The Communications system is divided into two parts, the amateur radio system and the spread spectrum system. The amateur radio system is used to download diagnostic data, upload control signals and transmit a beacon signal. The spread spectrum system is used for the email system and for downloading diagnostic and scientific data. In keeping with the philosophy for the design of the satellite, off-the-shelf products are the heart of the system.

The spread spectrum radio is an inexpensive commercial system from FreeWave. We chose the 900-930 MHz system because of international frequency allocations. These frequencies are within the Ham radio frequencies and are protected by international treaty. This means that governments can not legally bar their use. This simplifies the process of receiving permission for the portable stations. The spread spectrum radio interfaces to a standard RS-232 port. This makes it easy to interface with the email server.

Fig. 9—900 MHz FreeWave XCVR

For the amateur radio system, we chose a V-U (mode J) system. Using a 70cm antenna for downlink will make it easy for amateur radio operators to monitor the TUSat1 beacon. For the radio we have chosen a Yaesu VX-1 dual band. This radio is ideal due to its small size and low power requirements.

For the Terminal Node Controller (TNC), we used a PacComm PicoPacket. This TNC is very small and powerful. The PicoPacket includes a packet mailbox, but, due to power requirements, this feature is not being used.

Unlike other TNC's we considered the PicoPacket uses a digital carrier to detect the signal instead of relying on the radio's squelch. This simplifies the design by not requiring an external carrier detect circuit. To further reduce the noise on the TNC signal, a CTCSS, or Continuous Tone-Coded Squelch System, will be used on the signal transmitted to the satellite. In order to conserve power the TNC is turned off when not in use. Brain will monitor a pin on the radio. When it receives a signal, Pinky will turn on the TNC. After the connection is closed, the PIC will turn the TNC off.

The satellite will transmit a beacon every few minutes if it has enough power. The beacon will identify the satellite and include basic diagnostic information. Pinky will automatically power up the TNC and send an unconnected protocol beacon containing the proper information.

As mentioned above, there are three antennae on the satellite: two amateur radio antennae and one spread spectrum antenna.

The spread spectrum provides the gain needed for the system. Our satellite will be flying relatively close to the ground so we have to make sure the gain is high enough to get through the atmosphere but low enough to have a usable pass time. Our antenna effectively meets this trade off between pass time and signal strength.

For the amateur radio antennae, two quarter wavelength 70cm groundplanes are being used. These antennae are angled for a broad signal transmitted mostly below the satellite. Because the stations that are receiving this signal are permanent, large antennae are used on the ground. The 70cm antennae will also function as receiving antennae for the 2m amateur system.

Standard amateur radio equipment is being used for the main ground station. The ground station will be programmed to automatically track the satellite passes using data given by the North American Aerospace Defense Command (NORAD). During each pass, the computers in the ground station will automatically connect to the satellite to download diagnostic and scientific data as well as email.

Because of the size, weight, and power restrictions on the satellite, the ground station is designed to compensate for any communications problems. Because of this, we used a high-gain antenna system for the ground station. To account for the Faraday rotation of radio signals, we used circularly polarized antennae. Because of the unpredictability of Faraday rotation, we used both right-hand and left-hand polarized antennae.

Helical antennae were chosen for mechanical considerations. The best length for the 2m antenna is ten feet. To simplify the design, and for better stability and gain, all antennae are ten feet long.

The antenna system consists of two sets of antennae mounted on either side of a rotor. Each set consists of a 2m, a 70cm, and a 33cm antenna. The 33cm antennae are made from ten-foot lengths of four-inch schedule 20 PVC. The 70cm antennae are each made from a ten-foot section of eight-inch schedule 40 PVC. The outside diameter of this pipe is the exact size needed for the frame of a 70cm helical antenna. The weight of two ten-foot sections of schedule 40 PVC is much higher than the rotor can handle. To reduce this weight, we cut large sections out of the pipe. This removed the majority of the weight without significantly reducing its strength. The 2m antennae are built around the 70cm and 33cm antennae. Rods extending from the common axis support the heavy wire of the 2m helix. To reduce any possibility of interference between the antennae we wound the 2m and 33cm antennae opposite of the 70cm antenna.

One of the goals of TUSat1 is to test the miniaturization of satellite communications systems by using standard components. This goal requires small portable ground stations capable of communicating with the satellite. These portable stations must be small, inexpensive, durable, and have low power requirements. They must also be easy for less technically skilled people in the field to use. The system should be a "black box" that the user aligns and connects to their computer. The only hard part of the setup is making sure that the antennae are aligned well enough to track the satellite.

The portable stations will be fully functional even when not connected to the user's computer. To do this, the system is based around a small 386 board identical to the one used in the satellite. This board will take care of all the tracking and data handling for the user. When the satellite is not overhead, the system will run in a low power mode. Shortly before the satellite comes over the horizon, the computer will power-up the spread spectrum radio. We used the same spread spectrum radios in the ground stations as in the satellite. The radio is programmed to attempt to connect with the satellite upon startup. Once a connection is established the ground station will automatically download all email addressed to it, upload all of its outgoing email, download any software updates required, and download the latest NORAD tracking elements.

The user's computer will connect to the ground station using an Ethernet link. This simplifies the portable stations by eliminating the need to deal with multiple types of connections.

Because of the simplicity required in the portable stations, a complex tracking system like the one used in the ground-station is unusable. Instead, we used a system of several small directional antennae. These antennae will have much lower gain than the helicals used at the ground station. The design of the antenna system will allow the alignment to be done by simply using a compass to point an arrow on the antenna box north.

Using an omni-directional antenna on the ground station is not an option, because of the low power output allowed to unlicensed operators on the spread spectrum radios. The best solution to the complexity problem is a system of several small directional antennae. To remove the need for a rotor, we used an electronic switch between the antennae.

The computer in the portable station must know its approximate location. This is used along with tracking information downloaded from the satellite to control the spread spectrum radio and the antenna switch. During each pass of the satellite, the latest tracking information will be downloaded to the groundstation. The tracking system will automatically turn the radio on shortly before it is needed. This will provide a safety factor in case the portable station is unable to connect during several passes. This extra time will also allow the ground station to work if its exact position is not known. As the satellite passes overhead, the tracking program on the 386 switches the antenna.