4.0 Communication Subsystem The primary goal of the communication subsystem is to provide a link to relay data findings and send commands to and from the CubeSat. Telemetry and command subsystems will ensure continuous communication between the ground station and the CubeSat after ejection from the ARLISS rocket. To better understand the basics of the CubeSat communication architecture, the theory behind the system will be presented first, followed by a timeline of the progress that has been made throughout the semester. 4.1 Background The CubeSat communication system is composed primarily of the telemetry and command systems, which send and receive data, respectively. Analog and digital data collected by the sensors and payload of the satellite must be relayed to the ground station via the telemetry system, which is composed of a transmitter that acts much like a “modem in a computer”. The microcontroller will accumulates data from the sensors and convert these inputs into a stream of 8-bit binary numbers. This numerical string is encoded into AX.25 protocol by the terminal node controller (TNC), which serves to “packetize” the information and key the transmitter. The transmitter then sends the signal to the ground station through the satellite’s antenna [Dominguez and others, 2002]. A radio operating in the ultrahigh frequency (UHF) band at the ground station will receive the data signal and encode the stream to a form that may be interpreted by software on a laptop.Another vital aspect of the communication subsystem is the command/uplink portion. From the ground, moderators must be able to send commands to the system. All incoming signals from the ground station will be compared to all other inputs, and any errant signals are discarded [Dominguez and others, 2002]. The Satellite Solutions CubeSat design will implement commercially available transmitter and receiver packages that operate in UHF, and therefore, care must be taken to ensure that the correct radio-data protocol is followed for the transmission to be efficient, reliable, and robust. Also, the frequency of the signals must be transmitted within the correct FCC license regulations for the system. The most common protocol is AX.25, which was originally developed for amateur radio use as the basis for applications in mobile and radio-data transmission [Thorcom, 1998]. 4.2 Requirements and Constraints As mentioned previously, the CubeSat system must be no larger than 10 cm on each side and weigh less than 1 kg. Therefore, the internal components must be scaled to fit within these constraints. Since only a fraction of the 1000 cm3 volume is allotted to communications, the team must select a transmitter and terminal node controller (TNC) that are miniaturized. The entire system must also be relatively inexpensive and operate within the designated frequency band allowed under the Federal Communications Commission (FCC) amateur radio guidelines. Finally, for adequate communication time, the data transmission rate is desired to be at least 9,600 baud, and the amount of working amperage drawn by the communication subsystem must not exceed the available power provided by the batteries or solar cells.4.3 Options and Evaluation One of the largest obstacles for the Satellite Solutions team was the overall lack of experience in radio communications. Therefore, the CubeSat communication subsystem will consist primarily of commercially available off-the-shelf (COTS) components for both the internal and ground station systems, in order to simplify the amount of modifications necessary to build a working system. Given the requirements listed above, several COTS options have been researched to aid in selection. For the internal CubeSat communication system, an Alinco DJ-C5 transmitter was investigated. Figures 9 (a) and (b) show the external and internal view of the Alinco device. (a) (b) Figure 9: (a) Outside view of Alinco DJ-C5 transceiver prior to modifications [RigPix, 2001]. (b) Internal view of Alinco DJ-C5 transceiver prior to modifications. [PacComm, 2003].The DJ-C5 is small, lightweight, and versatile, as it can transmit in the 144-146 and 430-440 MHz range [RigPix, 2001]. The CubeSat team disassembled and modified the transceiver to allow for further miniaturization and increased performance. First, the front and back plastic covers were removed. The flexible antenna was unscrewed from its connection point, and the Lithium-ion battery pack was disconnected. Next, a team member cut both sides of the transmitter’s case with a Dremel tool to reduce the length of the device by approximately 3 cm. Figures 10 (a) and (b) show the Alinco DJ-C5 after the modifications were made. (a) (b) Figure 10: (a) Outside view of Alinco DJ-C5 transceiver after modifications. (b) Internal view of Alinco DJ-C5 transceiver after modifications. After all modifications were complete, the DJ-C5 was connected to the previous CanSat electronics and a power supply, to ensure that the transmitter still had the capability to send a signal despite the changes. The test was successful; the transmitter could indeed still send a data signal.Next, different terminal node controllers were researched to find one that would work with the rest of the system, including the Alinco DJ-C5 transmitter and the Atmel microcontroller. The best option for this electronics package was the PacComm PicoPacket. This TNC would operate in transparent mode to control the flow of data to and from the microcontroller, as well as radio transmission and reception. The PicoPacket (Figure 11) was attractive because of its electronic capabilities, and since it was the only TNC found that could fit within the strict volume limits, with a total volume of approximately 180 cm3 [PacComm]. Figure 11: PacComm PicoPacket miniature TNC [PacComm]. Another interesting possibility for the communication system employed a faster, more inclusive package manufactured by MaxStream. The MaxStream XStream 900 MHz Wireless OEM (Original Equipment Manufacturers), as shown in Figure 12, is a frequency-hopping module that allows for wireless communication and can sustain a continuous data stream at a given data rate.Figure 12: Size of MaxStream XStream 900 MHz wireless OEM module. This device has several advantageous features, which include [MaxStream]: 