Human-Competitive Results: Evolved Antennas for Deployment on NASA’s Space Technology 5 Misson

1. Human-Competitive Results:Evolved Antennas for Deployment on NASA’s Space Technology 5 Misson Jason D. Lohn Gregory S. Hornby Derek S. Linden2Evolvable Systems GroupComputational Sciences DivisionNASA Ames Research CenterMountain View, CA USA2JEM Engineering, Laurel, MD USA GECCO-2004, June 2004

2. Human Competitive • CRITERIA D: (D) The result is publishable in its own right as a new scientific result - independent of the fact that the result was mechanically created. • Genetic Programming Theory and Practice Workshop 2004 • IEEE Symposium on Antennas and Propagation, June 20-26, 2004

3. Human Competitive • CRITERIA E&G: (E) The result is equal to or better than the most recent human-created solution to a long-standing problem for which there has been a succession of increasingly better human-created solutions. (G) The result solves a problem of indisputable difficulty in its field. • DIRECT COMPETITION: The antenna designed by the contracting team of human designers for the Space Technology 5 mission -- which won the bid against several competing organizations to supply the antenna – did not meet the mission requirements while the evolved antennas did meet these requirements.

4. Main Points • Interesting Evolutionary Design (not an optimization!) • Evolved Antenna Scheduled to Fly in Space • One of the Top Evolvable Hardware Results to Date • Rapid Re-design Due to Requirements Change • 4 weeks from start-to-first-hardware

5. ST5 Mission • Three nanosats (20in diameter). • Measure effect of solar activity on the Earth's magnetosphere.

6. Transmit: 8470 MHz Receive: 7209.125 MHz Gain: >= 0dBic, 40 to 80 degrees >= 2dBic, 80 degrees >= 4dBic, 90 degrees 50 Ohm impedance Voltage Standing Wave Ratio (VSWR): < 1.2 at Transmit Freq < 1.5 at Receive Freq Fit inside a 6” cylinder Transmit Frequency: 8470 MHz Receive Frequency: 7209.125 MHz Antenna RF Input: 1.5W = 1.76 dBW = 31.76 dBm VSWR: < 1.2 : 1 at the antenna input port at Transmit Freq, < 1.5 : 1 at the antenna input port at Receive Freq Antenna Gain Pattern: Shall be 0 dBic or greater for angles 40 <= theta <=80; 0 <= phi <= 360 Antenna pattern gain (this shall be obtained with the antenna mounted on the ST5 mock-up) shall be 0.0 dBic (relative to anisotropic circularly polarized reference) for angles 40 <= theta <=80; 0 <= phi <= 360, where theta and phi are the standard spherical coordinate angles as defined in the IEEE Standard Test Procedures for Antennas, with theta=0 to direction perpendicular to the spacecraft top deck. The antenna gain shall be measured in reference to a right hand circular polarized sense (TBR). Desired: 0 dBic for theta = 40, 2 dBic for theta = 80, 4 dBic for theta = 90, for 0 <= phi <= 360 Antenna Input Impedance: 50 ohms at the antenna input port Magnetic dipole moment: < 60 mA-cm^2 Grounding: Cable shields of all coaxial inputs and outputs shall be returned to RF ground at the transponder system chasis. The cases of all comm units will be electrically isolated from the mounting surface to prohibit current flow to the spacecraft baseplate. Antenna Size: diameter: < 15.24 cm (6 inches), height: < 15.24 cm (6 inches) Antenna Mass: < 165 g. Original ST5 Antenna Requirements

7. ST5 Quadrifilar Helical Antenna Prior to our work, a contract had been awarded for an antenna design. Result: quadrifilar helical antenna (QHA). Radiator Under ground plane: matching and phasing network

8. 1st Set of Evolved Antennas Non-branching: ST5-4W-03 Branching: ST5-3-10

9. Ultimate Goal Small Amount of Antenna Design Expertise • Transmit Frequency: 8470 MHz • Receive Frequency: 7209.125 MHz • Antenna RF Input: 1.5W = 1.76 dBW = 31.76 dBm • VSWR: < 1.2 : 1 at the antenna input port at Transmit Freq, < 1.5 : 1 at the antenna input port at Receive Freq • Antenna Gain Pattern: Shall be 0 dBic or greater for angles 40 <= theta <=80; 0 <= phi <= 360 • Antenna pattern gain (this shall be obtained with the antenna mounted on the ST5 mock-up) shall be 0.0 dBic (relative to anisotropic circularly polarized reference) for angles 40 <= theta <=80; 0 <= phi <= 360, where theta and phi are the standard spherical coordinate angles as defined in the IEEE Standard Test Procedures for Antennas, with theta=0 to direction perpendicular to the spacecraft top deck. The antenna gain shall be measured in reference to a right hand circular polarized sense (TBR). • Desired: 0 dBic for theta = 40, 2 dBic for theta = 80, 4 dBic for theta = 90, for 0 <= phi <= 360 • Antenna Input Impedance: 50 ohms at the antenna input port • Magnetic dipole moment: < 60 mA-cm^2 • Grounding: Cable shields of all coaxial inputs and outputs shall be returned to RF ground at the transponder system chasis. The cases of all comm units will be electrically isolated from the mounting surface to prohibit current flow to the spacecraft baseplate. • Antenna Size: diameter: < 15.24 cm (6 inches), height: < 15.24 cm (6 inches) • Antenna Mass: < 165 g. Requirements Algorithm Antenna Design Small Amount of Algorithm Design Expertise

10. Branching EA • Generational. • Rank-based selection with elitism of 2. • Population=200 (sometimes seeded w/ individuals from previous run). • Recombination, mutation mutually exclusive. • Recombination: 1pt, 50%. • Mutation: 50%: • Add/delete node (for adds: terminals remain terminals), or • Mutate node type/parameter.

11. rx f 5.0cm f rx rz f f f 2.5cm Feed Wire Antenna Genotype • Genotype is a tree-structured encoding that specifies the construction of a wire form • Genotype specifies design of 1 arm in 3-space: • Branching in genotype results inbranching in wire form

12. rx f f rx rz f f f Antenna Construction Commands • Commands: • forward(length radius) • rotate_x(angle) • rotate_y(angle) • rotate_z(angle) • Forward() command can have 0,1,2, or 3 children. • Rotate_x/y/z() commands have exactly 1 child (always non-terminal).

13. VSWR Fitness Function • Antenna designs are evaluated by NEC4 running on Linux Beowulf supercomputer (80 processors) • 4 evaluations (3 w/added noise) for each design • Fitness function (to be minimized):F = VSWR_Score * Gain_Score * Penalty_Score • VSWRs from both freqs arescaled and multiplied • Gain is sampled at 5 degree increments between theta=40 and theta=90 f = 0 if gain > 0.5 dB f = 0.5 – gain if gain < 0.5 dB • Penalty: proportional to # gain samples less than 0.01 dB

14. Measurements At NASA Goddard Space Flight Center

15. Comparison: Pattern Analysisfor Antennas @ 7.2 GHz Conventional Antenna Evolved Antenna Shaded Yellow Box Denotes Area In-Spec, According to Original Mission Requirements

16. Comparison: Pattern Analysisfor Antennas @ 8.47 GHz Conventional Antenna Evolved Antenna Shaded Yellow Box Denotes Area In-Spec, According to Original Mission Requirements

17. RHCP Gain Coverage forAntenna ST5-3-10 and QHA Evolved ST5-3-10 QHA 8.47 GHz 40 < theta < 80

18. New Mission Requirements • Launch vehicle change: spacecraft will go into LEO (low-earth orbit) • New requirements: • Deep null at zenith not acceptable; No way to salvage original evolved design • Desire to have wider range of angles covered with signal: • Gain: • >= -5dBic, 0 to 40 degrees • >= 0dBic, 40 to 80 degrees • Quadrifilar helical antenna still ok

19. Rapid Response to Changing Requirements • Within 4 weeks we re-evolved new antennas & had prototype hardware • Because of the dual antenna configuration, initial testing leads us to believe that these antennas may yield better coverage • Currently undergoing test

20. 2nd Set of Evolved Antennas EA 1 – Vector of Parameters EA 2 – Constructive Process

21. AXIS OF ROTATION Evolved Antenna on ST5 Mockup BOTTOM DECK TOP DECK GROUND PLANE

22. Measurements

23. Gain vs Theta for Phi=90 degrees QHA-QHA: 38% efficiency EA104-QHA: 80% efficiency EA6303-QHA: 74% efficiency EA6303-EA6303: 97% efficiency

24. % Coverage vs Theta, 7209 MHz

25. Measured Data QHA + QHA (phi 1 = 0 deg, phi 2 = 90 deg): 28% total efficiency, 8470 MHz EA short + QHA (phi 1 = 0 deg, phi 2 = 90 deg): 63% total efficiency, 8470 MHz

26. Benefits • Potential Lower Power: evolved antenna achieves high gain (2-4dB) across a wider range of elevation angles: • allows a broader range of angles over which maximum data throughput can be achieved. • may require less power from the solar array and batteries. • More Uniform Coverage: Very uniform pattern with small ripples in the elevations of greatest interest (40 – 80 degrees): • allows for reliable performance as elevation angle relative to the ground changes. • Inexpensive Design Cycle: 3 person-months to run algorithms and fabricate the first prototype as compared to 5 person-months for contractor team. • On Schedule to be First Evolved Hardware in Space when mission launches in 2005.

27. Conclusion • Evolved an antenna for ST5: • Meets mission requirements. • Better than conventional design. • Currently undergoing space qualification. • Evolutionary design/optimization: • Fast design cycles save time/money. • Fast design cycles allow iterative “what-if”. • Can rapidly respond to changing requirements. • Can produce new types of designs. • May be able to produce designs of previously unachievable performance.