THE UNIVERSITY OF MICHIGAN 2764-7-R 2764-7-R = RL-2075 24 January 1961 To: Cliff McLain, ARPA and Orville Woodyard, USASRDL From: W.E. Burdick and K.M.Siegel Subject: Report of Major Event In accordance with the requirements of ARPA Order 120-60, and Contract DA 36-039 SC-75041 under which this research was carried out we are reporting a significant theoretical result which we feel constitutes a "Major Event" under these requirements. In the most recent Radiation Laboratory report 1 on this research it has been shown theoretically that the radar cross section of an object traveling at satellite altitudes and velocities is enhanced by several orders of magnitude due to the plasma sheath surrounding the object and that this enhancement is quite aspect sensitive. This brief report compares the predictions of this theory with the measurements reported by Kraus and his associates of The Ohio State 2-8 University. Numerical results have been computed for only one set of conditions: A spherical satellite of 1 meter radius moving with a velocity of 8 kilometers per second at an altitude of 500 kilometers and a radar frequency of 20 Ncs (X = 15 m). With these conditions the radar cross section is computed as:

2764-7-R o a(in m2) 90~ 3.6 105 60~ 5.4 45~ 2.7 where(is the angle between the observer und the tangent to the satellite path. Thus this theory predicts that a satellite will have a large enhancement in radar cross section over a narrow range in azimuth near its point of closest approach to an observing radar. Many experimental investigators have reported observing unusually large radar cross sections of some ballistic missiles and satellites athigh altitudes. These results, in general, have not been well defined in the literature and are usually described only as "very large". Kraus has given several examples of his results which allow us to check them against this theory. These results consist, in part, of measurements of 20 Mcs WWV signal from the station near Washington, D.C. which show large bursts of signal which coincide with the passage of the satellite. In References 7 and 8 Kraus notes that these bursts of WWV occur at a time when the satellite is at or very near its point of closest approach to his receiver. Kraus, of course, has a bistatic situation but at the ranges involved the angle is usually small and will probably only spread the points of maximum return about the point of closest approach. In Reference 7 Kraus illustrates with four different sets of 2

2764-7-R data on Sputnik III; all of which show large bursts of WWV enhancement within a very few minutes of the point of closest approach. In Reference 8 he presents a composite plot of 50 passes of Sputnik III over a 14 night period which shows the highest peaks on either side but very close to the closest approach point. One of the most significant results reported by Kraus7 is due to the fortunate circumstance of the satellite, Sputnik III, having its point of closest approach on one pass fall on the center of his antenna beam. In this instance the enhanced burst pattern shows a very symmetric form and Kraus was able to calculate the radar cross section of the satellite as "several squarekilometers" (order of 106 m2). Thus we see that the theory of Reference 1 predicts 1. Enhancement of the radar cross section of the same order as observed by Kraus 2. That this enhanced signal is highly aspect sensitive being a maximum where the observer is normal to the tangent to the satellite path, also as observed by Kraus. We feel that this theoretical result represents a significant step in our understanding of the physics of satellite flight and in our ability to predict the radar returns to be expected from high velocity, high altitude objects. We realize, of course, that much additional theoretical work and comparison of these results with observations must be doneto 3

2764-7-R verify And extend this theory and to understand its possible limitations. This work is continuing. REFERENCES 1. Chen, Kun-Mu, Studies in Radar Cross Sections XLIII - Plasma Sheath Surrounding a Conducting Spherical Satellite and the Effect on Radar Cross Section, The University of Michigan Radiation Laboratory Report No 2764-6-T, October 1960. Contract DA 36-039 ARPA Order No. 120-60, SC-75041. 2. J.D. Kraus and J.S. Albus, "A Note on Some Signal Characteristics of Sputnik I", Proc. I.R.E., 46, 610-611, March 1958. 3. J.D. Kraus, "Detection of Sputnik I and II by CW reflection", Proc. I.R.E., 46, 611-612, March, 1958. 4. J.D. Kraus, R.C. Higgy and J.S. Albus, "Observations of the U.S. Satellites Explorers I and III by CW Reflection", Proc. I.R.E., 46, 1534, August, 1958. 5. J.D. Kraus and E.E. Dreese, "Sputniks I's Last Days in Orbit", Proc. I.R.E., 46, 1580-1587, September, 1958. 6. J.D. Kraus, R.C. Higgy, D.J. Scheer and W.R. Crone, "Observations of Ionization Induced by Artificial Earth Satellites", Nature, 185, 520-521, February 20, 1960. 7. J.D. Kraus, R.C. Higgy and W.R. Crone, "The Satellite Ionization Phenomenon', Proc. I.R.E., 48, 672-678, April, 1960. 8. J.D. Kraus, "Evidence of satellite-induced Ionization Effects Between Hemispheres", Proc. I.R.E., 48, 1913-1914, November, 1960. 4