NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: astrocompass still in use
From: Dan Allen
Date: 2002 Sep 25, 21:24 -0700
From: Dan Allen
Date: 2002 Sep 25, 21:24 -0700
On Wednesday, September 25, 2002, at 09:44 PM, Ken Gebhart wrote: > BTW a list member mentioned the MD-1 on the B-52. I think this was an > astro > TRACKER not an astro compass. It would lock onto stars automatically > and > translate to a lat/long for the navigator. It also gave the azimuth > of each > body observed, and that would translate automatically into a true > heading for > the airplane in order to check its gyro compass if desired, thus > performing > the function of an astrocompass. Do you have any of astro trackers in stock? I'll definitely buy one! The SR-71 Blackbird also had a very sophisticated star tracker. Here is an extract from "SR-71 Revealed, the Inside Story", by Richard H. Graham, pages 65-68. Richard Graham was an SR-71 pilot for many years. This is his own description of the system. --- Navigational Systems The SR-71�s high speed and sensitive missions demanded a navi-gational system that was highly accurate, reliable, and didn�t depend on inputs from other sources subject to electronic jamming. Patterned after navigational systems used on ICBMs, the SR-71�s Astro-inertial Navigation System (ANS) filled those requirements. Simplistically, the ANS was a star tracking navigation system. At least two different stars had to be tracked for optimum navigation performance. With a highly accurate chronometer (to the 100th of a second) supplying Greenwich Mean Time (GMT) and the Julian date, along with a 61-star catalog stored inside the ANS computer, it was possible to know precisely where SR-71 was over the ground. Selection of which star to track was made by the ANS computer a function of latitude, longitude, day of year, time of day, aircraft pitch and roll, and location of the sun. The computer selected a star by going through its star catalog, which was arranged in decreasing star brightness until it found a star. A telescope-like star tracker looked for the stars in an expanding rectangular spiral search pattern. The ANS window was located on top of the fuselage, just forward of the air refueling door and consisted of a round piece of distortion-free quartz glass (about 9 inch diameter) that allowed the star tracker to see through. On the cockpit ANS panel a star �ON� light indicated that a mini-mum of two different stars had been tracked within the last five minutes. Star tracking was automatic. However, the RSO could assist the system in overcoming conditions such as overcasts, changes of sky background brightness, long periods of ground time, and air refueling when the boom obscures the tracking window. Former RSO, Col. Phil Loignon (Ret), recalls a sortie he flew over North Vietnam that changed future ANS procedures. Jim Watkins and I launched on a operational sortie. We had solid cloud cover to 60,000 feet and no star lock on at coast in. A viewsight fix revealed a position error, so I updated the ANS. After exiting North Vietnam, the �STAR� light came on, and our track showed a 10 nautical mile error. The inquisition hy the 15th Air Force following that was some-thing to hehold. We had flown over Hanoi instead of 10 miles away. Our error had allowed intelligence to determine that a new device on the North Vietnam radar sites was actually an optical device for tracking low level fighters. Although I was thought to have �screwed up,� Lockheed came through with the determinations that the ANS tracked a light bulb in the hangar and had induced a heading error. We changed our ANS turn-on procedures as of that date. By comparing the position of the stars to their known location, and with the exact time of day, the ANS could then compute the aircraft�s precise position. A normal gyro compass alignment of the ANS required 36 minutes of warm-up time and provided the SR-71 with great-circle navigational accuracy of 1,885 feet (0.3 nautical mile) for up to ten hours of flying time. It still amazes me even today that astronomers have chart-ed our solar system so accurately that it allows the ANS to calculate the SR-71�s position so precisely. Things may change here on Earth from cen-tury to century, but the same stars guided both Christopher Columbus and Habus. The heart of the ANS was a large, self-contained unit�about half the size of a large refrigerator�called the Guidance Group. A computer inside the Guidance Group computed auto-navigation, guidance and avionics control, and maintained a continuously updated account of nav-igational status and coordinate values. The computer also stored instru-ment and mathematical coefficients, predetermined data references that defined the stars, and the mission flight plan. For continuous accuracy. the computer initiated and evaluated self-tests periodically throughout the flight. Software corrections to the star data were provided for the supersonic shock wave over the star tracker window that refracts the star light and for pressure and temperature gradients acting on the window causing optical lens effects. The aircraft�s flight plan and sensor operation for the entire mis-sion were contained on a wide tape punched with holes and loaded inside the Guidance Group computer memory. The tape was made by the 9th SRW�s Mission Planning Branch, a group of highly experienced Air Force officers who knew how to plan SR-71 missions down to the finest detail. Many former SR-71 RSOs worked as mission planners to provide exper-tise. As the tape ran inside the Guidance Group, the pattern of holes �told� the aircraft where to navigate, what bank angle for turns, when var-ious sensors were to turn ON/OFF, and where to have the sensors �look� for intelligence gathering. Prior to every flight, ANS maintenance personnel loaded the tape and ran the Guidance Group in their shop to insure the programming was correct. The Guidance Group was delivered to the aircraft several hours before flight. It was hoisted up by a crane assembly and slowly low-ered into its air conditioned bay located directly in front of the air refuel-ing door. Once inside its bay, numerous electrical, air conditioning, and computer connections were completed, mating the Guidance Group to the aircraft. An exterior aircraft panel containing the star tracker window bolted over the Guidance Group. The RSO had all the ANS controls in his cockpit. On the ANS panel, the RSO had a constant digital readout of longitude and latitude, wind direction and velocity, time to turn, and distance to the next turn point. By use of his keyboard a variety of other information was available from the ANS display panel, such as ground speed and true air speed. As long as everything was working satisfactorily, the RSO monitored the readouts to insure their accuracy. At any time, the RSO could manually override the ANS�s preprogrammed flight path and sensor action points, if required. It was an automatic abort if the ANS wasn�t working correct-lv, and since Don had first-hand knowledge of that, he had total respon-sibility in making abort decisions concerning our navigational accuracy. If we were in clouds or couldn�t achieve a satisfactory star lock-on, the SR-71 navigated by an inertial-only guidance system. The inertial system had to be aligned and was updated automatically by the ANS when it was navigating normally. By using fix points every hour, the inertial-only system maintained a navigational accuracy of two nautical miles per hour. On one occasion Don flew with another pilot, then Lt. Col. Bob Crowder (Ret), to take the SR-71 to a remote island in the Indian Ocean, called Diego Garcia. It was a test of Det l�s capability to support and fly reconnaissance missions out of a bare-base, remote island. Diego Garcia�s strategic naval location was also gaining popularity with the Air Force as a staging base to fly B-52s and tankers from. We wanted to prove our capability to fly the SR-71 from there as well. One of Beale�s SR-71 shel-ters was secretly torn down overnight and erected on the island days later, JP-7 was shipped in and stored, and large ground-handling equipment was flown in from Det 1 on C-141s. An advance party of Det 1 maintenance people were sent to receive the aircraft. Don really didn�t want to go on the trip because he had just planned his big Lieutenant Colonel promotion party about the same time he was due to return from Diego Garcia. His room was all ready for the party, so he left me with last minute instructions to have his promotion party regardless of whether he was back in time or not. After several air refuelings, the aircraft arrived in good shape. Although no sorties were flown out of Diego Garcia, maintenance prac-ticed all the necessary routines in order to prepare the aircraft for flight. After everyone was confident that the right equipment and supplies were in place to carry out numerous sorties, the return flight back to Okinawa was planned. Shortly after takeoff, Don�s ANS went haywire, and they returned to Diego Garcia. Navigation to each air refueling track was over thousands of miles of open ocean and required a high degree of reliance solely on the ANS. On their second attempt they had to abort on the ground, again because of a bad ANS. On the third try, Bob and Don were prepared for an alternative means of navigation in case the ANS failed a third time. It was called dead reckoning! The ANS did fail inflight; however, they successfully proceeded on to all their air refuelings by basically pointing the nose of the aircraft straight ahead. All the crews met Don at the aircraft and told him his party was �gang-busters!� He was one day late. --- Neat stuff!