Radar Sounding from Low-Earth Orbit at 45

MHz

Anthony Freeman, Essam Heggy, Xiaoqing Pi, Bryan Huneycutt, R. Jordan And Yonggyu Gim

CEOS SAR Cal/Val Workshop Nov 2011

Science Story Earth Orbiting Sounding radar: Understanding Ice Sheets and Fossil Aquifers Role in the earth current and paleo-climate Evolution

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Science Objectives

• Determine the thickness, inner structure, and basal boundary condition of Earth's ice sheets to improve models of future ice sheet response to climate change: Quantify contribution to sea level rise.

• Map the occurrence and distribution of fossil aquifers in desertic environments as key elements in understanding recent (Holocene-anthropocene) paleoclimatic changes

Ice sheet bed topography

• How is it currently measured?

Airborne Sounders

Bedmap flight lines

Antarctic bed topography

Kriging algorithm interpolation

Limited airborne coverage of few% resulting -> inaccurate bed map interpolation-> in accuracy in ice flow to ocean -> in accuracy in understanding seal level rise.

CReSIS flight lines

Ice sheet bed topography

• Why does it matter? – Subglacial hydrology (subglacial lakes, runoff, fresh water input to the

ocean etc.) – Basal friction, lubrication – Grounding line dynamics and its impacts on sea level rise – Thickness change (models)

Basal Friction under Pine Island Glacier (Morlighem et al. 2010)

Costal ice sheets (in purple) thickness and basal topography control the ice transport to the ocean

Modeled-inferred basal friction for Pine Island Glacier

Characterizations of Fossil Aquifers: Why it matter ? - Several hypothesis for their origin and evolution awaiting

for validation: (locally formed by meteoritic recharge or underground water transport)

- Unexplored Impact to understand earth paleoclimate: only Lake Vostok has benefit from such link - The depth variation of the water table and its correlation

to surface topography is an crucial to understand recharge, ground water transport processes and origin of the aquifer recharge that correlate to the paleo-climatic condition.

- There approximate number, occurrence and distribution remain largely unknown.

- The delineation of fossil aquifers is largely unknown. - Often studied to mitigate seasonal or temporal dry outs.

(Ogallala, USA, USGS)

(Nubian Aquifer, HEINL & BRINKMANN, 1987)

(A. Pensulia Aquifer system, Aderahman, 2006)

The Problem

• OASIS mission is proposed to systematically map from orbit shallow aquifers and the basal conditions of the major ice sheets (as MARSIS and SHARAD have done for Mars)

• 45MHz wavelength is selected as a balance between penetration depth and atmospheric transparency

• Major challenge for radar sounding at 45 MHz wavelength is the ionosphere, which is the subject of this presentation

12/15/2011

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OASIS Radar Block Diagram

DCG (digital chirp generator)

Command Decoder

Control/Timing Unit

Data Buffer Formatter

On-board processor

Matching Network (V)

Crossed Yagi with Horizontal and Vertical antenna elements

ADC

MARSIS type at a higher frequency and faster processing speed

SHARAD type at 50 MHz

State-of-practice, similar to Aquarius, SMAP or other

Yagi with multi-dipole elements

Matching Network (H)

Quadrature Hybrid Network

Input

Isolated

Direct

Coupled

Reflected power from target or load mismatch will be coupled via the quadrature hybrid to the Isolated port. Circuitry must provide proper termination, protect RX input, and not attenuate RX signal.

Receiver

Termination/ RX Isolation Switch

RF Subsystem

Digital Subsystem

Power Distribution Unit

Matching Network

Transmitter

State of Practice

System Parameters

Parameter Unit Frequency 45 MHz Bandwidth 10 MHz PRF 1.2 KHz Peak Radiated Power 100 W ADC 8 Bit One-way beam gain 10 dB On-board average in the frequency domain

150

Down conversion No Surface SNR at 400 km orbit with on-board presum and range-compression

60 dB

Chirp length 40 μs Data Rate 475 kBit/sec

Ionosphere Characterization

Electron Density Ne varies with altitude, sun cycle, and time of day

Critical frequency for solar min, at 4 am local time

Plasma Frequency

• Radio wave interaction with the ionosphere depends on the carrier frequency f and its relation to the plasma frequency of the ionosphere, fc, which is in turn dependent on the refractive index1, and is related to Ne:

• If f ≤ fc, then the radio wave will be reflected and such frequencies are used in radar sounding to determine the structure of the ionosphere.

• Provided f ≫fc the radar wave will pass through the ionosphere, with some residual distortion effects that are often used to provide information about the ionosphere.

1. Hargreaves, R. K., The solar-terrestrial environment, publ. Cambridge University Press, Atmospheric and Space Science Series, 1992

Ionospheric Propagation

• Ionospheric propagation effects: • Absorption • Polarization distortion • Propagation delays • Refraction • Loss of coherence time • Weak phase scintillation • Phase (and amplitude) variations due to strong scintillation (variations in electron density Ne) • All these distrotions are directly dependent on the Ne level in the

different layers of the ionosphere, and especially the column integral of Ne, the Total Electron Content (TEC).

• The lower the TEC value the better, which for a spaceborne sounding radar means as low an orbit altitude as possible, at around the time of solar minimum, with night-time data acquisitions.

Parameter Value Units Nmax 1.50E+05 electrons/cm3 fc 3.5 MHz TEC (for 400 km altitude) 5.0 TECU Coherence time 0.2 s Group Delay 10.0 µsec Phase path length change 4.2 km Phase change 6000 radians Phase stability (peak to peak) 60 radians Frequency stability (rms) 0.016 Hz Absorption (D and F regions) 0.08 dB

Refraction negligible

Absorption

• Absorption can be represented by the change in the amplitude of an electric field as it propagates through a layer of the ionophere, i.e.:

• where Eo is the initial electric field, α (∝TEC⁄f^2 ) is an absorption coefficient, and z the vertical coordinate.

• At VHF frequencies (100 MHz), observations have shown that typical night-time absorption values range from 0.005 dB to 0.02 dB2

2. Evans, J. V., and T. Hagfors, ‘Radar Astronomy’, publ. McGraw-Hill (1968)

Polarization Distortion

• For f ≫ fc, the change in polarization of a planar polarized radio wave, propagating through the ionosphere, can be characterized as a rotation through the Faraday rotation angle, Ω, where:

where HL is the longitudinal component of the geomagnetic field.

• In this frequency regime, significant changes in ellipticity will not occur.

• Faraday rotation at VHF frequencies can be many multiples of π, so the most commonly adopted solution when observing at longer wavelengths is to use circular polarizations [Evans and Hagfors, 1968].

Propagation Delays • Because the speed of light changes in the ionosphere, radio waves propagating

through the medium will exhibit a propagation delay, Δ,

• For a radar wave, the time delay will be twice this value, and this represents an error in the time an echo from the Earth’s surface (or beneath it) arrives back at the radar antenna.

• The corresponding (one-way) phase change is given by [Hargreaves, 1992]:

• For VHF pulse compression radars, the phase dispersion across the bandwidth B from f=fo+B/2 to f=fo-B/2 can be significant, leading to degraded range resolution and loss in peak SNR if left uncorrected [Safaenili et al, 2003].

• The Front Surface Reflection and Contrast optimization autofocus techniques3,4 successfully demonstrated for MARSIS that when f >> fc this problem can be corrected in range compression.

3. Safaeinili, A., W. Kofman, J-F. Nouvel, A. Herique, and R. L. Jordan, Impact of Mars ionosphere on orbital radar sounder operation and data processing, Planetary and Space Science 51, pp. 505-515, 2003

4. Safaeinili, A., Kofman, W., Mouginot J., Gim, Y., Herique, A., Ivanov, A., Plaut, J., Picardi, G. Geophysical Research Letters, Volume 34, Issues 23, December 2007, pp. 1235-1238

Coherence Length

• Turbulence in the ionosphere will limit the coherence length (over which radar returns are coherent with one another) The analytical approach described in [Ishimaru et al., 1999] estimated coherence length (CL) based on the wave structure function:

where κ is the wave number, di is the ionospheric thickness, J0 is the Bessel function, Фn is the spectrum of fluctuations of ionospheric refractive index, and the ρ parameter is a function of the along-track distance ys and the height as well as the slant range of the observation.

Coherence length estimates for side-looking radar vs. carrier frequency for 2 cases of ionospheric irregularities5.

OASIS case

5. Ishimaru, A., Y. Kuga, J. Liu, Y. Kim and A. Freeman, ‘Ionospheric effects on synthetic aperture radar at 100 MHz to 2 GHz’, Radio Science Vol. 34, No 1, pp. 257-268, Jan-Feb (1999)

Phase scintillations

• In 1979, Rino6 showed that the ionosphere can be modeled as a phase screen, and rms phase variations introduced by propagation through ionspheric irregularities represented by:

where ΔNe is the variation in electron density from its median value Ne, and

L is the effective length of the irregularity [Evans and Hagfors, 1968]. • In 1988, Basu et al7 found that at lower latitudes, phase scintillations peak before midnight (local time), and are lowest around 2-4 am.

6. Rino, C. L., A power law phase screen model for ionospheric scintillation 1. Weak scatter, Radio Sci., Vol. 14, pp. 1135-1145, 1979 7. Basu, S., MacKenzie, E. and Basu, S., Ionospheric constraints on VHF/UHF communications links during solar maximum and minimum periods, Radio Science, Vol. 23, No. 3, pp. 363-378, 1988

Phase Scintillations at VHF

Percentage occurrence of rms phase scintillations at VHF (150 MHz) averaged over a 24-hour period that exceed 10o for a high-latitude station at Ny-Alesund, Greenland for the period May 1992 to May 19938.

Percentage occurrence of rms phase scintillations at VHF that exceed 10o for Ny-Alesund, Greenland for Summer 1992, shown at hourly intervals throughout the day. The parameter Kp is a commonly used measure of geomagnetic activity.

8. Kersley, L., C. D. Russell, and D. L. Rice, Phase scintillation and irregularities in the northern polar Ionosphere, Radio Science, Vol. 30, No. 3, pp. 619-629, 1995

Rate of TEC Index

( ) [TECU/min] , ROT-ROTROTI 2= , ROT1

11

ii

iii tt

TECTEC−−

=+

++

To ensure that σφ≤30o at 50 MHz, for example, conditions where ROTI < 0.4 TECU/min will generally satisfy this constraint.

Summary

• OASIS mission is proposed to explore hidden aquifers and the base of the ice sheets

• Ionospheric propagation causes significant challenges, which can be mitigated by careful selection of: – Launch date (~2017) – Orbit crossing node (~ 4am) – Altitude (~400 km)

• Phase scintillation in high latitudes is the most difficult problem to deal with

• ROTI estimates from ground-based GPS can be used to flag acquisitions with high scintillation

• Budget up to 50% of data-takes will be corrupted by excessive phase scintillation in high latitudes