NEXRAD In Space (NIS) A GEO Satellite Doppler Weather Radar for Improved Observations & Forecasting of Hurricanes NIS concept study and design Simone Tanelli & Stephen L. Durden Jet Propulsion Laboratory April 10, 2007

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ 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

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ 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 spirals 86 revolutions per scan

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ 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

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ Flight Antenna concepts: trade-off 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:

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ Flight Antenna concepts: one example

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ Preliminary Radar Parameters Frequency (GHz) Range Resolution (m)250 Instantaneous Horizontal Resolution (km)12 (16 after 11 ms avg) Pulse Compression Sidelobes (dB)-55 Pulse Length (microsec)60 Data windowSurface 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

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ 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)

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ 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)

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ 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

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ Staggered PRF: Example n1 T u n2 T u n1 T u TuTu Sachidananda and Zrnic (2000) Sachidananda and Zrnic (2002) n1 = 4, n2 = 5

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ Staggered PRF: Rainfall Doppler Accuracy Mean Doppler Velocity of Rainfall Estimation after Clutter Filtering, Magnitude Deconvolution and Debiasing 101 Input velocities from -50 to + 50 m/s 1000 Monte Carlo simulations per input velocity Within the -40 to +40 m/s range: 70% are within  1 m/s 91% are within  2 m/s 1.6% are aliased estimates at  All estimates were obtained with T I = s and no range averaging

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ Staggered PRF: Clutter Suppression  R CSR dB dB dB dB  R CSR dB dB dB dB Residual CSR Rainfall 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)

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ From Line-of-sight Doppler to Hurricane Intensity Mass Contentln(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 NIS uniqueness: generates 3-D map once every hour

NEXRAD In Space (NIS): A GEO Satellite Doppler Weather Radar 04/10/ Rain Retrieval at Ka-band 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 Z m (r) profiles] 2)Surface-referenced [from P S ] 3)Non-radar [e.g., radiometric] dBZ Simulated NIS Radar Reflectivity (post clutter rejection) Mass Content (mg m - 3 ) dB