The amount of data that can be returned from the spacecraft, both in the form of telemetry from on-board instruments and radio science data from the microwave signals, is determined to a large extent by the capabilities of the DSN. Likewise, the accuracy of spacecraft navigation depends largely on DSN capabilities.
For the Voyager mission there were at least 17 first-time performance achievements by the DSN not accomplished anywhere before, and all strongly dependent on technology from the Advanced Systems Program. These first-time achievements are briefly described in the following paragraphs.
The Voyager mission required other unique capabilities of the DSN, also
depending on technology from the Advanced Systems Program:
Mariner 10 was the first JPL spacecraft
And the Mariner 10 Mission was the first JPL mission to use,
The design of the spacecraft's imaging data system (at S-band) provided a choice of just two uncoded data rates: 117.6 kb/s or 22.05 kb/s. The higher rate (117.6 kb/s) was determined to give effectively noise-free images [with a bit error rate (BER) of 5 ´ 10-3] from Venus distance using the standard 64-m antenna feeds and masers, with a comfortable SNR margin. With this configuration, the greater distance to Mercury would result in a decrease of 3.5 dB in SNR and would not permit use of the 117.6-kb/s rate for that encounter. The 22.05-kb/s rate, however, would permit the imaging of only a small portion of the visible part of Mercury rather than the entire visible part. (The Mercury encounter was scheduled for late March 1974, following the Venus encounter in early February 1974.)
So, in June 1973, the Mariner 10 Project Office requested the DSN to evaluate its capabilities to support a Mercury encounter with imaging data at 117.6 kb/s.
The resulting evaluation determined that a reduction in antenna operating noise temperature would improve the SNR enough for the higher data rate to be used at the Mercury encounter (with an increased BER that was near 0.02). And it was found that the antenna operating noise temperature could be reduced to 13.2 K (at zenith) if new 2.1-K super-low-noise masers (from the Advanced Systems Program) were installed in a receive-only configuration at the Goldstone and Canberra 64-m antennas.
In addition, the operating noise temperature of the Canberra 64-m antenna could be reduced to 12.5 K (at zenith), if an ultracone with a specially designed feed (previously developed by the Advanced Systems Program) could be equipped with the new low-noise maser and installed on the Canberra antenna.
The Mariner 10 Project accepted this improvement of the DSN's capabilities on a best-effort, mission-enhancement basis.
Unfortunately, a major spacecraft emergency occurred in 1973 on Christmas Day, seven weeks after launch and six weeks before Venus encounter, when the spacecraft antenna feed developed a problem that decreased the power output by 3 dB and changed the polarization from circular to essentially linear. The resulting mismatch with the DSN circularly polarized feeds caused an additional 3-dB loss in signal, for a total loss of 6 dB. Without correction, only the 22.05-kb/s rate could be supported at Venus, with a loss of over 80 percent of the imaging data. However, by using the antenna lower-noise capability (as just described for the Mercury encounter) and by installing emergency polarization equipment (from previous Advanced Systems Program work) at all three of the 64-m antennas to eliminate the 3-dB polarization loss, enough SNR was regained to yield very good results at the Venus encounter on February 5, 1974.
In the meantime the spacecraft antenna problem was going through fail-heal-fail cycles. On March 4, 1974, 25 days before the Mercury encounter, the spacecraft antenna problem disappeared and did not return. This allowed the two 64-m antennas with superlow-noise masers and the ultracone to carry out the original purpose of supporting the 117.6-kb/s rate at Mercury. (The entire illuminated disk could thus be imaged rather than a small part of it at the 22.05-kb/s rate.)
It was decided to extend the mission to include a second Mercury encounter (to occur on September 21, 1974). The range for this encounter was greater than for the first, and this caused an additional SNR reduction of 1 dB. The increased BER at 117.6 kb/s resulting from this reduced SNR put imaging at this rate in doubt. The Advanced Systems Program had been developing a process of signal combining for antenna arraying, and this process was quickly brought into the DSN on a best-effort basis during the months before the second Mercury encounter. At that encounter, one 64-m and two 26-m antennas were arrayed to increase the SNR by about 0.6 dB, which was enough to allow (at acceptable error rate) the full imaging provided by the 117.6-kb/s rate.
About five weeks before the second Mercury encounter (August 14, 1974), the Mariner 10 spacecraft tape recorder failed. From then on, all of the engineering and nonimaging science data had to be transmitted as it occurred, instead of being recorded for later playback, and this caused a considerable increase in workload for the DSN.
It was decided to extend the mission to include a third Mercury encounter, which was to take place on March 16, 1975. The primary purpose of this encounter was to gather more nonimaging science data. However, these data, transmitted at 2450 b/s simultaneously with the imaging data, took priority over imaging data. For instance, there were spacecraft attitude control problems that required additional DSN uplink activity, and more frequent radio metric data activities were needed for orbit corrections. Fortunately, improved radio metric data for the mission were provided by the Mu-II Ranging Machine that was developed by the Advanced Systems Program.
The research and development masers and ultracone installed on a best-efforts mission- enhancement basis had made possible the 117.6-kb/s imaging data for the first two Mercury encounters. Just before the third encounter, the ultracone maser cryogenic system at Canberra failed and the 22.05-kb/s imaging data rate had to be used; fortunately, the primary nonimaging science data were not impacted.
The highly successful MVM 73 mission depended very significantly on both established and "last minute" contributions of the Advanced Systems Program.
Significant cost savings have resulted from the use of less expensive optical fibers that replaced microwave links, coaxial cables, and expensive equipment that was used to mitigate degradation of signals by the replaced links and cables. The virtual lack of signal degradation by the fibers also reduces the number of required expensive items throughout a DSN complex, like hydrogen-maser or trapped-ion standards. Further important cost savings can be achieved by transmitting nearly all microwave signals through highly stable optical fibers. For the downlink signals, all of the equipment following the cryogenic low-noise amplifier module at every antenna could be moved to the signal processing center (SPC) of the DSN complex. Likewise, for the uplink signals, all equipment up to and including the exciter could be moved from every antenna to the SPC. Such a configuration would permit multiple cost savings (including elimination of antenna and/or station control rooms) and performance improvements.
The Advanced Systems Program began to monitor the development of fiber-optic technology in 1970, after Bell Telephone Laboratories announced that it had developed an optical fiber with low-enough loss for practical telecommunications applications. In the next years, the field of fiber-optic technology developed rapidly, resulting in lower-loss fibers and reliable semiconductor lasers that could furnish practical levels of optical input to fibers.
By 1978, the Frequency and Timing Systems (FTS) Research Group believed fiber-optic technology had reached a sufficiently practical level to propose a program for developing fiber-optic distribution of frequency and timing signals throughout DSN Complexes. With fiber-optic technology, the number of expensive hydrogen masers in a complex could be reduced to one plus spares. Also, from a central location such as Goldstone, frequency and timing could be provided at an unlimited number of antennas and at other locations in the complex. Fiber-optics could supply frequency and timing at least as well as a hydrogen maser at each location could, and with better interstation stability. In addition, fiber-optic links could easily carry all the interstation communications and eliminate the need for microwave links, which suffer from frequency allocation, reliability, and stability problems. Also, fiber-optic links consume much less energy than microwave links do, and they require no frequency allocations.
In 1979, the Advanced Systems Program provided initial funding for an in-depth study of state-of-the-art fiber-optic technology, which included the procurement of a laser diode, photodiode, and optical fiber cable to fabricate a basic system. Thermal coefficients of delay were determined for prototype single-mode fiber cable. The results of this study were promising, and in 1980 the Advanced Systems Program began long-term funding of fiber-optic research and development to meet future DSN needs. Over the first nine years the funding averaged approximately $250k per year, but has declined to half that as the technology has matured.
Initial funding by the Advanced Systems Program provided for installation and test of a 3-km fiber link at JPL, and in 1980, this link demonstrated its suitability for transmission of hydrogen-maser signals.
In 1981, the FTS Research Group designed and fabricated the first single-mode 1300-nm analog fiber-optic link using the first such lasers produced in the United States. Multimode fibers are not suitable for wideband signals because of velocity dispersion among the different modes. This link demonstrated the first fiber-optic transmission of a 1.25-GHz bandwidth analog signal to duplicate the function of a microwave link.
In 1982, the FTS Group installed the first fiber-optic link in the DSN connecting DSS 13 with DSS 12, a distance of 7 km. The plowed-in buried cable contained six fibers, two of which were among the first single-mode fibers produced by Corning Glass -- the other fibers were multimode. The FTS Group also worked with the Telecommunications and Data Acquisition (TDA) Office and the Quality Assurance Section to develop fiber-optic cable specifications so such systems could be deployed in the DSN.
In 1984, a measurement of the frequency stability of the round-trip fiber-optic link between DSS 12 and DSS 13 gave the result of 1 ´ 10
-14 for 1000 seconds averaging time without the expense of active cable stabilization (which can provide orders of magnitude of additional improvement in stability). In the absence of any available microwave-link frequency allocation, and to meet a spacecraft project deadline for a link between Goddard Spaceflight Tracking and Data Network (GSTDN) and SPC 10, a fiber-optic link was installed and made operational in 90 days. This illustrated the rapid maturing of fiber-optic technology in the DSN with the support of the Advanced Systems Program. Also in 1984, the FTS Group implemented several fiber-optic links to meet unique requirements at the JPL Oak Grove facility. These links included two matched-delay 45-Mb/s fiber-optic links to support synthetic aperture radar work, another 45-Mb/s link to support image processing, and several fiber-optic video links in the Space Flight Operations Facility (SFOF) to provide improved performance with cost savings.
In 1986, the frequency stability of the link between DSS 12 and DSS 13 was improved to 1 ´ 10
-15 over 1000 seconds to meet the needs of connected-element interferometry. This improvement was achieved without cable stabilizers by controlling reflections in the fiber. During 1986, the FTS Group planned and established the fiber-optic backbone system for the Goldstone complex. This system was based on a multifiber cable running from the Venus Station to the Mars Station (29 km) via the Echo and Apollo stations and involved a total cost of $500k (not provided by Advanced Systems Program funding).
In 1987, the Goldstone fiber-optic backbone installation was completed and an additional 12-single-mode-fiber cable was installed between DSS 12 and DSS 13. The FTS Group designed, fabricated, and tested fiber-optic terminals for frequency distribution between SPC 10, DSS 13, and DSS 12. An improved cable stabilizer was designed, fabricated, and tested. Theoretical work was completed on the cause-of-delay change in fiber-optic cable under flexure (as in an antenna "wrap-up"). It was found that most of the change in delay was due to reflections into the transmitter laser and not due to the fiber directly, as had previously been believed.
Subsequently, based on the new understanding of delay change resulting from fiber flexure, a commercial optical isolater was developed. A transmitter using the isolater was designed, fabricated, and tested at Goldstone, where fiber-optic cable delay variation due to flexure was virtually eliminated, thus opening the way to the use of fiber optics in antenna wrap-ups and other moving environments. Based on a frequency and timing study under the Advanced Systems Program, it was decided that hydrogen masers would be installed only at SPC 10 rather than at several locations. And frequency and timing reference would be distributed throughout the Goldstone complex by fiber-optic technology. This new technology also eliminated the need for moving radio-frequency equipment (e.g., the radio frequency interference trailer) to various locations at the Goldstone facility. Essentially, the interference signal was transmitted over an optical fiber to the trailer, rather than the trailer being moved to the location experiencing interference.
Also, in 1988, the Advanced Systems Program, through the FTS Group, funded a private company with $30k for a best-effort development of a commercial, isolated laser-diode transmitter. Although considerable work was done, the funding was not adequate to finish the task. Fortunately, money from another sponsor allowed the development to be completed, and the product made the company, Ortel, the world's leader in commercial analog fiber-optic systems, which are now used extensively in cable television.
In 1989, the FTS Group solved the problem of direct transmission of microwave signals over optical fibers by modulating the amplitude of the optical signal. This solution made it possible for all radio receiving equipment except the radio-frequency amplifiers to be removed from the antenna site , thereby enabling a future redesign of the radio system in DSN complexes. The initial work resulted in high-dynamic-range ultralow-noise fiber-optic links for microwave signal transmission at frequencies up to 20 GHz, with virtually no degradation. With later improvement in optical modulators, the range has been extended to include Ka-band.
In 1991, a method was developed using a fiber-optic link to measure the stability of a 34-m BWG antenna. The 14-GHz signal from a small adjacent reference antenna was carried over a fiber-optic link to the area of the feed of the large antenna, where the two signals were compared in phase. The 150-ft fiber-optic link had a nighttime stability of about 1 ´ 10-16 over 1000 seconds. The system was able to measure and verify antenna phase stability to the 1 ´ 10-15 level.
In 1993, a study of fiber-optic code-division multiple-access systems for potential use in the DSN was completed. A significant contribution was also made to a study of large antenna arrays. Also, a 12-km optical-fiber link was placed between the X-band output of the low-noise amplifier at DSS 13 and the downconverter. Station personnel measured the receiving system parameters and could not tell the link was there. The Magellan spacecraft was tracked with the link in place.
Fiber-optics and laser technology continue to evolve and open new options for DSN instrumentation. In 1994, a high-stability all-photonic microwave/millimeter-wave oscillator was developed. Photonic mixing can now be achieved by heterodyning a pair of lasers and using electro-optic modulators and fast photodiodes with responses into the millimeter wave region. This technology has supported the demonstration of one-step up or downconversion between microwave or millimeter-wave frequencies and conventional IF frequencies, where the converter inputs and the outputs can be either optical or electrical signals.
As a result of the long-term support of fiber-optic/photonic technology by the Advanced Systems Program (described in part above), there is now available for the DSN a technology that affords important cost savings and improved performance. The cost savings apply to both implementation and operation. By the nature of technology development and proof, the early applications in the DSN tended to be on a piecemeal basis, for example, merely replacing existing coaxial cables and microwave links with optical fiber. For the future DSN to fully realize the benefits of this technology, fiber-optic/photonics technology must be an integral part of DSN systems design. The resulting new design may well look quite different from that of the present DSN.
Many of the steps on this chart result from "cooperative" changes on the part of both the DSN and the spacecraft. Coding, for example is applied to the data on the spacecraft and removed on Earth. A change in frequency has resulted in some of the larger steps shown, by causing the radio beam from the spacecraft to be more narrowly focussed. Such change necessitates equipment changes on both the spacecraft and on Earth.
Other steps represent advances that are strictly spacecraft related, such as increases in return-link transmitter power or increases in spacecraft antenna size, which improves performance by more narrowly focussing the radio beam from the spacecraft.
Still other steps depict improvements strictly resulting from the DSN, such as reductions in receiving system temperature or increases in the size of the ground antennas, or the use of arrays of antennas, which increase the effective surface area for collecting signal.
The DSN Advanced Systems Program has contributed directly to all of the changes that are DSN-only in nature and has made possible the DSN contributions to the cooperative steps. If one considers coding developments as a contribution from the DSN arena, since all the coding work has been led by the DSN ASP researchers, then the technologies of the DSN have contributed 3.5 decades of the 12 decades of the growth shown. The remaining 8.5 decades have resulted from a combination of purely spacecraft developments and the cooperative frequency increases.
The logarithmic scale used to display the data rate gives the impression that the early improvements are more significant than the later improvements. This is because the steps represent fractional or percentage increases, rather than incremental increases. The latter would show the actual data rate increases which are much larger in the later improvements. If the value of the data were proportional to the amount of data, then the display of the incremental increases would be more meaningful than the logarithmic display.
--> Next SectionAdvanced Systems Program and the Voyager Mission
Like other outstandingly successful deep-space missions before and after, the Voyager mission to Jupiter, Saturn, Uranus, and Neptune and their moons and rings depended on the unique capabilities of the DSN, including the capability of arraying with non-DSN facilities at Parkes (Australia), the VLA (New Mexico), and Usuda (Japan). Many of these capabilities using technology previously provided by the Advanced Systems Program were in place at the time of the two Voyager spacecraft launches in 1977. However, other capabilities needed for the Uranus and Neptune encounters 9 and 12 years later were not available in the DSN in 1977 and, subsequently, came from technology provided by the Advanced Systems Program.
Advanced Systems Program and the Mariner 10 Mission to Venus and Mercury (1973-1975)
The Mariner 10 mission by design and, seemingly, by afterthought and accident pushed the capabilities of the DSN to its limits. Some of these capabilities were achieved only with developmental hardware and systems from the Advanced Systems Program--sometimes on rather short notice. Mariner 10, launched on November 3, 1973, and completed shortly after the third Mercury encounter on March 16, 1975, had many significant achievements, some of which are listed here.
Fiber-Optic/Photonics Technology
The introduction of fiber-optic technology into the DSN has resulted in cost savings and performance improvements that continue year after year. The addition of new fiber-optic/photonics technology in overall systems design will further increase cost savings and performance.
Development of Optical Fiber Use in the DSN
Fiber-Optic Research at JPL
Telecommunications Performance
One measure of the technological progress in deep-space communications may be seen clearly in the accompanying chart, which is informally identified as the "stair-step chart of telecommunications performance." The chart shows the growth in potential data rate for a space-to-Earth return link from Jupiter, through the years, and has appeared in a number of publications. The timeline begins in 1960 and attempts to forecast up to the year 2020. Actual Growth through 1995 is more than twelve orders of magnitude and includes contributions from both ground and spacecraft technology evolution.
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