The Suntracker is designed to point a solar cell at the sun as a balloon ascends and downlink data containing the cell short-circuit current, cell temperature and electronics module temperature. The suntacker includes a two-axis tracker and electronics module that together weigh less than thirteen ounces. The two-axis tracker has a collimator and two motors that are supported 5.06" above the electronics module by a 0.25" aluminum rod. The dimensions of the collimator were selected to prevent light scattered from the balloon,earth, moon or clouds arriving at the solar cell.
The Suntracker has two MicroMo series 1016 DC motors that point the collimator at the sun. Each motor assembly has a gearhead and magnetic encoder and weighs less than one ounce. The gearheads have a 377:1 reduction ratio enabling the motors and gearheads to deliver 13 oz-in torque. One motor controls the altitude angle of the collimator that ranges between 0o and 90o; the other motor controls the bearing angle between 0°C and 360°C. Each magnetic encoder produces two channels of square wave pulses in quadrature that are TTL/CMOS compatible; ten pulses are produced per revolution of the motor. The pulses are input to the electronic module to keep track of the position of the collimator.
The signal from the solar cell is input to an operational amplifier. One of the outputs from the operational amplifiers is input to a MIM module that was purchased from Clement Engineering; it is a 1200-baud, AX.25, transmit terminal node controller (TNC) that can telemeter five analog and eight digital signals at user selectable intervals. The MIM module is slightly larger than a credit card and programmable via a serial port connected to a personal computer (PC). It draws less than 15 mA and is powered during the flight by regulated 5.0 VDC from a battery pack. The unit is programmed to activate the push-to-talk function of the transmitter, and transmit data and a beacon message. The MIM module encodes data in the AX.25 format and inputs it to VX-1 YAESU radio.
The signal from the solar cell is used to control the Suntracker in order to produce a maximum in short-circuit current. The design specification is to control the Suntracker in seeking the maximum in solar cell short-circuit current to better than 1 %. This is done with a controller consisting of an eight bit analog to digital converter (ADC) and a Parallax, Inc. Basic Stamp 2 microcontroller (BS2). The solar cell signal from the operational amplifier is input to an 8-bit ADC. The digital signals from the ADC are input to the BS2 which is programmed to control the altitude and bearing DC motors. The BS2 is programed in PBASIC via a PC serial port.
The Suntracker package measures about 10” in diameter and 10“ high. It has a wall thickness of about one inch and weighs close to six pounds including electronics, batteries, antennas etc. The package was fabricated using a two-part urethane pour foam and molds. The collimator and motors are located on top and the electronics inside the package. The video camera can be seen on the left-top side of the package. The batteries are mounted on the wings of the package to stabalize the package during the flight. The antenna for the 70 cm and 2 meter transmitters are mounted below the package. During flights the package is suspended 150 feet below a latex meteorological balloon that is pressurized with helium. The train from the balloon to the package includes a swivel, parachutes attached to hoops and three 0.010” diameter shrouds tied to the hoop and package.
The payload includes the 2 meter and 70 cm systems. Each system has its own battery pack, GPS receivers, transmitters and antennas. The 2 meter system includes the Suntracker controller electronics, GPS receiver, 2 meter transmitter and 2 meter antenna. The video system is included in the payload in order to monitor the operation of the Suntracker and determine the stability of the payload during the flight. A CMOS model C3186A color camera provides live video that is downlinked using a 1 watt UHF TV transmitter connected to an antenna. A Garmin GPS receiver is used to determine longitude, latitude, heading, altitude and speed data. The GPS data is overlaid directly onto the live video with an Intuitive Circuits OSD-GPS video overlay circuit board. The electronics in the payload is powered by a battery pack that includes 4 SAFT LX 3457 lithium D-cells with a 14.4 VDC output and 7.5 Ah capability. Lithium cells were used because they maintain high efficiencies in the -60 oC temperatures encountered in the upper atmosphere. The batteries have the capacity to supply power to the payload for a minimum of 8 hours.
The Suntracker system is designed to downlink data with two transmitters operating on the 70 cm and 2 meter bands at frequencies of 439.25 and 144.34 MHz, respectively. Cost considerations result in a design criterion for the Suntracker that emphasizes the use of commercially available devices wherever possible. Separate communication systems were designed for each frequency in order to maximize the probability that position data be available throughout the flight in the event of a system failure. Each system has its own transmitter, GPS receiver, battery pack and supporting electronics.
Controller Board Description
The controller board used in the Suntracker I flight employed a through-hole prototype circuit. In an effort to improve reliability and as well as reduce the weight and area of the controller board for the following Suntracker flights, it was decided to use surface mount technology on a printed circuit board. The new controller board is composed of a 3” x 3.125” printed circuit board populated mostly with surface mount components. The printed circuit board area is about 11 in2 smaller than the through-hole prototype circuit. It weighs approximately 1.5 oz., about 3.0 oz. lighter than the prototype board.
The cell current and temperature, and the internal package temperature, are measured with devices that are mounted on the controller board. There are devices mounted on the controller board that position the collimator
and convert measurements to an AX.25 protocol for downlinking using a 2 meter transmitter. The two linear power supplies are mounted on the board. The low drop-out linear supply powers the BS2p microcontroller, A/D converter, operational amplifiers, motor driver and MIM module, all of which are mounted on the controller board. The low-drop out adjustable supply is used to vary the voltage to the motor driver in order to adjust the speed of the altitude and bearing motors. The motor driver is used to control direction and on/off states of both motors. Each motor has two control lines. By utilizing these control lines a motor can be energized in a forward or reverse direction. The driver also allows the motor leads to be connected together via an h-bridge to support dynamic braking. A built-in thermal shutdown is included in the motor driver to prevent damage to the circuit under an overload condition. Inputs to the motor driver are TTL compatible thereby simplifying the interface to the BS2p microcontroller.
Three operational amplifiers on the controller board are utilized for signal conditioning. One is used to linearize the output from a thermistor mounted on the back of the solar cell holder; the signal is used to determine the temperature of the solar cell. A second operational amplifier is used to amplify the voltage across a high precision resistor in series with the solar cell; it produces an output voltage that is proportional to the cell current. The third operational amplifier linearizes the output from a second thermistor that is used to determine the internal temperature of the Suntracker package. The conditioned analog signals from the two temperature sensors and cell current are routed to the analog input lines of the MIM module. The MIM module is an ARPS compatible packet-radio telemetry unit that converts the three analog signals to 8-bit numbers via an internal A/D converter (ADC). The MIM module also has a serial port that is available at a connector mounted on the board; the port is used to receive data from a GPS receiver and configure the MIM module. Output timing of the various signals, user call sign, and message formatting are some of the parameters that can be configured through the serial port. The MIM module packetizes the A/D data, GPS coordinates and user call sign, and sends the packets via a connector on the controller board to a 2 meter transmitter in an AX.25 protocol at 1200 baud.
The voltage from the operational amplifier that senses the solar cell current is also used as an input to the BS2p microcontroller. The analog voltage is converted to a digital form using an ADC configured in a free running mode. The ADC, once initiated, converts the output voltage from the operational amplifier to an 8-bit digital signal that is input to the BS2p microcontroller. The error rate of the ADC over the operating temperature range of 0°C to 70°C is +/- ½ bit. The BS2p microcontroller outputs logic signals to the motor driver while monitoring the VS line from the PIC 16F84 microcontroller.
Suntracker Base Station
Since Suntracker II flight the goasl is been to design and build a mobile system with all the equipment needed for a flight, including the base station, antennas, helium gas tanks, balloons and balloon inflation hardware. Base Station system was developed for the Suntracker II flight. The equipment for the flight was transported with a trailer and van. A trailer was used to transport the 18’x3.3’x4” antenna box and four 8’ tower sections. The trailer was equipped with a winch to raise the assembled tower, two-axis rotator and antennas. The yagi antennas measured about 17.5’ in length; the 70 cm and 2 meter antennas had 25 and 12 elements, respectively. The gain of the 70 cm antenna is 16.2 and the 2 meter antenna gain is 12.6 dBd. The antennas were assembled prior to transporting them because of the task of assembling them is tedious and time consuming. The antennas is about 35’ above ground level. The 70 cm antenna is mounted in the horizontally polarized position while the 2 meter antenna is mounted in the vertically polarized position. The two-axis rotator enables pointing the antennas at the package during flight in order to take advantage of the directional gain of the yagi antennas. The remote-control box for the rotator and the 2 meter and 70 cm receivers are located in the van. A 12 V deep cycle marine battery is used to power the base-station equipment. The data downlinked on the 70 cm system are viewed and saved with a 9” video/VCR unit. The data downlinked on the 2 meter system are viewed and saved using a notebook computer. Mapping software is used to overlay the position of the balloon on a map to facilitate recovery of the package.
Since Suntracker IV flight, base station is no longer being used because of set up time during the launch day. The mobile station is set up in the van with Diamond base antennas and LNAs. The data received during the flight is saved on 2 Notebook computers and TV/VCR system.