The shuttle orbiter S-band system operates in the S-band portion of the RF spectrum of 1,700 to 2,300 MHz.

S-Band Phase Modulation. The S-band PM system is used to transmit information to or receive information from the ground. This system can communicate either directly between the orbiter and ground or through the TDRS, FL or RL.

The S-band forward link (previously referred to as uplink) is phase modulated on a center carrier frequency of either 2,106.4 MHz (primary) or 2,041.9 MHz (secondary) for NASA and operates through the STDN or TDRS. The two S-band forward link frequencies would prevent interference if two orbiters were in operation at the same time, since one orbiter could select the high frequency and the other could select the low frequency.

The S-band return link (previously referred to as downlink) is phase modulated on a center carrier frequency of 2,287.5 MHz (primary) or 2,217.5 MHz (secondary) for NASA and operates through the STDN or TDRS. The two S-band return link frequencies also would prevent interference if two orbiters were in operation at the same. One orbiter could operate on the forward link high frequency of 2,106.4 MHz (primary) and a return link high frequency of 2,287.5 MHz (primary) through the STDN or TDRS, and the other could operate on the forward link low frequency of 2,041.9 MHz (secondary) and a return link low frequency of 2,217.5 MHz (secondary) through the STDN or TDRS.

The Department of Defense S-band forward link is phase modulated on a center carrier frequency of either 1,831.8 MHz (primary) or 1,775.5 MHz (secondary) from the Air Force Satellite Control Facility through its own ground stations and does not operate through the TDRS because the S-band power amplifiers are not powered in the AFSCF mode. The two S-band forward link frequencies would prevent interference if two orbiters were in operation at the same time; one orbiter could select the high frequency and the other could select the low frequency.

The Department of Defense S-band return link is phase modulated on a center carrier frequency of 2,287.5 MHz (primary) or 2,217.5 MHz (secondary) through the Air Force ground stations to the AFSCF and does not operate through the TDRS because the S-band power amplifiers are not powered in the AFSCF mode. The two S-band return link frequencies also would prevent interference if two orbiters were in operation at the same time. One orbiter could operate on the forward link high frequency of 1,831.8 MHz (primary) and a return link of 2,287.5 MHz (primary) from the AFSCF and its ground stations, and the other orbiter could operate on the forward link of 1,775.5 MHz (secondary) and a return link of 2,217.5 MHz (secondary) through the Air Force ground stations and the AFSCF.

The S-band PM forward link originates from MCC-H through the NASA STDN ground stations used for launch, lift-off, ascent or landing or through the WSGT via the TDRS system to the orbiter. The DOD S-band PM forward link originates from the AFSCF through its own ground stations to the space shuttle. NASA and the Department of Defense AFSCF have a choice of two forward link frequencies for data transfer, but both frequencies cannot be used at the same time.

The S-band PM forward link transfers a high data rate of 72 kilobits per second, consisting of two air-to-ground voice channels at 32 kbps each and one command channel at 8 kbps, two-way Doppler and two-way tone ranging.

The S-band PM forward link transfers a low data rate of 32 kbps, consisting of one air-to-ground voice channel at 24 kbps and one command channel of 8 kbps, two-way Doppler and two-way ranging. The two-way ranging does not operate through the TDRS.

The S-band PM return link can originate from one of two S-band PM transponders aboard the orbiter. Each transponder can return link on a frequency 2,287.5 MHz (primary) or 2,217.5 MHz (secondary), but not both at the same time. The S-band return link from the orbiter transmits the data through the NASA STDN ground stations used for launch, lift-off, ascent or landing or through the TDRS and TDRS system via the WSGT to the MCC-H. The DOD S-band return link from the orbiter transmits the data through the Air Force ground stations to the AFSCF.

The S-band return link high data rate of 192 kbps consists of two air-to-ground voice channels at 32 kbps each and one telemetry link of 128 kbps, two-way Doppler and two-way ranging. In the high-data-rate mode, incoming signals are directed to a preamplifier before they reach the transponder, and outgoing signals go to the preamplifier after leaving the transponder. The two-way Doppler and two-way ranging are operative only when in view of the NASA STDN ground stations at launch, lift-off, ascent or landing or in view of the Department of Defense AFSCF ground stations. The two-way ranging does not operate with the TDRSS.

The S-band return link low data rate consists of one air-to-ground voice channel at 32 kbps and one telemetry link at 64 kbps, two-way Doppler and two-way ranging. In the low-data-rate mode, incoming signals flow directly to the transponder, and outgoing signals go directly to the S-band antenna switch without amplification. As noted, the two-way Doppler and two-way ranging are used in the same manner as in the high-data-rate mode.

Four quadrant S-band PM antennas are located on the forward fuselage outer skin of the space shuttle approximately 90 degrees apart. The antennas are covered with a reusable thermal protection system. In the orbiter, on the flight deck viewed through the forward windows, the quadrant antennas are to the upper right, lower right, lower left and upper left. These antennas are the radiating elements for transmitting the S-band PM return link and for receiving the S-band PM forward link. Each quad antenna is a dual-beam unit that can look forward or aft for both transmission and reception without any physical movement. The orbiter S-band PM antenna switch assembly performs the signal switching among the two S-band transponders and any one of the four quadrant antennas. The proper antenna to be used is selected automatically, under onboard SM computer control, by real-time command from the ground or manually by the flight crew from the displays and controls on the spacecraft flight deck panel C3 with the S-band PM antenna rotary switch on panel C3 in GPC. The antenna selection is based on the computed line of sight to the NASA STDN ground station used for launch, lift-off, ascent or landing; the AFSCF ground station; or the TDRS in view. The antenna switching commands are sent to the switch assembly through the payload multiplexers/ demultiplexers.

GPC control can be inhibited to permit ground control to select an antenna other than the one currently selected by the GPC. The ground sends a real-time command to inhibit GPC control and then a second RTC to select the desired antenna. GPC control is restored by sending an RTC to enable the GPC mode. The eight antenna beam position selections available are upper left forward or aft, upper right forward or aft, lower right forward or aft, and lower left forward or aft.

Two identical S-band PM transponders in the orbiter function as a multipurpose, multimode transmitter/receiver. The transponder can simultaneously transmit/receive, transmit only or receive only. Only one transponder operates at one time; the other transponder is a redundant backup. The selected transponder transfers the forward link commands and voice to the network signal processor and receives the return link telemetry and voice from the NSP. The transponders may be cross-strapped. Transponder 1 or 2 may be used with network signal processor 1 or 2. The radio frequency sections of either transponder can be used with either preamplifier and power amplifier 1 or 2.

The dual S-band preamplifier, used in the TDRS mode for amplification, is required full-time for the FL radio frequency because of the much greater distance and, consequently, greater radio frequency loss from the TDRS to the orbiter (minimum of about 22,300 miles) than from the STDN to the orbiter (typically, slant ranges are in the low hundreds of miles). The preamplifier is not used in the AFSCF mode. One of the two units is used at a time, and the output of either unit can be cross-strapped to feed either transponder.

The selected transponders also provide a coherent turnaround of the PM forward link and PM return two-way Doppler and two-way tone ranging signals. The two-way Doppler and two-way ranging signals are operative when the orbiter is in view of the NASA STDN ground stations at launch, lift-off, ascent or landing or the AFSCF ground stations. The two-way Doppler operates through the TDRS, but the two-way ranging does not.

Two-way Doppler is used by ground stations to track the orbiter. The S-band PM forward link and PM return link are directly proportional to the forward link frequency (two-way Doppler). The S-band transponder provides a coherent turnaround of the forward link carrier frequency necessary for the two-way Doppler data. The transponder operates only when in view of the NASA STDN ground stations during launch, lift-off, ascent or landing or in view of the AFSCF ground stations. By measuring the forward link and using return link frequencies expected from the orbiter, the ground tracking station can measure the double Doppler shift that takes place and can calculate the radial velocity (range rate) of the orbiter with respect to the ground station. Because these links are PM, the S-band carrier center frequency is not affected by the modulating wave. It would be impossible to obtain valid Doppler data of the S-band carrier center frequency if it were affected by the modulating technique.

The S-band transponder also provides a subcarrier for two-way tone ranging. It is used to determine slant range from a known point to the orbiter and operates only when in view of the NASA STDN ground stations during launch, lift-off, ascent or landing or in view of the AFSCF ground stations. This capability does not operate through the TDRSS. The ground station forward-links ranging tones at 1.7 MHz and computes vehicle slant range from the time delay in receiving the return link 1.7-MHz tones to determine the orbiter's range. The orbiter azimuth is determined from the ground station antenna angles. A C-band skin-tracking mode also is provided from the ground station to track the orbiter and, again, is used only in view of the NASA STDN ground station associated with launch, lift-off, ascent or landing or in view of the AFSCF ground stations. This capability does not operate through the TDRSS.

The two onboard network signal processors receive commands (forward link) and transmit (return link) telemetry data to the selected S-band transponder. Only one NSP operates at a time; the other provides a redundant backup. The selected NSP receives either one or two analog voice channels from the onboard audio central control unit depending on whether one (in the low-data-rate mode) or both (in the high-data-rate mode) of the air-to-ground channels are being used, converts them to digital voice signals, time-division-multiplexes them with the telemetry from the pulse code modulation master unit, and sends the composite signal to the S-band PM transponder for transmission on the return link. On the forward S-band PM link, the NSP does just the reverse. It receives the composite signal from the S-band transponder and outputs it as either one or two analog voice signals to the audio central control unit. The composite forward link also has ground commands that the NSP decodes and sends to the onboard computers, which route the commands to the intended onboard systems.

S-Band Frequency Modulation. The S-band FM system cannot receive information; it is used to downlink data from seven different sources, one at a time, directly to the ground when there is a line of sight between the orbiter and STDN or Air Force ground stations. The S-band FM return link (previously referred to as downlink) can originate from two S-band transmitters aboard the space shuttle. Both transmitters are tuned to 2,250.0 MHz. The S-band FM return link can be transmitted simulta neously with the S-band PM return link to the STDN ground station or MCC-H or to the Air Force ground station at the AFSCF. It is planned that the S-band FM system be kittable for Department of Defense missions and removable for NASA missions when the TDRSS and the Ku-band system are fully operational.

The S-band FM signal processor aboard the orbiter receives inputs and processes data from the three space shuttle main engine interface units, the video (television) switching unit, the operations recorders for recorder dump, the payload recorder for recorder dump, payload analog, payload digital or Department of Defense.

During ascent, space shuttle main engine interface unit data are sent to the S-band FM system to be transmitted in real time to the ground. These data also are routed to the operations recorders for non-real-time transmission. On orbit, one of the other six services may be selected and routed through the S-band FM signal processor.

The FM signal processor is commanded to select one of these sources at a time for output to the S-band FM transmitter, which transmits it to the S-band FM return link through the STDN ground station used for launch, lift-off, ascent or landing or the DOD AFSCF ground station. The S-band FM return link transfers real-time SSME data from the engine interface units during launch at 60 kbps each or real-time video or operations recorder dumps of high- or low-data-rate telemetry at 1,024 kbps or one-track dumps of 60-kbps SSME data at 1,024 kbps or payload recorder at 25.5 kbps or 1,024 kbps or payload analog at 300 hertz or 4 MHz or payload digital data at 200 bps or 5 mbps or DOD data at 16 kbps or 256 kbps in real time or 128 kbps or 1,024 kbps of playback. The S-band FM return link does not operate through the TDRS system.

Only one of the two FM signal processors is used at a time. FM signal processor 1 interfaces with FM transmitter 1, and FM signal processor 2 interfaces with FM transmitter 2. The transmitters and processors cannot be cross-strapped.

Two hemispherical S-band FM antennas are located on the forward fuselage outer skin of the orbiter approximately 180 degrees apart. The antennas are covered with a reusable thermal protection system. In the orbiter on the flight deck, the hemispherical antennas are above the head (upper) and below the feet (lower) and radiate the S-band FM return link.

The S-band antenna switch assembly aboard the orbiter provides the signal switching among the two S-band FM transmitters and either of the two hemispherical antennas. The proper antenna is selected automatically by onboard or real-time command from the ground, computer control or manual flight crew selection from displays and controls on flight deck panel A1. In the GPC mode, the onboard SM computer selects the proper hemispherical antenna to be used whenever an S-band FM transmitter is active. The antenna selection is based on the computed line of sight to the NASA STDN ground station used for launch, lift-off, ascent or landing or the Department of Defense AFSCF ground stations.

The basic difference between the orbiter's quadrant and hemi spherical antennas is that the hemispherical antennas have a larger beamwidth while the quadrant antennas have a higher antenna gain. The hemispherical antennas are so named because there are two of them, one on top of the orbiter and one on the bottom. The quadrant antennas are so named because there are four of them, two on each side of the orbiter, one on the upper half and one on the lower half of each side, which provides nearly total coverage in all directions.

The hemi antenna switch has a port that can route RF television from the astronaut's extravehicular mobility unit to the orbiter's closed-circuit television system. An EMU TV unit can transmit television on one hemi antenna/antenna switch path to the orbiter while the S-band FM system is routing FM downlink telemetry to ground on the other hemi antenna/antenna switch path.