MANPORTABLE and PERSONAL MI[LSATCOM TERMINALS

0. Jones and A. C. Smith'

ABSTRACT

This paper addresses those aspects of Military satcoms relevant to the development and operation of man- portable and personal satellite communications equipment. The principal differentiating factors between civil and military solutions tend to focus on the need for physical and electronic robustness. This, coupled with the need to design solutions for military frequency bands and the inevitable short'pmduction runs, has tended to make milsatcom an expensive proposition. With the rapid increase in the use aif satcom since 1990 it is perceived many small and lightweight satcom terminals, with data and speech facilities into Government Networks, will be the next step in providing world wide communications to echelons below that of Headquarters Staffs. The likely numbers of terminals to meet this need will reverse the trend of short production runs and in consequence offer the potential for lower unit costs.

INTRODUCTION

Manportable satcom terminals are not new. The UK has been at the forefront of this technology since 1981 [11*r213 when the first designs for a 'manpack' were published. Since then the UK has fielded manpack satcom for use by the Army[4 fig. 1, but now this technology is dated and a new generation of terminal is being specified and designed. However, the new generation of field terminals are un1ik:ely to offer the apparent weight savings offered by the newer technology since these savings are mitigated by an increased communications requirement. The UWPSC 505[31 provided a world-wide, low daita-rate primary capability with low quality voice in the high gain footprints of appropriate spacecraft. The successor to this terminal, the UwPSC504, is required to provide, as its principal function, secure speaker-recognisable-voice and, secondly, a significantly increased data capability.

At the introduction of the uWPSC505 in 1987 it was possible to dedicate spacecraft resources on a permanent basis for each terminal. This is no longer possible and automatic access management techniques are necessary. Also as the number and type of circuits has increased - to over 200 times the occupied bandwidth since 1990 - the manportable terminal has to vie with other users for the allocation of space segment.

Factors which tend to differentiate civil from military satcoms are the wide variety of modulation schemes, data rates, power levels and terminal types which not only have to coexist on the same transponder but also may need to be access-planned for the duration of a mission. In the case of rnanportable terminals the allocation of spacecraft resource needs to be as dynamic as the traffic profile demands. This is an area where the military can learn from commercial developments.

LINK ASSESSMENTS

Fig. 2 shows an infinite number of link budgets in a graphical form. Here we have a transmitter's figure-of- merit, EIRP, as the ordinate and a receiver's figure-of-merit, G/T, as the absc.issa. The curves relate to constant signal-to-noise-density ratio, CAT, and in this figure are shown in 3clB steps from 36dBHz to 48dBHz for a hypothetical spacecraft transponder, assumed linear.

' 0 Jones and A. C. Smith are with the Defence Research Agency, Defford.

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It is immediately evident that for a small teiminal, with a small EIRP, to transmit effectively over the spacecraft the signals must be received by a much larger - in GiT terms - terminal. Since, for any transparent linear and transponder the necessary spacecraft EIRP is proportional to the uplink EIRP, we see that the demands on the spacecraft for power are quite modest. However for the retum link from the large terminal to the smaller terminal, the spacecraft power necessary to support it will be much more significant fraction of the available power. Similar arguments apply for personal satcom terminals. As any system designer will be aware it is the balance between the Operational Requirement and the system demands which are almost always in conflict, particularly if small terminals need to be specified.

In military systems one is nearly always power limited. For small terminal systems the spacecraft EIRP must be shared and managed in a way that is both efficient and timely for the user to meet his need. In current military spacecraft several beams are provided, each with their own characteristics and coverage areas. Each beam has to be assessed separately so as to optimise system performance.

The use of personal satcom, when supported by Geosynchronous Earth Orbiting (GEO) spacecraft, for other than low data rate communications will inevitably require higher power densities at the personal receiving terminal. This can be achieved by the use of higher EIRP from the Spacecraft, which could impact on the requirements for station-keeping, if high gain beams are to be employed.

An alternative solution to the use of GEO could be the use of Low Earth Orbits (LEO) or Highly Inclined Orbits. These have their own design implications, such as the handover of traffic and control from one spacecraft to another as they enter and exit the field-of-view. LEO appears to be one of preferred options likely to be adopted within the commercial sector.

ACCESS MANAGEMENT

Spacecraft resource is managed at several different levels: from managing the entire spacecraft through its TT&C channel, access planning of circuits, to the allocation and de-allocation of circuits for the duration of a message - whether data or voice. For our small terminals we need only address dynamic allocation and this is encapsulated in the term Demand Assigned Multiple Access[41.

We will not go into the DAMA protocol, since there are many possible variations, but there are some common features with them all. In particular, the field terminal (User) has to: affiliate to the system controller; demand resource; receive the allocation instructions; implement the allocation; communicate and, finally, relinquish the resource in a managed way. The System Controller has to: monitor a request order wire; confirm any request through an outbound link, establish that the destination address is available; signal to both parties that resource has been allocated; implement the anchor station communications parameters; permit communications and, finally, respond to the end-of-trafic message by closing down the circuit and returning the resources back to the resource pool.

Whatever protocol is implemented it must be matched to the expected traffic profile and this is, perhaps, the most significant element in identifying and sizing the system resource requirements for new services. The profile might take the from of a connectivity matrix containing the expected frequency and duration of each message type between different user groups. A basis for trying to estimate the requirements would be to assess the communications need during typical operations. The analysis will identify a number of parameters to size the system including, inter alia, mean duration of voice calls, average data message length and the numbers of trunks. However, until the system enters service any traffic profile will necessarily be an approximation.

ANCHORS STATIONS and INFRASTRUCTURE

From a UK perspective, the equipment to be installed within the United Kingdom Military Satellite Communications System (UKMSCS) ground segment will consist of two elements: that to be installed within the Satellite Network Control Centre (SNCC) and SateiIite Ground Stations (SGS’s). Equipment at the SNCC will be limited to providing bulk trunks whilst at the SGS will be the modems and associated traffic routing

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hardware.

It is seen as a primary objective that the UKMSCS provides little or no intervention in the minute-by-minute management of traffic. Most of the dynamic traffic control activities will take place at the System Control Centre. Allocation of resources and routing of traffic would be under the control of the System Control Centre. Figure 3 shows an overall architecture of a proposed system implementation.

The system would be controlled under the authority of a single System Control Centre. The System Control Centre would need to be equipped to carry out a number of functions including:

a. b. c. d. Traffic Management and Routing. e. f. g.

Operation and management of User Groups. Management of security related issues . Authentication of the requests for accesses to the system.

System Configuration and Performance Management. Allocation and Examination of user Precedence. Data message store and forward.

The above hnctions fall into two categories: those associated with the allocation of satcom resource and the control of the resource when allocated.

SUBSCRIBER TERMINALS

The manportable terminal can be defined as a complete man-packable SHF militruy satcom terminal. When operating within the SKYNET 4 satellite earth cover footprints, a basic terminal would be expected to be capable of providing user selection of secure speaker-recognisable-voice or secure: data. A major impact on the UKMSCS that the introduction of a population of manportable terminals would have would be associated with the use of DAMA to control access to the system and, as a consequence, the system architecture to support it.

The terminal itself would need to be simple to use with minimal operator intervention. The technology required to produce this type of terminal currently exists but packaging, and the use of materials to ensure that weight is minimised, represents a significant challenge to the designer.

For a military personal terminal it is of interest to note that the old UwPSC505 (figure 1) could be reduced considerably in size, for substantially the same communications specification, using current technology. Voice comms., from hand-portable terminals, puts additional challenging constraints onto the terminal and system design[’]. Parallel developments in a number of technologies including: component miniaturisation and integration; component power efficiency; battery capacity and improvements in vocoders will need to be made to meet this challenge. These disciplines are currently being pursued and need to be brought together.

Personal satellite programs are already in the planning stages for TRW/Teleglobe’s Odyssey, Motorola’s Iridium and Loral’s Globalstar. With this interest, and potential market, equipments are likely to be available within the next few years. Military users my well wish to take advantage of these developments for humanitarian and peace-keeping operations.

SUMMARY.

What has been described here are a number of issues relevant to the successful operation and implementation of a military satellite communications system architecture for small man-portablle and personal subscriber terminals. The entire system, whilst not showing any one high risk area, does involve the detailed designing of large and complex components to make it work. The need to break down the architecture into functional units and maintain system integrity across the various system boundaries is a significant challenge.

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Recent events on the world stage have resulted in the deployment of UK Forces outside of the traditional NATO theatres. These have contributed to the acceptance of satellite communications as being the primary long range bearer. A continuation of these changes could see the satcom terminal becoming the standard issue communications device. Within the next generation of military spacecraft a tactical satcom terminal will be found in every soldier's kit-bag. This will require the use of small and lightweight satcom terminals, with data and speech facilities. The likely numbers of terminals will reverse the trend of short production runs and in consequence offer the potential for lower unit costs.

0 HMSO, London. 1995.

The views expressed in this paper are those of the authors and do not necessarily represent the views of the Defence Research Agency or the Ministry of Defence.

References

1. C. H. Jones, 'A Manpack Satellite Communications Earth Station', Rad. & Elec. Eng., 51(6), 1981,

2. P. J. Skilton and A.C. Smith, 'Miniature Portable Satcom Terminals', Colloq. Small Terminal Satellite Communication Systems, Nov., 1986, 14-1 to 14-7. IEE, London.

3. P. J. Skilton and 1. L. Westall, 'Manpack SHF Satellite Ground Terminal', Military Microwaves Conference, London, 1982.

4 S . L. Kota, 'Demand Assignment Multiple Access (DAMA) Techniques for Satellite Communications', National Telecomms Conf Innovative Telecomms - Key to the Future. 2, 1981,

P. Wells, A. C. Smith, M. W. Holloway and D. P. Robinson, 'MILPIC0 Terminals - Military Personal Satcom', 3rd. European Conf. Satellite Communications, Manchester, November, 1993. IEE, London.

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Figure 3 Possible System Architecture for Manportabie and Personal Satcoms

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