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

Ames Research CenterAmes Research Center

Jason D. Lohn Jason D. Lohn Gregory S. Hornby Gregory S. Hornby

Derek S. LindenDerek S. Linden22

Evolvable Systems GroupEvolvable Systems GroupComputational Sciences DivisionComputational Sciences Division

NASA Ames Research CenterNASA Ames Research CenterMountain View, CA USAMountain View, CA USA

22JEM Engineering, Laurel, MD USAJEM Engineering, Laurel, MD USA

Jason D. Lohn Jason D. Lohn Gregory S. Hornby Gregory S. Hornby

Derek S. LindenDerek S. Linden22

Evolvable Systems GroupEvolvable Systems GroupComputational Sciences DivisionComputational Sciences Division

NASA Ames Research CenterNASA Ames Research CenterMountain View, CA USAMountain View, CA USA

22JEM Engineering, Laurel, MD USAJEM Engineering, Laurel, MD USA

Human-Competitive Results:Human-Competitive Results:Evolved Antennas forEvolved Antennas for

Deployment on NASA’s Deployment on NASA’s Space Technology 5 Misson Space Technology 5 Misson

Human-Competitive Results:Human-Competitive Results:Evolved Antennas forEvolved Antennas for

Deployment on NASA’s Deployment on NASA’s Space Technology 5 Misson Space Technology 5 Misson

GECCO-2004, June 2004


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


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.


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


ST5 Mission

• Three nanosats (20in diameter).

• Measure effect of solar activity on the Earth's magnetosphere.


Original ST5 Antenna Requirements

• 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.


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

ST5 Quadrifilar Helical Antenna


1st Set of Evolved Antennas

Non-branching:ST5-4W-03

Branching:ST5-3-10


• 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.

Ultimate Goal

Alg

ori

thm

AntennaDesign

Requirements

Small Amount of Antenna Design

Expertise

Small Amount of Algorithm Design

Expertise


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.


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

rx f

f

f f

rz rx

f

2.5cm

5.0cm

Feed Wire


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).

rx f

f

f f

rz rx

f


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 are

scaled and multiplied

• Gain is sampled at 5 degree increments between theta=40 and theta=90

f = 0 if gain > 0.5 dBf = 0.5 – gain if gain < 0.5 dB

• Penalty: proportional to # gain samples less than 0.01 dB

VSWR


Measurements

At NASA Goddard Space Flight Center


Comparison: Pattern Analysisfor Antennas @ 7.2 GHz

Max and Min Gain vs Theta for 7.2 GHz

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

0 10 20 30 40 50 60 70 80 90

Theta (deg)

Gai

n Max

Min

Conventional Antenna Evolved Antenna

Shaded Yellow Box Denotes Area In-Spec, According to Original Mission Requirements


Max and Min Gain vs Theta for 8.47 GHz

-35

-30

-25

-20

-15

-10

-5

0

5

10

0 10 20 30 40 50 60 70 80 90

Theta (deg)

Gai

n Max

Min

Comparison: Pattern Analysisfor Antennas @ 8.47 GHz

Conventional Antenna Evolved Antenna

Shaded Yellow Box Denotes Area In-Spec, According to Original Mission Requirements


RHCP Gain Coverage forAntenna ST5-3-10 and QHA

RHCP Gain Coverage Statistics

0

10

20

30

40

50

60

70

80

90

100

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Gain (dBic)

Co

ve

rag

e %

7.2 GHz

8.47 GHz

QHA 8.47 GHzEvolved ST5-3-10

40 < theta < 80


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


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


2nd Set of Evolved Antennas

EA 1 – Vector of Parameters EA 2 – Constructive Process


Evolved Antenna on ST5 Mockup

TOPDECK

BOTTOMDECK

GROUNDPLANE

AXIS OFROTATION


Measurements


Gain vs Theta for Phi=90 degrees

QHA-QHA: 38% efficiencyEA104-QHA: 80% efficiency

EA6303-QHA: 74% efficiencyEA6303-EA6303: 97% efficiency


% Coverage vs Theta, 7209 MHz


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


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.


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.

Documents

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