nexrad in space (nis) a geo satellite doppler weather radar for improved observations &...

NEXRAD In Space (NIS)A GEO Satellite Doppler Weather Radar forImproved Observations & Forecasting of

Hurricanes

NIS concept study and design

Simone Tanelli & Stephen L. DurdenJet Propulsion Laboratory

April 10, 2007

04/10/2007 2

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar

NIS Notional Concept and Innovations

• Operating in geostationary orbit (alt. ~ 36,000 km)• 35-GHz, 4˚ spiral scanning radar to cover 5300-km diameter earth disk (equivalent to coverage of 48˚ latitude and 48˚ longitude)

• Deployable spherical aperture antenna to obtain 12 to 14 km horizontal resolution

• Innovative antenna scan strategy:• 1 transmit feed and 1 receive feed with fixed spacing to compensate for pulse delay

• Scan by motion of 2 pairs of transmit/receive feeds on spiral path• Advantage over 2-D electronic scan, which requires millions of phase shifters

• Advantage over mechanical rotation of entire antenna, which creates unacceptable torque• Advantage over S/C rotation, which requires custom-made,

usually very expensive S/C• Vertical resolution of 250 m using pulse compression• Rain detection sensitivity: ~ 5 dBZ

• ~12 dB more sensitive than the TRMM radar• Line-of-sight Doppler velocity: 0.3 m/s rms accuracy• One 3-D full-scan image once per hour• Real-time processing to reduce downlink data volume/rate

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Key Design Considerations - Scanning

• 1 3-D full-scan image every hour GEO• Target of interest: Hurricanes Operating frequency at 35

GHz or less• Footprint of 12 km from GEO Ka-band (35 GHz) & 35 m antenna• Coverage of ~2600 km radius ~200 thousand beams• … mechanical scanning preferred vs electronic scan• … feed motion preferred over antenna or spacecraft motion• … speherical main reflector guarantees high gain and scan

angle• … Baseline scanning feed concept

Two feed pairs sliding on a boom

• Each pair has one transmit and one receive feed (spaced apart to compensate for feed motion during pulse roundtrip time of about 0.24 s).

• The boom rotates with variable rpm (less than 15 rpm) so that the tangential velocity of each feed remains almost constant (small variation to compensate for variations in round-trip time).

• Combined motions generate two precise spiral trajectories at constant linear speed.

• Feedback control systems used for accurate speed and position control

2 spirals86 revolutions

per scan

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Scanning Mechanism: concept to prototype

• A full-scale prototype of the scanning mechanism has been implemented and tested for control accuracy.

• One full scan completed in 1 hour with linear speed of ~17 cm/s to obtain homogeneous coverage over the disc

04/10/2007 5


Flight Antenna concepts: trade-off

Concept 1 Concept 2 Concept 3a Concept 3b Concept 3c Concept 4 Concept 5 Membrane AstroMeshReflector Surface Accuracy 1 6 6 6 6 6 6 5 1 9Support Structure Accuracy 1 9 9 9 9 9 9 7 3 10Controlled Deployment 0.8 6 7 8 8 9 9 7 3 10Packing 0.8 8 8 8 8 8 7 7 10 7Mass (Specific Mass) 0.6 8 8 6 6 6 7 9 10 6Scalability 0.6 9 9 8 8 8 8 5 2 6Ground Testability 0.6 9 9 9 9 9 9 2 1 9Design Flexibility 0.6 9 8 6 7 7 6 5 4 5Complexity/Reliability/Risks 0.8 8 8 7 7 7 7 5 5 6Cost 0.4 8 7 4 4 4 7 5 7 4

56.8 56.6 52.4 53 53.8 54.2 41.8 31.4 54.6

1 2 7 6 5 4 8 9 3

Total

Rank

NEXRAD in Space Spherical Reflector Trade Study

Concept DesignationWeightTrade Parameters

Trade Study Parameters:

Concept (1), ranked no.1, selected for preliminary study

(1) Reflector surface accuracy, (2) Support structure accuracy, (3) Controlled deployment, (4) Packing, (5) Mass, (6) Scalability, (7) Ground testability, (8) Design flexibility, (9) Complexity/reliability/risks, (10) Cost

Trade Study and Result:

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Flight Antenna concepts: one example

04/10/2007 7


Preliminary Radar Parameters

Frequency (GHz) 35.605

Range Resolution (m) 250

Instantaneous Horizontal Resolution (km)

12 (16 after 11 ms avg)

Pulse Compression Sidelobes (dB)

-55

Pulse Length (microsec) 60

Data window Surface to 20 km

Pulse Repetition Frequency (kHz)

variable, staggered < 7 kHz

Bandwidth (MHz) 2

Sample Frequency (MHz) 10

Antenna Aperture (Illuminated) (m)

28

Beamwidth (deg.) 0.02

Max scan angle (deg.) 4

Scan time (hours) 1

Minimum Detectable Reflectivity (dBZ)

5

Radial Doppler Velocity (m/s)

~0.3

04/10/2007 8


Surface Clutter

• Even with large antenna, surface clutter in main beam can interfere with rain signal from surface up to several km, depending on incidence angle and rain rate.

• Notch filtering of the Doppler spectrum is routinely used by ground-based radars to suppress clutter.

• For NIS the clutter is also at zero Doppler but can have larger width due to possible spacecraft slow motion and motion of ocean waves.

• Expected precipitation spectrum has width of several m/s due to possible spacecraft motion plus effects of varying fall velocities, turbulence, cross-beam effects, and non-uniform beam-filling.

• A simple Matlab simulation was developed to examine the effect of a simple, 3rd-order IIR high-pass filter on clutter suppression; a reduction in clutter power of 19 dB was obtained.

• A more realistic simulation has also been developed (following slide).• The output of a mesoscale numerical model for a

mesoscale convective system (MCS) is used to generate radar reflectivity and Doppler spectra

• The result is placed in a simulated NIS beam, and frequency domain notch filtering at 0 Hz was performed (with spectral density interpolation)

04/10/2007 9


Clutter Simulation

• At left, geometry of NIS beam intersecting simulated MCS.

• Lower left, Simulated Doppler spectrum versus altitude (clutter is dark red, extending to 3 km altitude).

• Below, clutter (blue), true rain (red solid), clutter removed (red dashed)

04/10/2007 10


Velocity Aliasing

• For a uniform PRF of 7 kHz the maximum unambiguous velocity is 15 m/s; larger velocities will be folded into this interval.

• The images below show a simulated hurricane at 28 degrees with maximum wind speed of 70 m/s; true speed at left, true radar component center, aliased radar right.• Grid is 300 km by 300 km, radar is assumed to be

looking downward and north

• Color scale shows velocity magnitude (brightest is highest velocity); radar velocities also have sign, negative on left side of each image.

• Structure of storm may make de-aliasing in ground processing possible

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Staggered PRF: Example

n1 Tu n2 Tu n1 Tu

Tu

V = CE

where : C is the Convolution Matrix corresponding to

staggered sampling at n1/n2 rate

E

Ec = E f I1 +E f 'I 2I vZ

Debiasing (in the Magnitude Domain)

• Sachidananda and Zrnic (2000)• Sachidananda and Zrnic (2002)

n1 = 4, n2 = 5

Ef = C−1 (C−C f1CEI1 −C f 2CEI 2 )

Clutter Filtering and Magnitude Deconvolution

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Staggered PRF: Rainfall Doppler Accuracy

Mean Doppler Velocity of RainfallEstimation after Clutter Filtering, Magnitude Deconvolution and Debiasing

101 Input velocities from -50 to + 50 m/s1000 Monte Carlo simulations per input velocity

Within the -40 to +40 m/s range:70% are within 1 m/s91% are within 2 m/s1.6% are aliased estimates at

All estimates were obtained with TI = 0.011 sand no range averaging

N2 * vmax

(n1+n2)

04/10/2007 13


Staggered PRF: Clutter Suppression

R

CSR 1 2 3

0 dB -19.7 -12.1 -4.2

10 dB -20.8 -11.5 -4.4

20 dB -0.1 -11.4 -4.6

30 dB -0.1 -10.8 -4.6

R

CSR 1 2 3

0 dB -1.9 -0.1 -4.4

10 dB -2.8 -0.2 -5.1

20 dB -3.5 -0.4 -6.1

30 dB -4.8 -0.6 -6.8

Residual CSRRainfall mean Doppler velocity = 0 m/s

Residual CSR Rainfall mean Doppler velocity = 5 m/s

• All quantities in dB• Clutter to Signal Ratio (CSR)• Black: CSR after Clutter Filtering and Debias• Blue: Small rejection of Rainfall Return (Residual Signal/Original Signal Ratio)• Red: Large rejection of Rainfall Return (Residual Signal/Original Signal Ratio)

04/10/2007 14

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather RadarFrom Line-of-sight Doppler to Hurricane

IntensityMass Content ln(g m-3) Horizontal Wind

velocity

Line-of-sight Doppler (including aliasing)

Simple Retrieval (only position of the hurricane center as ancillary data)

m s-1

m s-1m s-1

NIS uniqueness: generates 3-D map once every hour

04/10/2007 15


Rain Retrieval at Ka-band

k(r) =Zm(r) α 'e−0.46⋅PIA( )⎡⎣ ⎤⎦

1/β '

1+0.46β '

e0.46⋅PIA

α '⎛⎝⎜

⎞⎠⎟

1/β '

⋅ Zm1/β '(r)ds

r

rS

∫

Radar rain retrieval algorithms constrained by Path Integrated Attenuation (PIA)

NIS simulation• WRF simulated Hurricane IVAN

(mass contents and 3D wind field)• 3D High Resolution Spaceborne

Doppler Radar Simulator (reflecitivities and Doppler spectra)

Methods to estimate PIA:1) “Surface-less” [From the Zm(r) profiles]2) Surface-referenced [from PS]3) Non-radar [e.g., radiometric] dBZ

Simulated NIS Radar Reflectivity(post clutter rejection)

Mass Content

(mg m-3)dB

nexrad in space (nis) a geo satellite doppler weather radar for improved observations &...

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