Site: Aerospace Corporation
Los Angeles, CA 90009-2957
WTEC: William T. Brandon (report author), Neil Helm, Chris Mahle Ed Senesack (Observer, DOD, Office of Space Architect)
Aerospace Corporation is a federally funded research and development center, established to provide technical support to the U.S. Air Force's programs in space and missiles.1 This report focuses on satellite communications.
The discussions touched on a wide range of subjects: the emerging LEO and MEO systems; launch technology and launch costs; need for frequency re-use in military bands; impact of wavelength division multiplexing in fiber; some problems with reliance on cable; protocol issues in the use of satellites for data transmission; need for terminal phased arrays; the widely diverging views of technology evidenced in proposed commercial designs; problems and prospects for military use of commercial satellite systems; acquisition reform in satellites; some advanced system studies; and some innovations in contracting.
Based on such observations as the interest France had shown in EHF technology during planning activities for possible joint, international military satellites, it was suggested that the United States was in a leading position in EHF and onboard processing for communications.
Aerospace maintains a very competent on-orbit support capability for launch certification and analysis of parts failures (on-orbit). In addition, independent end-to-end testing has also been accomplished.
Research has been severely de-scoped (due to short-term focus and general budget reductions). However, a small technology demonstrator project featuring micromachines and effort in support of reusable launch vehicles are ongoing.
Planning activities for military satellite communications since 1992 were briefly overviewed (e.g., internal studies, the attempt to define and negotiate an international SHF program, and recent efforts of the DOD Space Architect). The major U.S. military satellites (UHF, SHF, EHF) reach end of life at approximately the same time. While it would be tempting to embrace quantum change, important constraints exist: the investment (dollars and time to deploy) in military terminals` creates a critical need for backward compatibility; advances in waveforms and desire for future flexibility must be balanced with the difficulty of changing waveforms with a processing satellite repeater; and there is a need for higher data rates per channel and more channels, all in a satellite launchable with a medium size or smaller launch vehicle. Effort now addresses an advanced wideband system, intended as an evolution of the EHF system (for increased capacity and link data rates). Envisioned as a four GEO constellation, this is viewed as a transitional architecture. The satellites would carry phase 3 global broadcast transponders. Because of the personal communications revolution, decisions to extend or replace UHF systems will be deferred slightly.
A major aspect of newer constellations in lower orbits is the management problem of large constellations, including handovers during sessions. A trend indicated in these systems was a tendency to make use of existing technology in revolutionary ways, rather than require new technology. An example of activity to extend battery life in hand-held radios was cited. Lithium ion batteries may offer twice the energy density of nickel hydrogen for GEO satellites, but industry is focused on small consumer appliances. Government agencies around the world (e.g., USAF and NASA) are trying to support development of larger (20 to 40 ampere-hour) sizes for space.
Aerospace performed a study for the European Space Agency (ESA) on the communications payload for a satellite providing mobile services to determine if an all digital processing repeater for about 5,000 voice circuits was feasible. The study identified several alternative architectures and how they scaled with the number of circuits. The conclusion was that an FFT polyphase filter bank with narrowband channelization (i.e., three 4 MHz L-band channels), implementable with available chips, resulted in the minimum power and was attractive. The study is believed to have influenced the ICO system design.
In addition to systems using lower altitude orbits and larger numbers of satellites, Internet via satellite and DirecPC signal a change in direction. Protocols used for cabled communications require revision for integration or interoperation with satellites.
The roles of fiber optic and satellite communications for military communications remain uncertain. While performance of fiber (BER 10-10 ) had been equaled by COMSAT/INTELSAT with concatenated error correction codes, the fact remains that more powerful codes introduce sharp thresholds in performance, (i.e., once the threshold is reached by a jammer, the error rate will increase dramatically, but the jammer power required to reach this level is somewhat higher than in the absence of coding). Similarly, the assumption that fiber optic cable will suffice for all military communications needs to be examined in terms of likelihood of reaching critical areas, landing rights, vulnerability of cable landing points, and cable physical vulnerability.
Aerospace foresees a pressing need for frequency re-use technology in military satellite frequency bands due to increases in traffic demand and low probability of greater allocations. This suggests smaller spot beams, digital beam forming, and phased arrays.
Since an objective of reform is to reduce development time, and the transition architecture goals include performance enhancement, it was proposed that an engineering model of a next generation EHF communications payload be funded for risk reduction. The proposed effort was limited to the baseband part of the payload. The general reduction in budgets and the tendency to focus on program specific problems have transformed the nature and quantity of technology development and research performed by Aerospace. A recent Aerospace initiative that runs counter to this trend is a proposal to build and fly an experimental/demonstration microsatellite using nano-electromagnetic components.
Several parallel funded studies (PRDA contracts) were awarded to industry to address the questions of the use of commercial satellite systems for military purposes, namely, is this feasible, what are the issues, and what modifications would be suggested to make this more attractive? Both satellite manufacturers and non-manufacturers were included. The findings indicated clear lack of commercial interest in antijam provisions and modifications, including hardening for prompt nuclear effects.
Requiring satellite delivery three years after contract award led to process reviews. The conclusion was that fixed requirements and assured funding were both needed for reduced development time. There is a tendency for military requirements to be unrealistic at the beginning of a program but also to grow during development. Despite increases in the requirements, the annual funding levels are often not increased; instead, more years are added in development to reduce risk and cover the higher costs. The lead time for developing a contract for military versus commercial satellites was also studied: The need to convince investors that a development is low risk leads to about three years of development prior to commercial program inception; whereas this development is normally a part of a military procurement. The processes are similar but the point at which the clock is started for timing the development is not consistent.
For advanced technology, rather than performing research, an Aerospace site office advises the Air Force Phillips Laboratory on technology development programs for space, including satellite communications.
Performance measures or metrics have come under discussion. When a new military system is proposed, the "requirements" are reviewed by the Joint Requirements Oversight Council. In addition to what is the estimated cost, a pervading question is, "what (or how) good is the new system?" Military commanders desire to quantify the "military utility," which is often difficult. In a commercial context, the market resolves the issue by determining what price is acceptable for a service.
Significant progress has been achieved in lowering cost per kbps for communications satellites. However, cost/pound in orbit has shown little or no progress. This is because neither launch costs nor development costs have dropped significantly. Each pound of communications payload has become more complex due to advances in antenna beam forming, digital signal processing and migration to higher frequency bands.
Aerospace Corporation. March 1997. Selected briefing charts on the Aerospace Corporation's organization, objectives, history, program responsibilities, areas of technical leadership and vision.
N. Feldman, J. Han, D. Ksienski, K. Soo Hoo, T. Tam and K. Woo, Aerospace Corporation, "Tradeoff Study of OnBoard Digital Signal Processing for Satellite-Based Personal Communications," IAF-94-M.2.282, 45th Congress of the International Astronautical Federation, October 1994.