Phase B & Prototype

Nov. 17, 1998

The development plan for the LAR comprises three parts: Phase A is a collaboration between NRC, universities and industry, which is approximately half way finished. It consists of analytical and computer simulations of each of the critical areas of the design. Phase B is a 3-year program of field tests and proofs of those sub-components that must be demonstrated and perfected before feasibility can be completely established. An attractive aspect of the LAR concept is that major parts can be completely tested in isolation, thus permitting low-cost proof of feasibility. The final phase is the construction of a full operational prototype, which will be evaluated as a potential element of the Square Kilometer Array (SKA), and possibly used as a powerful new single-antenna telescope, possibly one of the largest.

Work on the LAR started at the end of March, 1998. About 50% of this portion (feasibility and initial cost study) of the LAR study is complete. The project is divided the work into 7 main packages ñ allocated to groups in universities and industry as outlined in Table 1, with collaboration from HIA. Almost all groups are working with firm concepts, except for The Airborne Platform Control Systems and Feed Design projects, which are slightly less developed at this stage.

As might be expected for a project with such scope for invention, extraordinary creativity is emanating from each area,. and there are frequent expressions of interest in becoming involved - from researchers, companies, and others (see below). Table 2 is a list of people who are actively contributing to the project.

*not yet officially allocated. **expressed interest in becoming involved in this area.

The following is a list of reports written to date:

1. B. Veidt & P.E. Dewdney, Steady State Analysis of a Triple Tethered Balloon Platform, 1996
2. B. Veidt, Air Turbulence Above the DRAO Site, 1996
3. P. Dewdney & B. Veidt, The Development of the LAR for the Square Kilometer Array - Specifications & Guidelines for Research, 1997.
4. B. Veidt, Modal Analysis of a Triple Tethered Aerostat Platform, 1997.
5. P. Dewdney & B. Veidt, The Beam Pattern and Efficiency of the Large Adaptive Reflector (LAR), 1997.
6. P. Dewdney, The Large Adaptive Reflector (LAR), Geometry of the Reflector, 1998.
7. P. Dewdney, A Simplified Comparison of Streamlined and Spherical Aerostats for the Large Adaptive Reflector (LAR), 1997.
8. Coast Steel Fabricators, LAR Project, LAR Reflector Support Study Report, 1998.
9. P. Marttala, The Geometric Efficiency of the LAR Antenna, 1998.
10. LSCI Ltd. Structure Group, LAR Project, Adaptive Reflector Structure Report #2, 1998.
11. B. Carlson and P. Dewdney, Preliminary Feasibility Study of a Radial Rib Structure for the Large Adaptive Reflector, 1998.
12. B. Veidt, Feed Options for the LAR, 1998.
13. M.E. Cannon, Y. Gao, and G. Lachapelle, Assessment of Equipment and Techniques for Precise LAR Aerostat Positioning ñ Phase I, 1998.
14. P. Marttala, Errors in the Finite-Difference Time Domain Method Due to Small Time Steps, 1998.
15. B. Veidt, J. Fitzsimmons and P. Dewdney, A Steady State Analysis of a Multi-tethered Balloon Platform, 1998 (in preparation).
16. B. Veidt, B. Carlson, and P. Dewdney, The Feasibility of the LAR for Deep Space Communications, 1998 (in preparation).
17. M. Meng, J. Smith, Large Adaptive Reflector Project ñ Advanced Robotics and Teleoperation Lab: Lab Model Report #1, 1998.
18. J. Fitzsimmons, LAR Lab Model Sizing Recommendations, 1998.
19. B. Carlson, Geometry Study and Simulation of a Laser-Free Surface Measurement Technique for the Large Adaptive Reflector, 1998.

* Graduate students whose theses are tied to the project.

A few important potential collaborators have emerged:

1. TetherCam Inc. of Ottawa: This company manufactures and flies stabilized video cameras aboard aerostats. Their business is sports broadcasting, security, and defense related applications. We have already assisted them in predictions of whether they could operate their small aerostat system in Colorado at high altitudes. They are seriously interested in developing their expertise to serve both a potential LAR market and markets that could develop for them as a result of improvements to their aerostats, including multi-tethered stabilization. This is a small company that is expanding quickly, and would like to pursue funding for development of a multi-tethered platform.
2. NRC/IAR: This group is interested in collaboration, and has promised to analyze the wind loading on the reflector, once the structural concept is complete. They have also agreed to provide feedback on numerical modelling of the aerodynamic behaviour of the LAR aerostat system.
3. NRC/IMTI: Several members of the Innovation Center group in Vancouver are very interested in the reflector control system, diagnostic analysis, and vibrational analysis of the flexible reflector surface.

The present funding takes the project to Mar. 31, 1999. At that point we expect to have developed concepts for each critical area, established feasibility as much as possible on paper, and provided a rough cost estimate for the LAR. In order to be as efficient as possible with research funds, we must balance the need to make each step as small as possible, without extending the time to provide answers indefinitely. The following is an outline of five projects that can be carried out over the next three years, following which we would be technically able to build a full working prototype of the LAR.

1. Half-scale prototype of multi-tethered aerostat system (Figure 1)

• This would consist of an aerostat with approximately 1/8 of the lift of the full-scale prototype, capable of carrying a modest payload in the desired configuration.
• This size would be cost effective, yet permit a reasonable extrapolation to the full size. Note that the extrapolation will not be completely straightforward; careful analysis will be needed.
• The most likely configuration would be three (or more) tethers fixed to the aerostat, control winches on the ground, an airborne position measuring system, and a control system governing the motion of the aerostat and the tethers. A ground handling system for the aerostat would also be required.
• It is anticipated that TetherCam can supply the aerostat portion of this prototype.
• The setup would be available for about 2 years, to allow the control of the aerostat to be perfected, and to get experience with the technology.
• The goal is to implement and refine the tether control system for the airborne platform, so that we can experimentally determine the behaviour of a controlled aerostat, thus determining the requirements for other "layers" of stabilization in a hierarchical control system.

1. Actuated Triangular Reflector Section (Figure 2).

• This would be one 20-m triangular backing structure frame with panels mounted on top.
• The frame would be supported by primary actuators, and the panels by secondary actuators mounted on the frame.
• A panel measuring system and a control system would be used to perfect and demonstrate that the reflector can be controlled to the required degree of precision (0.7 mm rms).
• Panels would be mostly blanks, but would possibly be upgraded to real panels after panel casting techniques have been perfected.

1. Panel Casting Experiment.

• It must be demonstrated that the panels can be repeatably manufactured with consistent surface errors, gravitational deflection, and thermal deflection.
• A series of tests would be carried out to make the panels and measure their properties in the field.
• These steps could be carried out in parallel with the construction of the prototype reflector section.

1. Continued Work on feed design.

• The feed design is a critical part of the LAR project.
• Phased arrays will likely be a vital element of feed design. Large phased arrays for radio telescopes are leading edge technology, and will require considerable research effort to develop and test.
• Small prototypes will be needed for testing at an antenna range or anechoic chamber.
• Larger (possibly full size) prototypes will be required later.

1. Focal package payload development.

• After electromagnetic evaluation of possible feed configurations, an airborne payload containing the optics and receiver elements must be designed.
• Light mechanical systems will be needed, possibly including an inertial stabilization platform (It is not yet clear whether this will be required).
• Over time, different techniques will be developed for different wavelength ranges (probably ground-fed Cassegrain for short wavelengths, locally-fed Cassegrain for intermediate wavelengths, and prime focus phased arrays for long wavelengths.

Funding: The present HIA budget would continue to fund 2 FTE indefinitely each at $100K/yr for salaries, benefits, overhead, and travel. To maintain our contribution to these studies we should increase our staff base by two engineers totaling $200K/yr indefinitely. In addition we will need specific funds for Phase B as shown in Table 3 amounting to $2380K over 3 years. Over the 5 years from 2001 to 2005, HIA would need an extra $3380K.

By "breaking the gravity barrier", the LAR promises an enormous leap forward in the design of large radio telescopes (and antennas). Nevertheless, it is important to emphasize that, even though the LAR concept is simple, it is wholly new design territory for radio telescopes. In utilizing advanced technology in novel ways, it presents a huge, unexplored parameter space that must be investigated. In carrying out the Phase A study, we have made every effort to be efficient, choosing to work on critical components in priority order, without serializing the program too much. The goal of the Phase A study is to show feasibility and provide a cost estimate. Phase A studies are expected to be completed in April, 1999, at which time we expect to have a rough order-of-magnitude estimate of the total cost of a prototype. The Phase B program is prototyping only critical elements to yield information on feasibility, cost and performance, needed to proceed to a full prototype.

* Partially supported by DRAO budget; ***Supported by Radio Futures budget.

The cost target for the completed SKA is $400-600US per square meter; obviously we are aiming for the lower end of this range, while providing a high-efficiency telescope over a wavelength range from 1.5 m to 1.5 cm. A prototype might be expected to be more expensive per m², although it is hoped that the prototype would be within the upper end of the range. Nevertheless, this cost is 10-20 times lower than possible with traditional designs. We do not yet have cost values for the entire LAR. Very preliminary estimates for the 200-m reflector itself indicate that its installed cost could be $C 13M ($US 270/m²). A preliminary estimate of the cost of the bare aerostat and ground handling system is about $C 4M. At this stage we do not have a value for the rest of the telescope, but it might be reasonable to assume that it would cost another $C 4-5M. A reasonable working number for the total cost might be $C 22M.

It is inconceivable that the LAR would be selected as the technology for the SKA without a thorough shakedown as a single-antenna telescope. This is also true of competing technologies for the SKA. Because obtaining the best cost-to-performance ratio is crucial for the SKA, continuous design optimization will be needed during the prototype stage, until it is clear that the limits of efficiency, frequency range, sky coverage, etc., are known.

Some important cost factors clearly favour a large antenna. The strategy of the LAR design is to amortize the expensive components, such as the aerostat and feed, over a large aperture. This is the basic method by which the cost can be reduced far from that of traditional designs. For this reason we have selected a 200-m diameter antenna for study (30,000 m² area). The cost of these components is reduced for smaller diameters, but the reduction is not proportional. On the other hand, a factor that does scale with diameter is the size of panels (for a given f/D ratio, and upper frequency limit). This means that the number of panels is roughly invariant with the diameter ñ thus a smaller telescope contains about the same number of panels as a larger one, although they are smaller. Again the cost of a telescope with smaller panels is lower, but not in proportion to size. These factors alone could make a difference of a factor of two in the cost per unit area.

The prototype is expected to be a fully operational instrument. Construction is expected to take about three years, after which at least a year will be needed for engineering tests. Then there could be an opportunity to gain operational experience while reaping a scientific dividend from a world-class instrument. As such, it could be the basis of a scientific program similar to those of the worldís largest steerable reflectors ñ the Effelsberg 100-m telescope and the Green Bank Telescope (GBT, also 100 m). Its sensitivity would be much greater than either one, although its upper frequency limit would be lower than that for the GBT. The science program could include pulsar research, masers (H₂O, OH, CH₃OH), sensitive H I studies (low-luminosity galaxies, primordial H I searches, H I in galaxies, clusters, and groups, Galactic halo gas, high-redshift H I absorption), high-redshift CO (J=1-0), stellar radio sources, probing molecular clouds using molecular lines at wavelengths longer than 1.4 cm (e.g. NH₃, thermal OH and CH₃OH, CH, H₂CO), Zeeman splitting (OH and H I in absorption), thermal and non-thermal continuum at long and short wavelengths. The particular topics will depend on the receivers available. In addition, operation as an interferometer element in VLBI mode will be necessary to demonstrate its capabilities for the SKA. This would also yield scientific dividends in concert with other telescopes for high-sensitivity VLBI observations. The operating cost of a telescope of this size would likely be $C 1-2M per year, i.e. about $C 3M for the remaining 2 years of the 5-year interval.

To reach smooth operation of the prototype will require ten years of steady development from now. Well before this time, it will be necessary to select the technology for the SKA. If the LAR technology is selected, a decision will be required at that time as to whether to continue to operate the prototype, dependent on available resources. The key people who have been involved in the construction of the prototype LAR will be needed to guide the design and construction of the SKA. In addition there will be a substantial payback to the Canadian industry developers of the LAR concept.

If the LAR technology is not selected, then a decision as to whether to continue a domestic science program, based on the LAR would have to be made.

Figure 1: Half-scale prototype of the multi-tethered aerostat.

Figure 2: An illustration of an actuated triangular section of the LAR reflector.