Computers in Spaceflight: The NASA Experience

- Chapter Eight -
- Computers in mission control -
 
Umanned mission control computers
 
 
[258] Mission control of unmanned spacecraft is significantly different from that of manned spacecraft. Most important of the differences is the long duration of many unmanned flights. Except for Skylab, no American manned flight has lasted more than 2 weeks. In contrast, when Voyager 2 encounters Neptune in 1989, it will have flown for 12 years. During that time, the Voyager Project staff must monitor the health of the spacecraft and gather and interpret the data it is collecting. Few of the original engineers will still be associated with the mission, so conceptually mission planning for a long-duration unmanned flight must concentrate on an extended view of operations and the development of detailed documentation69. Another difference is that the manned mission control centers are used for one project at a time, whereas the unmanned centers may be controlling a wide variety of missions. So far, there has been no overlap in the manned programs in the sense that no Mercury flights continued after Gemini flew, and so on. In contrast, the Jet Propulsion Laboratory (JPL) commanded Surveyors, Lunar Orbiters, Pioneers, and Mariners all at once in the mid-1960s, and has continuously been responsible for multiple missions.
 
 
Control of Near-Earth Missions at the Goddard Space Flight Center
 
 
NASA formed Goddard Space Flight Center with the Naval Research Laboratory's Vanguard Project team as a nucleus. After Vanguard ended, use of the IBM 704 in downtown Washington ceased, and a model 709 was installed at Goddard on May 23, 1960, as a replacement machine for use in working with earth-orbiting satellites. Within 2 months, the first of six 7090 computers also arrived. Folklore has it that Goddard soon housed 1% of the total computing power in the entire United States. Although two of the 7090s and later [259] other computers supported Mercury flights, Goddard's most substantial customer base has been the plethora of scientific, navigational, communications, mapping, and weather satellites launched in the last quarter of a century.
 
Goddard pioneered the use of dedicated small computers for specific missions, thus eliminating the complexity of handling multiple missions on a single mainframe. This occurred in spite of the presence of large numbers of big computers. Some command and control and definitely navigation calculations are carried out on large machines, but each project has a small computer to handle data reduction and the day-to-day operation of the spacecraft. As examples, the Nimbus weather satellite program used Control Data 160A computers, the Orbiting Geophysical Observatory had Scientific Data Systems SDS 910 and 920 computers, and so did the Orbiting Solar Observatory70. These machines could be sent on to another project when their current job ended, and in fact some of the SDS machines had rather long lifetimes of nearly two decades. In addition to using small computers at the control center, Goddard installed UNIVAC 1218 computers in the Manned Spaceflight Network ground stations, originally for control of Gemini and Agena and later for Apollo. Both the 160As and 9105 were among the first products of their respective fledgling companies, and, with the 1218 and Digital Equipment Corporation's PDP series, the forerunners of the minicomputer boom of the 1970s.
 
Relatively little changed in the general techniques of mission control at Goddard for about two decades. As the 1980s continue, the trend is for the majority of unmanned satellites to be commercial rather than scientific in nature. Commercial satellites are controlled by their owners, although NASA provides orbit determination and some command services on a reimbursible basis. However, sufficient missions exist, such as the expected 17-year duration Hubble Space Telescope, to keep Goddard involved in ground control activities for some time, along with its continued commitment to NASCOM and STADAN.
 
 
To the Sky's Limit: Mission Control at JPL
 
 
As Goddard strove to standardize earth orbital operations and distribute its functions, JPL approached the similar problems in a different way, centralizing operations as much as possible. In many respects, Goddard and JPL are fraternal twins. Each has a set of ground-tracking stations, plus on-site control centers for a variety of missions. The difference is that JPL is responsible for deep space exploration. In fact, the lower limit of its responsibilities is set at 10,000 miles. For a short period, it did satellite work. JPL developed the [260] guidance system and propulsion for the Sergeant battlefield missile and studied adapting clusters of the motors as upper stages to the Redstone missile. The resulting Jupiter-C launch vehicle put America's first satellite into orbit on the night of January 31, 1958. Called Explorer I, the satellite carried JPL-developed instrumentation. A room near the office of laboratory director Dr. William Pickering became an active unmanned mission control center since it contained communications equipment connected to the tracking network that confirmed Explorer reached orbit. That same year NASA was formed and JPL became closely affiliated, changing its mission to deep space work.
 
In 1959, the early Pioneer flights aimed at the moon. JPL built a series of tracking stations, beginning at Goldstone in the high desert of California, to track the missions71. Unlike earth orbiters, whose closeness to the planet make it necessary to have a large number of stations to stay in contact, deep space probes needed only three stations spaced so that one would always face the spacecraft. Initially, the stations were located in Australia and South Africa as well as at Goldstone, but later one in Madrid replaced the African station and the Australian one moved from Woomera to Canberra72. The stations were collectively named the Deep Space Instrumentation Facility.
 
From the beginning, JPL considered using computers in the stations as data-gathering devices. One 1959 report suggested using IBM 650 machines, which were small computers73. In 1962, Dr. Eberhardt Rechtin, head of the Instrumentation Facility, sent Paul Westmoreland and Carl Johnson to evaluate the computers of Scientific Data Systems, a new company74. Westmoreland and Johnson thought that the SDS 910 could be used as the data gatherer, with the slightly more powerful 920 as a data processor. Accordingly, Rechtin directed that the machines be ordered and got the first 920 built and the second 910. The 910s and 920s still functioned in similar tasks as late as 1985!**
 
Functionally, the SDS computers took data received from the spacecraft and formatted and recorded it on magnetic tape. A computer at JPL processed the data more completely. Initially, an IBM 704 similar to the one used for Vanguard did the work. JPL installed the computer in late 1958 to use with Pioneer 3 and 475. Early Ranger lunar impact flights later had all data reduction done off tape on that machine. Data in analog form on the tapes would be translated into numbers that spewed out on teletypes and punched paper tape. Aerojet-General Corporation also owned a 704 that JPL used as a backup76.
 
[261] Planning for the first Mariner missions revealed that more computing power would be needed at JPL to handle the increased data generated both by more instrumentation and longer mission lifetimes. Dual 7090 computers similar to those installed at Goddard were bought for data reduction. To provide flight controllers with more up- to-date information about spacecraft telemetry, a Digital Equipment Corporation PDP-1 computer served as a near-real-time data processor. Data could be displayed on teletypes from 4.5 to 7 minutes after it was received77. By this time the Deep Space Instrumentation Facility could transmit data via NASCOM instead of having to wait for airmail to deliver the tapes. Operations with this equipment taught JPL at least one useful lesson: Power fluctuations in September and December 1962 caused both 7090s to go down at once, eliminating the redundant capability78. As a result, JPL built an auxiliary power generation facility, perhaps leading the manned Mission Control Center, under construction at this time, to do the same.
 
 
Centralizing the Effort
 
 
During the 1960s, NASA found itself about to be involved in a large number of critical deep space projects. Ranger would be followed by the Surveyor series of lunar landing missions. Mariners would continue to fly to Venus and Mars, with several targeted for Martian orbit and imaging duty. Lunar Orbiters would look for Apollo landing sites and Pioneers were aimed at deeper space. JPL did not have primary responsibility for all of these programs. Lunar Orbiter came from Langley Space Flight Center, and Pioneer from the Ames Research Center. If each responsible organization had to set up a control center for its spacecraft, considerable overlap and duplication would occur. Accordingly, in 1963, NASA decided to have JPL track and command all deep space missions, with the help of project personnel from home centers stationed at JPL79. On December 24, 1963, JPL's director William Pickering formally established the Deep Space Network80. Managed by William H. Bayley, with Eberhardt Rechtin as technical head, it would serve all of NASA, just like NASCOM did from Goddard81.
 
JPL was already building a Space Flight Operations Facility to house new, more powerful computers and the various teams from its own projects. Anticipating NASA's decision, Eugene Giberson, then of the Surveyor project, directed some of his money to help develop the centralized computer center82. The combination of the Operations Facility and the Deep Space Instrumentation Facility was the Deep Space Network. After opening on May 15, 1964, the Operations Facility supported Mariner Mars 1964 as its first flight83.
 
[262] Even though expected to handle all deep space missions, some organizations fought to retain mission functions. Ames set up the Pioneer Off-Line Data Processing System (POLDPS) in 1965 to handle non-real-time data recorded by the SDS 910s at the stations84. Both the Lunar Orbiter and Surveyor projects also wanted to record their telemetry data at the stations, so the Network bought dual SDS 920s for each site. Later, Pioneer 10 and 11 data were processed with these systems85. Langley originally wanted to control the Viking Lander, but costs and common sense forced that job back to JPL.
 
 

Figure 8-3.
 
Figure 8-3. The Space Flight Operations Facility central control room at the Jet Propulsion Laboratory. (JPL photo P3358BC)

 
 
Evolution of the Space Flight Operations Facility
 
 
JPL's Space Flight Operations Facility has had three generations of equipment. Beginning in 1964, two strings of solid-state computers formed the basis of the system. Each consisted of an IBM 7094 mainframe, an IBM 7040 medium-sized computer, and an IBM 1301 disk storage system placed between them. Later, a trio of System 360/75 computers replaced this configuration. More recently, the control center adopted a distributed computing strategy similar to Goddard's.
 
[263] As in the manned programs, during critical mission phases both strings of the original generation of equipment would be running at the same time but with the data from the stations only routed to one of them. If a 7094 failed, its associated 7040 could be connected to the other 1301 (and, thus, the second 7094), leaving the second 7040 as another layer of backup86. Later upgraded to a 7044, the smaller computer acted as a traffic cop on the incoming data. All inputs (teletype, telephone, microwave) went to the machine before they went anywhere else, and the software in the 7040 routed the data to active programs, inactive programs, or administration stations87. George Gianopolis of JPL, one of those charged with the responsibility of getting the system to work, remembers that the 7040s were especially difficult to install88. The 7040s deposited data on the 1301 disk storage system. A 54-megabyte hard disk, the 1301 served both the 7040 and the 7094 from the middle, so both could access data at identical addresses. This concept presages the network file servers in the modern office and the Common Data Buffer in the Launch Processing System. Airline reservation systems and other large data base operations utilized the same configuration beginning at about the same time. Using a smaller computer to handle resource-hungry input and output tasks and a common storage area is a standard network concept today. As for the 7094, the flight operations director could control its use by "percentage time sharing" in which higher priority jobs simply got more machine time89. The primary functions of the 7094 were telemetry analysis, tracking, predictive work for the stations, and maneuver commanding. UNIVAC computers in the JPL institutional computer center did the navigation calculations as batch jobs, separate from the Operations Facility computers90.
 
Although a powerful system, the 7040/7094 combination had to stretch to meet mission requirements. Upgrading it to 7044/7094 Model II status helped some, but the system could handle only a Mariner mission (two spacecraft) or a Surveyor but not both91. Surveyor project officials even had to add a PDP-7 as a front end computer to the front end computer, putting it between the stations and the 7044 and driving strip chart recorders92. More assistance came daring the Mariner Project when engineers realized the UNIVAC 1218 computers used in preflight testing of the spacecraft could also do engineering telemetry analysis93. This was not done until Mariner Mars 1971. Soon, though, the acquisition of more powerful 360 series machines ended the reign of the 7094s.
 
 
Monolithic Computer Systems
 
 
[264] In October of 1969, JPL installed its first System 360/75, a gift from the Manned Spacecraft Center, where it was considered surplus. A second machine arrived in April 1970, this one left over from the demise of NASA's Electronics Research Center in Cambridge, Massachusetts. JPL bought a third machine, which survived until August 198394. Each 360 had 1 megabyte of primary core storage and 2 megabytes of LCS, half that of an Apollo-configured machine95. Two of the 360s controlled missions as a redundant set, with the third used for development work. A special switch connected the 360s to the institutional UNIVAC 1108 mainframe computers so that tracking data could be directly transferred for use in navigational computations96. But the gift from Houston was not entirely welcome at JPL, for along with it came the Real-Time Operating System (RTOS) developed by IBM for the Apollo program. As Gianopolis saw it, "what we picked up from Houston was good for Houston, but not necessarily for us"97.
 
Unmanned spacecraft missions needed to create large data bases capable of handling the long series of telemetry signals that might go on for months or years. IBM's RTOS tried to keep all data in core memory, using disk storage as read-only devices. JPL needed to be able to write to the disks. Also, each Apollo computer concentrated on real-time functions and did not do development work. JPL wanted to run FORTRAN jobs on the machine, but RTOS could not handle it98. A crisis of sorts arose with the Mariner Mars 1971 orbital mission. During the cruise period to the planet the ground software failed every 5 hours. By the time Mariner reached orbit around Mars, the failure rate fell to once every 20 hours99. Still, something had to be done, so JPL contracted for an overhaul of the operating system, culminating in 1972 with the JPL/OS, which incorporated the needed changes.
 
Since the 360s lacked a small computer for a front end (original thinking being that the machine could handle the load by itself), JPL implemented the idea of using the preflight testing computers in mission support for Mariner Mars 1971100. Incoming telemetry went to the UNIVAC 1230/1219 set first. Then the 360s did commanding, tracking data evaluation, predictions for the stations, and engineering work. Besides the UNIVAC test set, the UNIVAC 1108s provided navigational data and, by then, the Image Processing Laboratory at JPL had its own 360/44 for processing planetary imaging101.
 
Viking, a much more complex project than Mariner, and with essentially four spacecraft (two orbiters, two landers) to control, stretched the 360s and their helpmates to the limit. JPL assigned the small UNIVACs to handle the Viking Orbiter data, since the spacecraft were built and tested at IPL and the software was in place. System 360s controlled the landers102. At peak, 700 controllers [265] worked the Viking mission, more than on any other space program to date (The count was double that of Skylab, the largest manned support group)103. Facing dual Voyager missions and Galileo, with the prospect of continuing Viking far past the original mission life estimates, JPL was again looking for a way to upgrade mission control.
 
 
Distributed Computing Becomes the Answer
 
 
As JPL discovered planning the computers for the Voyager onboard systems, functionally distributed small computers offered more reliability and cost savings than large single processor systems. The Laboratory implemented a distributed system to fill its Voyager ground control needs as well. Viking was the last mission to be supported from a large mainframe computer. By the time Voyager neared Jupiter, two strings of dedicated minicomputers performed the telemetry, tracking, and engineering monitoring functions. A single minicomputer shared by several projects did the commanding. Why did the change occur? First, the Deep Space Network was unhappy with the level of support it derived from a centralized system. Second, even though centralizing deep space mission control at JPL was a sound idea, putting too many missions on a single computer system was less so. No matter how much JPL tried to standardize things, each mission had its unique characteristics, calling for changes in the support software. With a distributed system, changes could be made without affecting other software. When missions neared critical phases, such as launch or encounter, software had to be frozen until the phase passed. With enough spacecraft aloft, the amount of time available to change software became quite short104.
 
NASA provided an additional impetus to switch to a distributed system. Acknowledging the Deep Space Network's concern over using the 360s in the JPL control center and worried that the Network could not monitor its performance when supporting projects originating at other centers (such as Pioneer), the Agency directed the Network to develop monitoring capability in separate computers. Between 1972 and 1974, a set of ModComp 2 minicomputers was connected in a local area network at JPL to implement this directive105.
 
In 1976, the control center itself converted from 360s to ModComp 2s and 4s in preparation for Voyager. Later the Laboratory added ModComp Classics and retained some of the UNIVAC 1218s and 1230s (renamed 1530s after upgrades)106. These computers are arranged in redundant sets. Each project (Voyager, Galileo, etc.) has its own telemetry machine and shares a command machine. A routing computer in the basement of the Space Flight Operations Facility building is the entry point of all data from NASCOM, sending the data to the appropriate computer. The command computer reverses the process for outgoing data.
 
 
 
[
266]
 
Figure 8-4.
 
Figure 8-4. A schematic of the components of the Space Flight Operations Facility. (JPL 333-660)

 
 
[267] By the early 1980s, the Deep Space Network was heavily into distributed computing. It converted from the 920s to ModComp 2s at the stations and ordered three Digital Equipment Corporation 11/780 VAK superminicomputers for use at JPL. Nearly 100 minicomputers were connected on an Ethernet. The use of high-level languages became the rule rather than the exception107. Key to the future success of the Deep Space Network is the inherent flexibility of distributed computing centers. They mirror the use of modules in software: interchangeable parts in a changing field.
 
 
Software Development in the Deep Space Network
 
 
Software development for the control center and the stations has always been a challenge, as programmers have struggled to use machines built primarily for commercial use in the arena of real-time control. In keeping with the centralization of the computers in 1963, the original software developers worked under Frederick Lesh in a program analysis group"108. JPL separated software development for mission operations from that of the network stations just before Mariner Mars 1969109. Also, at that time emphasis began to be placed on making the software more parameter-based and, thus, more flexible and capable of use on multiple missions110. A new management concept led to the assignment of a program cognizant engineer to each software system engineer. The software engineer would define requirements, prepare test cases, and oversee the program engineer, who would produce the code. This turned out to be quite successful and avoided the difficulties encountered when an engineer thought (wrongly) that he could do both jobs alone111. In microcosm, this is the "outside verification" concept used extensively in programming now.
 
Martin-Marietta Corporation, the Viking Lander contractors, had to do some dangerously unique software development when NASA decided to move control of the Lander from Langley to JPL. Since Orbiter software development and giving support to other missions tied up JPL's computers, Martin took the chance of developing the Lander software in a "minimal higher order language," specifically a hope- fully transportable subset of FORTRAN. Martin's solution reflected its recent migration to IBM 370 series and Control Data 6500 series computers at its Denver plant. These were technologically more advanced than the IPL computers and could not be trusted to produce directly transportable software112. The idea worked, but Martin admitted that the requirement for delivering mission support software 10 months before the flight provided strong motivation113.
 
 

[
268]
 
Figure 8-5.
 
Figure 8-5. The 64-meter antenna of the Deep Space Network at Goldstone, California. (JPL photo 333-5967BC)

 
 
As JPL moved to a distributed system, a concerted attempt at establishing software standards has resulted in a state-of-the-art set of documents114. Based on structured programming and software engineering principles, these documents and the decision to use more high-level languages such as HAL, C, and Pascal make the Deep Space Network one of the most sophisticated software organizations within NASA. A further decision to no longer change commercial operating systems (possible now that computers are more general purpose), will help ensure continued cost reduction and consistency115.
 
Mission control is the most computer-intensive part of spaceflight operations. From the beginning of both the unmanned and manned programs, the computer industry has been constantly forced to stretch the capabilities of both hardware and software in order to meet NASA's needs. In this way, NASA was a driving force in the development of multiprocessing operating systems and large computer complexes.
 

** After 1968, the SDS machines were known as XDS 910 and 920. Xerox bought out SDS and renamed the products "Xerox Data Systems."

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