/Pub/Company/Patents/Navigation_Patents/index.htm http://www.navcen.uscg.gov/ftp/GPS/status.txt http://www.pioneer.co.uk/uk/content/company/company/history.html http://gge.unb.ca/Resources/GLONASSConstellationStatus.txt http://www.lb.refer.org/sammuneh/ch2-6.htm http://www.foehn-aventure.com/indlandsis/GPS/sat-gps1.jpg http://www.obspm.fr/~unicom/magazine/article.php3?id_article=298&lang=fr

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