ON OBSERVATION OF LOWMASS AND SMALLSIZE SPACE OBJECTS USING THE GROUNDBASED OPTOELECTRONIC MEANSS.E. Zdor, V.I. Kolinko, V.V. TitenkoFSPC 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 smallscale satellites it was noted [1]: “…the strategy of research
and exploitation of the outer space using lowmass and smallsize space vehicles (SV) is
taking on special actuality. The share of lowmass and smallsize 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 smallscale 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 smallsize space vehicles at the
contemporary level cannot completely replace or displace the conventional space vehicles
which are multipurpose and multifunctional 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 wellgrounded scientific
and technical policy according to the principle of “wherever it is necessary and
advantageous”.
The given lengthy quotation from [1] defines the general trend of the smallsize SV
development. Table 1 gives the concrete information on smallsize SVs that were launched in
1981–2001.
Table 1
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.
Fig. 1.
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 r_{eq}, then it is expedient to set this dependence as r_{eq} = r_{eq} (M_{SV}) where M_{SV} 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 r_{eq} = r_{eq} (M_{SV}) for
minor space vehicles beginning from «Sputnik1» and «Avangard1» 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
«miniSV» the radius r_{eq} lies within the range of 0.3 through 0.9 m and for «picoSV» it is
within the range of 0.050.1 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 [3]. 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 smallsize 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:
(1)
where m_{Θ} is the visible Sun's magnitude in V system behind the atmosphere, m_{Θ} = – 26.78; α is the SO integral reflectance (the albedo) of the visible object surface averaged over it; r_{eq} 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; τ_{atm}(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
(2)
where: R_{e} = 6371·10^{3} m is the Earth mean radius, H is the SO flight altitude.
We can assume with sufficient degree of accuracy that
(3)
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 r_{eq} and located at different altitudes H has been performed. While
calculating, it was assumed that α = 0.3; τ_{atm} = 0.7. The calculation results are given in Table 2.
Table 2
Another unfavorable factor affecting minor loworbit SOs is their great apparent angular
velocities ω_{SO} 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.
Table 3
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 [5]. 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 lowmobility 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 m_{SO} ≤ m_{p} where m_{p} 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
smallsize SO.
CONCLUSIONS:
REFERENCES
Ðàçìåùåí 7 äåêàáðÿ 2006.
