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УДК 621.396.96
Zhakupova A. , Yergaliyev D. , Yessenov A. , Autalipova B. , Bimyrzayev A.
1Eurasian National University by L.N. Gumilyov, Astana, Republic of Kazakhstan
2JSC "National Company "Kazakhstan GharyshSapary ", Astana, Republic of Kazakhstan
RESEARCH OF ANTENNA SYSTEMS FOR REMOTE SENSING OF THE EARTH WITH THE HELP OF A HEXAPOD STAND
В настоящее время, чтобы достичь самой отдаленной точки нашей планеты, достаточно совершить 10-часовой полет на самолете. Однако такое путешествие вряд ли можно использовать в качестве крупного источника информации о регионе. Человек может быстро осмотреть территорию в пределах круга с радиусом 4-5 км, а для оценки ситуации на площади 100x100 км потребуется более одного дня. Тем не менее, существует более быстрый и надежный способ получения информации на территории с использованием космических снимков (цифровых изображений земной поверхности), сделанных с космических аппаратов ДЗЗ (дистанционное зондирование Земли). В дополнение к изображениям достаточно иметь компьютер и доступ в Интернет.
В данной статье рассматривается антенная система наземного комплекса управления спутником с высоким разрешением космической системы дистанционного зондирования Республики Казахстан, которая установлена в акционерной национальной компании «Цазацстан гарыш сапары», расположенной в национальном космическом центре Астаны, в Республике Казахстан.
Ключевые слова:
АНТЕННА, СТАНЦИЯ, СИГНАЛЫ ПРИЕМА, ПУТЬ ОБРАБОТКИ ПРИЕМА, ПРОГРАММНОЕ ОБЕСПЕЧЕНИЕ, КОСМИЧЕСКИЙ АППАРАТ, ГЕКСАПОД
However, for the professional application of remote sensing data, specialized software and hardware complexes are required that allow control, analysis management:
- Natural resources;
- With warning and liquidation of emergency situations;
- Real-time economic activity time.
The main element of remote sensing systems that allow monitoring of assigned areas in real time mode are the stations for receiving remote sensing data from the Earth. Typically, such stations are an integral part of ground-based data acquisition and processing complexes.
The station in cludes:
- antenna system reception signals from the spacecraft;
- receiving processing path (decoding and transformation of the received signal to the required level);
- software (data processing to the final information products);
- the corresponding acceptance licenses from a particular spacecraft.
The main purpose of the station is to receive the signal emitted by the spacecraft with the specified characteristics. Therefore, the choice of the station is made after the selection of the satellite. Currently, the modular system of station acquisition predominates, which allows it to be upgraded with minimal costs, depending on the requirements of the satellite. This is especially important, since the average life of the earth remote sensing satellite is 5 years, and the life of the ground station is 15 years.
The choice of an antenna for receiving satellite signals can be represented as a multi criteria optimization problem, in which there is a set of antenna requirements for an earth remote sensing satellite and the characteristics of antenna systems of different manufacturers. The solution of the problem is at the intersection of these two sets. Typically, several antenna systems correspond to the claimed requirements. In order to make the most rational decision,
We familiarize ourselves with the recommendations for choosing a mirror antenna for receiving data from a spacecraft. Let's consider the most important of them in more detail.
The operating frequency range of the antenna is determined by the choice of the spacecraft for remote sensing of the Earth. It should somewhat overlap the frequency of the signal broadcast from the satellite, because of the Doppler effect and the distortion of the propagation of electromagnetic waves along the path, the frequency of the received signal on the planet's surface differs somewhat from the frequency of the signal transmitted by the satellite. If it is supposed to work with several spacecraft, then it is necessary to cover all frequencies of the satellites.
The choice of the operating frequency range of the antenna also determines the material of its primary mirror. There are solid antenna mirrors. Mesh mirrors are used for antenna diameters of more than 5 m, because they are capable of with standing significant wind loads, have lower weight and size characteristics (which reduces the requirements for a rotary device) compared to continuous mirrors and are less prone to accumulation of atmospheric precipitation (Figure 1). However, when choosing a mesh antenna, it must be taken into account that the cell size must be smaller than the wavelength of the received signal, and the mirror itself is a set of rectangular plates, which reduces the reception efficiency.
Solid mirrors can be integral or composite (Figure 2). Compound mirrors are made with antenna diameters of more than 5 m, since they are difficult to transport with one- piece, and high accuracy of manufacturing of curvature is necessary. Solid mirrors are made of metal, aluminum and plastic with metal coating. Metal mirrors are strong, but are prone to corrosion and heavy; plastic - deformed from temperature and precipitation; aluminum - light, not rust, but soft and easily deformed. piece, and high accuracy of manufacturing of curvature is necessary. Solid mirrors are made of metal, aluminum and plastic with metal coating. Metal mirrors are strong, but are prone to corrosion and heavy; plastic - deformed from temperature and precipitation; aluminum - light, not rust, but soft and easily deformed.
If we compare roughly the two types of mirrors - mesh and solid, then the choice, unequivocally, will be in favor of a continuous one.
Since the problems of wind loads and precipitation are solved by using a radio-transparent dome, and there are no restrictions on the choice of the wavelengths of the received signals. It should be noted that precipitation in the form of snow and ice is also collected at the pole of
the radio-transparent dome that covers the mirror antenna 3.5 m in diameter. This prevents the penetration of electromagnetic radiation under the dome and may lead to a disruption of the communication session with the spacecraft.
Figure 1 - Mesh mirror antenna
Figure 2 - Solid mirror antenna
When we are talking about the choice of the operating frequency range of the antenna system, we are talking about the irradiator, which is placed in the focus of the mirror, namely, its ability to convert electromagnetic waves incident on the antenna mirror into electrical signals. Here we should pay special attention to the polarization of the signal emitted by the space vehicle. One of the most important parameters of the antenna is the diameter of its mirror. It is determined by the power level of the received signal from the satellite and the required data reception speed.
At the present time, the development of ERS spacecraft is proceeding along the path of maximum reduction of mass and dimensions characteristics, which allowsto save on the costs of putting spacecraft into orbit. Such trends are directly proportional to the power of the emitted signal. The lower the power of the signal emitted by the satellite, the greater the diameter of the receiving antenna's mirror. However, the larger the antenna diameter, the thinner the directivity pattern, which in turn requires a high accuracy of antenna guidance on the spacecraft. For example, to obtain a signal from a RapidEye ERS with a power of 10.6 dBW, the mirror diameter of the antenna should be 5 m. When operating with
multiple satellites, the antenna diameter is determined by the worst characteristics.
It is also necessary to take into account the expected life of the antenna. If it is about 15 years or more, which is 2-3 times longer than the service life of the spacecraft, one should think about the reserve of the effective antenna area, otherwise it will have to carry out an expensive upgrade of the ground data receiving and processing complex.
The type of the antenna's slewing bearing is determined by the mass of its mirror, the number of degrees of freedom (axes of rotation) required by the accuracy of guidance on the spacecraft and the speed of its tracking. The support of the slewing bearing is intended for targeting the main lobe of the antenna pattern to the satellite in real time, which allows obtaining the maximum radiated power. Usually choose two-plane slewing bearing. One of the leaders in the production of the slewing bearing with a hexapod pedestal is Zodiak (France) (Figure 3).
The main advantages of Hexapod:
- Hexapod can track the object in any direction, because there is no need to switch in azimuth and elevation. This provides better guidance accuracy than other antenna systems.
- The angular velocity of 3 ° / s is achieved in any position and direction.
Figure 3 - Hexapod
There are no restrictions on azimuth and unobstructed cable movement:
Hexapod can perform a constant rotation in azimuth without mechanical limitations. The cable between the antenna and its base has no restrictions in motion, because the azimuth is rotated without twisting the cable. This is the result of the kinematics of Hexapod, which produces a relative rotation of the antenna around its axis. It should be noted that all parts of the antenna drive system (motors, sensors, etc.) are located at the bottom of each of the six antenna posts. Thus, the antenna drive system is available for its maintenance or repair.
The location of the antenna is chosen taking into account the direct visibility of the sky in all directions, with the angle of inclination of the antenna to the plane of the horizon of 20, although the manufacturers of the Space Remote Sensing Equipment guarantee a safe reception of the signal at an inclination angle of 50. Therefore, if it is not possible to provide visibility of the sky at an angle of inclination antenna in 20, then this parameter can be reduced to 50.
Thus, when solving the problem of choosing an ideal antenna system (with the appropriate restrictions), we can distinguish the following:
- it is better to use a solid antenna mirror than a mesh antenna;
- a metal mirror with a resistant anti-corrosion coating is preferable to aluminum;
- the larger the diameter of the antenna, the better if the accuracy of the curvature of the surface is maintained and the requirements for the accuracy of tracking the spacecraft are met;
- A radio-transparent dome is needed;
- Requiring a Pivot Turning device with a high speed of tracking the spacecraft and the maximum possible number of degrees of freedom;
- the antenna should be installed in the line of sight of the sky in all directions at an angle of inclination of the antenna to the horizon plane of 20.
The receiving-processing path is a system designed to isolate a useful signal and transform it into a view sufficient for further processing by software. Basically, this decrease frequency, demodulation and decoding of the received signal. Each remote sensing satellite manufacturer uses its own methods of modulation and coding of a useful signal, which causes an expansion of the receiving path equipment to support work with several remote sensing satellites.
The choice of software is conditioned by the requirements for the final information products. The standard equipment of the station includes software that allows processing of the received data to low levels. The most common is the following gradation of pre-treatment levels:
- 0 - un treated (primary) data received from the spacecraft;
- 1A - data that has passed radiometric correction
and calibration;
- 1B -radiometrically adjusted and geographically related data;
- 2A - radiometrically and geometrically corrected data presented in a map projection.
ANTENNA SYSTEM DESCRIPTION
The KRS S+X Band Antenna System is located near ASTANA in KAZAKHSTAN.The antenna system description and operations procedures described in subsequent sectionsare applicable to this system (Figure 4-5).
Purpose and context
The Kazakhstan Remote Sensing (KRS) Antenna System provides tracking and imagery data collection capability from the HR satellite. The KRS Antenna System operates under instructions received from the Satellite Control Center (SCC),and supplies the received HR satellite data to the Kazakhstan GharyshSapary (KGS) Direct Archive System (DAS). In order to fulfill this purpose, the KRS Antenna System performs the following functions:
Receives High Resolution (HR) satellite Reception Schedules and Ephemeris Data from the SCC.
Tracks the spacecraft during its visible
pass.
Receives Image Telemetry (ITM) from the satellite at 8150 MHz carrier centre frequencies, 320 Mbps,
QPSK, with R/S)
- HDR will deliver data to DPS for HR satellite through TCP/IP or ftp interface, anddata to DPS for MR satellite through ECL output interface (cross-strapping).
Generates a pass report for each pass event and makes it available for retrieval.
-Provides operations personnel with a GUI that displays the status and activities ofthe KRS Antenna System.
-Provides an automated scheduled test capability to verify key antenna functions.
-Receives GPS satellite time signal and provides Network Time Protocol (NTP) timeto antenna system and to other nodes.
Antenna System Architecture
The 5.5m reflector is mounted on a hexapod assembly. (Figure 6) The hexapod assemblyand the drive motors are protected inside the radome. The Hexapod Lubrication Box is closeto the Hex-apod base plate.
X-Band Feed
The X-Band feed (Figure 6) is mounted in the reflector and provides the capability to receiv-eRF signals over the 8 to 8.4 GHz frequency range and includes Low Noise Amplifiers (LNA) for both Right Hand Circular (RHC) and Left Hand Circular (LHC) polarized signals. The feedalso provides selectable RHC or LHC auto-tracking signal.
Figure 4 - MCS (mission control software)
Figure 5 - Reflector, Hexapod S-Band Feed
The S-Band feed (Figure 6) is mounted in the reflector and provides the capability to receiv-eRF signals over the 2.2 to 2.3 GHz frequency range and provides the capability to emit RFsig-nals over the 2.025 to 2.12 GHz frequency range. The S-Band feed includes LNAs forboth RHC and LHC polarized signals.
Figure 6 - Dichroic
The sub-reflector is placed approximately at parabola's focal point between S Band feed andX Band feed. Dichroic is transparent for S band signal and reflecting for X bandAntenna Drive Unit (ADU) Cabinet the ADU cabinet is located in the control room and includes the Servo Control Unit(SCU) and servo drives for the six hexapod drive motors. It also includes a Dehydrator, DCpower supplies and circuit breakers.
Hexapod Lubrication Box
The lubricating box (Figure 7) includes an oil pump, a grease pump and associatedregulation and control devices. The lubrication cycles are monitored by the Monitoring andControl System (MCS).
Radome
The antenna system is protected against thermal conditions by a 10.7m radome.
Power Distribution
The PBB in the control room provides power distribution for the ADU (230V and400V).
S Rack
Figure 7 - Interiorof the radome
The S-band Rack is located in the control room and includes 2 S-Band to 70MHz frequency agile down-converters, both for data. The bandwidth of the down-converteddata is 100 MHz to accommodate reception of any channel within the 2.2-2.3 GHz range.The Rack also includes a 70 MHz to S-Band frequency agile up-converter for loop-backtest-ing.
There is also:
The BB unit (CRT) demodulates and reconstructs the TM frames, and provide them inTCP/IP format to the SCC. The signals from both polarizations are received simultaneouslyby the Cortex CRT.The S IF distribution elements (splitters), integrated in IF switching matrix. The 10 MHz drawer to synchronize CRT, HDR, ACU and RF converter.
X Rack
The X-band Rack is located in the control room and includes 3 X-Band to 720MHz frequency agile down-converters, 2 for data and the other for tracking channel. Thebandwidth of the down-converted data is +/- 200 MHz to accommodate reception of anychannel within the 8-8.4 GHz range. The RF Rack also includes a 720 MHz to X-Bandfre-quency agile up-converter for loop-back test-ing.The X Rack includes also the 200W SSPA unit with a load drawer.
Constantly growing needs and rapid expansion of economic activity require effective and careful distribution of the load on the ecosystem, accounting and management of which cannot be achieved without integrated monitoring of natural resources. These tasks can be solved by creating a ground-based complex for receiving and processing remote sensing data of the Earth.
A description of the stations for receiving the Earth remote sensing data is given. A procedure of creating ground complexes for the Earth remote sensing data receiving and processing is introduced with due consideration to the both tasks to be solved and spacecraft to be used for these purposes.
REFERENCES
www.zds-fr.com www.deimos-imaging.com www.seaspace.com www.sovzond.ru
www.spacetec.no
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