ANTENNAS

Overview

The key trends in spacecraft antenna technology are toward larger effective apertures, significantly higher numbers of beams, and integrating computationally-intensive beam forming and switching activities with other onboard processing functions. These trends are an integral part of universal efforts to raise spacecraft effective radiated powers (EIRP), make communications payloads smarter and more flexible, and make earth terminals smaller and cheaper. Table 3.2 provides a good indication of the near-term state of the art, illustrating the antenna systems that a representative sample of commercial Ka-band operators plan to fly in the 2000-2005 time frame. Many manufacturers offer competing proprietary technologies to build these antennas, and there is no clear world leader. Details of ongoing research and development efforts are generally proprietary. The situation has changed significantly from when large government research programs drove spacecraft antenna technology and quantitative information about the state of the art was reasonably available.

Table 3.2
Characteristics of Planned Commercial Ka-Band Communications Systems

System

Astrolink

Cyberstar

Euroskyway

East

West

Spaceway

Celestri

Teledesic

Sat orbit

GEO

GEO

GEO

GEO

GEO/MEO

GEO

LEO

LEO

Number

5

3

5

 

12.9

20

63

288

Coverage*

Pop. Centers

N.A. Eur., Asia

Eur., Afr., midEst.

Eur., Afr.

Eur., Afr., midEst

Pop. Centers

Global

Global

Beamwidth/pot

0.8°

~1°

~1°

 

0.6°

~1°

   

No. Beams

96

72

32

 

64

24

432u, 260dn

64

Type Sat Antenna

Horn fed

Horn fed

Horn fed

Horn fed

Horn fed

Horn fed

Array

Array

Market

Multimedia

Multimedia

Multimedia

Infrastructure

Multimedia

Infrastructure

Infrastructure

Infrastructure

On Board Proc.

Full

Baseband

Baseband

Baseband

 

Baseband

Full

Full

Through-put

7.7 Gb/s

4.9 Gb/s

   

6Gb/s

4.4 Gb/s

1.8Gb/s

13.3 Gb/s

ISL

V Band

Potentially V

V Band

 

Optical

V Band

6 optical

V Band

Terminals

Fixed

Fixed

Fixed

Fixed, mobile

Fixed

0.66m

Fixed

Smallest size

Not given

0.7 m

 

0.7 m, HH

0.7 m typ.

   

0.15



Source: Third ka-band Utilization Conference (see site report, Appendix B).

A major change noted since our 1992/1993 report is the improvement in the mechanical technology and manufacturing processes associated with spacecraft antennas. For example, while the ISL and gateway antennas (Figure 3.4) manufactured by COM DEV for the Iridium spacecraft represent state-of-the-art electromagnetics, their mechanical characteristics were what impressed the panel most. These antennas provide excellent pointing and tracking characteristics while coming off an assembly line on a one- set per week basis. This is a significant change from spacecraft antennas being individually hand-assembled by highly skilled engineers and technicians.


Fig. 3.4. Iridium satellite gateway antennas (Iridium 1997).

Large Reflector Antennas

There are a number of competing technologies worldwide for building large reflectors. The Lockheed Martin ACeS (Asia Cellular Satellite) spacecraft typify the current state of the art with two twelve-meter antennas and 140 total beams per satellite (see Figure 3.5). The ACeS system will offer GEO-based service to hand-held terminals at L-band.


Fig. 3.5. Prototype Lockheed Martin ACeS antenna (Mecham 1997).

While the 1992/1993 report emphasized efforts (Russian programs in particular) to orbit ever larger antennas, the decreasing coverage areas associated with larger apertures and smaller beamwidths today seem to obviate the commercial need for reflector diameters significantly larger than 12 to 15 meters. Accordingly there is less interest now in large inflatable antennas. In the future we anticipate that competition will be in minimizing mass, surface deviation, thermal distortion, and cost, and maximizing ease of deployment. Thus the French STENTOR spacecraft will carry what is described as an ultra-lightweight 2.4 m reflector.

A portion of the 1992/1993 report described two large-reflector technologies then being developed in Japan that differed from other approaches. One, using robotic assembly in space, has been discontinued. The other, Toshiba's modular approach, will be tested with the launch of the ETS VIII spacecraft.

Toshiba's design is based on hexagonal cross-section modules. Nineteen modules combine to make the ETS-VIII 15 meter diameter reflector with a total mass of less than 170 kg and 2.4 mm rms surface deviation.

Phased Arrays

Companies that the panel visited routinely cited phased array antennas as a critical technology area where cost breakthroughs are needed. Both direct radiating arrays and phased array feeds for reflectors are attractive for multibeam spacecraft antennas that must route traffic dynamically. All major spacecraft and antenna manufacturers seem to be working on phased arrays. Few would reveal any quantitative details, and none was aware of a potential breakthrough area where a sustained R&D program would have immediate impact.

The problems in phased array design remain what they were in 1992. Electromagnetically, the array must maintain the desired radiation pattern and polarization purity over the transponder bandwidth and the desired scan angle range. Electronically, the array must form and steer beams as fast as onboard traffic routing requires. Mechanically, the array structure must deliver control signals and DC power to (and often rf from) the radiating elements and dissipate heat while not screening the radiating elements. Most experts feel that the ultimate solutions to these problems lie in using photonic techniques to power and control the active elements in phased arrays.

As with large reflector antennas, most satellite manufacturers have competitive phased array technology but keep the details proprietary. Several of the present low earth orbit (LEO) systems (Iridium and Globalstar, for example) leverage technology developed for ACTS and formerly military technology to fly impressive phased arrays. ACeS (Figure 3.6,) will carry an impressive array feed generating 70 beams at L-band. Coming Ka-band systems like Teledesic will fly arrays developing hundreds of beams.


Fig. 3.6. Phased array feed for Lockheed Martin ACeS antenna (Mecham 1997).

In Japan, KDD is doing particularly interesting work on array antennas for mobile applications. A low profile is achieved using 2 layers of slightly overlaid patch radiators. The 3 x 3 array performs at both 2.3 and 1.6 GHz, as both transmit and receive, and was tested with ETS-V. The antenna uses a conventional beam-forming network; for more performance, an active phased array would be used. The second-generation model is a single layer with two element sizes on a high dielectric substrate. The axial ratio was not satisfactory, and a third generation model has been constructed. Similar to inverted-F, multiple short pins above each patch allow the sizes to be reduced to almost half; the patches can then be laid out without overlap in groups of four (transmit and receive for each band). There are 18 analog phase shifters (9 elements x 2 f bands), digitally controlled, and packaged into a small box. Transmit power is 250 mW per element.

A third array antenna, targeted for ICO and the Japanese Experimental Satellite (ETS VIII), uses a quadrifilar helix radiating element. The antenna will have 12 elements arranged in a triangular grid pattern (with corner elements missing from the grid). The antenna had just been delivered at the time of this WTEC visit, and patterns had not been measured. The feed electronics were packaged into four layers (for ease of further evolutionary changes). Diplexers comprise the first layer; LNAs the second layer; an analog beam forming network (BFN) the third layer; and down converters in the fourth layer. A design change is being introduced to substitute a digital beam forming network for the analog BFN. The feed network has one, two or three output ports. The antenna has 16 beam positions (switchable). Use of TDM downlinks might allow beams for two satellites. While ICO will use 6 kb/s links/user, thin route FSS multimedia is anticipated to operate at 64 kb/s, requiring about 10 dB more gain.

The major U.S. primes are working on phased arrays. Typical development models incorporate optical beam forming with true time-delay beam steering, and combine photonic and rf functions on the same chip. A representative example is a 96-element L-band single-beam array achieving 50 percent bandwidth and a 60 degree scan angle.

Several research satellites with impressive phased arrays are planned. For proprietary reasons, information about these is limited. France's STENTOR spacecraft will carry a direct-radiating array made up of 48 subarrays, each fed by its own SSPA. The STENTOR array will develop three independent beams. Japan's GIGABIT satellite array will develop five scanning spot beams, each with a 1.5 degree scan and 559 MHz bandwidth. Its total radiated power will be 500 W.

In 1992, Europe, Japan, and the United States had ongoing programs in phased array development. Since then, the end of the Cold War has brought former Soviet military technology into the commercial arena. The Moscow Aviation Institute, for example, is developing active phased arrays with multi-element transmit and receive amplifiers and hybrid optoelectronic signal processors.

Optical Beam Forming

Optics offers the potential for volume, mass, and power reductions with increased speed relative to similar subsystems implemented using electronics. There continues to be a tremendous amount of research in micro-optics, optical memory, optical signal processing, and optical communications throughout the world.

Diffractive optical components for use in free-space and bulk micro-optical systems are being studied for optical communications, information processing, optical computing and sensor applications. The subject components include high-efficiency blazed micro Fresnel lenses, high-efficiency chirped gratings, Bragg gratings, binary gratings, and arrays and composites of them. Integrated optics technologies are expected to play an important role in the development of new devices for future optical memory systems. There has also been considerable effort in the development of waveguide devices for communication use, including switches, mode splitters, mode converters and wavelength filters.

Hughes Research Lab, located in Malibu, CA, is jointly owned by Raytheon and Hughes. Research topics include communications, photonics, and microelectronics. Of particular interest is the work in optical beam forming for phased array antennas. The microelectronics staff is a vertically integrated team of experts in growth and diagnostics of III-V semiconductor materials and related compounds, development of microelectronics processing techniques, design of advanced device structures, modeling, rf/analog/digital circuit design, analysis, and evaluation all focused on delivering high performance digital, analog, linear, rf, optoelectronic and mixed-mode circuits for the next generation of microwave, millimeter wave, commercial wireless, and photonic systems.


Published: December 1998; WTEC Hyper-Librarian