Bistatic Radar Receiver for CubeSats: The RAX Payload John Buonocore Hasan Bahcivan SRI International 7 th Annual CubeSat Developer s Workshop 22 April 2010 Cal Poly San Luis Obispo SRI Proprietary
RAX Mission Overview Mission Payload Transmitter Support High-resolution Mapping of Auroral Ionospheric Irregularities UHF Radar Receiver Ground-based MW-Class IS Radar NSF CubeSat Program (NSF08-549) Selected September 2008 Delivered February 2010 Launch Orbit Lifetime TBD 2010, DoD Minotaur-4 from KLC 650 km circular, 72 deg. inclination 1 year primary + 5 year secondary
RAX Experiment Description RAX is bistatic - Receiver located far from Transmitter Maps irregularities with high spatial & angular resolution Ground-based ISR also measures background plasma state & E-Field Radio aurora backscatter cones RAX Receiver Poker Flat Advanced Modular Incoherent Scatter Radar (Alaska) Irregularities Incoherent Scatter Radar (ISR)
RAX Compatibility with Global ISRs Name Loc Lat Freq. MHz MW Beam width PFISR Alaska 78 449 2.0 1º RISR* Canada 81 442 2.0 1º PFISR MHO MUIR MHO Arecibo ESR Alaska Massachusetts Puerto Rico Norway 62 53 34 75 446 440 430 500 0.25 2.5 2.5 1.0 10º 0.6º 0.2º 0.6º Arecibo ESR
Challenges for RAX Radar Receiver Design Traditional (monostatic) Radar Synchronizes TX pulses with RX range gate timing Shares local oscillator (LO) with TX and RX subsystems Bistatic Radar Requires accurate independent synchronization scheme Requires LO stability - especially during ISR flyover Additional Requirements (in addition to CubeSat SWAP) Quick recovery from direct-path ISR illumination Extremely large dynamic range Immunity from bus-generated EMI (shared COMMS antenna) Efficient dissipation of thermal loads Avoidance of radiation sensitive components Tunability, gain control, housekeeping status
Synchronization Scheme GPS Based Receiver Sample clock is allowed to free-run Receiver samples I & Q data at 1 MHz Samples time-stamped via onboard GPS PPS Transmitter Timing standard is free-running Time-stamp first TX pulse to GPS time Time and drift values sent to spacecraft Overflow Based Direct-path TX signal saturates receiver every second Record ADC overflow bit on satellite TX signal time-stamp and drift values sent to spacecraft
Frequency Stability Considerations Transmitter and Receiver oscillators are free-running Short-term stability is very good A small frequency offset between TX and RX is okay Measure TX signal offset using Receiver
RAX Payload Receiver Primarily Analog Industrial Components Pulse (>2μS) or CW operation 426 510 MHz (1 MHz steps) 4-bands Adjustable Gain Internal Voltage Regulation Continuous Sampling at 14-bit Resolution In-phase and Quadrature (I/Q) Signals Internal 500 MHz Calibration Source ANT INPUT (SMA) POWER/DATA/CONTROL (50-pin MDR) Enclosure I/Q RX BOARD RF(CAL) RF(LO) RF(CLK) DC/CTL LO BOARD DC/CTL PWR CONV BOARD SHIELDED ENCLOSURE BOARD STACKUP Provides EMI Shield Thermally Dissipative 9.7cm x 9.7cm x 3.6cm Weight 320 g Power 2.6 W
I/Q Receiver Board Homodyne Design Direct Conversion (No IF) Calibration Internal 500 MHz source Preselector SAW Bandpass Filters 4 Bands: 426 434 MHz 437 445 MHz 443 452 MHz 483 510 MHz I2C Over Flow RF Input I/Q Output 1 MHz Clock Gain -4 to +58 db (2 db steps) Mixer Active I & Q outputs High Dynamic Range 2X LO input I/Q Filter Passive LC, 250 khz BW 10-pole Bessel function ADC Dual Interleaved I/Q 14-Bit, 1 MHz Sampling 500 MHz BAND Cal Signal GAIN PCB 4-layer FR-4 construction Thermal Transfer Perimeter 8.6cm x 8.6cm LO 10 MHz Input Clock
I/Q Receiver Performance Noise figure 3.8 db (400 K) Noise Floor -114 dbm Gain Range Single Frequency Dynamic Range (DR) Dual Frequency Spurious Free DR LO Radiation Max RF Input (no damage) Max Recovery Time Recovery to 1 db -4 db to +58 db 60 db 54 db < -80 dbm +20 dbm 30 μs 10 μs
I/Q Receiver Selectivity
Local Oscillator Board 10 MHz Reference Oscillator TCXO Digital compensation ±0.28 ppm, -40C to +85C Stabilization time ~200 sec. (cold start) I2C monitor for crystal temp I2C control for freq push/pull (1ppb or 0.01Hz) Freq Select I2C ADC Clock Buffered 10 MHz from TCXO Conversion to sinewave to reduce EMI 10 MHz ADC Clock 500 MHz Cal Signal Mixer Local Oscillator Phase-locked system 852-1020 MHz 2 MHz control resolution Cal Oscillator 500 MHz free running I2C on/off control LO Output to Mixer PCB 4-layer FR-4 construction Thermal Transfer Perimeter 7.6cm x 7.6cm
Power Converter Board PWM DC/DC Buck Regulators High efficiency (>90%) Wide input range (3-17V) Regulated outputs: +5V, +3V Integrated design low radiated EMI 5 V Out 7.5 V Input 3 V Out LC Filtering on Input and Outputs Voltage and Temperature Monitoring I2C Shielding Continuous external ground plane Compartment in housing PCB 2-layer FR-4 construction Thermal Transfer Perimeter 8.6cm x 8.6cm
Temperature Cycling in (Moderate) Vacuum Equipment 14L Enclosure with 0.6 T Pump 10cm X 10cm liquid coolant thermal Plate Temperature range (-60C to +60C) Setup RAX Receiver in 1U Pumpkin frame Multiple thermal sensors Procedure Baseline receiver operation RX OFF: Cold-soak @ -40C for 30 min RX ON: Start logging data Temperature ramp-up (240 min. to +55C) Post-test operational check Results TCXO drift <0.24 ppm (spec: +/-0.28ppm) PLL maintained lock RF gain variation ± 1.5 db Internal DC voltages stable
Crystal Oscillator Thermal Response RAX Payload Receiver Thermal Testing 1-27-10 Internal/External Temperatures and TCXO Drift vs. Time 60 1000000.05 50 40 1000000 30 999999.95 20 Temperature (deg C) 10 0-10 1 21 41 61 81 101 121 141 161 181 201 221 999999.9 999999.85 TCXO Frequency (Hz) Temp RF PCB (deg C) Temp PLL PCB (deg C) Temp PS PCB (deg C) Temp TCXO (deg C) Frame Temp. RTD (deg C) Bottom Plate Temp (deg C) 1MHz TCXO Freq (Hz) -20-30 999999.8-40 999999.75-50 -60 999999.7 Time (mins)
Board-Level Thermal Response RAX Payload Receiver Thermal Testing - 1/27/10 Internal Temperature and Voltage Monitoring vs. Time 8 60 50 7 40 30 6 RF PCB (Deg C) PLL PCB (Deg C) Temperature (deg C) 20 10 0-10 1 16 31 46 61 76 91 106 121 136 151 166 181 196 211 226 5 4 3 Voltage (V) PS PCB (Deg C) TCXO (Deg C) 7.5V bus (V) 5V bus (V) 3V bus (V) -20 2-30 -40 1-50 0 Time (mins)
Vibration Testing Pre-Test Sine-Sweep Baseline all resonances Range: 20-2000 Hz @ 0.5G Rate: 3 Oct/Min One sweep per axis Random Vibration Test 20-2000 Hz 10.4 G rms 1 minute/axis Post-Test Sine-Sweep Same as pre-test Compare pre/post resonances Results No change in resonances Pass - Visual inspection Pass - Post electrical tests
Functional Testing Radar Simulator Generates radar direct and scattered signals Variable timing (Pulse Width, IPP) Adjustable transmit frequency RF SWITCH Payload Interface Module (PIM) Simulator Direct interface to payload receiver Supplies power and control to payload Collects and stores I & Q samples Integrated Testing with Cubesat BUS PIM interface Antenna Interface I2C checkout Full-up EMI validation RF PULSE AMPLITUDE AT ANTENNA INPUT TX1 (Direct) INTER-PULSE PERIOD (IPP) TX2 (Scattered) TX1 TX2
The Path Ahead RAX Science Planning at 2010 NSF CEDAR Workshop Future Applications Antenna pattern measurements for large GB Radars Other bistatic radar experiments in the UHF band Global UHF LEO Noise Survey Ongoing life tests 11-week (24/7) operational burn-in No failures Future design modifications Investigate Digital down-converters Lower-power techniques Lighter materials Integrate PIM Functions within Structure Explore PnP Capabilities