Modular design of PERSEUS, a) Payload Module, b) Core Module,
c) Service Module

The primary structure of PERSEUS is the "backbone" of the whole satellite. Is has to withstand all the mechanical stresses during launch as well as the thermal and radiative influences in Earth's Orbit. During the design process the behaviour of the structure with regard to these demands is calculated via computer simulations and verified with real mechanical and thermal test.
The secondary structure accomodating all the satellite's components is devided into three different parts: the propulsion & power supply deck, the AOCS (Attitude & Orbit Control System) & communications deck and the optical payload deck. By seperating the individual components and organising them in these three funtional units the satellite will be easier to be integrated and tested because each unit will be independant to a certain extent.
It is the task of the satellite's thermal design to assure that all of the components can work in their specified temperature limits. This can be done by choosing the used materials and surface coatings in the correct way. But the two different missions of the satellite pose very different requirements for the thermal design. During the Engine Test Phase the thrusters, especially the arcjet, create extensive heat. At the nozzle of the arcjet a temperature of 1000°C was measured during laboratory tests. This heat needs to be prevented from reaching the satellite's interior and heating up other components. In the Science Phase the satellite's telescope needs to have a very constant and preferably low temperature. This point is important for the quality of the scientific data produced by the telescope.

The electrical power for the satellite is generated by three solar panels with triple junction Gallium arsenide (GaAs) solar cells. Two of the three panels will be deployed after the satellite has reached its orbit. Due to the high power demands of the electrical propulsion (ex. arcjet system: about 1000 Watts) PERSEUS will use lithium-ion batteries because of their good energy density (energy-to-weight ratio). On the other hand PERSEUS does not need a Ka-band communication like the Flying Laptop. So PERSEUS will not need a power-consuming travelling wave tube.

Like the Flying Laptop, PERSEUS is a 3-axis stabilized satellite with two sets of actuators (reaction wheels and magnetic torquers) and five different types of sensors to measure the position, the attitude and the rotation rates (GPS, sun sensors, star sensors, magnetometers and fiber optic gyros).
The four reaction wheels are aligned on the surface of a tetrahedron to provide some redundancy in the case of one wheel fails. By accelerating or decelerating the wheels the satellite is pointed towards its target and disturbance torques are countervailed.
The three magnetic torquers are aligned along the satellites X-, Y- and Z-axis. They consist of long conductive coils that produce an oriented magnetic field. In combination with the Earth's magnetic field the torquers can be used to control the attitude of the satellite or to “dump” momentum off the reaction wheels in case they reach their maximum rotation speeds.
The GPS (Global Positioning System) gives information about the position and the speed of the spacecraft and works just like any earth based GPS system. The position and speed is important to calculate the actual orbit of the satellite and to determine the location of the ground stations from which data will be send to PERSEUS and vice versa.
Sun and star sensors provide the satellite with information about its current attitude. Sun sensors give the attitude relative to the sun. This data can directly be used to prevent the telescope from looking into the sun or, by combining that data with the knowledge about the satellite's orbit and the current time, to calculate its orientation. Since sun sensors are not accurate enough for the high demands of the astronomical observation during the Science Phase, PERSEUS will also be equipped with two very precise star sensors to measure its attitude.
The two magnetometers (one redundant) measure the direction of the Earth's magnetic field. With some knowledge about the orbit the magnetic field and the current time give information about the satellites attitude and rotation.
Finally the fiber optic gyros (FOGs) measure the rotation rate of the satellite by using the sagnac-effect. Like the reaction wheels four FOGs are placed on a tetrahedron to provide redundancy.

The AOCS modes are similar to those of the Flying Laptop but sometimes with different requirements in accuracy, agility or stability.

Imaging Modes: A) Inertial- , B) Velocity- & C) Target-Pointing Mode

A - Inertial pointing: The astronomical measurements are taken in this mode. The telescope is pointed towards the target star and the spectrum is taken. During the data acquisition a very high attitude accuracy and stability of 7 arcsec (about two thousands of a degree) is needed. This is done by using Kalman Filters to combine rate information from the star sensors and the FOGs and to reduce the uncertainties in the measured rotation rates.

B – Velocity Pointing: In order to perform the orbital maneuvers the satellite has to be oriented with its thrust direction in the direction of flight or at a specific angle to the current velocity. In a perfectly circular orbit this is identical with Flying Laptop's nadir pointing mode. Since we do not know our final orbit, the nadir pointing mode has to be neglected and replaced by the velocity pointing mode.

C – Target Pointing: For PERSEUS the target pointing mode is only for communication with the ground station and not for image acquisition. Therefore the pointing accuracy is not that stringent.

During the Engine Test Phase PERSEUS will use several sensors for observation of the engine performances. Especially the arcjet thruster nozzle will heat up significantly during a burn and needs to be checked very carefully in order to prevent a failure. The more parameters of the engines are gathered the better the operational reliability during the Lunar Mission BW1 can be assured. The orbit change due to the burns will be detected by the AOCS-sensors and the ground stations and can be compared with the calculated values derived from the pressure and temperature sensor outputs.
In the Science Phase PERSEUS will use its telescope for astronomical observations. The telescope aperture will have a diameter of about 25 cm. It will concentrate the light of a target star on a spectrometer with a CCD (charge-coupled device) sensor. Both of them are designed to work in the spectral range from 120 nm - 180 nm. This means that they work in the part of the ultraviolet spektrum that is called VUV (vacuum UV). In this part of the spectral band many astronomical phenomena like supernovae can be observed by PERSEUS in a way that is not possible from Earth. This is due to the fact that the VUV light gets completely absorbed by Earth's atmoshere. So we absolutely need a satellite to gather the desired information. And PERSEUS will also have many other advantages compared to other observatories in space. The biggest advantage of PERSEUS will be that the Institute of Space Systems IRS has full control over the satellite and can choose the potential targets together with the Institute for Astronomy and Astrophysics IAAT on their own. This enables the satellite to react very fast on certain phenomena in the sky. Since many not yet fully understood astronomical events are unpredictable, this will be extremely important for monitoring the beginnings of these events. With PERSEUS it will be possible to observe even less favourable candidates for a much longer time then it is for other space observatories.

The on-board computer of PERSEUS consists of a reconfigurable, redundant and self-controlling field programmable gate array (FPGA) with high computational power as it is used for the Flying Laptop. Other components of the Flying Laptop will also be reused in order to increase the reliability of the satellite while reducing its costs and development time.
For more information on the on-board computer please have a look at the Flying Laptop homepage.

PERSEUS will use low gain UHF and VHF antennas for receiving commands from the ground station and sending back housekeeping data. Low gain S-band antennas for telemetry tracking and telecommands while high gain S-band come into operation for payload data. PERSEUS will not use Ka-band communication since the data capacity of Ka-band is not need and the qualification of the Ka-band antenna system will be done by the Flying Laptop. Hence a travelling wave tube will not be used for PERSEUS.

The system simulation and verification environment introduced and qualified by the Flying Laptop will also be used for the PERSEUS satellite: at first the hardware of the satellite will be simulated by software models and in further steps the software models will be replaced by the real hardware components. By doing so time-consuming and expensive tests with the real hardware can be reduced to a minimum.
For more information on the system simulation and verification environment please have a look at the Flying Laptop homepage.