karn@allegra.UUCP (Phil Karn) (02/29/84)
A Guide to the Architecture of UoSAT-B The following attempts to provide a guide to the sub-systems on the UoSAT-B spacecraft, due for launch on 1st March, 1984. Expanded specifications of individual modules will be made available for those experiments which supply data to the downlink directly or via the telemetry system. Mechanical Framework The spacecraft is constructed in a similar way to Oscar-9, that is with a square-section central core supporting rigid top and bottom plates. Solar cells are mounted on all four sides of these plates, enclosing a basic cuboid of dimension 35.5 * 35.5 * 58.5 cm. Two stacks, each of two module boxes of dimension 23.5 * 17.6 * 3.1 cm, are mounted on the outside of each face of the central core. A 'wing' extends the base of the spacecraft symmetrically by 16 cm on each side in one axis to permit the mounting of two SHF helical antennas, one on each side of the launcher attach fitting which is itself mounted in the centre of the bottom plate. The Navigation Magnetometer and Space Dust experiments are mounted above this wing, one on each side. Solar Cells Four solar arrays of dimension 49.5 * 29.5 cm are attached to the four sides of the spacecraft. These are capable of supplying up to about 0.9A at 28v when fully illuminated. The cells were manufactured by Solarex. Battery A solid octagonal block of aluminium, 14.9 * 14.9 * 10.2 cm, is fitted into the centre core of the spacecraft and is drilled to accept ten 'F'-sized Nickel-Cadmium cells, each 3.2 cm in diameter and 9 cm long. These cells, in series, form a 12v battery of 6.4Ah nominal capacity and are charged when the spacecraft is in sunlight in order to provide sufficient power to run the craft during peak load demands and its eclipse periods. Battery Charge Regulator Two redundant BCRs are responsible for accepting the 28v supplies from the solar cells (and a similar supply from the umbilical connector) and charging the battery as required depending on the current drain, the battery voltage and the battery temperature. Power Conditioning Module The PCM regulates the 12-14v battery bus supply to provide 10v, 5v and -10v supplies for powering the spacecraft systems and experiments. Power Distribution Module (PDM) The PDM switches the various regulated and unregulated rails to all the s/c systems and experiments, dependent on the commands which it receives from the Telecommand system. Each switch has an individual current foldback facility so that a faulty module is allowed to draw up to a pre-determined current before it is latched off, necessitating a power-down under positive command before resetting. Telecommand system The telecommand system comprises three uplink receivers, three data demodulators, a command detector and sets of command latches which hold the status of the command specified. The receivers are located in the 144MHz, 438MHz and 1268MHz amateur bands and the demodulators are robust devices which do not depend on phase-locked loops or other potentially unstable techniques. A command detector scans the three receivers according to a priority system and detects a valid set of command instructions, passing the data contained therein to the relevant latch. Some latches drive a set of multiplexer address inputs directly so that uplink and downlink path selection may be performed immediately on the command latch board. The 112 command latches drive the Power Distribution System, the remaining spacecraft systems and experiment functions. There is a parallel i/o port to the spacecraft 1802 computer for autonomous control of spacecraft operations in addition to serial data links with the 1802 computer and the DCE for backup operations. 145.825 MHz Beacon The 145 MHz beacon on UoSAT-B is nearly identical to the one flown most successfully on UoSAT-1. The modulation index has been increased in order to ensure more optimum reception on most radio amateur receivers. Modulation is by frequency-shift keying, as on UoSAT-1. 435.025 MHz Beacon This beacon is a completely new design which generates its frequency standard from a phase-locked synthesiser system. As a result, the DC to RF efficiency is much improved. In addition to frequency-shift modulation, phase-shift modulation is a switchable option. 2401.5 MHz Beacon When the original supplier of the 2.4GHz beacon was unable to meet his commitment, Colin Smithers, G4CWH, at the University of Surrey stepped in and designed and built the transmitter and power supply in under four weeks. The DC to RF efficiency has been improved by some 5 times over the UoSAT-1 implementation. Both AFSK and PSK modulation methods are possible. Telemetry System The basic output of the UoSAT-B telemetry system is very similar to that of UoSAT-1. However, 60 analogue channels, digitised to 3 decimal digits, and 96 status points encoded into hexadecimal digits are available together with a real-time clock for frame identification and the satellite identifier, 'UOSAT-2'. A checksum digit can also be added to each channel. A dwell facility has been added so that up to 128 channels can be output in rotation, combined with clock times and line feeds or frame ends in any combination. 1802 Computer & Digitalker The 1802 computer has been designed to support all the modules on the spacecraft, as well as to control the overall scheduling and be usable for specific communications experiments. To satisfy these requirements, the computer has access to many modules via parallel interfaces, and to some of the others and the receivers and transmitters via serial connections. In addition, there is a real-time clock and a total of 48kb of RAM for data storage. The Digitalker speech synthesiser is housed with the 1802 and has ROMs containing over 550 words. These will be used initially for 'speaking' telemetry. Navigation Magnetometer The Navigation Magnetometer is a three-axis flux gate device, much upgraded from the one flown on UoSAT-1. Indeed, the 14-bit resolution is very similar to that obtained from the much more complicated scientific magnetometer on the previous craft. The Nav. Mag. will be used for determining the attitude of the spacecraft during initial manoeuvres, as well as for experimental measurement of magnetic field disturbances once the attitude is stable. Magnetorquers & Boom Assembly Magnetorquers - coils of wire energised to act as electromagnets - are built into all 6 faces of the spacecraft, wound around the edges of the honeycomb panels supporting the solar cells and the top and bottom plates. The fields created interact with the earth's magnetic field to produce a torque which tends to rotate the spacecraft. When the spacecraft has been positioned so that the CCD camera end is pointing towards the earth - a long and complex process - a boom can be extended from the top of the craft. The boom looks similar to a steel tape measure, although nearly circular once it has been unrolled, and some 12 metres long. The boom carries a 2.5kg mass on the far end and this, in conjunction with the spacecraft body at the other end, creates a 'dumb-bell' configuration which naturally lines up with the earth's gravitational field so that one end points downwards, rather like a pendulum - it is, however, bi-stable! Any residual swinging motion can be damped with further controlled applications of the magnetorquers. Sun Sensors The sun sensors are made with specially fabricated solar cell substrates which are masked by grey-code stripes and illuminated by light passing through a slit in a metal foil in front. The mask coding on the cells can be used to derive the angle at which the incident light is falling on the slit. 6 such sensors are mounted around the top plate to provide complete 360 degree coverage. Horizon Sensors Built by a first year student at the University of Surrey, the Horizon Sensor is able to detect when only one of two photodetectors is illuminated. The detectors are housed in two narrow tubes of 4mm diameter and mounted at a small angle to each other so that the whole sensor thus detects the 'edge' of an illuminated object. This will be the earth, the moon or the sun and a fix can then be made on the object's position. Digital Communication Experiment The Digital Communications Experiment (DCE) was designed and built by AMSAT and VITA groups in the USA and Canada. It has two serial ports which can receive and transmit to the RF system and the 1802, as well as an NSC-800 CPU and nearly 128kb of CMOS RAM. The DCE will be used to investigate various packet radio protocols for use with a future digital 'store-and-forward' satellite being planned by AMSAT. In addition, the DCE has interfaces with the navigation magnetometer and the telemetry system for long-term data storage. Space Dust Experiment The Space Dust experiment was built by a group of students at the University of Kent, England. It has a dielectric diaphragm which, when punctured by a large particle, discharges the capacitance associated with it, therby indicating the impact. In conjunction with a piezo crystal microphone which detects particles of smaller size, correlation techniques can yield a measurement of the momentum of the incident particle. CCD Camera The CCD camera is a re-designed version of the device flown on UoSAT-1. Indeed, the CCD array at the centre of the camera is the same type as used before, although the later batches of this part are substantially improved over the early one used 2 years ago. This time the analogue electronics surrounding the array are also greatly improved. The active area of 384 pixels by 256 pixels is stored with seven bits of grey-level, in 96kb of RAM in the DSR experiment. The DSR is then responsible for the picture downlink, adding addresses and error correction and detection information as required. The DSR downlink is organised in packets of 128 bytes each, three across each imager line, so that two may be selected for display (using an extra digital filter) on existing UoSAT-1 CCD displays. The variable video amp gain and integration period of the CCD imager have been set up to provide the latitude required to photograph both land images and also auroral features, the latter being of interest in conjunction with the particle detector experiments. DSR Experiment The DSR stores data from the CCD imager, particle counter experiment or computer UART and outputs it in a checksummed format. The unit has 2 banks of 96k x 8 CMOS memory which can be used as two seperate banks or as one 192kb bank. The output frame consists of a three byte sync code, a two byte frame address, 128 bytes of data and 5 bytes of error detection/correction code. The data is sent in serial form with start bit, 8 data bits and selectable 1 or 3 stop bits. The data can be output at 1200, 2400, 4800 and 9600 BPS. Particle Detectors and Wave Correlator Experiment Three Geiger counters, each with different electron energy thresholds, similar to those flown on UOSAT-1, and a multi-channel electron spectrometer are mounted on the spacecraft to serve as a near-earth reference for magnetospheric studies to be carried out concurrently with the AMPTE & VIKING spacecraft missions due for launch later in 1984, and for ground-based studies of the ionospheric D, E and F regions being pursued with riometers and EISCAT. Data will be available in either real-time or, for more detailed analysis, from stored measurements over both polar auroral regions to professional scientists and radio amateurs. A data-base of the measurements acquired over the life of the spacecraft will be established at Surrey in conjunction with the SERC and will be available to approved experimenters. The modulations imparted on particles, as a result of wave-particle interactions in the magnetophere on auroral field lines, will be observed by a Particle Correlator Experiment designed around an NSC-800 microprocessor at the University of Sussex, England. The measurements will identify wave-modes responsible for accelerating electrons into the auroral beam and will also identify wave-modes which limit the further growth of the auroral beam. Roger M. A. Peel, G8NEF, University of Surrey, England.