ATLAS Radar Calibration 2002

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1313SEPTEMBER 2002AMERICAN METEOROLOGICAL SOCIETY |

uring the Radar Calibration Workshop at the

81st Annual Meeting of the American Meteo-

` rological Society in Albuquerque, New Mexico,

in January 2001, I was surprised at the relatively little

attention given to some of the simplest and proven

methods. This stimulated some extemporaneous re-marks that I presented toward the end of the work-

AFFILIATION:ATLASNASA Goddard Space Flight Center,

Greenbelt, Maryland

CORRESPONDING AUTHOR:David Atlas, Distinguished Visiting

Scientist, NASA Goddard Space Flight Center, Code 910,

Greenbelt, MD 20771

E-mail: [email protected]

In final form 22 May 2002

2002 American Meteorological Society

D

RADAR CALIBRATIONSOME SIMPLE APPROACHESSOME SIMPLE APPROACHESSOME SIMPLE APPROACHESSOME SIMPLE APPROACHESSOME SIMPLE APPROACHES

In considering new and promising methods to calibrate radar, it is worth remembering some of the

old and perhaps forgotten methods that were used over the last half century.

BYDAVIDATLAS

shop. While formalizing these remarks in writing I

thought it would be useful to elaborate upon them and

discuss some newer approaches. Thus this paper at-

tempts to synthesize a range of techniques. A com-

mon thread that runs throughout is the calibration of

the overall system by use of standard or well-definedtargets external to the radar.

In part, I was troubled by the apparent lack of fa-

miliarity of some of the younger generation with early

activities in this realm. I was also reacting to the re-

cent findings of the variability in the calibrations of

the Weather Surveillance Radars-1988 Doppler

(WSR-88Ds) around the nation that have been uncov-

Above: In the early 1970s, Atlas used BBs to cali-

brate the vertically pointing frequency modulated-

continuous wave (FM-CW) radar.


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1314 SEPTEMBER 2002|

ered by comparison with the radar measurements of

precipitation by the radar on board the Tropical Rain-

fall Measuring Mission (TRMM); (Bolen and

Chandrasekar 2000). The remarkable stability of the

TRMM precipitation radar has made it a traveling

standard against which ground-based weather radars

can be calibrated.

There were a few papers presented at the work-shop that resorted to the more traditional methods

such as calibration with a standard target. David

Brunkow of Colorado State University spoke about

the use of a metal sphere. Ron Rinehart of the Uni-

versity of North Dakota used an oscillating dihedral

corner reflector. Also Isztar Zawadzki recounted his

work with rain gauges and a JossWaldvogel (JW)

disdrometer. Surely, few of the participants were

aware that the early workers in Canada (Stewart

Marshall, Bob Langille, and Walter Palmer) and in

my group at the Air Force Cambridge Research Labo-ratories (Vernon Plank, Al Chmela, and I) used fil-

ter papers powdered with Gentian violet dye (which

left purple stains on our clothes and teeth) to mea-

sure the sizes of tens of thousands of drops by hand

in the late 1940s and early 1950s (Hitschfeld 1986).

Oh what a blessing it was to display the drop size dis-

tribution in a comfortable laboratory , while the J

W disdrometer was observing the size of each drop

automatically outdoors.

Historically, it was the Weather Radar Group at the

Massachusetts Institute of Technology (MIT), under

the leadership of Alan Bemis and the seminal workby Polly Austin and Ed Williams (1951), that found

the large underestimates of the radar echoes from

gauge measurements of rain in comparison to the

then-available theory. It was this difference that mo-

tivated Richard Probert-Jones (1962) in England to

formulate the proper radar equation for meteorologi-

cal scatterers (Hitschfeld 1986). For almost a decade

we all struggled to understand the source of this dis-

crepancy. And here we are today still struggling with

the optimum methods of radar calibration.

CALIBRATION METHODS.Frequency shift re-

flector (FSR).The FSR was invented by John Chisholm

(1963). It has been used mainly as a ground-based

target for precise locations on airports and geographi-

cal siting. It employs a parabolic reflector with a horn

at the focus that is shorted by a diode at a frequencyf

(e.g., 30 or 60 MHz). The frequencyfis generated by

a battery-driven modulator. The echo from the tar-

get is returned at Ff, where Fis the transmitted fre-

quency. The echoes at fare exactly 6 dB below that

corresponding to the known cross section of the an-

tenna. These frequencies are readily distinguished

from ground clutter and precipitation echoes. It is an

excellent calibration device because it is always avail-

able regardless of the weather.

BBs.We first used BBs fired vertically from a BB pis-

tol as standard targets to calibrate the vertically point-

ing frequency modulated-continuous wave (FM-CW)radar at the Naval Electronics Laboratory Center at

Point Loma, California (Stratmann et al. 1971). After

having failed to support a calibration sphere from a

balloon in a stable position on the axis of the radar

beam we searched for another approach. In a joking

manner I suggested the use of a BB gun. Although

there was no prior literature on the subject it was

cheap, straightforward, and worth a try. We were very

pleased by how well it worked. If enough BBs are used

(one at a time), the statistics of echo strength mimic

the radiation pattern of the beam. The maximum echocorresponds to the antenna gain on the beam axis.

When using a conventional radar, one should tilt the

beam close to the horizon outside the region of

ground clutter. With Doppler radar, the Doppler shift

can be used to distinguish the moving BBs from

clutter.

Metalized Ping-Pong balls.This is an extension of the

BB method. One can fly a light aircraft across and

above a fixed radar beam and drop the balls sequen-

tially at about 1015 m intervals so that only one tar-

get is in the beam at any time. The metalized ballsare good targets of known radar cross section. The

successive echoes present a quantitative measure of

the antenna pattern. Tracking of the aircraft and

timing of each drop positions each target relative to

the maximum echo on the beam axis. The Ping-

Pong balls are cheap and nonhazardous. One may

also use metalized wiffle balls (with holes in them).

The idea is to prevent either type of ball from falling

fast enough to create a hazard. Note that either of

these types of balls may be within the Mie region

depending on the radar wavelength so that theircross sections should be computed carefully. It is also

possible to release such targets sequentially from a

bucket carried on a constant-level balloon moving

with the winds perpendicular to the fixed radar

beam. A similar method was used to measure the

cross section of a free-falling artificial hailstone re-

leased from a balloon and measured by a tracking

radar (Willis et al. 1964).

Airborne modulated target.This approach combines the

concepts in the frequency shift reflector (FSR) and


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1315SEPTEMBER 2002AMERICAN METEOROLOGICAL SOCIETY |

Standard Target Radar (STADAR; Atlas 1967).

STADAR employs a rotating standard target on the

aircraft that modulated the total echo of the aircraft

and the target at a frequency corresponding to the

rotation frequency. The original idea was aimed at using

a simple CW radar to detect the range to the target by

the intensity of the echo from the rotating target of

known cross section using the radar equation to com-pute the range. However, it would be greatly improved

by using an FSR on board the aircraft so that the echo

is returned at a frequency that is different from that of

the carrier frequency and thus separated from the air-

craft echoes.

Balloon-borne or airborne standard target.This is an old

scheme that must go back to World War II. However,

we first used it in 1953 when we suspended a metal-

ized sphere below a helicopter and carried it across

the beam of our 24-GHz radar in a study of the radarcharacteristics of fog (Atlas et al. 1953). That study

was aimed at determining the relationship of the ra-

dar reflectivity to the liquid water content and drop

sizes of fog. Many others have used this method but

found it difficult to track the target in a narrow beam.

At the present time the use of the global positioning

system (GPS), either on the balloon or the airplane,

would facilitate tracking.

Calibration with a 24-in. metal sphere suspended

from a balloon was done quite reliably by Atlas and

Mossop (1960) by tracking the balloon with a long,

easily identified tail by theodolite. Today one mightmount a television camera on the bore sight axis of

the antenna and use the wide angle lens to find the

balloon and then change to telephoto mode to find it

accurately and adjust the radar position accordingly.

Metalized spherical target released from aircraft.During

experiments at Wallops Island, Virginia, to measure

the cross sections of individual insects and birds, the

latter targets were released from an aircraft flying into

the wind while being tracked by the radar (Glover

et al. 1966). The targets were released on countdownand the tracking gate was stopped until the aircraft

moved out of the gate and the unknown target could

be gated and tracked. Then the aircraft moved upwind

while the target moved downwind. This approach

requires the use of a tracking radar that can control

the weather radar. A metalized spherical, constant-

altitude balloon can be released from the aircraft and

expanded upon release by the use of a gas cartridge.

Tethered balloon or kytoon.Many investigators have

used metal spheres of known cross section suspended

from tethered balloons or kytoons. Some have used

three tethers to stabilize the position of the balloon.

During experiments in England we used a tethered

balloon with a standard 12 in. diameter metal sphere

and an ice ball (i.e., a simulated hailstone) of unknown

cross section suspended below the balloon at a suffi-

cient vertical spacing to separate the known and un-

known targets. Swinging the beam from one to theother allowed us to measure the cross section of the

simulated hailstone with accuracy of better than ~0.5

dB. This was more easily done at the time because of

the use of relatively wide beam height-finder radars

such as the MPS-4 and the TPS-10 (Atlas et al. 1960).

For greater use it is best to do this in the light winds

of early morning or evening.

Use of a radar profiler and disdrometer.The use of a

Doppler radar profiler (at vertical incidence) along-

side a disdrometer allows the measurement of thedrop size distribution (DSD) at the surface, computa-

tion of its associated value of reflectivity, and compari-

son to the reflectivity measured by the radar at heights

of 300400 m just beyond the radar recovery time.

This calibrates the radar remarkably well. The method

was first used by Joss et al. (1968). They measured the

reflectivity at a height of only 200 m above their zenith

pointing radar while measuring the rain and DSD with

gauges and a disdrometer. In 46 periods of uniform

stratiform rain they found excellent agreement between

the actual and the disdrometer-deduced values of Zwith

a standard deviation of only 6% or 0.25 dB in the ratiobetween the two. It is also remarkable that the radar

calibration was maintained to this accuracy for a pe-

riod of 4 months.

This approach has been extended by Gage et al.

(2000) and others. An analogous technique is that of

Kollias et al. (1999), who used a vertically pointing

94-GHz Doppler radar. At this frequency the Mie

backscatter function results in a well-defined mini-

mum in the Doppler spectrum at a specific drop size.

The difference between the measured Doppler speed

and the known fall speed for that drop size in still airis then a measure of the air motion; hence, the Dop-

pler spectrum in still air may be recovered and the

DSD and its reflectivity may be computed. The abso-

lute number of drops depends upon the overall radar

calibration and the attenuation by the rain. Thus one

still needs to use a disdrometer adjacent to the radar

to account for the attenuation. Once the zenith point-

ing radars are calibrated in this fashion, they may be

used as transfer standards for other radars.

Ulbrich and Lee (1999) have used the reflectivity

computed from drop size distributions measured with


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a disdrometer at the surface to check the calibration

of the WSR-88D at Greer, South Carolina, about

60 km away from their site at Clemson University.

They found that the radar gain was consistently 5 dB

too low. This is a straightforward technique, particu-

larly when used in relatively steady rainfall when the

bright band is high. It is similar to the schemes used

by Joss et al. (1968) and that reported by Zawadzki atthis workshop.

Measurement of DSD by aircraft.One may use obser-

vations of the drop size distribution on board an air-

craft for comparison to ground-based radar measure-

ments. This has been done by Marks et al. (1993) to

calibrate and obtain the ZRrelation in a hurricane.

In the latter case, the radar was on board the aircraft

and measured the reflectivity at a modest distance

ahead. The DSD was then measured a few minutes

later as the aircraft penetrated the radar-measuredlocation.

After 56 years of research in radar meteorology, we

have still failed to find a reliable and universally ap-

plicable method of radar calibration. Various radar

configurations require different approaches. I hope

that this brief essay will serve as a menu of simple

methods to fit the needs of various investigators and

operational users.

ACKNOWLEDGMENTS.I appreciate the discussions

with Dr. Merrill Skolnik, former Superintendent of the

Radar Division of the Naval Research Laboratories. He re-

mains skeptical about the accuracy that may be achieved

by some of the techniques described. This work was done

under the aegis of the NASA Tropical Rainfall Measuring

Mission.

REFERENCES

Atlas, D., 1967: STADAR, standard target radar. U. S.

Patent No. 3,357,014.

, and S. C. Mossop, 1960: Calibration of a weather

radar by using standard target. Bull. Amer. Meteor.Soc., 41,377382.

, W. H. Paulsen, R. J. Donaldson, A. C. Chmela, and

V. G. Plank, 1953: Observation of the sea breeze by

1.25 cm radar. Proc. Conf. on Radio Meteorology,

Austin, TX, Amer. Meteor. Soc., Paper XI-6.

, W. G. Harper, F. H. Ludlam, and W. C. Macklin,

1960: Radar scatter by large hail. Quart. J. Roy. Me-

teor. Soc., 86,468482.

Austin, P. M., and E. L. Williams, 1951: Comparison of

radar signal intensity with precipitation rate.

Weather Radar Research Tech. Rep. 14, Dept. of

Meteorology, Massachusetts Institute of Technology,

43 pp.

Bolen, S. M., and V. Chandrasekar, 2000: Quantitative

cross validation of space-based and ground-based

radar observations. J. Appl. Meteor., 39, 20712079.

Chisholm, J., 1963: Frequency shift reflector. U.S. Patent

No. 3,108,275.

Gage, K. S., C. R. Williams, P. E. Johnston, W. L.

Ecklund, R. Cifelli, A. Tokay, and D. A. Carter, 2000:

Doppler radar profilers as calibration tools for scan-

ning radars.J. Appl. Meteor., 39, 22092222.

Glover, K. M., K. R. Hardy, T. G. Hardy, W. N. Sullivan,

and A. S. Michael, 1966: Radar observations of insects

in free flight. Science, 154, 967972.

Hitschfeld, W., 1986: The invention of radar meteorol-ogy. Bull. Amer. Meteor. Soc., 67,3337.

Joss, J., J. C. Thams, and A. Waldvogel, 1968: The accu-

racy of daily rainfall measurements by radar. Pre-

prints, 13th Radar Meteorology Conf., Montreal, QC,

Canada, Amer. Meteor. Soc., 448451.

Kollias, P., R. Lhermitte, and B. Albrecht, 1999: Verti-

cal air motion and rain drop size distributions in

convective systems using a 94 GHz radar. Geophys.

Res. Lett., 26, 31093112.

Marks, F. D., Jr., D. Atlas, and P. T. Willis, 1993: Prob-

ability matched reflectivityrainfall relations for a

hurricane from aircraft observations.J. Appl. Meteor.,

32, 11341141.

Probert-Jones, J. R., 1962: The radar equation in meteo-

rology. Quart. J. Roy. Meteor. Soc., 88,485495.

Stratmann, E., D. Atlas, J. H. Richter, and D. R. Jensen,

1971: Sensitivity calibration of a dual-beam vertically

pointing FM-CW radar. J. Appl. Meteor., 10, 1260

1265.

Ulbrich, C. W., and L. G. Lee, 1999: Rainfall measure-

ment error by WSR-88D radars due to variations in

ZRlaw parameters and radar constant. J. Atmos.

Oceanic Technol., 16,10171024.Willis, J. R., K. A. Browning, and D. Atlas, 1964: Radar

observations of ice spheres in free fall.J. Atmos. Sci.,

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ATLAS Radar Calibration 2002