Rendezvous Solutions Research Articles

Approximate time-optimal low-thrust rendezvous solutions between circular orbits

Traditional planetocentric low-thrust solutions mainly focus on orbital rephasing or transfer maneuvers, while the rendezvous maneuver is more challenging to obtain and cannot be approximated by existing solutions in some mission scenarios. In this paper, approximate solutions to the time-optimal rendezvous problem between two close circular orbits are presented. First, the optimal control problem is established in terms of the true longitude by using the Sundman transformation, and a set of unified linearized equations of motion is derived to reduce the number of key parameters characterizing this problem. Then, we show that two key parameters can characterize the rendezvous problem, and that the rephasing (or transfer) corresponds to the case when one of the key parameters equals zero. Thus, a set of piecewise functions is developed to approximate the rephasing and transfer solutions. The rendezvous solutions are finally obtained by a traversal method and summarized in a set of contour maps. Numerical simulations demonstrate that the proposed solutions provide good initial guesses for solving the problem with nonlinear dynamics and accurately approximate the performance index for the preliminary mission design.

Non Volatile Low Power Wake up Radio Transceiver for Wireless Sensor Network

Wireless sensor nodes consume lots of energy during communication but huge power consumption has been observed during active listening in idle mode as source nodes can start data transmission at any time. Power saving can be achieved by establishing synchronization among end nodes. Many rendezvous solutions are available and out of which wake up receiver found extremely adroit. A non volatile wake up transceiver has been proposed in the present paper that works on the basis of ID matching. State of art using 4GB of memory to remember states of sensor nodes while proposed technique used only 60 bits of memory with very less false alarm probability. Power consumption for proposed model is only 59.47 nW. Hence this model is quite effective in terms of power consumption and memory usage as compared to trailing models.

Comprehensive asynchronous symmetric rendezvous algorithm in cognitive radio networks

Spectrum has become a scant quantity with recent upsurge in the field of wireless communication. Cognitive radio network (CRN) alleviates the overgrowing spectrum scarcity and underutilization problem by adequately sharing the frequency bands between licensed and unlicensed users. CRN allows unlicensed users or secondary users (SUs) to opportunistically utilize the free portion of the spectrum allocated to the licensed users or primary users. The fundamental process in the formation of CRN is the rendezvous process where SUs meet on commonly available channels and establish reliable links for effective communication. Existing rendezvous solutions based on the assumption of a common control channel (CCC) among the SUs are infeasible and less efficient in the dynamic environment of CRNs. Therefore, channel hopping (CH) technique without CCC support, often referred to as blind rendezvous, is usually employed for accomplishing the rendezvous between SUs. This paper presents a comprehensive asynchronous symmetric rendezvous (CASR) algorithm that does not require time synchronization and guarantees rendezvous of SUs in finite time. CASR algorithm exploits the MAC address of SU as the unique identifier (ID) and generates CH sequence based on the dynamic manipulation of ID according to the number of available communication channels. Leveraging the unique ID of each SU, CASR algorithm succeeded in rendezvous guarantee while perpetuating a good time to rendezvous. The efficiency of CASR algorithm is estimated theoretically and verified through various simulation experiments. Simulation results affirm that CASR algorithm performs better in terms of average time-to-rendezvous as compared with existing rendezvous algorithms.

Neighbor Discovery and Rendezvous Maintenance with Extended Quorum Systems for Mobile Applications

In many mobile sensing applications, devices need to discover new neighbors and maintain the rendezvous with known neighbors continuously. Due to the limited energy supply, these devices have to duty cycle their radios to conserve the energy and bandwidth, making neighbor discovery and rendezvous maintenance even more challenging. To date, the main mechanism for device discover and rendezvous maintenance in existing solutions is pairwise, direct one-hop communication. We argue that such pairwise direct communication is sufficient but not necessary: there exist unnecessary active slots that can be eliminated, without affecting discovery and rendezvous. In this work, we propose a novel concept of extended quorum system , which leverages indirect discovery to further conserve energy. Specifically, we use quorum graph to capture all possible information flow paths where knowledge about known-neighbors can propagate among devices. By eliminating redundant paths, we can reduce the number of active slots significantly. Since a quorum graph can characterize arbitrary active schedules of mobile devices, our work can be broadly used to improve many existing quorum-based discovery and rendezvous solutions. We comprehensively evaluate $EQS$ in three different scales of networks, and the results show that $EQS$ reduces as much as 55 percent energy consumption with a maximal 5 percent increase in latency for existing solutions. To test the real-world values of $EQS$ , we further propose a taxicab dispatching application called $EQS$ -dispatch to navigate taxicab drivers to the area with less competition based on the discovery results of nearby taxicabs.

Optimal cooperative power-limited rendezvous between coplanar circular orbits

Minimum-fuel rendezvous of two power-limited spacecraft is investigated. Both vehicles are active and provide thrust to complete the rendezvous. Total propellant consumption is minimized. A direct-minimization method, direct collocation with nonlinear programming, is used to obtain cooperative rendezvous solutions in an inverse- square gravitational field. Unconstrained and constrained circular terminal orbits are considered. The optimal solutions depend upon the power-to-mass ratios of the spacecraft, the initial orbits, and the specified transfer time. Optimal cooperative rendezvous solutions are compared with optimal active-passive solutions and with previously reported linearized solutions. ISTORICALLY, spacecraft rendezvous has involved one thrusting (active) vehicle and one coasting (passive) vehicle. A cooperative power-limited (PL) rendezvous is an orbital maneuver in which two PL spacecraft are active. With both vehicles thrust- ing, a reduction in total propellant consumption is obtained when compared to the traditional active-passive rendezvous. The practical application of cooperative rendezvous occurs if both vehicles have comparable size and propulsive capability. It would make no sense to consider cooperative rendezvous between a small spacecraft and a space station. Early studies of cooperative control considered general linear or nonlinear systems with various performance indices.14 Recently, cooperative rendezvous methodology has been applied to spacecraft maneuvering. Prussing and Conway5 determined the optimal termi- nal maneuver for a cooperative impulsive rendezvous. Mirfakhraie6 and Mirfakhraie and Conway7 developed a method for determin- ing fuel-optimal trajectories for the fixed-time impulsive cooper- ative rendezvous. Coverstone-Carroll and Prussing8 obtained an- alytical solutions for fixed-time PL cooperative rendezvous using the Hill-Clohessy-Wiltshire linearized gravity field to approximate the inverse-square gravity field. In this article, fixed-time cooper- ative and active-passive (noncooperative) rendezvous solutions are obtained for two PL spacecraft in an inverse-square gravity field. Transfer times, maximum exhaust powers, and the initial orbits of the spacecraft are varied to determine their influence on the optimal cooperative solution. Optimal solutions subject to circular terminal- orbit constraints are also examined. Necessary Conditions for Optimal Solution