ASSI Newsletter Number 3, Volume I, January 2000

Geostationary Orbit Optimization in the Future Space Commercialization

Prepared for One Day Symposium on Space Technology : Moving Toward New Era of Satellite Technology, organized by ASSI & TELKOM, Bandung September 27, 1999

1). As early as 1971 a resolution was passed by World Administrative Radio Conference (WARC) for Space Telecommunications – which convened under the auspices of the ITU, declaring that all countries were to have 'equal rights' in the use of both the radio frequencies allocated to various space radio communications services and the geostationary satellite orbit for these services, and that the radio frequency spectrum and the geostationary orbit were 'limited natural resources' which were to be most effectively and economically used [ Gorove].

2). Adherence to the above resolution should lead to every effort from all space society in developing space technology and its related business so as to make the resources be more accessible to every one at least costs with consequence of managing smartly the orbit location and spectrum in use.

3). Generally, the efforts fall in the following lines :

4) Utilization of higher frequency spectrum, especially those of Ka, K and V bands shall be enforced in view of unevenly exploitation of the spectrum resources. At present, Ku band is among the most used spectrum if we look at the ITU filing, followed by C-band, as shown by the following ITU-registered satellites classification in accordance with 1999 master file :
L-band : 202 satellites
S-band : 296 satellites
C-band : 164 satellites
Ku-band : 416 satellites
Ka-band : 12 satellites
In the recent publication however, there are big numbers of new proponents’ filing asked for more higher spectrum (especially Ka-band) for geostationary orbit satellites systems, representing wayforwards to evenly use the spectrum. The shorter wavelength enables the engineers to have easily design narrowed beam thereby higher gain at a relatively smaller diameter antenna on board the satellites. For example, both ACTs systems of NASA, Spaceway of Hughes, Teledesic, and others, are using Ka-band. The use of higher spectrums lead to the wider spectrum availability thus bandwidths , enabling satellite operator to acquire higher capacity satellites, allowing the use of New High Data Rate [Barker]. Furthermore as cellular-like spatialy frequency re-use technique to be combined with the cross polarization , higher channel capacity should be available. Satellite manufacturers have expended large amount of efforts to minimize antenna losses, costs, and masses, in all exploitable spectrum, with considerable success [Price]. Current efforts are focusing on active antenna designs such as direct radiating phased arrays with active elements in the antenna assembly, allowing the ease of integration and test and operational flexibility.

5) In view of acquiring higher capacity satellites, use of modular bus 3-axis stabilized platform is a must. Commercial buses are being configured to maintain launch envelopes as available today, but more competitive commercial communications satellite market has driven the development of a flexible and adaptive spacecraft bus with an ability of accommodating a variety of complex payloads. Such adaptable module bus does not require redesign for each new implementation thus reducing time to market and cost. The state-of-the art design of spacecraft bus is caracterized with the following features :

Those advantages maximize the available payload, mass, power and thermal dissipation capabilities [Taylor].

6) One among the most important and critical aspect for reducing the bus mass would be the use of certain efficient on-board propulsion technology. Propulsion is a dominant mass driver for satellite applications, and this is dictated by its system performance. High performance on-board propulsion systems can provide propellant mass saving across a range of applications including N-S station keeping for geostationary satellites. NASA sponsored technology (arcjet thrusters) has been operational, especially being flown over LM A2100 type of bus [McKinnon]. Using this technology, several hundreds kilograms of propellant can be saved, allowing an increase of payload mass capability, spacecraft life, or reducing launch costs [Callahan]. Incorporating advanced high performance electric propulsion system in portion of GTO to GEO transfer allows the increase of net mass of 20 to 45% whereas further implementation of the same SEP (solar electric propulsion) for NSSK can further increase the net mass by 13%. Omega bus of Loral is being equipped with electric propulsion subsystem to perform NSSK maneuvers by emplyoing the Russian’s SPT-100, a technology which is around for more than 25 years!

7) High Powered Spacecraft platform requires efficacy of thermal control of main spacecraft elements, main body, solar arrays and antennas. High Powered payload alone will have to dissipate some 40 to 60 % of the DC powered delivered to it. Common approach would be puting radiators on N-S panel outer surfaces, whereas multi-layer insulation (MLI) blankets are covering east, west, earth and anti-earth surfaces. Some design let the N-S panels be conductively coupled to reduce radiation panel area. Other common approach is the use of high emittance optical solar reflectors to achieve low solar absorbtion, as well as the use of heat pipes to isothermalized panels. The deployable heat pipes panels [Hughes 702] also being implemented to radiate even higher dissipative power. In conclusion, efficacy of thermal control of spacecraft has to be realized for an efficient and benign environment of high power platform .

8) Another method of making more efficient use of orbit location would be to collocate more than one satellite into one orbit position. The other drive of such an endeavour would be an operator’s concentration into one orbit location from which they could provide a majority of it’s television services, as well as providing multitude of other type of services from a single location. Collocation algorithm is being developed and applied enabling up to five satellites [Pattinson]. The idea resides in the way the orbit control strategy can assure collocation requirements generally as follows : a) same station keeping strategy for all satellites, b) avoidance of maneuver execution simultaneously, c) collision avoidance, d) 14 days of station keeping cycle, e) geometrical constraint due to RF interference between satellites, f) longitude and latitude windows to be respected. The key of the method lies on the following strategies : a) mean longitude separation, b) eccentricity vector separation, c) inclination vector separation, or d) combined eccentricity and inclination vector separation. Experience from Eutelsat for collocating the Hot Bird I and Eutelsat II FM1 using collocation parameter : Di = 0.05 , De = 3.646. 10-4 , a = 450, came up with the closest approaches of the two satellites at 3-4 km, and Monte-Carlo analysis has demonstrated that at 3s , the closest approaches of the satellites is 6 km. The company has also put , at this 130 E orbit position, three more satellites, Hot Bird 2,3 and 4.

For so doing, in addition, they are to make sure a collocation geometry assuring the rf interence contraints, namely neither satellite should fall within 50. Eutelsat can comfortably collocate five satellites, whilst satisfying all contraints of rf interference performance and avoidance of close inter-satellite approaches. The method used is a combined inclination and eccentricity vector separation, resulting intersatellite separations of 6 km at 3s.

9) High capacity payloads, fisrt of all , have to be measured in number of effective bandwidth available that the satellite payload have. In view of multiplying this scarce resource, the following frequency re-use technique commonly utilized with success:

The trend toward higher capacity in future satellite systems has imposed stringent demands on the resultant antenna design to provide more spectrum reuses. Those requirements put parabolic reflector on their limits, as it will be limited by mass as well as volume and performance. One way of alleviating this constraint would be to use active phased array, due to its ability to form multiple beams, and to rapidly reconfigure and repoint beam. The enabling technology which help this active phased array to be more practical would be MMICs , which could eliminate many of drawbacks in mass, DC-RF efficiency, and uniformity of performance associated with a large number of elements. MMIC will be used at the beam forming matrices for realizing phase shifters and attenuators [Sorbello]. Such a configurations have the potential for dramatic reductions in mass while providing increased efficiency, flexibility and capacity. Key issues driving the MMIC insertion in communications satellite transponders include advanced payload architectures with reconfiguration flexibility, increased eirp for communications with low-cost earth stations, increased satellite life, smaller size and mass to make communications satellites more cost efficient, reproducibility and uniformity of performance and reduced subsystem assembly costs [Gupta]. These insertion leads to key design issues including performance, power consumption and dissipation, chip/module integration level, packaging, interfacing/control and space qualifications. Comsat, for example, has addressed these key issues thoroughly, and has successfully applied Ku-band MMIC amplifiers flown in ITALSAT in 1991 . Superconductors, have the ability to conduct electrical current with zero resistance, no power loss, no generation of heat and greatly reduced levels of interference and noise. Those materials generally perform its superconductivity at critical (low) temperature where the electrons in certain materials pair-up and form a single quantum state, acting like a frictionless fluid, and become superconducting. For such a temperature liquid Helium is required and this is cumbersome. Fortunately , HTS has already been identified since 1986, which would be superconductive at 125 K requires less stringent technique for cooling. The experiments are being conducted to apply such a technology in satellite applications, namely : input multiplexer, antenna arrays, up/down – converters, LO/synthesizers, BFN/switches, baseband processors, receivers, RF power amplifiers. In is reported not only that a mass saving of 29% is achieved for communications payload when HTS are in use, but also it will increase payload performance[Lichtenberg]. HTS can enable higher channel capacity for about the same payload mass. Despite of these results, there is still a need for additional development of the HTS components. More complete test, characterization, and on-orbit life data is needed. Also there is still a need for better , longer-life cryocoolers and some cryo-engineering components. Once those requisites are done, HTS will revolutionary contribute to the more efficient space payload applications, in particular, for enhancing the millimeter wave satellite systems technology (Ka-band). Perhaps the biggest phenomena in the realm of satellite communications is Very Small Apperture Terminals (VSATs) and networking associated with them. It spanned for almost two decades this phenomena has 'mis en relief' its own success story when it began its service as slotted Aloha as well as X.25 terminals for connecting the distance data communications users through satellites at lower bit rates, making on overlay networking to its terrestrial partners . In the course of its evolution, mobile satellite terminals in the form of handheld have surfaced due to LEO / MEO technology. Iridium and sooner Globalstar are some of them. These type of terminals represent a metamorphose of the VSATs themselves. Nowadays, and perhaps also in the future, VSATs still are very important for data and telephone networking. They permeate even deeper into a total networking when VSATs society decide to plung them into IP based environment, encompassing then any sort of signal : data, voice, video, multi-media. It seemed that public response to this vast technology was very far ahead of the technology maturity. In view of the limited number or perhaps the high costs of high speed fiber access to the home, this technology immediately gives alternative to providing high speed access, up to T-1 type. There are however two approaches implementing the technology : 2-way symmetrical and assymmetrical. The later one relies on the use of copper cable for connecting the user to the ISP, whereas the other direction requires higher speed satellite link [Samejima]. Applying TCP/IP over satellites, however, require more deliberation to the protocol over satellite so as not give further burdens to the throughput performance [Orr]. The IP based multimedia is foreseen to reveal in the ATM environment. Again, for those users who are not having terrestrial access to ATM consider that ATM through satellite is an alternative to exercise [Jain].

In spite of blossomming phenomena of IP VSATs, it seems that concerted and thoroughly study on different aspects of Mesh VSATs need to be continued [Ivancic], especially in solving the issues of services and protocols, ground terminals, and needs of on-board processing.

 Within the VSATs context, it is important that the transponder bandwidth can be optimized in number of channels/Hz. Mitchel and Price have made a study and came up with the conclusion that the parameter in question depends whether the uplink consists of a single beam or a multiple beam [Mitchel]. In the case of single beam, FDMA is the most efficient, seconded by MF/TDMA, whereas in the case of multiple beams, due to inherent capabilities of MF/CDMA and MF/CPDMA in handling the interference, those alternatives are about twice as efficient as FDMA or MF/TDMA. This spririt of finding out the most efficient multiple access for MVSATs shall persist in view of scarcity of orbit and spectrum resources, to the benefit of as much users as possible. The following frequency bands (values in GHz) have been assigned by the ITU [RR] for implementing ISLs : 22.55-23.55, 32-33, 54.25-58.2, 59-64, 116-134, 170-182, and 185-190. Optical frequencies for ISLs looks also. promising [Tirro]. The use of higher spectrum for ISL will alleviate the attenuation problem caused by hydrometeors in the atmosphere, allowing a very large bandwidths may be used, and practically no restrictions due to international agreements. The techniques will also reduce multi-hop operations hence the delay of communications. 10) Needs for reliable and low costs launch services:

The launch costs of a satellite program still represent a predominant factor, in this can as high as 50 % to the program costs. In concert with the other space mission elements, namely spacecraft and mission operations, the launch costs have to be reduced to make space systems services be more accessible to the benefit of everyone on earth. The reduction of launch costs is having to do with what can we do with ability of decreasing the nonrecurrence costs. Despite poor economic performance of spaceshuttle [London], the concept of reusable part is still valid. For example its Orbiter lands on a runway, the Solid Rocket Boosters parachute into the ocean and are reused. The failure of this concept lies in its high design complexity, its high performance demands, and its higher development costs. Methods are proposed to further continue with this reduction of cost issue, see also [Greenberg], and [Sackheim].

High launch costs have an impact to the increase of spacecraft costs. Too much limited lift capability – because of high budget allocated for launch- will drive the high cost of payload due to forced sophisticated design of spcecraft : lighweight materials, high packaging densities, complex configuration related with deployable parts.

11) Regional collaboration :

Ploriferation of orbit location filling leads to total waste of the natural resource, if the associated real implementation of those satellite systems are not well planned in view of competitive environment. There are many examples in which big percentage transponders of some Asian Satellite Systems are not in use. Those transponders are ready to be used but at relatively high price to the users, which sometimes forced unhealty business by dumping the price, - an example of unhealty business practice. Or the survival is for those who can cut its total space mission cost down. Global initiative of LEO system are still to proof its worthiness to be a real solution so as to make people from developing countries find it to be fruitful infrastucture they asked for.

As an alternative, perhaps it is advisable that regionalization, instead of globalization of satellite system, be pursued. The neighboring countries are supposed to know better its surrounding that any counterpart at distant location.

12) Future direction

As a conclusion, an issue of optimizing the orbit location shall end up with the efforts of smarter technology and policy that the space user to deal with. Some outline to this end would be :

References:

[Barker] : K. Barker et al, Government Satellite Versus Commercial Communicationjs Satellite Procurement Cost Analysis, AIAA 16th International Communications Satellite Systems Conference, Washington DC, February 1996, p 1076
[Callahan] : L Callahan et al, On-Board Propulsion for Communications Satellites, AIAA 16th International Communications Satellite Systems Conference, Washington DC, February 1996, p 1293
[Gorove] : Stephen Gorove, Development in Space Law, Martinus Nijhoff Publishers, 1991
[Greenberg] : J. Greenberg, Selecting a Launch Vehicle for Geo Missions, , AIAA 16th International Communications Satellite Systems Conference, Washington DC, February 1996, p 1086
[Ivancic] : William D. Ivancic, NASA Lewis, Nasa Lewis Meshed VSAT Workshop – Meeting Summary, 15th AIAA ICSSC, February-March, San Diego, CA, 1994, p 622
[Jain] : Raj Jain, Rohit Goyal, Sastri Kota, Satellite ATM Network Architectural Consideration and TCP/IP Performance, The Ohio State University, 1999
[Lichtenberg] : C. Lichtenberg, G. Price, and M. Nisenoff, High Temperature Superconductivity Space Experiment : Communications and Satellite Payload Applications, AIAA 16th International Communications Satellite Systems Conference, Washington DC, February 1996, p 576
[London] : John R London III, Reducing Launch Cost, in Reducing Space Mission Cost, JR Wertz and WJ Larson (eds), AIAA Space Technology Library, 1996, p 115
[McKinnon] : Douglas McKinnon, A2100, Satellite of the Future, Here Today, AIAA 16th International Communications Satellite Systems Conference, Washington DC, February 1996, p 966
[Mitchel] : W. Carl Mitchel and Kent M. Price, Capacity Comparison of Uplink Multiple Access Techniques for Very Large Mesh VSAT Networks, , 15th AIAA ICSSC, February-March, San Diego, CA, 1994, p 137
[Mott] : R Mott, G Estep, W Kelley, I Yogev, L Di Fiore, J Talcott, A Williams and F Assal, Italsat In-Orbit Test Transponder Design and Performance, The 14th AIAA ICSSC, Washington DC, March 1992 , p 471
[Orr] : Michael Orr, Internet via Satellite : Problems and Solutions, ", Asia-Pacific Satellite Communications Summit at CommunicAsia 99, June 1999, Singapore
[Pattinson] : L Pattinson, EUTELSAT Satellite Collocation, AIAA 16th International Communications Satellite Systems Conference, Washington DC, February 1996, page 557
[Price]: K. Price and Y Lazear, Technologies to Enable Low Cost Satellite Communications, AIAA 16th International Communications Satellite Systems Conference, Washington DC, February 1996, p 1065
[Sackheim] : Sackheim et al, Propulsion Advancement to Lower the Cost of Satellite-Based Communications Systems, 15th AIAA ICSSC, February-March, San Diego, CA, 1994, p 179
[Samejima] : Shichi Samejima, Ph D, Multimedia Services / Satellite Internet Services to Business and Homes , "Internet in the Sky", Asia-Pacific Satellite Communications Summit at CommunicAsia 99, June 1999, Singapore
[Taylor] : Stuart C Taylor & Adi R Adiwoso, The Asia Cellular Satellite System, AIAA 16th International Communications Satellite Systems Conference, Washington DC, February 1996, p 1239
[Tirro] : Sebastiano Tirro (Ed): Satellite Communications Systems Design, Plenum Press, New York, 1993 , p 737
[Hughes 702] : Various presentation of HS702 bus technology by Hughes Space and Communications, in 1996-1997

Arifin Nugroho, PT. TELKOM.
arifin_nugroho@attglobal.net

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