Case 21

South Mountain (location A)

Spring 1967

Investigators: Low, Rush

BACK to Chapter 2


Operators of two airport radars reported that a target equivalent to an aircraft had followed a commercial flight in, overtaken it, and passed it on one side, and proceeding at about 200 knots until it left the radar field. No corresponding object was visible from the control tower. On the basis of witnesses' reports and weather records, explanations based on anomalous atmospheric propagation or freak reflection from other objects appear inadequate. The case is not adequately explained despite features that suggest a reflection effect (See Section III Chapter 6).


A radar traffic controller (Witness A) at an AF installation that serves as an airport for a nearby city (location A), telephoned the Colorado Project in the middle of May, 1967 to report an unexplained radar anomaly. The report was referred to Dr. Donald H. Menzel for comment, and Witness A and three other witnesses were interviewed at various times. The information so obtained is summarized in the next section.


Witness A, an air traffic controller of 20 years' experience, reported the following observations. At about 4:40 p.m., he and three other men were in the IFR (radar) room at the airfield. Two radars were in use: azimuth surveillance radar (ASR), used for early detection of arriving aircraft, and precision approach radar (PAR), used to monitor both azimuth and elevation of an aircraft approaching the runway (Fig. 2).

The controllers were monitoring the approach of a commercial Boeing 720. They got him onto the correct azimuth and glide path


Fig 2

Figure 2: ILS Runway Diagram

Click on thumbnail to see full-size image.


just as he broke through the 3,000 ft. ceiling about four miles from the radar receiver. Another commercial flight, a Viscount, showed on the surveillance radar about six mi. behind the 720. About the time the 720 appeared in the field of the precision radar, operated by Witness A, he noticed a very faint target on the elevation (glide path) screen about two mi. behind the 720. He adjusted the sensitivity of the instrument, and the unknown target became visible on the azimuth screen also. It appeared to be following the 720 on the glide path.

When the 720 had advanced about one mi., Witness A asked the operator of the surveillance radar, Witness B, whether he had the unidentified target; he did. Witness A then reported the object to the Viscount crew, about four mi. behind it. They saw nothing, though visibility under the overcast was 25-30 mi. He then reported the object to the visual control tower; but none of the three controllers there could see anything to account for it, even with binoculars. At this point, the departure scope man (the surveillance radar had duplicate screens for monitoring arrivals and departures) and the arrival data position man walked over to observe the precision scope. The target showed with equal clarity on both the elevation and azimuth screens. The unidentified object was overtaking the 720, and was about 0.25 mi. behind as the 720 passed the approach lighting system. At that point, the object pulled over, moved eastward, passed the Boeing on its right side, and continued on a parallel course at 200 ft. altitude and some 300 ft. east of the runway, until it passed out of the field of the precision scope. Unfortunately, no one thought to see whether the object appeared on the surveillance radar departure scope. At disappearance, it was about 1-1.5 mi. from the control tower. The controllers in the tower never saw anything to account for the target.

The Viscount came in normally on the radar, with nothing following. Its crew reported after landing that they had not at anytime during the approach seen anything between them and the 720.


Witness A observed that the 720 had not been visible as far out as six mi., where the "bogie" first appeared. It looked like an aircraft target, though weaker than usual, and became quite clear as it came nearer. He commented also that the bogie followed the correct procedure for an overtaking aircraft, and that, if a pilot is practicing an instrument approach but does not want to touch down, his prescribed procedure is to level off and cross the field at 200 ft., as the bogie appeared to do on the radar. In fact, the object showed the flight characteristics of a Century series jet fighter (F-l00, F-104, etc.), making an approach at a speed of 200-250 knots. However, such a jet makes a great deal of noise, and should have been heard even in the glass-enclosed tower.

Witness A was interviewed in detail when he first telephoned the project in Spring 1967, and questioned further on various aspects at several later dates. Other witnesses unfortunately were not contacted until Fall 1968.

Witness B, who had been monitoring the surveillance radar approach scope, was unable to recall details of the incident. He remembered only that it was "an odd thing" -- a radar target, but nothing visual.

Witness C was a controller of 15 years' experience, 11 on radar, who had been in the radar room when the sighting occurred, and had watched it on the precision scope. He recognized the difficulty in remembering accurately after such a time interval, but felt that his memory for the key details was good. He had been deeply impressed by the incident, and had discussed it with Witness A and others on various occasions.

He confirmed the account of Witness A in almost all respects. He was not certain that the bogie had come in on the ILS glide path which is indicated by a line on the elevation screen of the precision radar); it was following the Boeing and must have been on or near the glide path. Witness A had stated that the bogie overtook and passed the 720 at about the approach end of the runway. Witness C, however, recalled that the bogie had overtaken the 720 and flown alongside "like a wingman" (i.e., slightly behind and to the


right of the 720) for one or two miles before touchdown. Then, about a half mile from the runway, it had "pulled up" and flown on ahead. The 720's approach speed was about 140 knots.

Witness C emphasized that the bogie target was indistinguishable from an aircraft. He said that, if the bogie had appeared ahead of the 720, he would not have hesitated to warn the 720 off the approach.

He noted also that the surveillance radar was an old, faulty instrument that sometimes missed targets that were known to be in the field.

Witness D was a controller in the tower during the incident. He remembered that the radar crew phoned about the bogie; the tower men looked and saw the 720 coming in, but nothing else, even with binoculars. The conditions were such that he was confident that no such aircraft as the radars indicated could have come in without the tower crew having seen it.


The report of the project's consulting meteorologist follows:

Following is a brief summary covering the weather situation near . . . [the airfield in location A] at and near 1640 MDT ... [in the middle of] May ... 1967:


Hourly surface observations from - ... [Location A, location B, location C, location D, location E, location F]

Two and three hourly data from - ... [Location C, location H, location I]

Winds aloft and radiosonde data for ... [location D], at 12:00 noon and 6:00 P.M. MDT.


The general weather situation prevailing in ... [the general area] was a condition of drizzle and fog with low ceilings at most all stations east


of ... [location H]. Amounts of precipitation were generally light but the drizzle and fog continued for many hours at most stations.

Shortly after noon colder air moved in from a northerly direction in a layer from 1000 to 5000 feet above the surface. At ... [location D] the drop in temperature measured between the noon and 6:00 P.M. radiosondes was between 5o and 6o F. in this layer. This drop in cloud layer temperatures was accompanied by increasing winds near the surface. At 2:30 P.M. gustiness at ... [location Dl reached 30 knots. Similar increases in wind velocities began later at ... [location A, location B, location E, and location J]. Some snow and snow pellets fell at various stations as this mixture of colder air took place.


Two layers of scattered clouds, at 900 and 2400 feet respectively, would have been moving rapidly from north to south in an air flow having surface winds averaging nearly 30 mph. It occurred at 1630 MDT. Gustiness of 8-10 additional miles per hour was occurring at this time. A layer of overcast cloudiness was estimated at 4000 feet above the station. Visibility was greater than 15 miles.

A condition of very light drizzle had ended at 1530 MDT and light snow pellets began at 1710 MDT. The differences in surface temperatures was only lo (34 to 33) indicating that the greatest amount of change was taking place in the air at cloud level.

The snow pellets which began at 1710 MDT and intermittent snow showers continued past midnight. It is well known that water and ice surfaces mixed together inside clouds tend to intensify radar echo causing bright spots or bright lines to appear.


The snow pellets would have produced an increased intensity of the radar echoes in some small shower areas. Although snow pellets were not occurring at the station at 1640 MDT it is highly probable that some were in the vicinity.

Total amounts of precipitation were light. Only .03 inch was measured in the 24 hours ending at midnight.

At the same time that snow pellets and snow showers were observed at ... [the airfield, location B] reported no precipitation.


It is my opinion that fragmentary segments of two layers of scattered clouds moving at variable speeds beneath a solid overcast would have given a rapidly changing sky condition to any observer at or near the airport. Reflection of any lights could have caused greater or lesser brightness to the under surfaces of some of these scattered clouds. The strong gusty winds were not only capable of moving the clouds rapidly but could have carried some light substances, such as paper to an elevation similar to the lower cloud height. The shafts of snow pellets at a mile or more away from the base may have caused some distortion of visibility in directions concentrated to the west and northwest of the field.


Anomalous targets on radar generally are caused by instrumental defects, birds, anomalous atmospheric propagation (e.g. mirage effects), out-of-phase echoes, or multiple reflections. Instrumental defects appear to be eliminated in this case, since the bogie was seen consistently on the surveillance radar and both the azimuth and elevation beams of the precision radar. The speed of the bogie, its radar intensity, and the course it followed all appeared inconsistent with a bird.


Neither did this anomaly show any of the typical characteristics of the "angels" caused by anomalous propagation; moreover, weather data indicate no inversion was present. Both witnesses A and C had had many years of experience with all the usual types of anomalies. The fact that they were mystified by the phenomenon and considered it worth reporting indicates that it was an uncommon effect.

Sometimes a distant, strong reflector may return a radar echo so long delayed that it arrives after a second pulse has been emitted. It will therefore appear at a spuriously short range. This possibility appears to be precluded by the different pulse frequencies of the surveillance and precision radars (1000 and 5500 per sec., respectively), and by the behavior of the bogie, which appeared to relate it to the Boeing 720.

There remains the possibility of multiple reflections. After reviewing a report of the incident, Menzel suggested that the bogie had been produced by reflection of radar energy from the 720 to a fairly efficient reflector on the ground, back to the 720, and thence to the radar receiver. The superfluous echo would have appeared on the line of sight from radar antenna to aircraft, and beyond the aircraft the same distance as that from aircraft to reflector. Meuzel suggested that a structure involving a cube-corner -- e.g., a steel dump-truck body -- might act as a rather efficient reflector.

This hypothesis would explain some aspects of the observations. The bogie appeared about two miles behind the 720 when it was about four miles out, and gained on it at a rate roughly equal to the airplane's own ground speed of about 120 knots, as would be expected. This would imply that the reflector was about two miles ahead of the 720, which would place it about half a mile south of the approach end of the runway. The bogie then should have overtaken the 720 at that point.

Witness A said that it was about 0.25 mi. behind the 720 as the latter reached the approach light system; that would place the


reflector approximately at the approach end of the runway. Witness C, however (a year and a half after the incident), stated that the bogie caught up with the 720 "one or two miles" before touchdown, flew alongside, and pulled ahead about a half mile from the runway. That would place the reflector about 0.5 to 1.5 mi. south of the runway, differing by as much as a mile from the location resulting from Witness A's account.

So far, so good. Men who were a bit excited, or trying to remember details after such an interval, might differ by a mile in their estimates, particularly since the range scale on the precision radar scope is logarithmic. Incidentally, half a mile from the runway the elevation of the ILS glide path was about 200 ft. -- the elevation at which the bogie appeared to overfly the field.

However, a target produced by such a delayed reflection would not have appeared on the glide path. In elevation, the glide path was a line rising at an angle of 2.7o from the ILS transmitter 7,300 ft. south of the precision radar antenna. The line of sight from the radar to the Boeing four miles out thus intersected the glide path at a substantial angle, so the bogie reflection, seen on the radar line of sight, would have appeared about 0.25 in. below the line marking the glide path on the radar scope. It does not seem likely that an experienced controller would have failed to notice a discrepancy amounting to some 200 ft. in elevation that if not corrected would have been disastrous to an aircraft.

The shift of the unidentified object to the right as it overtook the 720 can be partially explained. If it is assumed that the bogie was a secondary echo from a reflector near the runway, then the bogie would have been always the same distance behind the 720 as the reflector in front of it, and would have appeared on the line of sight from the precision radar antenna to the 720. Since the antenna was about 400 ft. east of the runway, the bogie would have appeared projected to the west of the approach track. Its apparent course would have been a gradual swerve to its right.

However, the bogie would have nearly coincided with the radar image of the 720 as it passed low over the reflector; and immediately


thereafter, as the 720 passed beyond the reflector, the bogie would have stopped its forward motion and moved laterally to the west. This hypothetical behavior contrasts sharply with the statements of witnesses A and C, both of whom insisted that the bogie moved over and passed the 720 on the right (east), and that it continued on that course, ahead of the airplane, until it left the radar field.

The case is therefore not satisfactorily explained. In general, the association of the unidentified target with the 720 and the lack of a visible counterpart suggest strongly that it was a radar artifact. Yet the details of its course can be reconciled with the reflector hypothesis only by discounting the accuracy of reports by observers who were intimately familiar with the context in which they were working.