*Vega Group PLC, United Kingdom
** Directorate of Technical and Operational Support, ESOC, Darmstadt, Germany
Although the Ulysses mission is currently only one third of its way through its second orbit of the Sun, it is worth remembering that this very successful mission was launched 7 years ago. Many current and future missions with ESA participation (e.g. Hubble Space Telescope, ISO, SOHO, Cassini/Huygens, Rosetta) have one or more of the following in common with Ulysses: interplanetary-type trajectories; missions of long duration; multidisciplinary science; cooperation with other international space agencies. It would therefore seem prudent to pass on some of the lessons learned in these areas from the first 7 years of Ulysses operations.
The initial idea for a spacecraft which would orbit the Sun's polar regions was first proposed in 1959 by a group of interdisciplinary scientists, many of whom later became investigators with payloads on Ulysses. However, it was not until the early 70's that propulsion and mission design technology had advanced to the stage to make such a mission possible. In the mid-70's, Pioneer 10 and 11 visited Jupiter and Saturn, thus proving the concept of using planetary bodies to provide a gravitational 'slingshot' to spacecraft as they flew past. This allowed mission designers to plan orbital trajectories to greater distances and inclinations than would be possible by conventional means alone, and within reasonable timescales. The Ulysses mission, being a joint collaboration between ESA and NASA was initially planned as two spacecraft, one from each agency. Ulysses had to survive programme cuts, mission re-designs, programme cancellation and resurrection, and the Challenger disaster before finally being launched on 6 October 1990.
The goal of the Ulysses mission was to fly over the Sun's northern and southern poles at solar latitudes above 70°. This could only be achieved by first sending the spacecraft on a fast 16-month trajectory to Jupiter, using the planet's gravitational force to divert Ulysses into a new, high-inclination orbit around the Sun (see Fig. 1).
Figure 1. The Ulysses orbit
What has the Mission Operations Team gained from dealing with the challenges presented to them by Ulysses? This section summarises the many and diverse lessons learned so far.
Launching a spacecraft out of the sphere of the Earth's influence brings with it a host of new considerations for spacecraft operations. The effect of the deep space environment on spacecraft subsystems (especially power, thermal and TT&C), as well as the ground segment and planning and scheduling functions must be thoroughly understood. The major drivers are as follows:
One Way Light Times (OWLT)
At the maximum geocentric range of about 6.3 AU, Ulysses would experience OWLTs of 53 minutes. Traditionally, ESA missions use Flight Operations Procedures (FOP) to control spacecraft, but such long OWLTs make the use of FOPs very difficult. Procedures become very elongated, especially if commands have to be confirmed before proceeding further. While Ulysses daily operations were initially based on a set of FOPs, they have evolved into a daily command form based on a set of routine operations. Any periodic platform operations or experiment command requests are then added to the daily command form as required.
Another effect of the OWLT is a reliance on onboard safety and protection logic. Spacecraft rely on autonomous monitoring for safe operation of the power or attitude control subsystems. An additional concern for Ulysses was the thermal safety of the spacecraft, especially for the propellant in the Reaction Control System. During major power reconfigurations, thermal changes can occur quite quickly and reaction times, extended by the OWLT, can be long enough to allow temperatures to rise or fall outside of safety limits.
Power and thermal subsystems
In deep space the power and thermal subsystems will provide many, or perhaps all, of the mission drivers. Typically, the solar range changes greatly during the mission causing a large variation in the solar flux, thus affecting the thermal control of the spacecraft. Spacecraft which use advanced solar arrays for power will still receive reduced solar energy at large solar distances due to the 1/R2 effect. While providing power independent of solar distance, an RTG has a fairly low output (for Ulysses just 285W at beginning of mission) which decays, albeit in a fairly predictable fashion, throughout the mission. Either way, available power is a precious resource requiring careful allocation and management. Many passionate and energetic discussions took place in Ulysses meetings over the matter of a couple of watts of electrical power!
Telecommunications in deep space
Communications over large geocentric distances require a detailed understanding of the link budget. The Ulysses Mission Operations Team has frequently had to predict future link budgets and continuously monitor the performance of the TT&C subsystem.
Due to its trajectory and fast passage to Jupiter, Ulysses achieved large velocities relative to the Earth. This affected the uplink and downlink frequencies due to Doppler shifts. When difficulties were experienced in commanding Ulysses on 1 February 1991, it took a concerted effort by ESA spacecraft engineers and NASA ground segment engineers to find the exact adjustments needed to compensate for the Doppler effect. Partial commanding was regained after a few days, and full commanding capability was restored on 12 February.
The importance of the ground segment - a shared resource
NASA's Deep Space Network is a heavily used resource supporting many spacecraft in flight, launch campaigns, Space Shuttle missions and even ground-based observations such as high-powered radar and Very Long Baseline Interferometry.
Timely scheduling of antenna time is therefore critical. The use of shared resources demands early inputs by its users. Conflicts have to be identified early on so that solutions can be found and agreed upon.
At interplanetary ranges, the ground segment has a large effect on the planning, scheduling and execution of operations. The Mission Operations Team had to understand the influence of the ground segment on spacecraft operations. Some examples were:
Having to share resources with other missions (no dedicated antenna) means that there can be an operational impact from the ground segment. Examples include:
Operational flexibility is needed to cope with such resource losses. For example, in June 1993, a critical bearing of the DSN Standard 34 metre antenna in Madrid (DSS 61) failed. The resulting repairs lasted until mid-August and meant that the DSN had to serve its community with reduced capability. Even with additional coverage provided by the emergency use of the Weilheim 30 metre antenna in Germany, the Ulysses project lost a significant amount of tracking time. During this period, the rate at which the data was recorded on board the spacecraft was often reduced from 512 bps to 256 bps in order to avoids gaps in recorded data. Although this meant lower amounts of data overall, greater continuity of data was achieved, a trade-off the scientists preferred to make.
Long duration missions
Figure 2 details the Ulysses Mission Operations Timeline. Normally, after the Launch and Early Orbit Phase (LEOP), mission operations tend to settle down to a more routine nature, punctuated by the (hopefully) occasional anomaly. With Ulysses, there have been numerous activities to plan for and execute, many of them running in parallel. The LEOP phase, while very successful, uncovered the Nutation anomaly. Recovery from this and the subsequent investigation had to be balanced with the preparations for the Jupiter Flyby. Post-Jupiter checkouts revealed a significant anomaly in the redundant Central Terminal Unit (CTU2) which required further investigation and analysis. This began to overlap with efforts for the justification of a second orbit, swiftly followed by preparations for the first set of Solar Polar passes, and the associated return of Nutation.
Figure 2. Ulysses mission operations timeline
In a mission of such long duration, this busy and varied level of activity has been good for the Mission Operations Team. The interest level and the workload have remained high and this is reflected in the great morale and enthusiasm
of the team, and the low staff turnover. One example of the benefits of having a well motivated team can be seen in the consistently high data return for the mission, shown in Figure 3.
Figure 3. Ulysses mission data return
The stability of the team has benefitted the mission as a whole since mission-specific expertise and knowledge is retained. This tends to reduce the number of times you 're-invent the wheel' when a problem arises.
Ulysses was designed to provide a continuous set of science data between solar latitudes of ±80° (Fig. 3). Ulysses has consistently met and exceeded its mission goals of an average of 95% data return, with 33% of the data being at the higher data rate of 1024 bps. This has been achieved through the combined efforts of the members of the ESA and JPL Mission Operations Team who have worked closely to optimise the data return, and the DSN for providing such good support.
Before launch, it was assumed that an eight-hour pass per day would be sufficient to return eight hours of 1024 bps data. Once Ulysses was flying, however, it became obvious that there were significant periods (particularly at the beginning of a track) when 512 bps was required during each track to ensure that data continuity was maintained. Hence, there was not enough time to play back recorded data using a bit rate which would also provide the required 1024 bps real-time data. The resulting reduction in science data return was tremendous. Therefore, the Science Working Team passed a resolution at its meeting in Heidelberg, Germany, in April 1991, urging the two agencies to find a way to restore the expected data return. This was quickly achieved by increasing the Ulysses tracking requirements to ten hours per day. As a result, Ulysses has provided the heliospheric science community with not only a unique data set, but one of outstanding quality and consistency.
Software and hardware issues
Due to the length of the Ulysses mission and the rapid developments in computer hardware and software, the Ulysses Monitoring and Control System (UMCS) was in danger of running outdated software on unsupported hardware. Effort has therefore been devoted to developing new monitoring and command software using modern hardware which will ensure compatibility until the end of the mission.
A unique data set
Ulysses is providing the heliospheric science community with a unique set of field, particle and wave data over the complete latitudinal range of the Sun, and over ranges of 1 to 5.4 AU. With the Second Orbit, it will also have studied the Sun over one full 11-year cycle, thus observing the Sun during its minimum and maximum periods of sunspot activity.
Some trade-offs may be necessary when a single spacecraft is providing a platform for various experiments. A large part of the preparations for the Jupiter flyby were dedicated to ensuring that there was enough power available to guide the spacecraft safely through the Jovian radiation belts, at a time when the spacecraft was at its maximum heliocentric range (i.e. it was cold) but the onboard experiments were expected to use their peak power levels.
Another consideration is that some instruments are more sensitive to data rate changes than others, depending on how they acquire, process and transmit their data.
Extra science opportunities may arise during a mission, which were not planned for or anticipated in the design phase. Ulysses was able to perform additional Radio Science observations during the Joint Gravity Wave Experiment in March-April, 1993 and the Fast Latitude Scan of the Solar Corona in February-March, 1995. The STO experiment was also configured to listen for radio emissions during comet Shoemaker-Levy's collision with Jupiter in July 1994.
While bringing many benefits to both ESA and NASA, the joint partnership involved in running the Ulysses mission has also brought with it some management challenges*. Means have had to be found for coping with:
* Ulysses - An ESA/NASA Cooperative Programme, W. Meeks and D. Eaton, ESA Bulletin No. 63, August 1990
Operational and Communication Challenges during the Ulysses Odyssey, P. Beech, 47th International Astronautical Congress 7-11 October, 1996 Beijing, China.
At the highest inter-agency level, the cooperative components of the mission were bound together by the Memorandum of Understanding (MOU) signed by both ESA and NASA. Each agency provided a Project Manager and a Project Scientist, who had their associated 'chains of command'. However, to be a truly cooperative project, ways had to be found for the two sides to work together not only at agency level, but at all levels. This has been achieved by the formation of a decision-making body and various types of regularly scheduled meetings:
This structure has evolved into an efficient way of dealing with the inevitable small problems which 'crop-up' from time to time in running an international, interplanetary spacecraft mission.
Another contribution to the smooth operation of the Ulysses mission was the fusing of the ESA and JPL staff into an integrated Mission Operations Team. This team is headed by an ESA staff member as Mission Operations Manager with a JPL staff member in the role of Deputy. Operational responsibilities are divided into spacecraft operations (headed by an ESA Spacecraft Operations Manager) and ground segment operations (headed by a JPL Ground System Manager). Their respective teams are drawn from each organisation.
Co-location of the ESA and NASA elements of the Mission Operations Team at JPL has been central to the success of the Ulysses Mission, thus far. While modern communications can go a long way towards helping international teams to work together, a single location enabled the ESA and JPL staff in the integrated Mission Operations Team to develop a close working relationship. This has frequently resulted in the fast resolution of real-time problems and a consequent improvement in data return.
The inclusion of staff assigned to Ulysses for mission scheduling of antenna and network time and for resource negotiation has been an important factor. In these days of the 'multi-mission' approach to so many services, dedicated staff who know the technical aspects of the mission and who are personally committed to the overall goals of the project have greatly benefitted Ulysses.
With so much resting on cooperation between international partners and the extensive use of shared resources, it was always clear that advanced preparations would be a key to mission success. However, a flexible approach was also needed to cope with the realities of real-time operations in a shared-resource environment.
For spacecraft operations this meant that requirements had to be defined well in advance in order to schedule the necessary ground-segment resources. However, allocations had to be continuously reviewed since activities could be impacted by events such as launches (which have higher priority), equipment failures and spacecraft emergencies.
This need for early preparation translated into a busy workload for the Mission Operations Team. In addition to ensuring that routine operations achieved mission goals, future preparations were also ongoing.
Miscellaneous lessons learned
The Mission Operations Team members were cross-trained to learn each other's skills. This gives greater flexibility in meeting operational support requirements, and eases staffing problems during holidays or illness.
Outreach has become an important 'product' of today's missions. It can range from creating a Web site* with public access, giving lectures at local schools and colleges, designing and staffing museum exhibits, to giving visitors tours of the workplace.
Continuing self-assessment has been a beneficial activity. Review your way of doing things - is there room for improvement? Are there any persistent or repetitive problems? Visit other operations teams and see how they do things - you'll either learn something new or confirm that you are doing things well!
Ulysses has proven to be very robust, enabling it to cope with severe anomalies such as Nutation and the CTU2 problem, and to be run by a small operations team. When needed, excellent post-launch support was provided by both the project and industry, especially for the resolution of major anomalies. The rate of occurrence of anomalies has markedly decreased since the first two years of operations, as shown in Figure 4, and the propulsion system has demonstrated remarkably low gas generation rates**. The reliability and longevity of Ulysses have given it the potential to operate for a second and possibly third orbit.
Figure 4. The decreasing trend in the number of anomalies means long-term reliability for Ulysses
** Inflight Performance of the Ulysses Reaction Control System, A. McGarry, W. Berry & D. Parker, Second European Spacecraft Propulsion Conference, ESA SP-398, May 1997.
After seven years of successful operations, the Mission Operations Team has become very familiar with Ulysses. Seeking to use those long round-trip light-times, waiting for command confirmations, more productively and with the luxury of hindsight, we have often discussed what we would like to see in a 'Ulysses-2'.
A simulator would be useful in order to practise new procedures, rather than peer review followed by live tests. The inclusion of a 'time-tag buffer' has allowed routine activities to be loaded ahead of time on the spacecraft, thus reducing the real-time operations load. However, a larger one would be useful. More telemetry parameters would be valuable, especially to assist in the monitoring of the RTG power and thermal subsystem, and in the analysis of autonomous operations. The removal of imaging capability during the early design of the mission (due to budget constraints) may have reduced public awareness of the Ulysses mission. The ability to play back data more than once from the tape recorder would provide the opportunity to recover some data losses.
Ulysses is already a very successful mission. Hundreds of papers have already been published in scientific journals, several publications have dedicated special issues to Ulysses science, and scientists from many other missions are using Ulysses data. Ulysses is now well into its second orbit and feasibility studies have indicated that a third orbit is not only possible, but would continue to provide new and exciting data for scientists, and would challenge the skills of the Mission Operations Team.