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Tracking and Data Relay Satellite System
Demand Access System Augmentation
Keith Hogie, Ed Criscuolo – CSC
Asoka Dissanayaka, Bruce Flanders – Exelis
Haleh Safavi, Jeff Lubelczyk – NASA/GSFC
This material is a declared work of the U.S. government and is not subject to copyright protection in the United States.
Published by The Aerospace Corporation with permission.
TDRSS Full Coverage
• NASA's Tracking and Data Relay Satellite System (TDRSS) has multiple communication relay satellites in geostationary orbits with the ability to provide continual contact with low‐earth spacecraft • ISS communication is a prime
user of this capability
• Other missions use scheduled
blocks of time
• Each TDRS can support data rates up to 300 Mbps
• Ground terminals at:
275°
Indian
Ocean
GRGT
Pacific
Ocean
175°
– White Sands Ground Terminal (WSGT)
– Second TDRS Ground Terminal (STGT)
– Guam Remote Ground Termina,l (GRGT)
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WSGT
STGT
White Sands, NM
Atlantic
Ocean
45°
TDRS Antenna Systems
• Each TDRS has multiple
antennas
• Two 5 meter steerable dishes provide high‐rate
communication support
• However, these dishes are a
very limited resource that is
carefully scheduled among users
• There are also 30 small multiple access antennas that can support many simultaneous users at lower rates
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TDRS Multiple Access Service
• On most TDRS, the 30 signals from the MA antennas
are all sent to the ground for further processing
• Ground equipment can then
time shift and combine the
signals to form multiple receive beams aimed in different directions
• This allows supporting many more users but at lower data rates of up to 300 Kbps per user
• MA service operates on an S‐band frequency supporting simultaneous users with spread spectrum technology
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TDRS Demand Access Service
• The TDRS Demand Access Service (DAS) is intended to support multiple users for long durations with the MA multiple beam capability
• Satellites looking for events like gamma ray bursts use DAS for continuous, low‐rate communication
– Many DAS users operate at 1 to 2 Kbps, but 24 hours a day
• NASA's Space Network (SN) is investigating options for supporting many more MA users such as cubesats
• However, the current DAS hardware only supports 5 simultaneous users for each TDRS even though the MA signals can be used to form many more beams
• The desire to make more use of the MA signals and support many more simultaneous users led to a study to investigate augmentation options
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DAS Augmentation Concept
• With more DAS equipment, the SN could expand from 5 users to support 30 or more users on each TDRS
– The MA signals are continually sent to the ground, just need processing
• More equipment would allow permanently configuring dedicated equipment strings for all DAS customers at all sites to eliminate all need for scheduling systems
– DAS systems at all sites would continuously form beams, listen, collect, and forward any data received
– Users would just send updated orbital state vectors weekly
– Users could send requests to change data rates and some other settings • This highly automated DAS system would be standalone and data driven with almost no legacy interfaces
• Increased capacity might be useful for missions such as cubesat communication and locating missing cubesats
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Augmentation Key Components
• The key to DAS Augmentation is having enough low‐cost resources to allocate dedicated equipment to each user at all sites
• Key components:
– New beamformers to form more beams
• Already installed as part of TDRS Digital Signal Distribution upgrade
• Capable of forming 30 or more beams for each TDRS – Low‐cost DAS receivers (~$1K per receiver)
• Current receivers tend to cost $50K to $100K or more
– Low‐cost frame sync and user data formatting equipment
• Software based, allows unlimited copies at minimal cost
• All DAS components NASA developed hardware and software with no unit level licensing/maintenance fees
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Augmentation Key Components
•
•
30 MA Channels
SDDS Packets
6,300 Mbps
•
•
•
Beamformer
•
Data from TDRS antenna digitized at ground antenna
Beamformer ingests MA streams, forms beams, outputs Signal Data Distribution Standard (SDDS) IF packets
MA receivers despread, demodulate, remove coding, bit sync data, and outputs bits in UDP/IP packets
Frame sync ingests bits, locates frames, Reed/Solomon processing, and outputs with specified user headers
Monitor and control system monitors and commands components
TDSD IF Digitizer
•
•
Formed Beams
SDDS Packets
210 Mbps each
•
•
•
Raw User Data Bits
1‐150 Kbps
per channel
MA Receiver
Frame Sync
Despread
Demod
Bit sync
Frame Sync
Reed/Solomon
Output Header
Monitor & Control
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User Data Frames/Hdrs
1‐150 Kbps
per channel
Mission Network
TDRS Digital Signal Distribution
• TDSD already installed beamformers at all sites
– Each TDSD beamformer can form 30 beams
– STGT and WSGT each have 3 TDSD beamformers and each site has at least 2 TDRS's available for DAS
– GRGT has 2 TDSD beamformers and a TDRS available
– Each beamformer can access any TDRS at the site
• Each beamformer consists of a general purpose server and a custom FPGA system
– The server handles monitor and control of the FPGA system and feeds TLE orbit information to the FPGA
– The FPGA system receives 30 MA streams of SDDS packets at an aggregate rate of over 6 Gbps, uses configuration information sent to it to form beams, and outputs each beam as a stream of SDDS packets at 210 Mbps
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Low‐cost DAS Receivers
• Using 100 or more receivers requires low‐cost receivers
• DAS receivers ingest TDSD SDDS packets at 210 Mbps and output packets of raw data bits
– Ingest SDDS packets via UDP multicast on one Ethernet interface
– Despread, demodulate, optional Viterbi, bit sync, time tag, and output the bits in UDP/IP packets on a second Ethernet interface
– Receiver monitor and control also uses the second Ethernet interface
• Two prototypes being tested
– FPGA based hardware receiver version • Up to 3 receivers in a 1U chassis (~$3K per chassis)
• NASA owns the FPGA code
– Software receiver version running on a multi‐core server
• Up to 4 receiver instances running in a 1U server
• Software developed for NASA by Exelis
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Software Frame Synchronizers
• Frame synchronization software receives raw bits, locates frame boundaries, inverts bits if necessary, extrapolates time, performs Reed/Solomon if present, adds user header and sends the packets
– Software originally developed for Landsat in 1995
– Software reused for SOHO, WIND, POLAR, GEOTAIL level‐zero processing systems and control centers in 1998
• It can operate at multi megabit rates but only needs to handle up to 150 Kbps per stream for DAS.
– Supports over 100 DAS data streams on one server
– Uses 1% of one core at 150 Kbps random data
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DAS Augmentation Data Flow
30 MA Channels
SDDS Packets
6,000 Mbps
Beamformer
• Once IF signal is digitized at antenna, all other processing is digital over 1 Gbps and 10 Gbps Ethernet
• All equipment is connected using high performance layer 3 Ethernet switches passing data in UDP/IP packets using IP multicast and unicast
SDDS Time
I SDDS QTime
‐123 I SDDS
456 QTime
‐345‐123 I‐3
456 QTime
SDDS
4567
123 I‐3 456 Q
‐345‐123
SDDS Time
7324567
‐543
123 I‐3 456 Q
‐345
‐123
30467324567
‐3210
‐543
123 ‐3 456
‐345
‐123
…3046732
…4567
‐3210
‐543
‐345123 ‐3
…3046732
…4567
‐3210
‐543123
…3046732
…‐3210
‐543
…3046 …‐3210
…
…
MA Receiver
Len Seq
SDDS Time
11000101
11110001
00010110
11110000
10101001
Frame Sync
Frame Sync
Reed/Solomon
IPDU Header
Despread
Demod
Bit sync
Formed Beam
SDDS Packets
210 Mbps
User Data Frames/Hdrs
1‐150 Kbps
per channel
Raw User Data Bits
1‐150 Kbps
per channel
Monitor & Control
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IPDU Hdr
Time Etc.
. .
1ACFFC1D
DEADBEEF
DEADBEEF
DEADBEEF
Mission Network
WSC Data Distribution
SN
Network
Single Site Architecture
State Vectors, TLEs, Change Requests, UPDs
Site Monitor & Control
Session Control & Frame Sync
Web Browser GUI
Site Monitor & Control
TLEs,
SVs
Doppler
Offset
Session Monitor &Control
Apache
Web Pages/ Scripts
Config/
Doppler/
Status
MySQL
6 Gbps
Beam‐
former
Frame Frame Sync/Fmt
Frame Sync/Fmt
Frame Sync/Fmt
Sync/Fmt
•••
Layer 3 Network switch
MySQL DB
Doppler, Status, Config
Digitized IF Samples SN Network
150 Kbps or less
Multicast Join/
SDDS Packets
210 Mbps
Bits in
Receiver
UDP packets
Receiver
Receiver
Receiver
•••
Layer 3 Network switch
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Log
Log
Log
Log
Temp User Files
WSC Data Distribution
Mission Network
End Users
Formatted Frames for Delivery to Users
User Data
Monitor/Control
Full System Architecture
• With each of the three TDRS ground stations augmented, a single site is still needed to provide a central point of contact for user control centers
• Central software processes at WSC will collect data from all sites and forward to users as appropriate
• User performance data (UPD) will be collected from all sites and forwarded to users
• It will also accept configuration change requests and user satellite orbit updates and provide them to all three DAS sites
• Archival data storage will also be provided by storing recorded data files from each DAS site
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Full System Architecture
STGT
Site M&C
WSC DAS Central Control
Sync/Fmt
East ‐
West
User Input
Receiver
Receiver
Receiver
Receiver
Beamformer
State Vectors, Reconfig Requests
Temp Log
User UPD Delivery
WSGT
Site M&C
Data Delivery
Sync/Fmt
NASA
Mission Network
East ‐
West
Receiver
Receiver
Receiver
Receiver
Beamformer
User
Temp Log
Secure File Transfer
SN Network
GDIS‐R
GRGT
Beamformer
Site M&C
User data packets
User Files
UPDs
DAS File Store
Sync/Fmt
Receiver
Receiver
Receiver
Receiver
User
Temp Log
User Data
UDP Mulitcast per User
Monitor/Control
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Current Status
• A prototype system is currently being tested to determine if the low‐cost components can meet the DAS requirements
–
–
–
–
3 software receivers
3 hardware receivers
Frame sync server
Basic monitor and control • Testing is being done at White Sands using test data radiated from a ground based transmitter
• Also shadowing orbiting spacecraft and comparing with the legacy DAS system
• Data is being collected to compare the performance of the low‐cost receivers to the legacy ones
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Summary
• A decision on whether to proceed with the full DAS Augmentation will be made when the current testing and analysis is complete
• Enhanced service options also being considered
– Array multiple TDRS for signal gain to support spacecraft such as cubesats
• Combine WSGT & STGT MA signals to recover weaker signals
• Possible include GRGT by sending soft bits back to White Sands
– Frame Relay/IP support
• Spacecraft IP packets delivered direct to end users
– Tracking Service using multiple MA services
• Process multiple TDRS MA signals for triangulation
• More work to be done, but potential for supporting many more missions with low‐cost, automated support
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