Figure 1. The SOHO mission elements
Articles in previous ESA Bulletins have dealt with various aspects of SOHO and its development (e.g. No. 71 in August 1992, and No. 82 in May 1995), and the accompanying SOHO articles in this issue describe the most important elements and features of the mission in more detail. We will focus here on management aspects and, in particular, the multiple roles of the Project Team in the evolving life and needs of an international cooperation such as SOHO, which embodies several facets that are unique in the history of the European Space Agency.
SOHO's complex multi-disciplinary payload, which included novel experiments in the then still relatively young solar discipline of helioseismology, was being conceptually developed in parallel. This parallel develop-ment provided the correct foundation and interfaces for the industrial studies and the further contractual steps that the Agency would need to take vis-a-vis industry.
The eventual outcome of all of the foregoing activities was an ESA Solar Terrestrial Science Programme, or STSP, which however exceeded the European budget allocated to it at the time of selection. The STSP Programme Manager and the SOHO and Cluster Project Managers therefore initiated a dialogue with NASA on the possibility of their contributing in terms of the scientific payload, the onboard equipment or other mission elements (launcher, operations, etc.). Certain cooperative elements were identified as early as 1986 and potential US industrial suppliers were visited by NASA and ESA representatives. This activity led to an inter-agency Memorandum of Understanding (MOU) defining the framework and principles of the proposed cooperation between ESA and NASA. This was followed by a lower-level Programme Plan laying out the details of the MOU's implementation (deliverables, schedules, management guidelines, etc.).
This agreement with NASA, together with the descoping and rationalisation of several aspects of the mission and its payload, brought SOHO and Cluster within the allowable financial envelope. This in turn cleared the way for the industrial tendering phase. In the meantime, the overall STSP payload had been selected in March 1988, also after extensive discussion and descoping under the aegis of two scientific committees led by Profs. H. Balsiger and D. Southwood. All of the elements were then in place to initiate a joint Invitation-to- Tender (ITT) to European industry.
These three industrial-policy precepts were also applied in the selection of the industrial subcontractor teams for SOHO and Cluster.
The submission of the industrial Phase-B proposal was followed by an extensive evaluation by senior ESA management of all cost and schedule, management and contract, product assurance, technical, and test-facility aspects. Phase-B then began in December 1989.
The selection of the industrial teams at lower levels was pursued through a series of proposal and evaluation cycles, with the close involvement of the two ESA Project Teams and their Prime Con-tractors. The result was a good matching of the final industrial organisation with the mandatory geographical return, with limited fragmentation of tasks and interfaces vis-a-vis what would have resulted from the application of geographical-return considera-tions to the two projects (SOHO and Cluster) separately.
Figure 1. The SOHO mission elements
Overall progress and the resolution of the inevitable technical problems along the way have been closely monitored both by the Project Team and our colleagues from ESTEC's Technical Directorate, in a classical 'shadow engineering' role.
On several occasions, commercial-confiden-tiality claims were invoked and the SOHO team had to strive to balance those constraints with sound follow-up and evidence of technical acceptability, finding ad-hoc solutions agree-able to all parties involved.
SOHO's test campaigns were conducted using the coordinated European test facili-ties, requiring interfacing with the IABG, Ottobrunn (D) and Intespace, Toulouse (F) facilities for the structural- model and flight-model test campaigns, respectively. In this phase, the Project Team received dedicated resident support from ESTEC's Test Division.
The most demanding interfacing effort, however, was undoubtedly that between ESA and the Prime Contractor in order to cope with the large and complex payload that was finally selected for SOHO. The high number of separate experi-ment units -32 in all-unavoidably brought delays in the definition of exact interfaces between the instruments and the spacecraft bus. A formal control over variations and changes to the Experi-ment Interface Document was established at the outset. Each Experiment Team request was evaluated by the relevant Project Team experts and, if found valid, passed to the Prime Contractor for comment before a final ESA decision on implementation.
One area in particular where some difficult decisions had to be made at certain stages was the management of the experiment mass budgets. Thanks to the early introduction of formal control procedures, the original payload allocation of 610 kg needed to be increased by just 40 kg by the time that the final flight models of the experiments were delivered.
Another area of difficulty in terms of experiment-interface management was the difference in standards between US experi-ment Electrical Models (EM) and the European equivalents. The generally lower electrical fidelity of the US models resulted in an EM test campaign somewhat lacking in EMC representativeness, and also led to a mismatch in the completion of development of these units. This had repercussions, particularly in the case of the UVCS instrument, on the evolution of the flight-model test programme.
Another element that required a considerable coordination effort was the handover of the structural and thermal mathematical models from the twelve Experiment Teams to the industrial team. As Agency deliverables, all of these models had to be checked by ESTEC specialists for consistency and compatibility.
The primary preoccupation of the ESA Project Team throughout these efforts was to provide for the maximum of scientific results from the mission whilst keeping costs and schedule under strict control. A good example in this respect is the work done on minimising the effects on short-term spacecraft pointing of moving parts of the spacecraft (reaction wheels, tape recorder) or experiments (mechanism actuation), the so-called 'ijter'. This was addressed by a Pointing Review Board, periodically attended by experimenters, industry experts and the ESA Project Team; during these meetings the results of analyses (initially) were discussed and further steps proposed. This open approach led to improved balancing of SOHO's reaction wheels, to a series of tests on specific experiments and, finally, to an end-to-end test on the flight model in January 1995. The result is a common consensus that all possible measures have been taken to make the jitter as low as is technically feasible within the cost and schedule constraints on the project, and compliant with the scientific objectives.
Another element of the SOHO programme which has called for substantial managerial effort has been the relationship with NASA. As in other cooperative programmes between the two agencies, no exchange of funds was foreseen and decisions affecting both parties had to be taken by consensus. Nevertheless, SOHO is the first such cooperative programme in which overall mission responsibility rests with ESA. This in turn has meant that the Project Team has had to interface with three separate NASA centres:
The first two interfaces, with LeRC and KSC, have followed the normal evolution of such relationships, based on progressively more detailed requirement and interface docu-ments. The Launch Vehicle Interface Control Document, for example, passed through several drafts before its publication as a formally configured document. The spacecraft mechanical qualification tests, the separation shock test and the final matching and mating with the SOHO-specific launch adapter, all of which were conducted in Europe, were typical examples of the excellent co-operation with LeRC and Lockheed-Martin, whose representatives were present to witness key launch-vehicle/payload compati-bility tests.
The services offered by NASA at KSC were also agreed through a series of Ground Operation Working Group meetings, at which all contributing parties from Europe and the USA were present. These resulted in an ESA-produced Launch Operations Plan and Launch-Site Support Plan detailing the complex allocation of tasks between the US Air Force (supplier of the launch pad), the contributing elements of the KSC system, and the NASA industrial contractors.
Finally, the multiple GSFC interfaces absorbed a considerable amount of time, particularly at management level, for the SOHO Project Team. The three US experiments are in fact among the largest and most complex of the whole SOHO complement and their progress was therefore closely followed in the Quarterly ESA/NASA Management Meetings that have taken place during Phase-C/D.
Another novel interfacing aspect has stemmed from the fact that ESA is in charge of a mission for which the operations support is provided by a team and a facility not under its full direct control. This has required specific and continuing attention, particularly in terms of the transfer of knowledge of the spacecraft systems from the Prime Contractor and ESA to the Flight Operations Team, constituted by NASA contractors from Allied Signal.
Mission and Science Operations Working Groups, meeting several times per year, have been the typical forum in which these exchanges have taken place. The main formal vehicle for such exchanges has been the Spacecraft Users Manual issued by Matra Marconi Space, which documents all necessary technical aspects of the spacecraft and includes those experiment procedures under the direct control of the Flight Operations team.
The critical cycle of familiarisation of the US team with a spacecraft and its experiments sitting on the other side of the Atlantic, six time zones away, has been accomplished through a series of three carefully planned ground-segment compatibility tests, each lasting one to two weeks. A fourth and final test involving GSFC and KSC was conducted during the launch campaign. During these tests, the spacecraft and its payload has been commanded from GSFC by the same teams that will operate it in space, using the same ground hardware and software procedures. In this case also, the ESA Project Team has been the coordinating element between industry and the NASA infrastructure and Flight Operations Team.
Figure 2. The SOHO spacecraft, photographed at Matra Marconi Space in Portsmouth (UK)
ESA Bulletin Nr. 84.