published journal article
conference paper
The navigation performance with low Earth orbit (LEO) satellite signals is evaluated. The navigation framework used to perform this evaluation tightly integrates a vehicle’s inertial navigation system (INS) with Doppler and pseudorange measurements from LEO satellites. The following scenario is considered. A vehicle has access to global navigation satellite system (GNSS) signals and a priori, uncertain information about LEO satellite states. The vehicle navigates by tightly integrating GNSS pseudorange measurements with its onboard INS. During the period when GNSS signals are available, the vehicle tracks the LEO satellites from pseudorange and Doppler measurements, refining estimates about their states. Next, GNSS signals are assumed to be unavailable. The vehicle transitions to a simultaneous tracking and navigation (STAN) mode where it simultaneously tracks the LEO satellites and navigates by integrating pseudorange and Doppler measurements made on the LEO satellites with its onboard INS. The performance of this navigation framework is evaluated for two cases: when the LEO satellites periodically transmit their position and when the do not transmit such information. Simulation results with existing LEO satellite constellations pertaining to Orbcomm and Globalstar as well as the future satellite constellation pertaining to Starlink are presented. It was assumed that the LEO satellites are periodically transmitting their positions. These simulation results consider an unmanned aerial vehicle (UAV) equipped with a tactical-grade inertial measurement unit (IMU) navigating for 81.6 km in 600 seconds, in which GNSS signals were only available for the first 100 seconds. It is demonstrated that the final position error of the INS-Orbcomm-Globalstar system was 93.01 m while the INS-Starlink system was 9.81 m. The position root mean squared error (RMSE) of the INS-Orbcomm-Globalstar system was 58.59 m while the INS-Starlink system was 10.13 m. Experimental results with existing Orbcomm LEO satellites are presented in which only Doppler measurements were made on two available satellites. The experimental results were conducted on a ground vehicle equipped with a tactical-grade IMU that traversed 7.5 km in 258 seconds, in which GNSS signals were only available for the first 30 seconds. It is demonstrated that the final position error of the INS without GNSS signals was 3.73 km and the position RMSE was 1.42 km. On the other hand, the final position error of the INS-Orbcomm system was 233.3 m and the position RMSE was 188.6 m when the position of the satellite was decoded from its transmitted message. If such position was not decoded and was estimated only from the STAN framework, the final position error was 476.3 m and the position RMSE was 195.6 m.
research report
The paper analyzes the effects on the demand for owner-occupied housing that are likely to result from the Swedish 1983-85 tax reform. This is done by means of a microsimulation model which takes into account the dichotomous nature of the demand for housing: the consumers choose the mode of tenure (owning versus renting) as well as the quantity of housing conditional on the choice of the mode of tenure. The tax reform consists of a general reduction of marginal tax rates (i.e. an increase in disposable income) together with limitations in the deductibility of mortgage interests. The simulations show that this will cause an increase in home-ownership in the sense that more households will demand owner-occupied housing. At the same time, however, the households will demand smaller houses in the average, and thus aggregate demand for small and medium-sized units will increase while demand for large units will fall sharply.
conference paper
published journal article
published journal article
conference paper
Transit agencies are interested in integrating microtransit service with fixed-route transit (FRT) service to leverage the benefits of each mode. In an integrated FRT and microtransit system, fare structures for both FRT and microtransit services play an important role in influencing travelers’ behavior. This paper studies 10 different fare structures and their impacts on travelers’ behavior in the presence of an integrated FRT and microtransit system. We use a flexible agent-based FRT and microtransit modeling framework recently proposed by the authors to study fare structures. We analyze the impacts of fare structures on synthetic travelers in downtown San Diego and Lemon Grove, a small city in San Diego County. The results show that the intermodal transfer discount policy — where a traveler can use FRT freely when transferring from microtransit to FRT or receives a 50% discount on microtransit when transferring from FRT to microtransit — is the most promising. Under this fare structure, the subsidy per transit rider ($8.20 in downtown San Diego and $13.5 in Lemon Grove), transit user out-of-pocket costs ($1.27 in downtown San Diego and $1.71 in Lemon Grove), and auto mode share (80.6% in downtown San Diego and 86.5% in Lemon Grove) are the lowest of any fare structure.
conference paper
published journal article
published journal article