Magnetic sensor
Power Mass Volume Cost
80mW 6g 50x20x10mm ~£400

The system we are using is built around the Honeywell HMC1053 3-axis magnetometer. Honeywell technologies produce a number of magnetic sensing solutions, many of which have been space-rated. The 1053 is small but extremely resilient and capable of low-power operations. This chip can operate to a resolution of 120 microgauss, and we can expect a reading swing of between -0.6 and +0.6 gauss from the earth's magnetic field, given a low earth orbit. This is translated to an analog signal voltage that swings between zero and +4 volts (the output signal cannot swing the full 5V range, as our amplifiers distort the signal when it approaches their power supply voltage). Our onboard computer's ADC has an input range of between zero and +5 volts, and 12-bit resolution. The maximum useful resolution for this circuit would encompass 10000 graduations; wheras 80% of the computer's ADC range gives us roughly 3277 graduations. So a change in the ADC output of 1 corresponds to a change in field strength along that axis of ~366.18 microgauss with the current configuration.

For a detailed analysis of how this resolution affects data bandwith, see the data rates section.

Of course, the magnetic field is a three-dimensional vector, and so we use a three-axis sensor. The different field readings are amplified seperately to reduce distortion and improve redundancy, and then sent through an analog multiplexer - the Maxim electronics MAX4618. Maxim multiplexers based on MOSFET technology have been flown before on many other successful satellites, and the 4618 has a number of desirable features like built-in fault and overvoltage protection. The computer will directly communicate with the switch via two TTL input pins on the sensor board. These give four possible modes of operation; the first three pass the signal from the x, y or z axis through to the analog output pin, and the last mode is used to isolate the entire sensor from the computer, removing the need for a multiplexer if other subsystems need to use the sole ADC channel.

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