Figure 1. The SOHO Ground Data System (GDS)
The primary purpose of the SOHO mission is to investigate the Sun by focusing upon its solar wind, seismology, and coronal dynamics. Using SOHO's complement of twelve scientific instruments, particular emphasis will be placed upon:
These scientific investigations will be conducted with the SOHO spacecraft operating in a halo orbit about the L1 Lagrangian point while maintaining a fixed, three-axis stabilised Sun-pointing attitude.
The SOHO Ground Data System (GDS) has been designed and implemented to accommodate the following key operational features:
The primary responsibility of the SOHO GDS is to support the SOHO mission flight operations from launch through end-of-mission in the following functional areas:
The principal elements of the ground system are illustrated in Figure 1 and discussed in the following paragraphs.
Figure 1. The SOHO Ground Data System (GDS)
The ISTP Programme will provide a Flight Operations Team (FOT) to conduct and coordinate the Control Centre operations. The FOT will be responsible for:
Jet Propulsion Laboratory (JPL)/Deep-Space Network (DSN)
The DSN will provide primary SOHO telemetry, command, and tracking support during routine science operations (10 months per year) and continuous science operations (2 months per year continuous contact). The DSN will support SOHO operations utilising existing ground-station facilities at Madrid (E), Canberra (Aus) and Goldstone (Calif., USA). JPL (located in Pasadena, Calif.) will provide DSN interface coordination, central communications, scheduling, and facilities management. Figure 2 gives an overview of the DSN, JPL and GSFC interfaces.
Figure 2. The NASA/JPL Deep-Space Network (DSN) interfacing for SOHO
The JPL/DSN will support the SOHO space-craft telemetry data in both real-time and tape-recorder-playback modes. JPL/DSN tele-metry processing is limited to convolutional coding removal, error correction, packaging, time-tagging, and transmission to other elements in the SOHO GDS.
Radiometric tracking data will typically be collected simultaneously with telemetry data and forwarded to the Flight Dynamics Facility in real time.
JPL/DSN will support command operations in throughput mode. Commands will be received from the Payload Operations Control Center (POCC), de-blocked, processed and uplinked to the SOHO spacecraft at 2 kbit/s.
Telemetry-only backup will be provided by the following networks in the event of DSN coverage gaps:
Telemetry will be received by the ground station, decoded, and forwarded to GSFC via Nascom. Backup network support will only be required in the early orbit phase of the mission, during critical support periods.
NASA communications (Nascom)
Nascom institutional and mission-specific resources will provide communications support for the SOHO mission from pre-launch testing until the end-of-mission. Nascom a global telecommunications system that provides real-time operational communications support. Through its primary switching and control center located at GSFC, Nascom provides centralised management for all communications links. Figure 3 provides Nascom configuration overview for SOHO.
Figure 3. The Nascom configuration for SOHO
Payload Operations Control Center (POCC)
The POCC will be the focal point for all SOHO GDS operations. Continuously manned (24 h/day, 7 days/week) and operated by the FOT, the POCC will provide the capability to command and monitor the SOHO spacecraft and all its instruments. The POCC will have sole control over all commanding modes, including near-real-time commanding of the instruments from the Experiment Operations Facility (EOF).
The POCC consists of mission-specific hardware and software which provides a central data collection and control facility for operations, planning, monitoring, and commanding of the spacecraft. The POCC provides the following fundamental SOHO support capabilities for all mission phases:
The POCC equipment is physically located in two main areas at GSFC (Building 3):
The baseline architecture of the SOHO POCC is shown in Figure 4.
Figure 4. Baseline architecture of the SOHO Payload Operations Control Center (POCC)
Command Management System (CMS)
The CMS is co-located with the POCC and is also manned and operated by the FOT. The CMS will provide the functions necessary for constructing spacecraft and instrument command groups (real-time commands) and loads (software and stored commands). The CMS will also support near-real-time commanding of the instruments, and will serve as the focal point for
reporting and exchanging status on the near-real-time commanding link.
The primary function of the CMS is the generation of planned sequences of space-craft commands (command loads) to be transmitted to the spacecraft. The CMS helps ensure the safe operation of the spacecraft by checking that the command loads are free from known operational constraint violations or conflicts. The CMS also provides the following fundamental SOHO support capabilities for all mission phases:
Experiment Operations Facility (EOF)
The SOHO EOF will provide for the day-to-day coordination and execution of SOHO instrument observation programmes. The EOF will also serve as the principal data-processing centre for certain solar experiments. Through work stations in the EOF, investigators will be able to command their instruments in near-real- time mode, via an interface with the CMS and POCC, without FOT intervention.
Flight Dynamics Facility (FDF)
The FDF will provide all orbit support required for the SOHO mission, including mission analysis, orbit determination, attitude validation, and manoeuvre planning and implementation. The primary functions of the FDF are:
- Mission Analysis
- Orbit Determination
- Attitude Determination
- Manoeuvre Support
- Momentum Management Support
Definitive attitude will be computed onboard SOHO and down-linked within the telemetry. During spacecraft contacts, FDF will provide attitude support involving the real-time processing of attitude-sensor telemetry data, provided via the POCC, and displaying the results for FOT inspection.
During critical manoeuvre periods the FDF will provide real-time Doppler analysis and display support. This support compares actual Doppler data computed from real-time radiometric tracking data with predicted values created when the manoeuvre was planned. Results will be displayed in real-time for manoeuvre execution support.
Sensor Data Processing Facility (SDPF)
The SDPF comprises the following systems and their corresponding functions:
- Generic Block Recording System (GBRS)
- Packet Processor (Pacor)
- Data Distribution Facility (DDF)
These facilities provide the functions necessary for data capture, intermediate data processing, and the creation, compilation, and mailing of distributed data. Figure 5 gives an overview of the SDPF interfaces and products.
Figure 5.The Sensor Data Processing Facility (SDPF) interfacing for SOHO
Central Data Handling Facility (CDHF)
The CDHF will provide instrument-specific data-reduction support together with the capability for online access to the data. For the SOHO mission, the CDHF will receive orbit and, in the event of a spacecraft contingency, attitude data from the FDF on a weekly basis.
Nominally, the CDHF will reformat onboard-computed definitive attitude and generate user reports for distribution upon request.
The CDHF will also receive daily command history files, summary data, a time-anomaly file, and a data-accountability log from the EOF. The command history and orbit/attitude data will be maintained on line for electronic access by the users, and transferred to the DDF to be included with other distributed data.
SOHO Simulator
A dynamic SOHO spacecraft simulator has been developed to support the verification of the GDS elements, interfaces, and flight databases. The Simulator has also been utilised successfully to support FOT training, science simulations, and emulate spacecraft anomalies.
SOHO will be injected into a halo orbit around the L1 Sun Earth libration point, about 1.5 x 10(exp 6) km sunward from the Earth-Moon barycentre, following a four-month transfer trajectory. The halo orbit will have a period of 178 days and has been chosen because:
The Sun-spacecraft velocity along the Sun line will be measured to better than 0.5 cm/s.
The orbit and distance involved in the halo orbit presents some operational challenges. There will be a 5's (approx.) light time delay in communications each way to SOHO, which makes it difficult to perform command verification and still allow a high rate of commanding. Special ground software is required to allow pipelining of commands, whereby up to ten may be sent before the first is verified, without losing track of the command sequence. This is made necessary by the high rate of real-time science commanding and large amount of real-time contact required for the mission.
The operational mission is divided into three main phases:
Figure 6. The SOHO early-orbit phases
The parking orbit is nearly circular, at 150 200 km altitude and inclined at 28.5 degrees. The orbital period is 90 min.
The eclipse duration is approximately 37.5 min, but the cumulative time in eclipse can be a maximum of 58 min in the case of a long coast (10 min eclipse duration in parking plus 10 min eclipse after injection into transfer). Depending on the launch date, the SOHO/Centaur may remain for between a minimum of 10 min and a maximum of 100 min in the parking orbit.
During the LEOP phase, the SOHO/Centaur composite will be oriented with its X-axis towards the south ecliptic, with a roll rate of about 0.2 rpm. This attitude will be achieved and maintained by the Centaur upper stage. The latter will start the thermal roll soon enough after insertion into parking orbit and sufficiently late before transfer trajectory insertion for the spacecraft to be exposed to sunlight (from launcher payload fairing jettison until injection into transfer) for less than 8 min.
No communication with the ground is foreseen during this phase.
After the first three days of continuous contact, the normal operations contact sequence will begin. The communication scenario is based on three daily passes of 1.6 h each and one 8 h pass, as during the normal halo-orbit phase. This leaves the remaining 11.2 h of the day without contact during three periods of about 3.73 h typically (if divided evenly). However, for critical operations (initial Sun acquisition, solar-array deployment up to 12 h after transfer trajectory insertion, and orbit-correction manoeuvres), the ground segment will be available continuously for critical support.
The TTP is divided in the following four subphases (Fig. 6):
Three daily passes of 1.6 h and one 8 h pass are baselined for SOHO support during 10 months per year, the remaining 2 months having 24 h/day support. The onboard data recorder provides data storage during all non-contact periods. These contacts are sufficient for complete recorder-play-back data capture and to meet all telemetry, ranging and command requirements. During the operational phase, any combination of the ranging, telemetry, and telecommanding links will nominally be available.
SOHO possesses many unique features, such as the near-real-time commanding from the Experiment Operations Facility, an exten-sive number of telemetry and command parameters, and an innovative spacecraft design. Managing all of these features presents a special challenge to the Flight Operations Team. A combination of online and offline activities will be used for the complete study of the spacecraft's performance. The health of the spacecraft will be continually assessed by monitoring the appropriate telemetry during real-time contacts. This monitoring will be performed using telemetry processing, various software tools, and online expert systems. Offline data processing will also be performed to monitor selected parameters in detail and watch for more subtle changes and trends in performance and behaviour.
Figure 7. The SOHO mission planning and scheduling timeline
Only when all of the above inputs have been received, digested and verified and all of the inevitable conflicts have been resolved will the SOHO Activity Plan be finalised!
ESA Bulletin Nr. 84.