AMSAT-OSCAR 40 AMSAT-DL Journal

Frank Sperber, DL6DBN
(Translation: Jabi Aguirre, EA2ARU and Frank Sperber, DL6DBN/AA9KJ)

AO-40 Mini-Glossary

In the context with the works of the command-stations, words like squint, flight-attitude or MA often. They are explained shortly in the following article. Especially in the time, when AO-40 is stabilized as ?spinner? in self- rotation, these statements are of meaning.

ALAT

ALAT stands for Attitude-LATtitude, therefore a lattitudinal-statement (Lattitude) of the flight-attitude of the satellite. The reference-level of the flight-attitude-determination is the orbitplane of the satellite. The latitudinal-statement ALAT of the flight-attitude indicates how far the +Z-axis of the satellite shows out of this plane. Positive values show an inclination northwards, negative an inclination of the axis southwards. The main-lobe- direction of the antennas is in +Z-direction. The nearer ALAT therefore approaches to 0, the better are the antennas oriented in the north/south- direction to Earth?s center.

ALON

ALON stands for Attitude-LONgitude, therefore the longitudinal-statement (Longitude) of the flight-attitude. That reference is the semi-major-axis, that perigee, most near-earth point of the orbit, Earth's center and apogee (most distant orbit point) connects. Reference point is the apogee and the counting- direction goes in the same direction as the satellite move in its orbit. ALON indicates with values from 0 to 359 how the +Z-axis stands in reference by the semi-major-axis. ALON = 0 means for example that the antennas in the apogee show in direction Eearth, with ALON = 180 they point in the perigee to the Earth?s center. Values between 0 and 180 declare that the +Z-axis show to Earth on the way from the apogee to the perigee, and with values between 180 and 359 before the apogee.

ES1 and ES2

AMSAT-OSCAR 40 has two Earth-Sensors. The attitude of the satellite can be determined in spin-mode with them and the sun-sensors with respect to the Earth and to the Sun. From this values command-stations can generate ALON, ALAT and the rotation-speed (spin) of the satellite.

Flight-attitude

The flight-attitude declares the alignment of the satellite in space with two values (ALON/ALAT). The current flight-attitude is important for the command- stations in order to be able to program attitude-alterations to improve antenna- or solar-cell-alignment. Also for maneuvers for example with the arcjet-engine ATOS must be kept a certain flight-attitude, so that the thrust works into the correct direction.

The flight-attitude is transmitted via the beacon of the satellite for example. As the orbit in relationship to the Earth rotates by precission continuously, the flight-attitude-statement is applied to a certain reference-time. A good tracking-program takes this movement of the orbitplane into account and calculates the current flight-attitude in each case internally. Caution! A big number of older programs is still working with respect to the predecessors of AMSAT-OSCAR 40, AO-10 and AO-13, their antennas pointed in -Z-direction. ALON and ALAT must be changed accordingly for a correct calculation of the squint- angle of the antennas: ALATnew = -ALAT, ALONnew = 180 + ALON, values over 360 are normalized on 0.

IHU

The Integrated-Houskeeping-Unit is the central on-board-computer of AO-40. Via the IHU the measurements are grasped from inside the spacecraft, controls planned and the data for the beacon generated.

IHU-2

The IHU-2 is a technology-experiment for possible future on-board-computers. Also it can access measurements and can control the satellite. Additionally to the IHU the IHU-2 can access pictures of a camera (YACE), store them and send them delayed to the Earth. These pictures were already consulted additionally for the attitude-regulation of the satellite.

Illumination

The illumination describes how much percentage of the maximum energy the solar- cells can generate. If the sunlight falls on the solar-cells vertically, the illumination amounts to 100%. The more the sun shines on the solar-cells sideways, the more worse the illumination becomes. If the sunlight falls on the cells at the side, the illumination amounts to 0%. The context passes it: Illumination = 100 * cos (solar angle).

MA

The movement of the satellite around the Earth is subdivided into 256 synchronized parts, MA-values. They start counting with 0 in the perigee, reaches 128 in the apogee and in the next perigee 256. With approximately 1,269 orbits/day with AO-40, a MA-unit amounts to approximately 4 minutes and 26 seconds. Certain systems of the satellite (transmitter, receiver, experiments, attitude-control) can on the basis of the MA-value be turned on and off by the IHU. In the later business, certain transpondermodes can be declared in a timetable by MA-values.

Magnetorquer

In the satellite, several electro-magnets, also named magnetorquer, are distributes that can be used in the interplay with the Earth's magnetic field close to perigee for the attitude-control of the satellite. The satellite acts as the rotor of an electric motor while the magnet-field of the Earth forms the stator. The process of this movement is named as magnetorquing. With the magnetorquing, the flight-attitude of the satellite and the spin-speed can be changed during perigee-passes.

SEU

The Sensor-Electronic-Unit changes the signals of the sensors into a format readable for the IHU and transverts IHU-data for the control of the magnetorquers.

Solar-Angle

The solar-angle (SA) is determined with the sunsensors and indicates how much the satellite is tilted to the sun. At an SA of 0 deg sunrays enter the solar- cells perpendicularly giving best illumination. With 90 deg the solar-arrays do not see the sun.

Spin

AMSAT-OSCAR 40 can be stabilized in two different forms in its attitude: by self-rotation (spinning) of the whole satellite about its Z-axis and by three spin-wheels, that rotate inside the satellite, three-axis-control. For the time of the tests and preparations for the later normal-business, AO-40 works in spin-mode. Like with a toy gyro the stabilizing effect is the bigger the speed (spin)increases. However, the magnetic attitude-control becomes more difficult with increasing speed.

As the antennas of the higher bands, L, S, C, X, Ka, are not symmetrically placed on the satellite spinfading and spinwobbling can be observed. Spinfading originates at times of occultation at the antenna-side-lobes or distortions of the antenna-pattern by other parts on the satellite's surface. Spinwobbling is a small Doppler-effekt. Through the rotation of the satellite, some antennas come cyclically closer to and depart from the observer. By this relatively movement, a Doppler-effekt originates of up to 100 Hz according to band, speed and antenna-orientation. Also the speed of the satellite can be determined with the spinfading and particularly the spinwobbling.

Squint

The squint-angle or only squint describes how much the main-antenna-direction of the satellite is tilted away from the observer on Earth. A squint of 0 degrees means that the antennas are optimally oriented to the ground-station. The gain is maximum, and effects by spin are minimal. The higher the squint the worse the audibility of the satellite becomes. With a squint of 90 degrees, one sees the satellite from the side, with 180 degrees, one sees it from behind.

In dependence on the beam-width of the used satellite-antenna, an usefull business is possible at different squints. With the S2-antenna, for example good reception was reported with squint below 60 degrees and particularly below 35 degrees. Main-lobe and first side-lobe are received here. The S1-antenna has a smaller beam-width, but however also more gain, and will make sense only with even smaller squint-angles.

Some tracking-programs calculate the squint from the current flight-attitude, the satellite's position in space and the observation-location.

SS1 und SS2

These sun-sensors measure the orientation of the satellite to the sun in spin- mode. With this and the values of the earth-sensors the flight-attitude can be determined. The sun-sensors have a measuring-area of approximately +/- 45 degrees. If the sun-angle should therefore climb above 45 degrees, the sun cannot be detected by the sensors any longer. This condition steps theoretically all 220 days for a duration of 110 days caused by the travel of the sun around the orbit and the precission of the orbit approximately. This situation can be met by an active change of the flight-attitude. At times this countermovement cannot take place sufficiently quickly or becomes effected by other influences. In the phases, in which the sun-sensors see no sun no attitude-maneuvers by magnetorquing can be enforced for the lack of determinable flight-attitude. AO- 40 is in this case in a type of momentary "Hibernation".

WOD

With the transmission of Whole Orbit Data, selected telemetry is stored in the IHU and are transmitted via the beacon in one data-block. So telemetryanalysis is possible from orbit-phases, in which the command-stations have no direct access to the satellite.

YACE

YACE is the abbreviation for Yet Another Camera Experiment. It is a b/w-CMOS- camera with 512 x 512 pixels that is directly connected to the IHU-2. Beside the reception of Earth images it was already used to gain additional attitude- information from the pictures if Sun and Earth were in an unfavorable constellation for the other sensors.


© AMSAT-DL/DL6DBN; AMSAT-EA/EA2ARU, 05/2001