Ground-Based Remote SensingPart I: Active Remote Sensors Chap. 1: Introduction to radar 1/26/10

Active Remote Sensor: Transmits radiation of specific frequency (or frequency interval) and receives it via reflection or backscattering from a point target or distribution of targets (e.g., raindrops)

  1. Introduction to radar

Good overview:

1.1Background

Definitions: RADAR -- Radio Detection and Ranging

Range of wavelengths: 1 mm < < 10 m

Specific radar bands: refer to handout sheet

Doppler (coherent) radar – ability to measure change in phase of transmitted signal

Incoherent radar – phase change not measured, only magnitude of backscattered signal is measured

Radar strengths – ability to detect clouds, precipitation, and refractive index variations

Radar limiations – crude spatial resolution, spectral limitations, side lobe contamination, ambiguous signals

1.2Radar History (Rinehart, pp. 1-4)

Radar

Meteorological radar

Doppler radar

Polarimetric radars

1.3Radar hardware overview

1.3.1Radar types

Pulsed – radiation emitted in short pulses (~1 s) at some pulse repetition frequency (PRF)

Continuous wave (CW) – radiation emitted continuously

Bistatic – two antennas, one transmits and one receives

Monostratic – one antenna (transmits and receives), more common

1.3.2Radar components

Transmitter

Source of EM radiation; generates signal at specific radio frequency (RF)

Transmitter types

Magnetron – an oscillator tube with resonant cavities (Fig. 6.2, Skolnik)

Developed in 1939

Small, relatively cheap

Can transmit signals with peak energy of ~500 kW

coaxial magnetron – improved power, frequency stability, efficiency and life (Fig. 6.3, Skolnik)

Fig. 2.2 in Rinehart

Example: HSV WSR-74C radar (coaxial magnetron)

Klystron – power amplifier fed by a RF oscillator (true amplifier)

Also contain cavities (Fig. 6.9, Skolnik)

Larger and more power, peak transmit power up to 2 MW

Good waveform, purer (very stable) transmit frequency

Also termed MOPA – Master Oscillator Power Amplifier

Example: WSR-88D radar

Solid state transmitter

Much lower power (<1000 W ?)

Can be used in clusters to produce greater peak power

Example: 915 MHz Doppler radar profiler (UAH MIPS, ~500 W)

Modulator

Switches transmitter on and off

Controls the waveform of the transmitted pulse

Stores energy between pulses

Master cloud or timer (or computer)

Controls pulse repetition frequency (PRF) and pulse duration ()

PRF – number of pulses per sec

PRF ~ 103 s-1

Pulse Repetition Period (PRP) = 1/PRF (~1 ms)

PRF determines the maximum unambiguous range (Rmax) – more details later

Rmax = c/(2PRF)

For PRF = 1000 s-1, Rmax = 150 km

Recovery time of transmitter defines minimum range (Rmin), 0.1-2 km

Transmitted signal is ideally rectangular, but in reality more Gaussian in shape (Fig. from D&Z)

Pulse length – h = c/2 (for  = 1 s, h = 150 m)

Waveguide – a hollow metal conductor with rectangular cross section (Fig. 2.3 Rinehart)

Most efficient way of getting transmitted pulse (signal) to the antenna

Wires and coaxial cable experience more loss

Coaxial cable is used effectively in lower frequency radars (e.g., 915 MHz profiler)

Longer dimension is in the direction of the E field, the shorter in the direction of the H field

Waveguide dimension is /2

Pieces: straight sections with flange and choke joints, curved sections, and rotary joints

Antennna

Antenna “system” consists of feedhorn and dish reflector

Wave guide carries signal from transmitter to feedhorn

Directs the signal into a narrow beam, typically 0.5-2.0

Dish diameters 0.3-9 m; define the beamwidth for give ; larger  requires larger dish

Dish provides directional capability and gain (g or G)

g = Pmeas/ Pisotropic (maximum directional gain in linear units)

G = 10 log10(Pmeas/Pisotropic) (units in dB)

Isotropic power density Pisotropic given by

Pisotropic = Pt/(4r2)

Beamwidth and gain are related

G = 2k2/ k-shape factor (1),  are horizontal and vertical beamwidth

G = 2/2 for circular dish with parabolic cross section

For , G = 2 / (1 x /180) = 32400 = 45.1 dB

Antenna types (Fig from Battan, last time) – most common is circular with parabolic cross section

Antenna beam patterns are irregular and “messy”

Figs. 2.2, 2.3 and 2.4 of Rinehart

Main lobe, side lobes, back lobes

First side lobes typically –20 to –30 dB below peak

Beam width is defined by the angular distance between half power points (-3 dB down)

Approximate formula: 1 = 1.27/D (radian)

Main lobe shape

g = g0exp[-(/1)2] for a Gaussian beam (good approximation)

or in log scale, G = G0 –4.343(/1)2

Side lobe power ~1% of power in main lobe

Theoretical antenna beam illumination pattern (from Eq. 3.2a of D&Z)


where  is the angular distance from the beam axis and J2 is the Bessel function of the 2nd order.

[Note: refer to

Antenna typically scans in constant elevation mode (elevation is incremented to scan a volume)

PPI – plan position indicator; constant elevation scan (azimuth changes)

RHI – range height indicator, constant azimuth scan (elevation changes)

Duplexer (T/R switch)

Switches between transmit and receive modes for a monostatic radar

T output ~106 W (60 dB)

R input as low as ~10-10 to 10-11 W (-110 dBm)

Large ration, T/R ~ 1017 or 170 dB

Protects sensitive receiver components

Receiver – detects and amplifies weak incoming signals

Superheterodyne type – RF mixed with reference signal to convert to lower intermediate

frequency (IF) of 30-60 MHz, which can now be processed digitally with fast computers

(or signal processing chips)

RF amplifier – increases receiver sensitivity, not always used

Mixer – crystal diode that converts RF to IF (heterodyning); IF contains same frequency

and phase as RF

Local Oscillator – provides CW signal to mixer

IF amplifier – amplifies the IF signal

For a MOPA (klystron), there is a STALO and COHO, and no AFC is needed.

Display

A-scope

PPI

RHI

Aspects of the transmitted signal and received signal – refer to handouts from Rauber notes

1.4Example of radar systems

Preliminaries

Monostatic vs. bistatic

CW vs. pulsed

Coherent (Doppler) vs incoherent

Radars used in aviation

ARSR - enroute surveillance radars (L band)

ASR-9 – new Airport Surveillance Radar (S-band) that has a Doppler weather channel to monitor flow and

boundaries. HSV received one of the first ASR-9 radars.

TDWR – terminal Doppler Weather Radar (C band, 0.5 beamwidth), located at major airports

Aircraft radars – X-band (I think), located in nose section of aircraft); monitor weather ahead of aircraft

Weather radars

Precipitation radars: X- C- and S-band

WSR-88D is the NWS network S-band radar; specs in table below

X and C-band will experience attenuation in rain (more on this later)

Cloud radars (research): K band ( = 3 mm and 8 mm are most common)

Short wavelength is needed to detect small cloud particles (as demonstrated by radar eq.)

Wind profiling radars: UHF and VHF (915, 405 and 50 MHz)

Detect motions in clear air via scattering from refractive index irregularities (Bragg scatter)

Radar specifications (See Rinehart, Appendix D)

Table: WSR-88D specifications (taken from D&Z)

Antenna subsystem

Radome

TypeFiberglass skin foam sandwich

Diameter11.89 m

RF loss (two way)0.3 dB

Pedestal

Typeelevation over azimuth

AzimuthElevation

Scanning rate30 s-130 s-1

Acceleration15 s-215 s-2

Mechanical limits-1 to 60

Reflector

TypeParaboloid of revolution

PolarizationLinear horizontal

Diameter8.54 m

Gain44.5 dB

Beamwidth0.95

First sidelobe level-26 dB (with radome)

Transmitter and Receiver subsystem

Transmitter

TypeMOPA (klystron)

Frequency2700-3000 MHz

Wavelength10.7 cm

Pulse power (peak)1 MW

Pulse duration1.57 and 4.57 s

RF duty cycle0.002 maximum

PRFs

Short pulse320-1300 Hz (total of 8 selectable in this range)

Long pulse320 and 450 Hz

Receiver

TypeLinear

Dynamic range93 dB

Intermediate Frequency57.6 MHz

System noise power-113 dBm

Filter

Short pulseanalog filter:bandwidth (3 dB): 0.63 MHz

Bandwidth (6 dB): 0.80 MHz

Long pulseAdditional digital filtering; 3 samples

(spaced 0.25 km) of I and Q are averaged.

Output samples are space at 0.5 km intervals.

Radar constant: 58.4

System performance: minimum reflectivity factor of –20.7 dBZ at 50 km

Problem:

Find (web search) radar specifications for the following three research radars and compare with the WSR-88D radar specifications

a)CHILL radar

b)NCAR S-Pol radar (S-band)

c)Doppler on Wheels (DOW) X-band radar (Center for Severe Storms Research)

d)A K- or W-band cloud radar (NOAA ETL)

Radar systems

Block diagram for a simple radar (From Rinehart, Fig. 2.1)

Presentations: