Castor Satellite


The Satellite:

Castor is a 19 inch diameter aluminum sphere
with a mass of 63 Kg. It is as near perfect sphere as possible given the constraints of cost and manufacturability. The sphere is split in half with e delrin disc. The hemispheres are also the satellite antenna. For power, the satellite has 112 19AH lithium primary cells from Tadiran. This provides about 7000 watt-hours of power which has to last for the one year mission.

The electronics are designed to operate with minimal power consumption. Since the satellite is orbiting in such a low altitude, only industrial grade commercial components are used. The on-board computer is an Atmega128 microcontroller operating at 1.28 MHz and 3.3 volts. Power consumption of the on-board computer is less than 3 ma. The radio receiver operates at 10 ma. All subsystems are turned off when not used to minimize power consumption. Telemetry is transmitted at 30 second intervals. Initially, the transmitter will operate at 1 watt but can be reduced if reception is good enough.


The satellite has several different types of sensors. There are two main sensors, a Neutral particle wind and temperature spectrometer and an ion mass spectrometer.

A group of college students designed and built a MEMS sensor payload to test some commercial gyroscopes and a magnetometer. There are also six light sensors and six temperature sensors mounted in the satellite hemispheres.

Student Participation

Three university students were involved in testing subsystems and developing the MEMS payload. The MEMS payload consists of three gyroscopes and a three axis magnetometer. The gyroscopes are the ADXRS401 75 degree/sec gyroscopes made by Analog Devices. Breakout boards from Sparkfun are being used for the flight units. The magnetometer is the MicroMag3 made by PNI Corporation. The students wrote the software to measure the sensor rotation rate and temperature and to collect the magnetometer data. The data is part of the telemetry.


The satellite will transmit telemetry every 30 seconds at 1200 baud AFSK using the AX.25 protocol. All data is transmitted in one packet with a space between each data. The format is as follows:

TIME:     time in seconds from Jan 1, 1970. If satellite            

               resets, the time is reset to 1 second.

NEXT:    This is the pointer to the next scheduled command.

CMD:      Number of commands received.

TELEM:  Telemetry interval in seconds.

MODE:    Sensor mode, 1=raw data or 0=processed.

TEMP1:   Temperature sensor 1

LIGHT1:   Light sensor 1

TEMP2:   Temperature sensor 2

LIGHT2:   Light sensor 2

TEMP3:   Temperature sensor 3

LIGHT3:   Light sensor 3

TEMP4:   Temperature sensor 4

LIGHT4:   Light sensor 4

TEMP5:   Temperature sensor 5

LIGHT5:   Light sensor 5

TEMP6:   Temperature sensor 6

LIGHT6:   Light sensor 6

VOLT1:    Battery voltage - volts

VOLT2:    Voltage regulator output - volts

VOLT3:    Battery current - amperes

GYRO1:    Student MEMS gyro x-axis - units rpm

GYRO2:    Student MEMS gyro y-axis - units rpm

GYRO3:    Student MEMS gyro z-axis - units rpm

GTEMP1:  X-axis gyro temperature - units celsius

GTEMP2:  Y-axis gyro temperature - units celsius

GTEMP3:  Z-axis gyro temperature - units celsius

Vref:         Voltage reference

MAG1:       Student magnetometer x-axis - sensor count

MAG2:       Student magnetometer y-axis - sensor count

MAG3:       Student magnetometer z-axis - sensor count

Sample Telemetry:

KD4HBO-1>CQ,TELEM:SYS 870 65535 0 30 0 ffff ffff ffff ffff ffff ffff ffff ffff ffff ffff ffff ffff 13.3 5.0 15.4  3.01 -6.72 -9.19 -79.78 -79.78 -69.43 16 3 46

Light sensor: Signed hexidecimal. Larger value indicates more light.

Temperature: Signed hexidecimal. Value/256 = Temperature in C.

Sensor count: Check out the sensor datasheet.