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 Conceptand some issues

TransmitReceiveMixed (Beat Freq)

t

f

fB=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 Conceptand 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.

Longer range 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

10 m

Ice Sheet Surface

Internal Accumulation

Layers

Snow Surface Air-Snow Boundary

Ice Surface Snow-Ice Boundary

1 m

Background

1. Antenna FeedthroughTwo antennas

2. Fast ChirpFast PLL +VCODDS + Multiplier

3. LO feedthrough High isolation amp4. Multiple reflections

Minimize cables and connectors

Typical FM-CW Radar

Instrument Measurements Frequency/Wavelength/ Bandwidth

Power Antenna Aircraft

MCoRDS/I Ice ThicknessInternal LayeringImage Bed Properties

195 MHz, 1.5 m 30 MHz/80 MHz

800 W Dipole ArrayWing -MountedFuselage

DC-8P-3Twin-Otter

Ku-BandAltimeter

Surface TopographyNear Surface Layering

15 GHz2 cm6 GHz

200 mW HornsBomb Bay

DC-8P-3Twin-Otter

Snow Radar Snow on Sea IceSurface TopographyNear Surface Layering

5 GHz7.5 cm6 GHz

200 mW Horns DC-8P-3

Accumulation Internal LayeringIce Thickness

750 MHz40 cm300 MHz

10 W Dipole ArrayVivaldi ArrayBomb Bay

P-3Twin-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 2011

10 m

0.9 km

Snow Radar Ku-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

Instrument Measurements Frequency range / Bandwidth

Power Antenna Aircraft

MCoRDS/I Ice thicknessInternal layeringImage bed properties

140-600 MHz460 MHz

800 W or ~2 kW

Slotted-ArrayWing -MountedFuselage

BaslerOther aircraft??

Ultra wideband microwave radar

Surface topographyNear-surface layeringSnow on sea iceSnow on land

2-18 GHz16 GHz

200 mW Vivaldi array DC-8P-3Twin-OtterBasler

Temperate ice Sounder

Ice thickness and layers

14 and 30 MHz 10 W Short Dipole Basler

Sea Ice Sounder

Ice thickness 50-300 MHz 10 W Broadband 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