Optical reconnaissance (Space situational awareness)

The demands of the modern society need an increased and reliable global access to outer space. Satellite-based navigation applications, for instance, require adequate data-supply. Weather and climate observations ask for satellites, which monitor cloud coverage and wind speeds as well as the thickness and coverage of polar ice caps.

However, such augmented use of orbital satellites is jeopardized by a continuous growth of space debris. The number of tracked debris objects has grown significantly over the last decades. A contributing factor to the growth of the space debris population is the steady rise of rocket launches for LEO and GEO missions. Additionally to increased use of space, the Chinese anti-satellite missile test in 2007 and the 2009 satellite collision (Iridium 33 vs. Kosmos-2251, 789 km above the Taymyr Peninsula in Siberia) increased the debris density dramatically. Such events are not the only source of the increasing amount of debris objects. Collisions of debris itself create an additional quantity of space junk, which is, due to its huge momentum, severely hazardous to space missions. For these reasons, space faring nations seek to diminish the number of debris objects. The first step on this intention is the detection and tracking of such elements – or in other words the surveillance of space. Hence, space situational awareness (SSA) has become the generic term for the monitoring of space, addressing space debris, space weather phenomena and potential impacts of Near Earth Objects (NEOs).
The space debris objects originate from several sources; to mention are for instance explosions of disused rocket stages due to propellant expansion and mixing followed by self ignition, surface degradation due to the harsh environmental conditions in outer space, and the above mentioned collisions of satellites. This leads to an exponentially growth of debris flux over time, even in the absence of a net input in the system (Kessler syndrome). Despite the cascading effect, which was claimed to start in the year 2000, but does not seem to have taken place yet, the mentioned Iridium-Cosmos collision gives an impression of the need for protective measures necessary to secure space infrastructure.

The subject of „debris monitoring“ is tackled at the Institute of Technical Physics (DLR-TP) using laser based observation methods in the future. These methods promise an enhancement of the detection accuracy as well as detection efficiency. Both aspects are of great importance, since the debris objects in lower LEO orbit are slowed down by atmospheric friction. Thus, their orbital parameters change continuously, which necessitates so called follow-up measurements, which in particular benefit from an increased effectiveness of debris observations.

In the field of space debris monitoring, up to now mainly angular information on the debris position is used in order to deduce its position and therefrom the orbital parameters. These orbit data are organized in catalogues, which consists of so called “two line elements” (TLE). This standardised format contains (besides the object number and other administrative entries) information on inclination, right ascension of the ascending node (RAAN), and eccentricity. Hence, the position of the object at a given point in time can be calculated based on these entries. However, since the drag of the earth atmosphere on the debris influences the orbital parameters, so called follow-up measurements are necessary in order to keep the catalogue up to date. As a consequence, known objects have to be re-monitored. Additionally, techniques which are solely based on goniometry need several round-trips of the debris object around earth in order to gain enough measuring data. Hence, these follow-up measures as well as monitoring of newly detected debris is quite time consuming and thus ties capabilities.

A lot of potential to enhance both the accuracy and effectiveness of debris monitoring lies in the detection of the debris distance in addition to angular measurements. This can be done by making use of the well established technique of laser ranging, which facilitates the ranging by using the time of flight of short laser pulses reflected by the target of interest.

In order to track the movement of the prior detected debris object, different strategies can be pursued. The easiest approach is making use of the reflected solar radiation, which can be fed to a quadrant detector to monitor the angular position of the target. Together with the attitude of the telescope, one dataset for the given debris sample can then be recorded. However, if it is planned to use a laser ranging technique which necessitates the illumination of the debris in any case, it is self-evident to make use of the reflected laser radiation for the fine tracking as well. Since the angular position of the object is roughly known from the passive detection described above, the pulsed laser beam can be pointed onto it. The round-trip time of the photons for the upper bound of the LEO space debris distribution is approximately 10 ms, which means that debris objects travel about 70 meters within this period of time. Hence, the pointing and tracking of both telescopes for transmitting and receiving of light have to take this into account.

To tackle the threat of space missions by space debris, the Institute of Technical Physics of the German Aerospace Center (DLR) has developed a concept for laser-based monitoring of these objects. This concept relies on

1. passive optical detection of space debris,
2. fine tracking of these objects,
3. laser ranging to achieve distance information.

Whereas for all of these three steps the general technical approach can be more or less adapted from other methods (e.g. satellite laser ranging, optical free space communication), the fact that the monitoring system has to react instantaneously on appearing debris objects as well as their uncooperative nature imposes specific requirements, which have to be considered in the development. Namely, these requirements are high accelerations of the tracking telescopes of approximately 10 °s-2 to acquire and follow the object with fine tracking accuracy on the order of 1 microrad and a sophisticated pulsed laser system (pulse energy ~ 1 J, repetition rate ~ 1 kHz). In order to perform fine tracking and laser ranging, all relevant components have to be harmonised to be able to detect backscattering from the debris, which enables the time of flight measurement. Since the telescope aperture size is limited by manageability and the detector sensitivity by commercial availability, the characteristics of the pulse laser (beam quality and pulse energy) is of crucial importance for the functionality of a space debris monitoring system. Accordingly, parallel to the development and integration of other parts of the system, a suitable pulsed disk laser will be developed at DLR-TP, which delivers pulses in the range of several Joules with a repetition rate of ~1 kHz at a good beam quality (M2 < 3).

Monitoring of space debris by making use of laser ranging is a promising approach to increase the accuracy and the amount of catalogued entries. In principle, the longitudinal resolution is in the order of the pulse width (here: c • 10 ns ≈ 3 m), but can be improved by using information of the particular pulse shape. In addition to high precision angular data obtained by fine tracking, the range can be used in order to supplement known sets of two line elements and to effectively establish European catalogues of orbital data of space debris objects.

Principle of laser ranging on space debris

Platform for observation of space debris

Wolfgang Riede
German Aerospace Center

Institute of Technical Physics
, Active optical Systems
Tel: +49 711 6862-515

Fax: +49 711 6862-788

E-Mail: Wolfgang.Riede@dlr.de
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