Manfred Wittig

In-orbit measurements of microaccelerations of ESA's communication satellite Olympus

On ESAs large communication satellite OLYMPUS a set of three orthogonally arranged microaccelerometers is installed. The aim of this experiment is to characterize and observe the behaviour of the different mechanisms in space and to get measurements of the vibration levels which are of relevance for the design of optical communication payloads. During the commissioning phase of the spacecraft in October 1989 a lot of data recordings were made at ESTEC. The translation of the measured acceleration spectra into a base motion spectra shows a higher amount of vibration in the frequency region above 100 Hz, when compared with previously reported data from Landsat spacecraft. The main disturbing vibration sources, solar array drive and thruster firing, can not be characterized precisely enough before the launch. This confirms that measurements in space are mandatory before flying an optical payload. Therefore ESA plans to install this kind of equipment on several other spacecrafts which will be launched before launch of ESAs optical communication payload SILEX. In this paper the equipment on board OLYMPUS will be described followed by presentation of typical measured data. A specification for the micro-vibration environment with which an optical payload has to cope is then derived.

European SILEX project and other advanced concepts for optical space communications

The European Space Agency (ESA) is developing an optical inter-orbit communication system enabling a link between a low earth orbiting (LEO) and a geostationary (GEO) spacecraft. The link allows the transmission of 50 Mbps between LEO and GEO in an experimental and pre-operational mode. The system uses laser diodes of typically 100 mW optical power at a wavelength of 830 nanometer. Direct intensity modulation is applied. Telescopes of 25 cm diameter are used on both terminals. The breadboard phase has been completed and the launch of both terminals is scheduled for 1994. Other concepts for optical space communication links using Nd:YAG lasers and heterodyne receive systems are outlined.

European SILEX project: concept, performance, status, and planning

The European Space Agency (ESA) is developing an optical interorbit communication system enabling a link between a low earth orbiting (LEO) and a geostationary (GEO) spacecraft. The link allows the transmission of up to 65 Mbps between LEO and GEO in an experimental and preoperational mode. The system uses laser diodes of typically 100 mW optical power at a wavelength of 830 nanometer. Direct intensity modulation is applied. Telescopes of 25 cm diameter are used on both terminals. The breadboard phase has been completed and the launch of both terminals is scheduled for 1994.

Data relay for Earth, Moon and Mars missions

ESA is setting up the future sustainable European Data Relay mission in geostationary orbit. An outline of the driving user needs, the key system trade off and associated technological challenges as well as the overall partnership strategy with the commercial stakeholders is being described. In frame of the European activities for human exploration a mission concept for an early data relay service and a first Moon lander was elaborated as a joint mission proposal. This mission provides low power communication between the lunar surface and Earth as well as early positioning services on the Moon surface for the first lunar explorers. The Data Relay for Moon (DROM) spacecraft will be launched together with the Moon lander and the combined mission is called the Moon Orbiter and Lander (MOL). One attractive feature of the proposed concept is that the DROM spacecraft will establish a high data rate link between the Moon orbit and the Earth. However, high data rates are not required for the early part of the mission. The DROM payload will be used in his early life span to map the lunar surface with high resolution and will use the data relay link to deliver these data to Earth stations. The third part of this paper is to elaborate concepts for high data rate communication between the Mars surface and the Earth supporting Human exploration of Mars.

Tracking experiments with OTS using the mitrex modem

Abstract Ranging tests with the “Microwave Time and Ranging Experiment” module, MITREX, and the ESA telecommunication satellite, OTS, are described. Slant range measurements with a very-high-resolution (15 cm or better) were obtained by using only the MITREX modem and a time interval counter as additional ground station equipment. The transmission power used was equivalent to that required for one telephone channel. The link was shown to be very insensitive to interference, due to the fact that the signal-to-noise ratio for the pseudo-random ranging signal may be as low as − 10 dB within a bandwidth of 3 MHz. Four-way ranging using three earth stations was employed to demonstrate the practical ease of instantaneous position and velocity measurements to high accuracy using MITREX. The significance of the application of high-accuracy ranging in the orbit control of telecommunication satellites is discussed. It may be concluded that the MITREX has application in current telecommunication missions requiring more precise orbit control, as well as future concepts, such as geostationary platforms and clusters.

Generic communication user and system requirements for future space science missions

Since space missions have been flown, a proper data return to Earth was inherently required. The complexity level and the number of space experiments have increased over the last decades. As a consequence, higher data rates have been required. The most limiting factor on achievable data rates for interplanetary communications is the distance. It imposes challenges on all parts of the communication system. Communication user and system requirements must be carefully defined to fulfill the mission needs. These requirements are expected to increase in the future. Hence, we studied 13 generic future ESA space mission scenarios comprising 5 Lunar missions, 5 Mars missions, and three missions to special targets (Lagrange point L2, NEO, and the Jovian satellite Europa) based on planned and already flown missions in order to derive communication user and system requirements. These requirements were classified, scaled, adapted and validated by calculations for these missions defined in [1]. Within this paper we will present how communication user and system requirements for future space science missions can be classified, scaled and applied. Therefore, after introducing a proper classification of communication requirements we will present recommendations by means of concrete examples assuming an Asteroid Sample Return mission scenario. Some practical hints will be provided as well.

Link budget model and applications for laser communications through the atmosphere

One of the activities being undertaken in the Free-Space Optical Communications Program of the European Space Agency, is a detailed description of the Link Budget for Laser Communications through the atmosphere. A link budget model definition, for a system using a tracking system with a given probability of failure shall be presented here, as well as an application to a ground station to/from the geostationary ARTEMIS satellite optical link.

Inter-mountain measurement campaigns on atmospheric propagation in the near infrared

Propagation effects at around 830 nm were evaluated in an intermountain scenario in the Canary Islands (Spain) in the framework of an ESA Free Space Optical Communication program. It is concluded that the extinction effects in clear weather conditions do not limit possible ground experiments. The different turbulence-induced effects are found to be reasonably consistent with simplified models. Power scintillation and point-spread function widening can be disturbing effects on the horizontal path.

Effect of microaccelerations on an optical space communication system

Various image processing algorithms for acquiring and tracking an inter-satellite laser beam are assessed. These algorithms aim at detecting a faint point target of a few picowatts embedded in a background by means of acquisition systems based on CCD sensors. Simple algorithms for real-time operation have been designed. A model which takes into account the background, its spatial and temporal fluctuations as well as the opto-electrical parameters of the CCD has been developed. Fluctuation of the background and CCD noise appear to be the main perturbations that hinder obtaining the targets spatial coordinates. Two types of image processing techniques have been analyzed: single frame processing using spatial filtering and multiple frame processing using frame subtraction. The performed analyses show that in systems where fixed spatial fluctuations are the main constraints, the frame subtraction method is the most efficient in terms of probability of acquisition. Whereas the spatial filtering method is much simpler to implement, it can offer a significant improvement if the spectral energy of the fixed pattern parasitic background is low within the passband of the chosen spatial filter. The performances of these techniques are compared in terms of successful target acquisition.