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Groundstation
Details


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Groundstation Telemetry
& Tracking

There are four major elements to the the groundstation satellite/rocket tracker:

1. A widebeam tracking antenna
2. A high-gain communications antenna.
3. A motorised elevation-azimuth mount (& video camera)
4. The tracking electronics

and the prodution of all will be discussed here.

For a description of how the antennas and telemetry will work, see Telemetry Design.


Widebeam Tracking Antenna

The widebeam antenna is implemented with an axial-mode 15-turn helical design. A helixal antenna was chosen because of its circular polarity - its quite hard to judge the polarity of the signal coming from something that's potentially spinning in orbit, so this was the ideal choice.

The helix was wound using 8mm microbore copper pipe around 40mm plastic wastepipe with wooden dowel standoffs providing the correct radius.

This antenna has a characteristic impedance of 141 Ohms, so to match it into a 50 Ohm UHF connector and cable a quarter turn of copper plate was needed parallel to the groundplane at the connector end. A strip 10mm wide and separated 4mm from the groundplane (with an air gap) provided a matching impedance of 83.5 Ohms.

This matching stub therefore gives the finished antenna an impedance of 83.52 / 141 = 49.5 Ohms which is close enough.

Modelling of this antenna's characteristics shows it to have a forward gain of around 11dBi and a 3dB field of view of 29 degrees.


Hi-Gain Antenna

The high-gain communications antenna is an 80cm offset-focus dish aligned so that its focus conincides with the highest gain node of the helical antenna. The receiving element is a two-turn variant of the helical antenna with a smaller (three-quarter wave) groundplane.


The Motorised Mount & Camera

Both antennas are mounted on a common assembly which is in turn mounted on a motorised azimuth/elevation gimbal (see photo on right). Geared stepper motors control the both axes of movement and a slew rate of 3 degrees / second has been attained. The final electronics will double that to 6 degrees / second.

A video camera is located at the cente of the assembly. This ensures shake-free video recording which keeps the rocket/satellite in the centre of the field of view all the time tracking lock is present.


Control electronics

The groundstation electronics comprises a GPS receiver, telemetry transceiver and control processor. The processor's job is to provide an interface between the controlling PC (via the USB port) and the radio electronics. All of the tracking algorithms, logging, graphical interfaces etc are implemented on the connected PC.

The GPS receiver is included so that the groundstation may be located anywhere in the world and, once aligned to magntic north, it will be able to accurately set the antenna gimbals to point to the satellite/rocket's position in space.


The widebeam helical tracking antenna.
Click for a larger picture

The (almost) complete ground-station.
Click for a larger picture

Slewed around to point at the camera.
Click for a larger picture

Groundstation electronics.
Click for a larger picture

Software Launch Console.
Click for a larger picture
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