Frank van Graas

Design of an Electric Propulsion System for a Quadrotor Unmanned Aerial Vehicle

A quadrotor unmanned aerial vehicle has been developed at Ohio Universitys Avionics Engineering Center for use as a navigation sensor testbed. The quadrotor was designed with a 10 lb payload capacity, transforming what has typically been a lightweight airframe into a more robust platform. Specific design considerations included the characteristics of high-power brushless motors and electronic speed controllers, the variation of motor rotational losses with frequency, and the impact of heat dissipation within the battery packs. Simple feedback loops were sufficient to stabilize the platform. An accounting of the component efficiencies allowed for effective mission planning based on the desired payload. The quadrotor, with a demonstrated ability to lift up to 10.6 lb, provides a convenient way to flight-test new sensor technology.

Characterization of GNSS signal parameters under ionosphere scintillation conditions using software-based tracking algorithms

This paper analyzes the carrier tracking outputs of GPS L1 signals during ionosphere scintillations using a conventional PLL, a frequency-assisted PLL, and a Kalman filter based PLL. The data used in the analysis were collected by a radio frequency front end at Ascension Island in March 2001. Among the 8 GPS satellites in view during the 45 minutes data collection experiment, 6 of them experienced phase fluctuations while 7 of them showed signs of amplitude scintillation. The paper presents the detailed tracking algorithm implementations, tracking loop generated carrier to noise ratio and S4 index, detrended carrier phase measurements, and spectral analysis of both signal intensity and the detrended carrier phase for all SV in view during scintillations.

eDME architecture development and flight-test evaluation

Distance Measuring Equipment, DME, plays a crucial role in current and future aviation navigation. DME has good performance, wide-spread international coverage, decades of proven robustness, and dissimilar failure modes from satellite-based navigation systems. These characteristics reaffirm DME’s potential in current and future aviation Positioning Navigation and Timing (PNT). However, with the transition to Performance-Based Navigation (PBN) [1] the requirements for PNT become significantly more stringent. This warrants upgrades of the DME/N system to enhanced DME, or eDME. This paper presents various potential eDME architectures, proof-of-concept implementations, and flight-test results. Substantial DME performance enhancements can be obtained by evolutionary technology improvements, including increased number of transponders, more precise transponder timing and processing, increased transponder capacity, and modern interrogator technology. These improvements significantly enhance the DME system performance level beyond the currently specified 0.2 nmi total system ranging error [2]. An essential next step is the revision of the DME/N performance standards such that full credit can be taken for this improved performance. Further DME performance improvements can be obtained by more “revolutionary” advancements. The eDME concept is often associated with the addition of a UTC-synchronized “beat” signal broadcasted by the transponders [3], which will enable passive ranging (pseudoranging) and hence unlimited capacity, as well as the provision of time to the user. Data broadcast is generally also considered part of the eDME advancements. A dramatic performance improvement both in terms of accuracy and integrity can be achieved by DME carrier phase tracking [4]. Carrier phase tracking combined with pseudoranging enables a class of algorithms such as Pulse pseudorange Minus Carrier, Carrier Smoothed Pulse pseudoranging and Pulse-Noise-Multipath, which increases accuracy and simultaneously provides assurance for this performance. Robust, accurate, assured, and cost-effective time synchronization of a large number of DME transponder sites is not trivial. This paper presents DME-Next, an eDME architecture that does not require accurate time synchronization of the transponders. DME-Next is a combination of DME Carrier Phase, DME pseudoranging, and occasional two-way ranging that allows the receiver/interrogator to resolve the timing offsets between transponders. This proposed system dramatically improves the capacity of DME without the challenge of transponder time synchronization. Flight test results demonstrate the potential of this concept.

Time-frequency analysis of ionosphere scintillations observed by a GNSS receiver array

The objective of this paper is to present and compare performances of several time-frequency analysis algorithms that characterize and associate ionosphere scintillation events focusing on carrier phase measurements obtained from three relatively close spaced antennas in Alaska. The results will be used to derive plasma drift velocity and estimate the 2-dimension size of the scintillation patches. There are two challenges in detecting and associating scintillations among closely spaced receivers. First, we have to precisely estimate the start time and end time of the scintillation event captured by each receiver. Second, we have to distinguish the fluctuation caused by multipath and receiver clock from the effects of scintillation. Our approach is to analyze the time-frequency relations of GPS L1 signal observables among the receivers. Based on past experience, scintillation causes high frequency fluctuations in the range of 0.1 to 10 Hz, while multipath only introduce an extra frequency less than 1 Hz and are typically occurring at low elevations. Receiver clock errors are eliminated using differenced measurements from L1 and L2, or from a reference satellite without scintillation. Several algorithms have been studied to perform the time-frequency spectrum analysis of carrier phases: windowed Fourier transform (WFT), Morlet Wavelet transform (MWT), Hilbert-Huang transform (HHT), and an adaptive periodogram technique (APT). All four algorithms are evaluated using both simulated and real scintillation signals. Among them, APT provides the most optimal performance in terms of precision in detecting the occurrences of scintillation events in both time-domain and frequency-domain. However, APT also has a high computational cost. Morlet Wavelet transform offers complimentary performance with significantly lower computational cost. Thus, it is implemented as the level one scintillation filter before the events are finally analyzed by APT.

Camera models for the wright patterson air force base (WPAFB) 2009 wide-area motion imagery (WAMI) data set

This article provides an introduction to the Air Force Research Laboratory (AFRL) Wright Patterson Air Force Base (WPAFB) 2009 wide area image data set. The data set is useful for those doing research on projection, tracking, georegistration, and other image-related activities. The data set includes raw images, pose data, and projected images. For users that would like to project the data, a camera model and calibration are provided here. The camera model and calibration developed here provide a significant improvement over the projected images included with the data.

Field Programmable Gate Array-Based Attitude Stabilization

Abstract : A system for determining vehicle attitude using a field programmable gate array (FPGA) and low cost gyroscopes is presented. The method is intended to support the stabilization of a short duration, unmanned aerial vehicle. Using a microelectronmechanical system (MEMS) inertial sensor for the calibration and serial interface, the algorithm sidesteps concerns related to electronmagnetic interference and the impact of embedded, proprietary filters. An Allan variance analysis is used to characterize the sensor errors and predict system performance. A floating point representation using a direction cosine matrix is hosted on the FPGA alongside the platform stabilization feedback loops. Although prone to drifting without additional aiding, the derived attitude has been demonstrated to be effective in stabilizing a remotely piloted quadrotor.

Integration of a synthetic vision system with airborne laser range scanner-based terrain referenced navigation for precision approach guidance

Synthetic Vision Systems (SVS) provide pilots with a virtual visual depiction of the external environment. When using SVS for aircraft precision approach guidance systems accurate positioning relative to the runway with a high level of integrity is required. Precision approach guidance systems in use today require ground-based electronic navigation components with at least one installation at each airport, and in many cases multiple installations to service approaches to all qualifying runways. A terrain-referenced approach guidance system is envisioned to provide precision guidance to an aircraft without the use of ground-based electronic navigation components installed at the airport. This autonomy makes it a good candidate for integration with an SVS. At the Ohio University Avionics Engineering Center (AEC), work has been underway in the development of such a terrain referenced navigation system. When used in conjunction with an Inertial Measurement Unit (IMU) and a high accuracy/resolution terrain database, this terrain referenced navigation system can provide navigation and guidance information to the pilot on a SVS or conventional instruments. The terrain referenced navigation system, under development at AEC, operates on similar principles as other terrain navigation systems: a ground sensing sensor (in this case an airborne laser scanner) gathers range measurements to the terrain; this data is then matched in some fashion with an onboard terrain database to find the most likely position solution and used to update an inertial sensor-based navigator. AECs system design differs from todays common terrain navigators in its use of a high resolution terrain database (~1 meter post spacing) in conjunction with an airborne laser scanner which is capable of providing tens of thousands independent terrain elevation measurements per second with centimeter-level accuracies. When combined with data from an inertial navigator the high resolution terrain database and laser scanner system is capable of providing near meter-level horizontal and vertical position estimates. Furthermore, the system under development capitalizes on 1) The position and integrity benefits provided by the Wide Area Augmentation System (WAAS) to reduce the initial search space size and; 2) The availability of high accuracy/resolution databases. This paper presents results from flight tests where the terrain reference navigator is used to provide guidance cues for a precision approach.

Implications of C/A code cross correlation on GPS and GBAS

This paper presents a systematic discussion on GPS C/A code cross correlation and its impact on signal acquisition, tracking and Ground-Based Augmentation System (GBAS) performance. Three types of cross correlation effects are investigated: 1) between two satellites that have approximately the same Doppler frequency; 2) between two satellites that have an offset at an integer number of kHz in Doppler frequency; 3) between a C/A code signal and a signal that consists of alternating zeros and ones. The first type of cross correlation has been well studied in the past decade, and its impact is often found similar to that of multipath. There exist some subtle, but important, differences between cross correlation and multipath, which will be discussed in this paper. Cross correlation cannot be treated as a random interference source, since it is inherently constrained by the Doppler frequency difference between the two satellites. Based on this constraint, the cross correlation functions will be analytically modeled in the time domain and in the frequency domain for each of the three types. All three types of cross correlation are potential threats to weak signal acquisition, PseudoRange tracking and carrier phase tracking. There are impacts on both mobile users and ground reference stations. More specifically, the PseudoRange tracking error will likely not be common between a ground reference and a mobile user, which becomes a concern for differential systems like GBAS. The PseudoRange error is not only a function of the Doppler offset and signal strength, but is also dependent on the tracking loop configuration. For example, the relative motion between the satellites and the antenna, the tracking loop bandwidth, coherent integration time and the carrier smoothing time constant all play key roles in the PseudoRange error model. It has been discovered in previous studies that a sufficiently large time constant in carrier smoothing provides effective mitigation for cross correlation errors. Most GPS users are protected against cross correlation in signal acquisition and tracking. In some worst-case scenarios, however, meter-level PseudoRange errors in GBAS may occur. As a result, cross correlation must be carefully monitored for high-accuracy safety of life applications. It can also falsely trigger other GBAS monitors, which may include the low power monitor and the signal deformation monitor. An overview of the implications of cross correlation on GBAS is provided in this paper.

Impact of antenna group delay variations on protection levels

Requirements for single frequency airborne antenna group delays were developed after the completion of ground and airborne specifications for aircraft precision approach operations using the Global Positioning System (GPS). Airborne antenna group delays were not included in the aircraft position protection level calculations as they were assumed to be part of the allocation for aircraft pseudorange noise and multipath. Unfortunately, the dual-frequency antennas used for the characterization of aircraft pseudorange noise and multipath exhibit much smaller group delay variations as a function of signal arrival angle than the allowable errors for single frequency antennas. Therefore, antenna group delay variations must be treated as a new error source for high-accuracy aviation applications such as the Ground Based Augmentation System (GBAS), the Space Based Augmentation System (SBAS) and future Advanced Receiver Autonomous Integrity Monitoring (ARAIM) concepts. This paper provides an analysis of the impact of antenna group delay variations on aircraft position protection levels for single frequency users. In addition, it is shown that dual-frequency users experience similar group delay errors as single frequency users when an ionosphere-free pseudorange solution is used.

C/A code cross-correlation at a high doppler offset

C/A code cross-correlation affects signal acquisition and tracking in global positioning system (GPS) receivers. This paper investigates the cross-correlation-induced tracking errors, which can be substantial when the two Doppler frequencies are apart by an integer number of kilohertz. Carrier smoothing is an effective mitigation against the pseudorange errors, although the residual could still be significant if a short smoothing time constant is used.