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From SPIRAL to MAKS

Features of the Information Provision of the Aerospace System

Dr. Risenberg V.Kh.
There are considered some scientific and technical aspects of the information provision development for the Orbital Plane (Orbiter) starting from the subsonic Carrier-Plane. The information provision is based on the Universal Complex Navigating System including the Inertial Measuring Block and equipment of the Navigational Satellite Systems’ consumers.

The study of information provision for air launching started in NPO MOLNIYA in the end of 1970s. By that time separate specialists in parallel with intensive development of the BURAN orbiter’s navigating system began researches concerned organizational principals of information provision for the orbital plane (OP) of reusable aerospace system.

The major differences in requirements for navigating systems of OP and the BURAN orbiter were rapidly detected. First of all, it is:

  • global application of the orbital plane, including start point, by that, a limitation on local navigating systems’ application appears;
  • necessity to prepare a navigating system for functioning at start from mobile platform;
  • high-level operational efficiency at orbital plane’s application which considerably reduces preparation time and transition time from standby to high-accuracy mode of information issue;
  • non-powered automatic landing on the first-class airdromes with shorter and more narrow runway if comparing with runway in Baikonur;
  • rough requirements for system mass, dimensions and power capacity.

It became obvious that it was necessary, relying on general conception and development of the BURAN orbiter’s navigating system, to reconsider several principals of its structure. It should be taking into account providing the navigation’s independence from the starting and landing points and fast transmission of precise information about orbital plane’s motion parameters as well as considerable equipment mass and power consumption reduction.

Like for the BURAN orbiter, an inertial navigating system was chosen as an information kernel of the OP navigating system. This choice was caused by several very famous advantages of this system: continuous information issue, autonomy, noise immunity, possibility to get information during any flight phases.

Among new scientific and engineering problems, appeared for this project, it is to mention two of them, taking central points at navigating system development for the advanced orbital planes:

  • at orbiter’s air launch from carrier-plane, the navigating information’s formation for the injection into an orbit stage with accuracy practically equal to Baikonur launching site variant;
  • considerable reduction of variety and number of on-board and ground-based means used for navigating information formation aboard for different flight legs.

One of the most complicated problems of precise Inertial Navigating System’s (INS) functioning is the system’s preparation for work, so called INS initial tuning.

When a rocket launcher starts from the ground, starting point coordinates are well known with high precision and the object’s initial velocity relative to Earth surface is zero. The determination and adjustment of initial orientation in inertial space of instrument coordinate system connected with INS, including leveling and azimuth aiming, is performed by well-adjusted methods with precision reaching angular seconds. As a result, the initial tuning errors’ effect on the INS precision at injection leg is minimal. In general, the errors in inertial coordinate calculations are determined by errors in INS equipment.

The situation is turning around for air launch. In this case, the errors of INS initial tuning are dominating.

The analysis on different INS initial tuning methods has shown that if an initial tuning was done on Earth, before orbiter’s launch from the carrier-plane, a considerable errors due to INS bgyroscope drift would appear after long-term flight. In addition, it is necessary during precise initial INS tuning and calibration to stop actions connected with refueling and loading of other service components. The displacements of the staff, engaged in the orbiter’s pre-flight preparations, should be also stopped. All these demands lead to orbiter’s preparation time increasing. But a high-level immediacy of orbital plane application is very important feature of the MAKS system.

And as a result, by starting from carrier- plane, due to considerable errors of INS initial tuning, calculated precision of received orbiter’s flight parameters at launch trajectory has appeared to be considerably lower than for rocket launcher start from the ground-based launching site. Accordingly, the MAKS orbiter has lost precision of injecting into a given orbit. Due to this circumstance, it has become necessary to solve the problem of required precision at the INS tuning by the time moment when the MAKS orbiter starts from carrier-plane.

As a result of the long-term researches and mathematical simulation, the following method of INS tuning was suggested. The INS initial tuning is performed on an airdrome before carrier-plane take-off, supposing that its duration can’t break the schedule pre-flight preparations and additional tuning and calibration of sensors is performed in a flight.

To provide in flight the additional INS tuning and calibration, it was suggested to use means of the satellite navigating system - Global Positioning System (GPS). As the simulation has shown, in the case of satellite navigation system application in combination with optimal processing methods it is possible to reduce the errors in start point coordinates down to 10 meters and the errors of velocity vector components – under 0,1 meters per second. At the same time, a combined information processing from GPS and INS accelerometers makes it possible to estimate, with sufficiently high precision, and compensate the orientation errors of instrument coordinate system of the INS relative to a plane of local horizon.

All these operations are performed during the carrier-plane flight with orbiter above to the start point. However, the researches have shown that it was difficult to increase precision of the INS azimuth tuning, as far as at actual flight trajectories of the carrier-plane the errors of azimuth tuning are ‘weakly viewed’, as they are named in the theory of estimations.

To get these errors under view, to estimate and compensate it, it was suggested to use the carrier-plane’s ‘snake’, ‘parallel path reversion’ and ‘boost-breaking’ maneuvers before entering to the start point. In this case an acceptable precision of azimuth tuning was received.

Hence, continuous and sufficiently laborious researches have put the basis of the information preparation for air launch, ensuring high precision of terminal state when injection into a given orbit. This precision appeared to be practically the same as in the case of vertical rocket start from the ground-based launching site.

The major disadvantage of any INS is that the errors of inertial information are gradually increased along the flight trajectory. For the errors’ increase restriction, the corrections of the INS indication are performed by other measuring instruments, based on other physical principles, for example, radio principles.

The BURAN orbiter has used (at different flight legs) various correction facilities, considerably complicating the on-board equipment and required special expensive ground equipment. For example, in orbit flight various optic-electronic devices were used, at descent and landing - high-altitude and middle-altitude radio-altimeters, aneroid-altimeter, radio distance system interacting with ground distance relays placed in the basing airdrome zone, microwave landing system and (at emergency descent from orbit and landing) short-range navigation radio system.

The development of navigating system for the NAKS orbiter has shown a low productivity of such method and further researches were directed to an universal correction facility which information can be used throughout of the whole flight. And such facility was found. At that time, in 1980s, practically simultaneously Russia and the USA have finished development and started preparation for deployment and exploitation of satellite Global Positioning System (GPS) intended for precise consumer’s coordinate and velocity determination practically in any point of the world.

The positioning system equipment (PSE) placed aboard of the consumer is used for passive measure of time of propagation and Doppler frequency shift of signals radiated by the positioning satellites and created continuous radio navigation field.

The global positioning system developed in Russia was called GLONAS and in America - NAVSTAR.

Developed global positioning equipment of those days was used either in aircraft’ flying in limited range of speeds and altitudes or in space vehicles. As a result NPO MOLNIYA has started preparation the requirements to PSE for the orbital spaceship. The conducted researches allowed to get requirements specification on development of PSE for the MAKS orbiter, thus co-developer companies could start the development of particular equipment.

Created technical appearance of PSE for the orbital plane looks in the following way.

GPS equipment of the MAKS orbital plane works in the space conditions and in atmospheric flight. The receiving facility getting satellite’s navigating signals from GLONAS system is multi-channel device, functioning in dual-frequency band. Multi-channeling allows receiving information simultaneously from not less than four navigating satellites. The optimal set, including four navigating satellites, should be chosen by on-board calculator in accordance with minimal geometric quotient. Dual-frequency measure method may be used to eliminate errors caused by radio wave refraction in atmosphere. Multi-sectional antenna provides radio signal receiving from satellites with minimal constraints on the orbiter’s angular position.

After ‘Cold War’ completion Russia and the USA made an agreement about mutual application of GLONAS and NAVSTAR signals. This give us opportunity to increase redundancy of navigating measure and to provide required guarantee of precise and reliable orbiter’s velocity and location determination.

A non-powered automatic landing on the first-class airdrome from the first approaching makes high demands on precision and reliability of information provision and still doesn’t allow to decline microwave landing system. However, the specialists from NPO MOLNIYA continue work on providing autonomous landing by use of satellite positioning facilities. The major direction of this work is to use differential operating mode of GPS equipment. For that purpose correction stations are placed near landing airdrome (see figure). They include precise GPS equipment, correction information generator and transmitting apparatus. In addition to GPS equipment a device for correction information receiving and its entering into GPS equipment is placed aboard the orbiter.

Continuously receiving radio signals from navigating satellites an equipment correction station is able to distinguish forecast error of ephemeris and navigating satellites’ on-board time scale offset as well as other types of slow variable errors appeared in received radio signals.

Then distinguished measure errors are transmitted by radio link aboard of the orbital plane and are taken into account by on-board calculator at processing of signals received by GPS equipment. As a result a precision of determined orbiter’s coordinates is increased thus allowing to consider seriously a question concerned automatic landing of the orbital plane without use of traditional landing facilities.

One more prospective trend, which is being developed by NPO MOLNIYA specialists and other companies, is development of methods providing determination of orientation angle of orbiter by means of GPS. The researches conducted in our country and abroad showed a possibility to use GPS for determination of flying vehicle orientation based on phase-metric method of measuring the angular directions on the navigating satellites. The results have shown that in many cases such system can exchange traditional facilities for stellar supervision used on orbit to define orientation of the orbital plane.

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Scheme of GPS differential mode

Accordingly, nowadays all strains of NPO MOLNIYA specialists, working under information provision, are directed on solution of quite complicated scientific and technical problem concerned creation of universal navigating system which includes inertial measuring unit, GPS equipment and more perfect methods of data processing. Thanks to universality of such complex and its capability to function on all flight legs, a necessity to use optic-electronic and radio navigating systems to define orbiter’s coordinates and velocity at the orbital, descending and landing legs is eliminated. In that way hardware part of on-board navigating system is simplified. Its mass, size and power consumption is considerably reduced and number of ground expensive facilities for navigation is sufficiently reduced as well.

Taking into account that performance requirements presented for navigating system of the orbital plane, at great extent, exceed requirements for navigating systems of any other flying vehicles, it is expected that developed universal navigating system will be wildly used on other objects of aircraft and space industries.

Relying on experience gained in cooperation with other enterprises during development of the BURAN orbiter which occupies front lines in aerospace instrument-making industry, NPO MOLNIYA specialists continue to develop information provision systems for maintaining the flights of orbital planes of reusable aerospace systems.