Testing of spacecraft orientation and stabilization systems using starry sky simulators

Modern space exploration requires going beyond the limits of near-Earth orbits. As a result, the technical requirements for the means of ensuring the flight of a spacecraft (SC) are constantly increasing. In order to solve scientific problems, in particular, astrophysical ones, the requirements for ensuring the accuracy of determining the position and spacecraft orientation are constantly increasing[1; 2].

The developers of modern spacecraft are assigned with ensuring high accuracy of orientation and stabilization of the spacecraft in space. The success of the ongoing scientific experiments in space and the fulfillment of the target functions by the spacecraft depend on the solution of this problem.

Modern communication and navigation spacecraft are complex automated systems capable of solving a number of applied problems. There is a constant development of spacecraft in terms of their functional purposes, the complication of the applied and the introduction of new on-board systems, instruments and mechanisms.

A perfect example is the spacecraft orientation and stabilization system (SOS). SOS along with other methods for determining the spacecraft positioning, uses the method for determining the orientation relative to stars and planets, which is one of the main types of astroorientation [3].

There are several stages in the development of astroorientation. The first stage using visual orientation by the stars. It acquired special significance in the absence of clear landmarks on the ground such as among nomads and in navigation. Later there have appeared devices for more accurate measurements e.g. sextants, astrolabes, etc. Similar navigational skills and methods are still used today [4].

The second stage in the development of astroorientation started with the occurrance of aircraft (AC). Where after gyrocompasses became the main component of their orientation systems. These systems did not allow maintaining the orientation of the aircraft for a long time due to the so-called departure of gyroscopes - their precession under the influence of not fully taken into account external forces. Care control was carried out using star trackers (ST). The first generations of STs worked on several bright stars with known positions to solve problems of astroorientation. As a rule such STs had a system for pointing at a bright object and a system for keeping the direction to a captured object (fig. 1, а ). They appeared on board spacecraft and rockets and were used from the very beginning of space exploration. Already on the “Vostok” spacecraft they were used to align the spacecraft along the orbit. Information from the majority of devices of this generation was not processed on board the spacecraft, but was transmitted to the ground in the general telemetry stream. Orientation was calculated on the ground and then reported on board [5].

а

б                              в

г

Рис. 1. Приборы ориентации на звезды:

а – прибор ориентации на полярную звезду; б – БОКЗ-МФ; в – широкопольный звездный датчик 329К; г – звездный датчик SED-26; д – звездный датчик 348К (блок оптический)

д

Fig. 1. Star orientation devices:

a – polar star orientation device; b – BOKZ-MF; c – wide–field star sensor 329K; d – star sensor SED–26; e – star sensor 348K (optical unit)

The last stage in the development of the considered devices came with the advent of matrix radiation receivers: Charge Coupled Device(CCD) and Complementary metal-oxide semiconductor

(CMOS)video matrix. Such star sensors (SS) are not tied to individual, predetermined stars, but determine their orientation from the images of groups of stars visible in the field of view of the device (fig. 1, b–d ) [6].

Applications for star trackers

At the moment there are a number of tasks that can be solved with the help of a spacecraft attitude control system namely:

• 1)    determination of the orientation of the three turning angles sensor relative to some coordinate system;

• 2)    pointing some device installed on the spacecraft – an antenna, a solar battery, in a given direction.

This task is close to the previous one, but requires precise transfer of the orientation from the Star Sensor(SS) to the device;

• 3)    drawing on some image, for example, the image of the Earth, a coordinate grid.

This is a typical task of Earth remote sensing. Additional but very important requirements for the listed tasks are the accuracy of determining the coordinates, the time for which these coordinates are determined, as well as operating dynamic range of determined angular velocities of the spacecraft [7; 8].

Modern requirements for astronavigation systems

As it was mentioned before the development of spacecrafts continues constantly, they are faced with more and more complex tasks and their functionality is increasing e.g :

• –    the need to use lasers as information transmitters both from orbit to earth and between spacecrafts. The use of low-power lasers with small divergence angles requires the transmitter to be pointed at the receiver and held with an accuracy better than 0,05 ";

• –    modern meteorology requires obtaining maps of the Earth's cloud cover with a resolution of 10 m. For geostationary weather satellites, this corresponds to a resolution and orientation accuracy of at least 0,05 ";

• –    attitude control systems on fast maneuvering spacecrafts should determine their orientation at least 10 times per second with an accuracy of no worse than 0,1–1 " [8].

To fulfill the tasks set the developers of new spacecrafts are subject to higher requirements for new star sensors.

New generation sensors should provide:

• 1)    accuracy of determining the orientation of the spacecraft 0,1–0,01";

• 2)    navigation information update time 10–100 ms ;

• 3)    determination of coordinates and angular velocities at spacecraft rotation speeds up to 15–30 °/s .

• 4)    The star sensor must operate in a wide range of angular velocities of rotation of the spacecraft, from 0 to one rotation per few seconds, and at the same time provide high orientation accuracy at low rotation speeds and moderate accuracy at high rotation velocities. [8].

Development of star trackers in laboratory conditions

One of the main stages in the design and development of SOS spacecraft is the stage of developmental and autonomous tests on a hardware-software stand for half-life simulation. The purpose of this stage is experimental confirmation of the operation of the SOS onboard equipment under conditions close to operational ones. [9–11].

At the moment JSC “Academician M. F. Reshetnev “Information Satellite Systems” has a complex modeling stand (CMS) that uses methods of both mathematical and semi-natural modeling to carry out these types of spacecraft tests. [12–14]. The structure and principle of building a complex simulation stand provide opportunities for system checks that cannot be achieved by other means [15].

Since modern star sensors are faced with a wide range of tasks such as increasing the accuracy of determining coordinates at a high polling rate and successfully determining coordinates at high speeds of rotation of the spacecraft, this leads to the need to modernize the means of simulating the starry sky which are necessary for testing the performance of the SOS spacecraft.

At the very beginning starry sky simulators (SSS) creating the static light of a single star were developed. They imitated some kind of reference point on the celestial vault. Further sky simulators imitating light of a group of stars were developed. Both those and other sky simulators work by using a screen with holes and a long throw projection lens.

Then sky simulators of the previous generation were replaced by ones that dynamically simulate stars using monochrome displays with a wide dynamic range of luminosity, mainly manufactured for medical equipment. They made it possible to simulate the position and angular velocity of stars. (fig. 2) [16].

And as the final stage, there was the creation and development of starry sky simulators based on microdisplays (Fig. 3). The table shows the main characteristics of the starry sky simulators existing as part of a complex modeling stand (CMS) [17; 18].

Рис. 2. Динамический стенд имитации группы звезд (ДСИГЗ):

1 – к онтрольно-испытательная аппаратура (КИА); 2 – монитор оператора; 3 – персональный компьютер со звездными каталогами; 4 – станина; 5 – испытываемый прибор (329К);

6 – коллиматорный объектив; 7 – светозащита; 8 – цифровой монитор высокой четкости

Fig. 2. Dynamic simulation stand of a group of stars (DSSGS):

1 – control and testing equipment (CTE); 2 – operator monitor; 3 –personal computer with star catalogs;

4 – bed; 5 – test device (329K); 6 – collimator lens; 7 – light protection; 8 – high definition digital monitor

Рис. 3. Структура имитатора звездного неба на основе проекционных систем (ИЗН ПС):

1 – блок оптический звездного датчика 348К; 2 – плита установочная;

3 – оптический модуль ИЗН ПС; 4 – блок электроники звездного датчика 348К;

5 – к онтрольно-испытательная аппаратура

Fig. 3. The structure of the starrysky simulators based on projection systems (SSS PS):

1 – the optical unit of the star sensor 348K; 2 – the installation plate; 3 – optical module SSS PS;

4 – electronics unit of the star sensor 348K; 5 – control and testing equipment

The observed rapid development of technical means of virtual and augmented reality has led to the continuous improvement of technologies for creating microdisplays with a diagonal size of up to one inch, which made it possible to use them in the construction of means for optically simulating the starry sky of small sizes sufficient to be able to fix them on star sensor hoods. At present there is a transition to new types of starry sky simulators built on the basis of microdisplays. This leads to the need of modernization the existing methods for testing SOS AC including star sensors. In addition, the constant increase in the operating range of the measured angular velocities of rotation of the celestial vault by star sensors leads to the fact that microdisplays with an ever higher image refresh rate are required. This is because the screen refresh rate increases, the period decreases and therefore the maximum simulated angular velocity also increases, since it is equal to the ratio of the average angular size of the stars to this period value. At present, the limiting refresh rate of the microdisplay image can reach up to 90 Hz, which allows solving most tasks for working out the SOS of a spacecraft. .

Since star sensors are gradually being introduced to obtain position data at higher rotational speeds relative to the until recently considered normal values of angular velocities, there is a need to develop a method for dynamic imaging of the starry sky which would allow the star sensor to "see" the rotating celestial vault at these angular speeds.

Specifications of starry sky simulators

	DSSGS for SS 329К	CTE for SS SED 26	CTE for SS 348К	CTE for SS 348К	SSS PS
Working with SS optical blocks at the same time	1	–	–	2	4
Field of regard	22 ° x18 °	–	–	23 ° x23 °	19 ° x19 °
Range of simulated angular velocities of the spacecraft	No more than 1.6 deg./s	В рамках работы прибора	During the operation of the device	±1 deg./s	3 deg./s
Imitation of the starry sky sections	О	E	E	О	О
Ability to work with other SS	–	–	–	+ with revision	+ with revision

Примечание. O – оптическая имитация, Э – электронная имитация, ОБ – оптический блок. Note. O- optical simulation, E-electronic imitation, OB-optical block Conclusion

Modern tasks of space navigation require the creation of a new generation of stellar orientation and this in turn leads to the development of a methodology for testing spacecraft orientation and stabilization systems using starry sky simulators built on the basis of various types of microdisplays which will use the method of dynamic formation of an image of a star of the sky taking into account the peculiarities of the operation of matrix photodetectors used in SOS spacecraft star sensors.

This method will allow simulating the rotation of the spacecraft at speeds up to 15–30°/s which will lead to the possibility of testing the SOS of modern spacecrafts.