meteor shower characterization at kwajalein missile range · • close, hunt, minardi, and mckeen...

• CLOSE, HUNT, MINARDI, AND MCKEENMeteor Shower Characterization at Kwajalein Missile Range

VOLUME 12, NUMBER 1, 2000 LINCOLN LABORATORY JOURNAL 33

Meteor Shower Characterizationat Kwajalein Missile RangeSigrid Close, Stephen M. Hunt, Michael J. Minardi, and Fred M. McKeen

■ Approximately one billion meteors enter the Earth’s atmosphere daily, andtheir potential impact on spacecraft is not yet well characterized. KwajaleinMissile Range radar systems, because of their high sensitivity and precisecalibration, have contributed new information on meteor phenomena, includingobservations of the Perseid and Leonid meteor showers of 1998. Initially,Perseid data were collected by using the Advanced Research Projects Agency(ARPA) Long-Range Tracking and Instrumentation Radar (ALTAIR), which is atwo-frequency radar (VHF and UHF) uniquely suited for detecting meteor headechoes. ALTAIR transmits right-circular (RC) polarized energy and records fourchannels: left-circular (LC) sum, RC sum, LC azimuth difference, and LCelevation difference. The four channels facilitate the determination of apparenttarget position and the calculation of polarization ratios. Shortly after thePerseid observations demonstrated ALTAIR’s capabilities for meteor detection,Leonid data were simultaneously collected by using ALTAIR and otherKwajalein sensors at microwave and optical frequencies. This article contains ananalysis of Perseid data collected at VHF. Meteor head-echo statistics arepresented, with an in-depth analysis of a few select head echoes to estimatedecelerations and densities. Head-echo data collected at three frequencies (UHF,VHF, and L-band) and ionized meteor-trail data from the Leonid shower arealso presented. Radar cross-section measurements for both head echoes andionized meteor trails illustrate the frequency dependence of plasma reflections.

T bombarded by par-ticles of debris in solar orbit. Most of theseparticles are small meteoroids (i.e., meteors in

solar orbit), typically the size of a grain of sand, whichare captured by the Earth’s gravitational field and de-stroyed in the atmosphere before they reach theEarth’s surface. These meteoroids are often a greatdanger to orbiting satellites in the upper atmosphere.The collision of a meteoroid and a satellite can resultin significant damage, including mechanical crateringof the satellite surface or plasma and electromagneticpulse generation, which can lead to electronic noise,sudden current and voltage spikes, and softwareanomalies. Meteoroids enter the Earth’s atmospherewith considerable energy, between 11 and 72 km/sec.

The lower limit represents the kinetic energy of a par-ticle, initially at rest, that has fallen into the Earth’sgravitational field. The upper limit is a sum of theEarth’s orbital velocity and the solar escape velocity.This article examines meteor data collected at radarfrequencies in order to estimate meteor decelerations,densities, and size, which are used to characterize thedanger of the meteoroids to orbiting satellites.

Meteor activity consists of either sporadic meteorsor meteor showers. Sporadic meteors, which can beencountered anywhere in the Earth’s orbit, and at anytime, create a constant background flux. Meteorshowers are a heightened level of meteor activity thatoccurs when the Earth’s orbit intersects the orbit of adebris stream, typically created by a comet. Meteor


34 LINCOLN LABORATORY JOURNAL VOLUME 12, NUMBER 1, 2000

showers occur at the same time every year, and themeteors appear to an observer to be radiating from asingle point called the radiant. This effect occurs be-cause all the meteors, originating from a single object,are traveling on roughly parallel paths. Meteor show-ers take their name from the star constellation thatcontains the radiant point; for example, the Leonidshower takes its name from the constellation Leo.

When meteoroids enter the Earth’s atmosphere,they collide with and ionize (i.e., produce a transfer ofenergy that frees an electron) neutral air moleculesand atoms, which generates localized plasma regions.Ionization takes place in the E region of the iono-sphere (between approximately 80 to 140 km in alti-tude). Above 140 km the neutral particle densitytends to be too low to ionize, and below 80 km themeteor usually has been destroyed. Meteor ionizationin the E region is classified into two categories—theionized meteor trail and a localized spherical ionizedregion surrounding the meteor [1]. The meteor trailis cylindrical in shape and typically kilometers inlength and meters in diameter; at radar frequencies,trails are often modeled as a long conducting wire.Trail duration varies but is typically less than one sec-ond (although some trails last for many minutes) [2].Trails are stationary except for motion due to atmo-spheric winds. Specular reflection from these trailsoccurs if incident radio waves are perpendicular to thecylindrical trail, producing strong returns that havebeen studied since the 1940s.

The localized spherical ionization surrounding themeteor produces a much weaker type of reflectionknown as the head echo. Head echoes travel with thesame velocity as the meteor, with cross sections thatdepend on the size and shape of the meteor. Becausethe size of the meteor subsequently depends on therate of mass dissipation, which in turn depends onthe air density and meteor velocity, cross sections varyamong meteors and change rapidly as a meteor travelsthrough the ionosphere. By analyzing head echoes wecan deduce meteor decelerations and densities, whichare independent of assumptions about ionization.

The current high interest in meteors is due to therecent Leonid meteor storm in 1998. The annual Le-onid meteor shower, which occurs in November andwas created from the comet Tempel-Tuttle, developed

into a meteor storm in 1998 and 1999 and is ex-pected to develop into equally intense storm-like ac-tivity in 2000. The higher flux of these meteor stormsarises because comet Tempel-Tuttle crossed the or-bital path of the Earth in February 1998, as illustratedin Figure 1, and the greatest density of meteoroids iscollocated in the proximity of the comet. In addition,comet Tempel-Tuttle is inclined only 17° to theEarth’s orbital plane, which increases the duration ofthe meteor shower. Finally, the comet’s retrograde or-bit around the sun (opposite the direction of theEarth’s orbit) increases the brilliance and visible num-bers of the meteor shower because the Earth is collid-ing with the meteoroids at their maximum velocity.

Comet Tempel-Tuttle has a thirty-three-year or-bital period; thus the last Leonid storm occurred in1966, when few satellites were in orbit. At that time, apeak flux rate of forty meteors per second was de-tected by visual observations [3]. Because of the in-creased risk of a meteor shower to the current satellitepopulation of over seven hundred operational space-craft, a worldwide meteor data-collection effort wasinitiated to help characterize the Leonid storm and itspotential threat to satellite safety [4]. The AdvancedResearch Projects Agency (ARPA) Long-Range Track-ing and Instrumentation Radar (ALTAIR), which wasrequested by the U.S. Air Force Office of ScientificResearch to support this effort, collected data on boththe Perseid shower and the Leonid storm in 1998.

FIGURE 1. The intersection of the Earth’s orbit with thecomet Tempel-Tuttle in February 1998. This intersection, incombination with Tempel-Tuttle’s retrograde motion and loworbital inclination of 17°, produced an intense meteor stormin November of 1998 and 1999 and is expected to produce asimilarly intense meteor storm in 2000. These storms are ac-tively studied by radar researchers for information on meteorhead echoes and meteor trails.

Sun

Earth's orbit

Tempel-Tuttle'sorbit

17°



ALTAIR is located in the central Pacific Ocean onthe island of Roi-Namur in the Kwajalein Atoll, Re-public of the Marshall Islands, at 9° N latitude and167° E longitude. Shown in Figure 2, ALTAIR has amechanically steered, 46-m-diameter dish antennathat transmits a peak power of 6 MW with a 2.8°beamwidth at VHF and a 1.1° beamwidth at UHF.ALTAIR is part of a system of radars known as theKiernan Reentry Measurements Site (KREMS).KREMS mission areas include the support of missiletesting activities, such as operational tests of fieldedballistic missile systems and developmental testing ofmissile defense systems.

ALTAIR has a second and much larger missionarea, providing support to U.S. Space Command forthe space-surveillance mission. ALTAIR dedicates128 hours per week to providing data to Space Com-mand on nearly every aspect of space surveillance.The remaining forty hours per week are devoted tosystem maintenance and development. However, AL-TAIR is available with a fifteen-minute recall forhigh-priority tasks even during these maintenance pe-riods. Lincoln Laboratory fills the role of scientificadvisor to the U.S. Army Kwajalein Atoll, which op-erates the entire radar range. In particular, LincolnLaboratory has responsibility for the quality ofALTAIR’s space-surveillance mission.

ALTAIR’s high peak power and large antenna aper-ture combine to create high system sensitivity. By us-ing the most sensitive waveforms available, ALTAIRcan detect a –74-dBsm (decibels relative to a squaremeter) target at VHF and a –80-dBsm target at UHFat a range of a hundred kilometers, which is a typicalrange for meteor observations. This high system sen-sitivity makes ALTAIR uniquely suited for the detec-tion of small head echoes, which so far have been ne-glected relative to radar research on larger trail echoes.Because head echoes give direct measurements of me-teor velocities, important meteor parameters such assize and density can be inferred.

Perseid Meteor Observations

The first meteor data collection discussed in this ar-ticle occurred during the Perseid meteor shower. Thepurpose of the data collection was to determine thesuitability of ALTAIR for meteor observation. Data

were collected at two different times during the earlymorning hours of 12 August 1998. First, the ALTAIRantenna was pointed at the radiant point in the con-stellation Perseus when it was at its maximum eleva-tion of 40°. Ten minutes of data were recorded. Whilethe radiant point was still at its peak elevation, fiveminutes of off-radiant data were collected after mov-ing the antenna 30° in azimuth. While the antenna ispointing at the radiant, Perseid meteors follow pathsroughly aligned with the antenna beam, and theytherefore endure longer in the beam. The purpose ofcollecting the off-radiant data was to observe more ofthe sporadic meteor head echoes and to have a greaterchance of seeing returns from meteor trails.

Amplitude and phase data were recorded for eachof four receive channels: sum right circular (SRC),

FIGURE 2. The Advanced Research Projects Agency(ARPA) Long-Range Tracking and Instrumentation Radar(ALTAIR), located on the island of Roi-Namur in the Kwaj-alein Atoll, Republic of the Marshall Islands, in the PacificOcean. ALTAIR has a mechanically steered, 46-m-diameterdish antenna that transmits a peak power of 6 MW with a2.8° beamwidth at VHF and a 1.1° beamwidth at UHF.ALTAIR’s primary mission area is space surveillance, in-cluding observations of meteor activity, but it also contrib-utes to operational testing of fielded ballistic missile sys-tems and developmental testing of missile defense systems.



sum left circular (SLC), azimuth-difference left circu-lar (ALC), and elevation-difference left circular(ELC). Data samples were collected every seventy-five meters for ranges corresponding to altitudes be-tween 70 and 140 km. ALTAIR radiated a VHF 260-µsec (V260M) pulse fifty times per second. The pulsewas modulated with a 1-MHz linear frequencymodulation, which allowed the pulse to be com-pressed to 1 µsec after receive filtering. By using thiswaveform, ALTAIR can detect a –72-dBsm target at arange of a hundred kilometers.

Perseid Data Analysis

Figure 3 shows a representative range-time-intensity(RTI) image of 3.5 sec of SLC channel data. Imagecolor in the figure corresponds to the received signal-to-noise ratio (SNR). This data sample, which con-tains both meteor head echoes and an ionized trail re-turn, represents the raw data that we processed todetect and measure meteor head echoes (note therange sidelobes resulting from the strong return offthe trail). The amplitude and phase data were reducedby using MATLAB scripts. First, receive biases wereestimated for all four data channels by averaging thedata over an 80,000-sample region that was devoid ofdetections. After the receive biases were removed, thenoise floor in the SLC channel was estimated by aver-aging the data amplitude over the same window. A12-dB threshold (above the noise floor) was applied

to the SLC amplitude data, and the data from all fourchannels were saved whenever the threshold was ex-ceeded in the SLC channel. A high threshold waschosen to reduce the huge amount of data (on the or-der of gigabytes) to a manageable data set. Also, real-time observation of the data indicated a wealth of de-tections, so we felt there was no need to search thedata for small signals.

Next, we interpolated the SLC range samples tofind the peak signal strength and associated range. Anautomated search of the range-time maps was imple-mented to detect lines corresponding to meteor rangerates between –72 and –4 km/sec. Meteor head ech-oes with fewer than six detections were discarded.The resulting head-echo detections were then used tocompute histograms of meteor head-echo parameters.A final step was to use the computed range rates tocorrect the target ranges for range-Doppler coupling1.

To obtain meteor decelerations we extracted eachhead echo, fit a polynomial curve to its associated ve-locity profile, and interpolated the data samples toobtain finer resolution. The monopulse angle-offsetdata were then applied to each head echo to computeits apparent position from the radar slant range.Many head-echo returns were excluded because ofpoor-quality monopulse angle data that resulted fromlow SNR and low angle sidelobe detections.

Once the head-echo position had been calculated,the time rate of change of position was computed toobtain the apparent velocity of the meteor; a seconddifferentiation resulted in an estimate of the meteor’sdeceleration.

The next goal was to estimate the radius and den-sity of the meteor particle [5]. The meteor momen-tum reduction per unit time is

1 Range-Doppler coupling is a property of a chirp-typepulse. Doppler shifts of the radar echo cause an offset in theapparent range of the echo. The relationship between targetrange rate and the range-Doppler coupling range offset is∆r = (Tf0vr)/B, where ∆r is the range offset, T is thepulsewidth, f0 is the radar RF frequency, vr is the target radialvelocity, and B is the chirp bandwidth. The quantity (Tf0 )/Bhas units of time and is called the range-Doppler couplingconstant. The coupling constants for the ALTAIR wave-forms used in this study are 4.16 × 10–2 sec for V260M, 1.83× 10–3 sec for V40H, and 3.17 × 10–3 sec for U150.

FIGURE 3. Range-time-intensity (RTI) image showing threemeteor head echoes and a head echo with an associatedtrail formation.

1.5 2.0 2.5 3.0 3.5

105

100

95

90

85

80

75

Time (sec)

Alti

tude

(km

)

Head echo with trail formation

20

15

5

10

SN

R (d

B)

Head echoes



dvdt

vm

m m= σγ ρ 2

, (1)

where σ is the physical cross section of the meteor, ρ isthe air density, γ is the dimensionless drag coefficient,vm is the velocity of the meteor, and m is the meteormass. The velocity is further defined as

vdh dt

m =cos

,χ (2)

where h is the altitude of the meteor and χ is the el-evation angle. After substituting Equation 2 intoEquation 1 we obtain

dvdh

vm

m m= σ ρ χsec.

If we estimate the physical cross section to be 2πr2

and calculate the mass m of the meteor to be

m r= 43

3π δ ,

where δ is the density of the meteor, the final resultfor the radius-density product is

r vdvdhm

mδ ρ χ=

−32

1

sec .

Note that the velocity, air density, altitude, and eleva-tion change as a function of time.

Perseid Data Results

Figure 4 contains histograms showing head-echo datacollected when ALTAIR was pointed at the Perseid ra-diant [6]. Over 692 head echoes were detected ineleven minutes of data; again, this significantly highnumber includes only meteors approaching the radarand those with duration greater than six pulses (0.12sec). Meteors traveling perpendicular to the beam orwith positive range rates were excluded.

Figures 4(a), 4(b), and 4(c) are histograms of themean detection altitude, radial velocity, and radarcross section (RCS) of the 692 head echoes. The alti-tude profile in Figure 4(a) is roughly Gaussian inshape and is consistent with previous Perseid datataken at other sensors [7]. The radial velocity distri-bution in Figure 4(b) illustrates two peaks; the distri-bution with a velocity near zero may be attributed to

FIGURE 4. Perseid meteor-shower radiant histograms, including (a) mean detection alti-tude, (b) radial velocity, (c) radar cross section (RCS), and (d) polarization ratio.

Peak = –51 km/sec

Peak = –49 dBsm Peak = 18 dB

100

80

60

40

20

0

Mean detection altitude (km) Radial velocity (km/sec)

RCS (dBsm)

80 100 120 140

Peak = 103 km

–60 –40 –20

–20–30–40–50–60 –10 403020100–10 50

0

Polarization ratio (dB)

(a)

(d)(c)

(b)

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0



the background sporadic meteor detection. The RCShistogram in Figure 4(c) shows that the RCS with thehighest count is somewhat lower than previouslypublished data taken at other sensors [7]. This lowercount reflects the fact that ALTAIR’s sensitivity allowsthe observation of smaller head echoes. The sharpcutoff at low RCS is due to thresholding the data to12-dB SNR and is an artifact of the data-processingprocedure. The polarization ratio histogram in Figure4(d) represents the ratio of the LC and RC receivedpower. Head echoes with an SLC SNR less than 20dB are excluded from the histogram because theSRC-channel data must be above the noise floor tocalculate the polarization ratio. The peak count,which occurs at approximately 19 dB, is consistentwith the expected returns from a sphere-like objectsuch as a meteor.

Figure 5 contains histograms for the 30° off-radi-ant Perseid observations; 281 meteor head echoeswere detected in five minutes of data. The distribu-tion of the altitude histogram in Figure 5(a) remainsunchanged from the radiant data shown in Figure 4,as does the polarization ratio data in Figure 5(d). Theradial velocity samples in Figure 5(b) are more spread,due to the relative radar detection angle. The varia-

tion in RCS in Figure 5(c) can be attributed either tothe fundamental variations in head-echo shape andstrength, or to the broadside view of the Perseid headechoes (sporadic meteors show a more random distri-bution independent of pointing). The exact shape ofa typical head echo is unknown at this time; it is pos-sible, however, that the head echo is spherical in frontand spreads to a more teardrop shape in its wake.

As noted earlier in the Perseid data-analysis sec-tion, the monopulse angle data were used to computethe apparent position of the head echo. This compu-tation represents the true three-dimensional positionof the meteor in a radar-based reference frame. Mostof the head-echo detections had to be discarded be-cause of uncertainties (for example, in angle sidelobeand noise) in the monopulse data; many other datavalues were omitted because of low SNR and shortdurations. After this data-filtering process was com-plete, focus was placed on twenty head echoes tocompute decelerations and densities.

Figure 6 contains the apparent meteor velocity as afunction of time and the radius-density product as afunction of altitude for a single head echo that re-mained in the 2.8° VHF beam for nearly one second.The apparent velocity plot shows at least two distinct

FIGURE 5. Perseid meteor shower off-radiant (30°) histograms, including (a) mean detec-tion altitude, (b) radial velocity, (c) radar cross section (RCS), and (d) polarization ratio.

Peak = –48 km/sec

Peak = –46 dBsm Peak = 19 dB

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

Mean detection altitude (km) Radial velocity (km/sec)

RCS (dBsm)

80 100 120 140

Peak = 103 km100

80

60

40

20

0–60 –40 –20

–20–30–40–50–60 –10 403020100–10 50

0

Polarization ratio (dB)

(a)

(d)(c)

(b)



decelerations, namely, 2 km/sec2 and 16 km/sec2. Thepoint-to-point fluctuations at the beginning of thedata set are due to noise in the monopulse angle dataand were excluded from the estimation. This curve isthen utilized to calculate the radius-density product.Figure 6(b) shows that this product stays near a valueof 0.02 gm/cm2, except for a few outlier points. Table1 summarizes the measurement parameters associatedwith this head echo.

Figure 7 contains the results from the same analysisapplied to twenty head echoes. Figure 7(a) shows theapparent meteor velocity as a function of altitude(each meteor is represented in the figure by a distinctcolor). The average velocity is about 56 km/sec; thisnumber represents an average value over the lifetimeof each head echo, which is further averaged over alltwenty meteors. These data were then used to calcu-late the radius-density product shown in Figure 7(b).Table 2 summarizes the averaged measurement pa-rameters for these twenty head echoes. Note that theaverage duration of 0.3 sec is representative of the me-teor head echoes detected by ALTAIR.

Leonid Meteor Observations

Because of the successful collection of data during thePerseid shower, data were also collected during theLeonid meteor storm. Several adjustments were madeto the data-collection techniques. First and foremostwe decided to collect UHF data as well as VHF data.Some theoretical models of radar observations of me-

teors predict that the RCS of a head echo follows anf –2 dependency [8]. Therefore, head-echo observa-tions have been infrequently attempted at UHF be-cause head echoes are difficult to detect at such highfrequencies. Despite this theoretical difficulty, how-ever, the combination of an unusually high numberof head echoes, the high RCS of the target meteors,and the sensitivity of ALTAIR at UHF indicated thatmany head echoes would indeed be seen. Dual-fre-quency detections of head echoes are rare in the litera-ture [9]; such detections would be extremely usefulfor validating theories of meteor RCS.

Discussions with staff members of the Institute forMeteor Studies at the University of Ottawa indicated

FIGURE 6. (a) The apparent velocity for a single head echo (corrected by monopulse angle data) in the ALTAIRVHF beam as a function of time. These data show two distinct decelerations of 2 km/sec2 and 16 km/sec2. (b) Theassociated radius-density product as a function of altitude remains relatively constant at 0.02 gm/cm2 until justbefore the meteor burns up.

Table 1. Measurement Parameters fora Single Head Echo

Duration 0.95 sec

Maximum radar cross section –40 dBsm

Mean apparent velocity –62 km/sec

Deceleration 2 and 16 km/sec2

Mean meteor radius × density 0.02 gm/cm2

Mean radius * 0.02 cm

Mean mass 10–3 to 10–4 gm

* We assume meteor density is 1 gm/cm3

–60.0

–61.0

–62.0

–63.0

–64.0

–65.0

0.6 0.7 0.8 0.9 1.0

16 km/sec2

2 km/sec2

1.1 1.2 95100105110

App

aren

t vel

ocity

(km

/sec

)

Met

eor r

adiu

s ×

dens

ity (g

m/c

m2 )

Time (sec) Altitude (km)

0.09

0.07

0.05

0.03

0.01

(a) (b)



there was an interest in studying meteors during thefinal moments before they burned up in the iono-sphere. To examine these meteors, researchers at AL-TAIR selected shorter waveforms to increase thepulse-repetition frequency (PRF) to 333 Hz in orderto have a higher detection rate during the meteors’ fi-nal moments. Although higher PRF waveforms haveless energy, ALTAIR had sufficient return sensitivityto satisfy data-collection requirements.

Leonid observations were taken on 18 November1998 during a three-hour period designed to span thepredicted peak of the Leonid storm [10]. ALTAIR re-corded over 26 GB of data during this period. Bothon-radiant and off-radiant data were collected whenLeo was at 28° elevation and 77° elevation; Leo was atits highest elevation during the predicted peak. Forthis data-collection activity, ALTAIR collected simul-taneously at VHF (160 MHz) and UHF (422MHz,), and the PRF was increased to 333 Hz. Thewaveforms chosen for this experiment (V40H andU150) have a range sample spacing of 30 m at 160MHz, and 7.5 m at 422 MHz. The sensitivity of AL-TAIR with these waveforms allows it to reliably detecta –55-dBsm target at 160 MHz and a –75-dBsm tar-get at 422 MHz, at a range of a hundred kilometers.

As part of KREMS, ALTAIR has several sister ra-dars. The TRADEX radar operates at 1320 and 2951MHz, and ALCOR operates at 5664 MHz. Both ra-dars have narrower beams and somewhat less sensitiv-ity than ALTAIR. TRADEX and ALCOR were alsooperational during the Leonid meteor storms, and si-

multaneous data were collected at L-band, S-Band,and C-Band frequencies, as well as at optical sites.

During the Perseid meteor shower, ALTAIR de-tected a total of twelve head echoes with an RCS of–20 dBsm or greater. Assuming an f –2 falloff in RCS,those head echoes could have been detected by bothTRADEX and ALCOR if they had passed throughthe TRADEX and ALCOR beams. The Leonid me-teor storm was predicted to have ten times to a hun-dred times higher meteor activity compared to stan-dard meteor showers such as the Perseids. Ahundredfold increase in meteor flux would mean ahundred detectable meteors might pass through theALTAIR VHF beam every minute, so the likelihoodof at least one also passing through the ALCOR beam

FIGURE 7. (a) The apparent velocity (corrected by monopulse angle data) for twenty head echoes. Each meteor is rep-resented by a distinct color; the average velocity over the lifetime of each head echo is approximately 56 km/sec. (b) Theassociated radius-density product as a function of altitude for these twenty head echoes.

Table 2. Measurement Parameters Averagedover Twenty Head Echoes

Duration 0.3 sec

Maximum radar cross section –42 dBsm


Deceleration 0.54 km/sec2





–30

–40

–50

–60

–70

–8090100110120 9095100105110

App

aren

t vel

ocity

(km

/sec

)

Met

eor r

adiu

s ×

dens

ity

(gm

/cm

2 )

Altitude (km) Altitude (km)

100

10–1

10–2

10–3

10–4

(a) (b)



or the TRADEX beam was believed to be significant.The chances of a detection at a higher frequency war-ranted the use of TRADEX and ALCOR.

Leonid Data Analysis

To date, the Leonid data have not yet been reduced toobtain statistical information. From the real-timeRTI display at ALTAIR, we noted numerous headechoes and trails collected at both VHF and UHF.Figure 8 is an example of an RTI display that was re-corded near the end of the data-collection period,when ALTAIR was pointed at the Leonid radiant. Anexpanded view of the RTI display shows the meteorhead echo associated with the trail. This trail is mostlikely not a typical specular reflection, but instead is aview as we look down the tube of the trail. Its dura-tion was extremely long, nearly three minutes (mostionized trails last less than one second). The fre-quency dependence of the signal return from theionospheric plasma is apparent.

Because of the large quantity of data, at this timewe have completed an analysis of only one head echo,which was recorded during observations of the Le-onid radiant. Although many head echoes and trailswere visible at VHF and UHF, real-time observationof A-scopes (a display of amplitude versus range) andRTI displays at ALCOR and TRADEX revealed onlyone detection at a frequency higher than UHF. Asingle head echo was detected by TRADEX at L-band(1320 MHz). ALTAIR data were examined at thesame data-collection time and range as the TRADEXL-band detection. As expected, a large head echo wasrecorded at both UHF and VHF. This detection issignificant because we believe it is the only head echodetected simultaneously at three frequencies [11].

Figure 9 illustrates this three-frequency head-echodetection in the UHF, VHF, and L-band LC data.The slanted line that extends for less than 0.2 sec ineach of the three RTI images in the figure representsthe meteor head echo. The ionized trail is spread inboth range and time, and, again, most likely repre-sents a non-orthogonal view of the meteor trail. Thespacing between the head echo and the trail is due torange-Doppler coupling (range-Doppler couplingoffsets for a target range velocity of –59,000 m/sec are–108 m in VHF and –187 m in UHF). The Doppler

measurement of the trail is due to atmospheric windsand is typically less than 100 Hz at VHF, while theDoppler measurement of the head echo is associatedwith the velocity of the meteor and is between 12 and77 kHz. In Figure 9(d), a slice of the head echo is ex-tracted from both the VHF and UHF data at bothLC and RC polarization. As with the Perseid data, apolarization ratio of approximately 20 dB is apparentfor both frequencies. (The VHF sensitivity is downapproximately 6 dB because of operation at reducedtransmitter power.)

FIGURE 8. Snapshot of the RTI display at ALTAIR. The up-per image shows a long-duration ionized trail visible in bothUHF (left) and VHF (right). The expanded view of the VHFdata in the lower image shows the meteor head echo.

VHF

Meteor head echo

UHF

App

roxi

mat

ely

thre

e m

inut

es

75 km75 km



From these RTI images, we can see that the SNRof both the head echo and the trail is reduced as thefrequency increases. Table 3 contains the RCS valuesfor both detections as a function of frequency. Wealso estimated the radius-density product for theVHF return of this head echo. Figure 10 shows theapparent velocity of the meteor as a function of time;

Table 4 shows the corresponding meteor measure-ment parameters. Note that the duration of the headecho is only 0.15 seconds. This head echo has a muchhigher RCS and a much shorter duration than thePerseid head echo shown in Figure 6, with signifi-cantly less noise in the monopulse data (illustrated bythe relatively smooth apparent velocity curve in Fig-ure 10). The mean meteor radius-density product of0.05 gm/cm2 is also larger than the typical Perseidmeteor.

Summary

The Perseid meteor shower demonstrated the capa-bilities of ALTAIR for detecting head echoes. Thehigh sensitivity of ALTAIR resulted in an extremelyhigh number of head-echo detections, compared tothose seen by other radars. The altitude histogramsare consistent with data in the literature. The RCS

Table 3. Radar-Cross-SectionDependence on Frequency

Frequency Head Echo Trail

VHF –5 dBsm –18 dBsm

UHF –23 dBsm –51 dBsm

L-band –36 dBsm None detected

FIGURE 9. RTI images for left circular (LC) data at (a) VHF, (b) UHF, and (c) L-band. (d) The signal-to-noise ratio(SNR) of the head echo was extracted at UHF and VHF for both LC and right circular (RC) data and is plotted as afunction of time.

116

114

112

110

108

1060 0.1 0.2 0.3

Time (sec)

VHF

Ran

ge (k

m)

0.4

35

40

30

25

20

15

10

116

114

112

110

108

1060 0.1 0.2 0.3

Time (sec)

UHF

UHF

VHF

Ran

ge (k

m)

0.4

35

40

30

25

20

15

10

116

114

112

110

108

1060 0.1 0.2 0.3

Time (sec)

L-band

Ran

ge (k

m)

0.4

35

40

30

25

20

15

10

60

50

40

30

20

100 0.05 0.10 0.15

Time (sec)

LCRC

SN

R (d

B)

SN

R (d

B)

SN

R (d

B)

SN

R (d

B)

(a) (b)

(c) (d)



histogram shows a peak count at a much lower RCSthan previously seen, most likely because of ALTAIR’shigh sensitivity. The radial velocity distribution showsa peak consistent with what is expected for Perseidmeteors. Some of the spread in the velocity distribu-tion is most likely due to the presence of sporadicmeteors. The polarization ratios of meteor head ech-oes were reported for the first time. High polarizationratios (the peak of the distribution is near 20 dB) areconsistent with returns from a sphere-like object, sup-porting the theory that head echoes are approxi-mately spherical in shape. The monopulse angle datapermitted the determination of the true three-dimen-sional position of the head echoes that resulted in de-celerations, and estimates of the radius-density prod-uct of the meteor particle.

The Leonid data-collection activity, includingother sensors at the Kwajalein Missile Range, has yet

to be fully analyzed. By examining selected real-timeobservations, we detected numerous head echoes andtrails at both VHF and UHF. One head echo, de-tected at VHF, UHF, and L-band, has been analyzedto determine RCS dependence on frequency, as wellas its deceleration and radius-density product.

Acknowledgments

The authors would like to acknowledge the followingpeople for their contributions: William Ince and KurtSchwan, the site managers at Kwajalein; Tom White,the ALTAIR sensor leader; Tim Kirchner, the techni-cal evaluator for the Kwajalein Missile Range; JeffDelong, the KREMS technical leader; Scott Couttsand Mark Corbin for valuable input in both data col-lection and analysis; Andy Frase and Wil Pierre-Mikefor software and hardware support; and Peter Brownand Andrew Taylor for technical direction and input.The Leonid data collection effort, in particular, in-volved many Lincoln Laboratory and RaytheonRange Systems Engineering staff, including Bill Riley,Bob Foltz, Tim McLaughlin, Glen McClellan, DaveGibson, LeRoy Sievers, and Dave Shattuck.

R E F E R E N C E S1. T.R. Kaiser, “Radio Echo Studies of Meteor Ionization,” Adv.

Phys. 2 (8), 1953, pp. 495–544.2. G.R. Sugar, “Radio Propagation by Reflection from Meteor

Trails,” Proc. IEEE 52 (2), 1964, pp. 116–136.3. Leonids website, <http://www.skypub.com/sights/meteors/

leonids/king.html>, Nov. 1998.4. D. Jewell, private communication, U.S. Air Force Space Com-

mand, Colorado Springs, Colo., 1998.5. J.V. Evans, “Radar Observations of Meteor Deceleration,”

J. Geophys. Res. 71 (1), 1966, pp. 171–188.6. Meteor website, <http://medicine.wustl.edu/~kronkg/

perseids.html>, Aug. 1998.7. D.W.R. McKinley and P.M. Millman, “A Phenomenological

Theory of Radar Echoes from Meteors,” Proc. IRE 37 (4),1949, pp. 364–375.

8. A. Hajduk and A. Galád, “Meteoric Head Echoes,” Earth,Moon, and Planets 68, 1995, pp. 293–296.

9. D.W. McKinley, “The Meteoric Head Echo,” Meteors (A Sym-posium on Meteor Physics): Special Supplement (Vol. 2) to J.Atmos. Terr. Phys., 1955, pp. 65–72.

10. P. Brown, private communication, Meteor Physics Laboratory,Ottawa, Canada, 1998.

11. E. Malnes, N. Bjørnå, and T.L. Hansen, “Anomalous EchoesObserved with the EISCAT UHF Radar at 100-km Altitude,”Ann. Geophysicae 14 (12), 1996, pp. 1328–1342.

Table 4. Measurement Parameters forLeonid Three-Frequency Head Echo

Duration 0.15 sec


Deceleration 10 km/sec2





–50

–55

–600 0.05 0.10 0.15

App

aren

t vel

ocity

(km

/sec

)

Time (sec)

FIGURE 10. The apparent velocity corrected by monopulseangle data.



is an associate staff member atthe Kwajalein Missile Rangefield site, where her primaryduty is in space surveillance atALTAIR, including orbitalanalysis for operational spacelaunch tracking support,routine spacecraft monitoring,and unplanned space-surveil-lance events. Her interests alsoinclude ionospheric modelingand analysis, as well as meteorresearch. Previously sheworked for the AdvancedElectromagnetic Systemsgroup at Lincoln Laboratoryin the area of radar phenom-enology. She has a B.S. degreein physics and astronomy fromthe University of Rochester,and an M.S. degree in physicsfrom the University of Texas atAustin, where her graduatethesis involved the study ofgravity waves in plasmas.

. is a former staff member at theLincoln Laboratory radar siteat the Kwajalein MissileRange, where his research wasin radar systems, radar polar-ization and calibration, andradar meteor astronomy. He isnow a member of the researchstaff in the department ofelectrical engineering atWright State University, wherehis research is in the area oftarget recognition and battledamage assessment. Beforejoining Lincoln Laboratory in1996, he worked at the AirForce Research Laboratory,Sensors Directorate, at WrightPatterson Air Force Base inDayton, Ohio. He was anundergraduate at the Univer-sity of Dayton, where heearned a B.S.E.E. degree inelectrical engineering and aB.S. in mathematics. He alsoreceived M.S. and Ph.D.degrees in electrical engineer-ing from Georgia Institute ofTechnology.

. is a senior systems engineer inthe Raytheon ALTAIR Sys-tems Engineering group atKwajalein Missile Range. Thefocus of his recent research hasinvolved the operationalsupport and integration of theRAMP upgrade in the AL-TAIR system. Other researchactivities include operationalsupport of ALTAIR for datacollection efforts, includingreentry mission support, theSurveillance ImprovementProgram, and meteor showercharacterization efforts. Priorto joining Raytheon he waswith the Georgia Tech Re-search Institute (GTRI) foreight years, where he workedon electronic countermeasures,including expendable counter-measures. He also worked onradio frequency electroniccountermeasures at NorthropCorporation and Texas Instru-ments prior to joining GTRI.He received a B.S. degree inelectrical engineering fromBrigham Young University andan M.S. degree in electricalengineering from GeorgiaInstitute of Technology.

. is an associate staff member inthe Field Systems group. He isthe system analyst of theKwajalein Space SurveillanceCenter, which is used to con-duct the remote operation andmeasurement-data reductionfor the radar systems atKwajalein. He received a B.S.degree in physics fromWorcester Polytechnic Insti-tute and is currently pursuinga graduate degree in spacephysics at Boston University.Prior to his current position heworked in the Space Surveil-lance Techniques group, in-cluding several years at theKwajalein Missile Range. AtKwajalein he was a space-surveillance analyst, and heparticipated in a collaborativeeffort to characterize meteorsby using the ARPA LongRange Tracking and Instru-mentation Radar (ALTAIR).He was also responsible for thedevelopment and integrationof a real-time ionosphericmodeling system for ALTAIR.

meteor shower characterization at kwajalein missile range · • close, hunt, minardi, and mckeen...

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