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|Federal Highway Administration > Publications > Public Roads > Vol. 62· No. 4 > Surface Transportation and Global Positioning System Improvements: L5 and DGPS|
Surface Transportation and Global Positioning System Improvements: L5 and DGPS
by James A. Arnold
Several proposed improvements to the Global Positioning System (GPS) are likely to benefit surface transportation. The two primary improvements are L5, the second civilian GPS downlink frequency, and Differential GPS (DGPS).
Unfortunately, a great deal of confusion has surrounded them, and unless you have been following the debate closely, it is very easy to become confused. Nevertheless, because of their many potential surface transportation applications, L5 and DGPS are important concepts to the transportation professional.
This article dispels some of the myths surrounding these concepts and provides a clearer picture of their potential. In addition, the impact of setting Selective Availability (SA) to zero within the next 10 years is addressed from the perspective of its impact on both L5 and DGPS.
For an introductory article about GPS and its transportation applications, see "Navigating the Future," by James Arnold, in Public Roads, Autumn 1995.
Surface Transportation Applications
To understand L5 and DGPS, it is important to have an appreciation for the applications of GPS. GPS is a position-location and navigation system that is available nationwide. Depending on equipment configuration, GPS' accuracy ranges from less than a centimeter - 95 percent of the time - to about 100 meters. With a little imagination, applications of this technology to surface transportation begin to become obvious.
Consider for a moment how much effort goes into developing a highway infrastructure map for a local jurisdiction. With GPS, it is now possible to map a large area quickly and efficiently and then be able to return to that area and inventory and reinventory the various roadside structures (signs, guardrails, etc.) many times faster than was done previously. By doing this routinely, maintenance dollars can be applied not only more appropriately but also to areas or projects that are in greatest need. The output of such a mapping effort is a very accurate digital map of the highway infrastructure.
As a spin-off of this, consider what other uses an accurate and easily updated digital map may have. Consider school systems in areas of rapid growth. The school bus routes can change very quickly as new subdivisions are added, roads are realigned, and school district boundaries are updated. The ability to determine efficient bus routes based on quickly updated maps can reduce fuel consumption, reduce the time students are on buses, and decrease the wear and tear on the buses themselves.
A more critical, and perhaps immediate, application would be for emergency response personnel locating fire hydrants under snow. This can be a severe problem in the north central United States, where there were several blizzards last winter. A recent fire in Pierre, S.D., destroyed a warehouse containing sailboats, antique cars, and furniture. Local firefighters had trouble finding the fire hydrants under the 6-meter-high snowbanks. If the infrastructure had been mapped and if the fire response personnel had been equipped with GPS, they could have located the fire hydrant in seconds and perhaps saved much of the building and its contents.
Another application of GPS during snow emergencies is locating snowplows. In an effort to ensure that snow removal is accurate and that plows and salt trucks are used efficiently, these vehicles are equipped with GPS and data links to a central facility. Routes that have been plowed are mapped and updated to show the latest information. It is even feasible now to identify which lane of a multilane highway has been plowed. When this is grouped with weather information about snowfall rates and temperature, it is possible to forecast when a road will need to be replowed or sanded. Many states are now collecting this weather information with roadside sensors and will have the capability to provide more efficient plowing services in the near future.
In some highly accurate modes, GPS can be used to monitor stresses in bridges. Also, GPS has been shown to be useful in determining loading based on the fluctuations in a bridge's altitude. Research is being conducted to determine the benefits of continuously monitoring bridge structures. This technique could be used to monitor individual bridge components for fatigue that results in an overall change in the dynamics of the structure.
Another whole set of applications for intelligent transportation systems (ITS) is just around the corner. "Smart" systems apply advanced and emerging technologies in fields such as information processing, communications, control, and electronics to surface transportation needs.
The ITS program is focused on the development and deployment of a collection of user services. Thirty interrelated user services have been defined as part of a national program planning process. The services are in various stages of maturity. While some are available today, others will require significant research, development, testing, and advances in technology applications before they are ready for deployment. Table 1 lists the user service bundles and the user services that require some sort of geolocation system. GPS is most likely to be the system of choice.
From a public perspective, personalized public transit may be one of the most visible benefits of GPS. This user service is an on-demand public transit system, which permits individuals to call for transit service and which reroutes the appropriate transit vehicle to make the pickup. The transit organization can tell the rider which vehicle will arrive and when. GPS allows the transit organization to monitor the exact location of each vehicle and to better manage the fleet by having access to frequently updated maps that show new residences - an issue in rural areas where there may be little indication of a residence - and new routes for the vehicles to use.
Police and other emergency-response users are putting GPS in their vehicles. GPS permits the dispatcher to constantly monitor the location of each vehicle. When a policeman pursues a suspect, the dispatcher is able to track the police car and to alert others in the area so that the suspect is apprehended, hopefully before innocent bystanders are injured.
The availability and accuracy of several of these applications, including ITS, will be better than what is currently provided by the GPS Standard Positioning Service (SPS). L5 provides increased availability, and DGPS provides increased accuracy and integrity - the ability to provide timely warning to users if and when the system should not be used. These enhancements will benefit surface transportation users in ways that sound like science fiction today but will be reality tomorrow.
Global Positioning System Description
To understand L5 and DGPS, it is important to understand GPS.
GPS is a space-based, radionavigation system that is managed jointly by the departments of Defense and Transportation. GPS was originally developed as a military force-enhancement system and continues to play this role. However, GPS also has significant potential to benefit the civilian community in an increasingly large number and variety of applications.
In an effort to make GPS service available to the greatest number of users while ensuring that national security interests of the United States are protected, two GPS services are provided. The Precise Positioning Service (PPS) provides full system accuracy primarily to U.S. and allied military users. The Standard Positioning Service (SPS) is designed to provide accurate positioning capability for civil users throughout the world.
GPS has three major segments: space, control, and user.
The GPS space segment is composed of 24 satellites in six orbital planes. The satellites operate in circular 20,200-kilometer orbits at an inclination angle of 55 degrees and with a 12-hour orbit. The spacing of satellites in orbit is arranged so that a minimum of five satellites are in view to users worldwide, with a Position Dilution of Precision (PDOP) of six or less.
PDOP is a quality measure of the relative spacing of the satellites. The more evenly spaced around the sky they are, the lower the PDOP. A perfect PDOP implies one satellite directly overhead, and the remaining three positioned at 120-degree intervals just above the horizon.
The dilution of precision is a root mean square measure of the effects that any given position solution geometry has on position errors. Geometry effects may be assessed in the local horizontal position (HDOP), vertical position (VDOP), three-dimensional position (PDOP), or time (TDOP).
The GPS control segment has five monitoring stations and three ground antennas with uplink capabilities. The monitoring stations use a GPS receiver to passively track all satellites in view and accumulate ranging data from the satellite signals. The information from the monitoring stations is processed at the Master Control Station (MCS) near Colorado Springs, Colo., to determine satellite clock and orbit states and to update the navigational message of each satellite. This updated information is transmitted to the satellites via the three ground antennas, which are also used for transmitting and receiving satellite health and control information.
The GPS user segment consists of a variety of configurations and integration architectures that include an antenna and a receiver-preprocessor to receive and compute navigational solutions to provide positioning, velocity, and precise timing to the user.
Each satellite transmits three separate spectrum signals on two L-band frequencies: L1 (1575.42 MHz) and L2 (1227.6 MHz). L1 carries a Precise P(Y) Pseudo-Random Noise (PRN) code and a Course/Acquisition (C/A) PRN code; L2 carries the P(Y) PRN code. (The P(Y) in the Precise PRN code denotes that this PRN code can be operated in either a clear, unencrypted "P" configuration or in an encrypted "Y" configuration.) Both PRN codes carried on the L1 and L2 frequencies are phase synchronized to the satellite clock and are modulated (using modulo two addition) with a common 50-Hz navigation and data message.
Positioning and Navigating
The concept of GPS position determination is based on the intersection of four separate vectors, each with a known origin and a known magnitude. By using four satellite vectors to define four intersecting spheres, we can define a single point in three-dimensional space, providing latitude, longitude, and altitude based on the World Geodetic Survey 1994 (WGS-94) Earth-Centered, Earth-Fixed (ECEF) Coordinate System.
Vector origins for each satellite are computed based on satellite ephemeris, which describes where a satellite is and its velocity and direction at a given point in time. Vector magnitudes are calculated based on signal propagation time delay as measured from the transmitting satellite's PRN code phase delay. Given that the satellite signal travels at nearly the speed of light and taking into account delays and adjustment factors, the receiver performs ranging measurements between individual satellites and the user by multiplying the satellite signal propagation time by the speed of light. The measurements are combined to yield system time and the user's three-dimensional position.
Navigation can then be developed based on point-to-point fixes or from a single fixed point continuously measuring speed and heading based on the satellite's signal and Doppler shift.
Currently, the full accuracy of GPS is denied to stand-alone, non-PPS users of GPS for both navigation and time transfer through the implementation of Selective Availability (SA). SA has two functions: (1) fluctuation of the GPS satellite clock frequency, known as dither, and (2) transmission of incorrect ephemeris parameters in the navigation message, termed epsilon. SA affects all GPS observables, which include the C/A code and P-code pseudorange measurements and the L1 and L2 carrier phase measurements. The Department of Defense has stated that the degradation produced by SA will be limited to a value that maintains the 100-meter (with a 95-percent confidence factor) specified, stand-alone, horizontal accuracy of the SPS.1
Positioning and Timing Accuracy Standard
GPS positioning and timing accuracy will be provided in accordance with the tolerances shown in table 2.
Actual observed data, as shown in table 3, suggests that the Department of Defense is striving to maintain this standard.
L5 Description and Benefits
L5, also known as the second civilian downlink frequency, is planned to be an exact duplicate of the existing L1 downlink frequency. The contents of the navigation message include all the data necessary to determine a receiver's location. L5 provides redundancy and should fall within a band that has worldwide protection from interference. This is a very important point.
To a great extent, the benefits of L5 depend on the technology in use when L5 becomes available. If dual frequency receivers are available in the 2007-2010 time frame, then a small improvement in accuracy will be seen. If not, no improvement in accuracy will be observed. Much depends on how successful these firms are in marketing their service and how receiver technology changes in the next decade.
Nevertheless, placing the L5 downlink in a radionavigation band that is protected worldwide will result in an observable increase in reliability. Currently, L1 is only protected in the United States and its possessions. In addition, testing by the Federal Aviation Administration and the Defense Department shows that, as the Mobile Satellite Service (MSS) begins operation, the emissions mask that MSS is proposing will interfere with L1. The extent of this interference is unknown.
President Clinton, in the Presidential Decision Directive of March 1996, stated his "intent to terminate Selective Availability within 10 years." Table 4 shows the accuracy that could be achieved if SA is eliminated, or set to zero.
Next, if dual frequency receivers are available, improvements in ionospheric measurement will be available. This is a very big "if" and one that cannot be answered now. If current trends continue, it is likely that high-end survey users will use dual frequency receivers while consumer-grade users will opt for the more reliable frequency, either L1 or L5, to use for positioning.
Differential GPS Description and Benefits
Differential GPS or augmented GPS describes several methods for improving the availability, integrity, and accuracy of the GPS service. Although accuracy is one of the strong points of DGPS, it is not the only strong point. And even though augmentations to GPS can increase the availability - usually by adding another ranging signal - few, if any, other DGPS systems address this. Thus, the primary benefits of most DGPS systems are integrity and accuracy.
Integrity, or time to alarm, has been an issue with GPS for many years. The downlink data from each satellite contains health data from the full constellation of satellites. It is a way of telling users which satellites should be used for determining position and navigation data. The description of GPS included the fact that there are three uplink facilities worldwide. The implication is that the satellites must pass over one of these uplink facilities before the satellite health data can be updated. This means that if a satellite fails, the health data on that satellite and any other satellite cannot be updated as soon as the status has changed. It may, in fact, take several hours to update the health status information on all the satellites.
The implications of using invalid data are enormous. This affects all forms of position accuracy derived from GPS, as well as any timing that can be derived. The extent of the error depends to a great deal on what information in the satellite has been corrupted. Systems that depend on GPS for a benchmark accuracy could be severely impacted.
Because this has been a concern for some time, the Defense Department is attempting to fix this with the Block IIR satellites, which permit satellite crosslinking, the ability of the satellites in the constellation to "talk" among themselves. However, it is not certain at the present time if the capacity of this link will support updating satellite health data along with the many other necessary functions. More would be known about this from the first test of the Block IIR as part of the constellation, but it was destroyed shortly after launch in January 1997 because of a malfunction in the launch vehicle.
The integrity and accuracy of a DGPS system must be considered together. Simply stated, if a satellite is broadcasting invalid ranging data and you correct that data based on incorrect data, your correction is very likely to be inaccurate. Another way to say this is that your accuracy is only as good as your ability to project ahead. The correction data is valid for a relatively short time. Thus, the more dynamic a vehicle is, the sooner it needs to know when its corrections are inaccurate.
With that in mind, the DGPS system corrects for accuracy factors. These factors include SA, atmospheric errors, and clock and ephemeris errors. Table 5 shows what happens when these are perfectly corrected.
Figure 1 shows that the integrity requirements for surface transportation tend to be less stringent than for most applications, but the accuracy requirements tend to be more stringent.
As noted above, accuracy and integrity are linked. The more dynamic a platform, the more stringent its accuracy requirements. Integrity can be described as the assurance of navigational accuracy.
Most surface applications are not on highly dynamic platforms and thus do not have the high integrity requirements of, for example, aircraft. On the other hand, users tend to be more concerned with reliability and would tend to reduce their acceptance of a new technology if it were perceived to provide inaccurate data some portion of the time.
Accuracy requirements for surface transportation are mostly on the order of from one to 30 meters. Many opportunities for surface transportation open up when this degree of accuracy is available. For example, this provides the ability to conduct roadway infrastructure inventories faster and with more accuracy.
Several states have implemented their own DGPS system or use post-processing techniques to conduct inventories today. They are also using DGPS to develop more accurate maps, and they can provide more responsive emergency services as a result of increased knowledge of street addresses.
Availability is a factor that is harder to quantify for surface transportation users. Because a system with the accuracy, integrity, and availability of GPS has never been available before, it is seeing unprecedented applications development. As users rely more and more heavily on GPS, their availability needs are likely to increase. As additional radio services are approved, increasing the opportunity for interference, the availability of L1 is likely to decrease. The data on which a projection could be based is not sufficiently backed by operational experience to make a projection accurate enough at this time. As with integrity, when users begin to rely upon GPS more and more heavily, they may notice a reduction in its availability and become less sure about its reliability.
L5 and DGPS provide different, but complementary functions, all of which are important to surface transportation users. As we become more and more exposed to GPS and how it can be used, we are likely to see the benefits more clearly and gain from their use in our daily activities. As mentioned previously, many of the applications we can envision today sound like science fiction. The opportunities that GPS will offer tomorrow have not been dreamed of today.
James A. Arnold is an electronics engineer in the Federal Highway Administration's Intelligent Systems and Technology Division at the Turner-Fairbank Highway Research Center in McLean, Va. His experience includes development of military communications systems, commercial communications systems related to ITS, and technical evaluation of GPS systems. He received a bachelor's degree in electrical engineering from the University of Delaware and a master's degree in electrical engineering from the Florida Institute of Technology.
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