Note: Orbital analysis presented at this web page was generated using Two-Line Element sets obtained from the NASA OIG web service and not the newer Space-Track site.
In case the interceptor did not impact the ITV, the ITV was equipped with a Miss Distance Indicator (MDI). This was a continuous wave radar transceiver. It sent out a continuous signal, and received the doppler shifted returns from anything that flew by it. It did not have very long range, but it would give us a very accurate estimate of the scalar miss distance. I remember having a few good technical papers at the time detailing how we reduced the data. Those CW MIDIs are fairly standard. One just looks for the change in the slope of the doppler returned frequency and you know when the closest approach was. I think a few iterations and analysis of the slope and rate of change of the frequency is needed too, but the process was fairly straightforward. Xontec incorporated, a well-known company specialized in Test Range tracking and Best Estimate Trajectory analysis did the calculations for us. But of course we did not shoot at the ITVs so all we had is one run of simulated data that I believe AVCO had generated against a howitzer cannon rail test shot.
The ITV Miss Distance Indicator was not high power and was basically omnidirectional. It was designed to help out post test analysis of flights where we got close, inside the tracking resolution of the ground based tracking radars, but missed. The range tracking radars at the time could only reconstruct the trajectory and distances with an accuracy of 20-50 feet, depending on whose Best Estimate of Trajectory (BET) analysis you believed. So the Miss Distance Indicator was designed only to operate inside that range. If we missed by more than 100 feet, the range radars would certainly tell us that. I can't recall the frequencies that RF Miss Distance Indicator used. That was not important to my analysis so it is not coming to mind. I assume it was not in the same frequency band as the test range C-band tracking transponder radars to avoid signal interference.
Obviously, if we hit the ITV, we knew the trajectories coincided to within 6 feet, the diameter of the balloon. I remember seeing the canisters on the lab bench and the kevlar laying around. I calculated that infrared signature so many times getting ready for the actual intercept. I should remember that number as I needed the cross section area exposed to the interceptor!
Those wires on the surface would tell us where on the ITV we hit or the Miss Distance Vector. That was helpful for the guidance analysis of the interceptor. When one is trying for a hit-to-kill, 3 feet or a 1 foot miss distance is important, and the location on the balloon was supposed to help the terminal guidance people figure out exactly what the interceptor did.
Yes, we used to joke about the ITV's Miss Distance Indicator being used as a proximity fuse, especially when we heard that the test data tape of one howitzer shot from a test range was used to check out the Miss Distance Indicator. But it was not used as a proximity fuse, just a closest approach determiner.
The ITV was "inflated". It had a hydrazine catalyst generator that produced hot gases from a rheuthenium catalyst decomposition driven device. It was designed to heat and maintain the Kevlar balloon at a temperature that would emit the desired infrared signature in a controlled manner for our Interceptor to see. Post test telemetry analysis was used to calculate the IR signature of the balloon. There were many temperature sensors embedded in the kevlar in known locations.
The guidance of the MV was simple Direct Proportional Line of Sight. The MV had 56 full charge solid propellant rockets arranged around the circumference, and 8 half charge solid rocket motors for a "bang-bang" control system. The 56 motors were called divert motors, and the 8 other ones were supposed to be used in the end game phase of the intercept where the needed positional changes would supposedly be less.
To control or dampen coning or wobbling of the interceptor, the back end of the interceptor had four pods of attitude control rocket motors. They were little tiny squibs that would detect the wobbling or off central rotation of the interceptor by logic that would look at the strip detector data. I can't recall exactly how many charges were in each pod, but the number was not that large.
Sven Grahn's comments: I have been thinking about the intercept geometry of the F-15 ASAT. As I see it the two-stage booster could not have reached more than maybe 4-5 km/s at burnout, which is not enough to catch up with a satellite in orbit moving at 7-8 km/s. Therefore the intercept must have been made "head-to-head", i.e a head-on approach. Also, this would have made it necessary to launch the interceptor more or less into the orbital plane of the target. Of course the F-15 could fly to a point in the orbital plane of the target...within certain limits, of course. Also, the trajectory must have been chosen to make the necessary field-of-view of the IR telescope reasonably small, i.e. the angle between the interceptor and target trajectories must have been as small as possible. In this way the target moved little relative to the intrecptor's flight path vector.. I have tried to sketch this in the attached picture.
The interceptor detector was cooled by liquid helium prior to release. Yes, 4 degrees kelvin helium. We had a large helium dewar on the ground that was about the size of the Robot on the old Lost in Space TV show. We then had a large helium dewar on the F-15. We removed the ammunition drum and the back seat in the F-15 with a large tape recorder and the helium dewar. This allowed us a fair amount of time to do pre-mission checks, perform the flight from Edwards AFB to Vandenberg AFB. The upper stage of the missile also had a helium dewar that obviously was much smaller than the one on the aircraft. This dewar was connected to the interceptor by a supply and return line.
In space, after the missile was launched, these cryo lines were retracted, and then two spin motors located on the spin bearing assembly of the interceptor were fired and the interceptor was spun up. It is important to note that the interceptor did not work while not spinning. This was not a staring array. I have attached a crude drawing of the strip detector layout. Those spiral lines were symmetrical logarithmic spirals (spirals of archimedes as I recall). Some simple geometry and trigonometry would enable us by recording the time of the detector crossing, calculate the objects location in the sensor field of view.
The spin bearing was quite
a development problem too. Once the interceptor was spun up, we did
not have all day to deploy it, and spin-down was a problem as the interceptor
had to operate within a certain revolutions-per-second range. Since
my BS was in mechanical engineering I would stick my nose into those meetings
quite a bit.
Sven Grahn's comments:
The following flight tests were conducted with the system
|1||21 Jan 1984||Successful: missile tested without miniature vehicle|
|2||13 Nov 1984||Failed: directed at a star with miniature vehicle|
|3||13 Sept 1985||Successful: destroyed NLR satellite P78-1 Solwind (79-17A, Sat Cat Nr 11278)|
|4||22 Aug 1986||Successful: directed at a star|
|5||29 Sept 1986||Successful: directed at a star|
" ....By September 1985, all was finally ready for a test against an orbiting satellite. On Sept. 13, Maj. Wilbert D. "Doug" Pearson, the director of the F-15 ASAT CTF, took off on a crucial mission that required him to fly an extraordinarily exacting profile in order to arrive at a precise firing location at exactly the right time. Flying at Mach 1.22 some 200 miles west of Vandenberg Air Force Base, he executed a 3.8g pull-up to a climb angle of 65 degrees. The missile automatically launched itself at 38,100 ft. Minutes later, orbiting peacefully 345 miles above the Pacific Ocean, an obsolete satellite named P78-1 was suddenly shattered into pieces. Pearson had become the world's first pilot ever to shoot down a satellite. To this day, now Maj. Gen. Doug Pearson remains, as Air Force Materiel Command Commander Gen. Lester Lyles recently observed, the first and only "space ace ..." (1)
President Reagan gave the go-ahead for the test against a real target in space on 20 August 1985 (4). The test was originally scheduled for 4 September, but because the 15 days notice had not been given to Congress it was delayed 9 days (3). The target was Solwind P78-1, a gamma ray spectroscopy satellite weighing 850 kg (2) that had been launched in February 1979 into an initial orbit at 563-602 km at 97.6 degrees inclination. P78-1 was in a noon-midnight, Sun-synchronous orbit. The identity of the target was actually leaked before the test (3).
The most probable time
of intercept is at around 2040 UT on 13 Sept. 1985 when the Solwind passed
off the US West Coast from south to north. This time of intercept is also
given in (2). The local time in the
Pacific time zone was then 1240, i.e. the target satellite was illuminated
making the surface of the spacecraft warm and radiating infrared radiation.
The altitude of the target satellite at the probable time of intercept
was 530 km.
It is interesting to plot the trajectory of an ITV when passing straight over the ground station at Hawaii, where Greg Karambelas says the inflation command would be issued. It turns out that the inclination of the orbit seems to be deliberately chosen so that the target satellite would head straight for the southern California coast, indeed directly in the direction of Vandenberg and Edwards AFB where the F-15 was based (see figure below).
One can also see that the timeline was tight. After AOS at Hawaii it would take a few minutes to command and verify that the balloon had deployed properly, say five minutes. Then there would only be ten minutes before the target vehicle crossed the California coast. The F-15 would hardly be able to scramble and reach launch altitude in such a short time.
What happened to the target vehicles?
According to Gregory Karambelas the target vehicles were battery-powered and were kept dormant and uninflated in orbit following the ban on further testing of the ASAT system. Finally, permission was granted to test the inflation system. Evidence of this can be gathered from an analysis of the orbital period of the taget vehicles. This analysis shows that the ITV-2 (Sat Cat Nr 16329) target was actually inflated late on 17 December 1986 or early on 18 December, see figure on the right. The decay rate of the spacecraft suddenly increases as the cross section suddenly increases.
No corresponding sudden
increase in decay rate can be observed for the ITV-1 (Sat Cat Nr 16328)
vehicle. The ITV-2 target decayed after 604 days (on 9 Aug 1987) and the
ITV-1 target decayed after 1245 days (on 11 May 1989) (2).