ON OBSERVATION OF LOW-MASS AND SMALL-SIZE SPACE OBJECTS USING THE GROUND-BASED OPTOELECTRONIC MEANSS.E. Zdor, V.I. Kolinko, V.V. Titenko
FSPC JSC “S.A. Zverev Krasnogorskiy Zavod”
The existing general trend to reduce the mass, the size and the power consumption of technical devices and systems is also applicable to space systems in full measure. At the 3rd International Conference on small-scale satellites it was noted : “…the strategy of research and exploitation of the outer space using low-mass and small-size space vehicles (SV) is taking on special actuality. The share of low-mass and small-size SV in the orbital groups of spacecraft, that are launched now, increases continually. The forecasting evaluations show that this trend will grow in future”.
In view of the fact that minor space vehicles are now becoming the key element of the perspective space programs a lot of leading space companies of the USA, Russia, the UK, Germany, France and China carry on active works in developing new generation of space systems using micro- and nanotechnologies to solve problems related to the science, economics, defense and the state security.
The more active policy in the field of small-scale space vehicles is due to the fact that the developing potential of micro- and nanotechnology allows to greatly improve the performance characteristics of the conventional space vehicles.
At the same time it should be admitted that the use of small-size space vehicles at the contemporary level cannot completely replace or displace the conventional space vehicles which are multi-purpose and multi-functional in most cases and are designed to solve problems that cannot be “covered” by minor SV. Therefore, the transition to the use of small- size space vehicles should be gradual and successive based on the well-grounded scientific and technical policy according to the principle of “wherever it is necessary and advantageous”.
The given lengthy quotation from  defines the general trend of the small-size SV development. Table 1 gives the concrete information on small-size SVs that were launched in 1981–2001.
By the minor SV classification adopted by space vehicle designers the account of their mass is taken first of all. The size characteristics are not considered to be the principal ones, since the cost of launching into an orbit is usually estimated in «dollars per kilogram». When SVs are observed by the optoelectronics means quite other characteristics, however, begin to play the decisive part, and first of all the dimensions of space vehicles, their configuration, and the optical properties of their surfaces.
It is necessary, therefore, to determine the dependence between the mass of a SV and its characteristics affecting its brilliance, and first of all its dimensions. If it is assumed that a space object of any virtual configuration can be approximated by a certain equivalent sphere with the radius req, then it is expedient to set this dependence as req = req (MSV) where MSV is the mass of the SV. Unfortunately for a great many of SVs such dependence can be obtained only approximately. The diagram on Fig. 1 actually shows the dependence req = req (MSV) for minor space vehicles beginning from «Sputnik-1» and «Avangard-1» and ending with some vehicles that are in the design stage now. Though this diagram is not at all complete, it still allows to clarify the situation arising in the outer space. The obtained statistics show that for «mini-SV» the radius req lies within the range of 0.3 through 0.9 m and for «pico-SV» it is within the range of 0.05-0.1 m.
The above consideration concerns for the most part with the space vehicle observation. The important component of the general problem of minor space objects (SO) is also the observation of space refuse fragments . The operation of passive optoelectronic means for observation of both space vehicles and space refuse fragments, while being practically identical, still presents some difference. This difference is due to the fact that for the space vehicles it is necessary to obtain the «catalogue» data, while for the space refuse fragments it is possible to statistically estimate the space contamination at certain altitudes without running a catalogue for concrete minor SOs. This difference should be taken into account in further consideration.
It is known that for SO detection by a passive optoelectronic means the following conditions should be met:
For small-size SO the second condition is the most difficult to be fulfilled. For the quantitative estimation of the SO brilliance in the stellar magnitudes in the V (visual) system the following expression can be used:
where mΘ is the visible Sun's magnitude in V system behind the atmosphere, mΘ = – 26.78; α is the SO integral reflectance (the albedo) of the visible object surface averaged over it; req is the radius of the equivalent sphere; ψ is the phase angle which defines the quantity of the sunlight reflected in the direction of the optoelectronics means and depends on the mutual location of the SO, the Sun, and the observer; L is the slant range to the SO; τatm(0) is the integral atmospheric transmittance in the visible at zenith; z is the zenith angle at which the SO is observed from the optoelectronic means site; M(z) is the ratio of the air column mass at z angle to the air column mass at zenith.
In its turn
where: Re = 6371·103 m is the Earth mean radius, H is the SO flight altitude.
We can assume with sufficient degree of accuracy that
The SO albedo depends on the SO surface optical properties. In practice diffuse reflection is typical for most cases of the SO observation. As a rule the value of α is in the range of 0.1 through 0.4.
By use of the method exposed above the estimation of brilliance for some typical SOs of different radii req and located at different altitudes H has been performed. While calculating, it was assumed that α = 0.3; τatm = 0.7. The calculation results are given in Table 2.
Another unfavorable factor affecting minor low-orbit SOs is their great apparent angular velocities ωSO that reach 1.12 °/s at the altitude of 400 km and 0.089 °/s at the altitude of 4000 km. Such angular velocities impair the conditions for the SO recording, because, due to the lack of data related to their motion direction, the capability of the signal accumulation in photodetectors is reduced sharply.
The information about the SO average apparent angular velocities is given in Table 3. The velocities are given for the case of SO observation accounting for the Earth rotation. In the Table are given also the linear velocities of the SO image on the sensitive surface of the photodetector and the signal accumulation duration assuming that the focal length of the optical system is 2 m and the pixel size is 20 μm.
If the movement direction of a minor SO is unknown, the way out can be found by introduction of the special search and detection mode described in . The basis for this mode is the approach used by astronomers while photographing the meteors. The mode is as follows. The optoelectronic device field of view is adjusted to the reference point of thecelestial sphere, and then it is put in motion at a fixed velocity and in a fixed direction. The motion velocity is chosen to be equal to the SO velocity at the altitude to be monitored. During the motion the detection events occur repeatedly. When the area is examined the field of view is shifted to a new point and put in motion at the same velocity but in another direction.
In this mode the immobility or the low-mobility of a minor SO against the moving stars is used as a character by which they are automatically detected. As a result only those SOs are detected that move in the monitored direction at the monitored altitude and have the brilliance of mSO ≤ mp where mp is the device penetrability. In this mode there is almost no loss in the penetrability at low altitudes, but it has to be paid for by sharp reduction of searching capabilities.
The above consideration is based on the supposition that the sunlight reflection by a SO obeys the diffusive law, and the mirror reflectivity component is not taken into account. Such behavior is true for the most part of the monitored area. When operating near the Earth shadow boundary the mirror component increases significantly and can exceed the diffusive one by several times. This fact should be taken into account when planning the operations on small-size SO.
Ðàçìåùåí 7 äåêàáðÿ 2006.