UltraWideband Radars (FM-CW) for Snow Thickness Measurements

Outline Introduction Typical FM-CW Radar Background FM-CW radars design considerations Results –Sea ice –Land Future Conclusions

Basic FMCW Concept and some issues Transmit Receive Mixed (Beat Freq) t f f B =kτ Mixing the tx and rx signals produces single tone cw signals. FFT transforms into target delay. Transmitting and receiving at the same time. Transform has sidelobes (windowing and amplitude variations). Linear Chirp Considerations: for slow chirps and close targets, chirp non-linearities are near coincidental in tx and rx and tend to cancel. k FFT

Basic FM-CW Concept and difficulties with airborne applcations t f t f Airborne applications require fast chirps and far targets. Even slight non-linearities can cause major degradations. Loss in resolution and FFT sidelobes. Fast chirp to sample Doppler (High PRF) and avoid target de- correlation.

Ultra-Wideband >500 MHz FM-CW –Easy to implement –Low-sampling –Low power Disadvantages –Linearity –RX/TX Isolation –Sidelobes Application of FM-CW radars for snow studies started in 1970s –Used linearized VCOs –YIG oscillators Surface-based systems Interfaces mapping was demonstrated A number of systems have been built and used FM-CW Radar

We started our work in 2000 –Small grant from NASA to develop a system –Collaboration with Thurston Markus, GSFC Instruments being used on aircraft –Long-range –Short-range 2-18 GHz Fast chirp problems Surface-based Airborne, 2006 Background

Solved chirp problems in 2008 Fast-settling PLL + ultrawideband VCO Background

1.Antenna Feedthrough Two antennas 2.Fast Chirp Fast PLL +VCO DDS + Multiplier 3.LO feedthrough High isolation amp 4.Multiple reflections Minimize cables and connectors Typical FM-CW Radar

InstrumentMeasurementsFrequency/ Wavelength/ Bandwidth PowerAntennaAircraft MCoRDS/IIce Thickness Internal Layering Image Bed Properties 195 MHz, 1.5 m 30 MHz/80 MHz 800 WDipole Array Wing - Mounted Fuselage DC-8 P-3 Twin-Otter Ku-Band Altimeter Surface Topography Near Surface Layering 15 GHz 2 cm 6 GHz 200 mWHorns Bomb Bay DC-8 P-3 Twin-Otter Snow RadarSnow on Sea Ice Surface Topography Near Surface Layering 5 GHz 7.5 cm 6 GHz 200 mWHornsDC-8 P-3 AccumulationInternal Layering Ice Thickness 750 MHz 40 cm 300 MHz 10 WDipole Array Vivaldi Array Bomb Bay P-3 Twin-Otter Radar Instrumentation

MCoRDS, Accumulation, Snow, and Ku-band radars DC-8 and P-3B Antenna Configurations

Results – Arctic and Antarctic

Results – Snow and Ku-band Radar Comparison

Results – Greenland m 0.9 km Snow RadarKu-band Radar 10 m 0.9 km

Extend frequency range –2-18 GHz –1-17 GHz Quad polarization Digital beamforming –16-32 receivers –Measure backscatter as function of incidence angle Backscatter data at number frequencies Invert data to estimate snow density and particle size Combine these with sounder-mode data to determine snow thickness Future

InstrumentMeasurementsFrequency range / Bandwidth PowerAntennaAircraft MCoRDS/IIce thickness Internal layering Image bed properties MHz 460 MHz 800 W or ~2 kW Slotted-Array Wing -Mounted Fuselage Basler Other aircraft?? Ultra wideband microwave radar Surface topography Near-surface layering Snow on sea ice Snow on land 2-18 GHz 16 GHz 200 mWVivaldi arrayDC-8 P-3 Twin-Otter Basler Temperate ice Sounder Ice thickness and layers 14 and 30 MHz10 WShort DipoleBasler Sea Ice Sounder Ice thickness MHz10 WBroadband slot array Twin Otter Basler Future: Radar Instrumentation

2D infinite dual-pol array simulation Ideal 8-element array patterns at 5, 10 and 15 GHz Array Performance

Advances in RF and Digital technologies enabled us: –To develop low-power and high sensitivity ultrawideband radars –Wide use over sea ice –FCC permit to operate over land Conclusions