Bistatic Radar Receiver for CubeSats:The RAX Payload

John BuonocoreHasan Bahcivan

SRI International

SRI Proprietary

7th Annual CubeSat Developer’s Workshop22 April 2010

Cal Poly San Luis Obispo

Mission High-resolution Mapping of Auroral Ionospheric Irregularities

Payload UHF Radar Receiver

Transmitter Ground-based MW-Class IS Radar

Support NSF CubeSat Program (NSF08-549)

Selected September 2008

Delivered February 2010

Launch TBD 2010 , DoD Minotaur-4 from KLC

Orbit 650 km circular, 72 deg. inclination

Lifetime 1 year primary + 5 year secondary

RAX Mission Overview

RAX Experiment Description

Radio aurora backscatter cones

RAX Receiver

Incoherent Scatter Radar (ISR)

• 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

Poker Flat Advanced ModularIncoherent Scatter Radar (Alaska)

Irregularities

RAX Compatibility with Global ISRs

Name Loc Lat

Alaska 78

81

62

53

34

75

Canada

AlaskaMassachusetts

Puerto Rico

Norway

Freq.MHz MW

Beamwidth

PFISR 449 2.0 1º

RISR* 442 2.0 1º

MUIR 446 0.25 10º

MHO 440 2.5 0.6º

Arecibo 430 2.5 0.2º

ESR 500 1.0 0.6º

PFISR

MHO

AreciboESR

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-runReceiver samples I & Q data at 1 MHzSamples time-stamped via onboard GPS PPS

• TransmitterTiming standard is free-running Time-stamp first TX pulse to GPS timeTime and drift values sent to spacecraft

Overflow Based

Direct-path TX signal saturates receiver every secondRecord ADC overflow bit on satelliteTX 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

• 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

DC/CTLRF(CLK)RF(LO)

SHIELDEDENCLOSURE

LO BOARD

I/Q RX BOARD

BOARD STACKUP

POWER/DATA/CONTROLANT INPUT(SMA)

DC/CTL

(50-pin MDR)

PWR CONV BOARD

RF(CAL)

RAX Payload Receiver

• Provides EMI Shield • Thermally Dissipative • 9.7cm x 9.7cm x 3.6cm• Weight 320 g• Power 2.6 W

Enclosure

CalibrationInternal 500 MHz source

PreselectorSAW Bandpass Filters4 Bands:

426 – 434 MHz437 – 445 MHz443 – 452 MHz483 – 510 MHz

Gain-4 to +58 dB (2 dB steps)

MixerActive – I & Q outputsHigh Dynamic Range2X LO input

I/Q FilterPassive LC, 250 kHz BW10-pole Bessel function

ADCDual Interleaved I/Q14-Bit, 1 MHz Sampling

I/Q Receiver Board

500 MHzCal Signal

I/Q Output

1 MHz Clock

RF Input

I2C

BAND GAIN LO Input

10 MHz Clock

Homodyne Design -Direct Conversion (No IF)

PCB4-layer

FR-4 constructionThermal Transfer Perimeter

8.6cm x 8.6cm

Over Flow

I/Q Receiver Performance

Noise figure 3.8 dB (400° K)

Noise Floor -114 dBm

Gain Range -4 dB to +58 dB

Single Frequency Dynamic Range

(DR)60 dB

Dual Frequency Spurious Free DR 54 dB

LO Radiation < -80 dBm

Max RF Input(no damage)

+20 dBm

Max Recovery Time 30 μS

Recovery to 1 dB

10 μS

I/Q Receiver Selectivity

10 MHz Reference OscillatorTCXODigital compensation ±0.28 ppm, -40C to +85CStabilization time ~200 sec. (cold start)I2C monitor for crystal tempI2C control for freq push/pull (1ppb or 0.01Hz)

ADC ClockBuffered 10 MHz from TCXOConversion to sinewave to reduce EMI

Mixer Local OscillatorPhase-locked system852 - 1020 MHz 2 MHz control resolution

Cal Oscillator500 MHz – free runningI2C on/off control

Local Oscillator Board

PCB4-layer

FR-4 constructionThermal Transfer Perimeter

7.6cm x 7.6cm

Freq Select

I2C

500 MHzCal Signal

10 MHz ADC Clock

LO Outputto Mixer

PWM DC/DC Buck RegulatorsHigh efficiency (>90%)Wide input range (3 -17V)Regulated outputs: +5V, +3VIntegrated design – low radiated EMI

LC Filtering on Input and Outputs

Voltage and Temperature Monitoring

ShieldingContinuous external ground planeCompartment in housing

Power Converter Board

PCB2-layer

FR-4 constructionThermal Transfer Perimeter

8.6cm x 8.6cm

I2C

7.5 V Input

5 V Out

3 V Out

Equipment14L Enclosure with 0.6 T Pump10cm X 10cm liquid coolant thermal Plate Temperature range (-60C to +60C)

SetupRAX Receiver in 1U Pumpkin frameMultiple thermal sensors

ProcedureBaseline receiver operationRX OFF: Cold-soak @ -40C for 30 minRX ON: Start logging dataTemperature ramp-up (240 min. to +55C)Post-test operational check

ResultsTCXO drift <0.24 ppm (spec: +/-0.28ppm)PLL maintained lockRF gain variation ± 1.5 dBInternal DC voltages stable

Temperature Cycling in (Moderate) Vacuum

RAX Payload Receiver Thermal Testing 1-27-10Internal/External Temperatures and TCXO Drift vs. Time

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

1 21 41 61 81 101 121 141 161 181 201 221

Time (mins)

Tem

pera

ture

(deg

C)

999999.7

999999.75

999999.8

999999.85

999999.9

999999.95

1000000

1000000.05

TCXO

Fre

quen

cy (H

z) 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)

Crystal Oscillator Thermal Response

RAX Payload Receiver Thermal Testing - 1/27/10Internal Temperature and Voltage Monitoring vs. Time

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

1 16 31 46 61 76 91 106 121 136 151 166 181 196 211 226

Time (mins)

Tem

pera

ture

(deg

C)

0

1

2

3

4

5

6

7

8

Volta

ge (V

)

RF PCB (Deg C)

PLL PCB (Deg C)

PS PCB (Deg C)

TCXO (Deg C)

7.5V bus (V)

5V bus (V)

3V bus (V)

Board-Level Thermal Response

Pre-Test Sine-SweepBaseline all resonancesRange: 20-2000 Hz @ 0.5GRate: 3 Oct/Min One sweep per axis

Random Vibration Test20-2000 Hz 10.4 G rms1 minute/axis

Post-Test Sine-SweepSame as pre-testCompare pre/post resonances

ResultsNo change in resonancesPass - Visual inspectionPass - Post electrical tests

Vibration Testing

Radar SimulatorGenerates radar direct and scattered signalsVariable timing (Pulse Width, IPP)Adjustable transmit frequency

Payload Interface Module (PIM) SimulatorDirect interface to payload receiverSupplies power and control to payloadCollects and stores I & Q samples

Integrated Testing with Cubesat BUSPIM interface Antenna InterfaceI2C checkoutFull-up EMI validation

Functional Testing

TX1 (Direct)

TX2 (Scattered)

RF PULSEAMPLITUDE

AT ANTENNA INPUT

INTER-PULSE PERIOD (IPP)

RF SWITCH

TX1

TX2

RAX Science Planning at 2010 NSF CEDAR WorkshopFuture 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

The Path Ahead