The use of GPS for navigation-critical applications such as aircraft nonprecision approach or harbor and river crossings requires the navigation data to be both extremely accurate and extremely reliable. This paper describes a method for autonomous GPS satellite failure detection and isolation (D/I). The test statistic for the D/I algorithm is the range residual parameter for six or more satellites in view. Based on experiments conducted at Stanford, nominal carrier-aided pseudorange measurement errors are modeled as Gaussian random variables with mean in the range from −5 m to + 5 m and standard deviation of 0.4 m. The theoretical statistical distribution of the range residual is given. Monte Carlo simulations present results of applying the algorithm to measurement sets containing a biased measurement. With a 100 m biased measurement present, successful detection is achieved 99.9 percent of the time, and successful D/I is achieved 72.2 percent of the time. The user is always aware when isolation is not possible. User positioning errors resulting from application of the algorithm are always the same or better than with the all-in-view solution.

TechDemoSat-1 (TDS-1) is an experimental Global Navigation Satellite System Reflections (GNSS-R) satellite launched in 2014. The GNSS-R receiver onboard performs real-time navigation and generates delay-Doppler correlation maps for Earth-reflected Global Positioning System (GPS) L1 C/A ranging signals. This paper investigates the performance of the TDS-1 data for ocean surface altimetry retrievals. The analysis includes consideration of the transmitter and receiver orbits, time tag corrections, models for ionospheric and tropospheric delays, zenith to nadir antenna baseline offsets, ocean and solid Earth tides, and a comparison with mean sea surface topography. An error budget is compiled to account for each error source and compared with the experimentally derived surface height retrievals. By analyzing data sets covering global ocean surfaces over ±60° latitude, the current performance of spaceborne GNSS-R altimetry with the TDS-1 data set is experimentally established. In comparison with the mean sea surface topography, the surface height residuals are found to be 6.4 m, $1\sigma $ with a 1-s integration time. A discussion of the factors limiting this performance is presented, with implications for future GNSS-R altimetry missions designed for the observation of mesoscale ocean circulation.

This paper describes and demonstrates an assisted GPS technique, termed “Collective Detection,” for combining satellite correlograms to enable rapid acquisition and direct positioning. Correctly combining correlation values from multiple satellites reduces the required C/N₀ such that satellite signals, which cannot be acquired individually, can contribute constructively to a position solution. The acquisition search is performed in a position/clock space that directly yields the navigation solution. Results from a hardware simulator and live experiments are presented. The simulations compare combinations of 11 satellites and four satellites at C/N₀ levels of 40 and 20 dB-Hz. The outdoor experiments show horizontal position accuracies on the order of 50 m in open sky conditions and in a narrow courtyard environment using one millisecond of data. Collective detection and positioning is shown to be a promising approach for positioning in weak signal environments.

Ocean surface altimetry with GNSS reflectometry (GNSS-R) has the potential to improve the observation and retrieval of mesoscale ocean current flows by increasing the spatial density of ocean surface observations. The leading challenges in current GNSS-R altimetry investigations include precise delay retracking, correction of ionospheric effects, and spacecraft receiver positioning. Here, we present improved methods to account for these challenges. A reflection-model-based approach is presented for delay retracking that uses simulated delay-Doppler maps (DDMs) to retrieve the specular delay from measured DDMs. Global ionosphere maps are used to estimate the group delay effect along the direct and reflection paths. Precise orbit determination techniques are used to improve estimates of the receiving spacecraft position and reduce systematic intersatellite biases. We analyze altimetric height retrievals from two cases, strongly coherent or diffuse reflections. We study the residual systematic error signals with a focus on understanding the errors identified above. The strong coherent observations have narrow, symmetric delay waveforms and provide higher than average signal-to-noise ratio. The DDMs of diffuse observations, originate from relatively rough ocean surfaces and have the characteristic horse-shoe shape. Only measurements from the ocean and seas surrounding Indonesia are considered as this area provides a significant number of strongly coherent reflections. More than 50,000 single-point observations and week-long averaged results are presented to quantify the noise and systematic characteristics of the retrieved surface heights.

GPS has proven to be an extremely accurate positioning sensor for a wide variety of applications. However, in some situations, such as aircraft approach and landing, higher accuracy is required. Wide Area Differential GPS (WADGPS) is a system that could be used to meet such requirements. The WADGPS system comprises a master station, and local monitor stations distributed across the United States. The system calculates and transmits a vector of error corrections to users. This correction vector consists of parameters describing the three-dimensional ephemeris errors, satellite clock offsets, and ionospheric time delay parameters. The master station gathers GPS measurements made at each of the local stations, and estimates the errors using a combination of nonlinear static estimation and batch least squares. The performance of a 15-station WADGPS network was investigated by simulation for users at sites across the United States. The monitor stations were located at existing Loran or VOR stations. Simulation results indicate that normal GPS positioning errors can potentially be reduced by more than 95 percent using WADGPS.

Performing interferometry in an optical lattice formed by standing waves of light offers potential advantages over its free-space equivalents since the atoms can be confined and manipulated by the optical potential. We demonstrate such an interferometer in a one-dimensional lattice and show the ability to control the atoms by imaging and reconstructing the wave function at many stages during its cycle. An acceleration signal is applied, and the resulting performance is seen to be close to the optimum possible for the time-space area enclosed according to quantum theory. Our methodology of machine design enables the sensor to be reconfigurable on the fly, and when scaled up, offers the potential to make state-of-the art inertial and gravitational sensors that will have a wide range of potential applications. Published by the American Physical Society 2024