Very Large Reflectors for Multibeam Antenna Missions

Yves Demers MDA, 21025 Trans-Canada Highway, Ste-Anne-de-Bellevue, Qc, Canada

Tel: 1-514-457-2150, fax: 1-514-452-3002, yves.demers@mdacorporation.com Aiping Liang

MDA, 21025 Trans-Canada Highway, Ste-Anne-de-Bellevue, Qc, Canada Tel: 1-514-457-2150, fax: 1-514-452-3002, aiping.liang@mdacorporation.com

Éric Amyotte MDA, 21025 Trans-Canada Highway, Ste-Anne-de-Bellevue, Qc, Canada

Tel: 1-514-457-2150, fax: 1-514-452-3002, eric.amyotte@mdacorporation.com Edward Keay

Astro Aerospace Northrop Grumman Aerospace Systems 6384 Via Real, Carpinteria, California USA

Tel: +1-805-684-6641, fax: +1-805-684-3372, edward.keay@ngc.com Geoffrey Marks

Astro Aerospace Northrop Grumman Aerospace Systems 6384 Via Real, Carpinteria, California USA

Tel: +1-805-684-6641, fax: +1-805-684-3372, geoff.marks@ngc.com

Abstract

Mesh reflector technology has evolved significantly in the last few years. The manufacturing tolerances and the thermal distortion performance have been improved up to a point where it is now possible to use 6-m class mesh reflectors at Ka-Band frequencies and higher. Much smaller beams can now be generated with these large reflectors, resulting in higher gains and higher frequency reuse factors over a given coverage area. This new technology enables new multibeam mission scenarios that would be difficult or even impossible to realize with conventional reflector technology. Furthermore, the surface of these new mesh reflectors can be shaped to maximize gain, Tx / Rx beam congruency and C/I performance. Three new multibeam mission scenarios are presented in this paper. The first two scenarios take advantage of the very high gain and small cell size achievable with the new mesh reflectors. The first scenario consists of multibeam coverage of high rain rate areas of South-Asia at Ku-Band. With the new mesh reflectors, it is possible to generate at Ku-Band beams of sizes similar to those of state-of-the-art Ka-Band systems. Such a system at Ku-Band would therefore have frequency reuse factor and capacity similar to Ka-Band multibeam systems while minimizing rain fade issues. The second mission scenario consists of a Ka-band multibeam coverage targeting the densely populated areas of North-America. Using 5-m reflectors, edge of coverage gains of the order of 52dB or more in both Tx and Rx are achieved over small coverage cells of 0.16° of diameter enabling the use of very small User terminals. The third scenario takes advantage of both the large size and shaping capability of the new mesh reflector technology. In this scenario, a single large mesh reflector is used in a “Single Feed per Beam (SFB)” configuration to generate 18 contiguous beams over Mexico. In a conventional scenario, 4 smaller solid reflectors would have been needed in order to achieve a reasonable performance level. This reduction of the number of reflectors from 4 to 1 allows for additional missions to be accommodated on the same spacecraft.

1. Introduction

State-of-the-art Ka-Band multimedia multibeam satellite missions are typically geostationary satellite missions characterised by:

• Single Feed per Beam (SFB) configuration • Combined Tx and Rx operation with 4 reflectors antennas to generate the beams • Four colours frequency reuse plan • Cell diameters of the order of 0.5º

Because of the stringent requirements on thermal distortions and as-built surface accuracy, only solid shell reflectors have been used up to now on commercial multibeam missions at Ku- and Ka-band. This has set a lower limit on the achievable beam size, as this type of reflector is typically limited to an aperture size of less than about 3 m. With the recent improvements in mesh reflector technology, it is now possible to use 6 m class mesh reflectors at Ka-Band frequencies and higher. It is therefore now possible to generate much smaller beams, resulting in higher gain and higher frequency reuse factors over a given coverage area, giving rise to new or improved multibeam mission scenarios that would be difficult or even impossible to realize with conventional reflectors. Three of these new scenarios are presented in this paper. The advances in mesh reflector technology are also briefly discussed.

2. Mesh Reflector Technology

A typical AstroMesh®1 design is presented in Figure 1a). The mesh support structure consists of the front and rear nets and the tension ties. The front net is composed of a number of webs (composite tapes) that stretch between the nodes of the perimeter truss. The intersections of the webs are pinned together at precise positions so that the resulting structure, when correctly tensioned by the spring tension ties, forms the required shape. The mesh is stretched behind the front net into a series of triangular facets and so the "required" shape is that shape which best fits the series of facets to the desired parabolic shape per the optical prescription for the antenna. Several L-Band and S-Band mesh reflectors based on this design are operating flawlessly in space. In the last few years, a number of advances in mesh reflector technologies were made at NGAS to extend the operation frequency range of the mesh reflector to Ka-Band and higher frequencies. The main advances are:

o Mesh material - a denser mesh knit was developed to reduce RF losses and depolarization effects

o Smaller facets - to minimize grating lobes and improve reflector efficiency o Web materials - a new low CTE graphite composite web was developed to improve thermal

stability (the basic AstroMesh® design already addresses some of the problems of stability by having a perimeter structure with near net-zero thermal expansion properties and high stiffness derived from its basic structure)

o Manufacturing accuracy - a simple and effective method for adjustment of the as-built shape was implemented. A few iterations between photogrammetry measurements and tension adjustments in the ties (determined by Finite Element Model analysis) arrived at the correct surface accuracy.

A 5m mesh reflector development model featuring all these advances was built and is currently under qualification2 at NGAS. Based on the measured data, it is foreseen that very good overall performance will be achieved. The measured as-built accuracy on the 5m demonstration model is 0.15mm RMS. As shown in Figure 1b), the measured RF losses of the mesh are lower than 0.1 dB at 20.0 GHz and less than 0.2 dB at 30.0 GHz. The depolarization due to the reflector mesh will not be an issue for multibeam applications. The amplitude imbalance is negligible and the differential phase shift introduced by the mesh can be mostly compensated by biasing the feed design to achieve good performance in CP.

Flat TrussFlat TrussMeshMesh

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Flat TrussFlat TrussMeshMesh

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The three new multibeam mission scenarios using this mesh reflector technology are presented below.

3. Multibeam Mission for High Rain Rate Regions

Rain fade can be one of the main performance drivers for multibeam systems deployed in parts of the globe experiencing high rain rates. Operation at Ku-Band instead of Ka-Band could help alleviate these rain fades issues. However, part of the advantage of operating at Ku-Band to reduce rain fade is lost if larger coverage cells are used because of limited reflector size. The larger cells lead to lower frequency reuse factors and reduced capacity over a given coverage. With the new high performance mesh reflectors, it is now possible to generate small beams at Ku-Band and achieve performance levels similar to those of state-of-the-art Ka-Band systems while minimizing rain fade issues.

3.1. Mission Description

Figure 2a) shows a hypothetical coverage of South-Asia containing several high rain rate regions. The coverage consists of 69 cells having a diameter of 0.5º. The antenna system is based on a “Single Feed per Beam” approach and consist of 4 single-offset mesh reflector antennas illuminated by a total of 69 combined Tx and Rx feed chains. An artist rendition of a spacecraft with 4 deployed large mesh reflectors is shown in Figure 2b). A four colour frequency reuse plan is used to maximize system capacity. The colours of the beams in Figure 2 correspond to both the frequency allocation of the beams and their antenna allocation. The reflector diameter of the 4 antennas is sized to produce 0.6º beams to match the footprint of the cell enlarged by the pointing error (PE =0.05º). The 4 single-offset antennas are identical except for their orientation on the space-craft and have the following RF geometry: F = 6000 mm, O = 3600 mm and D = 3800 mm. The reflector surface consists of 558 triangular facets that are arranged to emulate a parabolic surface. To achieve optimal performance, high aperture-efficiency smooth-wall horns especially developed for multibeam antenna applications, are being used3. The horn primary patterns have been optimized to equalize the Tx and Rx footprints and maximize the Edge of Coverage gain and C/I performance. MDA’s proprietary dual-band high-efficiency smooth-wall horns have aperture efficiencies in excess of 81% in Tx and 84% in Rx. The antenna performance presented below is calculated using the following frequency plan for the Users: Tx= 11.7GHz – 12.2GHz, Rx= 14.0 GHz – 14.5GHz. Tx and Rx are in opposite circular polarization over the cells.

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Figure 2: a) South-Asia Coverage, b) Example of Reflector Accommodation

3.2. RF performance

The reflector antenna performance was analyzed with GRASP using a model of the mesh reflector surface that includes the facets to give a representative assessment of the performance achievable with this technology. Figure 3 shows the Tx and Rx patterns of Beams #52, #63 and #71. These

beams are scanned by 2º to 4º with respect to the antenna boresight centred on beam #29. The three beams are generated by the same reflector antenna and are operating in the same polarization and over the same frequency band. Therefore, they can interfere strongly with each other. Although, the antenna is a combined Tx and Rx antenna and the electrical aperture is much larger at Rx frequencies than at Tx frequencies, the Tx and Rx footprints are well equalized by a proper design of the high aperture efficiency feeds. In Figure 3, the difference between Tx and Rx EOC gain over the cell size enlarged by the pointing error (shaded area) is only of the order of a dB. The Tx and Rx patterns are distorted due to the beam scan but the isolation between the beams of Figure 3 is still quite good with C/I values in excess of 13 dB between the individual beams. The performance of one of the antennas (purple) is summarized in Table 1. The EOC gain performance in Tx and Rx is well balanced over the complete coverage with only 0.6 dB difference between the worst EOC gain performance in Tx and Rx for the nominal case. A loss budget of 1.1 dB in Tx and 1.3 dB in Rx has been assumed in these calculations. As it can be expected, the beams with the largest scan angles tend to have the worst EOC gain and C/I performance.

Figure 3: Tx and Rx Patterns of Beams #52, #63 and #71

Table 1: EOC gain and C/I performance of Purple Antenna (dB)

Beam No. Tx EOC Tx 95% EOC Tx 95% COI Rx EOC Rx 95% EOC Rx 95% COI

7 45.4 46.8 25.4 44.8 46.5 28.2

12 45.5 46.8 24.7 44.8 46.5 20.0

14 45.4 46.8 21.7 44.7 46.5 16.0

24 45.6 46.8 19.3 45.1 46.5 21.6

27 45.4 46.8 18.2 44.8 46.5 17.6

29 45.5 46.9 18.0 44.8 46.5 19.3

31 45.5 46.8 21.0 44.9 46.5 21.8

40 45.6 46.7 19.7 45.2 46.5 19.4

41 45.5 46.9 18.9 44.8 46.5 19.6

45 45.4 46.8 23.4 44.8 46.5 23.0

50 45.5 46.7 22.9 45.2 46.5 18.7

56 45.4 46.7 25.0 44.8 46.4 22.8

57 45.4 46.7 16.3 45.0 46.5 20.0

59 45.6 46.7 12.2 45.1 46.4 14.4

66 45.5 46.6 12.2 45.0 46.4 11.6

68 45.5 46.6 19.1 44.9 46.3 13.5

73 45.3 46.4 29.0 44.8 46.2 18.0

Average 45.5 46.7 20.4 44.9 46.5 19.1

WORST 45.3 46.4 12.2 44.7 46.2 11.6 The performance achievable using mesh reflector technology is very close to that of solid graphite shell reflectors (if these were available at that size). A comparison between the worst case performance for solid shell graphite reflectors and for mesh reflectors is presented in Table 2 for the purple antenna. The difference in EOC gain and C/I performance (95%) is only 0.2 dB and 0.3 dB respectively.

Table 2: RF Performance of Mesh vs Solid Graphite Reflectors Case Mesh Reflector Solid Reflector Difference

Worst Tx EOC (dB) 45.3 45.5 -0.2

Worst 95% Tx C/I (dB) 12.2 12.4 -0.3

Worst Rx EOC (dB) 44.7 44.9 -0.2

Worst 95% Rx C/I (dB) 11.6 11.9 -0.3 The performance of the mesh antennas was also analyzed for several thermal distortion cases. The nominal performance and the performance corresponding to the worst thermal case (EQ24) are

summarized in Table 3. In the performance analysis of the worst thermal case, the antenna orientation has been readjusted to eliminate any global depointing of the antenna due to thermal distortions. This approach is justified since any global antenna depointing would be eliminated by the active antenna tracking system (yielding residual PE=0.05º). The impact of the TEDs is fairly minor demonstrating the very good thermal stability of the AstroMesh® reflector.

Table 3: Impact of TED Case EQ24-Repointed Nominal Difference

Worst Tx EOC (dB) 45.1 45.3 -0.2

Worst 95% Tx C/I (dB) 11.8 12.2 -0.4

Worst Rx EOC (dB) 44.3 44.7 -0.4

Worst 95% Rx C/I (dB) 11.1 11.6 -0.5 The performance of this Ku-Band system is therefore similar to that of state-of-the-art Ka-Band systems operating with a similar cell size, but is less sensitive to weather conditions. The worst case EOC gain on the complete coverage varies from 44.3 dB in Rx to 45.1dB in Tx and the C/I values over 95% of the coverage of each beam is in excess of 11dB in Tx and Rx. The frequency reuse factor over the coverage area is approximately 17. This performance level is superior by far to the performance level achievable using conventional reflectors at Ku-Band due to larger electrical size. Since the aperture size of the conventional reflectors is limited to approximately 2.8m, the minimum cell size would be approximately 0.7°, leading to a very significant frequency reuse and system capacity reduction of a factor 1.5 to 2 (the value of the factor depends on shape of the coverage). Furthermore, larger User ground terminal antennas would be required to compensate for the 2.5 dB EOC gain reduction on the larger cells.

4. High Performance Coverage of Densely Populated Areas

Some business models indicate that the market for broadband satellite services is mostly concentrated at the edge of urban areas, in the near periphery of densely populated areas. The availability of very large reflectors now opens the possibility of generating very small beams with very high gains at Ka-Band targeting only these densely populated areas, reflecting more accurately the true market distribution. The advantage of this approach would be two-fold. Firstly, very small User terminals could be used without compromising the quality of service. Secondly, a higher capacity per beam should be achievable since the antenna gain is higher and the interference levels in a four colour frequency reuse scheme tend to be much lower in a sparse coverage.

4.1. Mission Description

Figure 4 shows a coverage consisting of 89 cells over densely populated areas of the United States. The ″Single Feed per Beam″ antenna system consists of 2 single-offset antennas located on opposite sides of the spacecraft and illuminated by 89 combined Tx and Rx feedchains. This coverage can be generated using only two reflector antennas because most of the cells are not contiguous. The two antennas have identical RF geometry: F = 6300 mm, O = 3600 mm and D = 5000 mm. The reflector surface consists of 1692 small triangular facets that are arranged such as to best emulate a parabolic surface. The use of small facets is mandatory at Ka-Band to minimize grating lobes and maximize antenna gain. The beam frequency allocation is colour coded on Figure 4. A four colour frequency reuse scheme is used to maximize system capacity. Two different cell sizes are used to better match the traffic distributions. The smaller cells have a diameter of 0.16º and correspond to a footprint on the earth of approximately 100 km. The larger cell diameter is 0.26º and it corresponds to a footprint of approximately 160 km. Assuming a pointing error of 0.03º, which is reasonable given the very small cell size and very large antenna gain, the corresponding beam sizes are 0.22º and 0.32º. Both sizes of beams are generated using high aperture efficiency smooth wall horns to maximize performance. The large beams are generated with larger horns having higher gain to under-illuminate the reflectors. The horn Tx and Rx primary patterns are optimized to equalize the Tx and Rx secondary patterns and maximize the Edge of Coverage gain and C/I performance. The smooth wall horns have Tx and Rx aperture efficiency in excess of 82%. The performance presented below was calculated over the standard User frequency bands: Tx= 19.7GHz – 20.2GHz, Rx= 29.5 GHz – 30.0GHz, with the four colors corresponding to 2 x 250MHz in RHCP and 2 x 250MHz in LHCP. Tx and Rx are in opposite circular polarization over the cells.

Figure 4: Multibeam Coverage of densely populated areas of North America

4.2. RF Performance

The antenna RF performance was evaluated using GRASP with a model of the mesh reflector surface that includes the facets. The Rx co-polarization gain pattern of beam #11 is presented in Figure 5. Because of the use of very small facets, the gratings lobes are fairly benign and their negative impact on the antenna C/I performance will be small. The grating lobes are approximately 28 dB below peak. The beam pattern of Figure 5 is not symmetrical as Beam #11 is scanned by approximately 8 beamwidths with respect to the antenna boresight.

Figure 5: Rx Co-Polarization Pattern of Beam #11 at f = 29.5 GHz

The EOC gain and C/I performance for 10 of the beams of Antenna #1 are presented in Table 4. The last two lines of Table 4 give the average and worst values for all the beams of Antenna #1. A loss budget of 1.3 dB in Tx and 1.6dB in Rx was used in the gain calculations. The Tx and Rx EOC gain performance is well balanced on both the small and the large cells. On the small cells, the Tx and Rx EOC gain performance is typically around 53 dB and remains close to 52dB even for the most scanned beams. On the larger cells, the gain performance is typically 3 dB lower, i.e. approximately in ratio of the beam areas, indicating that the horn design is nearly optimal. The gain performance evaluated on 95% of the cell area is approximately 1 dB higher in Tx and 2 dB higher in Rx. The aggregate C/I performance on 100% of the cell coverage in Tx and Rx is very good with typical values around 20 dB. This high level of C/I performance is unusual for a four colour frequency reuse scheme and is due to the use of mostly non-contiguous beams in the coverage. Combined with the very high EOC high gain, these high C/I values will result in a much higher system capacity than that of state-of-the-art Ka-Band system of 89 beams providing a complete coverage of the continental USA.

Table 4: RF Performance over North-America (dB) Beam No. Tx EOC Tx 95% EOC Tx COI Rx EOC Rx 95% EOC Rx COI BW

2 52.4 52.8 24.0 52.2 53.0 21.0 0.22

5 48.7 50.0 20.0 47.4 49.3 19.6 0.32

15 52.1 52.3 22.9 51.6 52.5 22.0 0.22

18 50.5 51.1 23.4 49.9 50.8 20.1 0.32

50 53.9 54.2 21.5 53.8 54.5 23.5 0.22

51 54.4 54.7 22.9 54.0 54.6 25.2 0.22

52 50.3 51.2 24.6 49.4 51.1 20.9 0.32

61 50.8 51.7 21.4 50.3 51.5 19.2 0.32

62 53.6 54.0 22.3 53.3 54.1 25.5 0.22

63 53.3 53.7 16.4 53.3 54.2 20.5 0.22

Average Antenna #1

BW=0.22 / BW=0.32 53.4 / 50.5 53.8 / 51.2 22.9 / 21.9 53.1 / 49.8 53.9 / 51.1 22.9 / 18.9

Worst Antenna #1

BW=0.22 / BW=0.32 51.7 / 48.7 52.7 / 50.0 16.4 / 18.1 49.2 / 47.4 51.4 / 49.3 17.2 / 16.3

5. Multibeam Piggy-back Mission

Accommodating a small piggy-back Ka-Band multibeam mission on a BSS or FSS satellite is often unpractical because of the real estate required for the multibeam antennas. Three or four fairly large conventional reflectors are required to provide reasonable performance over a typical multibeam coverage of contiguous beams, making the accommodation of the antennas of the other missions very difficult. As it will be shown in this example, a single large mesh reflector can be used to generate all the beams and alleviate the accommodation problems while providing nearly optimal performance.

5.1. Mission Description

A hypothetical coverage for a small Ka-Band multibeam mission over Mexico is shown in Figure 6. The coverage consists of 18 cells of 0.68º of diameter. A four colour frequency reuse plan is used to maximize system capacity. All the beams are generated by a single antenna illuminated by a cluster of 18 combined Tx/Rx feedchains. The multibeam antenna system would therefore occupy only one side of the spacecraft leaving the other side and the top floor available for other missions. The single-offset antenna RF parameters are as follows: F = 6050 mm, O = 4000 mm and D = 6000 mm. The reflector is largely oversized for the 0.78º beams (PE = 0.05º). In fact, a 6m parabolic reflector optimally illuminated at Ka-Band would produce beams smaller than 0.2º. The oversized reflector is required to minimize the spill-over losses resulting from having all the feeds packed in the focal plane of a single antenna. The reflector surface consists of 1692 small triangular facets that are arranged to recreate a non-parabolic shaped surface optimized to produce the right size of beams. The performance presented below was evaluated over the standard User frequency bands: Tx= 19.7GHz – 20.2GHz, Rx= 29.5 GHz – 30.0GHz, with the four colors corresponding to 2 x 250MHz in RHCP and 2 x 250MHz in LHCP. Tx and Rx are in opposite circular polarization over the cells.

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Figure 6: Multibeam Coverage of Mexico at Ka-Band

5.2. RF Performance

The Tx and Rx patterns of beams #3, #9 and #15 are presented in Figure 7. The patterns have a “flat top” shape over the cell areas (shaded areas in Figure 7) which results in very good EOC gain

performance for a cell of this given size. These flattened patterns come from a combination of reflector shaping and the use of the primary pattern sidelobes on the reflector. The EOC gain performance in Tx and Rx of all the beams is listed in Table 5. Because of the pattern flat-top shape, these values are typically better than the values obtained with a four antenna system with reflectors optimally sized for 0.78º beams. The shapes of the patterns, especially in Rx, are fairly distorted and need to be controlled by reflector shaping. The achieved C/I performance is still quite good, typically only 1 dB lower that the values achievable with a four antennas system with conventional reflectors. Since the mesh reflector technology imposes special restrictions on the reflector shapes that can be achieved, standard reflector antenna optimization tools cannot be used. The results presented here are based on a preliminary shape optimization but are representative of the performance level achievable with this mesh reflector technology.

Figure 7: Tx and Rx Patterns of Beams #3, #9 and #15

Table 5: RF Performance over Mexico

Beam No. Tx EOC Tx 95% EOC Tx 95% COI Rx EOC Rx 95% EOC Rx 95% COI

1 43.7 44.3 19.6 43.2 43.9 17.4

2 43.7 44.2 18.1 43.2 43.8 17.3

3 43.8 44.0 17.6 42.9 43.6 15.3

4 43.6 44.3 22.0 42.6 43.9 15.5

5 43.9 44.4 17.8 43.5 44.2 14.8

6 43.9 44.2 18.7 43.3 44.1 15.8

7 43.5 43.9 15.3 43.5 43.9 14.9

8 44.0 44.5 18.4 43.0 44.3 14.9

9 43.9 44.4 17.3 43.5 44.4 14.7

10 43.8 44.0 16.3 44.0 44.2 15.3

11 43.9 44.5 17.1 42.6 44.3 12.8

12 43.7 44.3 19.5 43.5 44.4 14.2

13 43.9 44.1 16.0 43.9 44.3 14.5

14 43.5 43.7 17.8 43.1 43.6 15.1

15 43.7 44.2 19.5 43.2 44.5 13.5

16 43.7 44.0 20.7 43.3 44.3 14.5

17 43.6 43.8 17.8 43.3 43.7 14.3

18 43.4 43.7 20.0 42.8 43.6 14.5

Average 43.7 44.1 18.3 43.2 44.1 14.9

Worst 43.4 43.7 15.3 42.6 43.6 12.8

6. Conclusion

6 m class mesh reflectors designed for operation at Ka-Band frequencies and above are now becoming available on the market. The preliminary calculations presented here indicate that the performance level achievable with these reflectors for multibeam applications at Ku-band and Ka-Band is very good, and in fact, comes close to that of conventional graphite reflectors, if conventional reflectors of comparable size were available. These large mesh reflectors with their very high gain, very small beam sizes and their shaping capabilities are opening the door to several new or improved multibeam mission scenarios.

References

1- AstroMesh® US Patent No. 5,680,145. 2- Geoff Marks and al., "Performance of the AstroMesh® Deployable Mesh Reflector at Ka-Band Frequencies and Above.", this conference. 3- E. Amyotte et al., 19th International Communications Satellite Systems Conference, Toulouse, France, April 2001, Section 31, p964.