Integer carrier phase ambiguity resolution is the process of resolving unknown cycle ambiguities of doubledifferenced carrier phase data as integers, and it is a prerequisite for rapid and high-precision global navigation satellite system positioning and navigation. Besides integer estimation, integer ambiguity resolution also involves validation of the integer estimates. In this contribution a new ambiguity resolution method is presented, based on the class of integer aperture estimators, which for the first time reveals an overall approach to the combined problem of integer estimation and validation. Furthermore, it is shown how the different discrimination tests that are currently in use in practice can be cast into the framework of the new approach.

/pdf/new-global-navigation-satellite-system-ambiguity-resolution-yqicyxk89u.pdf

New Global Navigation Satellite System Ambiguity Resolution Method Compared to Existing Approaches

The use of global navigation satellite system (GNSS) antenna arrays for applications such as interference counter-measure, attitude determination and signal-to-noise ratio (SNR) enhancement is attracting significant attention. However, precise antenna array calibration remains a major challenge. This paper proposes a new method for calibrating a GNSS antenna array using live signals and an inertial measurement unit (IMU). Moreover, a second method that employs the calibration results for the estimation of steering vectors is also proposed. These two methods are applied to the receiver in two modes, namely calibration and operation. In the calibration mode, a two-stage optimization for precise calibration is used; in the first stage, constant uncertainties are estimated while in the second stage, the dependency of each antenna element gain and phase patterns to the received signal direction of arrival (DOA) is considered for refined calibration. In the operation mode, a low-complexity iterative and fast-converging method is applied to estimate the satellite signal steering vectors using the calibration results. This makes the technique suitable for real-time applications employing a precisely calibrated antenna array. The proposed calibration method is applied to GPS signals to verify its applicability and assess its performance. Furthermore, the data set is used to evaluate the proposed iterative method in the receiver operation mode for two different applications, namely attitude determination and SNR enhancement.

https://www.mdpi.com/1424-8220/14/6/9669/pdf

Precise calibration of a GNSS antenna array for adaptive beamforming applications.

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.

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

The antenna of a zero-offset off-ground ground-penetrating radar can be accurately modeled using a linear system of frequency-dependent complex scalar transfer functions under the assumption that the electric field measured by the antenna locally tends to a plane wave. First, we analyze to which extent this hypothesis holds as a function of the antenna height above a multilayered medium. Second, we compare different methods to estimate the antenna phase center, namely, 1) extrapolation of peak-to-peak reflection values in the time domain and 2) frequency-domain full-waveform inversion assuming both frequency-independent and -dependent phase centers. For that purpose, we performed radar measurements at different heights above a perfect electrical conductor. Two different horn antennas operating, respectively, in the frequency ranges 0.2-2.0 and 0.8-2.6 GHz were used and compared. In the limits of the antenna geometry, we observed that antenna modeling results were not significantly affected by the position of the phase center. This implies that the transfer function model inherently accounts for the phase-center positions. The results also showed that the antenna transfer function model is valid only when the antenna is not too close to the reflector, namely, the threshold above which it holds corresponds to the antenna size. The effect of the frequency dependence of the phase-center position was further tested for a two-layered sandy soil subject to different water contents. The results showed that the proposed antenna model avoids the need for phase-center determination for proximal soil characterization.

Analysis of Horn Antenna Transfer Functions and Phase-Center Position for Modeling Off-Ground GPR

A new adaptive Kalman filter for navigation systems of carrier-based aircraft

The Joint Precision Approach and Landing System (JPALS) is being developed to provide navigation to support aircraft landings for the U.S. military. One variant of JPALS is the Shipboard Relative GPS (SRGPS), which will be implemented on an aircraft carrier. In order to meet strict accuracy, integrity, continuity, and availability goals in the presence of hostile jamming and in a harsh multipath environment, advanced technologies are required. One of those being studied is a controlled reception pattern antenna (CRPA) array with beam steering/adaptive null forming capabilities. The Stanford University GPS Laboratory has developed a software tool to study CRPA algorithms and their effects on GPS signal and tracking characteristics. A testbed has been constructed to investigate hardware issues including the phase center offset of the antenna elements and mutual coupling effects. This testbed consists of a 3 element antenna array with a baseline of 1m, using high-quality survey-grade or lower-quality patch antennas. Data has been taken using this array in conjunction with sufficient satellite constellation and antenna array motion to ensure complete azimuth and elevation signal coverage. A carrier phase-based attitude determination algorithm was used to generate inter-antenna bias residuals, allowing characterization of the virtual phase center of the array. Repeating the testing procedure both with survey-grade antennas, for which the phase center characteristics are well known, and with a patch antenna possessing unknown phase center behavior, allows characterization of the azimuth- and elevation-dependent properties of the patch antenna phase center. In addition, mutual coupling effects have been investigated by adding inactive patch elements around the active patch antenna. All results are compared to predictions from detailed simulation of the patch antenna used using an EM modeling software package.

Precise phase calibration of a controlled reception pattern GPS antenna for JPALS

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Precise phase calibration of a controlled reception pattern GPS antenna for JPALS