Experience of space flight control

of third-generation orbital complexes

V.A.Soloviev, V.I.Stanilovskaya

S.P. Korolev Space Corporation Energia

4, Pionerskaya st, Korolev, Moscow region, 141070, Russia

"Space flight control" means not only control of space vehicles traditionally developed by various groups of researchers, but control of the whole executed operations and furthermore performing all the actions ensuring accomplishment of all the tasks from the moment of putting a space vehicle into orbit and to the moment it ends to exist.

Third-generation orbital complexes (OC) like Mir and especially like the International Space Station (ISS) differ considerably from all the previous orbital stations that worked on orbit for a long time. Even the first stage of Mir flight made these differences obvious:

ћ the number of scientific and technical experiments conducted on-board increased dramatically;

ћ flight duration was extended substantially;

ћ dockings with scientific modules, transfer and cargo vehicles occurred continuously;

ћ necessity arose to provide space vehicles with permanently running attitude control system and consequently to include in that system attitude/translation control that worked according to prop-free principle;

ћ space vehicles docked to permanently crewed OC.

In view of the above, it becomes evident that the methods all the previous space stations were controlled through cannot assure effective functioning of a new-generation complex. It proved to be impossible to control the third-generation orbital station without many new problems being resolved. Thus, importance of working out new methods to control orbital manned flights was driven by transition to a new phase of space technology development.

At the same time, in the interests of long and effective running of the OC and with the purpose to meet the requirements of advanced reliability at all the stages of the flight, it was essential to build a control system that could counter decreasing efficiency of the OC caused by lasting operation and maintain high level of ground-onboard control complex as a whole.

Carrying out the task of the third-generation OC control covered development of techniques employed in long-term and execute planning, techniques for command and program control, for assessment and analysis of onboard systems status as well as for mathematical modeling of control processes.

From a technological point of view, OC control is a sequence of operations. A flight is preceded by planning process in the course of which in order to guarantee implementation of the flight plan in-flight operations sequence order is determined, the operations are tied to a single time standard, program of necessary control actions to be up linked is developed, and prioritized are the analyzed systems. General diagram of flight planning is shown in Figure 1. Displayed is two-level planning scheme worked through during Mir flight control and made up of strategic, long-term and tactical, and execute planning. It should be noted that unlike planning schemes used earlier (for the first- and second-generation stations), for planning on Mir and then on the ISS a lot of mathematical models, databases and expert systems were set in operation which in turn made it possible to switch from so-called session control (uplink of commands on every orbit) to daily control (uplink of commands to Mir station once a day).

In the flight all OC onboard systems should function in such a way as to ensure execution of the flight plan (Figure 2). To successfully fulfill all the objectives of a given flight stage, the list of controlled systems and time frames of real-time and post-session analysis is drown up. General scheme of flight control realized on a regular basis in the process of flight plan execution is given in Figure 3. A main distinction between the control scheme applied today and the methods used earlier is not including all possible parameters in control process but conducting the following operations during data receiving and processing:

ћ initial, panoramic review of the controlled systems;

ћ choosing of analyzed systems priority list;

ћ further data analysis being a comparison of data with regulations or with predictions.

It is worth saying that considerable difference of the scheme in question from control systems used for space vehicles in the past is employing databases that undergo modification in the course of the flight and are supplemented with new information.

In addition to measures taken to save and increase resources, which is caused by significantly longer duration of the flight, the third-generation OC control set new complicated tasks relating to frequent changes of OC configuration. Dockings with scientific modules, transfer and cargo spacecraft followed by moving them to their nominal ports noticeably complicated flight control for two reasons mainly. First one is the necessity to constantly take into consideration variable mass-inertia characteristics of the complex and conditions of solar arrays illumination; the second reason is the requirement to provide integration of newly docked module (or spacecraft) onboard systems into Base Block, which results in great and sometimes unexpected effect on OC systems logic. It is important to mention that during forming a new architecture of OC the developments onboard are rapid and a "safe" system goes into a category of abnormal ones. Active measures should be taken to cope with the off-nominal situation or to compensate for the consequences. These basic problems and a number of other ones that arise during flight control compelled developing new schemes of flight action, control and analysis, and using mathematical modeling of different complexity at all the stages of flight control.

New flight programs, especially those connected with international projects on OC promoted development of so-called iteration intermodelling. We think that mathematical modeling is most widely used for flight control in off-nominal situations and during activities to overcome them. Figure 4 shows the algorithm of actions in off-nominal situation onboard Mir station. It's natural that non-studied off-nominal situations pose more problems and accordingly they are of great scientific interest. Such situations happen quite often during operational process because of complexity of the systems serviced, diversity of operational modes, and because of OC configuration modifications.

Mir station control showed that even after you had managed a non-studied off-nominal situation you could face another unexpected off-nominal situation. That is why it is crucial to narrow down the list of possible non-studied situations. For that, a set of radically new mathematical systems databases were created that allowed the specialists to automate keeping track of system resources, to access operational data about all the off-nominal situations coped with in the past, and to have visual information of changes in major systems.

Using the results of space flight control enabled implementation of a large-scale program of scientific and applied research in the course of 15-year Mir flight. On Mir station were placed more than 240 items of scientific and payload equipment (made in 27 countries) weighing 11.5 tons in total. Twenty eight Russian Programs for main expeditions and 27 Programs in the framework of international cooperation were realized during 15 years of Mir flight. 31200 runs of scientific experiments were performed on the station with 1690 gigabytes of scientific data being obtained and 4700 kilos of the experimental results being returned to the ground.

Mir station's unique capabilities permitted the scientists from all over the world to accomplish long permanent observations and research, to carry out scientific and technical experiments in weightlessness, under conditions of cosmic radiation, vacuum, and other factors of outer space.

Resulting data contributed greatly to our knowledge of the Universe and the matter, of global factors that influence our planet and near-earth space, of human organism and forms of life evolution as a whole. Experience gathered in international programs on Mir station was later efficiently used for ISS control. Simultaneous control of two orbital complexes appeared to be particularly intense period.

New objectives of ISS control are connected to international status of the project and International Partners' Mission Control Centers working in parallel with each other (distributed control). The presence of several Mission Controls necessitated the automated exchange of data and flight plans between the Partners. Appropriate mathematical support was developed and introduced.

Mir station control over a long period of time, methods developed and the system of flight control had no analogues in the world. In the field of control methodology our leadership is recognized by all the countries and the experience acquired is now widely used in ISS control.