Antenna rotators for satellite tracking: a beginner's guide to az-el rotators

What a rotator does

A rotator is a motor unit that points an antenna. For satellites you need two axes: azimuth (the compass direction, 0° north, 90° east) and elevation (the angle above the horizon, 0° to 90°). Together they make an az-el rotator.

Software on a computer works out where the satellite is every second, from its orbit (TLE) and your location, and sends the angles to the rotator's controller. The antenna follows the pass by itself, your hands stay free for the radio, and a directional Yagi stays on target the whole time.

Do you need one?

Not at first. A handheld Yagi or a QFH covers most beginner targets. A rotator is worth it when you want to record passes unattended, use a bigger, higher-gain antenna that is too heavy to hold, or work linear satellites where both hands belong on the radio.

Options range from home-built units with stepper motors and 3D-printed parts (such as the open-source SatNOGS rotator) to commercial az-el rotators like the Yaesu G-5500 family, SPID units, and the compact network-controlled rotators now made for SDR stations.

How the computer talks to the rotator

Most rotators speak one of a few simple command languages over a USB serial port or the network:

• Yaesu GS-232: plain text such as "W180 045" (azimuth 180°, elevation 45°). Used by Yaesu's computer interface and many compatible controllers.
• EasyComm: text such as "AZ180.0 EL45.0". Common on home-built and SatNOGS rotators.
• SPID Rot2Prog: a short binary packet, used by SPID rotators.
• Hamlib rotctl: a network protocol, usually on port 4533. Hamlib's rotctld translates it into almost any rotator's own language, and some network rotators speak it directly.

The overhead-pass problem (the keyhole)

On a pass close to straight overhead, the satellite's azimuth swings by nearly 180° within a few seconds as it crosses the top. An ordinary rotator can't turn that fast, so the antenna lags and loses the signal at the best moment of the pass.

There are two common fixes. Rotators with 0–180° elevation can flip: instead of swinging round, the antenna tilts over the top and follows the satellite down the other side with almost no azimuth change. And for passes that cross north, rotators with 450° of azimuth travel (such as the G-5500) can keep turning past 360° instead of unwinding the long way round mid-pass. Good tracking software plans the whole pass in advance and picks the path that avoids both problems.

Calibration: making it point where it says

• True north, not magnetic north. A compass can be off by several degrees or more depending on where you live. Align the zero mark with true north using a map, the Sun's direction at local solar noon, or a distant landmark.
• Level the mast. A leaning mast tilts every angle; put a bubble level on the mounting plate.
• Check elevation zero: the antenna should be parallel to the ground at 0° and point straight up at 90°.
• An accurate clock and fresh orbit data. A computer clock a few seconds off, or a week-old TLE, moves the predicted position more than you would think. Keep the clock synced and update orbits daily.
• Test on the Sun. Point the rotator at the Sun and check that the antenna's shadow (or a small sight) lines up. If it is off, adjust the offset in the controller.

Practical tips

• Leave a loop of slack in every cable at the rotator so it can turn through its full range without pulling.
• Use a deadband of a couple of degrees: the rotator only moves once the satellite has moved that far. It saves wear and still keeps the satellite inside a Yagi's beam.
• Park the antenna where the next pass rises a minute or two before it starts.
• Mind the wind: a big array on a light rotator can strip gears in a storm.
• Our radar can drive a rotator straight from the browser (USB through Web Serial, or rotctl over the network) and plans the flip and north crossings for you.