tactical air navigation units determine slant range and magnetic
bearing of the orbiter to a TACAN or VHF omnirange TACAN ground
TACAN and VHF omnirange TACAN stations constitute a global navigation
system for military and civilian aircraft operating at L-band frequencies
is equipped with three TACAN sets that operate redundantly. Each
TACAN has two antennas: one on the orbiter's lower forward fuselage
and one on the orbiter's upper forward fuselage. The antennas are
covered with reusable thermal protection system tiles.
TACAN sets are used for external navigation and for the orbiter
during the entry phase and return-to-launch-site abort. Normally,
several ground stations will be used after leaving L-band communications
blackout and during the terminal area energy management phases.
TACAN's maximum range is 400 nautical miles (460 statute miles).
station has an assigned frequency (L-band) and a three-letter Morse
code identification. The ground station transmits on one of 252
(126X, 126Y) preselected frequencies (channels) that correspond
to the frequencies the onboard TACAN sets are capable of receiving.
These frequencies are spaced at 63-MHz intervals.
The TACAN ground
station beacon continuously transmits pulse pairs on its assigned
frequency. The orbiter TACAN receivers pick up these pulse pairs,
and the TACAN data processors decode them to compute bearing. The
onboard TACAN sets detect the phase angle between magnetic north
and the position of the orbiter with respect to the ground station.
The ground beacon is omnidirectional; when the orbiter is over the
ground station, or nearly so, it is in a cone of confusion. Within
this cone, bearing is unusable.
the onboard TACAN sets emit an interrogation pulse that causes the
selected TACAN ground station to respond with distance-measuring
equipment pulses. The slant range (orbiter to ground station) is
computed by the onboard TACAN sets by measuring the elapsed time
from interrogation to valid reply and subtracting known system delays.
As the orbiter approaches a ground TACAN station, the range decreases.
After a course has been selected, the onboard TACAN sets derive
concise deviation data.
The range and
bearing data are used to update the state vector position components
after the data are transformed by the TACANs in the entry phase
(or return to launch site) by navigation and for display on the
horizontal situation indicators on panels F6 and F8, as well as
for display of raw TACAN data on the cathode ray tube.
Each of the
onboard TACANs has an ant sel switch on panel O7. In the auto position,
the onboard GPCs automatically select the upper L-band antenna or
lower L-band antenna for that TACAN. The upper and lower positions
of each TACAN ant sel switch allow the flight crew to select the
upper or lower L-band antenna manually.
Each of the
onboard TACANs is controlled by its mode rotary switch on panel
O7. The modes are off, receive, transmit and receive, and GPC. In
the GPC mode, the onboard GPCs control TACAN ground station channel
selection automatically, and both bearing and range are processed
by hardware and software. In the transmit and receive mode, both
bearing and range are processed by hardware and software, but TACAN
ground station channels are selected manually using the four thumbwheels
for that TACAN on panel O7. The first three thumbwheels (left to
right) select the channel (frequency), and the fourth selects the
X or Y. In the receive mode, only bearing is received and processed
by the hardware; the thumbwheels for that TACAN would be used to
select the channel.
every 37 seconds, the selected ground TACAN station transmits its
three-letter identification to the onboard TACAN. In order for the
Morse code identification to be verified by the commander and pilot,
TACAN ID audio controls are located on panel O5 for the commander
and panel O9 for the pilot. The TACAN on/off switch is positioned
to on to transmit the TACAN identification. The TACAN 1 , 2 and
3 switch selects the onboard TACAN that will transmit the TACAN
identification code, and the TACAN on/off switch is positioned to
on to transmit the code to the orbiter's audio system, thus the
commander and pilot. Volume TACAN thumbwheels on panels O5 and O9
control the volume setting of the TACAN identification code to the
commander and pilot.
In the GPC
mode, 10 TACAN ground stations are programmed into the software
and are divided into three geometric regions: the acquisition region
(three stations), the navigation region (six stations), and the
landing site region (one station).
operations, landing sites are grouped into minitable and maxitable
programs. The maxitable programs provide data sets that support
a broad range of trajectories for contingency deorbits and enable
reselection of runway and navigation and data sets for those deorbits.
The minitable consists of three runways determined by the flight
crew, one of which is initialized as a primary runway. The minitable
is transferred from entry operations and becomes unchangeable. Entry
guidance is targeted from one of the three runways selected by the
crew, initialized with the primary runway for the well-defined trajectory
and nominal end-of-mission data sets. Since the TACAN units are
placed in groups of 10 and 10 TACAN units from one group (primary)
form the TACAN half of the minitable, the secondary and alternate
runways should be from the same group as the primary runway to assure
region is the area in which the onboard TACAN sets automatically
begin searching for a range lock-on of three ground stations at
approximately 160,000 feet. After one TACAN acquires a range lock,
the other two will lock on to the same ground station. When at least
two TACAN sets lock on, TACAN range and bearing are used by navigation
to update state vector until microwave scan beam landing system
selection and acquisition at approximately 18,000 feet.
When the distance
to the landing site is approximately 120 nautical miles (138 statute
miles), the TACAN begins the navigation region of interrogating
the six navigation stations. As the spacecraft progresses, the distance
to the remaining stations is computed. The next-nearest station
is automatically selected when the spacecraft is closer to it than
to the previous locked-on station. Only one station is interrogated
when the distance to the landing site is less than approximately
20 nautical miles (23 statute miles). Again, the TACAN sets will
automatically switch from the last locked-on navigation region station
to begin searching for the landing site station. TACAN azimuth and
range are provided on the horizontal situation indicator. TACAN
range and bearing cannot be used to produce a good estimate of the
altitude position component, so navigation uses barometric altitude
derived from the air data system probes, which are deployed by the
fight crew at approximately Mach 3.
If the microwave
scan beam landing system is not acquired, TACAN data can be used
until an altitude of 1,500 feet. When runways with MSBLS are acquired,
MSBLS operation can be automatic. The flight crew is provided with
the controls and displays necessary to evaluate MSBLS performance
and take over manually if required. The runways with MSBLS must
be in the primary or secondary slot in the minitable for the minitable
to copy the MSBLS data. The maxitable is an initial-loaded table
of 18 runway data sets and MSBLS data for runways and 50 TACAN data
sets. In orbital operations, the landing site function provides
the capability to transfer data from the maxitable to the minitable.
is processed in the TACAN subsystem operating program, which converts
range and bearing to units of feet and radians.
management consists of processing and mid-value-selecting range
and bearing data. The three TACAN sets are compared to determine
if a significant difference is detected. When all three TACAN sets
are good, redundancy management selects middle values of range and
bearing. If one of the two parameters is out of tolerance, the remaining
two will average that parameter. If a fault is verified, the SM
alert light is illuminated, and a cathode ray tube fault message
occurs for the applicable TACAN set.
The three convection-cooled
TACAN sets are located in the orbiter crew compartment middeck avionics
bays. Each set is 7.62 inches high, 7.62 inches wide and 12.53 inches
long and weighs 30 pounds.
The TACAN contractor
is Hoffman Electronics Corporation, Navigation Communication System
Division, El Monte, Calif.