Per Enge

The Global Positioning System: Signals, measurements, and performance

The Global Positioning System (GPS) is a satellite-based navigation and time transfer system developed by the U.S. Department of Defense. It serves marine, airborne, and terrestrial users, both military and civilian. Specifically, GPS includes the Standard Positioning Service (SPS) which provides civilian users with 100 meter accuracy, and it serves military users with the Precise Positioning Service (PPS) which provides 20-m accuracy. Both of these services are available worldwide with no requirement for a local reference station. In contrast, differential operation of GPS provides 2- to 10-m accuracy to users within 1000 km of a fixed GPS reference receiver. Finally, carrier phase comparisons can be used to provide centimeter accuracy to users within 10 km and potentially within 100 km of a reference receiver. This advanced tutorial will describe the GPS signals, the various measurements made by the GPS receivers, and estimate the achievable accuracies. It will not dwell on those aspects of GPS which are well known to those skilled in the radio communications art, such as spread-spectrum or code division multiple access. Rather, it will focus on topics which are more unique to radio navigation or GPS. These include code-carrier divergence, codeless tracking, carrier aiding, and narrow correlator spacing.

Wide area augmentation of the Global Positioning System

The Wide Area Augmentation System (WAAS) is being deployed by the Federal Aviation Administration (FAA) to augment the Global Positioning System (GPS). The WAAS will aid GPS with the following three services. First, it will broadcast spread-spectrum ranging signals from communication satellites. The airborne WAAS receiver will add these new ranging signals to the GPS constellation of measurements. By so doing, the augmented position fix will be less sensitive to the failure of individual system components, thus improving time availability and continuity of service. Second, the WAAS will use a nationwide ground network to monitor the health of all satellites over our airspace and flag situations which threaten flight safety. This data will be modulated on to the WAAS ranging signals and broadcast to the users, thereby guaranteeing the integrity of the airborne position fix. Third, the WAAS will use the ground network to develop corrections for the errors which currently limit the accuracy of unaugmented GPS. This data will also be included on the WAAS broadcast and will improve position accuracy from approximately 100 m to 8 m. When complete, the augmented system will provide an accurate position fix from satellites to an unlimited number of aircraft across the nation. It will be the primary navigation system for aircraft in oceanic routes, enroute over our domestic airspace, in crowded metropolitan airspaces, and on airport approach.

Spread-Spectrum Multiple-Access Performance of Orthogonal Codes: Linear Receivers

This paper analyzes a direct-sequence, spread-spectrum, multiple-access (SSMA) communication system which assigns a set of M orthogonal sequences to each user. With all direct sequence SSMA systems, K users share a channel by phase modulating their transmissions with signature sequences. However, the users of our system transmit log_{2}M bits of information/sequence. This contrasts classical SSMA schemes which use a pair of antipodal sequences and transmit 1 bit/sequence. In this paper, we assume that the channel noise is a combination of additive white Gaussian noise (AWGN) and multiple-access interference. We employ the optimum (single-user) demodulator for orthogonal signals in Gaussian noise. The multiple-user performance of this receiver is analyzed. We obtain approximations for the multiuser probability of error by using a Gaussian approximation for the multiple-access interference. We also obtain an upper bound on the exact probability by using characteristic functions. Our SSMA system is Well suited for application at the lower radio frequencies. Therefore, a companion paper describes a realistic model for low-frequency radio noise, modifies the receiver to include a zero-memory nonlinearity, and studies the performance of the nonlinear receiver.

Local area augmentation of GPS for the precision approach of aircraft

The Federal Aviation Administration (FAA) is investigating the feasibility of using the Global Positioning System (GPS), together with local augmentation as the primary navigation system for the Category II and III precision approach of aircraft. This augmented system would be known as a local area augmentation system (LAAS) because it would place subsystems on the ground at or near an airport to greatly enhance the performance of airborne GPS receivers approaching that airport. The LAAS would complement space-based augmentations of GPS, such as the wide area augmentation system. When complete, the integrated system will meet stringent requirements on accuracy, integrity, continuity, and availability. Indeed, the LAAS will improve the airborne accuracy from approximately 100 m for stand-alone GPS to better than 1 m. This high accuracy is required so that LAAS can guarantee the integrity and continuity of the aircraft guidance during the approach phase of flight. Integrity requires the LAAS to detect any situations which threaten the safety of the landing, and notify the aircraft within 2 s of such a threat. Continuity is a competing consideration-it requires that the total number of alarms, true plus false, sent to the aircraft is very small. High continuity guarantees that the total number of aborted approaches is manageably small. Finally, availability is the fraction of the time during which LAAS is operational-providing position fixes with the specified accuracy, integrity and continuity. This paper describes the LAAS which has been proposed by the FAA. It discusses the fundamental operation of the LAAS.

Civilian GPS: The Benefits of Three Frequencies

A third civil frequency at 1176.45 MHz will be added to the Global Positioning System (GPS). This new frequency will bring a number of benefits. The aviation user will be one of the prime beneficiaries because the new frequency is in a protected aviation band. Thus, the system will be more robust against interference and jamming.The carrier-phase differential user will also be a prime beneficiary as long as his application has a reasonably short baseline. It is this high accuracy use that is explored in some depth. The process of forming linear combinations of both the code and carrier-phase measurements is studied, and the benefits and problems are explained.

Worldwide Vertical Guidance of Aircraft Based on Modernized GPS and New Integrity Augmentations

In the 2020 time frame, the Global Positioning System (GPS) will be fully modernized, and other satellite navigation systems will be operational. With an additional layer of fault detection, these systems will provide vertical guidance worldwide. This capability will be born of three important technologies. First and foremost, avionics will receive signals on two frequencies: L1/E1 and L5/E5a. This frequency diversity will do much to obviate the impact of ionospheric storms that troubles aviation use of GPS today. Secondly, a multiplicity of data broadcasts will be available to convey integrity information from the ground to the airborne users. These will include the navigation satellites themselves, geostationary satellites, and possibly terrestrial transmitters. However, the most important change will be the most subtle. The fault monitoring burden will be split between the aircraft and the supporting ground systems in a new way relative to the fault-detection techniques used in 2008. This new integrity allocation and the associated architectures are the subject of this paper.

Differential operation of the Global Positioning System

The principles of differential operation of the Global Positioning System (DGPS) are discussed. DGPS error sources and their influence on spatial and temporal correlation are analyzed. The standards for the differential corrections and the auxiliary messages which have been developed by the Radio Technical Commission for Maritime Services are then described. The dependence of DGPS accuracy on satellite geometry is examined. Two possible DGPS data links are described, and the applications, advantages, and limitations of each are discussed. The first data link would use the extant marine radiobeacon network (a medium-frequency system) and provide coastal coverage. The second data link would use pseudosatellites and provide additional lines of position as well as carry the differential correction.<<ETX>>

Ionospheric Threat Parameterization for Local Area Global-Positioning-System-Based Aircraft Landing Systems

Observations of extreme spatial rates of change of ionospheric electron content and the characterization strategy for mitigation applied by the U.S. local area augmentation system are shown. During extreme ionospheric activity, the gradient suffered by a global navigation satellite system user a few kilometers away from a ground reference station may reach as high as 425 mm of delay (at the GPS L1frequency) per km of user separation. The method of data analysis that produced these results is described, and a threat space that parameterizes these possible threats to user integrity is defined. Certain configurations of user, reference station, global navigation satellite system satellite, and ionospheric storm-enhanced density may inhibit detection of the anomalous ionosphere by the reference station.

RAIM with Optimal Integrity and Continuity Allocations Under Multiple Failures

Among the receiver autonomous integrity monitoring (RAIM) algorithms treating multiple failures, multiple hypothesis solution separation algorithms (MHSS) - a type of solution separation algorithm - offer several advantages: First, the link between threat model, upper bound on the position error - the protection level and probability of hazardously misleading information is an easy and straightforward one; second, the calculation of the protection level does not involve complex steps. One of the critical steps in this algorithm is the allocation of the integrity and continuity budgets among the failure modes, as it determines the overall performance of the algorithm. After describing the baseline MHSS approach, we present an algorithm that simultaneously allocates the integrity and continuity budget among the failure modes to obtain the minimum protection level per satellite geometry. Then, we show how slope-based RAIM and solution separation RAIM are related through a little-known formula, which both unifies and highlights the differences between the two approaches. Finally, we apply the algorithm to evaluate the performance of RAIM for vertical guidance for a dual constellation, and find that even with a very large prior probability of satellite failure, vertical guidance can be achieved worldwide with high availability.

Paired overbounding and application to GPS augmentation

The relationship between range-domain and position-domain errors remains an open issue for GPS augmentation programs, such as the Federal Aviation Administrations Local Area Augmentation System (LAAS). This paper introduces a theorem that guarantees a conservative error bound (overbound) in the position domain given similarly conservative overbounds for broadcast pseudorange statistics. This paired overbound theorem requires that a cumulative distribution function (CDF) be constructed to bound both sides of the range-domain error distribution. The paired overbound theorem holds for arbitrary error distributions, even those that are non-zero mean, asymmetric or multimodal. Two applications of the paired overbound theorem to GPS augmentation are also discussed. First, the theorem is employed to construct an inflation factor for a non-zero mean Gaussian distribution; in the context of a simulation of worst-case satellite geometries for 10 locations in the United States and Europe, the required inflation factor for broadcast sigma is only 1.18, even for biases as large as 10 cm for each satellite. Second, the theorem is applied to bound a bimodal multipath model tightly; the result shaves more than 40% off the previously established inflation factor derived through a more overly conservative analysis.