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Space Shuttle Wireless Part 2

Space Shuttle Wireless Part 2

In a continuing report on the space shuttle and the critical role that wireless technology plays in its launch, orbit, and re-entry, WBT's David Geer takes a closer look this month at the program's S-band and Ku-band systems.

In last month's article about NASA's Space Shuttle, I introduced the shuttle program's S-band system, the wireless frequency used at those points in the mission when the shuttle's payload bay doors can't be opened to release the Ku-band antenna. The S-band is the range of RF spectrum from 1,700-2,300MHz. The NASA S-band system uses S-band PM (phase modulation) to send and receive communications to and from the ground and the shuttle.

S-band PM is a type of modulation like FM (frequency modulation) or AM (amplitude modulation). Modulation is used to add data to either an electronic/current-based signal carrier (as is the case here) or an optical signal carrier. With PM, instead of varying the frequency or the amplitude to perform modulation, you vary the phase in order to add data onto the signal carrier.

The S-band system uses two uplink and two downlink frequencies within the 1,700-2,300MHz spectrum. For uplink to the shuttle, either 2,106.4MHz or 2,041.9MHz is used, operating through the TDRSS or STDN (Space Flight Tracking and Data Network, as it is also called). These two frequencies are available so that if there were ever two shuttles in orbit, each would have its own frequency. The S-band PM forward link (as the uplink is now commonly called) sends a signal at a top rate of 72Kbps. This signal carries two air-to-ground voice channels and a command channel. The lower data rate is 32Kbps and carries one air-to-ground voice channel and one command channel.

The S-band PM return link (as the downlink is now commonly called) can come from either of two transponders aboard the orbiter. Either can downlink (or return link) on either of the two available frequencies. The data comes through the NASA STDN ground stations that are used for launch and landing. The data can also be transferred through TDRSS. This is sent through the WSGT (White Sands Ground Terminal) to the MCC-H (Mission Control Center-Houston). Data communications, on the other hand, transfer at a rate of 192Kbps over the S-band on the return link. In high data rate mode, incoming signals go to a preamp, and then on to the transponder. Outgoing signals also pass through a preamp, after leaving the transponder.

Antennas, Transponders and Transmission Modes
The space shuttle's S-band uses four antennas on the front fuselage outside the shuttle. The four-antenna switch assembly does the signal switching between two transponders and between each of the antennas. A computer on the shuttle automatically selects the best antenna for the job at hand. An antenna may also be selected manually from the ground or by the crew. Antenna choice is made based on computationally derived best line of sight down to the NASA STDN. It can also transmit to the AFSCF (Air Force Satellite Control Facility) ground station or by line of sight to TDRS (the nearest satellite). Switching commands for the antennas are forwarded to the switch assembly via the payload multiplexers/demultiplexers.

The two transponders are multipurpose, multimode transmitter/receivers. Either can simultaneously transmit and receive. (Just one transponder is used at any time.) A dual-band preamp is used in TDRS mode for amplification. It is in constant use due to possible signal loss over the great span of space between the orbiter and the particular TDRS that it is signaling. This distance is a minimum of approximately 22,300 miles.

There are two transmission modes available, which are selected according to vehicle operations and status and what is most expedient. (There is also a third mode with a lower data rate.) The S-band data traveling over the selected transmission mode includes two voice channels so that the ground control team can communicate with the crew; it also includes engineering data about operations on the orbiter. The data includes the experiments on the vehicle and command data going to the vehicle from the ground over the link that goes through the relay satellite down to the orbiter.

"There is some science data on it on some missions but it's very low-rate science data and typically it's more engineering data connected with the health and safety and operation of the vehicle," says Phil Liebrecht, associate director and program manager for Mission Services, NASA Goddard Space Flight Center. Once the signals reach the ground stations, they are transferred by an IT network to Johnson Space Center.

Ku-band
The Ku-band is of a much broader bandwidth. While the orbiter S-band links operate at 192Kbps from the orbiter to the satellite and 72Kbps from the satellite to the orbiter, the Ku-band operates at 50Mbps. The Ku-band carrier frequencies are 13.755GHz from the TDRS to the orbiter and 15.003GHz from orbiter to TDRS. The Ku-band antenna, deployed from the cargo bay, can also be used as a radar system for tracking objects in space. It can't be used for communications and for radar at the same time.

During the hand-off of communications from the S-band to the Ku-band system, the orbiter's onboard NSP (network signal processor) uses the Ku-band signal processor rather than the S-band transponder. The data stream then travels through the K-band signal processor and Ku-band antenna to the TDRS. The Ku-band transmits three data channels, one carrying the voice and telemetry otherwise processed by the S-band PM. Other channels handle payload analog data, payload digital data, payload recorder, operations recorders, television, spacelab, and other data.

The signal goes through the Ku-band signal processor where it is interleaved and sent on to the onboard-deployed electronics assembly. There, it is transmitted to the TDRS via the Ku-band antenna. Incoming signals travel from the antenna to a receiver, and from the receiver to an internal electronics assembly (there is one EA for communications and another for radar). The signal then travels to the Ku-band signal processor. Voice and command signals go to the NSP. An output from the Ku-band signal processor goes to the text and graphics system.

Ku-band Channels, Modes
Ku-band Channel One transmits 192Kbps of operational data. Some 128Kbps of Channel One is telemetry and interleaver data and 32Kbps is two air-to-ground voice links (and some additional data). The Ku-band system forward link consists of two modes transferred through the TDRS. Mode 1 is 72Kbps consisting of two air-to-ground voice streams at 32Kbps each and 8Kbps of command data (and over 128Kbps of other data). Mode 2 is similar without the additional data.

* * *

Note: In my next installment on the Shuttle, I'll address wireless security.

SIDEBAR

Some Specific RFs Used for Space Shuttle Applications

  • 259.700: AM shuttle voice frequency
  • 296.800: AM frequency used in the earth's atmosphere from the shuttle to the ground and from the orbiter to the space suits during EVAs
  • 279.000: AM frequency used from space suit to space suit and to the orbiter during EVAs
  • 243.000: The ultrahigh AM military frequency used only in emergencies, such as if the S-band should fail (which is highly unlikely)
  • 2287.500: The primary digital downlink
  • 2217.500: The secondary digital downlink
  • 2250.000: WFM TV during orbit with voice
  • 121.750: FM frequency used shuttle to Mir space station during rendezvous, docking, and undocking
  • 139.300: Another FM frequency used shuttle to Mir
  • 143.600: Still another FM frequency used shuttle to Mir
  • More Stories By David Geer

    David Geer is a contributing writer to WBT, a journalist, and a computer technician. He graduated from Lake Erie College in 1993 with a BA in psychology and has worked in the computer industry and in the media since 1998.

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