Amateur Radio Satellite Tracking

Submitted by Dave Matthews, KI4PSR

Using a Basic Stamp with Hobby Servo Motors for Amateur Radio Satellite Tracking:

One of the aspects of the Amateur Radio hobby is the use of Low Earth Orbit (LEO) satellites for communications. These are satellites placed in orbit for Amateur Radio use. These satellites pass over the earth with durations for effective satellite communications of an average of 10 minutes for a given location. During this time Amateur Radio operators (Hams) can communicate through the satellite to other Hams who are under the same satellite footprint. (More information is available at http://www.amsat.org/)

For best satellite communication it is important to use a directional antenna system that is pointed at the satellite position and tracks the satellite’s position as it travels through space. ‘Keplarian’ data is available from a few locations that describe the satellites positions, and Ham software tools are available to convert that information into azimuth and elevation positions for a given location on earth. Many companies offer tracking rotator systems that use that software produced information delivered by serial or USB computer connections to drive azimuth and elevation motors to keep an antenna array pointed tracking the satellite. Most of these systems are designed for fixed base station use, and typically require 120V ac power for operation.

My goal was to produce a battery powered azimuth and elevation antenna positioner, using inexpensive hobby servo motors and gearboxes, for portable operation. Although typical portable satellite tracking is done manually with hand-held small directional antennas, I wanted an automatic tracking mechanism to allow me to operate hands-free.

The Hardware:
The elevation rotator uses a ServoCity.com ‘Standard Servo Power Gearbox’ and a HiTech HS-7955TG servo motor. The azimuth rotator uses a ServoCity.com ‘HS-805/815BB Servo Power Gearbox’ and a HS-805BB servo motor. The rotators were assembled using the instructions included with the gearboxes, the azimuth gearbox modified to permit 360 degree rotation. During testing and program development, the elevation rotator often went to its limit of travel, with enough torque to shear teeth off of the gears! So metal gears replaced the stock nylon gears, and end-stop limit switches were added.

I mounted the rotator assembly on a sturdy Bogen tripod meant for heavy video camera use. With a compass and a level I can position the rotator assembly for magnetic north and horizontal positions. The control box I made has a few pushbuttons for manual operation, for example to park the antenna or test the system range of motion.

The Software:
The servo motors are controlled by a Parallax ‘Board of Education’ development board holding a BS-2 Basic Stamp. In addition, a Parallax ‘ServoPAL’ is used to refresh the servos so they will hold position while the BS-2 processor is busy receiving position commands. I used the Parallax Serial LCD model# 27977 to display operator prompts and show current azimuth and elevation positions. The servo motors use 6V power, so I built a small regulator assembly to reduce the 12V radio power to 6V. The BS-2 processor receives position commands from the rotator control program built into Ham Radio Deluxe (HRD). ( http://www.ham-radio-deluxe.com/ ) I chose the Alpha Spid output from HRD, and the BS-2 program extracts the position information from a serial data stream sent by HRD. HRD does all of the satellite rotator control, as well as the uplink and downlink radios frequency control to compensate for the Doppler shifting of transmit and receive frequencies as the satellite approaches and departs the station’s position.

Performance:
These hobby servo motors are adequate for this application but they have a good bit of deadband and backlash slop. The entire assembly will shake a bit while moving or when hit by a gust of wind. While the appearance looks unsteady, the result is perfectly acceptable tracking accuracy given the antenna’s beam width.

Schematic and Source Code Examples:
Contact Dave Matthews
Satellite Tracker Schematic (.pdf)
Satellite Tracker 07/16/09 (.zip)
Satellite Tracker 05/31/08 (.zip)