Nebula - n-Prize competition entrant No. 1

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Telemetry Electronics

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Helical Antenna characteristics

Telemetry Package

Telemetry from the Ambition rocket in motion is implemented with the use of a high-power radio modem operating in the unlicensed 868MHz band.

Antennas

At 500mW, the output power is sufficient for around 6Km line of sight communication using 1/4 wave dipoles so obviously something has to be done to get it up to the 200Km needed for reporting back from orbit.

Nothing much can be changed at the rocket end (can you imagine trying to gimbal a high-gain antenna on this project's budget), but a lot can be done at ground station level. Our design uses two antennas: One medium gain directional helical antenna for broad location, and one high-gain parabolic antenna for communication.

Both antennas are mounted on a single sub-assembly which is in turn mounted on a motorised azimuth-elevation telescope mount. The control electronics for the mount contains a small GPS receiver and flux-gate magentometer so that it can auto-orient wherever it is placed, and interfaces to a local laptop PC via a USB connection.

You can find photos of the finished antennas on the Groundstation page.

Mission Patch

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Data

Two telemetry modes are used: Synchronised and Unsynchronised.

The usual mode of operation is unsynchronised, where the orbiting craft is not in direct commnication with a ground station. A signal is transmitted from the orbiter once every 500mS that contains only the syncword, ID and 6 positional and velocity vector values from the IMU.

In synchronised mode, a two-way line-of-sight communications link must have been set up between the ground station and the craft. In this mode, the ground station can request packets of data that cover all aspects of the craft's operation from navigational data, through tank pressures and battery states, to image data. A special mode allows additional software routines to be uploaded if an update is needed.

Sensors

I won't go into the complete set of sensors that can have their data passed downstream (because its a boring list), but two of the most important for this mission are the fore and aft visual imaging sensors (cameras).

Both sensors have dual roles: Their output is used to determine the craft's orientation with respect to the earth; they also act as pure imaging devices. Before relaying images to the ground station, the pictures are JPEG encoded to compress the data - a single uncompressed image would take 64 seconds to download.

The forward looking imaging sensor has a special function in that it is used to prove that the satellite has deployed away from the body of the craft and is in independent orbit.

Power

Electrical power onboard the craft is provided by an array of High Capacity NiMH batteries that are designed to keep the craft functioning for up to 8 hours - not enough for the full mission time. The batteries therefore have to be topped-up by a ring of flexible amorphic silicon solar panels around the body of the craft, just aft of the main chute section. These panels will be clearly visible on the outer skin of the rocket before launch.