FORMATION FLYING PICOSAT SWARMS FOR FORMING EXTREMELY LARGE APERTURES

Transcription

1 FORMATION FLYING PICOSAT SWARMS FOR FORMING EXTREMELY LARGE APERTURES Presented at the ESA/ESTEC Workshop on Innovative System Concepts February 21, 2006 Ivan Bekey President, Bekey Designs, Inc Quarter Charge Dr. Annandale, VA 22003, USA I ;

2 WHY LARGE ANTENNAS IN SPACE Communications and radar space applications are requiring ever larger antennas There are many reasons why large space antennas are desired Minimization of spacecraft transmitter power for communications Reception of weak signals such as cellphones or data transmitters Formation of small spot sizes on the ground for high resolution Minimization of interference by forming low amplitude sidelobes Clutter rejection for radar applications Forming very narrow beams with very high gain for planetary applications Others The antenna sizes can become very large in LEO (many tens of meters) However many applications naturally prefer to be in GEO Constant dwell, near-hemispherical view, ease of control, etc. And in GEO the antenna sizes are times larger for the same performance Since the weight and cost of such anetnnas is bound to be huge, we are motivated to look for non-conventional construction approaches The most promising of these is coherent cooperation in swarms

3 ANTENNA SIZE REQUIRED IN GEO 100,000 10,000 1, GHz 1 GHz 10 GHz 10 Thuraya ,000 10,000 Spot size desired on the ground, kilometers

4 ACHIEVING A 1 km GROUND SPOT AT 1 GHz FROM GEO 12 km 12 km 12 km Conventional rib-mesh antenna, lightweigted Projected inflatable membrane antenna Projected adaptive membrane structureless antenna, 10% filled 15 kg/sq.m 1 kg/sq.m kg/sq.m Comparative total weight of a hypothetical 12 km diameter antenna 1,700,000,000 kg 110,000,000 kg 790,000 kg

5 SPARSE ARRAY APPROACH Use a very sparse, very large array Eliminate all structure and trusses to minimize mass Populate with very many self-contained, phase controlled picosat repeaters Repeated signals will add coherently in the receiver (or form a narrow beam if transmitter) Very narrow, high gain, mainbeam made possible by very large diameter of the constellation Addition of the many small picosat receive areas results in a large total capture area Beam diameter and aperture area are independent Receiver No truss No structure Very large but very sparse array of large numbers of picosat repeaters. No membrane, no structure From Earth

6 GENERAL CONCEPT OF SPACE-FED SPARSE ARRAY WITH TETHERED RECEIVER Receivers,central computer, downlink to ground km diameter array Constellation of ,000 picosat repeaters functioning as a phased array GEO GEO 100+ km tether kg picosats 100-1,000 m ground spot size Earth

7 HALO ORBITS CONSTELLATION Subsatellites orbit in a plane around the center of the constellation in relative coordinates All satellites are in Keplerian orbits Subsatellite orbit Normal vector of reference orbit Earth Normal vector of constellation plane 60 o Nadir vector 30 o Constellation plane (sub-orbit) Reference (center) orbit Constellation plane (sub-orbit) Constellation plane at t=1/2 p Constellation plane at t=0

8 THE PICOSAT HALO ORBIT CONSTELLATION Picosat positions randomized within the rings. Ring radii also randomized Central receiver Picosats as seen normal to their sub-orbit plane Picosat constellation plane Typical drifts are 100 m in 10 days. The V required for stationkeeping individual picosats is a small fraction of the 100 m/s/year which would be required to hold position in the constellation propulsively. Local horizontal 26.5 deg* maximizes constellation stability * Angle patented by The Aerospace Corporation, USA Picosats as seen from the ground or the central receiver

9 FUNCTIONING OF SPACE-FED PHASED ARRAY AT RF Incoming signals are in phase to λ 20 and add coherently. Final signal is sum of individual picosat signals Phase delays introduced by picosats are calculated based on precision position metrology relative to the central receiver. The actual picosat positions are not critical Spherical wavefronts Added phase delays, modulo 2π (ø4>ø5 ø6) Added phase delays, modulo 2pi (ø1>ø2>ø3) Reference plane ø6 ø5 ø4 ø1 ø2 ø3 Incoming plane wavefronts Picosats A global phase shift pattern is superimposed on the individual picosat shifts across the array to focus and steer the beam

10 PICOSAT POSITION METROLOGY Orbiting navigation reference units in HALO orbits about constellation center Tethered navigation reference units Picosats in plane Same TDOA principle as GPS, but much smaller, lighter, cheaper Several reference units with stable oscillators and low power (short range) transmitters Enables each picosat to determine its own position, and thus compute its own required phase delay 1 cm accuracy should be readily attainable: no ionosphere, no atmosphere, low relative velocities Navigation chips for picosats will be cheap. (Cell phone-mandated GPS chips now cost $10-30) Could use GPS cell phone chips almost as-is, just add shielding; or make new CMS chips Picosats can use same environment to precisely determine their own attitude using differentials

11 PICOSAT CONTROL REQUIREMENTS FOR COHERENT SIGNAL ADDITION Tethered receiver Frequency = 1 GHz Link to receiver Link to receiver >100 m 22 m 0.14 m >100 m 0.14 m Picosats >100 m Position tolerances needed for phase coherence within λ /20 From ground From ground

12 PICOSAT PHASE CONTROL AT RF Narrow band system in GEO assumed Need to control RF phase to about, worst case, for coherent signal addition λ 20 This is equivalent to about 1.5 cm of path length at 1 GHz Space-fed array with tethered receiver loosens the worst case to 14 cm, equivalent to 0.46λ Can use modulo 2 pi phase shift, so need only control phase to about 2 increments This implies a straightforward 1 bit phase shifter at 1 GHz Picosats need to determine and set their own phase, but only infrequently because: The picosats dynamic perturbations will be small and have periods of 6-12 hours Differential drifts of 100 meters in 10 days are typically expected Thus position must be determined and phase adjusted only about once/day Since the picosats compute their own phase, commands are only required to set the global phase for beam steering and focusing, and amplitude taper for sidelobe control

13 DETERMINING REQUIRED NUMBER OF PICOSATS Constellation diameter Individual picosat capture area a = G R λ 2 4π If there are n coherent picosats Equivalent total capture area of all picosats = n x a Distance = R Total capture area of all picosats = n x a The received signal power is P R = P TG T G R λ 2 (4πR) 2 = P T G T na 4πR 2 The receiver noise power is KTB n And if the S/N is to be 10 P Then T G T na =10 4πR 2 KTB n 40πR So that n = 2 KTB P T G T a 2 Signal source

14 IDEALIZED PATTERN OF SPARSE MICROWAVE ARRAY Width of main beam = constellation diameter 2.44λ / Grating lobes are suppressed by randomizing the picosat positions Amplitude of near sidelobes set by the aperture illumination taper Average amplitude of far sidelobes equals 1/number of picosats - Angle off boresight +

15 FOCUSING THE ANTENNA BEAM SPOT 12 km GEO Focusing (and scanning) is accomplished by superimposing a phase pattern across the entire antenna array Distance to spot is 37,000 km Focusing the beam is possible in the near field of the antenna, within a distance of 2D 2 λ = 960,000 km Diffraction limited near-field spot focused to 1 km Far field minimum spot size without focusing = 10 km

16 TOTAL COST OF MASS PRODUCTION 1,000, ,000 10,000 1,000 "Learning" ,000 10, ,000 1,000,000 Number of units produced

17 SUMMARY Extremely large antennas (kilometers) will be desired in the future, especially in GEO Filled apertures of such sizes will have astronomical weights, regardless of technology The only practical solution is to adopt very sparse arrays consisting of hundreds-to-thousands of coherently cooperating picosats/nanosats Sparse arrays allow the total collection area and aperture diameter to be set independently HALO arrays in Hills orbits allow low V formation flying, and form real apertures Sparse space-fed arrays appear the most practical for coherent addition of picosat signals by introducing phase shifts to the picosats to compensate for their inaccurate positions Picosat position can then be allowed to vary greatly (hundreds of meters) In a space-fed array the picosat positions must only be known to about 1/2 wavelength to attain coherence among picosats within 1/20 wavelength at 1 GHz Picosats can determine their own position and control their own phase, or a remote system can track them and relay phase commands to each picosat In the former case GPS could be used, or a local dedicated navigation environment set up Lightweight tethers can position receivers/transmitters far above array without propellants Array diameters of hundreds of kilometers, and collection apertures equivalent to filled antenna diameters of hundreds of meters appear readily attainable in GEO Constellation command and control (the 1,000-body problem) needs to be addressed