Proceedings of the 36th European Microwave Conference

Ground Stations of Arrays to Increasethe LEO Download Capacity

Sebastien Rondineau (1), Charles Dietlein (1), Zoya Popovic (1),

Richard Q. Lee (2), Felix A. Miranda (2), Robert R. Romanofsky (2),Mary Ann Ingram (3), William C. Barott (3), John Langley (4), Daniel Mandl (5)

(1) University of Colorado, Boulder, CO 80309-0425, USAtel+1 303 492 8719, sebastien.rondineau@colorado.edu

(2) NASA GRC, 21000 Brookpark Rd, Cleveland OH 44135, USA(3) Georgia Institute of Technology, Atlanta, GA 30332-0250, USA(4) Saquish Group, POB 3554, HalfMoon Bay, CA 94019, USA

(5) NASA GSFC, Code 584, Greenbelt, MD 20771, USA

Abstract: To lower costs and reduce latency, a network ofadaptive array ground stations, distributed across the UnitedStates, is considered for the downlink of a polar-orbiting lowearth orbiting (LEO) satellite. Assuming the X-band 105Mbps transmitter of NASA's Earth Observing 1 (EO-1)satellite with a simple line-of-sight propagation model, theaverage daily download capacity in bits for a network ofadaptive array ground stations is compared to that of asingle 11 m dish in Poker Flats, Alaska. Each adaptive arrayground station is assumed to have multiple steerableantennas, either mechanically steered dishes or phasedarrays that are mechanically steered in azimuth andelectronically steered in elevation. Phased array technologiesthat are being developed for this application are the space-fed lens (SFL) and the reflectarray. Optimization of thedifferent boresight directions of the phased arrays within aground station is shown to significantly increase capacity; forexample, this optimization quadruples the capacity for aground station with eight SFLs. Several networks comprisingonly two to three ground stations are shown to meet orexceed the capacity of the big dish. Cutting the data rate byhalf, which saves modem costs and increases the coveragearea of each ground station, is shown to increase the averagedaily capacity of the network for some configurations.

I. INTRODUCTION

A typical ground station for NASA's low-earthorbiting (LEO) satellites utilizes a single large (10 m- 11m) dish antenna, and tracks a single satellite at a time bymechanically scanning the antenna through as much as160 degrees. The downlink supports data rates rangingfrom 2 kbps up to 150 Mbps. To maximize contact withthese polar-orbiting but precessing satellites, the groundstations are near the poles. The ground stations cost from$2M to $4M each to build and have an associatedmaintenance cost.

It will be shown that it is possible to construct anetwork with lower-cost ground stations, not necessarilylocated near the poles, which will receive data from thesesatellites by adaptively combining several small antennas,thanks to the adaptive combining software developed atthe Georgia Institute of Technology [1]. When used with

current satellites, these networks can provide averagedaily data rates that meet or exceed the current rates of thelarge antenna ground stations and use as few as 7 or 8antennas per ground station, such that each antenna has anaperture size of 0.75m. Variable bit rate methods canfurther reduce the number of antennas for the entirenetwork. Furthermore, although the subarctic locations ofcurrent ground stations maximize the amount of data thatcan be received per day, it might be cost effective to placethe phased arrays at more accessible locations, such as ontop of buildings in urban areas. Also, since a distributednetwork increases the amount of time that a satellite cantransmit data, the bandwidth of the satellites can bereduced without compromising the average daily datarates.

II. DESCRIPTION OF THE PROJECT

Our vision is that the ground stations would beconnected via the Internet, so that any given LEO satellitecan be in nearly continuous communication with thenetwork on Earth. While the focus of the current project isfor a ground station to communicate with only onesatellite at a time, the architecture being studied is capableof a rapidly and electronically controlled reconfiguration,to enable fast switching from one satellite to anotherwithin the same constellation, or simultaneouscommunication with multiple satellites. Two phased arraytechnologies are investigated for use as the "smallantennas" in the proposed ground station. One, which wasdeveloped at the University of Colorado, is the space-fedlens (SFL) array, shown in Fig.1. The other, developed atNASA's Glenn Research Center, is the reflectarray (RA),shown in Fig.2.

A space-fed lens array, shown schematically in Fig.1,is a planar implementation of a Rotmann lens [4, 5] withthe capability of 2D scanning. The antenna array is fedthrough a spatial feed which is light-weight andinexpensive since it is fabricated using standard pcbtechnology [6-8]. It can be viewed as a hardware discrete

September 2006, Manchester UK2-9600551-6-0 (D 2006 EuMA 874

Fourier transform of an arbitrary linear combination ofplane waves incident on a focal surface. The wavefront issampled at the incidence plane by an array of antennaelements; each sample is then appropriately time-delayedand re-radiated onto a focal surface by a second array ofantennas. The fields on the focal surface are sampled sothat each focal-plane antenna element (also called adetector) preferentially receives waves incident from asingle direction. This is a discretized version of an opticaldielectric lens which is thicker in the middle, i.e. has alarger delay in the middle [5]. Beamforming through aSFL at the front end has advantages in terms of reducedcomplexity, cost of digital hardware, and computationalload, especially for large arrays.

The RA has a surface containing integrated phaseshifters and patch radiators; the surface is illuminated by asingle feed at a virtual focus. The signal passes throughthe reflect-mode phase shifters and is re-radiated as acollimated beam in essentially any preferred direction inthe field of view of the antenna.

Both approaches provide electronic beam steering inelevation, mechanical steering in azimuth, and are lowcost. It will be shown that the cost of the network isgreatly reduced if optimizations are performed for avariable bit rate (VBR) link or array pattern synthesis, asshown on Fig.3 and 4. Because the ground stationrequirements in a LEO communications system aredetermined by the maximum distance between the satelliteand the ground station, the system exhibits an excesssignal power of up to 12dB when the satellite is close tothe ground station. VBR and pattern synthesis methodsincrease the efficiency of the ground station by ensuringthat the received signal power remains near the desiredvalue regardless of the distance between the satellite andthe ground station. The advantages are significant; VBRmethods can increase the daily data rate of a groundstation by a factor of 2.5, and we will show that patternsynthesis can reduce the required number of antennas for aground station by a factor of ten. To the best of theauthors' knowledge, no other papers have investigated theuse of array pattern synthesis or variable bit rate methodsto maximize the downlink capacity of an array for LEOsatellite communications. Although the array described in[2] will have a dedicated subarray to increase the gain ofthe array near the horizon, this only mitigates theinefficiencies and it is not an analytically optimal solution.The X-band downlink on the satellite EO-1 was chosen asthe reference link for this analysis. This satellite is locatedat an altitude of 707 km and has a sun-synchronous orbitwith an inclination of 980. When this satellite is directlyover a ground station, the required gain over noisetemperature (G/T) for the ground station is 10.25 dB,accounting for required margins [3]. The results, in termsof signal strength are displayed on Fig.5 and 6.

III. CONCLUSION

Using a simple path loss model, we have shown thatthere exist several possible small networks of groundnodes utilizing phased array antennas that can provide anaverage daily download capacity for the satellite EO-1equaling or exceeding that of an lIm dish in Alaska. Twocandidate phased array technologies have been reviewed:the space fed lens array and the reflectarray. Optimizationof the boresight directions of the phased arrays has beenshown to effectively counter the degradation of thenetwork capacity caused by scanning loss.

Another technology Luneburg lens based [9] is nowunder consideration: it has no scan loss because of itsspherical symmetry, its focusing properties are frequencyindependent, and so it represents the most attractivecandidate for the ground station network application.

REFERENCES

[1] M.A. Ingram, W.C. Barott, Z. Popovi6, S. Rondineau, J.Langley, R. Romanofsky, R.Q. Lee, F. Miranda, P.Steffes, and D. Mandl, "LEO Download CapacityAnalysis for a Network of Adaptive Array GroundStations," presented at ESTC 2005. Symposium, June2005.

[2] B. Tomasic, J. Turtle, S. Liu, R. Schmier, S. Bharj, and P.Oleski, "The geodesic dome phased array antenna forsatellite control and communication- subarray design,development, and demonstration," presented at the 2003IEEE Int. Symposium, Oct. 14-17, pp. 411-416.

[3] Radio Frequency Interface Control Document (RFICD)Between the Earth Orbiter (EO)-1 Spacecraft and theSpaceflight Tracking and Data Network (STDN),National Aeronautics and Space Administration,Goddard Space Flight Center, November 2000.

[4] W. Rotman, R.F. Turner, "Wide-angle microwave lensfor line source applications, " IEEE Trans. AntennasPropagat., AP-11, pp.623-632, 1963.

[5] D.T. McGrath, "Planar three-dimensional constrainedlenses," IEEE Trans. on Antennas and Propagations, pp.46-50, Jan. 1986.

[6] S. Romisch, N. Shino, D. Popovi6, P. Bell, Z. Popovi6,"Multibeam planar discrete millimeter-wave lens forfixed-formation satellites," in the Proc. of the 27thGeneral Assembly of the International Union of RadioScience (URSI), Maastricht, Netherlands, Aug. 2002.

[7] R.Q. Lee, S. Romisch, Z. Popovi6, "Multi-Beam PhasedArray Antennas," 26th Annual Antenna ApplicationsSymposium, Sept. 2002, Monticello, Illinois.

[8] D. Popovi6, Z. Popovi6, "Multibeam antennas withpolarization and angle diversity," IEEE Trans. Antennasand Propagat., Special Issue on WirelessCommunications, pp. 651-657, May 2002.

[9] R.K. Luneburg, Mathematical Theory of Optics,University of California Press, Berkley and Los Angeles,California, 1964.

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Fig. 1. Photographs of the 952-element SFL: (a) the RHCP non-feed side array, (b) the linearly polarized detector array and its switchingnetwork and (c) the detector array and the linearly polarized feed-side array. Non-feed side antennas are RHCP because of the incomingsignal, and feed-side antennas are linearly polarized for design simplicity and reducing inter-element coupling.

Fig. 2. 615 element, 19 GHz ferroelectric prototype array. The diameter is 28 cm.

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Fig. 5. Received signal strength as a function of the subsatellitepoint for a single downlink station located at the Poker Flats site.

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Fig. 4. The normalized link gains of the optimized SFL array andthe uniform array as a function of elevation angle.

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Fig. 6. Received signal strength as a function of the subsatellitepoint for four downlink stations, located in Hawaii, Washington,and Maine. The shading scale range is different than Fig. 5 toenhance readability.

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