{ TREX 3 }

> Abstract

TREX 3 is an antenna and camera tracker designed to track suborbital rockets during flight. It is being developed as part of the S²OUTH project in order to receive data and record videos of the N₂ORTH rocket. To learn more about the parent project go to the official S²OUTH website.

> Payloads

The primary objective of TREX is to receive the 5.8 GHz Video downlink during flight. To achieve this, the tracker actively points a 5.8 GHz dish antenna towards the rocket, which transmits the video signal through four onboard patch antennas. In order to maintain a stable live video transmission throughout the mission, the dish antenna must continuously follow the rocket with a pointing accuracy in the medium single-digit degree range.

As a secondary objective, TREX also points a Yagi antenna towards the rocket to receive flight telemetry data. This enables the ground segment to continuously receive, process, and monitor flight data during the mission.

Finally TREX will also feature a camera with a zoom lens in order to record and livestream high resolution tracking videos of the launch.

> Hardware

The tracker is implemented as a two-axis system with independent horizontal and vertical movement. Both axes are driven by closed-loop NEMA 23 stepper motors. The horizontal axis uses a motor directly connected to a 20:1 gearbox, followed by a 3:1 belt drive, which rotates the tracker and dish antenna in azimuth. The vertical axis uses a NEMA 23 motor connected to a 10:1 gearbox, also followed by a 3:1 belt drive, to actuate the elevation movement.

The belt drives are used to minimize mechanical backlash while maintaining a dynamic and fast-mov- ing system. To improve positioning behavior, the belts are tensioned both statically and dynamically. Final position feedback is provided by Hall effect sensors, allowing the tracker to achieve an approx- imate repeatable pointing accuracy of 2–3 degrees.

> Trajectory estimation

TREX is designed as a mostly self-contained subsystem. For target estimation, the system uses a basic ballistic prediction model as an initial reference trajectory. During flight, this prediction is corrected using state estimation telemetry received from the rocket. The tracker compensates for telemetry delay by predicting the rocket position over a short time horizon before calculating the required azimuth and elevation angles.

The STM32H7-based control board calculates the target pointing angles and commands the motor drivers for both axes. The control system includes timeout and fallback behavior so that the tracker can continue short-term prediction during brief telemetry interruptions and enter a safe state if valid tracking data is lost for too long.