Joint Precision Approach And Landing System Research Articles

Model for JPALS/SRGPS Flexure and Attitude Error Allocation

This paper develops a linearized parametric error model for assessing the effects of structural flexure and attitude uncertainties on the shipboard variant of the joint precision approach and landing system (JPALS). The outputs of the error model are position domain error bounds on the estimate of the ship reference point (SRP) coordinates. The model is parameterized in terms of GPS antenna installation geometry and the covariance matrices capturing the statistics of GPS measurement, ship structural flexure, and ship attitude estimation uncertainty. The performance of the model is evaluated via a set of simulation studies. It is shown that when the attitude errors are small and the flexure statistics well characterized, the error model provides an accurate and convenient way of mapping attitude and flexure uncertainties into SRP position uncertainties. Estimation of SRP position errors is a nonlinear problem and when ship attitude uncertainties are large, the nonlinearities can be important. However, the bounds calculated by the error model developed can be inflated to deal with these nonlinearities. Finally, by analyzing data collected from ship trials, it is shown that perhaps a more challenging issue may be the potential for highly correlated, bias-like structural flexure uncertainties.

Measuring Aircraft Carrier Flexure in Support of Autonomous Aircraft Landings

This paper quantifies the experimental measurements of aircraft carrier flexure (deformation) at sea in support of the United States Department of Defense sea-based Joint Precision Approach and Landing System (JPALS) that aims to deliver automatic landing capabilities to inbound aircraft aboard aircraft carriers. The methodology for measuring ship flexure using the Global Positioning System (GPS) and inertial sensors is described. Results indicate that flexure on the aircraft carrier used for testing has a standard deviation of approximately 1-2 cm. For the ship tested, the most significant flexure effects are in the port/starboard direction and correlate best with roll or lateral acceleration.

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

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> 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 sinusoid's 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. </para>

Detecting RFI Through Integrity Monitoring at a DGPS Reference Station

Because GPS is a radio navigation system which has a very low power level, it is vulnerable to RFI. Excessive RFI could cause receiver performance degradation, such as degradation of position accuracy, loss of lock and increased acquisition time. After GPS modernization plans introduce dual-frequency civil signals to mitigate ionospheric errors, RFI will remain as one of the dominant threats for differential GPS navigation systems. Examples of safety-critical civil aviation and military missions threatened by RFI include the Local Area Augmentation System (LAAS) and the Joint Precision Approach and Landing System (JPALS). This paper focuses on RFI mitigation through integrity monitoring for a DGPS system like LAAS or JPALS. The mitigation strategy consists of two parts. First, the paper develops a new RFI detection method, using a raw divergence statistic. Second, the paper investigates strategies for maintaining integrity in the case that RFI is detected.To validate the utility of the divergence-based RFI monitor, this paper takes an experimental approach. The experiments assess the performance of the divergence metric and compare it to existing alternatives for RFI detection, such as metrics for Automatic Gain Control (AGC) and carrier-to-noise ratio (C/N0). Generating a monitoring threshold for these statistics proves challenging, because the threshold depends both on the type of RFI threat (e.g. continuous wave, narrow band, wideband, pulsed) and on environmental conditions, such as temperature. As experiments illustrate, the divergence statistic resolves these limitations, as divergence directly estimates ranging source error, independent of the type of RFI threat or the environmental conditions.

Development of Multipath Error Budgets for JPALS Ground Station Receivers

This paper develops code multipath error budgets for land-based Joint Precision Approach and Landing System (JPALS) ground station receivers. The approach is similar to that used for the civil Local Area Augmentation System (LAAS), with modifications for P(Y)-code tracking and the use of beam-steering antenna arrays. The budget includes two error allocations, one for receiver noise and one for code multipath, both of which are defined as a function of satellite elevation. The noise allocations are based on an analytical model that is fit to JPALS test data, while the multipath allocations are based on test data alone. The results show that JPALS ground stations have some advantages over LAAS ground stations due to antenna differences and the more limited multipath susceptibility of the P(Y)-code. We also investigate code multipath biases by way of simulations. The results indicate that the multipath allocations developed herein capture multipath biases for the expected environments.

Performance Analysis of Carrier-Phase DGPS Navigation for Shipboard Landing of Aircraft

: Shipboard-relative GPS (SRGPS) is a variant of the Joint Precision Approach and Landing System (JPALS) that is being developed to support automatic shipboard landings in zero-visibility conditions. At present, the navigation system requirements specify a vertical protection level of 1.1 m, an integrity risk of approximately 10−7, and a system availability of at least 99.7 percent. Because of the stringent nature of these specifications, differential carrier-phase solutions are being pursued. In this context, this paper gives a detailed analysis of the fault-free integrity and availability of SRGPS. The performance of single- and dual-frequency architectures is evaluated for both floating-ambiguity and fixed-integer carrier-phase differential GPS (DGPS) navigation. The sensitivity of SRGPS availability is quantified with respect to raw code and carrier measurement quality, spatial decorrelation of ionospheric and tropospheric errors, and broadcast service radius. Necessary conditions to achieve the desired navigation performance are defined.