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-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Paul Hirose
Sent: Sunday, March 31, 2002 10:16 PM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: Digital Sextant

I was told this astro inertial platform was nearly the same device the
SR-71 carried. You can see the same round port on top of the '71. I
also heard Northrop's expertise with this type of navigation was an
outgrowth of the old Snark intercontinental cruise missile. They had
to develop astro inertial because a pure inertial system would drift
too far during the missile's flight, much longer in duration than an
ICBM's.

paulhirose@earthlink.net (Paul Hirose)
B-2 flight test maintenance, 1987 - 97

---

Dan Allen replies:

Great information - thanks Paul!

From "SR-71 Revealed, the Inside Story",
by Richard H. Graham (a SR-71 pilot), p. 65f are more details
about the astro-inertial navigation system given below:

-

The SR-71's high speed and sensitive
missions demanded a navigational 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
minimum 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 something 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 warmup
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 charted 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 century 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 navigational
status and coordinate values. The computer also stored
instrument 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 mission 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
expertise. As the tape ran inside the Guidance Group, the
pattern of holes "told" the aircraft where to navigate, what
bank angle for turns, when various 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 lowered into its air
conditioned bay located directly in front of the air
refueling 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 correctly, and since Don had firsthand knowledge of
that, he had total responsibility in making abort decisions
concerning our navigational accuracy. If we were in clouds
or couldn't achieve a satisfactory star lockon, 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 barebase, 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 shelters 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 practiced 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 "gangbusters!" He was one day late.

-

Great stuff!  We need a portable star-tracker!

   

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