SPACE COMMUNICATIONS AND HIGH ALTITUDE EARTH OBSERVATION Stargel R. Doane Old Dominion University Dr. Ayodeji Demuren Advising ABSTRACT

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1 SPACE COMMUNICATIONS AND HIGH ALTITUDE EARTH OBSERVATION Stargel R. Doane Old Dominion University Dr. Ayodeji Demuren Advising ABSTRACT The principles surrounding space communications and high altitude earth observation are both fascinating and complex. To better understand this topic, a satellite ground station was constructed on the campus of Old Dominion University. It has the ability to automatically track, record, and decode NOAA weather satellite transmissions and communicate with other parts of the world via amateur radio satellites. The satellite ground station has been assembled from individual electronic components, each with an intended purpose. The collection of weather satellite images has been taking place over the past ten months, and all data is accessible through a searchable database on the World Wide Web. Since the satellite ground station is only one aspect of a space communications system, a simple remote sensing vehicle is currently being constructed. This vehicle is in fact a high altitude balloon, which when completed, will have the ability to record temperature and pressure variations in the earth s atmosphere and take high altitude earth images. This balloon is projected to reach altitudes of approximately 90,000 feet. All of the collected data will be continuously transmitted to the satellite ground station, thus illustrating a simple but complete space communications system. 1.0 INTRODUCTION 1.1 Space Communications To start off this seemingly complex task of creating a space communications system, it is important to identify the current uses of such systems. In today s society, we rely quite heavily on space communications and the data pertaining to it. The average person comes into contact with this technology without giving it much thought or concern. Everyday, people all over world receive information passing in some way or another through orbiting satellites. This data could be amateur radio voice communications, weather satellite images, global positioning signals, or earth observation data. It is important to note the intended role of any desired satellite communications operation, as this will directly affect the layout and design of the system. In this research project, the focus was placed on earth observation. It was found that the orbiting NOAA weather satellites produced signals, which were relatively easy to receive and decode. With each pass of the NOAA satellites, an image was obtained that detailed the current weather conditions over North America, the Caribbean, and Mexico. 1.2 Space Flight and Orbits When people are questioned how space communications works, most responses involve something to do with satellites, the Space Shuttle, rockets, etc. While these are obviously correct answers, a very important concept is almost never mentioned. All satellites are moving with respect to some reference point, whether it is on earth or in space. In the case of space communications involving the earth, orbiting satellites are either fixed in their positions, or they are moving at some velocity. This leads to a rather dynamic situation requiring that each satellite be tracked. In order for a satellite pass to be predicted, the laws of physics are called upon to construct an accurate orbital model. As a starting point, the major terms that are used to describe a spacecraft s orbital Doane 1

2 characteristics will be introduced and briefly defined. Apogee Period during which a satellite is at its maximum distance from earth. Perigee Period during which a satellite is at its minimum distance from earth. Inclination Orientation of the satellite s orbital plane with respect to the earth s equatorial plane. Right Ascension of Ascending Node (RAAN) Specifies the orientation of the satellite s orbital plane with respect to the fixed stars. Eccentricity Describes the shape of the satellite s orbit. Argument of Perigee Gives the location of the satellite s perigee. Mean Motion Represents the number of revolutions the satellite makes in one day. Decay Rate Describes how atmospheric drag affects the satellite s orbit. 1.3 Orbital Models Now that some basic terms have been identified, the method by which satellites are tracked can be more thoroughly detailed. Initially upon entering into orbit, a satellite has a known position, velocity, and orbital makeup. From this point, the position of the satellite can be defined in a set of orbital elements known as Keplerian elements. These numerical elements are essentially a snapshot of the satellite in its orbit. Along with the terms defined above, a few other quantities are included in a set of Keplerian elements. These additional terms are defined below. Satellite Identifier Gives the name of the satellite that the Keplerian elements are describing. Catalog Number A number assigned by NASA to identify a satellite. Epoch Time Gives the time at which the given elements were calculated. Gives the time the orbital snapshot was taken. Element Set The number of the Keplerian element set. Epoch Revolution The number of orbits the spacecraft has made up to the point at which the element set was computed. Since Keplerian elements can be easily obtained, they will form the basis of any satellite pass prediction or tracking attempt. From the data provided within these element sets, orbital models can be constructed and simulations run to determine the expected overhead pass of any given satellite. There are several different satellite tracking programs on the market today that can accurately predict, simulate, and visually plot a satellite s trajectory. Examples of such programs are SatPC, WISP, NOVA, and InstantTrack. The main difference between the program suites is the mathematical models (often referred to as propagation models), which are used to calculate a satellite s position. There are five well known propagation models for predicting a satellite s trajectory. They are SGP, SGP4, SDP4, SGP8, and SDP8. Ultimately, the main difference between all of these models is the overall accuracy of the prediction, with SGP being the least accurate and SGP8 being the most accurate. Also, it is worth mentioning that a satellite s trajectory is affected by its altitude. Because of this, the mathematical model representing the satellite s trajectory will need to be slightly different. Altitude is important because the closer the satellite is to earth, the more atmospheric drag it will encounter. This atmospheric drag will thus cause orbital decay. After reviewing the different propagation models given above, one will find that the names contain two different prefixes, SGP or SDP. The SGP model is used for near-earth satellites, and thus, incorporates drag terms. The SDP is for deep space and includes terms for the gravitational effects of the sun and moon. For reference, the full derivation for each propagation model is given in Spacetrack Report No. 3, by Felix R. Hoots and Ronald L. Roehrich. Since Doane 2

3 the satellites of interest in this project are considered near-earth, a SGP4 model will be used. The SGP4 propagation model is highly accurate, and is thus, sufficient for this application. 2.0 Satellite Ground Stations 2.1 Purpose and Design Most satellite ground stations have one or two specific applications in which the system is to specialize. This being the case, a satellite ground station will operate mission specific transceivers, antennas, transmission lines, amplifiers, and digital signal processing equipment. In the case of the satellite ground station, which was constructed in this research project (ODUSGS), the intended target was to communicate with orbiting weather satellites and amateur radio satellites. Thus, the station was designed to receive and transmit on VHF and UHF frequencies. A full description of the satellite ground station, which was constructed on the campus of Old Dominion University, has been given in the section below. This description includes definitions and overall purpose for all necessary components in the satellite communications system. 2.2 The ODUSGS System The main purpose of the ODUSGS was to communicate with amateur radio satellites and weather satellites and obtain earth images. The system was designed for operation on VHF and UHF frequencies. Specifically, the system was intended for the 135MHz-175MHz and 420MHz-480MHz frequency range. Within this specification, several other factors had to be considered. The first of these issues was the relatively low transmission power of the target satellites. Typically, amateur satellites are small and do not have very high output power. The output on most amateur satellites will fall between 1-10W, and thus, reception is not always easy (Davidoff, 2001). Along with the amateur satellites, the station has also been designed to receive data from the NOAA weather satellites. These satellites are similar to the amateur satellites in their orbits and have an output power of approximately 5W. The second issue concerning the design and construction of the satellite station was the orbital characteristics of the satellites. Both the weather and amateur radio satellites are considered low earth orbiting (LEO) satellites. These satellites are relatively close to the earth, have high velocities, and make complete orbits several times a day. Because of these characteristics, the satellite station was designed to predict, accurately track, and function automatically. The final issue concerning the design of the satellite ground station relates to data management. Since the target satellites make many revolutions per day, the amount of data collected can be quite high. This means that a reliable filing and searching system is needed in order to collect and organize data. 2.3 ODUSGS Components In the interest of completeness, a full listing of satellite ground station components has been provided below. This list is then followed by brief component descriptions. At the end of this section, a complete schematic diagram (figure 2.3L) has been provided for the entire ODUSGS system. Components of the ODUSGS FM Transceiver Pre-Amplifiers Antennas Antenna Rotators Computer Control and Tracking System Transmission Cables Modems and Data Decoders FM Transceiver The first and most important Doane 3

4 component of any satellite ground station is the transceiver. This item is responsible for providing the radio link to and from the satellites. Transceivers are designed to work on specific frequencies and modulation schemes. In the ODUSGS, an FM transceiver was selected, which transmits and receives on the 135MHz-175MHz and 420MHz-480MHz frequency range. Also, to account for the Doppler effect, the radio is linked to satellite tracking software to provide continuous frequency correction. This is required to clearly hear the satellite s downlink signal. This is accomplished via the use of a CI-V level converter. This component has been illustrated in figure 2.3A. A picture of the Icom 910H transceiver has also been provided below in figure 2.3B. This is the transceiver that the ODUSGS utilizes. satellite s signal and helps to eliminate any existing background noise. Many different types of pre-amplifiers are available on the market, each with different uses and specifications. For the ODUSGS, two mast mounted pre-amplifiers are used. Mast mounted means that the component is placed on the tower (or mast) as close to the antennas as possible. This is important because as the distance between the amplifier and the antenna increases, the greater the chance for interference. Each pre-amplifier is designed to work on a specific frequency range (see Figure 2.3C). This helps to eliminate amplification of background noise or other off-frequency emissions. The ODUSGS uses two Icom AG (figure 2.3D) series preamplifiers to provide the necessary signal amplification. ICOM Pre-Amplifier Specs. AG-25 AG-35 Figure 2.3A From: Frequency Range 144~148Mhz 430~450Mhz Gain 15db 15db Current Drain 200mA 200mA Maximum feed through RF Power Input/Output Impedance Figure 2.3C 100W 50Ohm 100W 50Ohm From: Figure 2.3B From: Figure 2.3D From: Pre-Amplifiers To receive both weather and amateur satellites, a suitable pre-amplifier was required. This component boosts the Antennas To further improve the overall performance of the satellite ground station, Doane 4

5 directional crossed yagi antennas were constructed. This particular type of antenna design allows the ground station to focus on a particular point rather than simply collecting radio transmissions from every direction. Crossed yagi antennas are considered to be high gain, meaning that they produce a much stronger signal when compared to other antenna designs. This characteristic makes the crossbeam antenna the best choice for smallscale space communications. A parabolic dish antenna would be the perfect choice; however, these are often expensive and hard to manage. See figure 2.3E below for a picture of the Hy- Gain UB-7030SAT and VB-216SAT antennas, which are currently in use with the ODUSGS. by connecting the antenna boom to two electrical motors. These motors are controlled by a remote system in the satellite ground station. The main design consideration for this system is the total torque required to move and stop the antenna boom. This is dictated by the type of antenna array and any possible wind loading. A close-up view of the Yaesu G-5500 rotators and control box has been given below in figure 2.3F. Figure 2.3F From: Figure 2.3E Antenna Rotators Because of the highly directional nature of the crossed yagi antennas, it is necessary to provide some means of accurately pointing the antenna arrays. This is accomplished by connecting the antennas to a boom, which can be rotated for azimuth and elevation control. Rotation is accomplished Computer Control and Tracking System As mentioned in the previous section, the two motors that are used to position the antenna arrays are controlled by a remote system within the satellite ground station. This system consists of an electrical relay interface to power the drive motors. This system uses potentiometers within the electric motors to sense the position of the array. Using an analog to digital interface in the control room, the voltage level from the potentiometers is sensed and converted into digital format. This value is then sent to a computer for monitoring and control purposes. This same interface is also used to power the motors and cause rotation. This is done via an on/off relay within the circuit. A voltage is sent from the control computer, the relay is activated, and power is sent to the motors. In the ODUSGS, Antenna Rotator Systems RCI- SE (figure 2.3G) is used as the A/D converter and computer interface. This component connects to the control box illustrated previously in figure 2.3F. Using satellite Doane 5

6 tracking software, the position of a satellite can be determined and the position of the antenna array matched to that same point in space. This setup allows for automatic tracking and leaves the ground station operator free to perform other tasks. While there are many different satellite tracking programs on the market, each one has unique characteristics. The issue or difficulty is finding the correct program to mesh with all of the existing hardware. In the case of the ODUSGS, the tracking program SATPC32 was chosen to provide both antenna positions and radio frequency control. This program uses the SGP4 prediction algorithm, which was mentioned earlier. A screenshot of this program has been provided in figure 2.3H. Transmission Cables This is perhaps one of the most important components in a satellite ground station s design. Whenever a radio signal is transmitted through a cable, it experiences some level of loss. The ultimate goal of the designer is to minimize this loss while keeping cost as low as possible. In the case of the ODUSGS, this was a very critical issue because of the required two hundred foot cable run from the antenna tower to the control room. Because of this, a rather expensive low loss cable was used. This cable is known as Andrews ½ Heliax and is of the highest quality. A cross-section of this cable has been provided below in Figure 2.3I. Figure 2.3I From: Figure 2.3G Figure 2.3H From: Modems and Data Decoders The final consideration for a satellite ground station is the necessary encoding and decoding of the satellite transmissions. In the case of the weather satellites and amateur satellites, relatively easy and inexpensive options are available. For the amateur satellites, most of the data is transmitted in amateur radio packet. Any off-the-shelf amateur radio modem will be suitable. An example of such a modem is the Kantronics KPC-9612plus, which is currently used in the ODUSGS (see figure 2.3J). This modem is capable of handling both 1200 and 9600 baud packet transmissions. For the weather satellites, two options exist. The main difference between the two is overall cost. The first option is to purchase a hardware modem, which has been specifically designed to decode weather satellite transmissions. This option is relatively expensive. The second option involves the use of a software package, which uses a Doane 6

7 computer s sound card to record and decode the weather satellite signals. These software packages can be obtained for free, or for more sophisticated programs, for around 100 dollars. The ODUSGS uses the software package WXtoIMG (see figure 2.3K) to automatically record, decode, and save each satellite pass. Figure 2.3J From: Figure 2.3K Overall ODUSGS Schematic Diagram Figure 2.3L Doane 7

8 3.0 Satellite Data 3.1 Data Variety As one can imagine, there are numerous satellites orbiting the earth, each with a specific purpose and function. Because satellites are not manufactured with the assembly line mentality, each satellite can be thought of as a unique system. Therefore, each satellite s data collection and transmission scheme is slightly different. This ultimately requires specific hardware and software for successful communications. For example, an amateur radio enthusiast who wishes to communicate with all of the analog (voice) transponders currently onboard the amateur satellites would need receiving equipment for 29, 145, 435, 2400, 10000, MHz (Davidoff, 2001). It is clear from this statement that satellites and their systems are quite unique. This equipment need is expanded if other modes of operation are required, such as for image reception and digital communications. 3.2 Weather Images As previously stated, the main purpose of the ODUSGS is to receive, decode, and store NOAA weather satellite images. Specifically, the ODUSGS receives the Automatic Picture Transmissions (APT) from the NOAA 12, 15, and 17 weather satellites. This is a continuous data stream, which can produce very large images of the earth. Reception of the weather image is line of sight. This translates to an image, which covers from the northern-most portion of Canada to the tip of Mexico. A typical satellite schedule consists of three to four quality passes per day, with each pass lasting approximately 14 minutes. Each raw image is then rendered into approximately fifteen different specialty images. These images show many things, such as cloud temperature, water temperature, and precipitation, just to name a few. A sample of the weather images received by the ODUSGS has been provided below in figure 3.2A. Figure 3.2A 3.3 Data Management Since new images are obtained three to four times in one day for a total of 60 images per day, data management is a serious concern. Initially, recording and filing the images is quite simple. After several months, however, searching through thousands of pictures can be quite tedious. Because of this downfall, the ODUSGS employs a server side search engine written in PHP to more efficiently view downloaded images. The search engine uses the month, day, and year, which are supplied by the user, to locate images. This system will then display the time of day for each image group, along with the corresponding pictures. 4.0 High Altitude Earth Observation 4.1 Overview High altitude earth observation can be interpreted in many different ways. In the Doane 8

9 case of this research project, it will be defined as viewing the earth from 100,000 feet and above. Currently, satellites are the main form of high altitude earth observation. These satellites can have different missions, such as spying on other countries or mapping of hard to navigate terrain. Whatever the case, the earth is being viewed from very high altitudes. This allows for large expanses of the earth to be covered and analyzed. The ODUSGS high altitude balloon project hopes to mimic this on a very simplistic and small scale. 4.2 High Altitude Balloons The concept of using high altitude balloons to make scientific measurements and images is in no way a new idea. The amateur radio community is a very active participant in high altitude ballooning. There are many clubs devoted to this interesting hobby throughout the U.S. and the world. In general there are three types of high altitude balloons. The first is the pilot or ceiling balloon, which is designed for flights up to 20 km. The second is the sounding balloon, which is capable of reaching altitudes of approximately 35 km. The final type is the cold weather balloon, which can achieve altitudes of approximately 40 km. In general, weather balloons are typically made of latex. The difference between the balloon types is the chemical formulation of the latex. Some balloons are made of very strong latex, which can allow for rough weather launches. The opposite of this is a latex formulation, which permits extremely high stretching and thus allows for ultra- high altitude flights. Within each category of weather balloons, there are slight variations in the balloon performance and payload. The maximum weight for a high altitude balloon can range from 250 grams to 1050 grams. These specifications are based on Kaymont Meteorological Balloons ( therefore, this data is not intended to identify the absolute maximum or minimum values for all weather balloons, but instead, give a rough estimate. 4.3 ODUSGS s High Altitude Balloon Program As mentioned previously in this report, the purpose of this research endeavor was to investigate space communications and high altitude earth observation. This being the case, a satellite ground station only makes up half of a complete space communications system. To complete the other half, a small satellite or high altitude vehicle should be constructed. A high altitude balloon was identified as the perfect candidate for such a project because of the overall simplicity and low cost. Power consumption, tracking, and communications are all important design aspects of a high altitude balloon payload. This makes high altitude balloons relatively similar to satellites. The intended mission of this balloon project is to collect data, such as temperature, pressure, and earth images. The initial tracking system is already under construction and has passed its initial testing. This system uses a GPS to report real-time location and altitude information. This data is then transmitted via an amateur radio transceiver to the ODUSGS. The ground station next records this data and automatically plots the position of the payload on a map. The complete payload will incorporate this subsystem with the data collection sub-system. The initial launch is expected to take place around the end of June The target altitude for all flights is expected to be around 90,000 feet. Conclusion This research project has demonstrated in a very simplistic manor the principles behind space communications and high altitude earth observation. A complete satellite ground station has been constructed Doane 9

10 and placed under automatic operation for approximately ten months without any system failures. A successful data management system has also been assembled and allows anyone to search the image archives, learn more about the environment, and understand the usefulness of satellite communications. The next aspect of this project requires the construction of a high altitude balloon. This should prove to be exciting, challenging, and a great opportunity to explore in greater detail the topics surrounding space communications. This portion of the project will also give an exciting opportunity for high school students to participate in experiment planning and overall mission planning for the high altitude balloon design project. Acknowledgements Special thanks should be given to the following individuals and organizations for their assistance in making this project a success. Thanks to Dr. Ayodeji Demuren for his devotion of time and contribution of great knowledge. Ed Williams for his very generous donation of a tower. His donation truly got this project off the ground. The Virginia Beach Amateur Radio Club for their interest in supporting my research with much advice and experience. Synergy Systems, LLC for allowing us to receive discounted GPS hardware and software. Mr. Justin Rice for his help in the construction of the ODUSGS and current participation in the high altitude balloon project. Mr. Thomas Hall for his current participation in the high altitude balloon project. Mr. Ken Pierpont for his initial guidance and technical knowledge, which helped make the ODUSGS a success. Virginia Space Grant Consortium for their financial support and backing of this research project. Mr. Richard Doane, my father, for his assistance in transporting many of the ground station components and for his current participation in the high altitude balloon project. Ashley, Scott, and Mr. Steven Czarny for all of their assistance in moving ground station components. References Davidoff, M., (2001). The Radio Amateurs Satellite Handbook. Newington: The American Radio Relay League. Hoots, F., & Parker, K. (1988). Spacetrack Report NO. 3. Retrieved July 25, 2004, from spacktrk.pdf Doane 10