An excerpt from JPL Document D-12104 presents additional information on the control system of a steerable microwave antenna that is mounted on the roof of a land vehicle and used to communicate with a geostationary satellite. The antenna and control system were described previously in "Steerable K/Ka-band Antenna for Land-Mobile Satellite Applications" NASA Tech Briefs, Vol. 18, No. 1 (January 1994), page 28. Under control by a computer mounted in the vehicle, a motor turns a rotary table to aim the antenna in azimuth toward the satellite, which transmits a pilot-tone signal that serves as an aiming beacon. (The radiation pattern of the antenna is wide enough in elevation so that steering in elevation is unnecessary.) Once the pilot tone has been acquired, an inertial turn-rate sensor mounted in the vehicle provides most of the information needed by an open-loop control subsystem that keeps the antenna pointed toward the satellite as the vehicle moves. Drift of the inertial-sensor bias is the source of most aiming error. Any such error is detected by sinusoidally dithering the rotary table by ±1° at a frequency of 2 Hz while measuring the resulting dither in the strength of the received pilot tone.
This work was done by Arthur C. Densmore of Caltech and Richard Renner of Cal Corporation for NASA's Jet Propulsion Laboratory. To obtain a copy of the excerpt, "Subsystem Design," access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Systems category, or circle no. 189 on the TSP Order Card in this issue to receive a copy by mail ($5 charge). NPO-19863
This Brief includes a Technical Support Package (TSP).
Control System For Steerable, Rooftop-Mounted Antenna
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Overview
The document presents a technical overview of a control system designed for a steerable, rooftop-mounted antenna developed by the Jet Propulsion Laboratory (JPL) for NASA's Advanced Communications Technology Satellite (ACTS). Launched in 1993, the ACTS satellite facilitates high-rate digital communications, including voice, video, and data, from mobile vehicles.
The control system is based on classical control theory and is analogous to a third-order phase-locked loop (PLL). It employs a microstepped electrical phase to drive a stepper motor, which is crucial for accurately tracking the satellite. The design addresses potential issues such as motor phase cycle slips, which can occur if the stepper motor is disturbed.
A key feature of the system is its ability to continuously and adaptively cancel the drift commonly experienced by low-cost 3-axis internal sensors. This drift can be induced by factors such as temperature and time, which can affect the accuracy of the antenna's pointing. The software algorithm integrates sensor data with low-bandwidth feedback to mitigate these drifts, ensuring high performance at a low cost.
The document outlines the subsystem design, detailing how the functional requirements of the antenna controller are met. It highlights the successful demonstration of the antenna system's capabilities in Southern California, where it enabled direct-dial communications while the vehicle was in motion. The system supports data rates of up to 64 Kbps, showcasing its efficiency and effectiveness in real-world applications.
Overall, the document emphasizes the innovative engineering behind the antenna control system, which combines advanced control techniques with practical applications in mobile communications. It serves as a testament to the ongoing efforts of NASA and JPL in developing technologies that enhance communication capabilities, particularly in mobile and dynamic environments. The work not only contributes to the field of aerospace engineering but also has implications for various industries that rely on robust communication systems.
