Radar clutter of multiple tall buildings observed from the ground

Ronald P. Larkin, Illinois Natural History Survey

George W. Swenson, Jr., University of Illinois Department of Electrical and Computer Engineering

29 May 2002

Abstract

We wanted to see if it would be possible to use a noncoherent radar to observe birds near very tall lighted buildings in a large city. One evening's test observations answered the question negatively.

Methods

Study site: A parking lot and grassy field at 41 52.344' N, 87 38.096' W, 100 m WNW of the corner of Wells and Polk, near the South Branch of the Chicago River in downtown Chicago, Illinois (see sketch ). We chose the site for an unobstructed view of a skyscraper at 311 S. Wacker and the even higher Sears Tower just behind it but many other buildings of great height were also visible. We looked up at the S sides of an array of buildings to the NNW, N, NNE and, to a lesser extent, E and NW. Except for antennas, tanks, and some architectural details, the visible surfaces at ranges of interest were almost entirely oriented E-W.

Conditions: A high pressure system dominated, with clear weather and a very light wind from SW or SSW before dusk changing to very light from NE (probably a lake breeze) and then calm at the surface by 2200. At this date in May in Northern Illinois, the bulk of spring "passerine migration" is past but birds of many passerine species and some other species are still migrating until about 1 June. Times are Central Daylight Time.

Radar and modified antenna: Noncoherent, monostatic X-band (3.19-cm wavelength) with 81.3-cm diam. parabolic reflector and linear horizontal polarization. We used two solid metal cuffs in attempts to provide better screening of clutter, both maintaining the 81.3-cm diameter:

Short cuff: 19 cm high (from rim of paraboloid) with the top edge rolled into a 14-mm tube. This cuff was rigid and strictly cylindrical.

Long cuff: 42 cm high, made of thin aluminum sheeting with no rolled edge. This cuff was asymmetrically distorted from a perfect cylinder by about 1 cm. With the antenna pointed at the zenith, no structures were visible from the tip of the antenna feed when this cuff was employed.

These and another cuff had been previously tested in open country and no difference was detected between the cuff with the rolled edge and a same-dimensioned simple cylinder made of sheet aluminum.

We observed an A-scope (rectified logarithmic receiver output) visually and recorded periodic 12-bit digital samples at 7.5-m resolution. In the line graphs, the dark curve plots mean receiver voltage (arbitrary units) vs. range in m and the gray curve plots mean + 1 S.D., from 2.7-s samples.

Results

Nexrad (WSR-88D) KLOT showed a healthy migration by 2123, with reflectivity 20-24 dBZ and the mode of movement toward about 350 degrees.

General 3-cm observations: Before dusk, almost no bird targets were visible; the presence of small insects usually seen in this area in late spring at low heights could not be verified because of appreciable clutter at short range even at the most favorable azimuths. We did not notice the sudden onset of migration usually seen in migration season in rural areas. The number of migrating birds and large insects steadily increased beginning with darkness and at 2200 amounted to a light/medium migration of a majority birds and a minority insects. Although no computation of cross sections has been done, birds were the size of "passerine" migrants observed on all such nights in May in Illinois and showed A-scope fluctuation characteristic of wing beats. No local wind data were taken.

Pointed away from the Chicago skyline at elevation 20 to 50 degrees (120 to 270 degrees azimuth), migrating birds and large insects were visible with both cuffs, although moderate clutter appeared to about 1000 m range especially with the 19-cm cuff. The largest migrating birds were usually <500 units receiver voltage but, as expected, most birds were at 200 to 300 units, i.e. not much above the noise.

Pointed toward or above the skyline at elevation 20 to 50 degrees (azimuths clockwise from 270 degrees to 120 degrees), severe clutter (mostly > 400 units) was present to >1200 m range at all azimuths. When the antenna was pointed at the largest buildings, they produced intense echoes. Moreover the 400 to 600-unit background clutter was visible at all even at elevations > 50 degrees. Several minutes of observation at 2114 confirmed that indeed birds could not be detected <1200 m range when pointed at the top of the building 311 S. Wacker.

Pointed vertically with the long (42-cm) cuff, antenna rotation made a difference. With the azimuth N or S (E-vector aligned E-W , parallel to the faces of most of the buildings) discrete spikes of clutter were prominent. With the azimuth at the orthogonal directions (E-vector aligned N-S), clutter was reduced to the point that counts of migrants were conducted without difficulty. Although we did not make measurements at other than cardinal compass points, the effect of azimuth on clutter as the antenna was rotated seemed smooth and monotonic.

Conclusions

Single large structures generate intense clutter at discrete ranges but birds can be observed on a simple radar within a few tens of meters of the structures (Larkin and Frase, 1988). However in the present brief study a group of large urban structures generated clutter so pervasive that birds could not be observed anywhere near them. Investigations involving radar and communications towers need to take into account the spatial distribution of structures and their potential for ruining research plans.

Antenna rotation had a large effect on skyscraper clutter when pointed at the zenith. The effect was symmetrical (N=S and E=W), whereas the imperfections in the long cuff were decidedly not bilaterally symmetrical.

A better screening method such as a radar fence (Seilman, et al. 1981) or a cuff of microwave-absorbing material may help to some extent in situations like this, but we suspect that a radar with MTI or Doppler capability is required to conduct radar ornithology or radar entomology investigations in such an urban environment and probably in other similar environments such as groups of communications towers.

Acknowledgements

Mr. Ken Wysocki instigated this visit to Chicago and provided local expertise. The short cuff was designed and built by the University of Illinois ECE Department Electronics Shop, supervised by Mr. Chuck Henderson. Mobility and mobile power were provided by the Illinois Natural History Survey Shop. Mr. Robert Diehl and Weathertap provided the 88D images.

----------------------

Larkin, R.P. & Frase, B.A. (1988) Circular paths of birds flying near a broadcasting tower in cloud. Journal of Comparative Psychology, 102, 90-93.

Seilman, M.S., Sheriff, L.A., & Williams, T.C. (1981) Nocturnal migration at Hawk Mountain, Pennsylvania. American Birds, 35, 906-909.