Communications Overview
The communications subsystem is the methods by which the satellite communicates from space to the ground. This is done through radio frequencies, where different frequencies allow for different transmission rates, and therefore, dictates how much information can be relayed within a given amount of time.
Phoenix will operate in the amateur frequency bands under an open, but limited communication link. UHF amateur bands (435-438 MHz) will be used for uplinking mission operations schedules and for downlinking all housekeeping telemetry and images to support the science objective. At a baud rate of 9600 kbps, it will take three full passes over the ground station to retrieve one full image at a size of 321 kB. The design originally incorporated S-Band communications but was de-scoped from the design due to time constraints during development.
Comms at a Glance:
- Operating frequency: 437.35 MHz
- ITU emission: 20K0F1D
- Packet protocol: KISS protocol with AX.25 & HDLC encapsulation
- Bit rate: 9600 kbps
Link Encryption
All uplinked commands will be encrypted with a rotating cipher to maintain operational integrity while the spacecraft is in orbit. However, all downlinks will be left open to abide by the amateur communication protocol. Once the spacecraft has completed the minimum mission objective, a set of limited commands will be made available on the project website so that anyone with an amateur license may listen to and communicate with the spacecraft.
Licensing
In order to communicate with the ground station, the spacecraft must be authorized by the FCC to transmit and receive information as well as be coordinated to operate in an open frequency band. Through working with the IARU, the FCC, and the ITU, Phoenix is licensed to operate in the amateur bands at 437.35 MHz. All mission operators will be registered with a ham radio license in order to transmit to the spacecraft in orbit
Ground Station
A ground station will intercept the telemetry (health data and images) from the satellite when it passes overhead as well as uplink operations schedules for taking images and performing other maintenance operations. The ASU Ground Station will be the primary resource for communicating with the spacecraft to uplink commands and downlink housekeeping telemetry and images. UHF communications are supported by the ICOM-9100 UHF transceiver, the KPC-9612+ TNC, and a set of Yagi antennas.
In addition to learning the cubesat hardware, it is the responsibility of the comms team to test the hardware with the ASU ground station and ensure that the spacecraft can communicate with the ground while it is in orbit. The ICOM and TNC were both used as part of a lab-based ground station setup to allow the team to get acquainted with the hardware as well as fine-tune the hardware settings to minimize packet loss as much as possible. After all, as the saying goes, test as you fly!
Embry Riddle Aeronautical University, located in Prescott, AZ will serve as the backup ground station for UHF communications in the event where the ASU ground station is not operable.
Ground Station Satellite Tracking
The ground station will utilize GPredict for handling doppler shifting and tracking the spacecraft as it passes overhead. Both yagi antennas are attached to a rotor system, which allows the antennas to track the spacecraft as passes overhead. GPredict can be programmed to operate the rotor system as well, allowing tracking to be precise and collect data without it being required that an operator is present at the time of the pass.
Link to Gpredict: http://gpredict.oz9aec.net/