Showing papers presented at "IEEE/ION Position, Location and Navigation Symposium in 2002"

Indoor GPS theory & implementation

Abstract: A Global Positioning System (GPS) that works in cell-phones, and indoors, is the key to consumer applications that will dramatically improve our quality of life. GPS applications will do this by increasing two priceless resources: our security and our time. In this paper we outline the key applications of GPS in cell-phones and similar wireless devices. Most of these applications require the GPS to work indoors, and we provide an outline of the theory of indoor GPS, showing that GPS can indeed be made to work in most places that cell-phones work. Indoor GPS, or more precisely high-sensitivity GPS, is a combination of assisted-GPS (A-GPS) and massive parallel correlation. This paper describes both these components, with examples of each, and provides test results showing the theory in practice.

Development and deployment of GPS wireless devices for E911 and location based services

Abstract: The Federal Communications Commission recognized as early as 1994 that the rapid adoption of wireless technology in the USA was destined to put additional strain on emergency dispatch and rescue personnel due to the absence of any location information associated with a wireless handset. Previously, the FCC had gone to considerable effort to ensure that all landline phones were associated with an unambiguous street address, so that rescue personnel could be dispatched efficiently during a 911 call. The FCC was compelled by their duties to establish a mandate for the development of "Automatic Location Identification" (ALI) technology by the wireless industry. This paper outlines a system that requires no changes to the wireless network and can provide a wide range of location capabilities, including precise location solutions with GPS. This system was designed for flexibility, and can be used for both commercial location-based services, as well as E911/PSAP solutions.

A direct RF sampling multifrequency GPS receiver

Abstract: The future satellite positioning/navigation systems (both GPS and Galileo) will provide civil signals on multiple frequencies, similar to that currently available only for military use. The multiple distinct frequencies will provide many advantages to users of the navigation systems. This paper presents a direct RF sampling front end design well suited for multiple frequency satellite navigation receiver design. No frequency down conversion is necessary, rather the particular frequency bands of interest are intentionally aliased using a wide band ADC. The resulting samples are passed, via a doubling buffering FPGA design, to the memory space of a host PC for storage as well as eventually processing of the multiple frequency transmissions. This paper describes the design of the front-end, validates its concept with collected data, and discusses the variations on the design of a generic multiple frequency GPS front end.

A new bit synchronization method for a GPS receiver

Abstract: We develop a maximum-likelihood bit synchronization algorithm. This algorithm computes an estimate of the bit energy for all possible bit boundary candidates and chooses that candidate which maximizes the energy. Means to estimate whether the received signal contains enough bit value changes are also presented. The effect of frequency error is analyzed. It is shown that small frequency errors can be tolerated. Furthermore, a way to deal with larger errors is developed. Computer simulations show that the new algorithm is efficient in handling a very weak received signal.

A network aided iterated LS method for GPS positioning and time recovery without navigation message decoding

Abstract: The paper suggests a network aided iterated least squares (LS) method for GPS positioning and time recovery when a navigation message from satellite vehicles (SV) is not available. Using an additional variable which defines the satellite positions, the estimates of time of transmissions and pseudoranges are obtained from measurements available from the tracking side of the receiver. LS iterations are modified to account for adjustment of SV positions on orbits, adjustment of pseudoranges, etc. As a result, the positioning is performed without time-of-week (TOW) information from the navigation message.

An integrated MEMS inertial measurement unit

Abstract: With MEMS technologies, instrument subcomponents are integrated in the design to form individual gyroscopes and accelerometers on a single chip. This replaces the assembly of individual subcomponents which forms conventional instruments. Similarly, MEMS instrument components can be integrated to form complete inertial measurement units (IMUs). Integration is the only practical approach to realizing low cost and small size units, largely because both instrument and IMU assembly and instrument alignment tasks are eliminated. The obstacle to success is the lack of fabrication process maturity which limits the yield of satisfactory devices which, in turn, very severely limits the yield and escalates the cost of systems that require six working devices on the same chip. The solution to this problem is to base the IMU on instruments with a common design so that the fabrication process variations produce similar effects on each of the instruments. The IMU performance can be optimized by exploiting the commonality between instruments.

An AutoPC for supporting in-vehicle navigation and location-based multimedia services

Abstract: With the rapid progress in the development of wireless technology, the bandwidth of wireless local area network (WLAN) is getting higher and its outdoor transmission distance is also getting longer. It becomes feasible to build a regional communication infrastructure in local areas, e.g., in the campus of an institute or a university, using WLAN. Thus, the design of new generation on-board PCs aims not only to provide conventional navigation services in rural or urban areas, but also to deploy integrated multimedia services in regional areas such as campuses. In this paper we present our design of in-vehicle navigation and location-based multimedia applications in an 802.11b wireless environment. It supports mobile communication based on IETF mobile IP standards with route optimization, smooth handoff, and fast handoff. Furthermore, in order to encourage fast deployment and to reduce system costs, we utilized open technologies in designing our prototype system. The developed AutoPC is a real-time embedded system platform on a single-board PC with GPS, featured by a downsized embedded Linux and a bilingual windows system. The platform supports applications such as in-vehicle navigation, real-time traffic information, MP3 player, and MPEG-4 streaming through mobile IPv4. This prototype is evaluated on a campus-wide 802.11b network, where two neighboring access points are placed about 150 meters apart from each other along the roadside such that a road in the campus is fully covered by 802.11b radio signals. Our preliminary experiments show that the low handoff overhead makes it possible for an in-vehicle AutoPC to run video streaming application seamlessly on the road even when the car speed is up to 50 km/hr. Besides, a 486-level platform is powerful enough to support in-vehicle navigation and to play MP3 smoothly, but a more powerful CPU is required to run our RTP-based MPEG-4 video streaming applications.

Low cost, high G, micro electro-mechanical systems (MEMS), inertial measurements unit (IMU) program

Abstract: Precision guided munitions (PGMs) are critical to the Maneuver Battalion Commander's success on future rapidly deployed battlefields. These PGMs will provide an increase in lethality, significantly improve survivability, reduce collateral damage, minimize non-combatant casualties, and decrease the logistics burden or increase the number of kills from the standard logistics load. PGMs all require inertial measurement sensors embodied in an IMU. Until now, they have been large, expensive, and could not readily survive the high G launch environments. MEMS technology provides inherent cost savings and size reduction and is ideal for application to navigation and control systems for small missiles and munitions. Using MEMS technology, gyroscopes, accelerometers and control electronics can readily be integrated to form a tightly packaged, low-cost, extremely small, high performance IMU suitable for munition, missile, and personnel guidance as well as other applications. This paper describes the current Army development underway and its partnership with industry to achieve the necessary requirements. The program goal is to survive a 20,000 g gun launch and meet a 1/spl deg//hr, 1/2 milli-g performance capability. The MEMS IMUs will significantly reduce the cost of precision-delivered missiles and munitions, as well as expand precision delivery capability to artillery ammunitions, and thus reduce the number of required rounds by more than 30% while meeting roughly 90% of the tactical weapons fleet navigational requirements.

A submarine navigator for the 21/sup st/ century

Abstract: Recent advances in technology on multiple fronts will enable new and improved approaches to precision navigation of submarines. Improvements in interferometric fiber optic gyro (IFOG) technology have made it possible to use IFOGs in a high accuracy navigator. The use of spherical gimbals for improved temperature control and inertial instrument error autocompensation has been made practical by computer-controlled machining. The use of sonar in certain covert situations will become more prevalent, facilitated by lower power sonar systems and advances in signal processing technology. Satellite based altimetry will make it possible to compensate for gravity deflections using lookup tables. Satellite altimetry will also allow us to make tables of gravity anomaly and ocean depth available to the overall navigation process. Using these advances, it is possible to use measured gravity anomaly from the ship's IMU and sonar soundings from its sonar array to estimate and correct navigation position errors. A navigator is currently being developed at Boeing that will incorporate these new developments as well as the traditional velocity log, pressure depth meter, and Global Positioning System aiding. The new inertial navigator outputs, as well as the set of currently available navigational aid outputs, are combined in a series of filters that allow for the detection and removal of transient errors to furnish the best available position, velocity and attitude to the ship's systems.

Integrated MEMS/GPS navigation systems

Abstract: This paper describes the development of integrated MEMS-based inertial sensor/GPS navigation systems by BAE Systems, Plymouth, UK. Underlying MEMS capability is outlined, and the significance of this technology as a component in low-cost, miniature, gun-hard navigation systems is described. The importance of concurrent MEMS developments driven by the commercial marketplace is also addressed. A series of integrated inertial sensor/GPS systems are described. The progression from first-generation, loosely-coupled systems, to higher performance, closely-coupled systems is outlined. A route-map for eventual progression to deeply-integrated, high anti-jam systems is also described. A modular and flexible Kalman filter employed in the integrated systems is described. The potential for exploiting this flexibility by employing alternative aiding sources, such as terrain referenced navigation (TRN), is also explored.

The application and future development of a MEMS SiVS/spl reg/ for commercial and military inertial products

Abstract: BAE Systems has been developing Coriolis vibratory gyros for over 15 years. The latest gyro to emerge from this development uses a silicon MEMS sensor, SiVSG/spl reg/. This is an evolutionary development from earlier ceramic cylinder and metal ring VSGs, produced by BAE Systems. SiVSG/spl reg/ is being mass-produced for the commercial and automotive rate sensor market. SiVSG/spl reg/'s immunity to vibration and shock has enabled its application in advanced braking systems on automobiles. BAE Systems has now leveraged the high volumes from the automotive and commercial market to introduce next generation, low cost gyro products into the military and aerospace sectors. BAE Systems is using these SiVSG/spl reg/ as the basis of a family of single-axis rate sensors (SiVSG/spl reg/) and Inertial Measurement Units (SiIMU/spl trade/) that are aimed at defense and aerospace applications. The robust structure of the gyro has allowed it to survive and operate after a 23,000g gun-launch shock. Variants of SiJMU/spl trade/ have been selected for a number of guided missile programs such as the UK Vertical Launch Seawolf Missile system and the US Navy Extended Range Guided Munition (ERGM). This paper describes the performance of SiVSG/spl reg/ technology as applied to SiIMU/spl trade/. Future technology and product development strategy is outlined. The development of a digital gyro drive circuit is described, and its implementation in both a rate integrating gyro (SiRIG/spl trade/) and a variable rate range gyro (SiVRG/spl reg/) for high roll rate platforms. Current and projected performance of SiIMU/spl trade/ is presented, together with the strategy for integration with GPS.

Using the information from reference station networks: a novel approach conforming to RTCM V2.3 and future V3.0

Abstract: In applications requiring centimeter accuracy, single baseline positioning methods are being superseded by those methods using information from permanent reference station networks. The well-known advantages afforded by reference station arrays include improved modeling of the remaining tropospheric, ionospheric and orbit biases. Methods and concepts show the improvements in performance and reliability in some closed system approaches. Standardization discussions underway within RTCM target the interoperability between reference station systems and roving receivers from various manufacturers. One obstacle in the discussion, and therefore in later interoperability issues, is the creation and proper description of the models used for deriving the biases noted above. This difficulty has to be mitigated and will vanish with time, but this interoperability is needed urgently. This paper details a different approach to utilize and distribute the information from permanent reference station networks in RTCM Version 2.3 compatible message types. Throughput calculations show their efficiency.

Evaluation of adaptive space-time-polarization cancellation of broadband interference

Abstract: It is demonstrated that an array of N dual-polarized antennas can cancel up to 2N-1 broadband interferers, while still maintaining a high probability of GPS satellite availability.

A merging of system technologies: all-accelerometer inertial navigation and gravity gradiometry

Abstract: Significant research has been conducted in recent decades on developing a practical all-accelerometer inertial navigation system and on developing a practical moving-base gravity gradiometer. The former strives to measure kinematic motion in the presence of unwanted variations in gravity; the latter endeavors to measure variations in gravity in the presence of unwanted motion. Thus, goes the adage, one person's signal is another person's noise. In this paper, a mathematical relationship is derived that links the two concepts together. It is demonstrated that a complete solution to real-time navigation and gravity gradient determination can be performed simultaneously and unambiguously using all-accelerometer inertial measurements only. Although the separation of gravity from kinematic motion appears to violate Einstein's principle of equivalence, this is not the case. Nonetheless, accelerometer technology is not yet of sufficient maturity to allow a practical implementation of this concept, but advances certainly are being made that could result in a practical implementation perhaps within a decade.

Design and implementation of MIMU/GPS integrated navigation systems

Abstract: With recent research, we outline the framework of a MIMU/GPS integrated navigation system with Micro-programming Controlled Direct Memory Access (MCDMA) technique and decoupled state and bias estimation applied in. The micro-miniature inertial measurement unit (MIMU) sensor module in this system is composed of three QRS14 gyroscopes and three 3140 accelerometers which are mounted on the three orthogonal basal planes of a hexahedron. Two sets of GPS receivers are served in the GPS module so as to calculate the azimuth angle during the system initial alignment. The integrated system has both advantages of loose and tight coupling. For less computation, the decoupled state and bias estimation is applied. The constant but unknown biases of inertial components are estimated accordingly with successful simulation results.

Field demonstration of a Mars navigation system utilizing GPS pseudolite transceivers

Abstract: Tasks envisioned for future generation Mars rovers-sample collection, area survey, resource mining, habitat construction, etc.-will require enhanced navigational capabilities over the Mars Sojourner rover. Many of these tasks will involve cooperative efforts by multiple rovers and other agents, adding further requirements both for accuracy and commonality between users. This paper presents a new navigation system (a self-calibrating pseudolite array) that can provide centimeter-level, drift-free localization to multiple rovers within a local area by utilizing GPS-based transceivers deployed in a ground-based array. Such a system of localized beacons can replace or augment a system based on orbiting satellite transmitters, and is capable of fully autonomous operations and calibration. This paper describes the prototype SCPA that has been developed at Stanford to demonstrate these capabilities and then presents new results from the most recent set of field trials performed at NASA Ames Research Center. These experiments, which utilize the K9 Mars rover research platform, validate both the navigation and self-calibration capabilities of the system.

An adaptive information fusion method to vehicle integrated navigation

Abstract: GPS can not be used continuously for city vehicle navigation due to the loss of satellite signals by obstructions from building, trees etc. Therefore, a kind of integrated GPS/dead reckoning (DR) system of continuous positioning is developed. The operational principle of this integrated vehicle navigation system is given, then calibration of each and switching between the GPS and DR in the system are discussed, and the application of federated Kalman filtering technology to the multisensor system is studied. Finally, we advance an adaptive information fusion method to achieve optimal calibration between the GPS and DR systems. Experimental results demonstrate the effectiveness of this integrated navigation system.

GPS tomography using the permanent network in Goteborg: simulations

Abstract: We will draw attention to a new technique for retrieval of 3D water vapor refractivity fields by using a small network of GPS receivers, referred to as GPS tomography. We present first results of the simulations of tomographic solutions using two different approaches and applied on a recently established network of GPS receivers in the area of Goteborg (Sweden). We study the network capabilities to resolve water vapor structures using grids with different vertical and horizontal resolutions. The sensitivity to resolve vertical structures and the impact of noise present in the data are also shown. The simulation results indicate the feasibility of the GPS tomographic method to be applied on the existing network.

International HARM precision navigation upgrade - a GPS/INS missile upgrade that improves effectiveness and minimizes friendly-fire accidents

Abstract: The High Speed Anti-radiation Missile (HARM) is an air to surface tactical missile designed to seek and destroy enemy radar-equipped air defense systems. Unfortunately, the HARM is "no respector of persons", and it has been known, most notably during the Gulf War, to attack "friendly" targets. The international HARM Upgrade Project is a tri-national missile technology project sponsored by the United States, Italian, and German governments. The HARM Precision Navigation Upgrade (PNU) program has as its goal, the development and installation of a PNU into the HARM that will improve the weapon's effectiveness, while nearly eliminating the likelihood of fratricide. The precision navigation system consists of a modem SAASM (Selective Availability Anti-Spoofing Module) based Global Positioning System (GPS) receiver (NavStrike/sup TM/, Rockwell Collins Government Systems), and an Inertial Measurement Unit (IMU), consisting of state-of-the-art Fiber Optic Gyros (FOG MFORS-1, Litef) and modem micro-machined accelerometers (B-290 Triad, Litef).

SAW filters for Global Positioning Satellite (GPS) receivers

Abstract: Interference, whether intentional (jamming) or unintentional, both in-band and out-of-band, can be a significant problem for receivers that operate in close proximity to high power transmitters. Out-of-band interference protection can be provided by high performance Radio Frequency (RF) filters used between the antenna and the receiver. These RF filters need to be low loss, have high out-of-band rejection and have nearly constant group delay to preserve the incoming signal characteristics. Well-designed superconducting filters can provide this level of performance and may also aid solutions for in-band interference, as the low loss of these devices allows their placement before the first Low-Noise Amplifier (LNA) in the system. We will summarize the results of experiments conducted with a set of electromagnetic HTS filters specially constructed for GPS frequency bands L1 and L2. We will also present recent data characterizing Lithium Niobate from room temperature to cryogenic temperatures. Although the temperature coefficient of delay of a Y-cut Z-propagating Lithium Niobate delay line is large at room temperature the coefficient of delay decreases as the temperature decreases until it becomes zero near 25K. Thus a combination of an ambient temperature quartz or Gallium Orthophosphate SAW bypass filter with either an HTS electromagnetic or an HTS Lithium Niobate SAW filter can provide linear phase filtering without impulse response ringing even in tactical applications.

Proposed IEEE Coriolis Vibratory Gyro standard and other inertial sensor standards

Abstract: A new Coriolis Vibratory Gyro (CVG) Specification Format Guide and Test Procedure is being developed by the IEEE/AESS Gyro and Accelerometer Panel for consideration by the IEEE Standards Association as IEEE Std. 1431. It provides guides for specifying CVG performance and gives test procedures for verifying compliance with requirements. It also describes the various types of CVG, ranging from vibrating shell macro-sized devices to low-cost silicon and quartz chip micro-sized devices. Model equations are given for the various CVG operating and readout modes: whole-angle, open-loop, force-rebalance, and ratiometric. Also discussed in this paper are the IEEE standards that have been published for single- and two-degree-of-freedom spinning wheel gyros, laser gyros, interferometric fiber optic gyros, angular accelerometers, linear accelerometers, accelerometer centrifuge testing, and inertial sensor terminology, and an inertial sensor test equipment and analysis document and an inertial systems terminology document that are under development.

Flight-test results of an integrated wideband-only Airport Pseudolite for the Category II/III Local Area Augmentation System

Abstract: This paper reports the results of a flight test that successfully demonstrated the integration of a wideband-only Airport Pseudolite (WBAPL) into a prototype Local Area Augmentation System (LAAS). An analysis of the error performance exhibited by the prototype LAAS, with and without the inclusion of the WBAPL in the system, is presented. The inclusion of the WBAPL is shown to not only increase the availability of the system, but also improve the error performance. The improvement in performance is seen to be most pronounced when a reduced GPS SV set is available, a state that would otherwise result in the unavailability of LAAS. This is the first demonstration of simultaneous increase in availability and accuracy from the inclusion of a WBAPL into a prototype LAAS.

Electro-optical ultra sensitive accelerometer

Abstract: We present a novel optical transducer concept in the initial stages of development that promises to be inexpensive, small, lightweight, highly sensitive and durable. The successful development of this sensor will result in an optical accelerometer with resolution under 1 /spl mu/g (1g=9.8 m/s/sup 2/), which is two to three orders of magnitude more sensitive than current state-of-the-art MEMS-based accelerometers. This accelerometer is also expected to have a wide dynamic range with a resolution under 1 /spl mu/g at 100 Hz and improved low frequency response over existing MEMS technologies. This will yield much improved velocity and acceleration aiding to GPS tracking loops under high dynamic conditions, permitting continued low bandwidth tracking, a concomitant mitigation of external noise, and an increased jamming immunity. Also, the successful development of this accelerometer may enable the use of Distributed Tactical Navigation Tools (DISTANT) IMUs where distributed ultra-sensitive accelerometers may replace one or more expensive gyroscopes in an integrated IMU system.

Thermal characteristics and thermal compensation of four frequency ring laser gyro

Abstract: This paper studies "warm-up" and controlled temperature ramp processes of a four frequency RLG and shows that temperature is the primary factor to bias the output of the gyro. The "warm-up" process can be divided into three sections corresponding to turn-on bias section (TBS), fluctuation bias section (FBS) and stabilization section (SS). For the controlled temperature ramp process, a similar phenomenon is obtained, plus a bias ramp. The experiment shows that frequency sum (SF) is approximately linear to temperature and can be calculated as the temperature strategic point. Bias can be compensated mostly by temperature, temperature gradient and temperature rate.

A combined GPS satellite/pseudolite system for Category III precision landing

Abstract: This paper presents the results of a computer simulation, which models a combined GPS, satellite/pseudolite navigation system for precision landing. The goal of this system is to meet the CAT III requirements exploiting a combined system of satellites/pseudolites under severe geometry, weather, and jamming conditions. The simulation includes the aircraft position/velocity estimation and probability of false alarm and misdetection utilizing the satellite and/or the pseudolite navigation system. Aircraft trajectory, GPS satellite constellation, pseudolite location, and the system noise input parameters form the system inputs. The system output is given in terms of the position/velocity estimation error vs. simulation time. Performance measure of the system integrity are also computed and presented. The overall system performance is the combination of the system navigation accuracy and integrity. It is well known that the GPS satellite navigation system lacks vertical information, which results in poor vertical accuracy. On the other hand a pseudolite navigation system provides very good vertical information thus improving the vertical accuracy which is crucial for CAT III precision landing. Both satellite and pseudolite systems are all weather navigation systems, which guarantees optimal performance under any weather conditions. The pseudolite system can perform navigation in the S band or bands different from the L band; therefore, enabling the aircraft landing under severe jamming conditions of the L band.

Microstrip antennas for GPS applications

Abstract: Design and testing of circularly polarized probe-fed single-layer microstrip patch antennas for GPS applications are presented. An-echoic chamber antenna range measurements on a variety of patch elements is discussed and tabulated showing the effects of element size, around plane size, symmetry, and proximity of scatterers (such as a battery). "Real world" performance of selected patch element and ground plane combinations are discussed and tabulated. These results are intended to compare the design of an element for its intended environment and its antenna range measurements to the performance of a complete system.

A highly flexible and scalable system for location determination of wireless devices

Abstract: Over the past several years numerous positioning technologies have been presented to the wireless industry for the location of devices in their network. The solutions have ranged from crude location capabilities that have little or no impact on the infrastructure of the wireless network to very precise location only after tremendous changes to the network. This paper will outline a system that requires no changes to the wireless network and can provide a wide range of location capabilities, including precise location solutions with GPS. This system was implemented to provide location-based service providers a platform upon which they can create solutions and deploy using existing wireless systems today.

Strategies for estimating tropospheric delays with GPS

Abstract: Until recently, the tropospheric zenith delay (TZD) was considered a nuisance parameter of the GPS observation model. However, as the requirements of the weather forecasting community have become better understood, the possible use of TZDs in weather forecasting has been recognized. There are a number of strategies available to process GPS data and produce TZDs. The complexity of some of these strategies can be significant and may even be prohibitive for non-GPS specialists. With suitable infrastructure and precise GPS products however, the complex strategies can be simplified for efficient production of TZDs. The Geodetic Survey Division (GSD) of Natural Resources Canada (NRCan) supports a GPS tracking station and communication infrastructure and computes GPS satellite products suitable for TZD estimation. This paper describes the GPS products, and different approaches, for TZD recovery and compares them in terms of accuracy, availability and operational merits.

Integer cycle ambiguity resolution under conditions of low satellite visibility

Abstract: GPS attitude determination systems use carrier phase measurements at three or more antennas to determine platform orientation. Since the antenna separation is usually more than half of the GPS carrier wavelength, the measurements contain an integer cycle ambiguity that must be resolved before attitude can be calculated. This paper will present a new algorithm to instantaneously and reliably resolve the integer cycle ambiguities under conditions of low satellite visibility using at least four antennas in a non-planar array. The new algorithm makes it possible to establish and maintain an attitude solution with as few as two satellites if clock differences are calibrated or three satellites if they are not. Analysis of antenna array geometry and algorithm performance using simulated data will also be presented.

WAAS performance in the 2001 Alaska Flight Trials of the high speed Loran data channel

Abstract: The wide area augmentation system (WAAS) enables the Global Positioning System (GPS) to provide the performance and integrity necessary for en route flight and many landing procedures. Currently, WAAS employs two geostationary satellites to provide coverage to the United States. However, operational WAAS will require a redundant broadcast of WAAS throughout the US. Additional geostationary satellites will be used to provide some redundancy. Loran can serve as a cost-effective additional broadcast channel for WAAS. While additional geostationary satellites are still necessary, Loran can further augment broadcast of the WAAS message particularly in terrain, urban canyons or high latitudes. Using Loran has some appealing features. First, Loran can be modified to attain the capacity to support the WAAS message. Second, it could also provide a reversionary positioning capability in case the GPS/WAAS signal is lost due to radio frequency interference (RFI). Loran can be particularly helpful in Alaska, where terrain combined with high latitudes could block the signal from the geostationary satellite that appears low in the Alaskan sky. This paper describes the flight and ground tests of the reception of WAAS from Loran and the geostationary satellite.