Very interesting thread, and some good replies. Some right angle turn is definately a manoeuvre that by itself would rule straight out a lot of
possible explanations, very well covered here by Karl.
I'd like to add something about an UFO showing some extremely strange carachteristics: I refer to the incident occurrerd in Belgium during the
1989-1990 UFO flap where the UFO's manoeuvres were tracked on radar, and the analysis on
data returned some surprising results: and in THAT case, the source was more than just reliable.
Glons radar confirmed the sighting of an unidentified object at an altitude of 3,000 meters. Semmerzake radar confirmed the Glons detection and
passed its confirmation onto the Air Force. The radar scans were compared with the previous Eupen radar sightings (see Eupen Case) by Semmerzake and
Glons and were found to be identical.
Several police patrols had witnessed the same phenomenon before. It was a massive triangular shape with the same lighting configuration as seen at
Eupen four months earlier.
Colonel Wilfred De Brouwer, Chief of the operations section of the Air Force, said: "That because of the frequency or requests for radar
confirmation at Glons and Semmerzake - and as a number of private visual observations had been confirmed by the police - it was decided that as these
parameters had been met, a patrol of F-16 aircraft should be sent to intercept an unidentified object somewhere to the south of Brussels"
As a consequence, two F-16 aircraft of the Belgian Air Force - registration
numbers 349 and 350 = flown by a Captain and a Flight-Lieutenant, both highly qualified pilots, took off from Bevekom.
Within a few minutes - guided by the Glons radar - both pilots had detected a positive oval-shaped object on their on-board radar at a height of 3,000
meters, but in the darkness saw nothing. This oval configuration, however, caused the pilots some concern. It reacted in an intelligent and disturbing
way when they attempted to 'lock-on' with their on-board radar.
Changing shape instantly, it assumed a distinct 'diamond image' on their radar screens and - increasing its speed to 1,000km/h - took immediate
and violent evasive action.
Photographs of the actual on-board radar of the F-16s recorded a descent of this object from 3,000m to 1,200 in 2 seconds, a descent rate of
1,800km/h. The same photographs show an unbelievable acceleration rate of 280km/h to 1,800km/h in a few seconds. According to Professor
Leon Brening - a non-linear dynamic theorist at the Free University of Brussels - this would represent an acceleration of 46g and would be
beyond the possibility of any human pilot to endure.
It was noted that in spite of these speeds and acceleration times there was a marked absence of any sonic boom. The movements of this object
were described by the pilots and radar operators as 'wildly erratic and step-like', and a zigzag course was taken over the city of Brussels with the
two F-16s in pursuit. Visual contact was not possible against the lighting of the city.
This same procedure was repeated several times, with this object - whenever an attempt at radar 'lock-on' was made - pursuing a violently erratic
course at impossible speed and losing its pursuers.
Colonel De Brouwer added "Immediatley after the operation, the pilots said they had never seen anything like it.
Certainly the flight pattern and
echo on their screens was in no way that of a conventional aircraft"
The Belgian Minister of Defence in the Belgian parliament stated that "The
Government did not know what they were".
Acceleration data
Radar data
Colonel W. De Brouwer, Belgian Air Force, with the radar videos of one of the F-16s at the press conference of July 11th
Speed changes of up to 410 knots in one second.
Heading changes of up to 70 degrees in one second.
Altitude changes of up to 3000 feet per second (1,777 knots) maintained for one second or less and typical ascent / descent rates of 1000 feet per
second (592 knots).
For instance, the F-4 Phantom is known be able to turn at only 11.5 degrees per second, less than 1/6 as fast as the observed UFO profile.
This graph shows the value of the changes:
Text file of the radar contacts of one of the F-16s
Not that easy to explain, at all.
Sources and references:
www.ufoevidence.org...
www.ufoevidence.org...
ufos.about.com...
www.ufoevidence.org...
www.geocities.com...
ufologie.net...
Karl, thanks for yet another great thread: with these great works, you are spoiling us! 
[edit to fix [b ] tag]
[edit on 20/2/2010 by internos]