The Communications subsystem has three primary tasks:
- Act as a beacon so that the position of the satellite can be tracked on Earth
- Downlink satellite health data and sensor data to ground stations
- Provide a means disabling the transmitter from a ground station via uplink
We have chosen to use an XDL Micro Transceiver from Pacific Crest Communications. This UHF radio transceiver is well-documented and thoroughly tested to thermal and vibrational standards. We will be using a low bit-rate 4800bps, with built-in forward error correction to improve the error rate. The downlink communications will consist of both beacon and data messages. Our beacon transmission will consist of our registered call sign. The data downlink will be automatically generated by the microprocessor to give updated reports of battery, solar panel, and LED health as well as sensor data from the infrared (Earth) sensors and other system monitoring sensors. When in receiver mode, the XDL Micro will listen briefly for a special shutdown code, as required by IARU (International Amateur Radio Union) regulations. This code will be available only to our registered ground stations.
- External Power Required: Regulated 3.6 VDC +/- 10%
- Power Required during RX: 0.45 W nominal @ 3.6 VDC
- Power Required during TX: 2.9 W @ 0.5 RF Output
- Dimensions: 69.8mm L x 46.6mm W x 11.2mm H
- Weight: 40g
- Operating Temperature: -40˚C to +65˚C
- Shock and Vibration: MIL-STD-810F
- Modulation/Link Rate: GMSK 4800 bps
- Frequency Band: 403-473 MHz
- Frequency Control: Synthesized 12.5 kHz tuning resolution
- Sensitivity: -110 dBm BER 10-5
Power Considerations and Duty Cycle
To minimize the amount of power required to operate the radio, a duty cycle of less than 5% will be imposed to only turn the radio on during brief transmission and reception intervals. The transmissions will be frequent enough to ensure reception of full messages via our ground stations during any pass (approx. 2 min long), even at low angles to the horizon. In the event of unforeseen power limitations (i.e. after launch), the microcontroller will be able to adjust the quantity and length of transmissions, preventing the radio from draining the batteries to unsafe levels.
We have designed our communications subsystem to transmit and receive signals in the 435-438 MHz band. This 70cm band is ideal for accessibility to Amateur Radio operators, as well as being licensable for use in amateur satellites, including CubeSats.
The primary ground station for EquiSat will consist of a receiving antenna and recorder device located at Brown University. This ground station is in development and will be able to continuously monitor EquiSat’s transmissions when the satellite is in range.
Meanwhile, EquiSat is designed so that ham radio operators will also be able to receive signals. This will be tested and accomplished using the Arrowhead Arrow II Antenna connected to the FUNCube Dongle. This combination of receiver antenna and driver/software has been successfully used by ITUPSat, which is currently flying the same BeeLine radio transmitter that EquiSat is using. It is important to note that any ham radio receiver-antenna setup will be able to receive transmissions if it is tuned correctly.
Because the antenna’s two poles must be longer than the side length of the cube (13 cm vs. 10 cm), the antenna is launched coiled on the top of the satellite and tied down with nylon fishing line. The fishing line is intersected by a thin piece of nichrome heating element that is heated electrically once in space to melt the nylon and deploy the antenna. The antenna is made from highly elastic nickel-titanium, so it easily springs into position when released, much like measuring tape.
Here is diagram showing how the nylon fishing line will be cut by the nichrome heating elements, releasing the antenna, and the fastener that will be used to mount the antenna to the satellite.
The dipole antenna leads will go through the fastener, and the antenna ends will be secured but the fastener’s top edges.
Antenna Deployment Tests
We’ve completed basic mechanical antenna deployment tests on a mock-up of the LED face of the satellite. A joint effort is currently under progress to finish the design and testing for the electronic release system for the dipole antenna.
Transceiver Signal Tests
These tests will include field-testing (on a 10-15 mile scale) of our transceiver setup to ensure that the transmitted signal will be strong enough
Satellite Reception Testing
Once our ground station is finished, we will calibrate it and use it to receive transmissions from amateur radio satellites.