The onboard tactical air navigation units determine slant range and magnetic bearing of the orbiter to a TACAN or VHF omnirange TACAN ground station.

The ground-based TACAN and VHF omnirange TACAN stations constitute a global navigation system for military and civilian aircraft operating at L-band frequencies (1 gigahertz).

The orbiter 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.

The onboard 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).

Each ground 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.

Periodically, 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.

Approximately 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).

During orbital 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 TACAN coverage.

The acquisition 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.

TACAN data is processed in the TACAN subsystem operating program, which converts range and bearing to units of feet and radians.

TACAN redundancy 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.