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A distributed orbital measurement instrument for link quality assurance
A distributed orbital measurement instrument for link quality assurance Lars Mehnen*, Bastian Preindl*
* Institute: Biomedical Engineering, University of Applied Science Technikum Wien, Austria (++43 1 333 40 77 -373, e-mail: [email protected]). Abstract: The development of Nano- and Picosattelites for educational and scientific purposes becomes more and more popular. As these satellites are very small, highly integrated devices and are therefore not equipped with high-gain antennas, the data transmission between ground and satellite is vulnerable to several ascendancies in both directions. Another handicap is the low earth orbit where the satellites are usually located as it keeps the communication time frame very short. To counter these disadvantages, the global educational network for satellite operations (GENSO) has been created. To optimize the communication between ground station and satellite, metrics and methods have to be identified to gain information about the satellite link quality during a ground pass. Such a global effort is hindered by utilization of very different hardware, protocols, frequencies and modulations. This paper shows a generic and widely applicable approach to passively measure and compare satellite link quality to be applied in a heterogeneous ground station and satellite configuration. Keywords: CubeSat, radio link, pattern classification, communication system performance
1. INTRODUCTION In fall 2006 the Global Educational Network for Satellite Operations (GENSO) project was initiated by the ESA Education Office as a consequence of the Cubesat concept, which has been developed at Stanford University and the California Polytechnic Institute. This concept allows a small 1kg satellite of 10x10x10cm to be launched into Low Earth Orbit (LEO) at remarkable low launch costs of about $40,000. The novel idea was that differently educated engineering students cooperate using all their skills and enthusiasms in solving every detail of the complex tasks. The GT-Sat project focuses primarily on the development of a specialized Cubesat for identifying the reliability of amateur ground stations and being a test-bed for educational satellite projects communication links and their interdependencies with environmental conditions.
As the available time for communication between satellite and mission control can be crucial for a mission, approaches took place to optimize the usage of ground stations and to significantly extend the time a satellite can communicate with the affiliated mission control centre. The most sophisticated approach for a world-wide interconnection of independent ground stations so far is the Global Educational Network for Satellite Operations (GENSO). Its aim is to share ground station capacity between different mission control centers by creating a hybrid, supervised and secure peer-to-peer network utilizing the internet.
1.1 Problem Within the last decade the educational and academic approaches in space science made huge steps forward. Driven by the development of small satellites for taking scientific payload of any kind into space, universities all over the world started to design, develop and launch small satellite projects based on the Cubesat standard [1]. Many small LEO satellites operate in the low earth orbit causing high communication intensities. The communication timeframe between a satellite and its affiliated ground station [2] is about 30 minutes a day (e.g. up to 15 min for each pass for a polar LEO) whereas the ground station is idle for the remaining time of the day.
Fig. 1. GENSO basic scheme Interconnecting a very large amount of satellite ground stations forms the base for novel scientific approaches in the
domain of link quality determination and prediction, overall optimization of space up- and downlinks as well as hardware utilization and the influence of environmental conditions on the radio communication. The low earth orbit is located between 200km and 1200km above the earth's surface and is the lowest possible orbit for spacecrafts of any kind. Other important orbits are the MEO (Medium Earth Orbit) and the GSO (Geo-Stationary Orbit, together with Elliptical Geostationary Transfer Orbits). The advantage of the LEO is that spacecrafts being placed in that orbit area do stay in a moderate temperature region and do not have to cross the "Van-Allen-Belt" and therefore are not exposed to high radiation. This allows the hardware on non-commercial, academic satellites to be of lower cost than for e.g. GSOs. Another advantage is the nearly infinite and free amount of orbits in LEO whereas orbits in GSO are very limited and cost-intensive. A further advantage is the short distance between spacecraft and ground station reducing the needed signal strength for up- and downlinks, which makes the very small spacecraft designs even more possible. The big disadvantage of LEO is however the short time frame available for communication between spacecraft and ground station due to a high velocity on one hand and the low distance between surface and spacecraft on the other hand which results in a very small communication interval. Beside this short communication time frame with a maximum of 15 minutes for each pass, the high ground track speed of approximately 7km/s (26.000km/h, not taking earth rotation into account) requires relatively fast-tracking and precise antenna and rotor facilities on ground to adjust elevation and azimut in time. Ground stations at higher latitudes (e.g. Svalbard in Spitzbergen) benefit from the high inclination of small satellites' orbits in relation to the earth rotation and have therefore an extended communication time frame. 1.2 Approach The Cubesat standard, originally defined by Stanford University and the Californian Polytechnical University, defines a small satellite having the size of 10x10x10 centimetres and a maximum weight of 1 kg. A majority of the current academic small satellite projects rely on the Cubesat design proposal as it standardizes also the deployment of the satellites in orbit and is the minimal easily detectable size for metallic objects by NORAD. Academic and small scientific satellites are usually additional payloads of larger commercial and scientific missions, which makes launches remarkable cheap and the amount of satellites deployable in orbit comparably high (the current maximum is 10 deployed missions during one rocket launch). The payload of Cubesats can be educational, scientific and also industrial. Latest industry-driven approaches in satellite development are towards small satellites (also referred to as pico satellites) being cheap in design and construction, standardized and easy to deploy.
Instead of designing full-sized satellites with enormous costs these are more and more substituted by redundant cheap pico and nano satellite constellations. Often (not only scientific) Cubesat payloads consist of measurement hardware (sensors) for numerous research topics, communication hardware, probes, novel spacecraft control hardware and many others. The typical framing hardware of a Cubesat consists of solar panels and small batteries for the power supply, satellite attenuation determination control facilities and radio communication facilities connected to an onboard computer for controlling the satellite (house-keeping data) and sending results of measurements via the transceiver radio. Many small satellites are also equipped with store and forward buffers for short messages exchange to support the amateur satellite-based packet radio network. Due to their design small satellites are equipped with comparable weak communication facilities as their production of energy is limited by the small area of the solar panels which are normally mounted on only 4 sides of the cube. These limits cause the up- and downlink to be very vulnerable to environmental influences and imprecise ground antenna calibration. It also causes low horizon passes to be very error-prone as the distance between the satellite and the ground station is highest at that constellation. This makes many satellite passes unusable and restricts the valuable passes to one with a high elevation above the horizon. The usual communication frequencies for small satellites in LEO are normally registered and coordinated in the amateur VHF, (beyond 400MHz), UHF (approx. 400-600 MHz) and S-Bands (2-4 GHz). Future developments take the usage of Ku-Band (10-15Ghz) into consideration. The higher the radio communication frequencies are, the higher is the necessity for a very accurate onboard attitude control system, since the antennas directional behaviour dominates more and more, resulting in the need for a directional control of the antennas towards the groundstation. GENSO is a project supported by the International Student Educational Board (ISEB) formed by ESA (European Space Association), NASA (National Aeronautics and Space Agency), JAXA (Japanese Aerospace Exploration Agency) and the CSA (Canadian Space Agency). The project is supported by AMSAT (the Satellite Radio Amateur league) [4] with several thousand amateur satellite ground stations beeing able to communicate with small satellites all over the world. The basic aim of the network is to interconnect the noncommercial ground stations all over the world to share resources and dramatically extend the time frame for communication with a satellite from a maximum of 30 minutes a day up to nearly 24 hours. The main focus lies on the support of LEO missions, however it is not restricted to LEO only. Not only educational satellites but all spacecrafts including for example the International Space Station (ISS) are supported by the network.
The basic concept of the network is to create a transparent layer between the traditional constellations of ground stations, mission controls and spacecrafts. The authentication server cluster is the network supervisor - it manages all participating ground stations, mission-controls and spacecrafts. It collects data about all traffic between ground stations and mission controls, which covers satellite pass booking information but also meta information about satellite passes. [9] The collected information about the network of groundstations and the spacecrafts and their passes forms the foundation for a scientific investigation on predicting and optimizing up- and downlink quality. Institutes which will join the network in operation are extremely interested in these scientific approaches include the Stanford University, USA, the EPFL, Switzerland, the University of Toronto, Canada, the University of Surrey, UK, the Hokkaido Institute of Technology, Japan and the TU Munich, Germany, to name only a few of them. GENSO constitutes the scientific base for a multitude of research fields and investigations. The proposal on hand aims on the identification and utilization of link quality information. For the first time the possibility is provided to gain focussed information about the quality of LEO satellite downlinks from a huge amount of independent ground stations throughout the world. The gained information can be processed and applied in various ways. As an example the optimisation of the groundstation network itself is one possibility to apply this information. The results of this computation can be applied continuously resulting in an adaptive but nearly optimal network structure – concerning the radio links. Each of these specific aims constitutes a novel approach in its field of science and can support space operations upcoming in the next decade and probably far beyond.
1.4 Communication Parameters The major amount of academic satellites uses the AX.25 protocol or at least parts of the high-level data link control standard (HDLC), only a few are using more advanced protocols, like SRLL. Usually the frequency shift keying (FSK) or audio FSK (AFSK) are applied as modulation schemes with symbol rates of 1200 and 9600 baud if usinf UHF bands. The most comprehensive collection about small satellite communication facilities provides Klofas et. al. in [24]. Common TNCs support primarily AX.25 protocol processing, other protocols require special firmwares or software decoders. TNCs can usually operate in different modes, the most common is the so-called KISS mode. The KISS mode only applies the CRC inspection and generation and bit stuffing operations of the HDLC part of the AX.25 protocol. Against common presumptions the KISS mode does not only forward the raw incoming and outgoing data but also processes the forwarded frames. It is therefor impossible to operate protocols not according to the HDLC standard regarding the frame check sequence (FCS), the bit stuffing process and the packet delimiters with KISS TNCs. KISS also describes a simple intermediate protocol for data exchange between the host machine and the TNC – after HDLC frame preprocessing the data is encapsulated in the socalled KISS frame which exchanges the original HDLC frame. Protocols different from standard AX.25 can be applied as long as the HDLC frame persists. This constraint limits the error detection to the CRC-algorithm of HDLC and eliminates the possibility of FEC. SMACK represents an enhancement to KISS as it adds a CRC checksum also for the communication between TNC and host machine. A simple KISS and SMACK frame counter in the ground station transcoder framework makes it possible to precisely count the amount of received good but not the errneous packets.
1.3 Space up- and downlink quality identification and determination As a basement for all further scientific approaches in this direction a metric has to be identified for measuring and comparing the quality of satellite links. This is complicated by application of different protocols [5,6], modulations and frequency [3,7] bands on one hand and the operation in a heterogeneous radio hardware environment on the other hand [8]. As the network itself is not able and also not permitted to be aware of the transferred communication payload between a mission control centre and a spacecraft this project will provide the possibility to spread only the information about the link quality in relation to environmental parameters in a reliable way. The investigations include a possible design for the determination of uplink quality for future application.
Fig. 2. This diagram shows the course of the S-meter levels of a typical satellite pass. The noise floor can be separated from the actual signal. Also, the peaks of the signal can be determined
1.5 Defining the measurements For gaining comparable and usable quality information the values have to be measured, preprocessed and compiled. For the SNR the driver libraries are going to be utilized and abstracted by the GSS software to retrieve the S-meter readings of the radio. Fig. 2 shows a single pass of AAUSAT II over Aalborg university recorded with an ICOM IC-910H radio [24] using a sample rate of 8 samples per second. The values collected between the beacon transmissions are used to determine the current noise level. For the PLR the retrieved data frames are counted and compared to the theoretical maximum amount of transferable packets during the pass. At a typical baud rate (e.g. 1200-9600) the packet rate is very low and the usage of beacons instead of constant streams of data decreases the amount of transmitted packets additionally. The proposed sampling rate for the packet count is therefore at least one minute. The reason why one overall count for one pass is not recommended is that it disables the possibilities for angle-based normalization, which is the reason why we do not read out the data transmission statistics that some TNCs provide. Based on the information which is provided for every spacecraft about baud rate, protocol, beacon length and interval the mean PLR can be calculated. The remaining problem is that - in case of transmitted beacons or discontinous transmissions - the measured Smeter values range from the noise strength to the actual received signal strength indication (RSSI). As a consequence the average value adducted for quality assertions over a certain amount of time would be in-between the noise level and the factual RSSI. Hence the values have to be devided into two categories, one containing values above an arithmetic mean value, representing the presumed RSSI values, and one containing the values below the arithmetic mean value, representing the noise level. By analyzing the samples located in the RSSI category for continous oscillations presumptions about the thumbling behaviour of the spacecraft can be extracted, which can be a valuable information for the spacecraft operator. Further investigations concerning the noise level exhibiting artificial, continous patterns could lead to assumptions about radio interferences, as an example. Although these investigations are not yet part of the current evaluation they can provide valuable information for future calculations and predictions (refer to [25] and [26] for further applications). 1.6 Automated identification and calibration of imprecise ground stations
By investigating differences between predicted and effective link quality after the pass of a spacecraft has taken place, software [19] can be used to identify ground stations with receiving and transmitting capabilities beyond a predicted level. The problem is more or less to gain this information in good quality and short time [11-16]. This can be managed by a sample satellite with known orbit and communication capabilities to normalize and test the communication points. The ground station operators then can choose whether he wants to be
informed if problems with their communication hardware arise. The network with this sample satellite - called GT-sat (GENSO Test Satellite) additionally offers the possibility of having a low-cost calibration facility for non-commercial ground stations. [21] If a problem arises in a participating ground-station and it is rechecked by the system that this problem really exists, then this entity is downgraded within the ground station network to avoid having broken or imprecise radio-stations to rely on in critical situations. This results in a concurrent network of ground-stations with known sending/receiving qualities and on/off-line times, which can be used by the Cubesat community [22] effectively.
Fig. 3. Basic block diagram of a system for transmission of PSK/MSK modulated signals.[23] The GT-Sat is also planned to be used as a predecessor for the upcoming QB50 project [27] which is addressing the lower thermosphere/ionosphere (90-320 km) which is the least explored layer of the atmosphere. Only five atmospheric explorers were flown by NASA in highly elliptical orbits. They carried experiments for in-situ measurements, but the time spent in the region of interest (below 320 km) was only very limited. Nowadays, sounding rocket flights provide some in-situ measurements. Whilst they do explore the whole lower thermosphere, the time spent in this region is also extremly short and therefore doesn’t provide information about dynamics of the thermosphere. Powerful remotesensing instruments on board Earth observation satellites in higher orbits (600–800 km) receive the backscattered signals from the atmospheric constituents at various altitudes. While this is an excellent tool for exploring the lower layers of the atmosphere up to about 100 km, it is not ideally suited for exploring the lower thermosphere as the density of the atmosphere is so thin that the reflected signal is extremely weak and therefore unreliable. The same holds for remotesensing observations from the ground with lidars and radars. The multi-point, in-situ measurements of QB50 is complementary to the remote-sensing observations by the instruments on Earth observation satellites and the remotesensing observations from the ground with lidars and radars. The results observed by the QB50 project will help to understand the dynamics of the thermosphere/ionosphere which will additionally improve the communication link quality prediction to increase the radio link capabilities, since
the ionosphere heavily affects radio communication between the satellites and the groundstations in lower bands.
9. 10.
1.7 CONCLUSION Heterogeneous ground station hardware environments, a wide range of very different spacecraft communication facilities and the utilization of different protocols together form one of the worst possible scenarios for obtaining meaningful, comparable quality measurement results. We have identified what link quality means in respect to non-commercial satellite communication and which possibilities are provided to collect raw information concerning the quality of an established space links. Furthermore we’ve demonstrated how the collected raw measurement data has to be sampled and processed to achieve normalized, comparable information. The introduced procedure establishes the required foundation for further investigations and novel approaches on link quality recordings and predictions. As integral part of groundstation networks link quality measurement facilities will offer both science and education the possibility of a distributed and accurate collection of communication meta-level information for the first time in history of non-commercial space-flight. As integral part of the QB50 project it will enable reliable communication facility for the downlink of the hughe ammount of science data produced by the CubeSat cluster.
11.
12. 13. 14.
15. 16. 17. 18.
REFERENCE 1.
2.
3.
4.
5. 6. 7.
8.
Stephen Waydo, Daniel Henry, and Mark Campbell. CubeSat design for LEO-based Earth Science Missions. In Proceedings of the IEEE Aerospace Conference, volume 1, pages 435-445, 2002. Tarun S. Tuli, Nathan G. Orr, and Robert E. Zee. Low Cost Ground Station Design for Nanosatellite Missions. In Electronical Proceedings of the AMSAT North American Space Symposium, 2006. Philip R. Karn, Harold E. Price, and Robert J. Diersing. Packet Radio in the Amateur Service. IEEE Journal on Selected Areas in Communications, 3(3):431- 439, May 1985. Keith Baker and Dick Jansson. Space Satellites from the World's Garage - The Story of AMSAT. In Proceedings of the IEEE National Aerospace and Electronics Conference (NAECON), volume 2, pages 1174-1181, May 1994. Subbarayan Pasupathy. Minimum Shift Keying: A Spectrally E_cient Modulation. IEEE Communications Magazine, 17:14-22, July 1979. Richard R. Parry. AX.25 [Data Link Layer Protocol For Packet Radio Networks]. IEEE Potentials, 16(3):14{16, August-September 1997. Irfan Ali, Naofal Al-Dhahir, and John E. Hershey. Doppler Characterization for LEO Satellites. IEEE Transactions on Communications, 46(3):309-313, March 1998. Steve Stearns. Antenna Modeling for Radio Amateurs. In Electronical Proceedings of ARRL Paci_con Antenna Seminar, 2008.
19. 20.
21.
22.
23. 24. 25.
C. Lorek. The ICOM IC-910 Transceiver. Radio Communication, 77(7):17, 2001. Harold E. Dinger and Harold G. Paine. Factors A_ecting the Accuracy of Radio Noise Meters. Proceedings of the IRE, 35(1):75-81, January 1947. Yoseph Linde, Andres Buzo, and Robert M. Gray. An Algorithm for Vector Quantizer Design. IEEE Transactions on Communications, COM-28(1):84-95, January 1980. National Weather Service. METAR Data Access. [Online]. Available: http://weather.noaa.gov/weather/metar.shtml. National Climatic Data Center. Global Surface Summary of Day. [Online]. Avail- able: http://www.ncdc.noaa.gov/oa/gsod.html. Space Weather Prediction Center. Real-time ACE Satellite Hourly Data and Spacecraft Location. [Online]. Available: http://www.swpc.noaa.gov/ftpmenu/lists/ace2.html. Advanced Composition Explorer (ACE). [Online]. Available: http://www.srl.caltech.edu/ACE/. Space Weather Prediction Center. Historical SWP Products. [Online]. Available: http://www.swpc.noaa.gov/ftpmenu/warehouse.html. Harald T. Friis. A Note on a Simple Transmission Formula. Proceedings of the IRE, 34(5):254-256, May 1946. Ingo Mierswa, Martin Scholz, Ralf Klinkenberg, Michael Wurst, and Timm Euler. YALE: Rapid Prototyping for Complex Data Mining Tasks. In Proceedings of the 12th ACM International Conference on Knowledge Discovery and Data Mining (SIGKDD), pages 935-940, 2006. Christopher M. Bishop. Pattern Recognition and Machine Learning. Information Science and Statistics. Springer, 2006. Helen Page, Bastian Preindl, and Viktor Nikolaidis. GENSO: The Global Educational Network for Satellite Operations. In Electronical Proceedings of the 59 th International Astronautical Conference (IAC), 2008. Bastian Preindl, Martina Seidl, Lars Mehnen, and Sebastian Krinninger. A Performance Comparison of different Satellite Range Scheduling Algorithms for Global Ground Station Networks. In Proceedings of the 61st International Astronautical Conference (IAC), 2010. Bastian Preindl, Lars Mehnen, Frank Rattay, and Jens Dalsgaard Nielsen. Design of a Small Satellite for Performing Measurements in a Ground Station Network. In Proceedings of the IEEE International Workshop on Satellite and Space Communications (IWSSC), pages 186-190, 2009. Robert Schürhuber, Master Thesis 2011, Technical University of Vienna, Fundamentals of Modulation Schemes for Low-Budget LEO Satellites. C. Lorek, “The ICOM IC-910 Transceiver,” Radio Communication,vol. 77, no. 7, p. 17, 2001. Bastian Preindl, Lars Mehnen, Frank Rattay, Jens Daalgard Nielsen, Sebastian Krinninger, and K. K. Sørensen, “A Global Satellite Link Sensor Network,” in
Proceedings of the 8th IEEE Conference on Sensors, Christchurch, New Zealand, 2009. 26. Bastian Preindl, Lars Mehnen, Frank Rattay, Jens Daalgard Nielsen, “Applying Methods of Soft Computing to Space Link Quality Prediction,” in Applications of Soft Computing. From Theory to Praxis, vol. 58 of Advances in Intelligent and Soft Computing, pp. pp. 233–242, Berlin Heidelberg, Springer, 2009.
* Institute: Biomedical Engineering, University of Applied Science Technikum Wien, Austria (++43 1 333 40 77 -373, e-mail: [email protected]). Abstract: The development of Nano- and Picosattelites for educational and scientific purposes becomes more and more popular. As these satellites are very small, highly integrated devices and are therefore not equipped with high-gain antennas, the data transmission between ground and satellite is vulnerable to several ascendancies in both directions. Another handicap is the low earth orbit where the satellites are usually located as it keeps the communication time frame very short. To counter these disadvantages, the global educational network for satellite operations (GENSO) has been created. To optimize the communication between ground station and satellite, metrics and methods have to be identified to gain information about the satellite link quality during a ground pass. Such a global effort is hindered by utilization of very different hardware, protocols, frequencies and modulations. This paper shows a generic and widely applicable approach to passively measure and compare satellite link quality to be applied in a heterogeneous ground station and satellite configuration. Keywords: CubeSat, radio link, pattern classification, communication system performance
1. INTRODUCTION In fall 2006 the Global Educational Network for Satellite Operations (GENSO) project was initiated by the ESA Education Office as a consequence of the Cubesat concept, which has been developed at Stanford University and the California Polytechnic Institute. This concept allows a small 1kg satellite of 10x10x10cm to be launched into Low Earth Orbit (LEO) at remarkable low launch costs of about $40,000. The novel idea was that differently educated engineering students cooperate using all their skills and enthusiasms in solving every detail of the complex tasks. The GT-Sat project focuses primarily on the development of a specialized Cubesat for identifying the reliability of amateur ground stations and being a test-bed for educational satellite projects communication links and their interdependencies with environmental conditions.
As the available time for communication between satellite and mission control can be crucial for a mission, approaches took place to optimize the usage of ground stations and to significantly extend the time a satellite can communicate with the affiliated mission control centre. The most sophisticated approach for a world-wide interconnection of independent ground stations so far is the Global Educational Network for Satellite Operations (GENSO). Its aim is to share ground station capacity between different mission control centers by creating a hybrid, supervised and secure peer-to-peer network utilizing the internet.
1.1 Problem Within the last decade the educational and academic approaches in space science made huge steps forward. Driven by the development of small satellites for taking scientific payload of any kind into space, universities all over the world started to design, develop and launch small satellite projects based on the Cubesat standard [1]. Many small LEO satellites operate in the low earth orbit causing high communication intensities. The communication timeframe between a satellite and its affiliated ground station [2] is about 30 minutes a day (e.g. up to 15 min for each pass for a polar LEO) whereas the ground station is idle for the remaining time of the day.
Fig. 1. GENSO basic scheme Interconnecting a very large amount of satellite ground stations forms the base for novel scientific approaches in the
domain of link quality determination and prediction, overall optimization of space up- and downlinks as well as hardware utilization and the influence of environmental conditions on the radio communication. The low earth orbit is located between 200km and 1200km above the earth's surface and is the lowest possible orbit for spacecrafts of any kind. Other important orbits are the MEO (Medium Earth Orbit) and the GSO (Geo-Stationary Orbit, together with Elliptical Geostationary Transfer Orbits). The advantage of the LEO is that spacecrafts being placed in that orbit area do stay in a moderate temperature region and do not have to cross the "Van-Allen-Belt" and therefore are not exposed to high radiation. This allows the hardware on non-commercial, academic satellites to be of lower cost than for e.g. GSOs. Another advantage is the nearly infinite and free amount of orbits in LEO whereas orbits in GSO are very limited and cost-intensive. A further advantage is the short distance between spacecraft and ground station reducing the needed signal strength for up- and downlinks, which makes the very small spacecraft designs even more possible. The big disadvantage of LEO is however the short time frame available for communication between spacecraft and ground station due to a high velocity on one hand and the low distance between surface and spacecraft on the other hand which results in a very small communication interval. Beside this short communication time frame with a maximum of 15 minutes for each pass, the high ground track speed of approximately 7km/s (26.000km/h, not taking earth rotation into account) requires relatively fast-tracking and precise antenna and rotor facilities on ground to adjust elevation and azimut in time. Ground stations at higher latitudes (e.g. Svalbard in Spitzbergen) benefit from the high inclination of small satellites' orbits in relation to the earth rotation and have therefore an extended communication time frame. 1.2 Approach The Cubesat standard, originally defined by Stanford University and the Californian Polytechnical University, defines a small satellite having the size of 10x10x10 centimetres and a maximum weight of 1 kg. A majority of the current academic small satellite projects rely on the Cubesat design proposal as it standardizes also the deployment of the satellites in orbit and is the minimal easily detectable size for metallic objects by NORAD. Academic and small scientific satellites are usually additional payloads of larger commercial and scientific missions, which makes launches remarkable cheap and the amount of satellites deployable in orbit comparably high (the current maximum is 10 deployed missions during one rocket launch). The payload of Cubesats can be educational, scientific and also industrial. Latest industry-driven approaches in satellite development are towards small satellites (also referred to as pico satellites) being cheap in design and construction, standardized and easy to deploy.
Instead of designing full-sized satellites with enormous costs these are more and more substituted by redundant cheap pico and nano satellite constellations. Often (not only scientific) Cubesat payloads consist of measurement hardware (sensors) for numerous research topics, communication hardware, probes, novel spacecraft control hardware and many others. The typical framing hardware of a Cubesat consists of solar panels and small batteries for the power supply, satellite attenuation determination control facilities and radio communication facilities connected to an onboard computer for controlling the satellite (house-keeping data) and sending results of measurements via the transceiver radio. Many small satellites are also equipped with store and forward buffers for short messages exchange to support the amateur satellite-based packet radio network. Due to their design small satellites are equipped with comparable weak communication facilities as their production of energy is limited by the small area of the solar panels which are normally mounted on only 4 sides of the cube. These limits cause the up- and downlink to be very vulnerable to environmental influences and imprecise ground antenna calibration. It also causes low horizon passes to be very error-prone as the distance between the satellite and the ground station is highest at that constellation. This makes many satellite passes unusable and restricts the valuable passes to one with a high elevation above the horizon. The usual communication frequencies for small satellites in LEO are normally registered and coordinated in the amateur VHF, (beyond 400MHz), UHF (approx. 400-600 MHz) and S-Bands (2-4 GHz). Future developments take the usage of Ku-Band (10-15Ghz) into consideration. The higher the radio communication frequencies are, the higher is the necessity for a very accurate onboard attitude control system, since the antennas directional behaviour dominates more and more, resulting in the need for a directional control of the antennas towards the groundstation. GENSO is a project supported by the International Student Educational Board (ISEB) formed by ESA (European Space Association), NASA (National Aeronautics and Space Agency), JAXA (Japanese Aerospace Exploration Agency) and the CSA (Canadian Space Agency). The project is supported by AMSAT (the Satellite Radio Amateur league) [4] with several thousand amateur satellite ground stations beeing able to communicate with small satellites all over the world. The basic aim of the network is to interconnect the noncommercial ground stations all over the world to share resources and dramatically extend the time frame for communication with a satellite from a maximum of 30 minutes a day up to nearly 24 hours. The main focus lies on the support of LEO missions, however it is not restricted to LEO only. Not only educational satellites but all spacecrafts including for example the International Space Station (ISS) are supported by the network.
The basic concept of the network is to create a transparent layer between the traditional constellations of ground stations, mission controls and spacecrafts. The authentication server cluster is the network supervisor - it manages all participating ground stations, mission-controls and spacecrafts. It collects data about all traffic between ground stations and mission controls, which covers satellite pass booking information but also meta information about satellite passes. [9] The collected information about the network of groundstations and the spacecrafts and their passes forms the foundation for a scientific investigation on predicting and optimizing up- and downlink quality. Institutes which will join the network in operation are extremely interested in these scientific approaches include the Stanford University, USA, the EPFL, Switzerland, the University of Toronto, Canada, the University of Surrey, UK, the Hokkaido Institute of Technology, Japan and the TU Munich, Germany, to name only a few of them. GENSO constitutes the scientific base for a multitude of research fields and investigations. The proposal on hand aims on the identification and utilization of link quality information. For the first time the possibility is provided to gain focussed information about the quality of LEO satellite downlinks from a huge amount of independent ground stations throughout the world. The gained information can be processed and applied in various ways. As an example the optimisation of the groundstation network itself is one possibility to apply this information. The results of this computation can be applied continuously resulting in an adaptive but nearly optimal network structure – concerning the radio links. Each of these specific aims constitutes a novel approach in its field of science and can support space operations upcoming in the next decade and probably far beyond.
1.4 Communication Parameters The major amount of academic satellites uses the AX.25 protocol or at least parts of the high-level data link control standard (HDLC), only a few are using more advanced protocols, like SRLL. Usually the frequency shift keying (FSK) or audio FSK (AFSK) are applied as modulation schemes with symbol rates of 1200 and 9600 baud if usinf UHF bands. The most comprehensive collection about small satellite communication facilities provides Klofas et. al. in [24]. Common TNCs support primarily AX.25 protocol processing, other protocols require special firmwares or software decoders. TNCs can usually operate in different modes, the most common is the so-called KISS mode. The KISS mode only applies the CRC inspection and generation and bit stuffing operations of the HDLC part of the AX.25 protocol. Against common presumptions the KISS mode does not only forward the raw incoming and outgoing data but also processes the forwarded frames. It is therefor impossible to operate protocols not according to the HDLC standard regarding the frame check sequence (FCS), the bit stuffing process and the packet delimiters with KISS TNCs. KISS also describes a simple intermediate protocol for data exchange between the host machine and the TNC – after HDLC frame preprocessing the data is encapsulated in the socalled KISS frame which exchanges the original HDLC frame. Protocols different from standard AX.25 can be applied as long as the HDLC frame persists. This constraint limits the error detection to the CRC-algorithm of HDLC and eliminates the possibility of FEC. SMACK represents an enhancement to KISS as it adds a CRC checksum also for the communication between TNC and host machine. A simple KISS and SMACK frame counter in the ground station transcoder framework makes it possible to precisely count the amount of received good but not the errneous packets.
1.3 Space up- and downlink quality identification and determination As a basement for all further scientific approaches in this direction a metric has to be identified for measuring and comparing the quality of satellite links. This is complicated by application of different protocols [5,6], modulations and frequency [3,7] bands on one hand and the operation in a heterogeneous radio hardware environment on the other hand [8]. As the network itself is not able and also not permitted to be aware of the transferred communication payload between a mission control centre and a spacecraft this project will provide the possibility to spread only the information about the link quality in relation to environmental parameters in a reliable way. The investigations include a possible design for the determination of uplink quality for future application.
Fig. 2. This diagram shows the course of the S-meter levels of a typical satellite pass. The noise floor can be separated from the actual signal. Also, the peaks of the signal can be determined
1.5 Defining the measurements For gaining comparable and usable quality information the values have to be measured, preprocessed and compiled. For the SNR the driver libraries are going to be utilized and abstracted by the GSS software to retrieve the S-meter readings of the radio. Fig. 2 shows a single pass of AAUSAT II over Aalborg university recorded with an ICOM IC-910H radio [24] using a sample rate of 8 samples per second. The values collected between the beacon transmissions are used to determine the current noise level. For the PLR the retrieved data frames are counted and compared to the theoretical maximum amount of transferable packets during the pass. At a typical baud rate (e.g. 1200-9600) the packet rate is very low and the usage of beacons instead of constant streams of data decreases the amount of transmitted packets additionally. The proposed sampling rate for the packet count is therefore at least one minute. The reason why one overall count for one pass is not recommended is that it disables the possibilities for angle-based normalization, which is the reason why we do not read out the data transmission statistics that some TNCs provide. Based on the information which is provided for every spacecraft about baud rate, protocol, beacon length and interval the mean PLR can be calculated. The remaining problem is that - in case of transmitted beacons or discontinous transmissions - the measured Smeter values range from the noise strength to the actual received signal strength indication (RSSI). As a consequence the average value adducted for quality assertions over a certain amount of time would be in-between the noise level and the factual RSSI. Hence the values have to be devided into two categories, one containing values above an arithmetic mean value, representing the presumed RSSI values, and one containing the values below the arithmetic mean value, representing the noise level. By analyzing the samples located in the RSSI category for continous oscillations presumptions about the thumbling behaviour of the spacecraft can be extracted, which can be a valuable information for the spacecraft operator. Further investigations concerning the noise level exhibiting artificial, continous patterns could lead to assumptions about radio interferences, as an example. Although these investigations are not yet part of the current evaluation they can provide valuable information for future calculations and predictions (refer to [25] and [26] for further applications). 1.6 Automated identification and calibration of imprecise ground stations
By investigating differences between predicted and effective link quality after the pass of a spacecraft has taken place, software [19] can be used to identify ground stations with receiving and transmitting capabilities beyond a predicted level. The problem is more or less to gain this information in good quality and short time [11-16]. This can be managed by a sample satellite with known orbit and communication capabilities to normalize and test the communication points. The ground station operators then can choose whether he wants to be
informed if problems with their communication hardware arise. The network with this sample satellite - called GT-sat (GENSO Test Satellite) additionally offers the possibility of having a low-cost calibration facility for non-commercial ground stations. [21] If a problem arises in a participating ground-station and it is rechecked by the system that this problem really exists, then this entity is downgraded within the ground station network to avoid having broken or imprecise radio-stations to rely on in critical situations. This results in a concurrent network of ground-stations with known sending/receiving qualities and on/off-line times, which can be used by the Cubesat community [22] effectively.
Fig. 3. Basic block diagram of a system for transmission of PSK/MSK modulated signals.[23] The GT-Sat is also planned to be used as a predecessor for the upcoming QB50 project [27] which is addressing the lower thermosphere/ionosphere (90-320 km) which is the least explored layer of the atmosphere. Only five atmospheric explorers were flown by NASA in highly elliptical orbits. They carried experiments for in-situ measurements, but the time spent in the region of interest (below 320 km) was only very limited. Nowadays, sounding rocket flights provide some in-situ measurements. Whilst they do explore the whole lower thermosphere, the time spent in this region is also extremly short and therefore doesn’t provide information about dynamics of the thermosphere. Powerful remotesensing instruments on board Earth observation satellites in higher orbits (600–800 km) receive the backscattered signals from the atmospheric constituents at various altitudes. While this is an excellent tool for exploring the lower layers of the atmosphere up to about 100 km, it is not ideally suited for exploring the lower thermosphere as the density of the atmosphere is so thin that the reflected signal is extremely weak and therefore unreliable. The same holds for remotesensing observations from the ground with lidars and radars. The multi-point, in-situ measurements of QB50 is complementary to the remote-sensing observations by the instruments on Earth observation satellites and the remotesensing observations from the ground with lidars and radars. The results observed by the QB50 project will help to understand the dynamics of the thermosphere/ionosphere which will additionally improve the communication link quality prediction to increase the radio link capabilities, since
the ionosphere heavily affects radio communication between the satellites and the groundstations in lower bands.
9. 10.
1.7 CONCLUSION Heterogeneous ground station hardware environments, a wide range of very different spacecraft communication facilities and the utilization of different protocols together form one of the worst possible scenarios for obtaining meaningful, comparable quality measurement results. We have identified what link quality means in respect to non-commercial satellite communication and which possibilities are provided to collect raw information concerning the quality of an established space links. Furthermore we’ve demonstrated how the collected raw measurement data has to be sampled and processed to achieve normalized, comparable information. The introduced procedure establishes the required foundation for further investigations and novel approaches on link quality recordings and predictions. As integral part of groundstation networks link quality measurement facilities will offer both science and education the possibility of a distributed and accurate collection of communication meta-level information for the first time in history of non-commercial space-flight. As integral part of the QB50 project it will enable reliable communication facility for the downlink of the hughe ammount of science data produced by the CubeSat cluster.
11.
12. 13. 14.
15. 16. 17. 18.
REFERENCE 1.
2.
3.
4.
5. 6. 7.
8.
Stephen Waydo, Daniel Henry, and Mark Campbell. CubeSat design for LEO-based Earth Science Missions. In Proceedings of the IEEE Aerospace Conference, volume 1, pages 435-445, 2002. Tarun S. Tuli, Nathan G. Orr, and Robert E. Zee. Low Cost Ground Station Design for Nanosatellite Missions. In Electronical Proceedings of the AMSAT North American Space Symposium, 2006. Philip R. Karn, Harold E. Price, and Robert J. Diersing. Packet Radio in the Amateur Service. IEEE Journal on Selected Areas in Communications, 3(3):431- 439, May 1985. Keith Baker and Dick Jansson. Space Satellites from the World's Garage - The Story of AMSAT. In Proceedings of the IEEE National Aerospace and Electronics Conference (NAECON), volume 2, pages 1174-1181, May 1994. Subbarayan Pasupathy. Minimum Shift Keying: A Spectrally E_cient Modulation. IEEE Communications Magazine, 17:14-22, July 1979. Richard R. Parry. AX.25 [Data Link Layer Protocol For Packet Radio Networks]. IEEE Potentials, 16(3):14{16, August-September 1997. Irfan Ali, Naofal Al-Dhahir, and John E. Hershey. Doppler Characterization for LEO Satellites. IEEE Transactions on Communications, 46(3):309-313, March 1998. Steve Stearns. Antenna Modeling for Radio Amateurs. In Electronical Proceedings of ARRL Paci_con Antenna Seminar, 2008.
19. 20.
21.
22.
23. 24. 25.
C. Lorek. The ICOM IC-910 Transceiver. Radio Communication, 77(7):17, 2001. Harold E. Dinger and Harold G. Paine. Factors A_ecting the Accuracy of Radio Noise Meters. Proceedings of the IRE, 35(1):75-81, January 1947. Yoseph Linde, Andres Buzo, and Robert M. Gray. An Algorithm for Vector Quantizer Design. IEEE Transactions on Communications, COM-28(1):84-95, January 1980. National Weather Service. METAR Data Access. [Online]. Available: http://weather.noaa.gov/weather/metar.shtml. National Climatic Data Center. Global Surface Summary of Day. [Online]. Avail- able: http://www.ncdc.noaa.gov/oa/gsod.html. Space Weather Prediction Center. Real-time ACE Satellite Hourly Data and Spacecraft Location. [Online]. Available: http://www.swpc.noaa.gov/ftpmenu/lists/ace2.html. Advanced Composition Explorer (ACE). [Online]. Available: http://www.srl.caltech.edu/ACE/. Space Weather Prediction Center. Historical SWP Products. [Online]. Available: http://www.swpc.noaa.gov/ftpmenu/warehouse.html. Harald T. Friis. A Note on a Simple Transmission Formula. Proceedings of the IRE, 34(5):254-256, May 1946. Ingo Mierswa, Martin Scholz, Ralf Klinkenberg, Michael Wurst, and Timm Euler. YALE: Rapid Prototyping for Complex Data Mining Tasks. In Proceedings of the 12th ACM International Conference on Knowledge Discovery and Data Mining (SIGKDD), pages 935-940, 2006. Christopher M. Bishop. Pattern Recognition and Machine Learning. Information Science and Statistics. Springer, 2006. Helen Page, Bastian Preindl, and Viktor Nikolaidis. GENSO: The Global Educational Network for Satellite Operations. In Electronical Proceedings of the 59 th International Astronautical Conference (IAC), 2008. Bastian Preindl, Martina Seidl, Lars Mehnen, and Sebastian Krinninger. A Performance Comparison of different Satellite Range Scheduling Algorithms for Global Ground Station Networks. In Proceedings of the 61st International Astronautical Conference (IAC), 2010. Bastian Preindl, Lars Mehnen, Frank Rattay, and Jens Dalsgaard Nielsen. Design of a Small Satellite for Performing Measurements in a Ground Station Network. In Proceedings of the IEEE International Workshop on Satellite and Space Communications (IWSSC), pages 186-190, 2009. Robert Schürhuber, Master Thesis 2011, Technical University of Vienna, Fundamentals of Modulation Schemes for Low-Budget LEO Satellites. C. Lorek, “The ICOM IC-910 Transceiver,” Radio Communication,vol. 77, no. 7, p. 17, 2001. Bastian Preindl, Lars Mehnen, Frank Rattay, Jens Daalgard Nielsen, Sebastian Krinninger, and K. K. Sørensen, “A Global Satellite Link Sensor Network,” in
Proceedings of the 8th IEEE Conference on Sensors, Christchurch, New Zealand, 2009. 26. Bastian Preindl, Lars Mehnen, Frank Rattay, Jens Daalgard Nielsen, “Applying Methods of Soft Computing to Space Link Quality Prediction,” in Applications of Soft Computing. From Theory to Praxis, vol. 58 of Advances in Intelligent and Soft Computing, pp. pp. 233–242, Berlin Heidelberg, Springer, 2009.