Methods for controlling orthogonal frequency division multiple access (OFDMA) communications over satellite links. The methods include estimating a differential delay (x ms) in a satellite spotbeam between a mean propagation delay (250 ms) in the spotbeam and a propagation delay ((250-x)ms) between a user terminal in the spotbeam and a satellite, estimating an overlap (215) between an OFDMA uplink frame and an OFDMA downlink frame as a result of the differential delay, and providing a guard band (215) in the OFDMA uplink frame and/or the OFDMA. downlink frame to reduce an overlap between remaining portions of the OFDMA uplink frame and the OFDMA downlink frame other than the guard band. Corresponding systems and devices are also disclosed.

Systems and methods for orthogonal frequency division multiple access (ofdma) communications over satellite links

A control system stabilizes a turret having a gimbal and base and pointing a weapon mounted within the turret. The control system includes computer executable modules to receive turret data and operator commands and to modify operator commands in accordance with a generated line of sight vector and ballistic data. The modules include a time optimal controller to generate modified operator commands. The modules further include a gimbal stabilization controller to generate motor commands to stabilize the turret and point the weapon.

Control system for a weapon mount

This thesis outlines the development of an agile, reliable and precise targeting mechanism
complete with free space optical communication (FSOC) capabilities for employment
in optomechatronic applications. To construct the complex mechanism,
insight into existing technologies was required. These are inclusive to actuator design,
control methodology, programming architecture, object recognition and localization
and optical communication. Focusing on each component individually resulted in a
variety of novel systems, commencing with the creation of a fast (1.3 ms⁻¹), accurate
(micron range) voice coil actuator (VCA). The design, employing a planar, compact
composition, with the inclusion of precision position feedback and smooth guidance
fulfills size, weight and power (SWaP) characteristics required by many optomechatronic
mechanisms. Arranging the VCAs in a parallel nature promoted the use of a
parallel orientation manipulator (POM) as the foundation of the targeting structure.
Motion control was achieved by adopting a cascade PID-PID control methodology in
hardware, resulting in average settling times of 23 ms. In the pursuit of quick and
dependable computation, a custom printed circuit board (PCB) containing a field
programmable gate array (FPGA), microcontroller and image sensing technology were
developed. Subsequently, hardware-based object isolation and parameter identification
algorithms were constructed. Furthermore, by integrating these techniques with the
dynamic performance of the POM, mathematical equations were generated to allow
the targeting of an object in real-time with update rates of 70 ms. Finally, a FSOC architecture utilizing beam splitter technology was constructed and integrated into the
targeting device. Thus, producing a system capable of automatically targeting an infrared
(IR) light source while simultaneously receiving wireless optical communication
achieving ranges beyond 30 feet, at rates of 1 Mbits per second.

An automated targeting mechanism with free space optical communication functionality for optomechatronic applications

U.S. military assets' increasing need for secure global communications has led to the design and fabrication of airborne satellite communication terminals that operate under protected security protocol. Protected transmission limits the closed-loop tracking options to eliminate pointing error in the open-loop pointing solution. In an airborne environment, aircraft disturbances and noisy attitude information affect the open-loop pointing performance. This paper analyzes the open-loop pointing and closed-loop tracking performance in the presence of open-loop pointing error and uncertainty in the received signal to assess hardware options relative to performance requirements. Results from the open-loop analysis are characterizations of the pointing error based on plant definition, aircraft motion, the control system, and a non-ideal GPS/INS. The closed-loop tracking analysis shows several results. The distribution of the noise power dominates (over the received signal power) the SNR distribution. The defined step-tracking algorithm reduces pointing error in the open-loop pointing solution for a pedestal experiencing aircraft disturbances and random errors from the GPS/INS. For initial pointing off-boresight, the performance of the step tracking algorithm depends on the antenna aperture size and the GPS/INS unit. The closed-loop tracking performance is primarily a function of the number of SNR samples and is for the most part independent of the hardware selection.

Airborne protected Military Satellite Communications: Analysis of open-loop pointing and closed-loop tracking with noisy platform attitude information

A satellite communication system may employ frequency tracking mechanisms at each earth terminal to control transmit frequency and tuned receiver frequency. The technique presented is applicable to time division multiplexed systems having burst transmissions. Frequency tracking can provide corrections for frequency offsets due to Doppler, oscillator inaccuracies, satellite inclination angle effects, and other factors. One objective of such frequency tracking is to minimize the width of the receive frequency window that must be searched. Reducing the width of this window will in general allow the use of a more efficient waveform, or will improve signal acquisition performance for a given waveform. A general technique is presented for characterizing any number of frequency error sources, and determining optimum constant correction factors based on the characterized error sources and parameters of a given system. Optimized correction factors and associated worst-case frequency offsets are then evaluated for example UHF networks. >

A frequency tracking technique for multiple access satellite communication networks

ABSTRACl Pointing and scanning control algorithms for a two axis gimbal, inertially stabilized, airborne antenna system are described and evaluated via computer simulation. Satisfactory performance is demonstrated in the presence of aircraft maneuvers, with some (expected) degradation for the very near-zenith antenna states. The antenna scanning algorithm presented herein is adaptive in its scan rate (in order to avoid scan distortions due to azimuth rate limiting imposed by the physical antenna system). is simple to implement, and properly accounts for aircraft maneuvering in order to maintain circular scanning trajectories in inertial space.

Antenna pointing and scanning control for a two axis gimbal system in

Pointing and scanning control algorithms for a two-axis gimbal, inertially stabilized, airborne antenna system are described and evaluated computer simulation. Satisfactory performance is demonstrated in the presence of aircraft maneuvers, with some (expected) degradation for the very-near-zenith antenna states. The antenna scanning algorithm presented is adaptive in its scan rate (to avoid scan distortions due to azimuth rate limiting imposed by the physical antenna system), is simple to implement, and properly accounts for aircraft maneuvering to maintain circular scanning trajectories in inertial space. >

Antenna pointing and scanning control for a two axis gimbal system in the presence of platform motion

A synchronous spread-spectrum communications system utilizing the resources of a nonprocessing satellite transponder is considered. The systemwide time and frequency standard (which is assumed to be maintained by the system master terminal) is transferred to the satellite, and from there it is disseminated and used by all slave terminals in synchronizing their downlink and uplink operations. The downlink and uplink time and frequency tracking processes that must be performed by each system terminal in order to achieve and maintain synchronization are described, analyzed, and evaluated. The mechanism that establishes system time and frequency reference at the satellite is also described. The authors present computer simulation results illustrating the transient as well as the steady-state performance capabilities of the synchronization mechanisms in the presence of satellite and platform motion. >

P.D. Karabinis

Papers

Antenna pointing and scanning control for a two axis gimbal system in

Antenna pointing and scanning control for a two axis gimbal system in the presence of platform motion

Downlink and uplink synchronization concepts for spread spectrum communications via satellite