Potential Uses of the Military Ka - Band for Wideband Milsatcom Systems

Since the designation in the 1970’s of a military satellite communications Ku-band (30-31 GHz uplink, 20.2-21.2 GHz downlink), these frequencies have held great potential to support U.S. forces and requirements. In 1995, the Ku band was chosen for the Global Broadcast Service (GBSJ which will be operational on the UFO satellites beginning in 1998. However, only a small percentage of the allocated 1 GHz of bandwidth will be used by GBS. During the recent M7L,VATCCM4 architecture reviews, the Ku band was again considered for the Advanced Wideband System (A WS) and the Gapjlller satellites. This paper will discuss the potential uses of the Ka band for the A WS and Gapjiller satellites. Included will be a technical discussion of the potential applications for both tactical andjxed infrastructure requirements in the 2005-to-201 O timeframe. Overview Information is essential for successfid military operations. This has become evident with the initiation of programs including the Navy’s IT-2 1, the Army’s digitization of the battlefield and other DoD information technology programs. By 2010, the DoD has projected a worldwide need to serve -1.8 Gbps of traff]c between fixed user sites that must be satisfied by DoD MILSATCOM. This level of traffic is more than 3 times the current fixed user traffic served by the DSCS III system. An even larger growth factor for tactical (transportable user) traffic is projected. Baseclon the highest levels of tactical user traffic which was carried by DSCS satellites during Desert Storm in 1992, a more than 50-fold increase, -6.6 Gbps is currently projected by 2010 for the two major theater of war scenario (MTW). The DoD is planning to accommodate these large growth factors by expanding the current X-band (DSCS) capabilities into a combined X and Ka-band system known as the Advanced Wideband System (AWS). Because the current DSCS III constellation will not last significantly beyond the 2004-2006 period, an interim system capability, known as the Gapfiller system, will be used to provide service during the phase out period of the DSCS III satellites until the AWS becomes available. One of the purposes of this paper is to explore options for the Gapfiller use of the Ka-band; i.e., which users, tactical, or fixed, (or both) are best served by Ka-band and by which types of terminals and satellite elements. Tactical Use of Ka Band To keep the cost of the Gapfiller within a limited budget, the likely solution will be a geostationary satellite system that complements any residual DSCS III satellite assets for tactical use. Fixed service may be complemented by packages on other systems. One option being considered is a package on the replenishment TDRS satellites planned for launch in the early 2000’s. Potential trade space for the Ka-band services include: (1) determine if onboard satellite processing is required or whether a simple transponder based system is adequate, (2) should AJ protected services be offered at Ka-band, (3) whether sufficient AJ protection is available through spread spectrum (frequency hopping, PN or hybrids) alone, or whether antenna spatial processing would be required. The next-generation Army tactical wideband earth terminal, STAR-T, will be deployed in support of Echelons above Corps (EAC). The Army is considering the addition of the Ka band to the STAR-T terminals. STAR-T’s are U.S. Government work not protected by U.S. copyright now in Limited Rate Initial Production (LRIP) with full production planned for the early 2000’s. LRIP versions will be contlgured with C, X and Ku bands. The C-band WOUIC1 be removed and replaced with Ka, making the terminal an X, Ku, Ka capability. An obvious advantage is the extensive bandwidth (1 GHz) available at Ka, This potentially could help satisfi the 6.6 Gbps of tactical traffic. An analysis was conducted to examine the suppcu-tabilityof requirements at Ka band using a modified STAR-T terminal. Assuming the STAR-T aperture size, the transmit HPA size (in watts), and the coverage area of the satellite remains the same (i.e., the same beamwidth for the satellite receive antenna), then moving to Ka-band increases the terminal antenna gain by F2, The free space loss also increases by the same factor; i.e., the net power received by the satellite remains the same under clear sky conditions. However, the uplink margins that must be added due to rain absorption and scattering effects increases from approximately 2.5 dB at X-band, to more than 12.5 dB at Ka-band thereby requiring almost 10 dB more transmit HPA power at Ka-band. This 10 dB increase in terminal HPA power for Ka-band operation can be avoided by reducing the satellite coverage area by a factor of -3.3 and thus achieving a 10 dB increase in satellite receive antenna gain. Thus, to base the required terminal HPA power simply on the uplink perilormance in thermal noise and rain, we can expect to need a ten times larger terminal HPA at Ka-band or we must be willing to sacrifice the satellite coverage to be less than 1/3 of the coverage at X-band. Payload Design Currently there is no specific design for the Wideband Gapfiller satellite. At this point in the process, the government has a Gapfiller concept to support trade studies and costing activities. The actual design will ultimately be determined by competing contractor proposals based upon government perflorrnancerequirements. The following discussion contributes to the ongoing dialogue on potential design implementations. Subsequent analysis is based upon an unstressed design with no consideration of jamming effects. The first aspect to be examined is the design of the Ka payload. Currently, the Gapfiller concept is a transponder system, Commercial ventures such as Teledesic@and Spaceway@ are considering a processed payload. These systems are investigating on-board processing to reduce the size and cost of customer earth terminals while providing user services in the 10’s of Mbps, and system throughput in the 10’s of Gbps. The curre)ntSTAR-T has a single HPA for C, X, Ku band transmission and is approximately 10% of the earth terminal cost. To maintain the same link availability and achieve data rates in the range of 5 to 20 Mbps, the required Ka band HPA could be 30-50 0/0of the earth terminal cost. Reductions in satellite coverage at Ka-band may be avoided by the use of onboard processing (demodulationkemodulation). Results of a technical trade between processed and transponder payload for the STAR-T requirements are shown in Figure 1.