Rotating PAR Risk Reduction Overview.pdf

9 May 2022 Department of Commerce // National Oceanic and Atmospheric Administration // 1

NOAA’s Phased Array Radar R&D Objectives Rotating PAR with Multiple Simultaneous Beams

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● Background of NOAA’s last 20 years of PAR R&D

● Weather Surveillance Radar – 1988 Doppler (WSR-88D)

● NSSL’s Advanced Technology Demonstrator

● Rotating PAR Risk Reduction

○ Spoiled Transmit Beam with Multiple Receive Beams

○ “Machine Gun” Mode

○ “Machine Gun” Mode with Frequency Diversity

● Classified Systems Issues

● Questions for Further Discussion

Agenda

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● NOAA has been investigating PAR technology applied to weather radar Since 2003

○ First platform was Navy SPY-1A converted to a weather radar

○ Scanned 90 degree sector and demonstrated benefits of 1 minute updates

● Involved in multi-agency programs and working groups (FAA/DOD/DHS)

○ MPAR 4-face planar PAR and Cylindrical PAR studies

○ Rotating PAR not investigated; required different update times for different missions

○ Developed dual polarization PAR technology; designed and built the

Advanced Technology Demonstrator (ATD)

● NOAA withdrew from SENSR program in 2018

○ Developed Weather Radar Follow-On Plan Report to Congress

○ Investigating Rotating PAR (less expensive alternative; new technical risks)

Background

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● A weather signal is a composite of echoes from a very large number of hydrometeors. Because one cannot resolve the individual scatterers, we resort to sampling the weather signal at discrete range time delays.

● The total number of samples (from a resolution volume) is determined by the PRT and dwell time. Because considerable correlation may exist from sample to sample, we determine the equivalent number of independent samples needed to reduce the estimate variance achieved by averaging.

● Estimates of meteorological variables require a large number of samples to minimize the variance.

Weather Radar: Parameter Estimation

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● Weather echoes behave statistically as narrow-band Gaussian noise

● Obtaining low-variance estimates requires semi-coherent averaging of many samples (~ 20-100)

● “Pulse-pair” processing often used owing to computational efficiency and desirable statistical properties

● Frequency domain based parameter estimation may be preferable when echoes are spectrally complex (e.g. contain moving clutter)

Parameter Estimation

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64 pulses integrated to form a beam

WSR-88D Waveforms PRF = 1000 Hz Pulse width = 1.57 µs Dwell time = 1 ms Number of pulses = 64 Antenna Rotation rate = 18 deg/s (3 rpm)

Typical Scan Parameters

Top-down view: Parabolic dish antenna is rotating clockwise

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Polarimetric Weather Radar

Polarimetric Radar Variables

• ρHV (Correlation Coefficient) is the correlation between the returns from the horizontal and vertical channels (a good measure of the variety of scatterers in the volume)

• ZDR (Differential Reflectivity) is the difference between the logarithms of return powers of the horizontal and vertical channels (a good measure of the aspect ratio of the scatterers in the volume)

• φDP (Differential Phase) is the difference between the phases of the horizontal and vertical channels (a good measure of the liquid content within the propagation path)

– KDP (Specific Differential Phase) is the change in differential phase shift between two specific range bins divided by the distance between the range bins. It can be used to estimate rain rate.

The dual linear polarization capability provides additional information necessary to differentiate among various precipitation types (and other airborne scatterers) as well as enhance the ability to determine rainfall rates.

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Differential Reflectivity

Larger drops -- Diameter ~ 4.0 mm

𝒁𝒁𝒉𝒉

𝒁𝒁𝒗𝒗

> 𝟏𝟏 Larger rain drops are oblate spheroids

Smaller rain drops are more spherical𝒁𝒁𝒉𝒉

𝒁𝒁𝒗𝒗

≅ 𝟏𝟏

Smaller drops -- Diameter ~ 0.5 mm

The Eh field needs to be parallel to the earth at all viewing angles and the Ev field needs to be perpendicular to Eh to effectively capture the aspect ratio of the rain drop.

ZDR =10log(ZH)-10log(ZV) > 0dB

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Phased Array Antenna

BroadsideBbeam (beam is perpendicular to the array face)

EV and EH are orthogonal 1.5°

EV

EH

Beam is circular in shape and H is parallel to the surface of the earth

Phased Array Antenna

Beam Off-Broadside (out of the principal planes) (beam is not perpendicular to the array face)

EV and EH are skewed

EV

EH

Beam is elliptical in shape and H is not parallel to the surface of the earth

H and V polarizations are skewed (and may be non-orthogonal) when looking off the principle planes. Will require calibration to correct.

Electronic Scanning

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64 pulses integrated to form a beam

PAR Electronic Scanning PRF = 1000 Hz Pulse width = 100 µs (utilizing pulse compression for increased detectability) Dwell time = 1 ms Stationary (non-rotating planar array antenna)

Top-down view: Antenna is stationary

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Spoiled Beam Transmit

R1 R2 R3 R4

Horizontally Spoiled Beam

Vertically Spoiled Beam

R

R

Multiple Receive Beams

R1

R2

R3

R4

Well established technology

• Reduced sensitivity due to power density being transmitted over larger antenna pattern

• Receive beam sidelobes are contaminated with returns caused by the spoiled transmit beam

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Dwell period … receive all phase positions and separate them in digital processing

Machine Gun Mode

Tx Pu ls e

2a

(p ha se p os iti on a

R es et p ha se s et tin gs

Tu rn o n

R ec ei ve rs

Tx Pu ls

2b se p os iti on b

Tx Pu ls

1a se p os iti on a

R es et p ha se s et o n

R ec ei ls

1b se p os iti on b

Tx Pu ls e a se p os iti on a

R es et p ha se s et o n

R ec ei ls e b se p os iti on b

2X Beam Positions

Phase positions electronically steered to nulls in antenna sidelobe pattern to minimize potential contamination.

• Twice as fast as WSR-88D

• Beams are symmetric / mirror images

Top-down view: Radar beams are electronically steered

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Machine Gun Mode 64 pulses at

2 beam positions

Receive all beams and digitally form 2 beams

2X Beam Positions and Mechanical Scanning

Top-down view: Radar beams are electronically steered while antenna is rotating clockwise

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Dwell period … receive all phase positions and separate them in digital processing

Machine Gun Mode

Tx Pu ls

1c se p os iti on c

Tx Pu ls se p os iti on a

Tx Pu ls se p os iti on b

R es et p ha se s et o n

R ec ei

R es et p ha se s et ls

2c se p os iti on c

Tx Pu ls se p os iti on a

Tx Pu ls se p os iti on b

R es et p ha se s et o n

R ec ei et p ha se s et

3X Beam Positions

Pu ls e c se p os iti on c

Tx Pu ls e se p os iti on a

Tx Pu ls e se p os iti on b

R es et p ha se s et o n

R ec ei et p ha se s et

Phase positions electronically steered to nulls in antenna sidelobe pattern to minimize potential contamination.

• 3 Times as fast as WSR-88D

• Outside beams are symmetrical, but different from the middle beam

Top-down view: Radar beams are electronically steered

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Machine Gun Mode (3X) 64 pulses at

3 beam positions

Receive all beams and digitally form 3 beams

3X Beam Positions and Mechanical Scanning

Top-down viewRadar beams are electronically steered while antenna is rotating clockwise

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Machine Gun Mode: Scan Strategy

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Dwell period … receive all phase positions and separate them in digital processing

Machine Gun Mode (3X)

Tx Pu ls

1c se p os iti on c

Tx Pu ls se p os iti on a

Tx Pu ls se p os iti on b

R es et p ha se s et o n

R ec ei et p ha se s et ls

2c se p os iti on c

Tx Pu ls se p os iti on a

Tx Pu ls se p os iti on b

R es et p ha se s et o n

R ec ei et p ha se s et

3X Beam Positions and Frequency Diversity

Pu ls e c se p os iti on c

Tx Pu ls e se p os iti on a

Tx Pu ls e se p os iti on b

R es et p ha se s et o n

R ec ei et p ha se s et f1 f2 f3 f1 f2 f3 f1 f2 f3

Frequency Diversity further isolates possible sidelobe contamination from pulses at other phase positions f1 f2 f3

Top-down view: Radar beams are electronically steered with different frequencies

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● Determine if these techniques to achieve multiple simultaneous beams are compatible with National Weather Service requirements.

○ Is it possible? What is the impact on dual polarization variables? What is the impact on spectrum usage?

● Compare “single beam” data to “multiple simultaneous beam” data

○ Establish scan strategy of Radar Test Article

■ Normal mode and Machine Gun mode

○ Collect data under various weather conditions

■ Normal / Machine Gun 2X / Normal / Machine Gun 3X / Normal

○ Compare Machine Gun mode data to Normal mode data

■ With and without frequency diversity

■ Examine impact on meteorological variables

Research Objective

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● NSSL’s ATD radar is subject to ITAR export controls

○ Old SPY-1A system contained classified components

● Systems developed for DOD customers/missions

○ Some components or technical details may be classified

○ System capabilities will likely be classified

○ DOD customer approval needed?

● Acquisition of classified systems for PAR R&D

○ May require more access control of test facilities

○ Clearances for personnel that need to have access to system

○ Proposals may need to be unclassified for DOC to process

■ Vendor may need to modify/declassify system during production

Classified Systems Issues

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● Is your radar capable of Machine Gun mode?

○ Both 2X and 3X?

○ Frequency Diversity?

○ What is the reset time to reconfigure phase shifters for transmit?

○ Ability to receive multiple beams simultaneously with different phase settings?

○ Higher radiated power would reduce pulse length necessary for sensitivity

● What modifications to your radar systems would be required to perform these comparisons?

● Are there classification concerns?

Questions for Vendors