Radar Blackout was a serious problem for Safeguard, particularly for its PAR radars, whose UHF frequencies were strongly absorbed by ambient ionization, fireballs from intercepts or salvage fusing, and remote regions. Ionization due to auroral effects is strong enough to cause radar degradation in certain seasons. Fireballs are generally ionization regions centered on the explosion. Remote regions involve beta rays (electrons) and fission fragments from explosions at higher altitudes that deposit at
50–60 km, where they produce enough ionization and absorption to affect radar and communication systems. Although they can deposit at lower altitudes than the explosions that produce them, they have similar system impacts, so Appendix D treats them together. Low-altitude nuclear bursts in the Sprint engagement altitude and yield region produce fireballs a few
kilometers in diameter that quickly achieve pressure balance, radiate to temperatures of a fraction of an electron volt. Their initial absorption at radar wavelengths is very strong and is maintained for several minutes. They are essentially black to UHF and lower frequency radars throughout structured attacks lasting a few minutes. However, such fireballs need not completely block radar operation. The fireball from a MT burst at sea level is about 1 km across, as is that from a Sprint-sized KT range explosion at 45 km, which would exclude a solid angle of about (1 km/45 km)^2 = 0.001 sr. Unless there were dozens of bursts in the radar’s field of regard, performance should not be severely degraded. However, a MT explosion at that altitude would produce a fireball initially about 10 km across, which would block about (10/45)^2 = 0.05 sr. A dozen large explosions could block the radars for endoatmospheric intercepts and reduce the flexibility of those for exoatmospheric intercepts. The uncertain coupling of energy into the low density ambient air at high altitudes by exoatmospheric explosions produces 10- to 100-fold uncertainties in predictions of the size of the regions affected by blackout and refraction from high-altitude explosions. Reducing these uncertainties would be difficult because of the lack of data. The U.S. detonated seven devices in the 10 to 250 km altitude region to be used for Safeguard defenses, but only two exoatmospheric nuclear tests relevant to Spartan. Neither tested the key coupling issues in those altitudes or the multi-burst phenomenology that would cause the greatest degradation and uncertainty in expected scenarios. Measurements were made of radar and communication degradations at various frequencies and ranges from the burst, but not of fireball interior ionization and absorption.
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The tests were recorded photographically with films only sensitive in the visible; thus, there is little basis for IR background predictions. Megaton explosions at altitudes of 150–250 km create hot, ionized fireballs 100s of kilometers across. Most ambient air molecules are stripped of some or most of their electrons, producing initial electron densities n o of about 10^9 to 10^12/cc. At early times, the fireball would form a reflective region of a solid angle of about (300 km/600 km) 2 = 0.3 sr. Placed in front of a PAR, an excluded angle that large would mask the trajectories of subsequent RVs the PARs would need to detect and track in 10s of sec onds. Such obscurations would be unacceptable against attackers spaced at short intervals. As affordable basing allowed little overlap in coverage between adjacent PARs, these obscurations could not be overcome by internetting PAR measurements. After a few 10s of seconds, the fireballs’ temperature should cool by radiation to temperatures of a few thousand degrees. As the fireball cools, the electron density falls. After that, the principal mechanism for the removal of ionization is radiative recombination, which is quadratic in electron density with rate coefficient C
R= 10^–12cc/s.
Recombination causes the electron density n e to fall as 1/CRt. After a time of about 300 s, the electron density drops below the critical density n c
of about 3 x 10^9/cc that would cause complete reflection at PAR’s UHF frequency. Even later, when the fireball is no longer reflecting, absorption losses could still be unacceptable. When electron-ion interactions are the dominant source of collisions, the absorption coefficient α(db/km) is approximately 0.1(n e/f)^2.
Figure D.1 shows absorption as a function of time after a high-altitude explosion for frequencies of 0.5, 2, and 10 GHz. At the PAR frequency of 0.5 GHZ, absorption is over 1,000 db at short times. By 200 s, it drops to about 10 db/km, which would produce losses in propagating through a 100 km thick fireball of about 100 km x 10 db/km or 1,000 db, which is quite opaque. The losses drop to about 0.4 db/km by 1,000 s, but even that would give a one-way loss of 40 db, or 10^4, which is unacceptable. Thus, PAR would not recover during an attack executed over 10 minutes.
The situation was more favorable at the roughly threefold higher frequency of MSR, which would stop reflecting in 30 s and drop to 1 db/km after about 100 s. However, MSR was sized to take tracks from PAR rather than search for itself, so it lacked the sensitivity and range to take advantage of its reduced absorption. X-band radars developed subsequently have critical frequencies 20-fold higher than UHF. Their critical electron densities of 1.2 x 10^12/cc would only be reached only near explosions at 150 km, so x-band radars probably would not be reflected, and their absorption losses would drop below 1 db/km after about 20 s, 0.1 db after 100 s, and 0.01 after 400 s. However, x-band radars were not available during Sentinel and Safeguard, and even those available today are better suited to tracking than to searching large volumes. Potential nuclear environments were complicated by the range of options open to the attacker, who could use precursor bursts to straddle and reduce the PARs’
effective viewing angle, and thereby reduce the value of its tracks to downstream radars and interceptors.
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22. Bethe, “Countermeasures to ABM Systems,” pp. 130–143.
23. C. Blank, A Pocket Manual of the Physical and Chemical Characteristics of the Earth’s Atmosphere (Washington D.C.:
Defense Nuclear Agency, 1974), p. 147.
24. Ibid., p. 247.
25. Bethe, “Countermeasures to ABM Systems.”
by Gregory H. Canavan, published by the Heritage Foundation.
.
