Aerospace Technologies Advancements 2012 Part 12 pptx

Design of Low-cost Telecommunications CubeSat-class Spacecraft 313 The size of the step index determines the output signal frequency. At the bit rate f b of 1200 Hz, an interruption of AIC is sent to the DSP. To generate the frequency f 0 = 1200 Hz (respectively f 1 = 2200 Hz), the sine table of size N = 120 is read with an integer step index equal to S 0 = 6 (resp. S 1 =11). For the implementing of the AFSK modulation on DSP, we used the sampling frequency of 24 KHz and data rate of 1200 bps which corresponds to 20 samples per bit. The steps and the sine samples are represented as 16 bit integer numbers. Fig. 17 represents the output of the AFSK modulator with the following bits of inputs [-1 1 1 1 -1 1 -1 -1 1 1]. Fig. 17. The AFSK signal 5.3.3 AFSK demodulation We used a bit-per-bit demodulation as the classical non-coherent demodulation scheme. The received AFSK signal is sent to DSP from the transceiver via the TDM serial port after being converted from analog to digital signal by AIC. The DSP implementation of the AFSK demodulator is illustrated in the Fig. 18. Fig. 18. General diagram of AFSK demodulation We used the Goertzel algorithm (Oppenheim, 1999) to demodulate the AFSK signal, which can be interpreted as a matched filter for each frequency k as illustrated in Fig. 18. The transfer function H k (z) corresponds to the kth Goertzel filter: () 21 1/2 /2cos21 1 )( −− − +− − = zzNk ze zH Nkj k π π (7) A further simplification of the Goertzel algorithm is made by realizing that only the magnitude squared of X(k), which represents the energy of the received signal, is needed for tone detection. It eliminates the complex arithmetic and requires only one coefficient, α k = cos(2πk/N), for each |X(k)|² to be evaluated. Since there are two possible tones to be X (k 2 ) X (k 1 ) Data Out FSK signal Matched filter f 0 (.) 2 Matched filter f 1 (.) 2 Decision (compare) Aerospace Technologies Advancements 314 detected, we need two filters described by (7). We conclude that the Goertzel algorithm is a Discrete Fourier Transform, calculated from a second degree recursive filter, easy to implement on DSP. In Our case, we compare only the two energies of the two AFSK frequencies to determine which AFSK tone has been received. The synchronization is performed by detecting the first change to the received signal by using the Syn_Rx module. After processing 20 samples for each bit and calculated the energy at each of the two frequencies, the Goertzel Algorithm then decides which AFSK tone has been received. The sampling frequency is chosen to be 24 KHz because it is the highest sampling frequency available in the AIC. Also to detect the frequency 1200 Hz (resp. 2200 Hz), we used k = 1 (resp. k = 1.83). For M = 20, we have α 1 = 0.951 and α 2 = 0.839, which are corresponding to frequencies 1200 Hz and 2200 Hz respectively. The format of each variable in the algorithm was being chosen suitably taking into account that we had used a 16 bit fixed point DSP. 5.3.4 GMSK modulation The GMSK modulation is a Continuous Phase Modulation (CPM) with a modulation index h=0.5. A modulated GMSK signal can be expressed, over the time interval nT b ≤ t ≤ (n+1)T b , as: () 0 () cos 2 2 t n kb k st A ft πhdgτ kT d π τ =−∞ −∞ ⎛⎞ =+ − ⎜⎟ ⎜⎟ ⎝⎠ ∑ ∫ (8) where d k : sequence of data information = ±1, and 1 () ( / ) () 2 bg b g t rect t T h t T =∗ with () 1 0,5rect t for t=≤ h g (t) is the pulse of Gaussian function, T b is the symbol period, B is the 3dB bandwidth of the Gaussian prefilter, and g(t) is the response of the transmitted rectangular pulse to the pre- modulation filter. By deriving the phase signal, the CPM can also be seen like Frequency Modulation (FM). The instantaneous frequency F i is given by: ∑ −∞= −+= n k bki kTtgdhftF )()( 0 (9) In the expression (9), h represents the proportionality constant of the modulator and is expressed in Hertz per volt. The baseband signal m(t) to be transmitted is written then, in the interval nT b ≤ t ≤ (n+1)T b , in the form of: () ( ) n kb k mt dgt kT =−∞ =− ∑ (10) In theory, the duration of Gaussian filter is infinite, but in practice, we limit the function h g (t) to the few period bits over which it is significantly not zero. This duration is inversely proportional to B. For a product BT b = 0.5, we consider that h g (t) is not zero over 2 bits. The convolution product of h g (t) with a rectangle function of duration T b lasts 3T b , which affects Design of Low-cost Telecommunications CubeSat-class Spacecraft 315 the half preceding bit and the half following bit. The Fig. 19 represents the response of Gaussian lowpass filter for BT b = 0.5 over three bits to a rectangular pulse of duration T. The implementation of filter convolution product requires multiple instruction processing inducing a lot of calculation time. To respect timing constraints we propose an optimized implementation code based on Lookup table of the Gaussian filter response (Fig. 19). For the implementing of the GMSK modulation on DSP, we used the sampling frequency of 24 KHz with 5 samples per bit which corresponds to data rate of 4800 bps. For data stream of [1 -1 1 1 1 -1 -1 1], the corresponding GMSK baseband signal is given by the Fig. 20. Fig. 19. Gaussian filter response in function with BT b parameter Fig. 20. Baseband GMSK output signal 5.3.5 GMSK demodulation We used the classical non-coherent demodulation scheme, which performs a bit-per-bit demodulation and it does not require recovery of the carrier phase and frequency. Analysis of the GMSK baseband signal (Fig. 20) permits the identification of eight types of shapes Time in bit p erio d m(t) -2 0 2 4 6 8 10 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 -1.5 -1 -0.5 0 0.5 1 1.5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 BT b =1 BT b =0,5 BT b =0,3 Time in half bit p erio d g(t) Aerospace Technologies Advancements 316 corresponding to binary states transition. The GMSK demodulator must extract the phase from the modulated signal and, by using a transition shape classification, decode the transmitted bit. According to Fig. 21, we have four transition shapes for a binary "1", and four transition shapes for a binary "0". We store only two predictive transitions, (b) and (f), on the DSP memory as look-up tables. Based on the lookup tables, the demodulator uses the Absolute distance d e , which shows the better performance, as matching function to classify the GMSK signal transitions, and determine the transmitted bit. Fig. 21. Eight binary states transitions () 1 d, n abs j j j x yxy = =− ∑ (11) The demodulation of the GMSK signal is processing to perform the shape comparison of binary transition based on the look up tables. The minimum Euclidean distance d e is evaluated and the decoded bit is determined. The synchronization is performed by using the Syn_Rx module. The C54x DSP family has a dedicated instruction for faster execution of the Absolute distance. 6. Conclusion As the satellite community transitions towards inexpensive distributed small satellites, new methodologies need to be employed to replace traditional design techniques. The ongoing research will contribute to the development of these cost saving methodologies. The goal of the integration of all the intelligences of the various satellite subsystems in only one intelligent subsystem is to minimize component expenditures while still providing the reliability necessary for mission success. Associating low cost ground terminals with a low cost Telecommunication CubeSat-class satellite will allow universities to access space communications with a very economical system. The present work, dealing with the design of the Low-cost Telecommunication CubeSat-class spacecraft, shows hardware and software solutions adopted to cut down the system cost. The hardware utilizes commercial low cost components and the software is optimized using assembler language. The On Board Computer unit is small device that can be mounted on any small satellite platform to serve telecommunications applications such as mobile localization and data collection. By using a single CubeSat satellite and low-cost Binary ‘0’ Binar y ‘1’ a b c d e f g h Design of Low-cost Telecommunications CubeSat-class Spacecraft 317 communications equipments, Telecommunications systems can be kept at the extreme low end of the satellite communications cost spectrum. 7. References Addaim, A.; Kherras, A. & Zantou, B. (2008). Design and Analysis of Store-and-Forward Data Collection Network using Low-cost Small Satellite and Intelligent Terminals, Journal of Aerospace Computing, Information and Communications, Vol. 5, No. 2, (February 2008) page numbers (35-46) Bahl, I. (2003). Lumped Elements for RF and Microwave Circuits, Artech House, first ed. Gérard, M. & Bousquet, M. (2002). Satellite Communication Systems, John Wiley & Sons; fourth edition Horan, S. (2002). Preparing a COTS radio for flight – lessons learned from the 3 corner satellite project, Proceedings of 16th Annual/USU Conference on Small Satellites, Logan, Utah, USA Hunyadi, G.; Klumpar, D.; Jepsen, S.; Larsen, B. & Obland, M. (2002). A commercial off the shelf (COTS) packet communications subsystem for the Montana EaRth- Orbiting Pico-Explorer (MEROPE) CubeSat, Proceedings of IEEE Aerospace Conference Jamalipour, A. (1998). Low Earth Orbital Satellites for Personal Communication Networks, Norwood, MA: Arthech House Lu, R. (1996). Modifying off-the-shelf, low cost, terrestrial transceivers for space based application, Proceedings of the 10th Annual AIAA/USU Conference on Small Satellites, Logan, September 1996, Utah, USA Milligan, T. (2005). Modern Antenna Design, second ed., Wiley Oppenheim, A.; Schafer, R. & Buck, J. (1999). Discrete-Time Signal Processing, second ed., Prentice Hall Paffet, J.; Jeans, T. & Ward, J. (1998). VHF-Band Interference Avoidance for Next-Generation Small Satellites, Proceedings of 12 th AIAA/USU Conference on Small Satellites, Logan, Utah, USA Pisacane, V. L., & Moore, R. C. (1994). Fundamentals of Space Systems, New York: Oxford University Press Poivey, C.; Buchner, S.; Howard, J. & Label, K. (2003). Testing Guidelines for Single Event Transient, NASA Goddard Space Flight Center, 30 June, 2003. Proakis, J. (1989). Digital Communications, McGraw-Hill, (Second Edition) Rotteveel, J. (2006). Thermal control issues for nano- and picosatellites, Proceedings of Space Technology Education Conference, Germany, May 2006, Braunschweig. TAPR, (1997). AX.25 Link Access Protocol for Amateur Packet Radio, TAPR, version 2.2 Texas Instruments, (1996). TLC320AC01 data manual single-supply analog interface circuit, SLAS057D Texas Instruments, (1997). DSKplus User’s Guide, SPRU191 Texas Instruments, (2001). TMS320C54X DSP: CPU and peripherals, SPRU131G. Texas Instrument, (2002). TMS320VC5416 DSK Technical Reference, Wertz, R. & Larson, W. (1999). Space Mission Analysis and Design, Microcosm, (third ed.) Aerospace Technologies Advancements 318 Zantou, B. & Kherras, A. (2004). Small Mobile Ground Terminal Design for a Microsatellite Data Collection System, Journal of Aerospace Computing, Information and Communications, Vol. 1, No. 9, (September 2004) page numbers (364–371) 16 Looking into Future - Systems Engineering of Microsatellites H. Bonyan Faculty of Energy Engineering and New Technologies, Shahid Beheshti University (SBU) Iran 1. Introduction Space age began with the launch of Sputnik-1 in 1957, by the Soviet Union. Initially, the spacecraft, especially the western ones, were rather small due to limited capabilities of the launch vehicles. With the increasing capabilities of rocketry in the US and USSR, the limitation was soon a part of history. From 1970s, several-thousands-kilograms satellites have been placed in orbits ranging from LEOs to GEOs and to interplanetary orbits. These large satellites have been the major payloads of launch vehicles until the very last years of the Cold War, the so-called “Super-power, government-only space era”. During the last two decades, however, there has been an ever-increasing interest within the private sectors in developed countries and, also, space agencies of developing countries to contribute to and take advantage of space market. It must be reminded that large satellites are not appropriate means to establish the required hardware-/software-expertise and infrastructure. Simply, the private sector is not able to afford the huge costs of large satellites and its immense complexity. This also holds true for government-funded project in many developing and third-world countries. Thus, most countries and space agencies have adopted microsatellite projects in order to initialize their space policy in order to obtain, establish and benefit from the rich space revenue. Thus, a “government/private-sector era” has been already initiated and almost established. In this methodology, microsatellites have served as “path-finders”, in order to pave the way of many nations and societies (top-class universities in developed countries, space-agencies in developing countries and so on) to obtain the space technology. In the space literature of the last two decades, microsatellites have been addressed as “hands-on experience” to facilitate consolidation of space technology in order to implement some “actual large satellite” programs. Microsatellites in the next decades, however, will be employed not only as “path-finders” and/or “hands-on experience” warm-ups, but also as actual projects with considerable financial Return on Investment (ROI). This requires fundamental reconsideration of system-level characteristics of microsatellite projects, such as mission definition, subsystem performance requirements, construction, test, launch and post-launch operations. The preceding issues are addressed in this chapter. 2. Mission definition Traditionally, microsatellites have served as engineering programs in order to pave the way for different communities (universities, organizations and/or nations) to acquire enough Aerospace Technologies Advancements 320 “hands-on experience” for establishment of actual several-hundred/several-thousand kilograms satellite programs. While this approach has considerably contributed to recent advancements in satellite technologies in many developing countries and elsewhere, it still utilizes few of enormous capabilities of microsatellites. Microsatellites developed in the said paradigm, mainly serve to educate highly-qualified space engineers and managers. However, once in orbit, these vehicles are utilized to an order of magnitude less than their full capability. There are evidences that some well-designed, built and launched microsatellites have been almost abandoned after a few months in orbit. However, if properly planned, these vehicles could have been actively in service for a few years rather than a few months. It must be reminded that the owner authorities of the satellites (mostly universities and space-industry) are reluctant to officially declare the ineffectiveness of the actual products of the spaceborne system i.e. microsatellite in orbit and mostly emphasize on educational achievements of such programs. However, according to [H.Bonyan, 2010]; [E.Gill et al., 2008]; [U.Renner & M.Buhl, 2008]; [G.Grillmayer et al., 2003] & [United Nations UNISPACE III, 1998], there are evidences that there will be an enormous enhancement in actual outcomes of microsatellite programs, from a practical-application and/or economical- value point of view. The enormous enhancement of products of microsatellite programs, stated above, is briefly described in the following paragraphs. During the last two decades, there has been an immense progress in the miniaturization of equipments incorporated in microsatellite technology. Miniaturization, in its broadest sense, is interpreted as provision of the same level of functionality via fewer resources. In satellite technology, resources are considered as mass, power and volume 1 . Today, with the increasing progress in computer technology, Commercial-Off-The-Shelf (COTS) units are accessible within the commercial space market. While these units are provided at fairly reasonable prices, they are as capable as their quite-expensive predecessors. For a given level of performance, these new units are also lighter and less power-hungry which, in turn, can be considered as extra financial benefit. Also, more efficient solar cells and battery units are now offered by suppliers of various communities. Furthermore, compact, light-weight and reliable reaction wheels and other attitude control actuators are provided by several suppliers [SSTL website, as of 2009]; [Sun Space website, as of 2009]; [Dynacon Inc. website, as of 2009] & [Rockwell Collins Deutschland website, as of 2009]. A complete list of these new components is not within the scope of this writing. It is being concluded that, at present and near future, microsatellites are and will be capable of fulfilling sophisticated missions, previously feasible only by several-hundred kilogram satellites. The preceding advancements, to some extent, are true for every engineering field. However, they are an order of magnitude more important regarding microsatellite technology. It is being reminded that mass and power are critical issues in space technology. At the present time (as of 2009), placing a kilogram of payload into Low Earth Orbit (LEO) can be as expensive as 5000-15000 US $ [Malekan & Bonyan, 2010]; [Futron Corporation Manual, 2002]. Consequently, there is an ever-increasing interest within the satellite design community to provide the same level of functionality via lighter equipments, thus avoiding 1 From a systems engineering point of view, all the three said items can be translated into dollars. Generally speaking, lighter, less power-hungry and smaller simply means cheaper! Looking into Future - Systems Engineering of Microsatellites 321 high launch costs. Also, purchase of solar cells required to generate 1 watt in orbit may be as expensive as 2500-3000 US $ [Larson & Wertz, 1992]. The typical prices are given here in order to help the reader realize the desire within the space community to provide the same level of functionality via equipments consuming less power. It is being concluded that any progress within the preceding arenas can be regarded as saving millions of dollars. Also, equally important, the unique feature of present and potential progress of microsatellite missions lies within the recent pattern of quality assurance developed within the microsatellite design community. Historically, quality programs applied in space programs have been rigorous and expensive. Also due to vastly-unknown nature of space environment, only few highly-qualified technologies have flown on space missions. Today, however, by the means of methods developed and/or established in the last two decades such as “qualification by similarity”,”Configuration control” and so on, much more responsive and cheaper qualification programs are available. Although these programs are not as precise as their predecessors, they still provide the required insight and confidence level required in most microsatellite programs. Also, due to the courageous microsatellite missions within the past, more components have been “space-qualified”. At this step, the author would like to draw the readers’ attention to the very point that, traditionally, there has been a considerable delay-gap in the technology-level utilized in space technology in comparison with commercial units available in the every-day market. As an example, in a microsatellite program, it is the ultimate wish of a Command and Data Handling (C&DH) designer to be able to incorporate a computer unit with equal capabilities as of a home- based Pentium-5. This delay-gap, however, is shrinking due to the recent missions accomplished mostly by top-class universities in US, Europe, Asia and Africa [Kitts & Lu, 1994]; [D.C.Maessen et al., 2008]; [Sabirin & Othman, 2007]; [Triharjanto et al., 2004 ]; [Kitts & Twiggs, 1994]; [Annes et al., 2002]. As a consequence, the technology-level of components employed in microsatellite technology is reaching that of hi-tech commercial market. Having considered the 10-20 years delay-gap of the space-qualified components and hi-tech COTS technologies, the importance of the new approach may be better understood. As a conclusion, Table 1 compares the system-level capabilities of microsatellites in the past and at the present/near-future. 3. System and subsystem performance requirements In this section, current status and future trends of various subsystems of microsatellites are discussed. Also, mutual effects of foreseen improvements of each subsystem on system performance are studied. 3.1 Payload mass ratio to total satellite mass A satellite payload is the main reason to launch the whole vehicle. Thus, from a top level point of view, the more ratio of payload mass to total satellite mass (PM/TSM), the better. In the first years of microsatellite re-appearance, limited PM/TSM was practically achievable. Today, however, with the ever-increasing progress in microsatellite technology, PM/TSM as high as 10-25% is achievable, at the present and in near future, respectively. Furthermore, at the present, more capable payloads are being developed and supplied at reasonable prices, in a non-military, non-governmental market. Thus, for a given PM/TSM, currently-available Aerospace Technologies Advancements 322 Table 1. System-level capabilities of microsatellites in the past and at the present/near-future payloads offer several-times better performance in comparison with their predecessors. Having considered the combined effect of the two preceding considerations, one may appreciate the potential applicability and ever-increasing interest of various communities in microsatellite technology. As an instance, Surrey Satellite Technology Ltd (SSTL) provides light-weight optical, navigation and communications payloads at exceptionally low prices [SSTL website, as of 2009]. A few of these capable payloads will be introduced in the following paragraphs. 3.2 Microsatellite in-orbit autonomy Highly-autonomous satellites are defined as those vehicles requiring minimum contact with external sources (Terrestrial and/or Spaceborne) to successfully accomplish their intended missions [H.Bonyan, 2007]. Most microsatellites are placed in LEOs, and communications gaps (time-intervals with no contact opportunity) are inherent characteristics of LEOs. Thus, logically, a given level of in-orbit autonomy must be accommodated within the orbiting vehicle to perform mission-specific tasks, when out of ground station visibility. Accommodation of a given level of onboard autonomy is a sophisticated systems engineering activity confined by inherent mass-/power-budget constraints of microsatellite missions and also by LEO characteristics. For a microsatellite mission, once in orbit, it is [...]... launch cost per pound (kilogram) for different medium (5,001 -12, 000 lbs to LEO) and intermediate (12, 001-25,000 lbs to LEO) launch vehicles, as of 1990-2000 Table 4 Launch cost per kilogram for different medium (5,001 -12, 000 lbs to LEO) and intermediate (12, 001-25,000 lbs to LEO) launch vehicles, as of 1990-2000 334 Aerospace Technologies Advancements Finally, Launch cost per kilogram for different... got the chief designer Werner 330 Aerospace Technologies Advancements Table 2 some construction-facilities/suppliers in various countries7 This table is not intended to provide a complete list of construction-facilities/suppliers and is only meant to name a few A comprehensive list can be found at EPPL (European Preferred Part List ), Issue 13; Issue Date: 2008-09 -12 7 331 Looking into Future - Systems... board of a Mexican LEO microsatellite, Acta Astronautica Journal, Elsevier Science, Volume 58, Issue 3, Pages 149-167 338 Aerospace Technologies Advancements A M Woodroffe and P Madle (2004), Application and experience of CAN as a low cost OBDH bus system, MAPLD 2004, Washington D.C USA Part V 17 An Aircraft Separation Algorithm with Feedback and Perturbation White, Allan L NASA Langley Research Center... powergeneration capability of microsatellites [Bonyan & Toloei, 2009] With this problem already removed, 2-3 times enhancement is foreseen in power generation capability of microsatellites 2 324 Aerospace Technologies Advancements (a) (b) Fig 2 The MTR-5, one of three magnetic torque rods available from SSTL (a) and SSTL 2axis DMC sun sensor (b) (a) (b) Fig 3 SSTL star tracker (a) and Sun Space star tracker... NATO (North Atlantic Treaty Organization), during the cold war US, by the first years of 1960's, had its NATO AFBs in several European countries, near to or neighbouring the soviet union 8 332 Aerospace Technologies Advancements During the first two decades after the first space flight, Launch-campaign was an issue mainly influenced by the political and military drivers From 1980s and afterwards, specifically... space activities and partially avoid high maintenance costs This is, in turn, one of the main reasons why microsatellites have gained more and more attention during the last 2-3 decades Microsatellites are obviously much cheaper and quite affordable to be launched into orbit, compared to conventional large satellites 10 Shtil launch costs are partially subsidized by the Russian Navy as part of missile launch... time-delay to be financially-valuable Non-real-time communication applications, yet Mbit-order data rates are now affordable within stingy mass-power- budget of microsatellite missions 3 326 Aerospace Technologies Advancements However, Most microsatellite missions, even in recent years, have been confined to some lowpower applications [United Nations UNISPACE III, 1998]; [Bonyan, 2007] This is, in turn,... Sky, USA H.Bonyan (2007), System engineering approach toward the problem of battery depth-ofdischarge of a LEO satellite, International Conference on Complex Systems (ICCS) Quincy MA USA 336 Aerospace Technologies Advancements H.Bonyan (2007), Systems Engineering Approach toward the Problem of Required Level of In-orbit Autonomous-operation of a LEO Microsatellite Mission, International Conference on... Propulsion Historically, microsatellites have not been equipped with propulsion systems Although there have been experiences of carrying propulsion systems onboard microsatellites, these 328 Aerospace Technologies Advancements experiences have been mainly for technology-demonstration and space-qualification purposes Realistically, practical applications of onboard propulsion systems for microsatellites... which was stated in terms of a moving average: no more than three incidents (of all types) over the last three years Since there are about ten million flights per year, this translates into 340 Aerospace Technologies Advancements one or fewer incidents per 10 million (107) flights The examination compares this moving average to a goal stated in terms of one year There are two comments First, future FAA . (third ed.) Aerospace Technologies Advancements 318 Zantou, B. & Kherras, A. (2004). Small Mobile Ground Terminal Design for a Microsatellite Data Collection System, Journal of Aerospace. signal Matched filter f 0 (.) 2 Matched filter f 1 (.) 2 Decision (compare) Aerospace Technologies Advancements 314 detected, we need two filters described by (7). We conclude that. 1.5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 BT b =1 BT b =0,5 BT b =0,3 Time in half bit p erio d g(t) Aerospace Technologies Advancements 316 corresponding to binary states transition. The GMSK demodulator