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.

Global Positioning System: Signals, Measurements, and Performance

Subscriber radio location techniques are investigated for code-division multiple-access (CDMA) cellular networks. Two methods are considered for radio location: measured times of arrival (ToA) and angles of arrival (AoA). The ToA measurements are obtained from the code tracking loop in the CDMA receiver, and the AoA measurements at a base station (BS) are assumed to be made with an antenna array. The performance of the two methods is evaluated for both ranging and two-dimensional (2-D) location, while varying the propagation conditions and the number of BS's used for the location estimate.

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Subscriber location in CDMA cellular networks

Although originally developed for the military, the Global Positioning System (GPS) has proven invaluable for a multitude of civilian applications. Each application demands specific performance from the GPS receiver and the associated requirements often vary widely. This paper describes the architectures and functions of civilian GPS receivers and then focuses on performance considerations. The fundamental receiver measurements are described and the quality of these measurements are related to the aforementioned receiver architectures.

GPS receiver architectures and measurements

The earth’s ionosphere can cause serious problems for
many radio applications: DOD C31 and navigation sys-
tems and commercial applications such as communica-
tions, radio astronomy and navigation. They can all be
seriously affected by ionospheric disturbances. Distur-
bance effects such as amplitude fading and phase scintil-
lation limit the ability to coherently integrate weak sig-
nals. One solution, which reduces the effective data
capacity of the channel, is the use of time-interleaved
modulation schemes in which data is sent repeatedly to
ensure its error-free reception. Another solution is to use
alternate resources during the times of predicted signal
outages due to scintillation fading. However, these pre-
dictions are generally based on monthly climatology and
are poor for a specific time in a particular region due to
the variability and dynamics ‘of the earth’s ionosphere. A
more real-time and local solution is required.
GPS signals provide an excellent means for measuring
scintillation effects on a disperse global basis because
they are continuously available and can be measured
through many points of the ionosphere simultaneously.
GPS signals are themselves affected, but because of the
spread spectrum properties of the signal, tracking
through disturbances with a GPS receiver is usually pos-
sible with reasonably wide bandwidth tracking loops, and
scintillation parameters can be extracted. There are GPS
applications, however, such as surveying where inte-
grated carrier phase measurements are used, in which
scintillation can easily disrupt operational activities [ 11.
This paper presents the result of a Small Business Inno-
vative Research (SBlR) Phase I study and testing of a
software-modified commercial C/A code receiver to per-
form this function. The result is a design of a low-cost,
portable Ionospheric Scintillation Monitor (ISM) being
developed on a follow-on SBlR Phase II project.

Ionospheric Scintillation Monitoring Using Commercial Single Frequency C/A Code Receivers

. With the recent increase in the satellite-based navigation applications, the ionospheric total electron content (TEC) and the L-band scintillation measurements have gained significant importance. In this paper we present the temporal and spatial variations in TEC derived from the simultaneous and continuous measurements made, for the first time, using the Indian GPS network of 18 receivers located from the equator to the northern crest of the equatorial ionization anomaly (EIA) region and beyond, covering a geomagnetic latitude range of 1° S to 24° N, using a 16-month period of data for the low sunspot activity (LSSA) years of March 2004 to June 2005. The diurnal variation in TEC at the EIA region shows its steep increase and reaches its maximum value between 13:00 and 16:00 LT, while at the equator the peak is broad and occurs around 16:00 LT. A short-lived day minimum occurs between 05:00 to 06:00 LT at all the stations from the equator to the EIA crest region. Beyond the crest region the day maximum values decrease with the increase in latitude, while the day minimum in TEC is flat during most of the nighttime hours, i.e. from 22:00 to 06:00 LT, a feature similar to that observed in the mid-latitudes. Further, the diurnal variation in TEC show a minimum to maximum variation of about 5 to 50 TEC units, respectively, at the equator and about 5 to 90 TEC units at the EIA crest region, which correspond to range delay variations of about 1 to 8 m at the equator to about 1 to 15 m at the crest region, at the GPS L1 frequency of 1.575 GHz. The day-to-day variability is also significant at all the stations, particularly during the daytime hours, with maximum variations at the EIA crest regions. Further, similar variations are also noticed in the corresponding equatorial electrojet (EEJ) strength, which is known to be one of the major contributors for the observed day-to-day variability in TEC. The seasonal variation in TEC maximizes during the equinox months followed by winter and is minimum during the summer months, a feature similar to that observed in the integrated equatorial electrojet (IEEJ) strength for the corresponding seasons. In the Indian sector, the EIA crest is found to occur in the latitude zone of 15° to 25° N geographic latitudes (5° to 15° N geomagnetic latitudes). The EIA also maximizes during equinoxes followed by winter and is not significant in the summer months in the LSSA period, 2004–2005. These studies also reveal that both the location of the EIA crest and its peak value in TEC are linearly related to the IEEJ strength and increase with the increase in IEEJ.

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Temporal and spatial variations in TEC using simultaneous measurements from the Indian GPS network of receivers during the low solar activity period of 2004-2005

Historically, conventional GPS receivers have used 1.0 chip early-late correlator spacing in the implementation of delay lock loop s (DLLs), However, there are distinct advantages to narrowing this spacing, especially in C/A-code tracking applications. These advantages are the reduction of tracking errors in the presence of both noise and multipath. The primary disadvantage i s that a wider precorrelation bandwidth is required, coupled with higher sample rates and higher digital signal processing rates. However, with current CMOS technology, this is easily achievable and well worth the price. Noise reduction is achieved with narrower spacing because the noise components of the early and late signals are correlated and ten d to cancel, provided that early and late processing are simultaneous (not dithered). Multipath effects are reduced because the DLL discriminator is less distorted by the delayed multipath signal. This paper presents the derivation of these narrow correlator spacing improvements, verified by simulated and tested performance.

Theory and Performance of Narrow Correlator Spacing in a GPS Receiver

Commercial Ionospheric Scintillation Monitoring Receiver Development and Test Results

The availability and continuity of accuracy and integrity are major concerns for GPS-based precision approach and landing systems without augmentation. Thus, following the lead of the FAA baseline Local Area Augmentation System (LAAS) architecture that includes pseudolites, RTCA Special Committee 159 Working Group 4a for GPS/Precision Landing Guidance has set up an Airport Pseudolite (APL) subgroup. The purpose of this subgroup is to develop LAAS pseudolite standards. This subgroup is responsible for trade studies necessary for the development of an APL signal structure. This development is well underway, starting with a set of basic assumptions to be considered in that development. Minimum performance criteria for a viable pseudolite signal structure are also being established. This paper presents these basic assumptions to be considered and the resulting minimum performance criteria for the APL signal structure. Then, the details of the proposed APL signal specification currently under review that meets those criteria are described. Also, the analytical results that justify the proposal against the defined considerations and minimum performance requirements are presented.

Proposed Airport Pseudolite Signal Specification for GPS Precision Approach Local Area Augmentation Systems

NovAtel, in conjunction with Stanford Telecom, have developed a prototype positioning system that combines multiple GPS and Pseudolite signals in a pseudo range/carrier based solution. The system is designed to give supplementary coverage during times of reduced GPS availability, to give better position accuracy during periods of poor geometry, to aid in ambiguity resolution enabling rapid centimeter level positioning, and to provide receiving remote stations with differential data collected at base stations. This paper describes the prototype system, including the components, the integration and theoretical and realized performance. Unique problems occur when using signals from pseudolites. These manifest themselves as both signal processing and as filtering problems. Transmitted multipath is a characteristic of pseudolite signals which does not occur, at least to the same extent, in GPS signals. The effects of this condition on both signal processing (tracking) and filtering algorithms are described and possible remedies are suggested. Furthermore, the geometry associated with pseudolite transmitters is significantly different than that expected from a set of GPS orbital transmitters. The effects of this geometrical difference, and the approach used to incorporate pseudolite measurements in a floating ambiguity position filter are described. Finally, simulation and real world test results from various manifestations of the system are presented along with descriptive analysis of the results generated in the different scenarios.

HAPPI - a High Accuracy Pseudolite/GPS Positioning Integration

The synergy between GPS and inertial navigation has been 
well known in the industry since the inception of GPS. In 
theory, the continuity of the inertial system can both fill in 
positioning gaps left by GPS satellite outages and reduce 
the effect of high frequency GPS errors, while the 
unbiased nature of the GPS signals can limit the size of the 
low frequency errors in the inertial system. NovAtel Inc. 
embarked on a development process more than 2 years 
ago in order to build a prototype GPS/INS integrated 
system designed to take advantage of the complementary 
nature of the two systems. 
The components of the prototype system were a 
Honeywell HG1700 IMU and a NovAtel Inc. OEM4 GPS 
receiver. The objective of the development was to provide 
a tightly integrated system at reasonable cost which could 
give positioning continuously at a 10 cm level provided 
the GPS signal outages were of short duration. The 
approach taken during the system development was to take 
advantage of the existing GPS navigation algorithms and 
supplement these with a set of inertial algorithms and to 
use these in a decentralised process that could run on the 
target processor on the OEM4 board. The result of this 
development is a modular system which fulfils the 
accuracy requirements noted and resides entirely on the 
NovAtel Inc. OEM4 receiver. 
A decentralised filter has both advantages and 
disadvantages compared to a centralised filter. The 
advantages of simplicity, modularity, component size and 
independence, are offset to a certain extent by the 
requirement that at least 4 satellite observations are needed 
to generate a GPS solution that can be used to control the 
inertial errors. Another drawback of the decentralised 
approach is related to eliminating the use of data with poor 
integrity. In addition, a decentralised filter does not 
incorporate integer ambiguity states with the states related 
specifically to the inertial system, so another method of 
aiding the ambiguity resolution process needed to be 
devised. Solutions to these problems are described. 
In an inertial system, the measurements of the system are 
used to provide the coefficients of the differential equation 
set which describes the dynamics of the system errors. 
Noise on these measurements tricks the Kalman filter into 
a condition of 'false observability', in which theoretically
unobservable states experience incorrect reductions in 
standard deviation. This condition was investigated and a 
solution to this problem was implemented. 
In this paper, the authors propose to describe the system 
architecture, the particulars of the system development 
noted above and to provide test results which 
demonstrates the system performance in various 
environments.

Pat Fenton

Papers

Theory and Performance of Narrow Correlator Spacing in a GPS Receiver

Commercial Ionospheric Scintillation Monitoring Receiver Development and Test Results

Proposed Airport Pseudolite Signal Specification for GPS Precision Approach Local Area Augmentation Systems

HAPPI - a High Accuracy Pseudolite/GPS Positioning Integration

OEM4 Inertial: A Tightly Integrated Decentralised Inertial/GPS Navigation System