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Ku-Band System

The Ku-band antenna aboard the space shuttle orbiter is located in the payload bay; thus, Ku-band can be used only when the orbiter is on orbit. The orbiter payload bay doors are opened and the Ku-band antenna is deployed. The Ku-band system operates in the Ku-band portion of the radio frequency spectrum between 15,250 MHz and 17,250 MHz. The Ku-band carrier frequencies are 13.755 GHz from the TDRS to the orbiter and 15.003 GHz from the orbiter to the TDRS. Once the Ku-band antenna is deployed, the Ku-band system can be used as a communication system to transmit information to and receive information from the ground through the TDRSS. The Ku-band antenna aboard the orbiter can also be used as a radar system for target tracking objects in space, but it cannot be used simultaneously for Ku-band communications and radar operations.

When the Ku-band antenna is deployed aboard the orbiter and handover from the S-band system to the Ku-band system occurs, the orbiter onboard NSP operates with the Ku-band signal processor rather than the S-band transponder. The data stream is then directed through the K-band signal processor and Ku-band antenna to the TDRS in view, to the TDRS system WSGT and to MCC-H on the return link. The process is reversed for the forward link. If the Ku-band forward link is lost, the system reverts (fail-safe) to S-band.

The Ku-band system can handle higher quantities of data than the S-band systems. It transmits three channels of data, one of which is the same interleaved voice and telemetry processed by the S-band PM system. Two of the seven possible sources of information sent on the other two channels are payload analog, payload digital, payload interleaver bent-pipe, payload recorder, operations recorders, television and Spacelab (if flown).

The three channels of data are sent to the Ku-band signal processor to be interleaved. This signal then goes to the onboard deployed electronics assembly, which contains the transmitter, to be transmitted to the TDRS through the Ku-band antenna. The incoming signal goes through the onboard Ku-band antenna to the onboard receiver in the DEA and then through an internal electronics assembly (EA 1 is for communications and EA 2 is for radar) to the Ku-band signal processor. Voice and commands are sent to the network signal processor. A separate output from the Ku-band signal processor is directed to the text and graphics system. (As previously mentioned, since TDRS-A's Ku-band forward link is not functional, TAGS cannot operate aboard the orbiters.)

The Ku-band deployed assembly is mounted on the starboard sill longeron in the payload bay of the orbiter. It is deployed and activated after the payload bay doors are opened. The deployed assembly consists of a two-axis, gimbal-mounted, high-gain antenna; an integral gyro assembly; and a radio frequency electronics box. The gimbal motors position the Ku-band antenna, and the rate sensors determine how fast the antenna is moving.

The Ku-band deployed antenna assembly is 7 feet long and 1 foot wide when stowed in the payload bay. The parabolic antenna dish is 3 feet in diameter and is a graphite epoxy structure. The deployed antenna assembly weighs 180 pounds. The weight of the entire system is 304 pounds.

The antenna can be steered in several selectable modes under manual control by the flight crew or automatically by the SM computer. It provides the interface with the TDRS when there is a line of sight between the orbiter and TDRS.

When the shuttle reaches orbit, before the Ku-band antenna is deployed, circuit breakers on panel R15 are closed to energize thermostatically controlled heaters for the deployed electronics assembly, gimbals and antenna assembly. They provide electrical power to the Ku-band electronic elements, electronics assemblies 1 and 2, the signal processor assembly and Ku-band portions of panel A2. Actual deployment involves the controls and associated talkback displays on panel R13L. The antenna is locked in the stowed position to clear the adjacent payload bay doors and radiators when they are closed or moving.

The SPA processes and routes Ku-band FL and RL data. EA 1, containing the communication data processor and antenna control electronics, provides data to onboard displays, meters and computers. EA 2 provides control signals to configure the system for radar operations in addition to receiving and processing return radar data.

Deployment and stowage of the Ku-band deployed assembly is controlled by flight crew switches at the aft flight station. Twenty seconds are normally required to deploy or stow the DA. In the deployed position, the DA forms a 67-degree angle with the orbiter X axis. Activating the Ku electronics frees the antenna gimbals by removing the locking pins.

The antenna dish is edge-mounted on a two-axis gimbal. The alpha gimbal provides a 360-degree roll movement around the pole or axis of the gimbal. The beta gimbal provides a 162-degree pitch movement around its axis. The alpha gimbal has a stop at the lower part of its movement to prevent wraparound of the beta gimbal control cable. Since the beta gimbal has only a 162-degree movement, there is a 4-degree non-coverage zone outboard around the pole and a 32-degree non-coverage zone toward the payload bay.

As described in the S-band discussion, there are times when the Ku-band system, in view of a TDRS, is interrupted because the orbiter blocks the Ku-band antenna's view to the TDRS because orbiter attitude requirements or payloads' radiation sensitivities prohibit its use. In addition, periodically the Ku-band antenna beta cabling may require positioning to ensure that it does not become twisted in a way that could cause the antenna to bind.

As discussed under S-band, the Ku-band system's narrow beam makes it difficult for TDRS antennas to lock on to the signal. Therefore, the orbiter uses the S-band system to lock the Ku-band antenna into position first because the S-band system has a larger beamwidth. The procedure for acquiring TDRS Ku-band communication from the orbiter is described in the S-band section.

The Ku-band system return link consists of channel 1, modes 1 and 2, plus one channel 2, modes 1 and 2, and one channel 3. Channel 1, modes 1 and 2, consists of 192 kbps of operational data (128 kbps of operational data telemetry and payload interleaver plus two air-to-ground voice links at 32 kbps each) plus one of the following selections from channel 2, modes 1 and 2: (1) payload digital data from 16 kbps to 2 Mbps, (2) payload digital data from 16 kbps to 2 Mbps, (3) operations recorder playback from 60 kbps to 1,024 kbps, or (4) payload recorder playback from 25.5 kbps to 1,024 kbps. It also includes one of the following from channel 3: mode 1 attached payload digital data (real-time or playback) from 2 Mbps to 50 Mbps, mode 2 television (color or black and white) composite video, or mode 2 real-time attached payload digital data or payload analog data.

The Ku-band system forward link consists of a mode 1 and 2 through the TDRS in view. Mode 1 consists of 72-kbps data (two air-to-ground voice streams at 32 kbps each and 8 kbps of command), 128-kbps TAGS (used in place of the teleprinter) and 16-kbps synchronization. Mode 2 consists of 72-kbps operational data (two air-to-ground voice streams at 32 kbps each and 8 kbps of command).

Like the S-band system, the Ku-band antenna must be stowed before the orbiter payload bay doors are closed in preparation for atmospheric entry. If the DA does not respond to normal stow operations, involving proper orientation and locking of the antenna, or to the stow operation itself, a direct stow switch on panel R13L is used. Setting this switch to on bypasses the normal stow control sequences and causes the DA to be driven inside the payload bay.

If neither the normal stow nor the direct stow can position the DA inside the payload bay, the DA can be jettisoned. To jettison the deployed assembly, the crew closes the circuit breakers on panel ML86B and activates the Ku ant arm and jett switches on panel A14, which causes a guillotine to cut the cables to the DA and releases a clamp holding the DA to the pivot assembly. The separation point is between the DA and deployment mechanism about 20 inches above the sill longeron. No ejective force is imparted to the DA; it is merely cut loose and the orbiter maneuvers away from it. The jettison operation takes approximately four seconds.

The space shuttle spacecraft transmits and receives through the S-band system, the TDRS in view and the TDRS system; thus, the WSGT and MCC-H are in the low-data-rate mode until the communications blackout in entry. After blackout, the space shuttle again operates in S-band through the TDRS system in the low-data-rate mode during descent to as low a view as possible until it reaches the S-band landing site ground station, which then transmits and receives in the high-data-rate mode on S-band.

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
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