DS-T DEVELOPMENTNASSER GOLSHAN
A fast-track prototype effort has begun to validate the concept of a fully automated, autonomous, ground station that simplifies implementation, operation, and reduction of the life-cycle cost of tracking deep space missions at the Deep Space Network (DSN). The origin of the Deep-Space Terminal (DS-T) is the successful development of the Low Earth Orbit Terminal (LEO-T), the prototype for a new class of low-cost ground station to reduce life-cycle costs of tracking the National Aeronautics and Space Administration's (NASA's) missions in low-earth orbit.
Development of the predecessor, LEO-T, was carried out in two phases by a small team of engineers at the Jet Propulsion Laboratory (JPL) and at SeaSpace Inc., a satellite ground terminal manufacturer in San Diego, California. Autonomous, unattended telemetry operation was successfully demonstrated by tracking two NASA science satellites-the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX), and the Extreme UltraViolet Explorer (EUVE), both operated by NASA's Goddard Space Flight Center, in Greenbelt, Maryland. In the second phase, command uplink capabilities were added to the prototype terminal by JPL. A successful, week-long demonstration of the automated unattended uplink and telemetry operation of the LEO-Terminal with the COBE (COsmic Background Explorer) spacecraft was completed at JPL on December 28, 1995. The LEO-T concept is now generally accepted by the NASA community as the ground station of choice for low-cost support of near-earth missions.
The DS-T effort will carry the LEO-T paradigm to NASA's Deep Space Network, enabling significantly lower costs for ground support of missions, as well as improved reliability and data flow. Lower costs are expected in ground systems development, implementation, maintenance, and operations.
The prototype DS-T is being implemented at the DSS-26 34-m beam waveguide antenna. In addition to the antenna system, DS-T includes an X-Band microwave system, a 4 kW transmitter, and a DS-T electronics rack. The latter combines all baseband telemetry and uplink functions, as well as all high-level monitor and control operations of the station for unattended, autonomous operations. Figure 1 shows a strawman block diagram for DS-T.
The DS-T electronic rack is being built entirely from commercially available subsystems. Use of Commercial-off-the-Shelf (COTS) equipment has enabled a rapid, low-cost, development cycle, and will ensure low recurring costs for future users. The electronics rack includes the telemetry receiver, command exciter, and an Ultra 2 Sun workstation, all COTS equipment.
The workstation provides for automated, unattended operations of the terminal, including scheduling, calculation of spacecraft trajectory and frequency predicts, automated uplink and telemetry operations, communication interfaces for remote command operations, as well as processing and distribution of spacecraft engineering and science telemetry data to the mission operations and science users of the data. In its current configuration, the terminal will receive telemetry at rates up to 1.2 Mbps, and uplink commands at rates up to 2 kbps. However, the operating frequency and the ceilings on telemetry and uplink rates of the terminal can be modified easily by replacing the appropriate modules of the terminal with other commercially available equipment consistent with the desired data rates and operating frequencies.
The X-band microwave system includes a corrugated horn, a horn diplexer, a commercial cryogenic low noise amplifier (LNA) package (including a high-electron mobility transistor (HEMT) amplifier and a cooled filter to reject the uplink RF leakage into the LNA), as well as microwave waveguides for uplink and downlink.
Based on preliminary measurements, the operating noise temperature of the DS-T system, with the HEMT amplifier at an antenna elevation angle of 30 degrees and 90 percent weather availablity, is estmated to be around 29 kelvins, both in receive-only and transmit-and-receive modes. This compares well with the operating noise temperature of 29 (39) kelvins for DSS-25, with MASER LNA and receive-only (or transmit-and-receive) conditions with similar weather and elevation.
Autonomous, Automated Operations
The software for automated operation of the terminal is based on the modifications to the LEO-T software. The latter was developed by SeaSpace Inc., with input from JPL, and is now available from the same vendor. It is expected that the automation software for DS-T will also be commercially available from a commercial vendor at the completion of the task.
The terminal provides TCP/IP interfaces for reliable networking with remote users over commercial communication links. Uplink commands can be sent in real time from the mission operations to the terminal for uplink to the spacecraft, or can be stored at the terminal for uplink during a future pass. Spacecraft telemetry received by the terminal is forwarded, electronically, to destinations designated for each spacecraft.
After the initial set-up, the terminal autodials an electronic navigation bulletin board at JPL (on a daily basis) and retrieves SPK (Space Planetary Kernel) files supplied by JPL Navigation. Based on the SPK files, the terminal automatically generates satellite view periods, antenna-pointing predicts, and receiver/transmitter frequency predicts.
The auto scheduler uses the view periods and user defined tracking priorities to continuously track multiple spacecraft of interest. For every scheduled pass of the spacecraft, the auto scheduler wakes up the terminal a few minutes before scheduled tracking. The terminal executes the automated, unattended, pre/in/post-pass uplink/telemetry reception routines and then waits for the next scheduled spacecraft.
Schedule and Demonstrations
DS-T implementation takes place in two phases. Phase one, already started, includes station autonomy and telemetry implementation, and is planned for completion by December 1997. A one-month telemetry reception demonstration is planned, using MGS as the spacecraft of opportunity. During this demonstration, the terminal will autonomously track the spacecraft, receive, and process telemetry data. Plans for the addition of uplink have been finalized; long lead items will be ordered in FY 97, and the work completed in FY 98. DS1 is considered the first spacecraft of opportunity for autonomous, uplink/downlink demonstration.
DS-T development provides an opportunity to explore low-cost approaches that reduce the hourly costs of DSN services for future DSN implementations. The DS-T concept is considered the baseline option for the DSN Network Simplification Plan, to simplify DSN operations and reduce costs. In addition, the DS-T prototype provides a flexible testbed for autonomous, unattended operations, user-initiated services, direct delivery of data to the principal investigator, and application of communication protocols to deep space.
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