Satellite Subsystems » Electrical » Telemetry

The purpose of the telemetry network aboard BLUEsat is to capture the readings of up to 64 sensors at various points around the satellite. The readings of all the sensors are assembled into a telemetry package, which is streamed down to earth continuously through the beacon. The telemetry package allows the satellite operators to diagnose or pre-empt potential problems and develop remedies, and simply to monitor that the satellite is functioning correctly throughout.

The sensors aboard BLUEsat take measurements of things like battery and solar panel voltage and current, temperatures at each face of the satellite and at internal points, and the statuses of various subsystems of the satellite, for example, receiver signal strength.

The network comprises a two-wire electrical bus using the PHILIPS I²C protocol, to which individual telemetry nodes are attached. Each node will have a bus interface connected, with appropriate logic, to a sensor. The reading of each sensor can be read by performing a read operation to the sensor's unique I²C bus address from the Flight Computer (FC). The FC is responsible for regularly collecting sensor readings, assembling a telemetry frame, and transmitting this down to earth on the beacon.

BLUEsat Telemetry Architecture

MAX 127: is an 8-channel 12-bit ADC (Analog to Digital Converter) with an I2C interface. It has pins to set three of the 7 I2C device address bits, allowing up to 8 MAX127s on a bus. There are two telemetry buses on BLUEsat for a total of (2 buses x 8 MAX127s x 8 channels =) 128 channels. Each channel measures a voltage. MAX 471: is a current sense amplifier that provides a dependable and accurate measurment of current flow. It outputs a voltage on one pin proportional to the current flowing through a pair of others. The voltage is measured by MAX127s. AD 590: is the temperature sensor we use. It is a current source whose output is linearly proportional to the absolute temperature. This current passes through a resistor to produce a voltage which is measured by a MAX127. PCF8574 IO Expander: is used in conjunction with the MAX127 ADC. It is used as an eight bit IO expander for the IIC bus. It allows the SA1100 to apply or remove power to individual AD590s to power them only when readings are being taken (current draw is otherwise large).

	Author: Chris Walsh © The Author, the BLUEsat project or UNSW. All rights reserved. Last Modified: September 18 2006 02:03:00.