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The Stretched-FrOnt-Leg (SFOL) pulse is a high-accuracy distance measuring equipment (DME) pulse developed to support alternative positioning and navigation for aircraft during global navigation satellite system outages. To facilitate the use of the SFOL pulse, it is best to use legacy DMEs that are already deployed to transmit the SFOL pulse, rather than the current Gaussian pulse, through software changes only. When attempting to transmit the SFOL pulse in legacy DMEs, the greatest challenge is the pulse shape distortion caused by the pulse-shaping circuits and power amplifiers in the transmission unit such that the original SFOL pulse shape is no longer preserved. This letter proposes an inverse-learning-based DME digital predistortion method and presents successfully transmitted SFOL pulses from a testbed based on a commercial legacy DME that was designed to transmit Gaussian pulses., Submitted to IEEE Transactions on Aerospace and Electronic Systems

The global navigation satellite system (GNSS) real-time kinematic (RTK) technique is used to achieve relative positioning centimeter levels among multiple agents on the move. A typical GNSS RTK estimates the relative positions of multiple rover receivers with respect to a single-base receiver. In a fleet of rover GNSS receivers, this approach is inefficient because each rover receiver only uses GNSS measurements of its own and those sent from a single-base receiver. In this study, we propose a novel GNSS RTK framework that facilitates the precise positioning of a swarm of moving vehicles through the GNSS measurements of multiple receivers and broadcasts fixed-integer ambiguities of GNSS carrier phases. The proposed framework not only provides efficient RTK positioning but also reliable performance with a limited number of GNSS satellites in view. Our experimental flight tests with six GNSS receivers showed that the systematic procedure of the proposed framework could maintain lower than 6 cm of 3D RMS positioning errors, whereas the conventional RTK failed to resolve the correct integer ambiguities of double difference carrier phase measurements more than 13% in five out of nine total baselines.

Position and time are some of the most vital pieces of information for convenience, security, and safety in our daily lives and have enabled innovative advancements in many science and engineering fields. Since Global Positioning System (GPS) became available to the public, GPS has been applied not only to smartphones and vehicular navigation systems that we use every day, but also to diverse areas such as Internet-of-Things (IOT), synchronization, cellular communications, military, geodesy, agriculture, mining, construction, remote sensing and more. However, the legacy GPS has only been successful in outdoor and open-sky applications, and the demand for positioning and navigation in indoor and urban street environments has been rapidly growing. A number of countries are carrying out their own plans to build advanced satellite-based navigation systems, called global navigation satellite systems (GNSS), with initial operation capability planned around 2020. GNSS will provide a number of new wideband signals with encoded data at multiple frequencies for the enhancement of accuracy, robustness, and signal availability in the GPS-denied environments, such as dense urban streets and indoor environments, as well as in the presence of cyber-attacks. On the other hand, there have been research activities around the world to develop indoor positioning and navigation technologies in diverse directions. One conventional approach is to use radio signals from wireless infrastructure such as WiFi, UWB, and LTE (i.e., cellular signals), and the other is to utilize non-radio signal measurements such as camera-vision, in-building magnetic anomaly, and inertial measurements.

The current autonomous navigation of unmanned aircraft systems (UAS) heavily depends on Global Navigation Satellite Systems (GNSS). However, in challenging environments, such as deep urban areas, GNSS signals can be easily interrupted, so that UAS may lose navigation capability at any instant. For urban positioning and navigation, Long Term Evolution (LTE) has been considered a promising signal of opportunity due to its dense network in urban areas, and there has recently been great advancement in LTE positioning technology. However, the current LTE positioning accuracy is found to be insufficient for safe UAS navigation in deep urban areas. This paper evaluates the positioning performance of the current network of LTE base stations in a selected deep urban area and investigates the effectiveness of LTE augmentations using dedicated short range communication (DSRC) transceivers through the optimization of the ground LTE/DSRC network and cooperative positioning among UAS. The analysis results based on simulation using an urban canyon model and signal line of sight propagations show that the addition of four or five DSRC transceivers to the existing LTE base station network could provide better than 4&ndash, 6 m horizontal positioning accuracy (95%) in the selected urban canyon at a position of 150 ft above the ground, while a dense LTE network alone may result in a 15&ndash, 20 m horizontal positioning error. Additionally, the simulation results of cooperative positioning with inter-UAS ranging measurements in the DSRC augmented LTE network were shown to provide horizontal positioning accuracy better than 1 m in most flight space, assuming negligible time-synchronization errors in inter-UAS ranging measurements.

Most conventional path generation algorithms search for an optimal path that avoids collisions with obstacles under the constraint of platforms' kino-dynamics. These conventional algorithms usually assume that a user position and obstacle locations are accurately known at any point in navigation environments. However, in a positioning network, the accuracy of a position estimate varies depending on, e.g., the ranging accuracy, network geometry, multipath error, and signal blockages which may lead to unexpected situations, including collision and low efficiency path planning. Therefore, positioning accuracy must be considered in path generation to ensure a reliable navigation capability and collision avoidance. To consider positioning accuracy in path planning, the proposed method in this paper uses a mixture of potential and positioning risk fields that generates a hybrid directional flow to guide an unmanned vehicle (UV) in a safe and efficient path. The results of simulations showed that the proposed method generated successful paths for around 90% percent of the tested routes, while using only the potential field method failed for around 50%. To demonstrate the effectiveness of the proposed local hybrid path planning method, we perform an experiment using a small-size quadcopter, and the results are analyzed and discussed.

Although Unmanned Aircraft Systems (UAS) have become popular, the auto land of a fixed wing UAS has still remained as a significant challenge. This paper presents a UAS auto landing system based on a Ultra Wide Band (UWB) positioning network. The geometry of the UWB anchors in the network is optimized to provide precise positioning accuracy during the course of UAS landing. The notable performance characteristics of the system is that the positioning of the UAS becomes more accurate as it approaches to the runway more closely. At the point of a landing flare, the landing system is expected to provide vertical positioning accuracy better than 20 cm in 95 percent of time. The paper discusses the methodology of designing the proposed UWB positioning network architecture and the positioning performance analysis through simulations.

Autonomous navigation of most unmanned aircraft systems (UAS) is entirely dependent on GNSS. However, in deep urban area, the weak signal of Global Navigation Satellite Systems (GNSS) can be easily lost, thus UAS has no other reliable means of navigation. For this reason, autonomous navigation or surveillance like Unmanned Aircraft System Traffic Management (UTM) in deep urban area remains as a challenging problem. To tackle this problem, there have been great interests in positioning using signals of opportunity (SOP) including such as Long Term Evolution (LTE). More recently, a software defined radio for LTE positioning demonstrated a successful UAS flight test with a few LTE base stations.Although there are usually plentiful LTE base stations in urban area, they are primarily deployed to provide coverage to mobile communication users such that the base station network layout is subject to be suboptimal for positioning. And, it is not certain that SOP based on LTE could provide an appropriate positioning performance to safely navigate in deep urban area. This paper investigates the positioning performance of the current network of LTE base stations and the required augmentation of the SOP network to meet desired positioning accuracy in deep urban area of Seoul, South Korea. The paper also considers DSRC as another possible SOP ranging source and looks into the benefit of cooperative positioning with nearby UAS.

Potential Field is one of the widely used path planning methods in robotics. This method assumes that a robotic system knows its current position and where the goal is. The location of obstacles is also known in advance for global path planning, or it can be detected from sensing in local path planning. Although the potential field method provides a convenient and simple way of generating paths, most previous approaches do not incorporate uncertainty of a platform position while it is moving. Unlike satellite-based navigation systems, a user positioning accuracy may significantly change in a local area if a positioning system basis on Ultra Wideband (UWB) or LTE/5G is used. Because the positioning accuracy varies while a vehicle navigates, an unmanned vehicle or a robot following a given path may experience dithering movement. In addition, this problem may lead to unnecessary oscillation and increase the probability of collision which is a critical treat for a safe navigation. To overcome the drawbacks, this paper proposes the PF-DOP hybrid path planning algorithms, which is a variant of potential field that takes into account a user positioning accuracy. A user positioning accuracy (UPA) in an interested navigation area can be conveniently represented by Dilution of Precision (DOP) field. From the mixture of the potential and DOP fields, the proposed approach generates a hybrid directional flow that is able to guide an unmanned vehicle to the safer and more efficient paths. The hybrid directional flow is made by mixing the two fields with weighting factors which is also used in traditional potential field method as a coefficient. The weighting factors will vary depending on the user locations, but as the positioning accuracy can be derived from the user location, the user positioning accuracy is also associated. The paper will describe the systemic procedure in generating the PF-DOP hybrid path and how to avoid local minimum by using this algorithm.

The 1030/1090 MHz frequency band is allocated for air traffic surveillance, including Secondary Surveillance Radar (SSR), Traffic Collision Avoidance System (TCAS), and Automatic Dependent Surveillance-Broadcast (ADS-B) systems. Concerns have been raised that the 1030/1090 MHz frequency band will experience significant congestion to the point of inhibiting ADS-B and TCAS from meeting performance requirement minimums by 2035 in the United States. Even today, intermittent SSR transponder availability has occurred in some high-density air traffic areas. These occurrences will likely escalate as air traffic increases, resulting in the inability of air traffic control to accurately track aircraft. This paper presents three alternative methods of using variable SSR interrogation powers with respect to azimuth sectors and ADS-B aircraft tracking data to mitigate spectrum congestion. Simulation results showed that the presented methods could reduce the spectrum congestion up to 92 percent in terms of an SSR transponder occupancy time. In addition, the proposed methods lessened the required number of Mode-S interrogator identification codes about 50 percent in the simulation environment.

The SFOL pulse is a high accuracy DME/N pulse developed from using Genetic Algorithms. This pulse complies with the current DME/N specifications and is significantly less susceptible to multipaths than previous DME pulses. Recent analysis of the SFOL showed that the multipath-induced ranging error of the SFOL pulse was five times and two and half times less than the conventional Gaussian and Smoothed Concave Polygon pulses. To implement DMEs transmitting the SFOL, first, it would be desirable if modern DMEs currently deployed in the field can be modified to transmit the SFOL pulse without significant changes. It is expected that the biggest modification is required in the pulse shaping process. Currently, we are investigating a pre-distortion of the SFOL pulse for the typical power amplifier of a DME designed to transmit Gaussian pulses. A successful development of the pre-distortion may enable Gaussian pulse-based DMEs to transmit the SFOL pulse with relatively little changes. Second, the terrain in South Korea consists of numerous high mountains that would make it difficult to plan effective DME/DME networks due to the limited line of sights between DMEs and aircraft. Considering the airspace and major airport locations in South Korea, we are developing search algorithms that will result in cost effective DME/DME network layouts in the difficult terrain conditions. The paper will introduce a promising pre-distortion of the SFOL pulse and a strategy in planning of optimal DME/DME networks in South Korea with a recommended network layout.

All the current SBAS systems broadcast the augmentation message based on the L1-only SBAS MOPS (Minimum Operational Performance Standards), which is described in the RTCA DO-229 as an international standard. The L1 SBAS message was originally designed to support multi-GNSS constellations, but all the SBAS systems except for the SDCM of Russia provide GPS-only augmentation message. There are several reasons (i.e. a low data-rate) why most SBAS systems cannot support the multi-constellation GNSS. In order to overcome the limitations of the existing L1-only SBAS MOPS and to improve the performance of the current operating SBAS systems, studies on the L1/L5 DFMC (Dual-Frequency, Multi-Constellation) SBAS are actively ongoing. FAA (Federal Aviation Administration) in United States and the ESA (European Space Agency) have held the SBAS IWG (Interoperability Working Group) and have continued to discuss the development of the L1/L5 DFMC SBAS. However, even if the development of the L1/L5 DFMC SBAS is completed, many users with a L1 single frequency and multi-constellation GNSS receiver, which occupies most of the GNSS receiver market share, cannot use the L1/L5 DFMC SBAS. Therefore, it is necessary to study on a method to improve the performance of the L1-only SBAS. We propose to remove the fast corrections and to assign additional time slots for providing the long-term corrections of GLONASS and other constellations. As we had already proven at the ION ITM 2017, the PRC (Pseudo-Range Correction) in MT (Message Type) 2 to 5 needs not to be broadcast every 6 seconds, and it can be included to the satellite clock correction term in MT 25 after being converted into the unit of second. If the fast correction is not broadcast, half of the time slots are available and enough to send the long-term corrections for other constellations than GPS. And, we generated the long-term corrections for GLONASS satellites using GLONASS satellite precise ephemeris from the IAC (Information and Analysis Center). GLONASS long-term corrections were packed in MT 25 and these additional messages were scheduled in the empty time slots obtained by removing the fast correction. In this paper, we suggested to remove the fast correction and to modify the broadcasting timing and very few parameters of the current L1 SBAS message. We also showed that even this minor modification is working with the live SBAS message and effective to reduce the size of the protection level by supporting the multi-constellation GNSS, which will also contribute to improve the performance of integrity, availability and continuity.

Google announced that raw GNSS measurements are available to apps in the Android N operating system, which means we can get pseudoranges, Dopplers and carrier phase from a phone or tablet through various APIs. Several studies show that pseudorange observations can provide meter-level accuracies, and phase measurements can potentially allow for a precise positioning. The process of reducing the code measurement noise should importantly precede to figure out the smart phone’s position within 1 or 2 meter as well as to make a set of cycle ambiguity candidate for the RTK or other precise positioning. In this paper, we introduces a suitable filtering algorithm which can be used to reduce the noise level of the single-frequency pseudorange from the Android N API without a severe divergence due to the ionospheric variation. The most general and convenient filtering method to smooth the single-frequency GNSS pseudorange is Hatch filter. The Hatch filter is a code-smoothing technique that uses the variation of the carrier phase. It can effectively reduce the noise of a pseudorange with a very simple filter construction, but it occasionally causes an ionosphere-induced error for low-lying satellites. To find out the optimal smoothing constant for the conventional Hatch filter, various algorithms have been recently suggested. It is difficult to estimate the ionospheric-error by SF (Single-Frequency) measurement for a single epoch, thus recently proposed methods focus on bounding the divergence due to the ionospheric error, rather than on reducing it. To minimize the effect of the ionosphere-induced error, we propose an SF divergence-free Hatch filter that can compensate for the divergence due to ionospheric variation by the grid ionospheric vertical error (GIVE) from the SBAS message type (MT) 26, which can be easily obtained from typical low-cost single-frequency GNSS receivers. Even though we assume that the SBAS has modelled current ionospheric delay sufficiently, it is necessary to consider the residual error due to the coarse resolution of the ionospheric vertical delay, 0.125 m. Considering the quantization error, we derived the error equation of the SF divergence-free Hatch filter, and found out the optimal smoothing width as a function of pseudorange CNMP (Code Noise and Multi-Path) and Obliquity factor. To verify the suggested SF divergence-free Hatch filter algorithm and to evaluate its performance in the Android N measurement, we had smoothed the raw measurement from Nexus 9 at the Sejong university, S. Korea and prevented a severe ionospheric divergence using the MT 26 information from the multi-functional satellite augmentation system (MSAS) of Japan. To apply the suggested divergence-free Hatch filter to the smart phone, we made a CNMP model function of the elevation angle which can fit the real CNMP errors of the Nexus 9. For the range-domain analysis, we did the double difference between Android device and Reference station at Sejong university, and then compared its residual errors with those of an U-blox receiver. The RMS (root mean square) of the Nexus 9 pseudorange noise error has been reduced from 5m to 0.6m for all the satellites, while the typical Hatch filter cannot prevent the divergence so that the RMS has risen up to 1.0m for the satellite of the low-elevation angle. Considering that current GNSS chipset supports only single-frequency measurement and is expected not to include multi-frequency receiving function, the suggested divergence-free Hatch filter is very practical to improve the performance while maintaining the recursive and simple form of the conventional Hatch filter.

An ad hoc localization system (AHLoS) is necessary at a time or place where global satellite navigation systems are not available and a permanent infrastructure is not needed. Previous work on such systems has typically focused on 2D horizontal navigation, but new applications such as those for unmanned aerial systems (UASs) require a 3D navigation capability. In this paper, we present an AHLoS utilizing aerial nodes in the form of quadrotors or similar platforms in addition to fixed, ground-based nodes to provide a precise 3D positioning capability. The nodes are equipped with ultrawideband devices to be used as ranging sources. Our mathematical analysis and simulated results suggest that the 3D AHLoS is capable of providing high positioning accuracy, most notably, the vertical accuracy that is better than a couple of decimeters for 95% of the time, making it ideal for various UAS applications.

The Hatch filter is a code-smoothing technique that uses the variation of the carrier phase. It can effectively reduce the noise of a pseudorange with a very simple filter construction, but it occasionally causes an ionosphere-induced error for low-lying satellites. To find out the optimal smoothing constant for the conventional Hatch filter, various algorithms have been recently suggested. It is difficult to estimate the ionospheric-error by SF (Single-Frequency) measurement for a single epoch, thus recently proposed methods focus on bounding the divergence due to the ionospheric error, rather than on reducing it. To minimize the effect of the ionosphere-induced error, we propose an SF divergence-free Hatch filter that can compensate for the divergence due to ionospheric variation by the grid ionospheric vertical error (GIVE) from the SBAS message type (MT) 26, which can be easily obtained from typical low-cost single-frequency GNSS receivers. Even though we assume that the SBAS has modelled current ionospheric delay sufficiently, it is necessary to consider the residual error due to the coarse resolution of the ionospheric vertical delay, 0.125 m. Considering the quantization error, we derived the error equation of the SF divergence-free Hatch filter, and found out the optimal smoothing width as a function of pseudorange CNMP (Code Noise and Multi-Path) and Obliquity factor. To verify the suggested SF divergence-free Hatch filter algorithm and to evaluate its performance, we applied MSAS correction to the U-blox M8, which is located in Sejong Unversity, Seoul, S. Korea. 3hr position-domain analysis shows that the optimal SF divergence-free Hatch filter with a smoothing constant of 500 can reduce the RMS error of the conventional Hatch filter with a constant of 300 by 4 cm. 95% percentile error also decreased from 1.28m to 1.19m by enlarging the averaging constant from 300 to 500 for the SF divergence-free Hatch filter. Considering that SF receivers dominate the GNSS market and that most of these receivers include the SBAS function, the filter suggested in this paper is very practical to improve the performance of most receivers while maintaining the recursive and simple form of the conventional Hatch filter. It is also of great value in that it can make the DGPS performance of the low-cost SF receivers comparable to that of DF receivers.

The FAA's recently announced 2016 Performance Based Navigation (PBN) National Airspace System (NAS) Navigation Strategy states that it will retain and expand Distance Measuring Equipment (DME/N) infrastructure to help ensure continued Area Navigation (RNAV) service during Global Navigation Satellite Systems (GNSS) outages. The targeted navigation performance using DME/DME in this strategy is RNAV 2.0 in the En Route Domain without requiring Inertial Reference Unit (IRU) and RNAV 1.0 in some large Terminal Areas. It is expected that DME will still play an important role as an alternative RNAV navigation source beyond 2030. It is, therefore, desirable to determine if the performance of DME can be improved to enable PBN of higher navigation accuracy to better meet future air traffic needs. This paper investigates the benefit of a newly developed DME/N pulse using Genetic Algorithms (GA) that can provide much higher ranging accuracy than the conventional Gaussian or Smoothed Concave Polygon (SCP) pulse while meeting the pulse shape requirements in current DME specifications. The primary benefit analysis will compare the achievable DME/DME positioning accuracy with a current DME ground network and compare the DME ground network requirements needed to meet RNAV 0.3 and 92.6 m surveillance positioning accuracy using the conventional Gaussian and the GA-based pulses in selected areas in Conterminous US (CONUS) NAS.

The Hatch filter is a code-smoothing technique that uses the variation of the carrier phase. It can effectively reduce the noise of a pseudo-range with a very simple filter construction, but it occasionally causes an ionosphere-induced error for low-lying satellites. Herein, we propose an optimal single-frequency (SF) divergence-free Hatch filter that uses a satellite-based augmentation system (SBAS) message to reduce the ionospheric divergence and applies the optimal smoothing constant for its smoothing window width. According to the data-processing results, the overall performance of the proposed filter is comparable to that of the dual frequency (DF) divergence-free Hatch filter. Moreover, it can reduce the horizontal error of 57 cm to 37 cm and improve the vertical accuracy of the conventional Hatch filter by 25%. Considering that SF receivers dominate the global navigation satellite system (GNSS) market and that most of these receivers include the SBAS function, the filter suggested in this paper is of great value in that it can make the differential GPS (DGPS) performance of the low-cost SF receivers comparable to that of DF receivers.

The Federal Aviation Administration's augmentation to the global positioning system for civil aviation is called the wide area augmentation system. This paper introduces the wide area augmentation system-based flight inspection system. This system is better optimized than the current flight inspection systems in terms of accuracy, cost, and efficiency. Although they meet the Federal Aviation Administration's flight inspection system accuracy requirements, the current flight inspection systems are either higher cost or less efficient. The innovative use of the wide area augmentation system with a radar altimeter and a television positioning system without a requirement for an inertial navigator makes it possible for the wide area augmentation system-based flight inspection system to have low cost and high efficiency in flight inspection for instrument landing system calibration. The wide area augmentation system-based flight inspection system meets the Federal Aviation Administration's accuracy requirements for CAT III ILS calibration and also provides firm integrity algorithms. The performance of the wide area augmentation system-based flight inspection system is confirmed with tests using flight data taken in collaboration with the Federal Aviation Administration.

This paper presents a methodology to suppress Distance Measuring Equipment (DME or DME/N) multipath using supervised learning and tightened DME pulse waveform variation. The current DME specifications allow relatively large variation in the DME pulse waveform in both of the interrogator and transponder. The allowed variation in the waveform makes it difficult to discern if the received signal is direct or deformed from added multipath. Therefore, the range of the allowed variation in the pulse shape needs to be tightened and that would facilitate multipath rejection using various algorithms. This paper applies a supervised learning technique to develop a time of arrival error estimator that compensates for the multipath impacts on the DME range measurements. The learning process utilizes the training data consisting of the numerous deformed DME pulse waveforms and the corresponding time of arrival errors. Considerable noise is also included in the training data such that the estimator behaves robustly in various situations. The theory behind and performance of supervised learning on the DME multipath problem are presented in this paper.

The Distance Measuring Equipment (DME, DME/Normal) based navigation method known as DME/DME positioning has been proposed as one of the possible Alternative Position, Navigation, and Timing (APNT) services for aviation during the outage of the Global Navigation Satellite Systems. A DME interrogator measures the slant range to a DME transponder by means of the elapsed time in exchanging a pair of DME pulses. A typical type of a DME pulse being used in practice is a Gaussian pulse, and the achievable DME ranging accuracy is primarily determined by the pulse shape. This paper presents an alternative DME pulse waveform that is able to provide much higher range accuracy than the conventional Gaussian pulse. The alternative pulse waveform is compliant with the pulse shape requirements in the current DME specifications to maintain the compatibility with existing DME ground transponders and avionics. This developed alternative DME pulse also takes into account the spectral density characteristics so that it does not cause interference to adjacent channels even when it is transmitted at the same high power of 1,000 Watts as the traditional Gaussian pulse. This paper will discuss the design approaches of the alternative DME pulse shape and show the improvement of range accuracy and multipath mitigation compared to the traditional Gaussian pulse. In addition, the paper will discuss the potential constraints in implementing the alternative pulse in the existing transponders and avionics.

DME/DME and Passive Wide Area Multilateration (P-WAM) are two proposed FAA's Alternative Position, Navigation, and Timing (APNT) architectures that provide navigation and surveillance capability to National Air Space during a GNSS outage. Although Distance Measuring Equipment (DME) and Multilateration (MLAT) system operate in a very different way, they use the same frequency band and can share the same antenna, which allows for one system integrating DME and MLAT electronics. Similarly, Automatic Dependent Surveillance-Broadcast (ADS-B) can be integrated with DME and MLAT. This DME/MLAT/ADS-B integrated system in one cabinet can be used to simultaneously provide DME and MLAT service for navigation and ADS-B for surveillance. With this dual navigation service capability of the integrated system, it is possible to develop a hybrid APNT architecture enabling both of DME/DME and P-WAM) service as a back-up of GNSS. The important advantage of the hybrid architecture is that the impacts on the avionics of General Aviation users and commercial jet airliners would be minimal by requiring only software and additional input/output connections. This paper describes the methodology and results for the feasibility study of the hybrid architecture and investigates the required ground station layout of the hybrid architecture that meets the anticipated APNT navigation and surveillance performance requirements by evaluating selected areas of the continental U.S. The required ground network will be determined through an optimization process called Coverage Analysis Method [1, 2] and be compared with the respective required ground networks for DME/DME and P-WAM based architectures to evaluate the additional number of ground stations needed in the hybrid architecture. In addition, the capacity of the DME/MLAT/ADS-B integrated systems in the hybrid architecture will be evaluated.

An optimized DME/DME network is one of the Federal Aviation Administration's (FAA) proposed Alternative Position, Navigation, and Timing (APNT) architectures. In comparison to other FAA-proposed APNT architectures, namely DME pseudolite network and passive Wide-Area Multilateration, airline operators find DME/DME more attractive for navigation back-up, as this solution requires no change to avionics used by nearly all commercial aircraft, thus reducing equipage costs to private companies. It is also advantageous because the absolute DME range measurements likely require a lower number of stations than the other architectures, thereby minimizing the cost to the public of installing new infrastructure. However, the insufficient range accuracy of the traditional DME (DME/N) has caused hesitation in actively pursuing this solution. U.S. and ICAO DME range accuracy standards of 0.2 nm is insufficient to support RNAV/RNP 0.3 nm operations, the performance the FAA has defined as needed for APNT. However, these standards are based on antiquated DME designs and have failed to account for advancements in both aircraft and ground station radio designs and performance. Recent flight inspections of DME range determined that the accuracy of the current state-of-the-art DME (DME/N) ground transponder is much better than 0.2 nm using current-day DME avionics. This enhanced accuracy presents an opportunity to leverage this technology and the network of DME ground stations for APNT. The DMEs in most stations in the National Airspace System (NAS) have been in service for more than 20 years and are due for replacement. It is thought that replacing DME legacy radios with modern state-of-the-art DMEs could support RNAV/RNP 0.3 operations, as well as optimize coverage with minimal addition to the DME network. This supposition leads to the feasibility study of the proposed APNT optimized DME/DME network. Using the expected range accuracy of the state-of-the-art DMEs, this paper investigates the feasibility of this proposed APNT solution by answering the following two key questions. First, what would be the optimal DME/DME ground station network that enables RNAV/RNP 0.3 operation for navigation and surveillance? Second, will the DME/DME network have sufficient capacity to support high density air traffic such as the 2020 LA basin model? The paper provides preliminary research results, by evaluating selected areas of the continental United States. Recommendations for network operation are based on the sample areas.

The FAA currently uses its Automated Flight Inspection System (AFIS) to check the accuracy of Instrument Landing Systems (ILS) and other navaids. It is desirable to measure the deviations of the ILS to within 0.015 degree accuracy. Therefore, a flight inspection system requires a high level of accuracy in determining position which is computed by a Flight Inspection Truth System (FITS). The AFIS has a navigation grade inertial navigation system (INS), a barometric altimeter, a radar altimeter, GPS, and a TeleVision Positioning System (TVPS). The AFIS is self-contained in that it does not require any facilities on the ground. The primary sensor in the AFIS is the INS whose error characteristics are limited by drifts. These drifts are measured by using a TVPS and a radar altimeter, and then the measurements from each sensor are fused with proprietary filtering techniques to result in the best possible accuracy in position. The error characteristics of the navigation grade INS causes the position computed from the current AFIS to have errors at distances far from the runway that are larger than desirable. This paper discusses techniques that will improve the accuracy and allow for better efficiency in the flight inspection procedures. The FAA has recently commissioned the Wide Area Augmentation System (WAAS), which provides corrections to GPS through a network of 25 reference stations throughout the U.S. WAAS accuracy (95%) over the U.S. is better than 1 meter in horizontal and 2 meters in vertical. The current standalone WAAS does not meet the ILS calibration accuracy requirements by itself, but WAAS can be used as a complementary sensor to the INS and radar altimeter for the flight inspection system because the position error in WAAS for the duration of an approach is approximately constant. This research focuses on the developments of the enhanced flight inspection system aided by WAAS, which does not require significant changes in the current AFIS. It has been found that the fusion of WAAS, radar altimeter, and INS makes it possible to obtain the required position accuracy. Another advantage is that an inspecting aircraft no longer has to fly over both ends of the runway in order to calibrate the INS drifts. The TVPS may also be eliminated for the inspection of CAT1 or CAT2 ILS. Preliminary results using the data from flight tests assure the WAAS-aided flight inspection system provides drift-free accurate positions.