Swarm Communications Using a Sparse Array of...

Swarm Communications Using a Sparse Array of

Small Antennas

February 2018

Jason Weaver, Greg Shreve

GSAW 2018

Northrop Grumman Corporation

Approved For Public Release #18-0133; Unlimited Distribution

© 2018 by Northrop Grumman Corporation.Published by The Aerospace Corporation with permission.

Overview and Motivation for MIIMIO

• Microsats have a small power budget, which intrinsically limits downlink transmitter power and data capacity

– Microsats may collect more data than they can downlink– Large downlink bandwidth requires a large receiving aperture on the ground– Microsat swarms compound the downlink problem

• The Multiple Independent Inputs, Multiple Independent Outputs (MIIMIO) sparse ground receive array concept provides microsatswarm downlink advantages

– Provides large receive aperture, to enable wideband downlinks with low transmitter power

– Enables simultaneous downlinks from all microsats in the swarm, using same frequency

– Microsats transmit independently, without coordination– Low cost ground antennas point open loop, providing the field of regard and total

aperture gain

2Approved For Public Release #18-0133; Unlimited Distribution

MIIMIO Downlink Overview System Drawing

3

Satellite swarmSwarm orbit

trajectory

Effective beamforming aperture –antennas may be distributed over

hundreds of meters

Sum of aperture area equivalent

to one large dish

Relatively wide beamwidthof each MIIMIO antenna

(the Field of Regard)

Narrow beamwidth of one MIIMIO effective receive aperture

Traditional approach – one large antenna with equivalent aperture gain• Can only point at one or a couple of adjacent

satellites in the swarm• Needs frequency division multiplexing

Downlink footprint of each satellite

MIIMIO Sparse Ground Receiver Array

• Digital beamforming creates a narrow high gain receive beam for each satellite in the swarm

The swarm may subtend angle of 1 degree or more, as seen from ground array


MIIMIO Features

• Transmitters on the microsats are independent – no coordination required among them

• The initial MIIMIO application is for reception of separate, re-used frequency downlinks, from a swarm of closely-spaced microsats

• Sparse array of receiving antennas on the ground– Similar to high resolution sparse array interferometers in radio astronomy– Ground antenna elements are typically 100 to 1,000 wavelengths apart– Compute very narrow beams to isolate each downlink– Total receiving area (antenna aperture x number of array elements) sets link

budget– Each microsat can downlink a large bandwidth

• This is -not- MIMO; no need for channel modeling


Sparse Aperture Digital Receive Beamforming Details

5

DigitizeBufferD/C

Network

Digital Beamforming(& Nulling)

Demodulators

N Beams

Ethernet Network from Antennas

MIIMIO arrayM Antennas

N Outputs

Digital true time delay signal combining

Downconverters and digitizer sampling clocks can & should be freq/phase locked for all antennas

Software demods

N satellites

Each satellite downlink

Electronics at each antenna:Downconverter, digitizer, memory buffer

MInputs

Frequency & Timing for Iterative Trajectory Reconstruction

Swar

m

Proc

essi

ng

Linear beamforming and nulling matrix


Numerical Example

• X-band downlink, 9 GHz

• 36 antennas, 1.8 meter diameter, open loop pointing– Half-power beamwidth (HPBW) provides 1.2° field of regard

• Equivalent single dish collection area = 10 meter diameter– Total RX gain = 56 dB

• Sparse array distributed over 150 meters diameter– Each formed beam HPBW = 0.015°– On average, only one satellite in the swarm will be in the full-gain boresight

• Nulls can be computed to further reduce co-channel interference in sidelobes and grating lobes


Notional Nulling in Swarm

• Can use traditional receive beamforming to accomplish reduction of interferers by nulling

7

X

XXX

X

X X

O

Zenith

Horizon

Swarm

Array gain peak on

Satellite n

Other satellites nulled


Notional Simulation - Beamforming Plus Multiple Nulls

• Sharp nulls can be computed for each beam, to null some or all of the other satellite downlinks in the swarm to further reduce the cross-channel interference

• A number of well-formed nulls can be multiplied with little reduction of main beam gain, while providing significant reduction of interferers

8

‐20

‐15

‐10

‐5

0

‐2 ‐1.5 ‐1 ‐0.5 0 0.5 1 1.5 2

Gain in

dB

Boresight angle in degrees

Synthesized Beams with Nulling

Beam 1 Beam 2 Beam 3

Clutter Rcvr Noise Field of Regard

NULL 1 NULL 2

Nulls not adaptively centered; have little effect on beam gain

Nulls centered on beams not of interest; suppression of >15 dB

‐20

‐15

‐10

‐5

0

‐2 ‐1.5 ‐1 ‐0.5 0 0.5 1 1.5 2

Gain in

dB

Boresight angle in degrees

Synthesized Beams with Nulling

Beam 1 Beam 2 Beam 3

Clutter Rcvr Noise Field of Regard

NULL 1 NULL 2

Synthesized Beams without Nulling


Simulation - Comparison of Transmitted Signal vs. Recovered Signal

• In simulation, use time domain (burst transmissions) to show received signals relative to interference and noise

9

Mag

nitu

de (V

)M

agni

tude

(V)


Simulation - Signals Received at Antenna Array

• Interference Signals at [30, 45.5 ] & [31, 45.5] ([az,el])

10

Mag

nitu

de (V

)


Normalized Power (dB), Broadside at 0.00 degrees

Elevation Cut (azimuth angle = 45.0°)

-40

-30

-20

-10

-150

0

30

60

-120

90

-90

120

-60

150

-30

180 0

Simulation - Polar Radiation Plot of 36 Element Array and Parabolic Pattern Superimposed

11

This pattern includes a superimposed antenna element gain with < 3° HPBW

Swarm


Conclusions

• Mature sparse array beamforming provides swarm support

• Microsats are independent and can be as simple as possible– No crosslinks needed for on-orbit coordination

• Beams can be computed after the LEO flyover– Iteratively optimize LEO tracking, formed beam gain, nulling and cross-channel

cancellation across the flyover arc

• Low cost ground– Small antennas with open loop pointing– Commodity digitizers– Offline beam computing


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