Stand‐alone GPS receiver velocity algorithms and flight test results are presented. A dual‐frequency GPS receiver was subjected to aircraft dynamics of up to 1 g (9.8 m/s2), with aircraft bank angles of up to 45 deg. Velocity errors were determined for static and in‐flight conditions. In‐flight results were evaluated against a postprocessed, differential kinematic GPS solution and by an assessment of the velocity estimation residuals. Differences between the stand‐alone velocity solution and the postprocessed differential velocity solution were found to be at the 2–4 mm/s level (1 s̀) for horizontal velocity components and 9.7 mm/s (1 s̀) for vertical velocity. Since these differences include noise contributions from three GPS receivers, stand‐alone velocity errors are expected to be smaller than the results reported here. This is also indicated by the stand‐alone velocity residual biases, which were found to be on the order of 0.1 mm/s or less, while residual standard deviations ranged from 0.8 to 3.2 mm/s.
The paper describes a fully autonomous relative navigation solution for urban environments (indoor and outdoor). The navigation solution is derived by combining measurements from a two‐dimensional (2D) laser scanner with measurements from inertial sensors. This derivation relies on the availability of structures (lines and surfaces) within the scan range (80 m depth, typically). Navigation herein is performed in completely unknown environments. No map information is assumed to be available a priori. Indoor and outdoor live data test results are used to demonstrate performance characteristics of the laser/inertial integrated navigation. Relative positioning at the cm‐level is demonstrated for indoor scenarios where well‐defined features and good feature geometry are available. Test data from challenging urban environments show position errors at the meter‐level after approximately 200 m of travel (between 0.6% and 0.8% of distance traveled).
A deeply integrated GPS/low-cost IMU system is developed and applied to re-acquire and track GPS signals with low carrier-to-noise ratio (CNR). The system provides continuous carrier phase tracking without requiring the knowledge of navigation data bits. Flight data with a 90-degree turn are applied to demonstrate re-acquisition and continuous carrier phase tracking of 15 dB-Hz GPS signals.
Many applications can be envisioned for accurate, robust, and reliable navigation solution in challenging urban environments. Examples of existing and prospective applications include, but are not limited to, navigation, guidance, and control of autonomous vehicles (including both ground and aerial vehicles) for urban surveillance and reconnaissance; collection of geographical information system (GIS) data in cities; monitoring of urban infrastructure for situational awareness; and, precise automotive applications such as automated lane keeping. If used by themselves, none of the existing navigation technologies have the potential to fully satisfy the requirements for reliable and accurate navigation in urban environments. Hence, this paper develops a multi-sensor integrated solution that combines the complementary features of the Global Positioning System (GPS), laser scanner feature-based navigation, and inertial navigation for urban scenarios.GPS and laser scanner-based navigation ideally complement each other for urban navigation. The laser scanner-based navigation relies on the availability of structures (lines and surfaces) within the scan range (80 m, typically). Features (such as lines) are first extracted from laser scan images and then used for position and attitude determination. In urban areas, if there exists a building wall that blocks GPS signals, this wall creates a feature in the laser scan image. On the other hand, for open streets with limited features, the GPS signal is generally unobstructed. Thus, GPS and laser data can be combined into integrated solution architecture. The system architecture developed also exploits INS navigation states for improved solution robustness: e.g., for robust feature association between scan images and for coasting through instances where sufficient number of combined GPS/laser measurements is unavailable. A tightly coupled GPS/laser scanner/INS mechanization is developed and applied for centimeteraccurate trajectory reconstruction.The paper uses live urban data to demonstrate that combined GPS and laser scanner data generally support the observability of navigation states at any part of the urban trajectory; and, for those limited cases where insufficient GPS/laser measurements are available, the INS coasting option can be efficiently utilized. Test results presented also show that the developed tightly coupled GPS/laser scanner/INS solution provides accurate trajectory reconstruction capabilities (one sigma error residuals are at a cm-level) in challenging urban environments.
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