Antenna Control Systems: Step-Track and Antenna Tracking Fundamentals

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Author, Chris Steph

Ground stations and teleports around the globe are filled with antennas and control systems tracking satellites for a multitude of purposes. When it comes to tracking Geosynchronous Earth Orbiting (GEO) satellites, one of the most common methods used in the industry is called “step-track”. While this is a simple idea, there is more to it than a cursory glance will reveal, and the devil is truly in the details of how this process is set up in your controller.

Step-track may be used as a stand-alone tracking method or combined with more sophistication to develop orbital elements for observed satellite positions over time. When an antenna is pointed at a target satellite with some form of signal power feedback like a beacon receiver, the system can maximize the received signal amplitude via step-track.

When an antenna is pointed at a target satellite with some form of signal power feedback like a beacon receiver, the system can maximize the received signal amplitude via step-track.

How Step-Track Works in Practice

For an elevation (EL) over azimuth (AZ) mount, this is done by periodically “stepping” the antenna in the clockwise direction for azimuth. If the signal amplitude decreases, drive the antenna the same angular distance counterclockwise from the original starting point. If the signal amplitude decreases, step back to the original starting point where the signal was the strongest. This process is repeated for the elevation axis. If the AZ/EL starting position represents the maximum signal amplitude, then stop and wait for the next periodic step cycle to begin. How easy was that! Sounds simple, as described!

Alas, nothing is as simple as it first appears! For example, how was the angular distance of the first and subsequent steps defined? How long should I wait between steps for the signal to settle and give me an accurate reading? If the signal amplitude increases when the first step is taken instead of decreasing, what then? Which axis (AZ or EL) should be chosen for the first step? Which direction for that axis should we choose first? What do we know about the satellite being tracked and should that affect our parameter value choices? Suddenly, the simple idea of step-track begins to get complicated. But we are getting ahead of ourselves!

Why -3dB Beamwidth Is Foundational to Tracking Performance

One of the fundamental parameters in any step-track system is the half power beam width (HPBW) or the -3dB beamwidth. A good starting point for any system is to know the HPBW as accurately as possible. Modern control systems utilize the HPBW or -3dB beamwidth value as the basis for many tasks.

Corrective steps tend to be based on an internal, mathematical model of the beam shape, which is built using the HPBW value, so if this value is inaccurate, the system will step too far or not far enough when taking corrective steps to maximize the signal level. Also affected are the step sizes which tend to be taken as a percentage of the HPBW value, so again, if this value is not

accurate then the step sizes taken will be too large or too small and will affect the tracking performance.

When and Why Beamwidth Should Be Verified

When changes are made over the service life of an antenna system such as feed replacement or modification, panel adjustments/replacements or replacing the antenna controller itself, it is a good idea to verify the -3db beam width for the antenna prior to returning it into service.

The actual measurement is the best source for getting the most accurate value for optimum step-track performance. If you see poor tracking performance, this should be one of the first verification steps taken to ensure that you are not wasting time chasing other issues when this parameter was the real culprit all along. Getting this foundational piece of information correct is of critical importance due to the number of tracking tasks it affects in the control system.

A Measurement-Based Path Forward

Because the -3dB beamwidth influences so many downstream tracking decisions, relying on theoretical values introduces unnecessary risk into an otherwise well-understood process. A repeatable, measurement-based approach removes that uncertainty and ensures the control system is working from data that reflects real antenna performance. To support this, the Radeus Labs engineering team has documented a clear, step-by-step method for measuring and validating -3dB beamwidth in the field.

Download our guide to -3dB beamwidth measurement to walk through the equipment requirements, setup considerations, and measurement process needed to establish accurate beamwidth values for your antenna system.