Boris Pervan

Civilian GPS: The Benefits of Three Frequencies

A third civil frequency at 1176.45 MHz will be added to the Global Positioning System (GPS). This new frequency will bring a number of benefits. The aviation user will be one of the prime beneficiaries because the new frequency is in a protected aviation band. Thus, the system will be more robust against interference and jamming.The carrier-phase differential user will also be a prime beneficiary as long as his application has a reasonably short baseline. It is this high accuracy use that is explored in some depth. The process of forming linear combinations of both the code and carrier-phase measurements is studied, and the benefits and problems are explained.

Worldwide Vertical Guidance of Aircraft Based on Modernized GPS and New Integrity Augmentations

In the 2020 time frame, the Global Positioning System (GPS) will be fully modernized, and other satellite navigation systems will be operational. With an additional layer of fault detection, these systems will provide vertical guidance worldwide. This capability will be born of three important technologies. First and foremost, avionics will receive signals on two frequencies: L1/E1 and L5/E5a. This frequency diversity will do much to obviate the impact of ionospheric storms that troubles aviation use of GPS today. Secondly, a multiplicity of data broadcasts will be available to convey integrity information from the ground to the airborne users. These will include the navigation satellites themselves, geostationary satellites, and possibly terrestrial transmitters. However, the most important change will be the most subtle. The fault monitoring burden will be split between the aircraft and the supporting ground systems in a new way relative to the fault-detection techniques used in 2008. This new integrity allocation and the associated architectures are the subject of this paper.

Paired overbounding and application to GPS augmentation

The relationship between range-domain and position-domain errors remains an open issue for GPS augmentation programs, such as the Federal Aviation Administrations Local Area Augmentation System (LAAS). This paper introduces a theorem that guarantees a conservative error bound (overbound) in the position domain given similarly conservative overbounds for broadcast pseudorange statistics. This paired overbound theorem requires that a cumulative distribution function (CDF) be constructed to bound both sides of the range-domain error distribution. The paired overbound theorem holds for arbitrary error distributions, even those that are non-zero mean, asymmetric or multimodal. Two applications of the paired overbound theorem to GPS augmentation are also discussed. First, the theorem is employed to construct an inflation factor for a non-zero mean Gaussian distribution; in the context of a simulation of worst-case satellite geometries for 10 locations in the United States and Europe, the required inflation factor for broadcast sigma is only 1.18, even for biases as large as 10 cm for each satellite. Second, the theorem is applied to bound a bimodal multipath model tightly; the result shaves more than 40% off the previously established inflation factor derived through a more overly conservative analysis.

Code-Carrier Divergence Monitoring for the GPS Local Area Augmentation System

Code-carrier smoothing is a commonly used method in Differential GPS (DGPS) systems to mitigate the effects of receiver noise and multipath. The FAA’s Local Area Augmentation System (LAAS) uses this technique to help provide the navigation performance needed for aircraft precision approach and landing. However, unless the reference and user smoothing filter implementations are carefully matched, divergence between the code and carrier ranging measurements will cause differential ranging errors. The FAA’s LAAS Ground Facility (LGF) reference station will implement a prescribed first-order Linear Time Invariant (LTI) filter. Yet flexibility must be provided to avionics manufacturers in their airborne filter implementations. While the LGF LTI filter is one possible means for airborne use, its relatively slow transient response (acceptable for a ground based receiver) is not ideal at the aircraft because of frequent filter resets following losses of low elevation satellite signals (caused by aircraft attitude motion). However, in the presence of a code-carrier divergence (CCD) anomaly at the GPS satellite, large divergence rates are theoretically possible, and therefore protection must be provided by the LGF through direct monitoring for such events. In response, this paper addresses the impact of the CCD threat to LAAS differential ranging error and defines an LGF monitor to ensure navigation integrity. Differential ranging errors resulting from unmatched filter designs and different ground/air filter start times are analyzed in detail, and the requirements for the LGF CCD monitor are derived. A CCD integrity monitor algorithm is then developed to directly estimate and detect anomalous divergence rates. The monitor algorithm is implemented and successfully tested using archived field data from the LAAS Test Prototype (LTP) at the William J. Hughes FAA Technical Center. Finally, the paper provides recommendations for initial monitor implementation and future work.

Parity space methods for autonomous fault detection and exclusion using GPS carrier phase

Kinematic carrier phase positioning provides navigation integrity. The sub-centimeter precision of carrier phase measurements can be used to leverage receiver autonomous integrity monitoring (RAIM) in the sense that extremely tight fault detection thresholds can be set on the least-squares residual (ensuring navigation integrity) without incurring high false alarm rates. In addition, the high precision of carrier phase, when compared with code phase, lowers the integrity risk associated with the fault identification process. This is true because carrier phase provides a much cleaner observation of the effect of a given failure on the residual. Thus, for the same improvement in navigation continuity (obtained from fault isolation), misidentification will be less likely. This paper is focused on the use of parity space methods to investigate the limits of high-integrity and high-continuity GPS performance. In this regard, prototype algorithms for receiver autonomous fault detection and exclusion were developed with the goal of maximizing navigation continuity subject to the constraint of maintaining high integrity (by repressing mis-identifications). Fault detection and exclusion performance was demonstrated through analysis and extensive simulation. In addition, the prototype algorithms were implemented in a real-time airborne kinematic positioning architecture and tested by deliberately inducing failures in the post-processing of raw flight data.

Paired overbounding for nonideal LAAS and WAAS error distributions

A significant challenge in fielding space-based and ground-based augmentation systems (SBAS and GBAS) for GPS involves the validation of navigation integrity, which requires the establishment of error bounds for aircraft position. This paper introduces a new approach to validating position-domain integrity by using two-sided envelopes for each ranging source. This paired-bounding approach allows for error distributions of arbitrary form and thus improves on earlier integrity validation approaches restricted to zero-mean, symmetric, and unimodal distributions

Autonomous fault detection and removal using GPS carrier phase

This paper is focused on the use of carrier phase measurements and parity space methodology to investigate the limits of high-integrity and high-continuity satellite-based navigation performance. In this regard, a new algorithm for receiver autonomous fault detection and removal is developed with the specific goal of attaining the high levels of integrity and continuity required for aircraft precision approach and landing applications. Fault detection and removal algorithm performance is demonstrated through analysis and simulation, and the results of tests using deliberately induced failures in raw night data are presented.

High integrity carrier phase navigation for future LAAS using multiple civilian GPS signals

In near future, there will be two more civilian GPS signals, at 1227.60 MHz and 1176.45 MHz, in addition to the present one at 1575.42 MHz. Of many possible benefits from the three civilian signals, this paper focuses on how to achieve high integrity and continuity in local area differential navigation by using multiple combinations of the carrier wave frequencies of the signals. The carrier frequencies of the three civil signals can be used to form measurements with different wavelengths, both longer and shorter than their own, depending on the specific combination used. By using these combinations in cascade manner, first finding the integer of the signal with the longest wavelength then moving on to a shorter one, the L1 integer can be resolved without using redundant measurements. Since integrity and continuity of differential carrier phase navigation depends on the validity of the integer solution, an integer rounding criterion based on wavelength and measurement noise is developed to verify the solution. Analysis of the integer rounding criterion shows that multiple levels of integrity and continuity for differential carrier phase navigation are possible, depending on the measurement noise level. Sensitivity analysis is carried out to study the effect of receiver noise, multipath delay, ionosphere delay and change in distance between an user and a reference receiver.

Autonomous airborne refueling of unmanned air vehicles using the global positioning system

Autonomous airborne refueling requires that the position of the receiving aircraft relative to the tanker be known very accurately and in real time. To meet this need, highly precise carrier-phase differential global positioning system solutions are being considered as the basis for navigation. However, the tanker introduces severe sky blockage into the autonomous airborne refueling mission, which reduces the number of visible global positioning system satellites and hence degrades the positioning accuracy. In this paper, we analyze the autonomous airborne refueling navigation problem in detail, define an optimal navigation architecture, and quantify navigation system availability. As part of this work, a high-fidelity dynamic sky-blockage model is developed and used to plan autonomous airborne refueling flight tests. The flight tests were conducted to obtain time-tagged global positioning system and attitude data that were processed offline to validate the blockage model.

Navigation, Interference Suppression, and Fault Monitoring in the Sea-Based Joint Precision Approach and Landing System

The United States Navy seeks the capability to land manned and unmanned aerial vehicles autonomously on an aircraft carrier using GPS. To deliver this capability, the Navy is developing a navigation system called the Sea-Based Joint Precision Approach and Landing System (JPALS). Because standard GPS is not sufficiently precise to land aircraft on a shortened, constantly moving runway, Sea-Based JPALS leverages dual-frequency, carrier-phase differential GPS navigation. Carrier phase measurements, derived from the sinusoidal waveforms underlying the GPS signal, are very precise but not necessarily accurate unless the user resolves the ambiguity associated with the sinusoids periodicity. Ensuring the validity of ambiguity resolution is the central challenge for the high-integrity, safety-critical JPALS application. Based on a multi-year, multi-institution collaborative study, this paper proposes a navigation and monitoring architecture designed to meet the guidance quality challenge posed by Sea-Based JPALS. In particular, we propose a two-stage navigation algorithm that meets the aggressive integrity-risk requirement for Sea-Based JPALS by first filtering a combination of GPS observables and subsequently exploiting those observables to resolve the carrier ambiguity. Because JPALS-equipped aircraft may encounter jamming, we also discuss interference mitigation technologies, such as inertial fusion and array antennas, which, with appropriate algorithmic modifications, can ensure integrity under Radio Frequency Interference (RFI) conditions. Lastly, we recommend a fault monitoring strategy tailored to the two-stage navigation algorithm. Monitoring will detect and isolate rare anomalies such as ionosphere storms or satellite ephemeris errors which would otherwise corrupt ambiguity resolution and positioning in Sea-Based JPALS.