The Thule J-site <a href="//commons.wikimedia.org/w/index.php?title=BMEWS&amp;action=edit&amp;redlink=1" class="new" title="BMEWS (page does not exist)">BMEWS</a> station's detection arcs of 200°<sup id="cite_ref-1" class="reference"><a href="#cite_note-1">[1]</a></sup> was a missile warning "fence" created by 4 radars' separate arcs: each AN/FPS-50 created 2 arcs (shown) centered at 3.5° and 7° elevation@<a href="//commons.wikimedia.org/w/index.php?title=User:BMW&amp;action=edit&amp;redlink=1" class="new" title="User:BMW (page does not exist)">BMW</a> (exagerated in illustration.)  Each arc was created by a smaller radar beam ~1° wide x 3.5° high at a "horizontal sweep rate…fast enough that a missile or satellite cannot pass through…undetected"@<a href="//commons.wikimedia.org/w/index.php?title=User:BMW&amp;action=edit&amp;redlink=1" class="new" title="User:BMW (page does not exist)">BMW</a> (e.g., "Lower Fan" and "Upper Fan") --concerns in 1962 of "ERBM's (Extended Range Ballistic Missiles)" were that missile speeds after burnout would be higher than the initially-deployed Soviet ICBMs@<a href="//commons.wikimedia.org/w/index.php?title=User:NORAD1958B&amp;action=edit&amp;redlink=1" class="new" title="User:NORAD1958B (page does not exist)">NORAD1958B</a> and prevent the sweeping "fans"(revisit time of 2 sec)@<a href="//commons.wikimedia.org/w/index.php?title=User:Skolnik&amp;action=edit&amp;redlink=1" class="new" title="User:Skolnik (page does not exist)">Skolnik</a> from detecting the missiles.
A missile within the lower arc (~1.75-5.25° elevation) would be detected at a "Lower Fan Q Point" (black dot) and then by the upper fan (black dot with jagged outline), which allowed the impact area to be estimated from "where the object crossed the two fans and the elapsed time interval between fan crossings"@<a href="//commons.wikimedia.org/w/index.php?title=User:BMW&amp;action=edit&amp;redlink=1" class="new" title="User:BMW (page does not exist)">BMW</a> (displays showed the uncertain impact point as an elliptical area.)   The free flight range of the missile outside the atmosphere (burnout to reentry) depends on the flight path angle and on the missile's parametric value of Q calculated from altitude and speed--additional ballistic range within the atmosphere to an estimated burst altitude was determined from computerized look-up tables in the Missile Impact Predictor.@<a href="//commons.wikimedia.org/w/index.php?title=User:BMW&amp;action=edit&amp;redlink=1" class="new" title="User:BMW (page does not exist)">BMW</a>
USAF
1961
Eyes of the North film