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4 Goal Architecture—NAVSATCOM-21 Drawing upon the review of communications requirements related to Naval missions (Chapter 2), as well as the assessment of the capabilities of current and planned communications satellite systems, the shortfalls and issues associated with these systems, and the potential new capabilities afforded by emerging technologies (Chapter 5), the panel synthesized a goal Naval Space Communications Architecture for the Twenty-first Century (NAVSATCOM-21). This chapter describes the approach used to develop NAVSATCOM-21, emphasizing the enhanced communications services the architecture supports in a flexible, fiscally constrained manner. It contains suggestions for influencing the DOD military satellite communications decisions scheduled throughout the 1990s. It concludes with a list of recommended actions for implementing NAVSATCOM-21. 4.1 APPROACH Four considerations significantly affected the development of NAVSATCOM-21: Naval strategy and missions, command and organizational structure, functionality and performance requirements, and system infrastructure. The goal architecture was designed to support the evolving naval strategy and mission responsibilities, which include flexibly projecting a U.S. presence and (as appropriate) responding to crises worldwide. It was recognized that future crisis response may involve the deployment of naval forces in concert with those of other U.S. and allied services. Accordingly, provisions for interoperable communications within a region of operations as well as connectivities to remote locations (e.g., CONUS) are provided. Naval as well as joint task force organizational structures are accommodated by NAVSATCOM-21. As shown in Figure 4.1, the communications connectivities provided by the goal architecture can be partitioned into a four-tiered hierarchical structure. Large, geographically distributed networks with layers of interconnecting and interoperable regional and local area networks are included. The highest or global backbone level includes communications paths among the NCA and the national agencies. The second or theater level involves connectivities within and among CINC organizations. Provisions are also included for interactions with appropriate global and regional-level modes. At the regional level, NAVSATCOM-21 supports communications among JTF constituents. The lowest or tactical level provides connectivities among force elements, engaging units, and weapon and support platforms. NAVSATCOM-21 was designed to provide communications both within and among the four levels shown in Figure 4.1. At the global level such connectivities supplement several other means of communications, such as terrestrial systems and commercial communications satellites. However, as one proceeds through the tiers toward the tactical level, the installations become more transportable and, eventually, highly mobile. At these lower levels, NAVSATCOM-21 connectivities may provide the only reliable communications. 44

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— GLOBAL — THEATER — REGIONAL — TACTICAL FIGURE 4.1 NAVSATCOM-21 structure. The panel's methods for developing the goal architecture evolved from a vision of transition in the 1990s from the current communications systems to the enhanced capabilities of NAVSATCOM-21. The approach was selected to leverage the significant investments in the current systems as well as to be synergistic with DOD's efforts to modernize, during the same time frame, other military satellite communications capabilities. A number of constraints were recognized as affecting the transition plans. First, the goal system had to correct the performance shortfalls of the current communications satellite systems (capacity and protection; Chapter 3). However, the limited space available for antenna and electronics installations on many of the Navy's platforms, as well as the likelihood of future funding difficulties, dictates solutions that are low cost and involve small terminals. Accordingly, the panel investigated ways for the Navy to maximize the utility of major fielded/planned terminal programs, to augment critical communications services in a cost-effective manner, to incorporate dual-use systems, and to affect DOD's opportunity for modernization decisions on its scheduled military satellite communications. To correct, within the above constraints, the performance shortfalls of current systems, the panel established five goals for NAVSATCOM-21: increased capacity; improved interference (antijam) and detection (low probability of intercept) protection; interoperability; flexible connectivity; and small terminal size. To meet these goals, the four enabling techniques listed in Table 4.1 were used. As indicated, spacecraft processing and switching contributes to accomplishing all five goals. Similarly, this technique, as well as use of higher frequencies, standard/robust transmission formats, and directive antennas, is integral to achieving robust 45

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communications connectivities, especially at higher data rates and/or from small terminals. The use of standardized transmission formats is a key element in achieving interoperability among naval units and with U.S. and allied forces. TABLE 4.1 Implementation Approaches " ^~_^_^^ Enabling Techniques Architecture Goals ~~ — ^.^ Standard/Robust Transmission Formats Spacecraft Processing and Switching Higher Frequencies Directive Antennas Increased Capacity X X X Improved Interference (AJ) and Dete- ctability (LPI) Protection X X X X Interoperability X X Flexible Connectivity X Smaller Terminals X X X Directive antennas on a communications satellite concentrate their receiving and transmitting capabilities on regions of interest and thereby permit capacity increases and/or terminal size reductions. They also discriminate against interference sources that are outside the region. (Such antennas can also be given the capability to null interference sources located within a region of operations.) Finally, directive satellite antennas contribute to LPI protection by allowing the use of lower power terminals. Figure 4.2 shows many of the considerations involved in selecting military communications satellite frequencies. Uplink bandwidth allocations increase considerably with the higher frequencies, ranging from approximately 100 MHz at UHF (300 MHz) to 500 MHz at SHF (X-band; 8 GHz) to 2,000 MHz at EHF (44 GHz). The increased bandwidth can accommodate larger capacities and/or spread spectrum modulations for improved AJ and LPI protection. As indicated, the uplink interference level required to disrupt a specific data rate from a fixed-size terminal is highest at EHF and lowest at UHF. Uplink satellite antenna discrimination can raise the required interference level by an additional 20 to 30 decibels (dB), but the relative antijam performance of the three frequency bands is the same. These results are the consequence of the wider bandwidth at EHF. Similarly, the detection footprint within which an airborne interceptor must be located to detect a small terminal's transmission is much smaller at EHF than at the lower frequencies. On the other hand, weather effects (especially rain) have a much larger impact on the EHF uplink (44 GHz) and upper SHF downlink (20 GHz) frequencies than on those hi the X-band or UHF regions. These frequency considerations suggest that a system needing significant levels of AJ and/or LPI protection could use the EHF band and incorporate sufficient link margin to overcome the effects of weather. 46

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UPLINK ALLOCATIONS FREQUENCY BANDWIDTH (MHz) UHF SHF EHF 100 500 2000 NOMINAL LINK MARGIN ALLOWANCES (dB) FOR WEATHER* FREQUENCY UPLINK DOWNLINK UHF 0 0 SHF 1 1 EHF 12 5 INTERFERENCE PROTECTION • 99% AVAILABILITY; 20' ELEVATION ANGLE; MID-LATITUDE LOCATION (REGION D) DETECTION FOOTPRINTS UJ o z IU oc UJ u. oc Ul WITH AJ WAVEFORM ANTENNA DISCRIMINATION WITH AJ WAVEFORM IU _J 5 UHF SHF EHF UPLINK FREQUENCY DIRECTION TO SATELLITE t 8 GHz MILES FUTURE AJ/COVERT SYSTEMS USING EHF FOR INCREASED CAPACITY AND ROBUSTNESS; INCORPORATING SUFFICIENT LINK MARGIN FOR WEATHER FIGURE 4.2 Frequency considerations. Another significant factor in the design of a satellite communications architecture is the type of satellite processing used. The fundamental choice is between a transponding and a processing pay load. In the former, the signals received on the uplink are shifted in frequency, amplified, and relayed on the downlink. The processing payload removes any spread spectrum features and demodulates the uplink signal before on-board routing and downlink modulation. As indicated in Figure 4.3, processing pay loads offer significant advantages when multiple users are simultaneously accessing a satellite. The processing removes the need to carefully power balance the users, allows the downlink power to be concentrated on the disadvantaged services, permits efficient on-board routings, and allows independent optimization of uplink and downlink resources. Dynamic networks with a changing mix of large and small terminals are much easier to accommodate with processing payloads. In addition, only a processing payload can prevent interference sources from robbing downlink power and thereby denying a small terminal the full AJ protection that the system bandwidth should afford. 47

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SATCOM PERFORMANCE IMPROVED BY SIGNAL PROCESSING Multiple Access — Reduces need to power balance — Allocates downlink power where needed — Connects users in different narrow beams — Optimizes uplink and downlink resources independently f CURRENT UHF, SHF, ( COMMERCIAL N^ SYSTEMS Jamming i J TRANSPONDER — >. S PROCESSOR EHF SYSTEMS Prevents power robbing Allows small terminal to get full antijam protection of system bandwidth FIGURE 4.3 Processing versus transponding satellites. Although more capable, processing communications pay loads are more complex than transponding ones, to date, most UHF, SHF, and commercial communications satellite systems have been implemented with transponders. The developing EHF systems are incorporating processing for the increased protection and flexibility discussed earlier. In defining NAVSAT- COM-21, the panel sought to incorporate an effective mix of both types of satellite payloads to enhance performance and leverage from current systems in a cost-effective manner. 4.2 ENHANCEMENT OF FUNDAMENTAL SERVICES The requirements discussion hi Chapter 2 mentioned three types of services: hard core, soft core, and general purpose. The first two types need AJ and (in some cases) LPI; the third does not. Table 4.2 shows qualitatively the mix (by volume) of most of these service types at each of the four hierarchical levels of NAVSATCOM-21 connectivities. As indicated, the required hard-core capacity is low at every level. However, the soft-core capacity needs are 48

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greatest at the tactical level; the general-purpose capacity requirements peak at the global and theater levels. In addition, the panel observed that the total throughput needs are much larger (about two to three orders of magnitude) than the hard-core capacity requirements alone and that soft core is a significant portion of the total capacity needs. (The possibility of high- or very- high-data-rate [HDR/VHDR] links to sensor/weapon platforms is addressed separately in Section 4.3.) TABLE 4.2 General Navy Communications Mix (by Volume) LEVEL HARDCORE SOFT CORE GENERAL PURPOSE Global Low Low High Theater Low Moderate High Regional Low Moderate/High Moderate/High Tactical Low High Low/Moderate Implication: Total throughput > > Hard core NOTE: HDR/VHDR links to sensor/weapon platforms considered separately. Table 4.3 summarizes the requirements to satisfy the objectives the panel established for NAVSATCOM-21. For capacity, the goal was to increase the link capacity available to most units from the typical current level of a few kbps to the Mbps range. At the same time, the current limited AJ/LPI protection was to be extended to the levels required for the hard-/soft- core services. Additional goals included flexible, easily reconfigurable, and interoperable connectivities. Achieving these objectives at limited cost requires leveraging planned DOD and commercial communications satellite efforts, minimizing the number of types of shipborne terminals, and maintaining compatibility with current Navy communications and networking modernization efforts (Copernicus and CSS). TABLE 4.3 Requirements Satisfaction FEATURE CURRENT NAVSATCOM NAVSATCOM-21 GOALS Network Capacity Protection Connectivity Interoperability kbps Limited Mbps Hard/Soft Core Flexible Transmission/Baseband Standards NAVSATCOM-21 achieves its goals at limited cost by Leveraging planned activities in DOD and commercial SATCOM Minimizing the number of shipborne terminal types Maintaining compatibility with Navy communications and networking modernization efforts (Copernicus, CSS) 49

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4.2.1 Hard-Core Communications As described in Chapter 2, hard-core communications need the highest protection. In addition, they are typically low-data-rate (<9.6 kbps per channel) but often have to be supported from small, mobile terminals. The Navy (and DOD) are planning to implement hard-core capabilities using processing communications satellites operating at EHF. Two test pay loads (the EHF packages on FLTSATCOM-7 and 8) are currently on orbit and have been used to verify the potential of this frequency band and satellite-based signal processing. MILSTAR, which should be launched in 1993, is planned as the backbone, worldwide, hard-core system for all of DOD. These satellites will also have crosslinks and highly protected Fleet Broadcast (FLTBDCST) injection capabilities. In addition, the Navy is developing an EHF package that will be carried by the UHF Follow-On satellites beginning with the fourth flight, which is scheduled for 1994. These packages will augment hard-core communications and protected FLTBDCST injection capabilities. An extensive Navy deployment of EHF terminals is under way. More than 150 have been procured, and more than 60 additional terminals will be procured within the next 5-year budget cycle. The panel endorses the Navy's approach to implementing hard-core communications at EHF. In addition, the panel observes that polar coverage was not available from the planned MILSTAR and UFO deployments. Such service could be provided by a modest payload (200 to 400 pounds [lb]) that could be a secondary payload on an appropriate host or the primary payload on a small satellite. Due to the critical nature of hard-core communications, the panel also addressed the role of crosslinks in maintaining connectivities. Most ship-to-ship and other links within a region of operations can be provided by a single satellite and, hence, would not use crosslink relays. However, many fleet-to-shore links are longer range than those in a regional operation. Such links can reach U.S.-controlled territory (CONUS, Hawaii, or Guam) with a single satellite hop if there are few constraints (e.g., due to international frequency coordinations) on orbital locations. If orbital placements are seriously restricted, at least one additional gateway/relay site in the Indian Ocean area is required (i.e., Diego Garcia). Crosslinks become essential for worldwide connectivity if properly placed gateway/relay sites are unavailable. 4.2.2 Soft-Core Communications Soft-core connectivities require considerable antijam and LPI protection, yet the required data rates (up to the Mbps range) are one to two orders of magnitude higher than those for hard- core communications. An important observation is that all Navy platforms that have a soft-core requirement also have a hard-core one. At the time of this study, there was considerable debate within the DOD community about the best method for satisfying soft-core requirements. Accordingly, the panel considered a number of options, including military communications satellites operating at EHF and SHF and commercial SATCOM systems. 50

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Representative soft-core communications capabilities that could be realized at EHF are indicated in Figure 4.4. Terminal antenna and transmitter parameters (1-m aperture and 250 W, respectively) corresponding to the LDR shipborne units now being procured were assumed. A processing EHF payload with a range of uplink (44 GHz) and downlink (20 GHz) antenna coverage capabilities was used. The coverage areas included earth coverage (implying an 18° beamwidth for payloads at geosynchronous altitude) and 800- and 400-nautical-mile (nmi)- diameter theaters of operation (2° and 1° beamwidths, respectively). As shown in Figure 4.7, the example terminal can achieve mid-range MDR services across an 800-nmi satellite uplink beam (2° beamwidth) and full-range MDR services from a 400-nmi theater beam (1° beamwidth). With a very modest 10-W, 20-GHz satellite transmitter, multiple soft-core uplink streams can be carried on the downlink to the example terminal (or to other users). Of course, shore terminal installations with larger antennas and/or transmitters could achieve greater capabilities. It should also be noted that the initial plans for the MILSTAR II (MDR) system include at least four times more downlink capacity than the payload in Figure 4.4. PROCESSING PAYLOAD T COVERAGE RATE EARTH 10 KBPS 800 NMI 500 KBPS 400 NMI 2MBPS 10W 44 GHz 20 GHz COVERAGE RATE EARTH 40 KBPS 800 NMI 2M8PS 400 NMI 8MBPS • SHORE LINKS HAVE GREATER CAPABILITY • MILSTAR II OFFERS FOUR TIMES GREATER DOWNLINK CAPABILITY FIGURE 4.4 Soft-core communications: representative EHF capability. 51

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EHF MILSATCOM systems were originally envisioned as providing highly protected (LDR) links to small, mobile terminals. As such, all planned EHF systems incorporate directive antenna and processing payloads. Hence, adding MDR capabilities to these systems primarily requires incorporating the appropriate signal processing features into the terminals' modems and the payloads' demodulation/remodulation subsystems. Furthermore, DOD has already restructured the MILSTAR program to incorporate an MDR capability on the fourth satellite, which at the tune of this study is scheduled for launch in late 1998 or early 1999. The panel also observed that emerging pay load technologies would permit small LDR/MDR packages (100 to 200 Ib) to augment EHF service in polar regions or other critical areas. In addition, a proposed ARPA space demonstration1 (ASTEC; 1996 launch) would provide an early MDR EHF terminal test opportunity. To exploit this EHF option for soft-core service, the panel believes that the Navy should consider the following four actions: • Develop an MDR modem enhancement for the LDR terminals now being deployed on all combat ships. • Influence DOD MILSTAR II planning activities to ensure the incorporation of appropriate capabilities for Navy needs. • Consider, at an appropriate programmatic tune, an MDR enhancement for the EHF package on the UFO satellites. • Explore possibilities for polar service. The second option the panel addressed for soft-core communications capabilities involved satellite systems operating at SHF (8-GHz uplinks/7-GHz downlinks) with a transponder-type payload. As shown in the example configurations in Figure 4.5, the assumed shipborne SHF terminals' parameters2 (4-ft antenna apertures and 500-W transmitters) are consistent with recent Navy development/procurement planning activities. The assumed payload uplink antenna coverage capabilities include Earth Coverage (18° beamwidth for payloads at geosynchronous altitude) and a 1,200-nmi theater of operations (3° beamwidth), while the downlink antenna coverage capabilities include Earth Coverage and 3° beamwidths as well as a 5° beamwidth (2,000-nmi theater of operations). As shown in Table 4.4, the DSCS III SHF communications satellites can produce all of these coverages, although the uplink beams are typically configured for Earth Coverage. With the 10-W payload downlink transmitter3 in Figure 4.5, the example terminal can only achieve the low end of MDR data rates (60 kbps) when operating into Earth Coverage uplink and downlink antennas. However, by using a typical configuration involving an Earth Coverage uplink beam and a 5° downlink beam (2,000-nmi coverage), this terminal 'ARPA did not receive the requested funding for a fiscal year 1993 initiation of ASTEC. 2Although these terminal parameters are larger than the corresponding values used in the above soft-core EHF service example, the panel does not believe that the differences would significantly affect relative comparisons among the options being considered. 3DSCS III has four 10-W transmitters and two 40-W transmitters. 52

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can potentially achieve mid-range MDR rates (500 kbps). Alternatively, by using the most directive pay load beams assumed in Figure 4.5, multiple MDR channels could be supported (5 Mbps total rate). As with the previous EHF example, larger sized shore-based terminals could achieve greater capacity. On the other hand, it is important to note that other users in the SHF pay load's transponders could limit the achievable ship-to-ship rates to 10 percent or less of the values shown in Figure 4.5. The indicated example capacities were obtained by assuming that the terminal had exclusive use of a transponder. TRANSPONDER PAYLOAD 10 W UPLINK/DOWNLINK COVERAGE RATE EARTH / EARTH EARTH / 2000 nmi- 1200nml/1200nml 60 Kbps 500 Kbps 5 Mbps • OTHER USERS MAY UMIT SHIP-TO-SHIP RATES TO 10% OR LESS • SHORE LINKS HAVE GREATER CAPABILITY FIGURE 4.5 Representative SHF capability. TABLE 4.4 DSCS III Antenna Coverage Capabilities COVERAGE UPLINK ANTENNA(S) DOWNLINK ANTENNA(S) Earth (18° beam) 61-EC Horn EC Horn 61-beam Multiple Beam Antenna (MBA) 19-beam MBA 2,000 nmi (5° beam) 61-beam MBA 19-beam MBA 1,200 nmi (3° beam) 61-beam MBA Gimballed Dish Antenna (GDA) NOTE: Typical MBA configurations are 61-beam uplink MBA -» earth coverage (18° beam) 19-beam downlink MBA -» 5° beam 53

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The SHF service example led the panel to conclude that an SHF/transponder-based satellite system (e.g., DSCS III and several allied SHF satellite systems) could potentially provide soft-core communications service, but the capacity achieved by a particular terminal is affected by the presence of other users more than with processor-based payloads. There are enough DSCS III and allied satellites either on orbit or in storage (DSCS Hi/construction, allied) for service into the next century. However, most of these systems, which have been configured primarily for connectivity among geographically distributed large ground terminals, are already significantly loaded with other users. Thus, the capacity that would be available to Navy shipborne terminals would typically be limited to the lower end of the MDR range. Improved Navy afloat SHF service would require some combination of the following two items: • Larger allocation of pay load power/bandwidth (e.g., dedicated transponder[s]). • Increased satellite transmit power/antenna directivity (DSCS replenishment/ follow-on deployments are not currently planned until 2000 4-, although DOD is considering modifying some of the DSCS III satellites now in storage). In addition, the Navy would need to procure significantly more SHF terminals if this option is selected as the primary method for soft-core service. At the time of this study, approximately ten SHF terminals were installed on ships, and an additional 31 were budgeted in the Program Objective Memorandum (POM) for fiscal years 1994 through 1998. The third option the panel considered for soft-core service used commercial communications satellite systems. Of the current operational systems, only those at C-bands (6- GHz uplinks and 4-GHz downlinks) and Ku-bands (14-GHz uplinks and 12-GHz downlinks) have sufficient bandwidth for the data rates associated with MDR. Directive shipboard antennas would be required in these frequency bands, as is the case in the military SHF and EHF bands. The commercial payloads are transponder based, and, consequently, the performance achieved by a particular terminal would be subject to the loading in the transponder that it is using. Because of the proximity in frequency of the C-, military SHF, and Ku-bands, as well as the transponder-type payloads used in all three of these frequency bands, a C- or Ku-band ship terminal would be similar in size to the one in the military SHF example (Figure 4.5) and the resulting capacity would be approximately the same (i.e., the lower end of the MDR range with earth coverage pay load antennas and the upper end with payload antennas with high directivity). Such soft-core, commercial communications satellite terminals would have to be significantly more capable than the commercial terminals currently installed hi Navy ships, which provide only LDR services. Two other factors could be important in the Navy's considerations of this soft-core option: • Much of the service may be from international organizations (such as INTELSAT or INMARSAT), since domestic communications satellites have limited coverage of ocean areas. • Several militarily significant features (e.g., highly protected telemetry and command systems, adaptive payload antennas, and military control of resources) may not be available on commercial service satellites. 54

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After carefully considering each of die above iiree crams. 3ie the Navy use EKF as its primary soft-core sysan. T-.'s rcorc j< ranrs rro^s: recaps; re higher annum LPI thresholds ai EHF and payicvac rrrczssr^: ric: .ibrcurxes rerfrnnKacs frrcr. user mix/ Also, the EHF option could be i — -';-— t—igid r> erra-iap grrrrak -*>» X^vr ias already procured or programmed for hard-core LDR service.. Avroirs satellite communications anrmna* on many of die Xrry"$ piarjcnns m*$ ac. rnxrao: consideration to the panel. The panel recognizes that DOD-s omen pians far XCLSTAS 3 ar- ^tx ctZ frr deplo>Tnem of ^fDR-capabk paylcvads irnnl hie 1??5 or or;y 1??$. recommeods that the Na\y deploy an innhii SHF sofi-cacr cacabiirv Due to the DSCS HI adaptive uplmk aiceina •el-bean MSAi aa± afjcary coccri syaan. interim serv-ice at this frequency band wi3 be becser ptmeced rh«r if r -»ee ir cnozKreiL: bands. In addition- the panel recommends thai me Savy r^cscpc a lr*»-ccs_ frequency-band xn 1.5 XTr per channel). However, there are no protection requirements. All Na\y sirs are eo UHF satellite communications terminals that provide LDR general-parpose c (typically at 2.4 kbps). The planned deplo>-ment of yrr.-wnarv- ffa^nOTv^ asscaec n^^jrpit access (DAMA) capabilities will provide a three- to fourfold capacity increase <.to aKxc IT ttcs per terminal). In addition, some ships have INMARSAT terminals mat provjae aoiaroal LDR sen-ice at a few kbps. Several other commercial mobQe satellite oommcnicaboaxs s> 'Satellite-based signal processing has been incorporated into onN EHF saeLnr ajiiiiiiinivaajag syacns x As time. It could potentially also be employed on SHF or co^nerr.£ systems, bm n wouic pctihtti:> IK mMnracihit with the large numbei of terminals already deployed in these bands. 55

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in the planning stage. They could be used to further enhance shipborne LDR capacity. However, even with all of the above systems, the panel discerned a shortfall in meeting the projected general-purpose capacity requirements. The panel recommends that the Navy consider the following two actions: • Improve the efficiency of UHF spectrum utilization. With modern technology, achieving modulation efficiencies of 2 to 3 bits/Hz should be possible for a general-purpose satellite communications system. This would imply 50 kbps (or more) through a 25-kHz UHF payload transponder such as those on the FLTSATCOM and UHF Follow-On satellites. • Utilize additional commercial capabilities for those platforms with remaining general-purpose requirements. Again, the panel observes that the use of a low- cost ("commercial style") multiband terminal (C-, X-, Ku-bands) would enhance flexibility by permitting connectivity via a number of satellite communications services (i.e., NASA's TDRSS, DOD/allied SHF, and commercial satellites). Also, a multibeam capability would further enhance the utility of such a multiband terminal. 4.3 SERVICE EXTENSION OPTIONS The panel believes that a key aspect of NAVSATCOM-21 is incorporation of the enhancements of fundamental (hard-core, soft-core, general-purpose) communications services discussed in Section 4.2. However, the panel also wants the goal architecture to be readily extendable to include other connectivity needs that might arise. This section contains descriptions of two such possibilities involving satellite communications with airborne Navy assets. 4.3.1 HDR/VHDR Sensor Connectivity The first scenario includes a high- to very-high-data-rate relay (HDR/VHDR; up to approximately 300 Mbps) of information from an airborne sensor. This is a difficult task for a satellite communications system and should be seriously considered only if line-of-sight communications links are not available. As shown in Table 4.5, two frequency bands (EHF and Ku) were considered. For both, the use of Common Data Link (CDL) signaling formats and transponder-based pay loads was assumed.5 Two types of airborne platforms were envisioned. One involved an aircraft-mounted sensor with a terminal that has a 2-ft aperture and a 100-W transmitter. This platform was assumed to be flying at an altitude of several thousand feet, so that it is above much of the weather that could degrade EHF links (i.e., a modest link margin allowance was included). The other platform involved a cruise missile that could accommodate 5A processor-based payload for HDR/VHDR would require considerable payload power. Also, the current Ku- band TDRSS system is transponder-based. 56

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only a 6-in. aperture satellite communications terminal with a 100-W transmitter. This second platform was assumed to be flying at low altitudes, so that full EHF weather margins had to be included in the link calculations (Figure 4.2). TABLE 4.5 HDR/VHDR Sensor Connectivity Two frequency bands considered; both use CDL signaling • Selected high-altitude aircraft (AC; 2 ft, 100 W) • Low-altitude cruise missile (CM; 6 in., 100 W) EHF 6-ft S/C antenna (100 nmi footprint) 4 Mbps from CM 300 Mbps from AC Downlink to < 10-ft terminal Ku-band (TDRSS) 16-ft S/C antenna (120-nmi footprint) 30 Mbps from CM 300 Mbps from AC Typically downlinks to CONUS ground terminal NOTES: Data compression would ease communications impact Multiple beams needed for large area of operations Larger terminals/lower data rates -»smaller satellite antennas TDRSS availability/EHF transponder deployment considerations As shown in Table 4.5, a 6-ft EHF satellite antenna could support the relay of 4 Mbps from a cruise missile to a 10-ft ground terminal. Alternatively, 300 Mbps could be relayed from an aircraft installation. Similarly, the 16 ft Ku-band antenna on NASA's TDRSS could support 30 and 300 Mbps from cruise missiles and aircraft, respectively, to a large ground terminal (currently based in CONUS). The footprints of the two example satellite uplink antennas are very narrow (100 nmi for the 6-ft EHF antenna; 120 nmi for the 16-ft Ku-band antenna). This means that multiple antennas would be needed for concurrent coverage of large areas of operations. The panel also made the following observations: • Data compression techniques on the sensor platform would ease the impact on the communications system. • High data rates imply low protection. • Larger terminals (perhaps difficult for airborne platforms) and/or lower data rates would result in smaller satellite antennas (i.e., lower cost, more easily implementable pay loads). • TDRSS availability and/or EHF transponder deployments would have to be considered. 57

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4.3.2 LDR/MDR Beyond-Line-of-Sight (BLOS) Connectivity The second extension service scenario considered also involved satellite communications connectivities to airborne assets (e.g., cruise missiles, UAVs, tactical aircraft, helicopters). However, in this scenario the desired data rate is in the LDR (< 10 kbps) to MDR (< 1.5 Mbps) range. Two possible approaches were considered. One involved the use of directional antennas (6-in. aperture at EHF or SHF) on the airborne platform, while the other used smaller omni- directional antennas (at UHF) that do not require a stabilized pointing mechanism. In both approaches, a 10-W airborne transmitter was assumed. As shown in Table 4.6, LDR service can be provided either by an EHF approach with an Earth Coverage satellite uplink antenna or by a UHF technique. Alternately, the use of a directive EHF uplink antenna (1° beamwidth; 400-nmi coverage) would result in mid-range MDR capabilities. Similar results would be achieved with an SHF pay load, but the protection would be somewhat lower and the payload antennas would be physically larger. Since corresponding payload antenna directivity at UHF would not be feasible, the EHF (or SHF) option should be pursued if MDR connectivities to airborne platforms are required. In addition, EHF (or SHF) equipped aircraft at altitude (6-in. aperture, 10-W transmitter) could achieve data rate increases of approximately two orders of magnitude (to the upper end of the MDR range with 1° payload antenna beams). TABLE 4.6 LDR/MDR BLOS Connectivity EHF* UHF* Uplink Rate Downlink Rate FLTSAT MUBL** Earth Coverage 400-nmi Coverage 150 bps*** 25 kbps*** 600 bps*** 120 kbps*** 1.2 to 2.4 kbps 2.4 kbps Terminal Parameters 6-in. antenna; 10 W Tx OMNI antenna; 10 W Tx Cost Comparable to cruise missile < < cruise missile j Multipart! Degradation Negligible Severe Negligible Antijam High None Moderate Detectability Low High Moderate * Similar results for SHF (except less protection) ** Multiple path BLOS communications system (ARPA/Army-funded R&D activity) *** lOOx capability increase for aircraft at altitude (6 in.; 10 W Tx) Other points of comparison between the approaches are also shown in Table 4.6. Antijam and signal detectability performance, as well as resistance to multipath degradations, is significantly superior in the EHF (or SHF) option. However, the cost of an EHF (or SHF) 58

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terminal with its directive airborne antenna may approach the cost of a cruise missile. On the other hand, the cost of a UHF terminal (with an omni-directional antenna) is modest, but the protection afforded in this frequency band for the example LDR signals is low. The cost advantages of UHF could possibly be preserved and the protection increased to moderate levels by using the Multiple Path BLDS Communications System (MUBL) concept, a CDMA-type technique that is compatible with existing UHF pay load transponders. This technique is presently an ARPA/Army-funded R&D activity. 4.4 INTEROPERABILITY/NETWORK CONTROL CONSIDERATIONS The NAVSATCOM-21 goal architecture accommodates a variety of link and baseband signal standards to promote increased interoperability among multiple-service forces of the United States as well as with appropriate allied units. As shown in Table 4.7, the architecture provides for interoperable EHF/processor-payload-based service at both LDR (MILSTD 1582C) and MDR (MILSTD 188-136). The former has been published by the United States and has been shared with the United Kingdom, Canada, and France. It is expected to become the basis for an eventual NATO Standardization Agreement (STANAG). MILSTD 188-136 is currently under development in the United States, and because of international interest in interoperable EHF/MDR systems, preliminary planning discussions have already been held with the United Kingdom and Canada. TABLE 4.7 Interoperability Multiservice and allied Link and baseband standards required — EHF: MILSTD 1582c (LDR) Shared with United Kingdom, Canada, France NATO addressing STANAG MILSTD 188-136 (MDR) United States developing Discussions with Canada, United Kingdom — SHF: Universal modem (up to 64 kbps) United States, United Kingdom, France developing STANAG being prepared Other interoperable modems exist — UHF: DAMA and other standards exists (25/5 kHz channels) Being incorporated into equipment — High/very high data rates: CDL standard Compatible with transponded systems at SHF, EHF, C-, Ku-bands At SHF, the United States, the United Kingdom, and France are developing the Universal Modem, which will provide interoperable service over SHF/transponder-based pay loads at data rates up to 64 kbps. A NATO STANAG based on the Universal Modem signal formats is being 59

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prepared. The Universal Modem features are fully compatible with NAVSATCOM-21. In addition, there are other interoperable SHF modems and several evolving SHF DAMA techniques that the architecture accommodates. Similarly, the emerging UHF DAMA and other link and baseband standards for both 25-kHz and 5-kHz UHF SATCOM channels can be used with NAVSATCOM-21. The architecture is also compliant with the provisions of the CDL standard for HDR/VHDR service over transponded systems at SHF, EHF, C-, and Ku-bands. NAVSATCOM-21 has four provisions for improved network control: • UHF (and SHF) DAMA techniques for more efficient use of transponder-based satellites. • Signal processing "switchboard" features inherent in all planned EHF payloads. • Compatibility with other Navy communications network modernization activities (e.g., Copernicus/CSS). • Utilization of GPS for accurate location and timing information in order to permit rapid spatial acquisition (for platforms with directive antennas) and fast COMSEC/TRANSEC equipment synchronization. Obviously, there would be operational advantages if all Navy SATCOM-capable platforms were equipped with GPS receivers. 4.5 DEPARTMENT OF DEFENSE CONTEXT The panel envisioned that NAVSATCOM-21 would be implemented as part of a DOD- wide MILSATCOM architecture evolution. Figure 4.6 shows the overall plan in which the existing baseline architecture (involving four space segments) would be modernized during the 1990s to better satisfy communications requirements, improve user flexibility/interoperability, and reduce costs. Over this period, there would be key decision opportunities at which the configuration of the next generation of a segment of the architecture would be decided. Concurrently, technologies for improved spacecraft, terminals, and networking and control would be developed and demonstrated. As indicated, technology insertion opportunities for the space segment portions generally correspond with the decision opportunities. Some of the planned key DOD MILSATCOM decision opportunities are shown in Table 4.8. The panel thinks it is important for the Navy to leverage these to expedite implementation of portions of NAVSATCOM-21.6 6In the fall of 1992, Congress directed a restructuring of the MILSTAR system to increase its use by tactical forces. In addition to reducing its nuclear survivability, an MDR capability was added to the system. In the spring of 1993, as a result of an OSD MILSATCOM Bottoms-Up review, the MILSTAR system was reconfigured as a two LDR and four MDR satellite. The issue regarding polar coverage was deferred to a later date, and UHF and commercial systems were recommended to support general-purpose communications requirements. These actions reinforce the NAVSATCOM-21 architecture recommended in this report. 60

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BASELINE ARCHITECTURE COMMERCIAL UFO DSCS MILSTAR MODERNIZATION DECISION \S OPPORTUNITIES KEY TECHNOLOGY DEVELOPMENT AREAS TECHNOLOGY INSERTION OPPORTUNITIES SPACECRAFT TERMINALS NETWORKING AND CONTROL DEMONSTRATION OBJECTIVES: • SATISFY COMMUNICATIONS REQUIREMENTS • IMPROVE USER FLEXIBILITY/INTEROPERABILITY •REDUCE COST FIGURE 4.6 MILSATCOM architecture evolution. TABLE 4.8 DOD MILSATCOM Decision Opportunities OPPORTUNITY TIME FRAME EXPECTED EMPHASIS 1 1A : 3 4 1992 1993/1994 1993/1994 1997/1998 1998 Mid-1990s EHF systems Polar coverage SHF replenishment and commercial Post-2000 EHF systems UHF replenishment Two joint ARPA/service technology initiatives planned • ASTEC small LDR/MDR/VHDR EHF satellite • IMPACT terminal technology program 61

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4.6 FINDINGS AND RECOMMENDATIONS The panel concludes that an adaptable, affordable, global satellite communications system for evolving Navy requirements is feasible. This overall finding is supported by the following results from the areas investigated: • Projected EHF capability (augmented with MDR) satisfies Navy hard- and soft- core requirements. • UHF can be enhanced, for tenfold increase in general-purpose capacity. • SHF can provide near-term, limited soft-core/general-purpose MDR capability for selected platforms. • Commercial SATCOM could augment general-purpose service. INMARSAT currently utilizes LDR. Other mobile services possible for LDR. Ku-/C-bands for MDR. • Standards for interoperability are well under way. • Direct (single hop) connectivities are possible to U.S. territory with most satellite deployments. • Links to small airborne platforms are feasible. • There are benefits to having GPS on all SATCOM-capable Navy platforms. Figure 4.7 contains an overview of the NAVSATCOM-21 goal architecture for Navy satellite communications. The basic services are obtained via EHF (hard /soft core), SHF (interim soft core on selected platforms; possibility for long-term backup via multiband terminal), UHF (general purpose), and commercial space segments (general purpose). All of the links conform to interoperability standards, and connectivities can be provided to airborne platforms. It is noteworthy that only two basic types of terminals are required for most ships: EHF for hard-/soft-core communications and UHF for general-purpose service. Selected platforms may have additional installations for interim soft-core communications at SHF and/or augmented general-purpose service via commercial satellites. As previously noted, a multiband terminal (C-, X-, and Ku-bands) would offer enhanced flexibilities for selected platforms. The panel developed the following five general recommendations pertinent to the implementation of NAVSATCOM-21: • Foster utilization of EHF. — Primary hard-/soft-core backbone for mid to far term. • Improve efficiency of DOD UHF. • Continue SHF soft-core/general-purpose service for major combatants. — Near-term interoperable LDR/MDR soft core. — General-purpose MDR capability. • Use commercial services for general-purpose supplement if — Additional services/capacity is needed. — Service cost is low. — Shipboard installation is affordable. 62

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Investigate low-cost, multiband (SHF and commercial), perhaps multibeam, terminal as flexibility-enhancing feature. SHF UHF EHF COMMERCIAL SATELLITES • EHF: MILSTAR/MILSTAR II UFO PACKAGES AUGMENTATION: POLAR, ETC • UHF: FLTSAT, UFO •SHF: DSCS, ALLIED •COMMERCIAL CONNECTIVITY TO AIRBORNE PLATFORMS • HARWSOFT CORE: EHF • INTERIM SOFTCORE: SHF -POSSIBILITY FOR LONG TERM BACKUP V)A MULTI- BAND TERMINAL • GENERAL PURPOSE UHF COMMERCIAL • LDR: UHF OR EHF •MDR: EHF •HDR/VHDR: MODIFIED EHF OR Ku BAND (TDRSS) ALL LINKS CONFORM TO INTEROPERABILITY STANDARDS ALL NODES WITH FULLY INTEGRATED GPS MOBILI ERMINAL TYPES AIRCRAFT -UHF -EHF ._ - MULTI-BAND SHF - COMMERCIAL FIGURE 4.7 NAVSATCOM-21 overview. To achieve the potential of NAVSATCOM-21 in a timely manner, the panel developed the suggested actions listed in Table 4.9. As noted, these suggested efforts were keyed to the general recommendations as well as to enhancing capabilities to airborne platforms and to leveraging other DOD MILSATCOM activities. The suggestions have been partitioned into 63

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three time periods: near term (1992-1997), mid-term (1997-2005), and far term (2005-2015). Some of the potentially highest leverage activities are the suggested near-term efforts to • Demonstrate an MDR upgrade for the LDR EHF terminals being deployed. • Consider an MDR upgrade/backfit for the UFO/EHF package. • Demonstrate low-cost/multiband/multibeam SATCOM terminal. • Continue SHF DAM A developments. TABLE 4.9 Suggested Actions NEAR TERM MID-TERM (1997-2005) FAR TERM (1992-1997) (2005-2015) Foster EHF Continue LDR deployment Demonstrate terminal upgrade for MDR Consider MDR backfit to UFO Deploy MDR Augment space segments Add HDR Add EHF buoy for subs Deploy airborne terminals Exploit DOD UHF Demonstrate and develop 10-fold enhancement Field upgraded modem Shift to commercial Utilize DOD SHF Track universal modem development Demonstrate low-cost multiband/beam terminal Install universal modem (if affordable) Field multiband/beam terminal Exploit as available Commercial Services Selective installation for additional general-purpose capability Small, Airborne Platforms Develop cruise missile terminal Deploy cruise missile terminal Increase data rates Participate in DOD Activities Technology Programmatic ARPA satellite and terminal initiatives Wideband gateways EHF buoy EHF beam hopping MILSTAR II + augmentation SHF replenishment Commercial utilization SHF DAMA Post-2000 EHF UHF replenishment Low-cost replenishment terminals The panel believes very strongly that NAVSATCOM-21 provides economical corrections for current shortfalls in Navy satellite communications (capacity, protection, interoperability, connectivity) as well as the flexibility to evolve in response to changing requirements and technology advances. Aggressive implementation is recommended. 64