Global telecommunications needs for the long duration balloon environment

Aav.Space Res.Vol.3,No.6,pp.59—66,1983 Printed in Great Britain.A11 rights reserved.

0273—1177/83 $0.00 + .50 Copyright © COSPAR

GLOBAL TELECOMMUNICATIONS NEEDS FOR THE LONG DURATION BALLOON ENVIRONMENT Stephen L. Waymire National Scientific Balloon Facility ~, Engineering Department, FM Rd. 3224, Palestine, TX 75801, U.S.A.

p.o. Box 1175,

ABSTRACT The Long Duration (LD) balloon environment significantly complicates the means of providing effective telecommunications support when compared to support possible within the conventional zero—pressure (ZP) environment. This paper will discuss general aspects of supporting two—way telecommunications between ground based facilities and multiple LD balloon payloads. A summary of the LD environment (general operational and NSBF support) is presented as a basis for discussing generic network characteristics. General LD telecDmmunication needs are highlighted and a preliminary systems model of an ideal” LD telecommunication network is introduced. INTRODUCTION The technical problem of providing effective two—way telecommunications with earth—orbiting balloons has long been recognized as a primary obstacle in establishing operational usefulness of LD ballooning [1],[2]. Also, the relative merits of using satellite—relay for primary LD telecommunications has been recognized. Though indeed, alternate methods, of direct line—of—sight (LOS) and HF, are appropriate candidates for auxiliary and/or emergency backup telecommunications. In recent years, there have been several constructive and thought provoking discussions (between members of the scientific ballooning community, NSBF, and others) concerning many facets of this complex and challenging problem. However, yet to emerge from these discussions are: 1) a realistic definition of LD telecommunications needs, 2) a thorough description of telecommunication network attributes needed to satisfy general LD ballooning requirements, and 3) a comprehensive trade—off analysis of recent satellite ~elecoimnunication alternatives. Toward these ends, this paper will present an overview of the LD operational environment, the planned NSBF support environment, and a summary of LD telecommunication needs. Then, a preliminary systems model of an ‘ideal” LD telecommunication network will be introduced. No attempt is made to provide a thorough treatise of all LD telecommunications issues since space limitations preclude such an in depth discussion. Rather, only generic attributes needed in a first—pass comparative evaluation of satellite telecommunication alternatives will be highlighted as they relate to LD ballooning. Also, references will be cited which describes each attribute in a general and more thorough manner. BACKGROUND

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CONVENTIONAL ZP ENVIRONMENT

Since the 1960’s, NSBF has supported scientific experiments with various configurations of flight instrumentation packages [3]. Presently, NSBF provides experiment support through a Consolidated Instrumentation Package (CIP). These support packages have, to varied degrees, provided two—way telecommunications between ground facilities and balloon payloads operating within a conventional ZP environment.

*

The National Center for Atmospheric Research is sponsored by the National Science Foundation. An~ opinions, findings, and conclusions or recommendations expressed in this publication are those of the author and do not necessarily reflect the views of the National Science Foundation.

JASR 3/frS

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ZP Operational Environment The conventional ZP operations environment is, for the most part, limited to ballasted balloon flights of less than 2 days duration. The geographical scope of ZP flights are generally constrained by the need to maintain direct LOS visibility (to maintain telecomniunica— tions) with one or more ground stations and by continental boundaries. The vast majority of these flights are launched from NSBF and remain within LOS telecommunication range of either NSBF or down—range stations located at Pecos, Texas and Tuscaloosa, Alabama. CIP Telecommunications Continuous LOS visibility of ZP flights allows CIP telecommunications to be supported using straight forward techniques. A CIP will normally provide a scientific experiment and flight operations COPS) with shared use of a multi—channel telemetry link and a real—time commanding link. Moreover, the modest to heavy payload weight capacities of ZP balloons allow reasonable freedom in accommodating extra weight and power, when necessary to support special experiment requirements not normally supported by the CIP. A CIP telemetry link is established by an L-band transmitter which is FM—FM modulated to obtain standard tRIG subcarrier channels. 1/ These IRIG channels may accommodate analog signals or digital PC1~! bit streams, with channel allocations defined according to experiment support requirements. Certain channels of a primary telemetry link are reserved for flight OPS requirements but remaining channels may be freely allocated. An auxiliary telemetry link is frequently established to accommodate additional channel capacity and/or special requirements. A serial P~Mbit stream, up to 80 kb/s, occupying one channel may accompany information of other channels on the same link. Modulation information bandwidths of up to 1 Nj~zare used by these links. The command link utilized by ZP balloon payloads is a single—channel FSK modulated VHF link. This link must be time shared between all balloon experiments and flight OPS; therefore, command transmitters are keyed on only when information is being sent to a balloon platform. Discrete conunands and 16—bit data words are accommodated by this link. Command allocations between flight OPS and all scientific experiments are accomplished by assigning unique destination addresses which must accompany all commands. Discrete commands and data may be sent to balloon payloads at an approximate rate of 1 item per second. LD OPERATIONAL ENVIRONMENT Stated in simple terms, the general LD operational environment may be characterized by balloon systems floating for as long as 90 days and being allowed to overfly vast geographical regions. The geographical scope of LD flights will be primarily constrained by the need for successful payload recovery and by international overflight agreements. Therefore, LD balloon systems must remain within two geographical regions illustrated by Figure 1. The Northern Hemisphere region is between 100 and 45°N latitudes and bounded about the United States by the West coast of Europe and the East coast of Asia. LD balloon systems flown in this region may generally achieve flight durations of only 6 to 8 days. The Southern Hemisphere region is between 10°and 400 S latitudes and encircling the Southern Hemisphere. LD balloon systems flown in the Southern Hemisphere will circumnavigate the globe in approximately 10 to 14 days. Multiple orbits are possible depending on flight requirements and the disposition of stratospheric winds at the time of the flight. This multi—orbit capability may allow flight durations of up to approximately 90 days. LD launch OPS will be accomplished from various remote sites appropriately chosen within each hemisphere. Payload recovery on Southern Hemisphere flights will generally be accomplished within the vicinity of the originating launch site after completing full orbits. Payload recovery on Northern Hemisphere flights will necessarily be accomplished from locations remote from the originating launch site since full orbits are not permissible. A LD flight campaign to a selected launch site will usually involve multiple experiments from one or more scientific investigative groups and multiple balloon systems. A Southern Hemisphere campaignwill generally be conducted during the months of December, January, and February. A Northern Hemisphere campaign will generally be conducted during the months of June, July, and August. PLANNED NSBF SUPPORT ENVIRONMENT NSBF is pursuing development of specialized support needed, in common, by scientific 1/ IRIG is Inter—Range Instrumentation

Group and associated

standards

[4].

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investigators who fly experiments within the general LD operational environment. In addition to LD balloon systems, this specialized support will involve commonly needed services provided by onboard NSBF support subsystems and by ground support facilities. A brief description of the planned NSBF support environment is presented as a basis for discussing interface characteristics of a LD telecommunications network. When viewed in abstract and functional terms, basic support services commonly needed in the LD environment are essentially equivalent to those needed in the conventional ZP environment, which includes: 1) an appropriate balloon system, 2) onboard power, 3) two—way telecommunications between ground facilities and a balloon payload, 4) navigational tracking, 5) experiment data quick—look analysis support, 6) experiment control, 7) flight OPS support, and 8) pre—flight check—out. However, extended flight durations, payload weight constraints and global scope of the LD operational environment have far—reaching effects on the means of satisfying LD support requirements. It should therefore be recognized that actual implementation of LD support facilities and services will be unavoidably more complex than for conventional ZP support. NSBF plans to supply LD balloon systems and provide onboard support through a Computer Augmented Platform for Telecommunication and Navigation (CAPTN) and a Solar Power Subsystem (SPS). Ground facilities will consist of a LD Operations Control Center (LDOCC) and a Remote Integration and Monitor Station (RIMS). Balloon Vehicles Present LD balloon vehicles include Sky Anchor [5] and RACOON [6]. The nature of these vehicles are quite different when viewed purely as balloon systems. However, in terms of providing effective telecommunication services, significant differences in balloon system characteristics may be limited to: 1) useful experiment observation time, and 2) payload weight capacity. Sky Anchor (SA) balloon vehicles provide continuous experiment observation opportunity due to very modest diurnal altitude variations. This implies the need for continuous support of experiment OPS including telecommunications and power. On the other hand, SA systems are significantly constrained in total payload weight capacity. This limitation requires basic support equipment be optimized to allow adequate weight resources to accomplish useful science. ZP balloon vehicles using the RACOON technique can provide greater payload weight capacities at the expense of limited useful experiment observation time. Experiment observations from RACOON vehicles may be limited to only portions of day—time hours due to extreme diurnal altitude excursions. This implies that experiment telecommunications support requirements may be satisfied in one of two modes as follows: 1) real—time science telemetry support may be duty—cycled consistent with experiment OPS intervals, or 2) real—time science telemetry data rate may be reduced to an average information rate using an onboard storage buffer operating in an asynchronous first—in—first—out (FIFO) mode. Onboard Balloon Support Equipment CAPTN. The Computer Augmented Platform for Telecommunications and Navigation (CAPTN) is being developed to support scientific experiments and flight OPS with telecommunications and onboard navigational tracking services while operating within a LD environment. Emphasis during development will be placed on satisfying SA support requirements since this support environment will generally be more demanding and constraints more severe than for a RACOON support environment. The CAPTN will provide primary telecommunications, with the NSBF, via interfaces to a satellite telecommunication network. CAPTN to satellite interfaces will be supported by a forward (NSBF to balloon payload) link and a return (balloon payload to NSBF) link. The CAPTN will support auxiliary telecommunications, with the remote launch site, using direct LOS techniques. Both a forward link and a return link will be supported. These links will be essentially equivalent to those presently used within the ZP environment. SPS. The Solar Power Subsystem (SPS) will support onboard power requirements of the balloon CAPTN and the scientific experiment. The SPS concept is to supply modular building blocks which can be used to configure a semi—custom solar power subsystem to satisfy unique power requirements of each LD flight. The SPS will provide two isolated battery charging buses. One bus will be used to charge batteries residing in the CAPTN. The other bus will be available to the experiment platform. Firm estimates are not yet available on SPS characteristics. However, it should be noted that the weight penalty of a high continuous payload power demand can consume significant portions of total payload weight allowed by a LD balloon system. Minimizing the payload

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(both CAPTN and science experiment) power demand is therefore considered to be a crucial aspect of reducing the total weight of basic support equipment. Ground Support Facilities LDOCC. The LD Operations Control Center (LDOCC) will be located at NSBF and will be the focal point for in—flight support of multiple LD balloon systems. It will accommodate personnel, equipment, and software needed to adequately ensure safe and effective in—flight support of experiment OPS and flight OPS. Experiment OPS activities are performed by scientific investigators who are responsible for effective utilization of their in—flight experiment(s) to accomplish planned scientific objectives. Flight OPS activities are performed by NSBF personnel who are responsible for flight safety and general health of all LD balloon systems and onboard support subsystems. RIMS. The Remote Integration and Monitor Station (RIMS) will be a portable facility located at the remote launch site. The RIMS will provide equipment and services needed for preflight check—out activities, involving: 1) CAPTN readiness testing, 2) experiment/CAPTN intergration testing, and 3) experiment OPS testing (through CAPTN supported services). It will also support launch OPS, auxiliary telecommunications during ascent, and emergency flight terminate control. LD TELECOMMUNICATIONS NEWS Given present day satellite telecommunication technology and the general LD environment, it seems logical to assume that utilization of a satellite telecommunication system (network) should be considered basic to providing effective primary telecommunications between the NSBF control center and all in—flight LD balloon payloads. Conventional LOS techniques would be appropriate only for auxiliary telecommunications during pre—flight, launch, and recovery phases of balloon payload support. Experiment Support Needs A LD ballooning questionnaire, dealing with many support issues, was recently distributed to members of the scientific ballooning community. Responses to this NSBF user survey represent the support needs of 34 experiment “designs” from 27 principal investigators. In summary, experiment telecommunication needs vary from essentially none to continuous (real—time) telemetry, of 72 kb/s data rate, in conjunction with frequent (real—time) commanding. The ability to send blocks of data to a balloon payload is also a significant experiment support need. Flight OPS Support Needs General telecommunication needs of flight OPS are for near—real—time house—keeping telemetry, of a few b/s average data rate, in conjunction with occasional commanding. These needs should remain relatively constant from flight to flight. LD TELECOMMUNICATION NETWORK MODEL It is likely that no existing satellite telecommunication system (network) can be considered as “ideal” in supporting LD primary telecommunication needs. However, TDRSS, INMARSAT, MARECS (search and rescue mode) [7], and GOES/METEOSAT are all capable of supporting LD needs; albeit, each with particular capabilities and unique constraints within the general LD environment. Before evaluating merits of a candidate satellite system, it is instructive (and indeed necessary) to construct a system model of an “ideal” LD telecommunication network. This system model should be developed as an abstract network, illustrated in Figure 2, independant of any particular candidate network. Network attributes should be treated in generic terms as they apply to: 1) general LD telecommunication needs, 2) general LD operational environment, and 3) the planned NSBF support environment. With this in mind, attributes needing emphasis are those which: 1) are of a general nature, 2) reflect views of the abstract network from external interfaces, 3) represent end—to—end channel characteristics, and 4) significantly impact requirements of network access equipment onboard the balloon payload. It is not important, at least for first—pass modeling and evaluation purposes, how the network implementation is achieved internally. General Attributes Geographic coverage. Network geographic coverage should be viewed as the ability, of the network, to support LD telecommunications on, or up to 50 km above, referenced geographic regions. Within this context, the network should allow access (support telecommunications) from anywhere within regions highlighted by Fig. 1. A zone—of—exclusion (ZOE) defines an area which excludes telecommunications support. The effects of telecommunications interrup—

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tion(s), caused by ZOE area(s), should be given due consideration as to the impact on balloon payload (experiment and flight) OPS requirements. ZOE region(s) may also affect remote launch/recovery site selection criteria. Physical network transparency. Physical network transparency allows users to consider the combination of all physical links between two access locations as merely a logical end—to—end channel. Stated in another way, it is the extent to which users may be un—aware and un—concerned about a network’s implementation details and internal operational characteristics. This physical transparency brings geographically dispersed network access locations into close logical proximity, as illustrated by the abstract nature of the LD telecommunications network depicted in Figure 2. Network availability. Network availability may be expressed as a percentage resulting from the ratio of time periods in which telecommunication services are supported by the network to the time periods in which these services are needed. This availability may be affected by many factors related to operational availability of either terrestrial or satellite facilities [8], [11]. Overall network availability of something greater than 99 percent should be a reasonably obtainable performance objective. Network Interface Attributes Direct (RF link) multiple access. A network supporting multiple access (MA) will permit multiple geographically dispersed user stations to share channel resources, of a satellite involved in the network, via direct access RF links. Note, however, that this does not necessarily imply that all user access locations will have direct access privileges. Direct access may be reserved for mobile, maritime, airborne, or space applications. Land—based access locations may require land line interfaces. In any event, direct (RF link) MA is required for both forward and return links with LD balloon payloads. The principle of MA may be implemented in three ways; namely, frequency division multiple access (FDMA), time division multiple access (TDMA), or code division multiple access (CDMA). These methods relate to ways of sharing satellite transponder bandwidth among multiple user channels. The FDMA method allocates mutually exclusive access to slots of transponder bandwidth for each user channel being supported. TDMA refers to assigning mutually exclusive time slots of the total transponder bandwidth for each channel being supported. CDMA is a method allowing multiple user channels to simultaneously share the total transponder bandwidth by separating channels in a pseudo—random code space. In CDMA, the user interface transmitter utilizes a high—rate pseudo—random cover sequence, generated based on a unique identification code, to perform sufficient spreading of a user’s data spectrum to occupy the full transponder bandwidth. User interface receivers synchronize to their unique pseudo—random cover sequence and despread received composite spectrum into the user’s original data spectrum using code correlation techniques. In principle, any of these techniques could support LD network needs. However, many consequential factors will need evaluation with selection of any of them. FDMA and TDNA are covered in references [8], [9]. COMA is discussed in reference [10]. Multiple channel access. User access to multiple channels may be accomplished via either direct RF link(s) or land line link(s). Additionally, various techniques may be used with either network access method. A simple technique would be to establish multiple links with the network, each providing access to one channel. Each user channel would then function independently of other channels. Though this method is straight forward, it: 1) requires redundant network access equipment, 2) does not allow efficient link utlization, and 3) is inflexible. More desirable multi—channel techniques involve various forms of multiplexing or concentration. In addition to eliminating the need for redundant network access equipment, multiplexing or concentration techniques offer improvements in link utilization efficiencies and/ or network interface flexibility. A direct RF link may use frequency division multiplexing (FDM) [8] or time division multiplexing (TDN) 18], [9] techniques. Network access via land lines may use: synchronous TDM, statistical TDM, message concentration, packet concentration, and other multi—channel techniques [11], [12]. A particularly noteworthy form of packet concentration is based on the X.25 packet switching protocol. Since the LD environment involves multiple balloon payloads, multiple channels must be supported through LDOCC interfaces with the network. A multi—channel network interface from balloon payloads would be desirable but not necessarily essential. Channel assignment method. MA channel resources of a network may be limited (and usually are due to satellite and/or ground segment design trade—offs) when compared to the total number of authorized network users. The channel assignment method establishes the means of channel resource allocation between users wanting to establish end—to—end channels through the network. The network may either assign channels on a user demand basis or require assignment

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based on some type of pre—use schedule. Demand assignment allows network channel allocations in a time—varying fashion according to user demands of the moment. Though not essential to LD support, demand assignment is desirable. End—to—End Channel Attributes Channel capacity. The capacity of an end—to—end channel to transport data may be expressed in terms of the user’s effective data bit rate. However, actual information rates (or signaling rates) on physical links within the network, may be different due to: 1) modulation techniques, 2) coding techniques, or 3) error control techniques [8], [9], [11]. Since user- data is always transported in the presence of some form of noise, a trade—off may exist between achievable user data rate and acceptable error rates [8], [9]. Channel error rate. An ideal network would transport user data through the logical end—to— end channel without change in information content. In reality however, this goal is never completely achieved. Therefore, in context of:an “ideal” LD network model, uncorrected channel bit errors should not exceed an “acceptable” level. Note that since the term “acceptable” is dependant on a particular application, varied experiment UPS support requirements may make it difficult to establish a firm bit error criteria for the general LD support environment. Therefore, somewhat extensive error control measures may be necessary to achieve error rates as low as reasonably possible. Channel data corruptions may be caused by many factors producing varied error characteristics. However, for present purposes, and end—to—end channel may be characterized as exhibiting an average bit error rate (BER). A BER of 10 ~ (1 bit error in 10’ bits) is relatively common for satellite.links designed for digital data traffic. Satellite links designed for voice traffic, and some terrestrial links, may exhibit a HER as high as 10 ~. These high error rate links will require error control in the form of either continuous automatic request for repeat (ARQ) or forward error correction (FEC). Descriptions of channel errors and error control techniques are available in [8], [11]. Channel time delay. A network which utilizes geosynchronous satellites will have a minimum end—to—end channel delay of approximately 270 ma. This delay corresponds to the propogation time required to make a “single hop” (up and down) [8]. Some networks internally group serial user data bits into blocks and use intermedIate data buffering while routing these data blocks through the network [11], [12]. This will add additional delays which may be a function of channel bit rate. Network delays may also exhibit random distributions about the average value. Network end—to—end delays of a few seconds (real—time) and delay distributions such as to allow continuous user data bit stream synchronization at network interfaces are required for LD telecommuncations. Channel availability. The availability of a particular end—to—end user channel may be limited by numerous factors not related to network equipment availability. These factors are generally associated with limitations of a satellite’s ability to support a particular end—to—end channel during certain conditions. Sun outages and satellite solar eclipse Outages are~conmionexamples [8]. Duty cycle limitations of a satellite transponder may require intermittent channel activity. Also, shared use of limited satellite channel capacity may require schedule availablity of channel support. Note that these factors are considered separate from network availability since network equipment will still be operable (available). Attributes Impacting Balloon Payload Power and weight. As previously emphasized, balloon payload support equipment power demand and weight requirements should be minimized. It is expected that onboard network access equipment supporting primary telecommunications will be the largest single contributor to the CAPTN’s total operational power demand. Also, the power subsystem weight penalty associated with telecommunication support, when added to the actual weight of telecommunication equipment will be a significant portion of the total onboard support weight. Return link EIRP requirements. Characteristics of network satellites will impose effective isotropic radiated power (EIRP) requirements on the CAPTN’s return telemetry link which will require a trade—off between transmitter RF power, antenna gain, channel (user data) capacity, and channel BER [8], [9]. Transmitter RF output power requirements will directly impact DC input power demand. Antenna gain requirements will dictate antenna beamwidth, and thus, antenna pointing and tracking accuracy requirements. SUMMARY Much discussion and detailed definition yet remains before a comprehensive evaluation of LD telecommunication needs and implementation alternatives may be accomplished in an objective manner. It is hopeful that the generic model presented herein may serve as a common terminology base and a preliminary structure relating to these future activities.

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References 1.

J.D. Kurfess, W.N. Johnson, D.M. Saulnier, Proposal, Development and Test of a Satellite Relay Communications System for Long Duration Flights, (March 1975), NRL, Washington, D.C.

2.

R.G. Berard, L.W. Campbell, R.H. Jennett, A.H. Katz, M.C. Poppe, Interim Report, System Definition Study on Long Duration Balloon Flight Electronics, (May 1975), Raytheon Co., Sudbury, Mass.

3.

E.E. Smith, Flight Electronics — 1963 to Present, Atmospheric Technology,5 Published for National Center for Atmospheric Research (NCAR), p. 14

4.

E.L. Gruenberg, Handbook of Telemetry and Remote Control, McGraw—Hill, New York, 1967.

5.

I.S. Smith, Development of the Sky Anchor Balloon System. Tenth AFGL Scietttific Balloon Symposium Proceedings, (March 1979), AFGL, Hanscom AFB, Mass., p. 81

6.

V. Lally, The Radiation—Controlled Balloon (RACOON), this volume.

7.

J.N. de Villiers, A Review of Satellite Alternatives for Retrieving Data from Long Duration Balloon Flights, this volume.

8.

3. Martin, Communications Satellite Systems, Prentice—Hall, Englewood Cliffs, 1978.

9.

J.J. Spilker, Digital Communications by Satellite, Prentice—Hall, Englewood Cliffs, 1977.

(March 1975),

10. R.C. Dixon, Spread Spectrum Systems, John Wiley, New York, 1976. 11. D.R. Doll, Data Communications; Facilities, Networks and Systems Design, John Wiley, New York, 1979. 12. D.W. Davies, D.L.A. Barber, W.L. Price, C.M. Solomonides, Computer Networks and Their Protocols, John Wiley, New York, 1979.