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 Laboratory University of Toronto Institute for Aerospace Studies 11 August 2004
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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
Building Canada’s Future In Space
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
• 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
Building Canada’s Future In Space
JPL Array Signal Combination Techniques
RF → IF →Baseband
RF → IF →Baseband
Carrier Demodulation
SubcarrierDemodulation (if required)
Symbol Synch
Carrier Demodulation
SubcarrierDemodulation (if required)
Symbol Synch
ΣTelemetry
Symbol Determination
RF → IF →Baseband
RF → IF →Baseband
Delay & Phase Shift
Delay & Phase Shift
Cross-Correlator Σ Telemetry
Demodulation
Symbol Stream Combining (SSC)
Full Spectrum Combining (FSC)
Time DomainCorrelation Only
Building Canada’s Future In Space
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
More channels enablesfrequency domain correlation which allows use of less accurate oscillators.
Orthogonality ensures spectral efficiency
Building Canada’s Future In Space
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)
+
RF → IF →Baseband
RF → IF →Baseband
Delay & Phase Shift
Delay & Phase Shift
Cross-Correlator Σ Telemetry
Demodulation
Building Canada’s Future In Space
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).
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
Building Canada’s Future In Space
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)
Mars Global Surveyor
(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
Mars Global Surveyor
(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
Building Canada’s Future In Space
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
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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
Building Canada’s Future In Space
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
Building Canada’s Future In Space
Natural Resources
Building Canada’s Future In Space
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
Building Canada’s Future In Space
Frequency Correlation: OFDM Signal
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
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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
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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
Building Canada’s Future In Space
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
Building Canada’s Future In Space
Frequency Correlation: OFDM Signal
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
Building Canada’s Future In Space
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