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).
Background:
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.
Investigation:
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
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 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
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.
Weather:
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:
SOURCES OF DATA
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.
GENERAL WEATHER SITUATION
The general weather situation prevailing in ... [the general area]
was a condition of drizzle and fog with low ceilings at most all
stations east
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.
MOST PROBABLE WEATHER AT 1640 MDT AT ... [THE] AIRFIELD
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.
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.
SUMMARY
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.
Hypotheses:
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.
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
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
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.
Figure 2: ILS Runway Diagram
Click on Thumbnail to see Full-size image.
of ... [location H]. Amounts of precipitation were generally light
but the drizzle and fog continued for many hours at most stations.
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.