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ISS communications enhancement plan
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Acta Astronautica Vol. 50, No. 11, pp. 697–703, 2002 Published by Elsevier Science Ltd. Printed in Great Britain S0094-5765(02)00002-4 0094-5765/02/$ - see front matter
ISS COMMUNICATIONS ENHANCEMENT PLAN† FRANK T. BUZZARD‡ and ORON L. SCHMIDT NASA Johnson Space Center, Houston, Texas 77058, USA (Received 10 June 1998)
Abstract—The capabilities of the International Space Station (ISS) communications system will be expanded to enhance the capability of scientists and principal investigators in controlling and obtaining data from their payloads. Payload user requirements exceed the current capabilities of the ISS communications system. This paper describes a phased approach that will signiAcantly expand the ISS communications system capability and enable payload telescience objectives. A Ku-Band forward link will be added for uplink voice, video and commands to support telescience. Increased demands on return link bandwidth will require the use of video compression to free up more bandwidth for payload data. Standard video compression algorithms such as MPEG 2 will be used to facilitate transmission compressed digital video to the end user. Ground data processing capabilities upgrades will be pursued to increase the downlink data rates from 50 to 150 Mbps. A new communication outage recorder will store payload data when the Space Station is not in view of a data relay satellite. The enhanced communication capability and ground data handling capability enhancements will be carefully matched to the expanding payload user requirements. The Anal phase of the ISS communication enhancement plan will investigate the feasibility of greatly increased bandwidth using commercially developed, state-of-the-art systems. Published by Elsevier Science Ltd.
inside Node 1 and two-phased array antennas outside on the port and starboard hatches (see Fig. 1). The ECOMM subsystem will communicate with the ground control center via the tracking and data relay satellite (TDRS). Other Station elements will displace the antennas for ECOMM and its operation will cease when the US Laboratory comes up.
1. SPACE STATION COMMUNICATION PHASING
The International Space Station (ISS) communications system will evolve through three primary phases: Phase I—initial S-Band and Ku-Band communications for early assembly, Phase II— Ku-Band enhancements and Phase III—upgrading Ku-Band communications through the use of commercial communications hardware.
2.2. Operational S-Band
2. PHASE I: ISS BASELINE COMMUNICATIONS
The ISS communications capability will grow to meet the requirements for each assembly phase beginning with S-Band for the early assembly of Node 1 and Ku-Band for the US Laboratory. 2.1. Early communications subsystem
The early communications (ECOMM) subsystem provides a command, telemetry, video conferencing and Ale transfer capability for Node 1 which is the Arst US manned module. ECOMM will have an S-Band transceiver and data processor
The operational S-Band subsystem provides commands, voice and telemetry via the TDRS to=from Mission Control Center—Houston (MCC-H). Two strings of hardware will be provided for redundancy (see Fig. 2). Two channels of air-to-ground voice are provided by the audio distribution subsystem (ADS) and commands and telemetry are exchanged with the command and control multiplexer demultiplexer (C&C MDM). 2.3. Ku-Band subsystem-baseline
The Ku-Band subsystem processes the wideband payload data and Station video for transmission through TDRS to the ground. The subsystem shown in Fig. 3 is comprised of Ave separate units: (1) the video baseband signal processor (VBSP), (2) the high rate frame multiplexer (HRFM), (3) the
†Paper IAF-97-T.3.06 presented at the 48th International Astronautical Congress, Oct. 6 –10, 1997, Turin, Italy. ‡Corresponding author. 697
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Fig. 1.
Fig. 2.
Fig. 3.
high rate modem (HRM), (4) the space-to-ground transmitter=receiver controller (SGTRC) and (5) the space-to-ground antenna (SGANT). VBSP. Up to four channels of analog video from the video distribution system (VDS) are dig-
itized by the VBSP and fed to the HRFM. The VBSP represents video compression technology of the early 1980s. If a standard television channel is digitized by the VBSP, the output bit rate is 42 Mbps. The maximum throughput of the HRFM is 43:2 Mbps. Therefore, a full-motion (30 frames per second) video channel will occupy most of the Ku-Band downlink bandwidth. The VBSP can digitize a video channel to a slower frame rate of 15 frames per second to cut the bandwidth in half. How does the VBSP achieve the lower bandwidth? It simply throws half of the frames away! Today’s video compression algorithms are much more eJcient and can provide full-motion video in as little as 6 Mbps of bandwidth. It is no wonder then why the payload community wants the VBSP replaced with modern video compression hardware. Another disadvantage of the VBSP resulted from a cost cutting exercise which removed the audio unit that was combining the audio with the video inside the VBSP. As a consequence, the audio comes down on the operational S-Band link. Sometimes the S-Band link is lost and the Ku-Band link is still good and vice versa. It was also assumed that the audio and video could be recombined on the ground. However, both the audio and video are digitized and arrive on the ground at randomly varying times. HRFM. The HRFM accepts eight channels of high rate data from the automated payload switch (APS) and four channels of digitized and packetized video from the VBSP. The output rate will be set at 50 Mbps because this is the highest rate that the ground distribution and processing hardware can handle. The HRFM (and the on-board Ku-Band system) can currently support 75 and 150 Mbps. HRM. The HRM accepts a serial data stream from the HRFM and modulates it onto an intermediate frequency carrier which is transmitted by the SGTRC and the SGANT. The VBSP, HRFM and HRM are all mounted in an avionics rack inside the US Lab module. SGTRC. The modulated carrier signal from the HRM is upconverted and ampliAed by the SGTRC’s transmitter and sent to the SGANT. The monopulse receivers allow the SGANT to autotrack the TDRS. The sum channel of the monopulse receivers is downconverted and presented to an output port on the SGTRC. This output signal is carried into the US Lab on a coaxial cable and will be fed into a future receiver=demodulator to restore the Ku-Band uplink. SGANT. The Ku-Band antenna is a 6-foot parabolic dish mounted on an azimuth=elevation gimbal system. The SGANT and SGTRC are both
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Fig. 5. 3.2. High-rate data
High-rate payload data are gathered and processed to a single packetized data stream in the payload rack. If the data are destined to go to the ground via the Ku-Band system, it will conform to one of two protocols: bitstream or frame format. 3.3. Tracking and data relay satellites Fig. 4.
mounted on a boom which extends out from the Z1 truss. The antenna can point anywhere within a hemisphere but structure such as solar panels surrounding the antenna causes blockage up to 25% of the time during TDRS passes.
3. PAYLOAD DATA
3.1. Standard payload rack
International standard payload racks (ISPRs) will be used for all internal payloads. The ISPRs will have standard interfaces to carry wideband data, video and command data. The US Laboratory provides an interface to each ISPR location from the payload multiplexer=demultiplexer (P=L MDM) via a standard 1553B bus, to=from the payload ethernet hub gateway (PEHG) via ethernet, to an internal video switch unit (VSU) via optical Aber and to an automated payload switch via optical Aber (see Fig. 4). P=L MDM data. The payload (P=L) MDM transmits commands, ancillary data and Ales to the payloads and collects health and status data and requests for service from the payloads. Ethernet data. An ethernet bus connects payloads to payloads and payloads to the ground through the PEHG. There is currently no path to move data up from the ground to payloads via the ethernet. Video. Operating a payload remotely from the ground can be accomplished by positioning a video camera in the payload and sending the video to the ground. A Aber optic input is provided for every payload rack to carry the video to the video distribution system and then on to the ground via the Ku-Band system.
The TDRS system provides the capability for the Station to communicate with the Mission Control Center (MCC-H) in Houston, TX and the Payload Operations Integration Center (POIC) in Huntsville, AL. 4. PHASE II: KU-BAND EXPANSION PLANS
4.1. Adding Ku-Band forward link receiver
The Ku-Band forward link (the link from the TDRS to the Station) receiver=demodulator was removed during the Freedom program. The sum channel of the SGTRC monopulse autotracking receivers is provided at an intermediate frequency (IF) of 385 MHz at an output connector. The current Space Station Program OJce has recently decided that the forward link should be restored to provide video conferencing and Ale transfers to an onboard Ale server that will establish a wireless ethernet within the space Station modules (see Fig. 5). Restoring the Ku-Band forward link will also support uplink commands and voice when a future forward link data demodulator is installed. 4.2. Forward link video compression
The Ku-Band uplink will support a digital signal of at least 6 Mbps. Transmitting video up to the Station will require extreme video compression. An early S-Band communications system in Node 1 will use Intel’s ProShare software running on a laptop computer to provide video conferencing, white board graphics exchanges and Ale transfers. This system will operate at 128 Kbps total bandwidth on the uplink and downlink. The video will be conAned to a small window on the laptop screen and will not be full-motion. Payload users and in-Might maintenance (IFM) ground teams demand full-screen, full-motion video displays on board. This will require the latest
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compression algorithms such as motion picture entertainment group 2 (MPEG 2). One MPEG 2 channel currently requires 6 Mbps of bandwidth. Several diNerent algorithms will be implemented in plug-in modules on board to facilitate tailoring an algorithm to speciAc video performance requirements. The modules will also allow new algorithms to be phased in without modifying the Station video hardware. There are user requirements for up to three simultaneous uplink video channels. Video decompression algorithms with higher ratios in the range of 50 : 1 to 100 : 1 will be needed and coding techniques will be employed on the Ku-Band forward link to meet these requirements.
Fig. 6.
4.3. Downlink video compression
Payload investigators have learned that mounting a camera in their payload rack is an inexpensive way to monitor their experiments from the ground. There are other sources of video such as that from the external cameras to monitor Station assembly and for public aNairs. All of this video must be digitized by the payload or the VBSP and eventually comes to the Ku-Band HRFM to be sent to the ground. There are two bottlenecks: the maximum total HRFM data rate of 43:2 Mbps and the VBSP compression ineJciencies. The MPEG 2 compression algorithm has a compression ratio that is 7 times higher than that of the VBSP. There are plans to replace the VBSP with a video digital processor that has four channels of highly compressed video. Each channel would have a plug-in module for each available algorithm such as MPEG 2 or 4. Additional modules would be available on board to customize the compression algorithm to the video application and for replacement in the event of malfunctions. Standard MPEG algorithms also embed the audio with the video prior to digitizing and compression which automatically synchronizes the audio with the video. An audio interface unit (AIU) would be added to bring up to four channels of audio from the audio distribution subsystem to be combined with four channels of video from the video distribution subsystem in the video processing unit (VPU) shown in Fig. 6. The VPU output would use the current four channels of video input to the HRFM coming from the VBSP. The VBSP is scheduled to be replaced by the VPU in 2002. 4.4. Telescience commanding
All commands currently come up the operational S-Band subsystem. If the commands are destined for the Station core systems, they are processed by
Fig. 7.
the C&C MDM. If they are destined for a payload, the C&C MDM forwards them to the PayLoad (P=L) MDM and they are routed to the correct ISPR. Payload data comes down the Ku-Band subsystem. The times that the S-Band link is up does not always overlap the time that the Ku-Band link is available. Performing telescience with payloads requires simultaneous transmission of commands while observing the real-time data. Therefore, a command capability is needed for the Ku-Band uplink. A payload command decryption and decoder function is planned for Utilization Flight 5 (UF-5), scheduled for launch in June of 2002. All station commands are secured by encryption. Fig. 7 shows the path that Ku-Band forward link commands would take to get to an ISPR. 4.5. Communications outage recorder
The TDRSs are positioned in synchronous orbits such that all satellites are in view of White Sands, New Mexico. The Station will be in a high inclination orbit and there will be times when a TDRS is not in view or a line of sight path to a TDRS is blocked by structure. For these reasons, a single Ku-Band antenna cannot provide continuous real-time coverage for payload data. A communication outage recorder (COR) will be installed in the Station to record wideband data (not digitized video from the VBSP) from the automated payload switch (APS) for later playback into the HRFM. Playback data can be used on the ground to build
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a continuous data stream for a payload. The COR will store a minimum of 43:2 Mbps for 1 h with a large growth potential for higher data rates and=or longer record periods. The COR is scheduled to go up on UF-3 in September of 2001. 4.6. Increasing Ku-Band downlink bandwidth
The downlink data rate bottleneck is on the ground. Tests performed over a year ago showed that the on board Ku-Band subsystem can Mow 150 Mbps of data through TDRS with positive link margins. The downlink data are carried by a satellite from the White Sands ground station to the control centers. The front end processors (FEPs) at the control centers are also limited in capacity. The FEP in the Houston control center (MCC-H) would have to be modiAed for 75 or 150 Mbps. The Huntsville control center’s FEP will have to be modiAed to handle 150 Mbps. Using the VBSP to send more than one video channel to the ground will put immediate pressure on the ground to go to 75 Mbps. High payload data rates and additional video will require 150 Mbps to the ground between UF-5 and UF-7. 4.7. Digital television
Good quality digital cameras are appearing in the commercial market place. Their performance is superior to that of the older analog cameras but they have a standard analog composite output and will work with the current video distribution system on the station. Commercial oN-the-shelf camcorders will be Mown Arst on the space shuttle orbiter and then Mown on the Station. Special care was taken in designing the video interfaces on the Station to ensure that any camera, whether it was used inside or outside, could also be Mown on the Station. The shuttle video engineers have perfected the process of taking oN-the-shelf cameras purchased at a local home video store and running them through environmental tests to determine what modiAcations are necessary to My them. This same process will be used to My high deAnition TV (HDTV) cameras, editing equipment and recorders on the Station. HDTV is a much greater technical challenge because of the high output data rates. Every necessary step will be taken to My commercial hardware to keep costs down and to bring the latest in video technology to the Station. 4.8. Digital video distribution
The analog composite video signal from external and internal cameras is pulse frequency modulated
Fig. 8.
(PFM) before being transmitted over optical Abers in the VDS. If an internal analog video signal needs to be routed over the VDS, the conversion from a copper path to a Aber optic path and the PFM modulation is performed by a common video interface unit (CVIU). Conversion from a Aber optic path to a copper path and PFM demodulation are also performed by a CVIU. There will be some degradation to a digital camera’s video quality if it is converted into an analog composite signal for distribution and display. If a digital camera output can be distributed and processed in a digital format, the original video quality can be preserved with none of the signal degradation seen in an analog system. Better performance will be achieved by leaving the digital video camera output in its digital format and distributing the video as high-rate digital data. Tests will be run on the Aber optic VDS components to determine what changes will have to be made to carry digitized and compressed camera outputs in excess of 100 Mbps. It is hoped that the current optical Aber and the video switches will carry up to 125 Mbps of serial video data. Both the external and internal switches should already be capable of passing digital data because the Aber path is converted into a copper path inside the switch and the video stays in its PFM state as it passes through the crossbar switches and gets converted back to a Aber optic signal before it leaves the switch. A digital video interface unit (DVIU) would be used instead of a CVIU. A DVIU will accept a digital video input on a copper path, PFM the digital signal onto output Aber for distribution over the VDS and can take a digitized video signal oN a Aber, pulse frequency demodulate it and output the digital video data on a copper path. The DVIU output can be sent directly to a digital monitor or editor. Digital compression will likely be required prior to recording on board or transmitting digital video to the ground. Compression and adding synchronous audio will be performed by the VDP. A typical path for a payload digital video camera output destined for the ground is shown in Fig. 8. This approach will handle an HDTV camera output if the digital camera output is compressed to less than 125 Mbps.
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F. T. Buzzard and O. L. Schmidt 5. PHASE III: COMMERCIAL APPLICATIONS
5.1. Why commercial communications hardware?
All of the development costs for the space communications hardware for the Apollo lunar program and the Space Shuttle program were paid for by the United States Government. Some of the early shuttle payloads were commercial communications satellites. Much of the technology built into these early communications systems was based on the Shuttle communications technology. The Station communications technology is over 10 years old and can also be traced back to Shuttle era technology. In the mean time, commercial satellite bandwidth requirements have exploded because of the need to direct broadcast video to millions of homes. Next came the Iridium satellite constellation to relay cellular telephone conversations anywhere in the world and the Teledesic constellations to relay Internet sessions and video conferencing anywhere in the world. Commercial communications corporations have risen to the challenge and have developed space communications hardware whose performance will exceed that of the Space Station communication hardware for a fraction of the per unit cost. These commercial communications corporations are now in position to lease new space technology back to the Government at a cost much lower than it would cost the Government to develop and manufacture small quantities of new technology hardware. 5.2. Carrier frequencies
The Ku-Band frequencies are getting crowded and interference is becoming more common. Ka-Band is the choice for future wideband satellite communications. Goddard Space Flight Center (GSFC) will build three new TDRSs that have Ka-Band capability in addition to the current S-Band and Ku-Band. The satellites will be designated H, I and J and will have a forward link bandwidth of 50 Mbps and a return link bandwidth of 600 Mbps. The Teledesic commercial satellite constellation will operate at Ka-Band. For the Arst time, a large portion of the development costs for Ka-Band space communications hardware will be paid for by commercial ventures. 5.3. Phased array antennas
Phased array antennas have been used by the Department of Defense for radar on land, on ships and on airplanes. A phased array and a parabolic dish are both directional antennas but the parabolic dish has to be mechanically pointed with motors
and gears whereas the phased array is pointed electronically and consequently has no moving parts. A phased array antenna is frequently built up from hundreds of cells each containing an antenna, a transmitter and a receiver. A narrow beam is generated and pointed by electronically changing the relative phase of the antenna characteristics of each of the cells. As the number of cells increase, the antenna beamwidth goes down and the resulting antenna gain goes up. Phased array antenna reliability can be very high because there are no moving parts to fail in a hostile environment like outer space and an array containing hundreds of cells will see minimal degradation as cells fail one by one. On the downside, a phased array antenna’s ◦ coverage area is typically a 90 cone, whereas a parabolic dish antenna’s coverage can easily be a hemisphere. Phased array antennas also consume more power. The power needed for several arrays to replace one parabolic dish can be as much as 5 or 6 times as much needed for the parabolic antenna system (includes transmitters and receivers). 5.4. Phased array antennas on the Station
The high reliability of phased arrays and their lack of moving machinery make them very attractive for space communication systems. Their power consumption will keep them oN the Station until all of the solar arrays are in place and there is suJcient power to operate them. Two or more array antennas properly located on the trusses could provide more TDRS coverage than the current Ku-Band dish. 5.5. Ka-Band communications for the Station
Barring any catastrophic failures, the current Ku-Band subsystem will meet the Station’s wideband communications requirements until assembly is complete. The Ka-Band commercial technology will be closely watched in the mean time. The data rates that the commercial satellite constellations can handle will rapidly increase until it becomes economically feasible to lease data terminals and install them on the Station trusses. The most diJcult challenge will be to gather up the wideband payload and video data from inside the Station inhabited modules and get it to the data terminals outside on the trusses. There are no spare Aber or coaxial cable hull penetrations to move the extra data. One approach would be used to the existing coaxial cables going between the SGTRC outside on the Z1 truss and the HRM in the US Laboratory. Two or more Ka-Band phased array data terminals would replace the current Ku-Band antenna and transmitter=receiver. Another high-rate
ISS communications enhancement plan
data multiplexer would be added to the HRFM inside the US Lab to gather up more data and a replacement multiplexer for the HRM would be added that could handle the higher data rates. The exploding communications technology eNorts are being driven by commercial industry. This allows
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a signiAcant paradigm shift where the National Aeronautics and Space Administration (NASA) will look for commercial “oN-the-shelf” communications solutions before embarking on another costly Ka-Band development program for the Station’s future wideband communications needs.
PII:
Acta Astronautica Vol. 50, No. 11, pp. 697–703, 2002 Published by Elsevier Science Ltd. Printed in Great Britain S0094-5765(02)00002-4 0094-5765/02/$ - see front matter
ISS COMMUNICATIONS ENHANCEMENT PLAN† FRANK T. BUZZARD‡ and ORON L. SCHMIDT NASA Johnson Space Center, Houston, Texas 77058, USA (Received 10 June 1998)
Abstract—The capabilities of the International Space Station (ISS) communications system will be expanded to enhance the capability of scientists and principal investigators in controlling and obtaining data from their payloads. Payload user requirements exceed the current capabilities of the ISS communications system. This paper describes a phased approach that will signiAcantly expand the ISS communications system capability and enable payload telescience objectives. A Ku-Band forward link will be added for uplink voice, video and commands to support telescience. Increased demands on return link bandwidth will require the use of video compression to free up more bandwidth for payload data. Standard video compression algorithms such as MPEG 2 will be used to facilitate transmission compressed digital video to the end user. Ground data processing capabilities upgrades will be pursued to increase the downlink data rates from 50 to 150 Mbps. A new communication outage recorder will store payload data when the Space Station is not in view of a data relay satellite. The enhanced communication capability and ground data handling capability enhancements will be carefully matched to the expanding payload user requirements. The Anal phase of the ISS communication enhancement plan will investigate the feasibility of greatly increased bandwidth using commercially developed, state-of-the-art systems. Published by Elsevier Science Ltd.
inside Node 1 and two-phased array antennas outside on the port and starboard hatches (see Fig. 1). The ECOMM subsystem will communicate with the ground control center via the tracking and data relay satellite (TDRS). Other Station elements will displace the antennas for ECOMM and its operation will cease when the US Laboratory comes up.
1. SPACE STATION COMMUNICATION PHASING
The International Space Station (ISS) communications system will evolve through three primary phases: Phase I—initial S-Band and Ku-Band communications for early assembly, Phase II— Ku-Band enhancements and Phase III—upgrading Ku-Band communications through the use of commercial communications hardware.
2.2. Operational S-Band
2. PHASE I: ISS BASELINE COMMUNICATIONS
The ISS communications capability will grow to meet the requirements for each assembly phase beginning with S-Band for the early assembly of Node 1 and Ku-Band for the US Laboratory. 2.1. Early communications subsystem
The early communications (ECOMM) subsystem provides a command, telemetry, video conferencing and Ale transfer capability for Node 1 which is the Arst US manned module. ECOMM will have an S-Band transceiver and data processor
The operational S-Band subsystem provides commands, voice and telemetry via the TDRS to=from Mission Control Center—Houston (MCC-H). Two strings of hardware will be provided for redundancy (see Fig. 2). Two channels of air-to-ground voice are provided by the audio distribution subsystem (ADS) and commands and telemetry are exchanged with the command and control multiplexer demultiplexer (C&C MDM). 2.3. Ku-Band subsystem-baseline
The Ku-Band subsystem processes the wideband payload data and Station video for transmission through TDRS to the ground. The subsystem shown in Fig. 3 is comprised of Ave separate units: (1) the video baseband signal processor (VBSP), (2) the high rate frame multiplexer (HRFM), (3) the
†Paper IAF-97-T.3.06 presented at the 48th International Astronautical Congress, Oct. 6 –10, 1997, Turin, Italy. ‡Corresponding author. 697
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Fig. 1.
Fig. 2.
Fig. 3.
high rate modem (HRM), (4) the space-to-ground transmitter=receiver controller (SGTRC) and (5) the space-to-ground antenna (SGANT). VBSP. Up to four channels of analog video from the video distribution system (VDS) are dig-
itized by the VBSP and fed to the HRFM. The VBSP represents video compression technology of the early 1980s. If a standard television channel is digitized by the VBSP, the output bit rate is 42 Mbps. The maximum throughput of the HRFM is 43:2 Mbps. Therefore, a full-motion (30 frames per second) video channel will occupy most of the Ku-Band downlink bandwidth. The VBSP can digitize a video channel to a slower frame rate of 15 frames per second to cut the bandwidth in half. How does the VBSP achieve the lower bandwidth? It simply throws half of the frames away! Today’s video compression algorithms are much more eJcient and can provide full-motion video in as little as 6 Mbps of bandwidth. It is no wonder then why the payload community wants the VBSP replaced with modern video compression hardware. Another disadvantage of the VBSP resulted from a cost cutting exercise which removed the audio unit that was combining the audio with the video inside the VBSP. As a consequence, the audio comes down on the operational S-Band link. Sometimes the S-Band link is lost and the Ku-Band link is still good and vice versa. It was also assumed that the audio and video could be recombined on the ground. However, both the audio and video are digitized and arrive on the ground at randomly varying times. HRFM. The HRFM accepts eight channels of high rate data from the automated payload switch (APS) and four channels of digitized and packetized video from the VBSP. The output rate will be set at 50 Mbps because this is the highest rate that the ground distribution and processing hardware can handle. The HRFM (and the on-board Ku-Band system) can currently support 75 and 150 Mbps. HRM. The HRM accepts a serial data stream from the HRFM and modulates it onto an intermediate frequency carrier which is transmitted by the SGTRC and the SGANT. The VBSP, HRFM and HRM are all mounted in an avionics rack inside the US Lab module. SGTRC. The modulated carrier signal from the HRM is upconverted and ampliAed by the SGTRC’s transmitter and sent to the SGANT. The monopulse receivers allow the SGANT to autotrack the TDRS. The sum channel of the monopulse receivers is downconverted and presented to an output port on the SGTRC. This output signal is carried into the US Lab on a coaxial cable and will be fed into a future receiver=demodulator to restore the Ku-Band uplink. SGANT. The Ku-Band antenna is a 6-foot parabolic dish mounted on an azimuth=elevation gimbal system. The SGANT and SGTRC are both
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Fig. 5. 3.2. High-rate data
High-rate payload data are gathered and processed to a single packetized data stream in the payload rack. If the data are destined to go to the ground via the Ku-Band system, it will conform to one of two protocols: bitstream or frame format. 3.3. Tracking and data relay satellites Fig. 4.
mounted on a boom which extends out from the Z1 truss. The antenna can point anywhere within a hemisphere but structure such as solar panels surrounding the antenna causes blockage up to 25% of the time during TDRS passes.
3. PAYLOAD DATA
3.1. Standard payload rack
International standard payload racks (ISPRs) will be used for all internal payloads. The ISPRs will have standard interfaces to carry wideband data, video and command data. The US Laboratory provides an interface to each ISPR location from the payload multiplexer=demultiplexer (P=L MDM) via a standard 1553B bus, to=from the payload ethernet hub gateway (PEHG) via ethernet, to an internal video switch unit (VSU) via optical Aber and to an automated payload switch via optical Aber (see Fig. 4). P=L MDM data. The payload (P=L) MDM transmits commands, ancillary data and Ales to the payloads and collects health and status data and requests for service from the payloads. Ethernet data. An ethernet bus connects payloads to payloads and payloads to the ground through the PEHG. There is currently no path to move data up from the ground to payloads via the ethernet. Video. Operating a payload remotely from the ground can be accomplished by positioning a video camera in the payload and sending the video to the ground. A Aber optic input is provided for every payload rack to carry the video to the video distribution system and then on to the ground via the Ku-Band system.
The TDRS system provides the capability for the Station to communicate with the Mission Control Center (MCC-H) in Houston, TX and the Payload Operations Integration Center (POIC) in Huntsville, AL. 4. PHASE II: KU-BAND EXPANSION PLANS
4.1. Adding Ku-Band forward link receiver
The Ku-Band forward link (the link from the TDRS to the Station) receiver=demodulator was removed during the Freedom program. The sum channel of the SGTRC monopulse autotracking receivers is provided at an intermediate frequency (IF) of 385 MHz at an output connector. The current Space Station Program OJce has recently decided that the forward link should be restored to provide video conferencing and Ale transfers to an onboard Ale server that will establish a wireless ethernet within the space Station modules (see Fig. 5). Restoring the Ku-Band forward link will also support uplink commands and voice when a future forward link data demodulator is installed. 4.2. Forward link video compression
The Ku-Band uplink will support a digital signal of at least 6 Mbps. Transmitting video up to the Station will require extreme video compression. An early S-Band communications system in Node 1 will use Intel’s ProShare software running on a laptop computer to provide video conferencing, white board graphics exchanges and Ale transfers. This system will operate at 128 Kbps total bandwidth on the uplink and downlink. The video will be conAned to a small window on the laptop screen and will not be full-motion. Payload users and in-Might maintenance (IFM) ground teams demand full-screen, full-motion video displays on board. This will require the latest
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compression algorithms such as motion picture entertainment group 2 (MPEG 2). One MPEG 2 channel currently requires 6 Mbps of bandwidth. Several diNerent algorithms will be implemented in plug-in modules on board to facilitate tailoring an algorithm to speciAc video performance requirements. The modules will also allow new algorithms to be phased in without modifying the Station video hardware. There are user requirements for up to three simultaneous uplink video channels. Video decompression algorithms with higher ratios in the range of 50 : 1 to 100 : 1 will be needed and coding techniques will be employed on the Ku-Band forward link to meet these requirements.
Fig. 6.
4.3. Downlink video compression
Payload investigators have learned that mounting a camera in their payload rack is an inexpensive way to monitor their experiments from the ground. There are other sources of video such as that from the external cameras to monitor Station assembly and for public aNairs. All of this video must be digitized by the payload or the VBSP and eventually comes to the Ku-Band HRFM to be sent to the ground. There are two bottlenecks: the maximum total HRFM data rate of 43:2 Mbps and the VBSP compression ineJciencies. The MPEG 2 compression algorithm has a compression ratio that is 7 times higher than that of the VBSP. There are plans to replace the VBSP with a video digital processor that has four channels of highly compressed video. Each channel would have a plug-in module for each available algorithm such as MPEG 2 or 4. Additional modules would be available on board to customize the compression algorithm to the video application and for replacement in the event of malfunctions. Standard MPEG algorithms also embed the audio with the video prior to digitizing and compression which automatically synchronizes the audio with the video. An audio interface unit (AIU) would be added to bring up to four channels of audio from the audio distribution subsystem to be combined with four channels of video from the video distribution subsystem in the video processing unit (VPU) shown in Fig. 6. The VPU output would use the current four channels of video input to the HRFM coming from the VBSP. The VBSP is scheduled to be replaced by the VPU in 2002. 4.4. Telescience commanding
All commands currently come up the operational S-Band subsystem. If the commands are destined for the Station core systems, they are processed by
Fig. 7.
the C&C MDM. If they are destined for a payload, the C&C MDM forwards them to the PayLoad (P=L) MDM and they are routed to the correct ISPR. Payload data comes down the Ku-Band subsystem. The times that the S-Band link is up does not always overlap the time that the Ku-Band link is available. Performing telescience with payloads requires simultaneous transmission of commands while observing the real-time data. Therefore, a command capability is needed for the Ku-Band uplink. A payload command decryption and decoder function is planned for Utilization Flight 5 (UF-5), scheduled for launch in June of 2002. All station commands are secured by encryption. Fig. 7 shows the path that Ku-Band forward link commands would take to get to an ISPR. 4.5. Communications outage recorder
The TDRSs are positioned in synchronous orbits such that all satellites are in view of White Sands, New Mexico. The Station will be in a high inclination orbit and there will be times when a TDRS is not in view or a line of sight path to a TDRS is blocked by structure. For these reasons, a single Ku-Band antenna cannot provide continuous real-time coverage for payload data. A communication outage recorder (COR) will be installed in the Station to record wideband data (not digitized video from the VBSP) from the automated payload switch (APS) for later playback into the HRFM. Playback data can be used on the ground to build
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a continuous data stream for a payload. The COR will store a minimum of 43:2 Mbps for 1 h with a large growth potential for higher data rates and=or longer record periods. The COR is scheduled to go up on UF-3 in September of 2001. 4.6. Increasing Ku-Band downlink bandwidth
The downlink data rate bottleneck is on the ground. Tests performed over a year ago showed that the on board Ku-Band subsystem can Mow 150 Mbps of data through TDRS with positive link margins. The downlink data are carried by a satellite from the White Sands ground station to the control centers. The front end processors (FEPs) at the control centers are also limited in capacity. The FEP in the Houston control center (MCC-H) would have to be modiAed for 75 or 150 Mbps. The Huntsville control center’s FEP will have to be modiAed to handle 150 Mbps. Using the VBSP to send more than one video channel to the ground will put immediate pressure on the ground to go to 75 Mbps. High payload data rates and additional video will require 150 Mbps to the ground between UF-5 and UF-7. 4.7. Digital television
Good quality digital cameras are appearing in the commercial market place. Their performance is superior to that of the older analog cameras but they have a standard analog composite output and will work with the current video distribution system on the station. Commercial oN-the-shelf camcorders will be Mown Arst on the space shuttle orbiter and then Mown on the Station. Special care was taken in designing the video interfaces on the Station to ensure that any camera, whether it was used inside or outside, could also be Mown on the Station. The shuttle video engineers have perfected the process of taking oN-the-shelf cameras purchased at a local home video store and running them through environmental tests to determine what modiAcations are necessary to My them. This same process will be used to My high deAnition TV (HDTV) cameras, editing equipment and recorders on the Station. HDTV is a much greater technical challenge because of the high output data rates. Every necessary step will be taken to My commercial hardware to keep costs down and to bring the latest in video technology to the Station. 4.8. Digital video distribution
The analog composite video signal from external and internal cameras is pulse frequency modulated
Fig. 8.
(PFM) before being transmitted over optical Abers in the VDS. If an internal analog video signal needs to be routed over the VDS, the conversion from a copper path to a Aber optic path and the PFM modulation is performed by a common video interface unit (CVIU). Conversion from a Aber optic path to a copper path and PFM demodulation are also performed by a CVIU. There will be some degradation to a digital camera’s video quality if it is converted into an analog composite signal for distribution and display. If a digital camera output can be distributed and processed in a digital format, the original video quality can be preserved with none of the signal degradation seen in an analog system. Better performance will be achieved by leaving the digital video camera output in its digital format and distributing the video as high-rate digital data. Tests will be run on the Aber optic VDS components to determine what changes will have to be made to carry digitized and compressed camera outputs in excess of 100 Mbps. It is hoped that the current optical Aber and the video switches will carry up to 125 Mbps of serial video data. Both the external and internal switches should already be capable of passing digital data because the Aber path is converted into a copper path inside the switch and the video stays in its PFM state as it passes through the crossbar switches and gets converted back to a Aber optic signal before it leaves the switch. A digital video interface unit (DVIU) would be used instead of a CVIU. A DVIU will accept a digital video input on a copper path, PFM the digital signal onto output Aber for distribution over the VDS and can take a digitized video signal oN a Aber, pulse frequency demodulate it and output the digital video data on a copper path. The DVIU output can be sent directly to a digital monitor or editor. Digital compression will likely be required prior to recording on board or transmitting digital video to the ground. Compression and adding synchronous audio will be performed by the VDP. A typical path for a payload digital video camera output destined for the ground is shown in Fig. 8. This approach will handle an HDTV camera output if the digital camera output is compressed to less than 125 Mbps.
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F. T. Buzzard and O. L. Schmidt 5. PHASE III: COMMERCIAL APPLICATIONS
5.1. Why commercial communications hardware?
All of the development costs for the space communications hardware for the Apollo lunar program and the Space Shuttle program were paid for by the United States Government. Some of the early shuttle payloads were commercial communications satellites. Much of the technology built into these early communications systems was based on the Shuttle communications technology. The Station communications technology is over 10 years old and can also be traced back to Shuttle era technology. In the mean time, commercial satellite bandwidth requirements have exploded because of the need to direct broadcast video to millions of homes. Next came the Iridium satellite constellation to relay cellular telephone conversations anywhere in the world and the Teledesic constellations to relay Internet sessions and video conferencing anywhere in the world. Commercial communications corporations have risen to the challenge and have developed space communications hardware whose performance will exceed that of the Space Station communication hardware for a fraction of the per unit cost. These commercial communications corporations are now in position to lease new space technology back to the Government at a cost much lower than it would cost the Government to develop and manufacture small quantities of new technology hardware. 5.2. Carrier frequencies
The Ku-Band frequencies are getting crowded and interference is becoming more common. Ka-Band is the choice for future wideband satellite communications. Goddard Space Flight Center (GSFC) will build three new TDRSs that have Ka-Band capability in addition to the current S-Band and Ku-Band. The satellites will be designated H, I and J and will have a forward link bandwidth of 50 Mbps and a return link bandwidth of 600 Mbps. The Teledesic commercial satellite constellation will operate at Ka-Band. For the Arst time, a large portion of the development costs for Ka-Band space communications hardware will be paid for by commercial ventures. 5.3. Phased array antennas
Phased array antennas have been used by the Department of Defense for radar on land, on ships and on airplanes. A phased array and a parabolic dish are both directional antennas but the parabolic dish has to be mechanically pointed with motors
and gears whereas the phased array is pointed electronically and consequently has no moving parts. A phased array antenna is frequently built up from hundreds of cells each containing an antenna, a transmitter and a receiver. A narrow beam is generated and pointed by electronically changing the relative phase of the antenna characteristics of each of the cells. As the number of cells increase, the antenna beamwidth goes down and the resulting antenna gain goes up. Phased array antenna reliability can be very high because there are no moving parts to fail in a hostile environment like outer space and an array containing hundreds of cells will see minimal degradation as cells fail one by one. On the downside, a phased array antenna’s ◦ coverage area is typically a 90 cone, whereas a parabolic dish antenna’s coverage can easily be a hemisphere. Phased array antennas also consume more power. The power needed for several arrays to replace one parabolic dish can be as much as 5 or 6 times as much needed for the parabolic antenna system (includes transmitters and receivers). 5.4. Phased array antennas on the Station
The high reliability of phased arrays and their lack of moving machinery make them very attractive for space communication systems. Their power consumption will keep them oN the Station until all of the solar arrays are in place and there is suJcient power to operate them. Two or more array antennas properly located on the trusses could provide more TDRS coverage than the current Ku-Band dish. 5.5. Ka-Band communications for the Station
Barring any catastrophic failures, the current Ku-Band subsystem will meet the Station’s wideband communications requirements until assembly is complete. The Ka-Band commercial technology will be closely watched in the mean time. The data rates that the commercial satellite constellations can handle will rapidly increase until it becomes economically feasible to lease data terminals and install them on the Station trusses. The most diJcult challenge will be to gather up the wideband payload and video data from inside the Station inhabited modules and get it to the data terminals outside on the trusses. There are no spare Aber or coaxial cable hull penetrations to move the extra data. One approach would be used to the existing coaxial cables going between the SGTRC outside on the Z1 truss and the HRM in the US Laboratory. Two or more Ka-Band phased array data terminals would replace the current Ku-Band antenna and transmitter=receiver. Another high-rate
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data multiplexer would be added to the HRFM inside the US Lab to gather up more data and a replacement multiplexer for the HRM would be added that could handle the higher data rates. The exploding communications technology eNorts are being driven by commercial industry. This allows
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a signiAcant paradigm shift where the National Aeronautics and Space Administration (NASA) will look for commercial “oN-the-shelf” communications solutions before embarking on another costly Ka-Band development program for the Station’s future wideband communications needs.