Analysis of Perseid meteor head echo data collected using the Advanced Research Projects Agency Long-Range Tracking and Instrumentation Radar (ALTAIR)

Transcription

1 Radio Science, Volume 35, Number 5, Pages , September-October 2000 Analysis of Perseid meteor head echo data collected using the Advanced Research Projects Agency Long-Range Tracking and Instrumentation Radar (ALTAIR) S. Close and S. M. Hunt MIT Lincoln Laboratory, Lexington, Massachusetts M. J. Minar all Department of Electrical Engineering, Wright State University, Dayton, Ohio F. M. McKeen Raytheon Range Systems Engineering, Kwajalein Atoll, Republic of the Maxshall Islands Abstract. The 1998 Perseid meteor shower was observed using the Advanced Research Projects Agency (ARPA) Long-Range Tracking and Instrumentation Radar (ALTAIR) in order to study potential meteoroid impact on orbiting spacecraft. ALTAIR is a dual-frequency radar that operates VHF and UHF, and its high sensitivity and precise calibration make it uniquely suited for detecting meteor head echoes. ALTAIR is dually polarized and records left-circularly and right-circularly polarized signal returns, which allow the determination of polarization ratios. ALTAIR uses a multihorn feed and interferometry to measure target angle of arrival. This paper contains analysis on Perseidata that were collected at VHF (158 MHz). Meteor head echo statistics are presented, including mean altitude, radial velocity, radar crossection (RCS), and polarization ratio. ALTAIR's VHF detection rate was approximately 1 head echo per second. An in-depth analysis on select head echoes to estimate meteor decelerations and densities has also been included. 1. Introduction The Earth is continually bombarded by debris that are captured from solar orbit by the Earth's gravitational field. These very small meteoroids, typically the size of a grain of sand, are destroyed before they reach the Earth's surface. Meteoroids from solar orbit enter the Earth' s atmosphere with geocentric speeds between 11 and 72 km/s. The lower limit represents the kinetic energy of a particle, initially at rest, that has fallen into the Earth's gravity well; the upper limit is a sum of the Earth's orbital velocity and the solar escape velocity at 1 AU. Meteor activity is classified as either sporadic or Copyright 2000 by the American Geophysical Union. Paper number 1999RS /00/1999RS shower: Sporadic meteors create the constant background flux; meteor showers are a heightened level of meteor activity that occurs when the Earth's orbit intersects the orbit of a debris stream, typically created by a comet. Showers occur at the same time every year, and the meteors appear to an observer to be radiating from a single point; all the meteoroids, which originate from a single object, are traveling on roughly parallel paths. Meteor showers derive their names from the star constellation that contains the radiant point. As meteoroids enter the Earth's atmosphere they collide with and ionize neutral air molecules and atoms, generating localized plasma regions. Ionization takes place approximately in the E region of the ionosphere (80- to 140-km altitude); above 140 km the neutral particle density is too low to ionize. Meteor ionization in the E region is broken down into two categories, the ionized trail and a localized ionized region surrounding the meteor [Kaiser, 1953]. The meteor trail is 1233

2 1234 CLOSE ET AL.: ANALYSIS OF PERSEID METEOR HEAD ECHO DATA cylindrical in shape and typically kilometers in length and meters in diameter; for radar purposes, trails are often modeled as a long conducting wire. The duration of the meteor trail will vary but is typically less than 1 s; some can last for many minutes [Sugar, 1964]. Trails are stationary except for motion due to atmospheric winds. Specular reflection occurs when the radio waves are perpendicular to the cylindrical trail, producing very strong returns that have been studied since the 1940s. The spherical ionization surrounding the meteoroid produces a much weaker type of reflection, known as the head echo. These head echoes travel with the same velocity as the meteoroid itself, and their cross sections are dependent upon its size and shape. As the size of the meteoroid is subsequently dependent upon the rate of mass dissipation, which, in turn, is dependent upon air density and meteoroid velocity, cros sections will vary between particles and change rapidly as a meteoroid travels through the ionosphere. From analysis of head echoes one can deduce meteoroid decelerations and densities, which are independent of ionization assumptions. The current high interest in meteors is a result of the recent Leonid meteor storm; a storm can be defined as showing a zenith hourly rate (ZHR) greater than The annual Leonid meteor shower, which takes place in November and was created from the comet Tempel- Tuttle, was expected to hit storm-like activity in 1998, 1999, and Tempel-Tuttle has a 33-year orbital period; therefore the last Leonid storm occurred in 1966, when there were few satellites in orbit. At that time a peak rate of 40 meteors per second was detected using visual observations. Because of the increased risk to the satellite population, which currently exceeds 700 operational spacecraft, a worldwide meteor data collection effort was initiated to help characterize the Leonid storm and its potential threat [P. Brown, University of Western Ontario, private conununication, 1998; D. Jewell, U.S. Air Force Space Command, private conununication, 1998). ALTAIR was requested by the Air Force Office of Scientific Research to support this effort and collected data on both the Perseids and Leonids in Located on the island of Roi-Namur in the Kwajalein Atoll, Republic of the Marshall Islands, ALTAIR resides in the central Pacific at 9 ø N and 167 ø E (Figure 1). ALTAIR is a powerful, dual-frequency radar that allows precise measurements of small targets at long ranges. ALTAIR has a 46-m diameter mechanically Figure 1. Photograph of ALTAIR.' steered dish antenna that transmits a peak power of 6 MW. A complex feed arrangement allows simultaneous operation at both VHF and UHF. The VHF side of ALTAIR was used in the observations described in this paper; unless otherwise noted, the remainder of this discussion will concern VHF only (ALTAIR UHF capabilities are essentially identical to VHF). Targets are illuminated with right-circularly (RC) polarized energy in a narrow 2.8 ø beam (1.1 ø at UHF). After reflecting from a target the electromagnetic energy returns to the antenna where it is focused by the main dish onto a dually polarized feed horn that receives leftcircularly and right-circularly polarized returns. These two channels are denoted sum left circular (SLC) and sum right circular (SRC). ALTAIR also has four additional receive horns that receive LC returns. The horns are offset from the focus of the main dish and are processed to produce two additional channels of data, including left circular azimuth difference (ALC) and left-circular elevation difference (ELC). ALC and ELC are combined in a process known as amplitude comparison monopulse [Skolnik, 1990], a form of interferometry, to measure the angle of arrival of the radar return to a small fraction of the beam width. Combined with a range measurement derived from the time delay of the target return, ALTAIR can fix an object position in three dimensions. ALTAIR is part of the Kwajalein Missile Range (KMR), whose mission areas include support of missile testing (such as operational tests of fielded ballistic

3 CLOSE ET AL.: ANALYSIS OF PERSEID METEOR HEAD ECHO DATA 1235 missile systems) and developmental testing of missile defense systems. ALTAIR itself has a second and much larger mission area, which is to provide Army meteors and to have a greater chance of seeing returns from meteor trails. Amplitude and phase data were recorded for each of supporto U.S. Space Command for space surveillance. four receive channels: sum right circular (SRC), sum ALTAIR dedicates 128 hours per week to Space left circular (SLC), azimuth difference left circular Command and provides data on nearly every aspect of space surveillance. The remaining 40 hours per week (ALC), and elevation difference left circular (ELC). ALC and ELC are the monopulse difference pattern are devoted to system maintenance and development; responses for azimuth and elevation, respectively. however, ALTAIR is available with a 15-min recall for Samples were collected every 75 m for ranges high-priority tasks even during these maintenance and corresponding to altitudes between 70 and 140 km. development periods. Massachusetts Institute of ALTAIR radiated a VHF 260-/ s pulse 50 times per Technology (MIT) Lincoln Laboratory fills the role of scientific advisor to the U.S. Army Kwajalein Atoll second; this pulse repetition frequency (PRF) will discriminate against many short-lived meteors. The (USAKA), which operates the range, and, in particular, pulse was modulated with a 1-MHz linear frequency Lincoln Laboratory is responsible for the quality of ALTAIR's space surveillance mission. ALTAIR's high peak power and large antenna aperture combine to create high system sensitivity. At modulation, which allowed the pulse to be compressed to 1 / s after receive filtering. Using this waveform, ALTAIR can detect a -72-dBsm target at 100-km range. a range of 100 km (a typical range for meteor observations), ALTAIR can reliably detect a target as 3. Perseid Data Analysis small as -74 decibels-relative-to-a-square-meter Figure 2 shows a representative (dbsm) at VHF (-80 dbsm at UHF). This high system (RTI) image of 3.5 s of SLC channel data. Image sensitivity makes ALTAIR suited for the detection of shading corresponds to received signal-to-noise ratio small head echoes (head echoes have been studied less, (SNR). This data sample, which represents the raw relative to conventional meteor trails) [Matthews et al., data that were processed to detect and measure head 1997; Chapin and Kudeki, 1994; Pellinen-Wannberg echoes, contains both meteor head echoes and an and Warmberg, 1994; Zhou et al., 1998]. Because head echoes give direct measurements of meteoroid velocities, important meteor parameters can be inferred, such as size and density. Head echoes range-time-intensity 2. Perseid Observations The first meteor data collection occurre during the Perseid meteor shower, with the purpose of determining ALTAIR's suitability for meteor observation. For this experiment it was decided to collect data only in VHF as the radar cros sections (RCS) are much larger. Data were collected at 158 MHz at two different times during the early morning hours of August 12, First the antenna was pointed at the radiant point in the constellation Perseus when it was at its maximum elevation of 40 ø, and 10 min of data were recorded. While the radiant point was still at its peak elevation, 5 min of off-radiant data were collected after moving the antenna 30 ø in azimuth. While the radar is pointing at the radiant, Perseid meteors follow paths roughly aligned with the antenna beam and will therefore endure longer in the beam. The purpose of the offradiant data was to observe more of the sporadic : ; ' 3, -. ;. "- ' E 95. : - <,. 90' 85 80' 75 " : 7-:-< -: , Time (sec) Head echo with trail formation 2O < 15 ' Z 10 U) Figure 2. Example range-time-intensity (RTI) image showing three meteor head echoes and a head echo with associated trail.

4 1236 CLOSE ET AL.' ANALYSIS OF PERSEID METEOR HEAD ECHO DATA ionized trail return. (Note the range sidelobes resulting from the large SNR of the trail; this is represented by the vertical line dissecting the trail.) The amplitude and phase data were reduced using MATLAB scripts. First, receive biases were estimated for all channels by averaging the data over an 80,000-sample region that was devoid of detections. After the biases were removed, the noise floor in the SLC channel was estimated by averaging the amplitude (412 + Q2 ) over the same window. A 12-dB threshold (above the noise floor) was applied to the SLC amplitude, and the data from all four channels were saved whenever the threshold was exceeded in the SLC channel. The high threshold was chosen to reduce the huge amount of data (gigabytes) to a manageable data set; also, real-time observation of the data indicated a wealth of detections, so we felt there was no need to "mine" the data Ibr very small signals. Next the SLC range samples were interpolated to find the peak signal strength and associated range. An automated search of the range-time maps was implemented to detect "lines" corresponding to meteor range rates between -72 and -4 km/s. Meteors with less than six detections were discarded. The resulting head echo detections were then used to compute histograms of meteor head echo parameters, including mean detection altitude, radial velocity, RCS, and polarization ratio. The polarization ratio is defined as the ratio of the power returned in two orthogonal receive channels. ALTAIR is calibrated to the RCS and polarization ratio of a known target, such as a conducting sphere. The absolute RCS measurement capability of ALTAIR is within 0.5 db, and the polarization ratio limitation of the ALTAIR system operating at VHF is on the order of db. The final step in the data reduction process was to use the computed range rates to correct the target ranges for range-doppler coupling. (Range-Doppler coupling is a property of a chirp-type pulse. Doppler shifts of the radar echo cause an offset in the apparent range of the echo. The relationship between target range rate and the range-doppler coupling range offset is Ar = (Tfovr)/B, where Ar is the range offset, T is the pulse width, fo is the radar RF frequency, Vr is the target radial velocity, and B is the chirp bandwidth. The quantity (Tfo)/B has units of time and is called the range-doppler coupling constant. The range-doppler coupling constant for the ALTAIR waveform used in this study (V260M) is 4.16 x 10-2 s.) To obtain meteor decelerations, we extracted each head echo and fit a polynomial curve to its associated time-of-flight velocity profile, interpolating to obtain finer resolution. The data from the two angle difference channels (ALC and ELC) were then processed to determine the angular offset of the detections from the radar boresite (in units of radians). By multiplying the angular offsets by the range to the detection, the position of the detection in the plane perpendicular to the radar boresite was determined. Monopulse angle measurements are only valid when detections are made in the main beam; therefore care was taken to ensure that the detections that were used did not occur in angle sidelobes. The detection of this condition is problematic. We only examined head echoes with large SNR, which almost ensures that the detection occurred in the main radar beam. The gain of the two-way ALTAIR antenna falls by more than 30 db outside the main beam. When the meteor head passes from the main beam into the first sidelobe, the apparent RCS would decrease, and the arithmetic sign of the angle error of one or both of the channels would suddenly change (as if the detection "hopped" to the other side of the beam). Once the position of the head echo was calculated, the time rate of change of position was computed to obtain the apparent velocity; a second differentiation resulted in an estimate of the meteor' s deceleration. (ALTAIR is a coherent radar, and Doppler processing of the head echo data to directly measure closing velocity was considered. However, the Doppler processing was not pursued because the low PRF of 50 Hz and the ALTAIR wavelength of approximately 2 m give an unambiguous velocity interval of approximately 0.05 km/s. This was considered too small of an interval for targets with range velocities of the order of km/s.) The next goal was to estimate the radius and density of the meteor particle [Evans, 1966]. The meteoroid momentum reduction per unit time is dv m c:r2',o V m 2 dt m (1) where cr is the physical cross section of the meteoroid,,o is the air density, 2' is the dimensionless drag coefficient, V,n is the velocity of the meteor, and m is the meteoroid mass. The velocity is further defined as dh / dt Vm =, (2) cosz where h is the altitude and Z is the elevation. After substituting (2) into (1) we obtain

5 CLOSE ET AL.: ANALYSIS OF PERSEID METEOR HEAD ECHO DATA 1237 dv m _ (O'Vm?',O sec j ') 100 (a) - m (3) ø i If we estimate the physical cross section to be 2nr 2 and so I calculate the mass of the meteoroid to be 4o Peak = 103 km,øø / (b) -48 km/s m = (4 / 3)n'r36, (4) where 6 is the density of the meteoroid, the final result is rry = 3 dv m. vp sec ;((- -- -) -1 (5) 2O Mean Detection Altitude (kin),øø I 60 (c) dbsm 100[ 8O 6O Radial Velocity (kin/s) (d) 19 db The velocity, air density, and elevation will change as a function of time. 4. Perseid Data Results O "" : RCS (dbsrn) Polarization Ratio (db) Figure 3 contains histograms for head echo data collected when ALTAIR was pointed at the Perseid radiant. A significant result is the high number of meteor head echoes that were detected: over 692 in 11 min of data. Again, this number reflects only meteors approaching the radar and those with duration greater than six pulses (0.12 s); therefore meteors traveling perpendicular to the beam or with positive range rates were excluded. Figures 3a through 3c are 60 / (a) peak = 103 km (b) -51 km/s Mean Detection Altitude (kin) Radial Velocity (kin/s) 8O 4O (c) (d) -49 dbsm 19 db I RCS (dbsm) Polarization Ratio (db) Figure 3. Perseid radiant histograms, including (a) mean detection altitude, (b) radial velocity, (c) radar cross section, and (d) polarization ratio. Figure 4. Perseid off-radiant (30 ø) histograms, including (a) mean detection altitude, (b) radial velocity, (c) radar cross section, and (d) polarization ratio. histograms of the mean detection altitude, radial velocity, and RCS of the 692 head echoes. The altitude profile is Gaussian in shape and is consistent with previous Perseid data taken at other sensors [McKinley and Millman, 1967]. The radial velocity distribution illustrates two peaks; the distribution with a velocity near -40 km/s may be attributed to the sporadic background meteor detection [Hajduk and Galad, 1995; McKinley, 1955]. The RCS histogram shows a peak count that is somewhat lower than has been reported in previously published data taken at other sensors [Malnes et al., 1996]. This lower peak count reflects the fact that ALTAIR's sensitivity allows the observation of smaller head echoes. The sharp cutoff at low RCS is an artifact of the data-processing procedure that is a result of thresholding the data to 12-dB SNR. Finally, the polarization ratio contained in Plate 3d represents the ratio of the LC and RC received power. Head echoes with an SLC SNR less than 20 db were excluded, because the SRC channel data must be above the noise floor to calculate the polarization ratio. The peak count, which occurs at approximately 19 db, is consistent with returns from a sphere-like object. Figure 4 contains the histograms for the off-radiant observations; 281 meteor head echoes were detected in 5 min of data. The distribution of the altitude

6 1238 CLOSE ET AL.' ANALYSIS OF PERSEID METEOR HEAD ECHO DATA -$ ,07 ½ I:: 0.03 O 0.0 e (a) 16 km/s 2. eee., 0 ' 4o5 Time (sec) (b) Altitude (kin) 11o 11o product as a function of altitude for one head echo that remained in the 2.8 ø VHF beam for nearly 1 s. The apparent velocity plot shows at least two distinct decelrations, including 2 and 16 lcm/s 2. The point-topoint fluctuations at the beginning of the data set are a result of noise in the monopulse angle data; these fluctuations were excluded from the estimation. This curve is then used to calculate the radius-density product. Figure 5b shows that this product stays near 0.02g/cm 2 until decelration increases when the meteoroid reaches the end of its life. At this point the radius density product increases to 0.05 g/cm 2 before it disintegrates. Table 1 summarizes the parameters associated with this head echo. Figure 6 contains the results from the same analysis applied to 20 head echoes. Figure 6a shows apparent velocity as a function of altitude where each meteor is represented by a different symbol. The average velocity is approximately 56 lcm/s; this number represents an average value over the lifetime of each head echo, which is further averaged between all 20 Figure 5. The (a) apparent velocity as a function of time corrected by monopulse angle data and the (b) associated radius density product as a function of altitude for one head echo. -30 (a) o. o. + histogram remains unchanged from the radiant data, as does that of the polarization ratio data. The radial velocity curve is more spread because of the relative radar detection angle. The variation in RCS can be attributed to either the fundamental variations in head echo shape and strength or to the broadside view of the Perseid head echoes (sporadic meteors show a more random distribution independent of pointing). The exact shape of a typical head echo is unknown at this time. As noted in section 3, the monopulse angle data were used to compute the apparent position of the head echo with the radar slant range. The result represents the true three-dimensional position in a radar-based reference frame. Most of the head echo detections had to be discarded because of uncertainties (e.g., angle sidelobe, noise) in the monopulse data; many others were omitted because of low SNR and short durations. After this discarding process was complete, focus was placed on 20 head echoes from which to compute decelrations and meteoroid densities. Figure 5 contains the apparent head echo velocity as a function of time and the meteorold radius-density _o -50 c o 10 '1 10' ;0 o o Ooo o.... o.o,.ooo.,...'o'j;' ++ + o o +. o-.o. o o o...,o;.oi.,: ;..,o..6 ' Altitude (km) (b) + o o o. o K. o4b o o. o o. ; o. ;o... oo o o ' ). '. oø-øi" + o o Altitude (km) Figure 6. The (a) average apparent velocity as a function of altitude corrected by monopulse angle data and the (b) associated radius-density product as a function of altitude for 20 head echoes. o o 11o 11o

7 CLOSE ET AL.: ANALYSIS OF PERSEID METEOR HEAD ECHO DATA 1239 Table 1. Summary of Parameters for One Head Echo Parameter Value Duration 0.95 s Maximum RCS -40 dbsm Mean apparent velocity -62 km/s Deceleration 2, 16 km/s 2 Mean meteor radius-density product 0.02,0.05 g/cm 2 Mean radius* 0.02,0.05 cm Mean mass 10 ' *Assume meteor density is 1 g/cm 3. meteors. These data were then used to calculate the meteoroid radius-density product (again each shape represents one head echo). The averaged parameters are listed in Table 2. Note that the average duration of 0.3 s is representative of the head echoes detected by ALTAIR. 5. Summary The Perseid meteor shower demonstrated ALTAIR's capability to detect head echoes. The high sensitivity resulted in an extremely high number of head echo detections compared with the number seen by other radars. The altitude histograms are consistent with data in the literature. The RCS histogram shows a peak that is much lower than previously seen, most likely because of ALTAIR's high sensitivity. The radial velocity distribution shows two peaks, including one consistent with what is expected for Perseid meteors and another associated with the presence of sporadic meteors. The polarization ratios of meteor head echoes were reported for the first time. High polarization ratios (with the peak of the distribution near 20 db) are consistent with returns from a sphere-like object, supporting the theory that head echoes are approximately spherical in shape. The monopulse angle data permitted determination of the true threedimensional position of the head echoes that resulted in decelerations, as well as estimations of the radiusdensity product of the meteor particle. Analysis of the Table 2. Summary of Parameters Averaged Over 20 Head Echoes Duration Parameter Value 0.3 s Maximum RCS -42 dbsm Mean apparent velocity -56 kngs Deceleration 0.54 kngs 2 Mean meteor radius-density product 0.01 g/cm 2 Mean radius* 0.02 cm Mean mass 10 '3-10 '4 *Assume meteor density is 1 g/cm 3 Leonid 1998 and 1999 storms is in progress. A collection campaign designed to characterize sporadic meteors is also currently planned. Acknowledgments. The authors acknowledge the contributions of the following people: William Ince and Kurt Schwan, the Site Managers at Kwajalein; Tom White, the ALTAIR sensor leader; Scott Coutts and Mark Corbin, for valuable input in data collection and analysis; Ken Roth and Chris Moulton from M1T Lincoln Laboratory; and Andy Frase and Wil Pierre-Mike for software and hardware support. The Leonid data collection effort, in particular, involved many M1T Lincoln Laboratory and Raytheon Range Systems Engineering personnel from the Kwajalein sensors, including Jeff DeLong, Bob Foltz, Tim Mclaughlin, Glen McClellan, Bill Riley, Dave Gibson, LeRoy Sievers, and Dave Shattuck. This work was sponsored by the Department of the Army under Air Force contract F C Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the U.S. Army. References Chapin, E., and E. Kudeki, Radar interferometric imaging studies of long-duration meteor echoes observed at Jicamaraca, J. Geophys. Res., 99, , Evans, J., Radar observations of meteor deceleration, J. Geophys. Res., 71, , Hajduk, A., and A. Galad, Meteoric head echoes, Earth Moon Planets, , Kaiser, T. R., Radio echo studies of meteor ionization, Adv. Phys., 2, , Malnes, E., N. Bjorna, and T. L. Hansen, Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude, Ann. Geophys., 14, , Matthews, J. D., D. D. Meisel, K. P. Hunter, V. S. Getman, and Q. Zhou, Very high resolution studies of micrometeors using the Arecibo 430 MHz radar, Icarus, 126, , McKinley, D. W., The meteoric head echo, J. Atmos. Terr. Phys., 2, 65-73, McKinley, D. W., and P.M. Millman, A phenomenological theory of radar echoes from meteors, Proc. IEEE, 37, , Pellinen-Wannberg, A., and G. Wannberg, Meteor observations with the European incoherent scatter UHF radar, J. Geophys. Res., 99, 11,379-11,390, Skolnik, M., Radar Handbook, McGraw-Hill, New York, Sugar G.R., Radio propagation by reflection from meteor trails, Proc. IEEE, 52, , Zhou, Q.-H., P. Periliar, J.Y.N. Cho, and J.D. Mathews, Simultaneous meteor echo observations by large aperture VHF and UHF radars, Radio Sci., 33, , 1998.

8 1240 CLOSE ET AL.: ANALYSIS OF PERSEID METEOR HEAD ECHO DATA S. Close and S. M. Hunt, MIT Lincoln Laboratory, 244 Wood Street, Lexington, MA (Sigfid(,,giI.mit.edu) F. M. McKeen, Raytheon Range Systems Engineering, P.O. Box 1706, APO-AP M. J. Minardi, Department of Electrical Engineering, Wright State University, Dayton, OH (Received October 25, 1999; revised February 7, 2000; accepted April 21, 2000.)