ARTEMIS: Low-Cost Ground Station Antenna Arrays for Microspacecraft Mission Support. G. James Wells Mark A. Sdao Robert E. Zee

Transcription

1 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

2 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

3 Deep Space Communications Mars Global Surveyor Nominally 34 m dish antenna only Galileo Low-gain antenna Arraying needed Deep Space Network Overbooked Time is expensive 70m Antenna Goldstone, CA, USA 70m, 34m Antennas Canberra, Australia

4 Communication Link Disparity Spacecraft Transmitter Comparison Ground Station Receiver Comparison Galileo Mars Global Surveyor Microsatelliteclass Radio Frequency (MHz) Spacecraft Transmit Power (mw) Spacecaft Antenna Gain (dbi) EIRP (dbm) Microsat omni antennas lower power 70m Goldstone 3m Antenna 6.1m Antenna Frequency (MHz) Ground Station G/T (db/k) Microsat Ground Station Low sensitivity

5 Ground Station Arrays as a Solution Effective improvement without incurring dramatic price increases while allowing for large aperture areas - costs scale linearly for increasing effective aperture Estimated Cost Comparison between Array & Single Dish Ground Station Combine signals at baseband Very Long Baseline Interferometry (several JPL correlation techniques available) - sharing same oscillator mitigates frequency errors, but is inflexible Estimated Ground Station Cost ($kcdn) SNR Improvement (db) Single Ground Station Array Ground Station

6 JPL Array Signal Combination Techniques RF IF Baseband RF IF Baseband Carrier Demodulation Carrier Demodulation Subcarrier Demodulation (if required) Symbol Stream Combining (SSC) RF IF Baseband Full Spectrum Combining (FSC) Delay & Phase Shift Subcarrier Demodulation (if required) Cross- Correlator Symbol Synch Σ Symbol Synch Σ Telemetry Symbol Determination Time Domain Correlation Only Telemetry Demodulation RF IF Baseband Delay & Phase Shift

7 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 baseband Frequency 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

8 Frequency Correlation: OFDM Signal Frequency Division Multiplexing: Multiple Low Data Rate Channels for easier correlation Orthogonal Frequency Division Multiplexing: More channels enables frequency domain correlation which allows use of less accurate oscillators. Orthogonality ensures spectral efficiency

9 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. RF IF Baseband Delay & Phase Shift Cross- Correlator Σ Telemetry Demodulation RF IF Baseband Delay & Phase Shift Low-Cost Ground Equipment + JPL Arraying Techniques (VLBI, Correlation) + Digital Signal Processing (OFDM)

10 ARTEMIS Microspace Scenarios Scenario Microsatellite (LEO) Microspacecraft (Moon) Microspacecraft (Mars Scenario 1) Microspacecraft (Mars Scenario 2) Frequency (MHz) Spacecraft Transmit Power (mw) Spacecaft Antenna Gain (dbi) EIRP (dbm) Path Loss (dbm) Diameter of Antennas in Ground Array (m) Individual Ground Station G/T (db/k) Data Rate (bps) Individual Receiver Eb/No (db) 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).

11 ARTEMIS Experimental Hardware & Software Development Spacecraft Transmitter DSP Input Data Spacecraft OFDM Signal Generation Digital-to- Analog (DAC) Interface IF Wired Link Array Output Correlation & BER Logs Ground Correlation, Correction & Combination Noise and Array Offsets (Frequency, Time) Analog-to- Digital (ADC) Interface (digital upconversion to IF of up to 38.4 khz) Ground Station Central Site DSP All modules developed in software and running on TI floatingpoint DSPs (optimized for I/FFT function) In future: implement wireless RF link between transmitter and receiver

12 Results Applied to ARTEMIS Microspace Scenarios Microsatellite (LEO) Microspacecraft (Moon) Microspacecraft (Mars Scenario 1) Microspacecraft (Mars Scenario 2) Scenario Diameter of Antennas in Ground Array (m) Data Rate (bps) Individual Receiver Eb/No (db) Required Number of OFDM Channels for Frequency Correlation 128 / / / 128 >4096 / 2048 Min. Array Size to Achieve 2 db Eb/No Equivalent Single Antenna Size (m) 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

13 ARTEMIS DSN Scenario Mars Global Surveyor (DSN) Mars Global Surveyor (small station) Scenario Frequency (MHz) Spacecraft Transmit Power (mw) Spacecaft Antenna Gain (dbi) EIRP (dbm) Path Loss (dbm) Diameter of Antennas on Ground (m) Individual Ground Station G/T (db/k) Data Rate (bps) Individual Receiver Eb/No (db) Mars Global Surveyor Scenario (small station) Diameter of Antennas in Ground Array (m) 6.1 Individual Receiver Eb/No (db) Required Number of OFDM Channels for Frequency Correlation >4096 / 256 Min. Array Size to Achieve 2 db Eb/No 62 ARTEMIS can augment or replace DSN antennas with arrays of small antennas or facilitate the arraying of large antennas to improve ground station performance

14 Current Hardware Experiment Concurrent Time & Frequency Correlation JPL 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

15 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 correlator site) on ground

16 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, 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

17 Natural Resources

18 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

19 Frequency Correlation: OFDM Signal 4-Channel OFDM Symbol Viewed in the Time Domain Orthogonal: k th channel has k cycles Σ Inverse Fast Fourier Transform (IFFT) used to create OFDM Symbol Each channel contains 1 bit of info Time length of each OFDM symbol is 4 times the bit time

20 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

21 Hardware Experiment Results 1st Frequency Correlation Method Frequency Correlation Error (Hz) Single Channel Signal 8-Channel OFDM 16-Channel OFDM 32-Channel OFDM 64-Channel OFDM 128-Channel OFDM 256-Channel OFDM 512-Channel OFDM Individual Receiver Signal Eb/No (db) -500

22 Hardware Experiment Results 2nd Frequency Correlation Method Frequency Correlation Error (Hz) Single Channel Signal 8-Channel OFDM 16-Channel OFDM 32-Channel OFDM 64-Channel OFDM 128-Channel OFDM 256-Channel OFDM 512-Channel OFDM -500 Individual Receiver Signal Eb/No (db)

23 Frequency Correlation: OFDM Signal Carrier Oscillator Freq. = F 1,0,1,1 Y bits/sec. Modulation Symbol Generation (eg. BPSK) -1,+1,-1,-1 D symbol/sec. X Upconvert to RF Transmit Carrier Oscillator Carrier Oscillator Carrier Oscillator Carrier Oscillator IFFT Serial-to-Parallel N-Channels Freq. = D/N Freq. = 2D/N Freq. = 3D/N Freq. = D 1, 0,1,1 Y bits/sec Modulation Symbol Generation (eg. BPSK) X X X X Σ Upconvert to RF Transmit Y/N bits/sec. D/N symbol/sec.

24 OFDM Signal Generation Divide entire transmission into groups of N bits BPSK: Symbols are either +1 or -1 N bits Modulation Mapping eg. BPSK N symbols IFFT One OFDM symbol x(t) Transmitter Receiver N bits Demodulation Mapping eg. BPSK N symbols FFT One OFDM symbol y(t) Combine every group of N bits to recover transmission

25 ARTEMIS Ground Station Block Diagram Remote Sites Antenna 1 Antenna 2 Antenna n a Central Site a 1 (t) a 2 (t) a na (t) Geo. Time Correction & Doppler Correction a 1 (t) a 2 (t) a na (t) Correlator Frequency then Time Offset Corrections Applied to Signals Summation of Corrected Signals A(t) FFT A(f) Correction 2 OFDM Output Σ Dechannelization Correction n a Perform BPSK Demodulation

26 Frequency Correlation Block Diagram a 1 (t) a m (t) FFT a 1 (f) a m (f) Cross-Correlation Algorithm FFT Multiplication IFFT Frequency- Domain Cross- Correlation Function c 1m (υ) Find υ max such that c 1m (υ max ) is a maximum υ max is the frequency offset correction for the m th antenna Frequency correction for m th antenna

27 Time Correlation Block Diagram a 1 (t) a m (t) FFT a 1 (f) a m (f) Multiplication IFFT Time- Domain Cross- Correlation Function c 1m (τ) Find τ max such that c 1m (τ max ) is a maximum τ max is the time offset correction for the m th antenna Time correction for m th antenna Shift a m (f) by υ max (frequency offset)

28 OFDM BER Curve Building Canada s Future In Space

29 OFDM BER Curve With FEC