Wouter Pelgrum

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.

A non-uniform DFT-based batch acquisition method for enhanced DME (eDME) Carrier Phase: Concept, simulations, and flight test results

This paper proposes a novel batch acquisition method for eDME Carrier Phase based on non-uniform DFT. The theoretical false acquisition, failed acquisition, and wrong acquisition upper bounds are derived analytically and validated by Monte Carlo simulations. Actual flight test data recordings suggest the existence of substantial RF interference. Two algorithms are proposed to mitigate this interference. The resulting acquisition performance shows zero false and wrong acquisition and a very acceptable failed acquisition rate for the entire flight test.

Using single-camera geometry to perform gyro-free navigation and attitude determination

This paper focuses on using an image-based optical sensor (e.g. a digital camera) to estimate relative position and attitude without the explicit dependence on Inertial measurement Unit (IMU) gyro measurements, thus, avoiding sensitivities to gyro drift errors. The method uses a priori known point features and is an alternative to existing resection methods used in photogrammetry that can be easily integrated with previously developed tight optical integration (TOI) method. The key principle involved in this approach is the application of geometric constraints derived from multiple point features and multiple views. The envisioned application of this method is cooperative urban navigation, for ground and airborne vehicles.

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.

L band ionosphere scintillation impact on GNSS receivers

Ionosphere scintillation is a natural interference encountered by RF signals propagating through the ionosphere. It can affect the performance of Global Navigation Satellite Systems (GNSS) signals and receivers. Since 2009, our research team has established several ionosphere scintillation monitoring and data collection system in Alaska, Singapore, and Hong Kong to collect both naturally occurring and artificially controlled L band scintillation data. As we enter the current solar maximum period, these data has provided us with a good opportunity to obtain statistical impact of high-latitude and equatorial scintillations on GNSS receivers.This paper presents the analysis results based on measurements obtained from a GNSS array in HAARP, AK and commercial receiver measurements from Singapore and Hong Kong. For the HAARP, AK setup, scintillation event triggers have been implemented to initialize RF front ends data recording systems during strong scintillations. A conservative event filter was created to allow us to extract all scintillation events with amplitude scintillation index S4 greater than 0.12 and phase standard deviation sigma phi greater than 6 degrees [3]. The low filter cutoff values are set to automatically flag both strong and weak scintillation events for further analysis. We are interested in both strong and weak scintillation because strong scintillation events have major impact on robustness of GNSS receiver operation, while the weak events are good indicators of ionosphere irregularities occurrence and plasma drift.

Total Electron Content measurements with uncertainty estimate

High-accuracy Total Electron Content (TEC) measurements are important for several applications, including GNSS time transfer with accuracies better than 10 ns. In addition, it is important to provide uncertainty estimates to algorithms that use the TEC measurements. Dual-frequency TEC errors are dominated by satellite inter-frequency biases, satellite inter-code biases, receiver inter-frequency biases, receiver noise and multipath, higher-order ionospheric errors, and receiver inter-code biases for certain receivers. To evaluate different algorithms and error mitigation methods, data were collected from four different GPS receivers in two locations at the Arecibo Observatory in Puerto Rico. Error mitigation methods include receiver hardware bias calibration, comparison with TEC maps provided by the Jet Propulsion Laboratory (JPL), receiver comparisons, TEC calibration during low TEC count as determined by the Arecibo radar, and multipath mitigation using the Code Noise Multi-Path (CNMP) algorithm [12]. The CNMP algorithm enables the definition of rigorous multipath error bounds on the TEC bias estimation.

H-field antenna considerations for eLoran aviation applications

The U.S. Department of Homeland Security has designated enhanced Loran (eLoran) as a backup for the Global Positioning System and will pursue development and delivery of eLoran services to aviation, maritime, and time and frequency users. Many of the improvements, much of the analysis, and the operational modifications necessary to transition the current Loran system to eLoran have been established in the process of modernizing Loran. However, new performance standards and approval test procedures are needed for certifying eLoran avionics. More specifically, for aviation applications, precipitation static (p-static) has historically been a significant interference effect for Loran when E-field antennas have been used and was the primary reason why Loran-C could not support the approach and landing phase of flight. However, research conducted over the last decade clearly indicates that by using an H-field antenna the effects of p-static for Loran applications can be mitigated. This work begins the specification process for an eLoran H-field antenna that may be suitable for a minimum operation performance specification (MOPS). This paper presents a number of considerations for the design, installation, and use of an H-field antenna in eLoran aviation applications and puts forward parameters for specification, proposed values, configurations, and test methodologies. The objective of this paper is to present various considerations and options for an eLoran H-field antenna performance specification to stimulate discussion within the government, industry, and academic position, navigation, and time communities and to receive feedback in the specification process.