DEVELOPMENT OF TRACKING SYSTEM FOR SATELLITE GROUND STATION

Transcription

1 8th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING April 2012, Tallinn, Estonia DEVELOPMENT OF TRACKING SYSTEM FOR SATELLITE GROUND STATION Winter, E.; Dahl, J.; Nordling, K.; Praks, J.;Kiviluoma, P.; Kuosmanen, P. Abstract: A satellite, in this context, is a man-made object that orbits around the earth. Aalto-1 is the first Finnish satellite and its design follows the popular CubeSat nanosatellite standard. The Aalto-1 is designed to use S-bandwidth which is not typical in small size satellites. To handle the communication with such a small satellite orbiting the earth, a ground station with an accurate tracking antenna is needed. In this paper we present ideas how to design, build and calibrate a low-cost and simple S- band satellite tracking system for educational purpose. The goal was to achieve a tracking accuracy better than one degree. Key words: ground station, S-band, pointing calibration, helix antenna, nanosatellite 1. INTRODUCTION At this very moment hundreds of satellites in all sizes are orbiting and observing the earth and giving navigation and weather information. Aalto-1 is the first Finnish satellite and its design follows the popular CubeSat nanosatellite standard. The dimensions of the satellite are 340x100x100 mm. One of the main payloads is a spectrometer which takes pictures of the earth in different wavelengths [1]. Because of the large data-packages sent from the spectrometer the satellite needs a high speed wireless connection to the ground station. The Aalto-1 is designed to use S-bandwidth that ranges from 2.4 GHz to 2.5 GHz which is not typical in small size satellites. To handle the communication with such a small satellite orbiting the earth, a ground station with an accurate tracking antenna is needed, to communicate with the satellite while it passes over the station. Highly directional antennas are getting more and more popular on satellites because of the need for massive amount of data-transfer [2]. This affects the ground station design so that a highly accurate pointing calibration method is needed. An accurate mechanical structure should also be designed to make this possible. For pointing calibration on S- band frequencies one can use radio objects in the sky e.g. radio galaxies, remains from supernovas or the sun as reference. A problem with highly directional antennas is the side lobes from the antenna which are getting power from surrounding environment. Most of the wireless disturbance around us is in the S-band frequency range and S-band antennas can pick up disturbance from these sources. Another method for calibration is to use an optical device to track the sun. This method can be sensitive for weather and there is inaccuracy caused by the atmosphere. Light scatters and refracts in the atmosphere so that the sun s position is not actually where it is observed. The refractions are zero at zenith and increases to about 0.6 degrees when watching the horizon [3]. Therefore the calibration should be executed when the sun is as high in the sky as possible. In this paper we present ideas how to design, build and calibrate a low-cost and simple S- band satellite tracking system for educational

2 purpose. This system consists of an antenna connected to a computer controlled rotating actuator, attached to a mast. The design of the mast has to be lightweight but strong enough to hold the wind load from the antenna. To make the ground station more versatile the mast has to be compatible with different kind of antennas. The goal was to achieve a tracking accuracy less than one degree. To test the accuracy of the ground station, signal strength from several tracked satellites can be measured. Different pointing accuracy calibration methods are evaluated to see which method would provide the most accurate calibration. The aiming accuracy depends mostly on the mechanical structure where the antenna is attached and on the calibration of the antenna. It is not possible to compensate for the displacement caused by wind load on the mechanical structure since it is a varying load. Therefore the mechanical structure has to be rigid enough not to deform under high wind load. A storm in Finland is defined for wind speeds over 20 m/s [4]. A wind load of 31 m/s was used for a safety margin. Higher wind speeds have been measured in Finland but designing the mechanical structure for higher values is not reasonable since it would add unwanted weight to the structure. 2. METHODS 2.1 Structure The ground station consists of an antenna connected to a computer controlled rotating actuator, attached to a mast. The aiming accuracy of the mechanical structure can be improved by making the structure stiff enough to reduce displacement caused by wind load on the antenna. By examining current solutions and their features one could by brainstorming choose between the different features and finally make a design that would suit this project. A three legged design (Fig. 1) was chosen since it is both simple and stable. To ensure adequate stiffness on the mechanical structure for the ground station a FEM analysis was performed. Fig. 1. The mechanical structure of the ground station The mechanical structure was also designed to be adjustable. This makes it possible to compensate for inaccuracy from small manufacturing errors. All of the legs on the mechanical structure have adjustment screws. This makes it possible to tilt the vertical centre axis of the mechanical structure. Both rotating axes on the rotating device are also adjustable. The rotating device is an azimuth and elevation rotator. It is possible for the rotator to turn around two axes, the azimuth and elevation axis. Azimuth is the angular measurement for turning along the horizon and elevation is for the height above the horizon. The rotator used in the testing of the calibration had a pointing accuracy of ±1 degree.

3 2.2 Calibration For the calibration part different types of calibration methods were examined to see which one would provide us with the best result and the simplest procedure. Four different kinds of calibration methods were found. Using photoresistor and signal measurements from antenna to calculate the position of the sun. Using radio objects (active galactic nuclei, supernova remnant). Using geostationary satellites. Maximising strength of the received signal from tracked object. All these objects have known positions in the sky and any of these objects can be used as a reference point when calibrating antennas. Two of the above mentioned calibration methods were disqualified. Geostationary satellites are below the horizon in Finland, and using received signal does not help getting information about how accurate the rotator is. In this paper a proof of concept for the sun tracking calibration system is presented. The calibration method used involves locating the position of the sun by measuring the brightness with a photoresistor. The schematic of the measuring equipment is illustrated in Figure 2 below. It consists of a photoresistor in a tube with a narrow opening to reduce the spreading angle of the photons from the sun (Fig. 3). The photoresistor is connected to a microcontroller that measures the voltage over the resistor through an analog-to-digital converter. The voltage over the photoresistor changes according to the intensity of the light hitting it. The microcontroller then sends the measurement to a computer on request. The computer stores the received data and plots it in a diagram after the measurements are done. The data contains the values for the intensity of the light and the azimuth and elevation rotation angle of the rotator at that moment. By this method the brightest spot in the measured area which is expected to be the sun can be located. Measurements from the sun produce data that should match a Gaussian function [5]. When the centre of the sun is found the results can be compared to an online database for the position of the sun at the moment of the measuring. From this the offset of the rotator can be found and adjusted so that it points accurately at the tracked object. Fig. 2. The principle of the solar locating system Fig. 3. Attachment of the photoresistor to the calibration tube The calibration test was done by first determining the position of the sun with the solar locating photoresistor and then by using a helix antenna for getting different

4 kinds of data to compare. With the data achieved from the two readings a calibration error can be estimated. Calibration starts by setting the zero point of the rotator to the north or south with the help of a compass. Then the edges of the sun were measured by the photoresistor and the centre position of the sun was calculated. The results were compared to values from Stellarium software [6]. Stellarium is an online software that gives the altitude and azimuth of the sun s position both in units of degrees. The next step was to check if it is possible to measure the signal strength from radio objects in the sky. Two objects were chosen for this, Cygnus A and Cassiopeia A [7]. Both of these are circumpolar objects in Finland, so they are always above the horizon [8]. It was approximated, based on information gathered from NASA/IPAC extragalactic database [7], that these objects are radiating at 2.4 GHz frequency with enough intensity so that they can be detected with a helix array antenna according to calculations below. Calculations are done for a helix antenna array consisting of 16 helix antennas. This array is the equivalent of a 3m dish antenna. Equation (1) below is used for calculating the brightness needed for observing an object in the sky, as well as for calculating the brightness achieved by the chosen objects. This gives us approximately minimum brightness of 172 Jy 1 Jy = W m 2 Hz needed of an object to be observed with our antenna. Cygnus A radiates at 2.7 GHz with 785 Jy and Cassiopeia A with 1495 Jy at 2.8 GHz. This means that the objects are bright enough to be observed by the antenna. 3. STATUS AND RESULTS A FEM analysis of the mechanical structure was made for a worst case scenario with a wind speed of 31 m/s on a 3 m in diameter mesh antenna. An image from the results can be found in Figure 4. The displacement caused by the wind load is so small that the attachment point for the antenna is just 0.04 degrees off-centre during the worst case scenario. The bending of the mechanical structure is therefore almost insignificant under normal weather conditions. S = 8kk sysk s ηπd 2 (tb) 0.5 (1) Where: k = Boltzmann constant k sys = system temperature k s = radiometer constant t = measurement time B = bandwidth η = efficiency Fig. 4. Results from FEM analysis The weather in Finland is very demanding in point of view of sun calibration. In winter the sky is cloudy most of the time, which makes it very hard to use the sun as

5 calibration. Below are the results from measurement of the amount of light hitting the photoresistor at a given turning angle. The equipment used in the testing allowed a ± 0.27 degree spreading angle of the photons travelling from the sun to the photoresistor. Figures 5 and 6 are presenting measurements from elevation angle and azimuth angle. Figure 5 represents the measurement results in azimuth direction. The maximum value on the fitted function can be found at 46.7 degrees on the rotator. Measurements were done on at 11:20 local time GMT +2. Fig. 6. Elevation measurements with fitted function 4. DISCUSSION Fig. 5. Azimuth measurements with fitted function Figure 6 represents the measurement results in elevation direction. The maximum value of the fitted curve can be found at degrees, which gives a corresponding value of degrees on the rotator. These measurements were done on 11:35 GMT +2. From the measurement values in Figures 5 and 6 one can easily read at which turning angle the light intensity is the strongest. The rotator was not reset to north before the testing so any values of the turning angle are not to be taken into account. The mast which was used for testing was built for VHF/UHF antennas and the beam width on VHF/UHF antennas are huge compared to the S-band antenna beam width. By making the antenna stiffer and by doing more sweeps over the sun the results could be improved. It is also possible to make the measurement equipment more light sensitive by reducing the size of the hole from where light is allowed to pass to the photoresistor. According to calculations it is not possible to achieve desired pointing accuracy with the equipment used in the tests. The goal was to achieve a pointing accuracy less than ±1 degree, which is the same as the accuracy on the rotating device. A more accurate rotating device should be obtained before

6 doing more serious testing but the present testing proves that the concept is valid. 5. FURTHER RESEARCH Next step would be to estimate how accurate the calibration is by measuring sun s position multiple times at different times of the day. One can then compare those results with the actual position of the sun on the Stellarium software and estimate the real pointing accuracy with this calibration method. For improving the accuracy, the used rotator has to be renewed. The same model used in the test is also available as a high resolution model, which has an accuracy of just ±0.2 degrees. The ground station is intended to be a part of the global educational network for satellite operations (GENSO) which connects ground stations all over the world and enables contact with satellites even when they are not in sight of local ground station. This allows scientist worldwide to access their satellites through this ground station. 6. REFERENCES 1. Kestilä, A. et al, Aalto-1, a Finnish hyperspectral remote-sensing nanosatellite: current progress,.4th European CubeSat Symposium, Modrzewski, R., Highly directional patch antenna for CubeSat applications, 4th European CubeSat Symposium, Allen, C. W., Astrophysical Quantities, 3rd ed., Athlone, London, Finnish Meteorological Institute [WWW] ( ) 5.Verma, R., Gregorini, L., Prandoni, I., Orfei, A., Pointing calibration campaign at 21 GHz with K-band multi-feed receiver, IRA 441/11, Stellarium, open source planetarium [WWW] ( ) 7. NASA/IPAC extragalactic database [WWW] ( ) 8. Karttunen, H., Kröger, P., Oja, H., Poutanen, M., Donner, K.J., Fundamental Astronomy, Springer, Helsinki, CORRESPONDING ADDRESS Panu Kiviluoma, D.Sc. (Tech.), Post-doc researcher Aalto University School of Engineering Department of Engineering Design and Production P.O.Box 14100, Aalto, Finland Phone: , panu.kiviluoma@aalto.fi 8. ADDITIONAL DATA ABOUT AUTHORS Winter, Edward, B.Sc. (Tech) Phone: edward.winter@aalto.fi Dahl, Johan Phone: johan.dahl@aalto.fi Nordling, Kalle Phone: kalle.nordling@aalto.fi Kuosmanen, Petri, D.Sc. (Tech.), Professor Phone: petri.kuosmanen@aalto.fi Jaan Praks, M. Sc. University Teacher Department of Radio Science and Engineering Aalto University School of Electrical Engineering Phone: jaan.praks@aalto.fi