Some control problems for low-orbit satellites of Earth remote sensing R.N.Akhmetov State Research and The article herein tackles crucial issues of remote
sensing satellites motion control in normal conditions and contingencies. Approaches
to ensuring survivability on the basis of autonomous control of operation state
of onboard systems have been studied. Maintaining autonomy of an unmanned satellite is of
paramount importance at all phases of its life-cycle, and, first of all, it is
linked to intellectualization of control procedures. Therefore it is
interesting to consider the following problems concerning the Earth remote
sensing satellites: -
satellite
autonomous attitude control; -
autonomous
solution to ballistic and navigation tasks; -
in-flight
coordinates calibration; -
improvement
of satellite survivability; -
development
of signature remote sensing; -
development
of criteria for estimation of a remote sensing satellite autonomy. Pattern of a remote sensing satellite control
procedure is in many respects determined by the selected control method of aiming
of the observation equipment (sensing equipment - SE) optical axis at observed
objects. Historically, the first control method for aiming of the
SE line-of-sight at a target was software-based temporal method when the
satellite work program was formed at the ground mission control center and time
of imaging equipment actuation as well as duration of its operation were transmitted
aboard via the radio channel. Attitude control of modern remote sensing (RS)
satellites is performed by coordinate-temporal method which, on the one hand, assumes
assignment of coordinates of survey routes from the Earth via radio link, and
on the other hand, autonomous creation of an attitude control program (ACP) for
imaging aboard the satellite. All data necessary for ACP autonomous aboard
synthesis are also determined and calculated independently, i.e. without
support from the Earth. These data include: -
motion
parameters (coordinates, velocities) of the satellite mass center (MC) in the
Greenwich coordinate system (autonomous navigation task), time; -
satellite
angular position and angular velocities in the inertial coordinate system
(attitude control system (ACS)); -
distance
from the Spacecraft MC to a point on the Earth surface to be imaged; -
prediction
of the Spacecraft MC motion and attitude for the moments of imaging. Note that ACP should allow for
the peculiarities of control both on the routes and on inter-route gaps. To
provide effective control on inter-route gaps it is advisable to minimize
satellite slew time and also time needed to meet the second of the above
mentioned limitations. Satellite attitude and stabilization relative to MC is
done in an orbital coordinate system with respective rotation of satellite body
axes relative to the orbital ones as per each of the three axes. Modern remote
sensing satellites make use of booster guidance, meaning that all calculations
are executed in an inertial system of absolute coordinates. The logic of target acquisition and tracking depends
on the adopted method of imaging. There are: target and route imaging, ground
area imaging, stereoscopic and random-azimuth imaging. Note that aiming of SE
axis-of-sight at the route central line and guaranteeing the required velocity
of sighting in the longitudinal direction is performed with programmed pitch
and roll rotation; and guaranteeing minimum velocity of optical image crosswise
motion in focal plane is performed with a programmed yaw turn. For autonomous ACP synthesis one should know and
predict a Spacecraft MC motion parameters and results of image motion
parametric analysis, and that requires solving the ballistic and navigational
tasks. Analysis of Spacecraft flight control shows that
ballistic and navigational information is required in general for rapid
coordinate-and-temporal referencing of a satellite in the course of its orbital
operation. It is performed by integration of GLONASS and GPS sensors in the
Spacecraft control circuit thus creating an onboard satellite navigation system
(SNS) for efficient execution of the following tasks: -
navigation
measurements according to global navigating satellite system signals and direct
navigational definitions of the satellite MC motion parameters (MCMP); -
refinement
of the satellite MCMP by results of statistical processing of direct
navigational definitions of the satellite MCMP; -
updating
the satellite MCMP for users of the onboard control system; -
generation
and accumulation of navigating and checking information for downlink to the
ground mission control center. Resurs-DK1 onboard navigational system consists of software installed in
the onboard control computer system, and measuring hardware, i.e. onboard
time-coordinate synchronizer Motion parameters are updated in the onboard control
system according to results of SNS periodically with the corresponding interval
in order to keep their required accuracy. Every day of flight the onboard SNS compiles several
files with motion parameters for the given moments of time. These files are
incorporated in data to be downloaded to the ground mission control center.
Minimum and maximum time for statistical processing of direct navigational
definition outcomes and, correspondingly, periodicity of motion parameters
update in the onboard control system is determined pursuant to: -
necessity
of urgent schedule and coordinate satellite control within a time interval,
comparable with one revolution along the orbit; -
reaching
the required accuracy of motion parameters by the beginning of time period
allotted for fulfillment of a certain functional task; -
possibility
to conduct sessions for navigational data definition taking into account
discontinuity of GLONASS navigating field, operating at the current moment of
time of the Spacecraft orbital flight. Over three years the navigational satellite system built according to
the indicated principles has successfully supported fulfillment of target tasks
and continuous autonomous operation of Resurs-DK1. To increase autonomy and survivability of the
satellite it is necessary to guarantee durable fulfillment of navigational
tasks in conditions of partly deployed GLONASS system and possible local
malfunctions of GPS system. Besides, usage of radio navigational fields of these
systems does not contribute to high-precision positioning of a spacecraft.
Therefore, more challenging is creation of integrated inertial-satellite
systems able to cope with navigational and positioning tasks. This may be feasible in case of multipurpose
utilization of measuring information received from inertial sensors, star
trackers and global navigational satellite system. Inertial sensors and star
trackers are the basic sensing elements of modern Spacecraft motion control
systems. At the present stage of remote sensing satellites
development a contradiction appeared between attained pinpoint accuracies
provided by the onboard hardware (seconds of arc) and mutual misalignment of
their sensitivity axes when operating as a part of a satellite, attaining
several minutes of arc that results in reduction of the satellite orientation
accuracy and aggravates quality of the target information. It is not obviously possible to eliminate such a
contradiction barely by certification of the onboard hardware in ground environment.
The effective approach to this problem consists in geometrical coordinate
calibration for the onboard hardware (including imaging equipment) directly
during satellite orbital flight. We consider in-flight
coordinate calibration for the onboard hardware as a verification
process performed aboard satellite aimed at detection and estimation of onboard
hardware intrinsic errors including installation errors of imaging equipment
and onboard segment of the attitude control system, their mutual alignment and
certification, with the subsequent consideration of these ambiguities while
controlling satellite attitude in real time. Our company has been studying scientific and applied
aspects of in-flight calibration of satellite onboard hardware since the 80s of
the last century, in order to reduce deviations of the satellite strap down
attitude control system based on standard measuring instruments (sensors) of
its angular velocity. There were developed calibration procedures for other
gyroscopic measuring devices using sensors acquiring external information, in
particular, dynamic star trackers. In modern remote sensing satellites, hardware-algorithmic structure of the attitude
control system has undergone considerable changes that have demanded new approaches
to the in-flight calibration. Due to constant toughening of quality requirements on
imagery data received by modern remote sensing satellites, increased attention
is paid to calibration of the onboard equipment. Research guidelines in this
sphere are the following: -
calibration
of attitude control system instruments (namely, EBG, SLSST, FOARM gyroscopic
and star trackers) directly by developers at the ground special-purpose
test-benches; -
definition
of the satellite configuration and implementation of corresponding design and
technological procedures allowing for correct calibration (e.g. placing of
sensitive elements of an attitude control system on a common platform - thermo stabilized plate, imaging equipment base
frame, mutual alignment of the instruments' base axes, etc.); -
calibration
of onboard hardware during the orbital flight performing special calibration
procedures and specification of models and algorithms hardwired in the
instruments' processors; -
development
of special angular maneuvers for satellite calibration and estimation of errors
of attitude control system and imaging equipment that differ in their nature; -
development
of special onboard systems and certain calibration modes that allow combined
operation of the attitude control system and imaging hardware; -
development
of algorithmic support for in-flight calibration procedures via virtual
attitude control system devices (i.e. their algorithmic models); -
development
of in-flight calibration techniques implementing regular angular maneuvers
performed by satellite in the process of its functioning. There were considered techniques and results of EBG in-flight
geometrical coordinate calibration performed during Resurs-DK1 flight
qualification test. Such calibration is carried out by stellar monitoring and
taking into account a corresponding deviation mathematical model (DMM) adapted
for conditions of orbital flight and developed at EBG design stage. DMM
coefficients were defined at EBG development tests. DMM and corresponding
coefficients are stored in memory of the EBG central computer and underlie the
calculation algorithm of gyro deviation during an orbital flight. Peculiarity
of this procedure is the following: during calibration and reference orbital
revolutions the satellite angular position is simultaneously measured by EBG
and STC, the obtained data are down linked via telemetry to a Ground Mission
Control Center and after processing are up linked to be used during satellite
real-time attitude control. At stellar monitoring and adjustment of EBG gyros
measuring axes position, the data are used which were obtained from external
orientation sensors (that is from onboard STC star coordinators) as reference
values of satellite angular position. However, accuracy of EBG measuring axes
positions relative to inertial coordinate system is not the same for different
axes: accuracy of optical axis positioning (direction perpendicular to the
sensor placement location) is by an order of magnitude higher than the positioning
accuracy of two other axes. Required accuracy of satellite attitude position
may be reached if the information is used from two synchronously operating STC,
mounted aboard satellite at different angles. But the period of time when
synchronous operation of two sensors is possible, is limited as well (due to the
Sun, the Moon, the Earth overexposure). Some works deal with calibration of the outer positioning
sensors, STC in particular, in a novel way using generation and implementation
of a "virtual" device (STC mathematical model). It guarantees accuracy
of the Satellite attitude position definition corresponding to the accuracy of two
synchronously operating STC. In this case, having defined discrepancies in
readings of the real and "virtual" STC, it is possible to control satellite by
measurements of a single SLSST. Note that the number of periods for which it is
possible to have two synchronously operating STC is considerably below the
number of periods when one STC may operate. Such an approach allows to consider
errors of STC sensitivity axes set-up relative to the Satellite axes and start a
quasi-continuous control of the satellite based on STC measurements. Mathematical aspects of the in-flight geometrical
coordinates calibration based on data from both the space telescope and star
tracker system are considered early. Analyzing heritage of Resurs-DK1 it is
possible to conclude the following: -
it is
necessary to provide an automatic onboard calibration of EBG without on-ground
processing of telemetry data; that will essentially raise efficiency of calibration
and will increase productivity of a satellite; -
at designing of new satellites and in order to reduce offset of EBG and
STC measuring axes it is expedient to provide possible installation of these
sensors on a common platform. The essence of geometrical coordinate calibration for
the onboard hardware of attitude
control system and imaging equipment mounted on a uniform platform (main
frame or thermo stabilized plate) consists in the following. SLSST directly
determines satellite position (or SLSST instrument axes) relative to the
inertial coordinate system. By means of FOARM (or EBG) the satellite position is
determined at the intervals between stellar monitoring by satellite angular
velocity measured relative to FOARM sensitivity axes (by integration of the
known satellite motion equations at the initial angular position measured by
SLSST). In the case under concern two modes are analyzed.
These are: the so-called mode of stellar monitoring and matching of star
tracker imaging equipment axes (SMMA) when position of their measuring axes is
determined by stars. And there is a mode when the current position of the main
axes of platform/imaging equipment is controlled by means of an autocollimation
measuring system and matching of axes (ASMA). The SMMA technique incorporates
two phases. The first phase is implemented directly during the satellite flight
and includes imaging of celestial map simultaneously by star trackers and
imaging equipment, i.e. an optron telescope system. The second phase implies combined on-Earth
processing of celestial imagery data acquired by star trackers and optron
telescope. Here the data received by ASMA technique are also considered. The ASMA
technique guarantees matching of the attitude control system onboard hardware
platform and optron telescope optical axes with an error of 3 seconds of arc. However, the time interval from the moment of
reception (measurement) of data up to their application turns to be considerable;
that leads to unsuspected mismatch of the onboard equipment axes. The problem
to be solved is elimination of this interval transferring this task aboard the
satellite. On completion of SMMA mode the orientation of optron
telescope base line and star trackers base line for a certain instant of time
relative to inertial base line I turn
to be the known values. Orientation of the platform (P) relative to optron
telescope base line is determined by results of ASMA mode. On the basis of SMMA
mode a current relative position of the optron telescope and star trackers base
lines was calculated. Position of attitude control system onboard equipment
relative to optron telescope optical axes may be computed basing on data
received in SMMA modes; whereas variation of the optron telescope and star
tracker current position is estimated by means of ASMA. The acquired data are
used in the attitude control system to control satellite position and promote
the higher quality imagery. The strategy of survivability improvement includes the
following: -
in-flight
on-board monitoring of satellite functioning; -
effective
testing of satellite systems fitness; -
compilation
of an onboard database of contingencies; -
autonomous
analysis of satellite systems status in contingencies; -
compilation
of onboard correct reference points database; -
autonomous
rollback to reference points in contingencies; -
autonomous
rearrangement of controls to recover satellite functions; -
balance
of centralized /decentralized control principles; -
implementation
of standard functional patterns (modes) of the satellite onboard tools in
contingencies; -
autonomous
calibration of the satellite onboard hardware. Signature imaging is remote sensing with imagery data
preprocessing aboard a satellite before its transmission to the Earth; consequently
only a part of imagery data is transmitted which demonstrates changes in the
state of targets if compared to previous instant, or it may not be transmitted
at all if no changes were revealed. Such an approach allows to transmit only
useful information and not to stuff the downlink channel with useless data. On
the other hand, it demands availability of a dedicated system aboard the
satellite which would allow real time determination of changes in the status of
targets and other elements of the scene under observation. An object signature is considered to be an object characteristic (descriptor) with the help of which the object can be found out,
discriminated, classified and identified. As a rule, it is not enough to have
only one characteristic, therefore an object should have the whole set (tuple)
of signatures. A tuple of signatures characterizing an object and used for its
detection, discrimination, classification and identification is called a
pattern (template). Thus, signature imaging is obtaining of information on
signatures of real targets, their comparison with the earlier accumulated and
stored in the knowledge base signatures (templates) of objects, discrimination
of objects, determination of changes if any, and decision making about transfer
of the appropriate information to the user. The user can receive either
complete information on the observed scene (objects, background conditions,
obstacles), or partial information about changes (signatures, parameters), or a
message on absence of changes, or no messages at all. There are geometrical, time-space, spectral
signatures, and also power, dynamic and fractal signatures. Development of onboard autonomous systems of signature
imaging requires a great many technological aspects to be properly resolved,
namely: 1.
Compilation
of an onboard knowledge base including: -
a priori
spectral characteristics of objects, backgrounds, obstacles (background -
target - obstacle environment) for various spectral bands, observation
conditions (flight altitude, sun aspect angle, observation angle, atmospheric
status, and contrast), modification of the optron system parameters and recognition
system parameters; -
methods,
algorithms and criteria for automatic spectral selection of objects in various
background-target-obstacle environment and observation conditions; -
minimum
necessary tuple of standard reference signatures sufficient for detection,
recognition of the object class, type, size, structure, status, which on the
one hand, supports the required quality performance of the onboard autonomous
system of signature imaging, and on the other hand, does not complicate its
design (ensuring specified probability of recognition and fitting restrictions
on weight, power consumption, fabrication cost and system maintenance). 2.
Software
for the onboard autonomous system of signature imaging (operational
environment, reduction of message redundancy and compression of imagery data,
decision making on imagery data modification and transfer to the ground-based
imagery data reception and processing center, interface with the satellite
onboard / ground-based control system); 3.
Imagery
data receivers (multispectral optron systems, hyperspectrometers), onboard
special purpose computers. 4.
Ground-based
center of the onboard autonomous system of signature imaging optimization, test
and support. 5.
Onboard
and ground-based training systems with self-correction, self-adjustment and
self-training elements. So far criteria estimation for autonomy of a remote
sensing satellite and its onboard systems has not been duly covered in
scientific research. Below some approaches are discussed to determination of
autonomy criteria. Criterion of power autonomy. The criterion is characterized, on the one hand, by an
autonomous set of devices and equipment necessary to maintain the required
energy profile of the satellite: chemical power sources (nickel-cadmium,
nickel-hydrogenous, lithium-ionic accumulator batteries), photovoltaic cells
(silicon with 14.5% efficiency, arsenide gallic with 26-28% efficiency), solar
arrays, power supply systems, automatic voltage stabilizers, and on the other
hand, onboard software which activates autonomous modes of power balance
maintenance. These are: solar arrays deployment technique (two degrees of
freedom), special modes of satellite turning with the purpose of power
accumulation and reaching max cosα, maintenance of a power balance and
fault recovery in power supply systems. Criterion of functional
autonomy. This is determined as relation of a scope of tasks
competed by an onboard control system to the total amount of tasks fulfilled by
the satellite autonomous control system (onboard and ground-based control
system). Such distribution of tasks became possible due to transfer of attitude
control, satellite MC motion control, satellite control in various modes, and
great number of testing and diagnostic tasks aboard the satellite. Criterion of informational
autonomy. From the informational approach
point of view, two different circuits should be discriminated: imagery data
circuit and satellite/orbital constellation control loop. Let us discuss the
first one. Imagery data circuit may be
considered in two aspects: in respect of creation, accumulation, compression and
down linking of imagery data; and in respect of improvement of imagery data
descriptiveness (resolving capability, ground resolution). As for the first
aspect, at first sight it may seem that now this circuit operates in autonomous
mode. However, not everything has been done here. For example, acquired imagery
data require ground-based processing, namely - matching of imagery fragments.
It is not absolutely acceptable for users who need real-time information.
Besides, currently signature imaging technique is under research. As for improving of imagery data descriptiveness,
special imagery restitution methods shall be implemented for denoising (for
example, elimination of smearing effect). Satellite and orbital constellation control tasks are
distributed between the ground control system and the onboard control system.
They interact via information flows between mission control center, receiving
station and satellite, satellite and special purpose center. Here, besides
transmission of control operations during communication sessions and operating
programs, for arrangement of onboard and research hardware functioning and
single commands, the telemetry information and housekeeping data are
transmitted. Autonomy in this case is possible only when informational
interchange between ground-based and onboard control systems fails by some
reason. In case of telemetry data lack, necessary data may be obtained together
with the housekeeping data. If it is impossible to uplink data about the
scheduled areas to be imaged, it is feasible to have satellite operating
completely autonomously, i.e. imaging the previously scheduled regions. Criteria of in-built
self-integration level. Control system response to modifications normally
leads to consequences suppression. Therefore it is necessary to develop control
methods responding not only to modifications, but also to the rate of these
modifications. Here comes the necessity to develop self-organizing systems. In
the case under consideration it is possible to have structural, engineering,
parametric and probably informational self-organization, autonomous
reconfiguration of onboard systems, functional and control processes. It is
especially urgent in case of anomaly, when it is necessary to maintain
integrity and continuity of functional tasks solution. Numerically this criterion can be determined by
various indices, namely: -
coefficient
of satellite operational readiness to execute basic functional tasks; -
minimal
time required for a satellite to perform restorative function in case of
anomaly; -
quantity
of possible structures and ways of reconfiguration of satellite systems; -
relative
volume of onboard software realizing satellite control pattern in case of
anomaly; -
number
of descriptors (faults in the satellite hardware) necessary for switching from nominal
flight to anomaly mode; -
amount
of checking information about satellite functioning sufficient for application
of "reverse engineering" during analysis and recovery of the satellite functionability; -
minimum
intensity of satellite switching from nominal flight to anomaly mode; -
decrease
of the ratio of severe faults to the total number of failures. To formulate an integrated criterion for estimation of system autonomy
it is necessary to standardize all partial criteria, e.g. to make them
dimensionless. Then the integrated criterion may be determined by any known way
(e.g. additive or multiplicative convolution of partial criteria, distance
estimation between alternative under consideration and its ideal
representation). Criteria estimation of autonomy by a numerical method
(criteria convolution) is problematic, as observed criteria are of different
nature and possess multifactor indeterminate form. In this case application of
artificial intellect and, in particular, fuzzy logic methods where not exact
numbers but loose linguistic variables are used, is challenging. The paper covers a wide range of problematic aspects
regarding autonomous functional control of low-orbit satellites, in-flight
coordinate calibration for the attitude control system, onboard signature
imaging systems, adaptive autonomous remote sensing space systems with
intelligent control. The approaches are suggested for determination of
autonomous navigation criteria. Not all the aspects are comprehensively studied. Many
of them require more thorough theoretical and experimental research, involving
some dedicated companies. Ravil Nurgalievich Akhmetov, First Deputy Director General (FSUE SSP RSC
"TsSKB-Progress"); Chief Designer, Head of "TsSKB-Progress"); Candidate of
technical sciences; Full Member of K.E.Tsiolkovskiy Russian Academy of
Cosmonautics, Full Member of the International Academy of navigation and motion
control, Corresponding Member of the Academy of Technological Sciences. His
professional interests: design, control and survivability of complex
rocket-and-space systems. |
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