ARTEMIS - DigitalCommons@USU

Building Canada’s Future In Space

ARTEMIS:Low-Cost Ground Station

Antenna Arrays for Microspacecraft Mission Support

G. James Wells Mark A. Sdao Robert E. Zee

Space Flight LaboratoryUniversity of Toronto Institute for Aerospace Studies

11 August 2004


Overview• Low S/N due to extreme range or high data rate and

low spacecraft power. E.g. microspacecraft.

• Deep space communications – JPL antenna arraying

• Low cost antenna arrays – issues and solutions

• ARTEMIS: software & hardware prototyping

• Experiments and scenario analysis – microspace & JPL missions

• Conclusions: applications of ARTEMIS to low cost arrays and DSN augmentation


Deep Space Communications

70m Antenna Goldstone, CA, USA

70m, 34m Antennas Canberra, Australia

Mars Global Surveyor Galileo

Low-gain antennaArraying needed

Nominally 34 m dish antenna only

Deep Space Network• Overbooked• Time is expensive

http://deepspace.jpl.nasa.gov/dsn/gallery/images/goldstone4.jpg


Communication Link Disparity

GalileoMars Global

SurveyorMicrosatellite-

class RadioFrequency (MHz) 2290 8400 2232

Spacecraft Transmit Power (mW) 15000 10000 5000

Spacecaft Antenna Gain (dBi) 7 42 0

EIRP (dBm) 47.8 82.0 36.0

70m Goldstone 3m Antenna 6.1m AntennaFrequency (MHz) 2290 2232 2232

Ground Station G/T (dB/K) 59.02 10.71 16.80

Spacecraft Transmitter Comparison

Microsat• omni antennas• lower powerGround

Station Receiver

ComparisonMicrosat Ground Station• Low sensitivity


Ground Station Arrays as a Solution

• Effective improvement without incurring dramatic price increaseswhile allowing for large aperture areas - costs scale linearly for increasing effective aperture

• Combine signals at baseband• Very Long Baseline Interferometry (several JPL correlation

techniques available) - sharing same oscillator mitigates frequency errors, but is inflexible

34

134

234

334

434

534

634

734

834

934

0 2 4 6 8 10 12

SNR Improvement (dB)Es

timat

ed G

roun

d St

atio

n C

ost

($kC

DN) Single Ground

StationArray GroundStation

Estimated Cost Comparison

between Array & Single Dish Ground

Station


JPL Array Signal Combination Techniques

RF → IF →Baseband


Carrier Demodulation

SubcarrierDemodulation (if required)

Symbol Synch

Carrier Demodulation

SubcarrierDemodulation (if required)

Symbol Synch

ΣTelemetry

Symbol Determination



Delay & Phase Shift

Delay & Phase Shift

Cross-Correlator Σ Telemetry

Demodulation

Symbol Stream Combining (SSC)

Full Spectrum Combining (FSC)

Time DomainCorrelation Only


Ground Station Arrays as a Solution

• VLBI array must be capable of compensating for errors such as :

Phase offsets due to each antenna receiving the signal at a different time due to their different geographic locations (Atomic clocks, GPS to solve this)Phase offsets introduced when downconverting the signal from RF to basebandFrequency and phase offsets between the various local oscillators of each antenna in the array and the spacecraft(DSN uses hydrogen masers to give high accuracy)

• Array decorrelation will occur unless these errors can be detected and corrected


Frequency Correlation: OFDM Signal

• Frequency Division Multiplexing:

• Orthogonal Frequency Division Multiplexing:

Multiple LowData Rate Channelsfor easier correlation

More channels enablesfrequency domain correlation which allows use of less accurate oscillators.

Orthogonality ensures spectral efficiency


Low-Cost Alternative: ARTEMIS• ARraying Techniques for Enhanced Multiplexing of Interferometric Signals• JPL arraying techniques: Full Spectrum Combining• Orthogonal Frequency Division Multiplexing (OFDM)• Use both Frequency Domain and Time Domain Correlation.• Time domain correlation easier due to multiple low data rate carriers.• Can use sloppier equipment or improve correlation capabilities.

Low-Cost Ground

Equipment+

JPL Arraying Techniques (VLBI, Correlation)

Digital Signal Processing (OFDM)

+



Delay & Phase Shift

Delay & Phase Shift

Cross-Correlator Σ Telemetry

Demodulation


ARTEMIS Microspace ScenariosScenario

Microsatellite (LEO)

Microspacecraft (Moon)

Microspacecraft (Mars Scenario 1)

Microspacecraft (Mars Scenario 2)

Frequency (MHz) 2232 2232 2232 2232Spacecraft Transmit

Power (mW) 400 4000 4000 4000Spacecaft Antenna

Gain (dBi) 0 0 0 0EIRP (dBm) 26.0 36.0 36.0 36.0

Path Loss (dBm) -172.9 -212.2 -265.0 -265.0Diameter of Antennas

in Ground Array (m) 3 3 6.1 6.1Individual Ground Station G/T (dB/K) 10.3 10.7 16.8 16.8

Data Rate (bps) 4000000 14400 1 15Individual Receiver

Eb/No (dB) -4.1 -8.5 -13.6 -25.4

• ARTEMIS provides both frequency and time domain correlation and allows for use of low-cost commercial-grade equipment (commercially available oscillators) in a ground station array.

• Better correlation gets arraying gain closer to theoretical combining gain limit (determined by aperture area).

http://www.utias-sfl.net/microsatellites/MOST/MOSTIndex.html


ARTEMIS Experimental Hardware & Software Development

Spacecraft OFDM Signal

Generation

Digital-to-Analog (DAC)

Interface

Analog-to-Digital (ADC)

Interface

Ground Correlation, Correction & Combination

Spacecraft Transmitter DSP

Ground Station Central Site DSP

Input Data

Array Output

Correlation & BER Logs

Noise and Array Offsets

(Frequency, Time)

IF Wired Link

(digital upconversion to IF of up to 38.4

kHz)

• All modules developed in software and running on TI floating-point DSPs (optimized for I/FFT function)

• In future: implement wireless RF link between transmitter and receiver


Results Applied to ARTEMIS MicrospaceScenarios

ScenarioMicrosatellite

(LEO)Microspacecraft

(Moon)Microspacecraft

(Mars Scenario 1)Microspacecraft

(Mars Scenario 2)Diameter of Antennas in

Ground Array (m) 3 3 6.1 6.1Data Rate (bps) 4000000 14400 1 15

Individual Receiver Eb/No (dB) -4.1 -8.5 -13.6 -25.4

Required Number of OFDM Channels for

Frequency Correlation 128 / 16 512 / 32 2048 / 128 >4096 / 2048Min. Array Size to

Achieve 2 dB Eb/No 4 12 38 500Equivalent Single Antenna Size (m) 6 10 38 136

• Results of scenarios extrapolated using hardware experimental results; they also validated the original software simulations done in Matlab

• 2 dB combined signal Eb/No will give a bit error rate of 10-5 for a BPSK modulated OFDM signal assuming Reed-Solomon & convolutional forward error correction is used


ARTEMIS DSN Scenario

• ARTEMIS can augment or replace DSN antennas with arrays of smallantennas or facilitate the arraying of large antennas to improve ground station performance

Scenario

Mars Global Surveyor

(DSN)


(small station)Frequency (MHz) 8400 8400

Spacecraft Transmit Power (mW) 10000 10000

Spacecaft Antenna Gain (dBi) 42 42

EIRP (dBm) 82 82Path Loss (dBm) -276.56 -276.56

Diameter of Antennas on Ground (m) 34 6.1

Individual Ground Station G/T (dB/K) 51.77 25.56

Data Rate (bps) 42667 42667Individual Receiver

Eb/No (dB) 9.51 -16.70

Scenario


(small station)Diameter of Antennas in

Ground Array (m) 6.1Individual Receiver

Eb/No (dB) -16.70Required Number of OFDM Channels for

Frequency Correlation >4096 / 256Min. Array Size to

Achieve 2 dB Eb/No 62


Current Hardware Experiment

• Concurrent Time & Frequency CorrelationJPL FSC, used by the DSN to correct time offsets, would fail if any significant frequency offset were to occur between the oscillators The structure of the OFDM signals makes it possible for ARTEMIS to perform both time & frequency correlation even if both types of offsets are present


Future Hardware Experiments

• Addition of an RF link to current experimental apparatus

• LEO flight experiment on a future SFL mission

OFDM transceiver in orbit (S-Band)ARTEMIS array (including central correlatorsite) on ground


Conclusion• Deep Space Communications:

ARTEMIS as a low-cost alternative to DSN

• For new ground stations, can use low-cost RF equipment (e.g. inexpensive oscillators, 100-1000 times less stable)

• Can create ad-hoc array with existing antenna infrastructure (large or small) using low-cost equipment.

• Microspace Applications of ARTEMIS

• High data rate LEO missions

• Greater range: Interplanetary Microsats

• Concept Study Completed.

• Hardware & Software Prototyping in Progress and Nearing Completion

• LEO flight experiment on future SFL mission


Natural Resources


What Does ARTEMIS Offer?

• ARTEMIS offers a low-cost alternative to the DSN for spacecraft communications

Enables higher data rate microsatellite LEO missions

Enables interplanetary microspacecraft missions

• ARTEMIS can also provide accurate ranging & tracking of interplanetary spacecraft

• Retrofit existing assets or develop low-cost new assets



4-Channel OFDM Symbol Viewed in the Time Domain

Orthogonal: kth channel has k cycles

Inverse Fast Fourier Transform (IFFT) used

to create OFDM Symbol

Σ

Time length of each OFDM symbol is 4 times the bit time

Each channel contains 1 bit of info


Frequency Correlation Techniques Tested

1) Correlate, between individual receivers, every OFDM symbol

2) Correlate, between each receiver and the central site DSP, an OFDM symbol with a known structure (ie. training symbol) periodically inserted into the data transmission


Hardware Experiment Results

• 1st Frequency Correlation Method

-500

0

500

1000

1500

2000

-25-20-15-10-5051015

Individual Receiver Signal Eb/No (dB)Fr

eque

ncy

Cor

rela

tion

Erro

r (H

z)

Single Channel Signal8-Channel OFDM16-Channel OFDM32-Channel OFDM64-Channel OFDM128-Channel OFDM256-Channel OFDM512-Channel OFDM


Hardware Experiment Results

• 2nd Frequency Correlation Method

-500

0

500

1000

1500

2000

-25-20-15-10-5051015

Individual Receiver Signal Eb/No (dB)Fr

eque

ncy

Cor

rela

tion

Erro

r (H

z)

Single Channel Signal8-Channel OFDM16-Channel OFDM32-Channel OFDM64-Channel OFDM128-Channel OFDM256-Channel OFDM512-Channel OFDM



Modulation Symbol

Generation (eg. BPSK)

-1…,+1,-1,-1D symbol/sec.

X

Carrier Oscillator

Freq. = F

Upconvertto RF

TransmitY bits/sec.1…,0,1,1

XX

X

X

Σ

IFFT

Modulation Symbol

Generation (eg. BPSK)

110

1

Y bits/sec.1,…0,1,1

Y/N bits/sec.

Serial-to-Parallel N-Channels

-1-1+1

-1

D/N symbol/sec.

Upconvertto RF

Transmit

Carrier Oscillator

Freq. = D/N

Carrier Oscillator

Freq. = 2D/N

Carrier Oscillator

Freq. = 3D/N

Carrier Oscillator

Freq. = D


OFDM Signal Generation

Modulation Mapping eg. BPSK

N bits IFFT

Transmitter

Receiver

x(t)

One OFDM symbolN symbols

Demodulation Mapping eg. BPSK

N bits FFT

y(t)

One OFDM symbolN symbols

Divide entire transmission into groups of N bits

Combine every group of N bits to recover transmission

BPSK: Symbols are either +1 or -1


ARTEMIS Ground Station Block Diagram

Antenna 1

Antenna 2

Antenna na

Remote Sites

Central Site

a1(t)

a2(t)

ana(t)

Geo. Time Correction

&

Doppler Correction

a1(t)

a2(t)

ana’(t)

Correlator

Frequency then Time

Offset Corrections Applied to

SignalsCorrection 2

Correction na

Σ Output

Perform BPSK Demodulation

Summation of Corrected Signals

A(t)FFT

OFDM Dechannelization

A(f)


Frequency Correlation Block Diagram

a1(f)

am(f)

FFT Multiplication IFFT

Frequency-Domain Cross-

Correlation Function

c1m(υ)

Find υmax such that c1m(υmax) is a

maximum

υmax is the frequency offset correction for

the mth antenna

FFT

a1(t)

am(t)

Frequency correction

for mth

antenna

Cross-Correlation Algorithm


Time Correlation Block Diagram

a1(f)

am(f)

Multiplication IFFT

Find τ max such that c1m(τ max) is a

maximum

τ max is the time offset correction for the mth

antenna

Time-Domain Cross-

Correlation Function

c1m(τ)FFT

a1(t)

am(t)

Shift am(f) by υmax

(frequency offset)

Time correction

for mth

antenna


OFDM BER Curve


OFDM BER Curve With FEC

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