1、Lessons Learned Entry: 1843Lesson Info:a71 Lesson Number: 1843a71 Lesson Date: 2008-02-19a71 Submitting Organization: JPLa71 Submitted by: David Oberhettingera71 POC Name: Helenann Kwong-Fu (Spitzer Project); John Kennedy (DSN)a71 POC Email: Helenann.H.Kwong-Fujpl.nasa.gova71 POC Phone: 818-354-2107
2、 (Helen); 818-354-0168 (John)Subject: Science Data Downlink Process Must Address Constraints Stemming from Fixed DSN Assets Abstract: Given their minimal ability to mitigate DSN resource limitations, flight projects must consider mission design and mission operations improvements that may help to ac
3、hieve Level 1 requirements, such as the 9 measures effectively employed by the Spitzer project.Description of Driving Event: The plethora of NASA missions transmitting science data to Earth via the NASA Deep Space Network (DSN) is challenging the capability of the aging DSN facilities to help fulfil
4、l mission requirements. The DSN is a network of very large antennas placed 120 degrees apart around the globe to provide telecommunications linkage with deep-space (and some Earth-orbiting) spacecraft and observatories. One such DSN customer that has addressed this issue is the Spitzer Space Telesco
5、pe project, an infrared telescope that orbits the Sun to return science data on stars, galaxies, and planetary discs. The Spitzer mission has a typical Level 1 requirement for downlink telemetry of 98 percent of observations (i.e., percentage of planned downlinks) and 99 percent of science data (i.e
6、., percentage of data generated onboard the spacecraft). However, the globe-spanning DSN facilities are very complex; DSN anomalies and planned downtime that interrupt communications are not infrequent (Reference (1). Furthermore, Spitzer cannot always utilize the available DSN uptime because: a71 S
7、ome data is lost by the spacecraft Command and Data Handling (C&DH) subsystem due to software faults, commanding errors, and procedural anomalies. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 DSN linkage is also used to uplink commands to the
8、spacecraft, and a missed telemetry pass could place Spitzer in safe mode and interrupt downlink. Reference (2) characterizes 330 DSN anomalies during normal operations affecting Spitzer downlink over 3 years of Spitzer mission operations. Most of the outage times ranged from 1 to 1-1/4 hours. The fa
9、ults are varied: the most common are attributed to Transmitter or Downlink Channel Control Processor failures (each accounting for 15 percent of the anomalies), but DSN operators are not always able to determine the root cause. DSN services are likely to remain fully subscribed for the foreseeable f
10、uture because: 1. DSN resources are strained by the large number of missions competing for fixed DSN assets. Figure 1 depicts the 26 missions utilizing DSN downlink on the day this lesson learned was approved. 2. Demand for DSN services is subject to significant peaks. For example, Spitzer has incre
11、ased its daily DSN usage as the spacecraft has moved farther away from Earth, requiring either larger antennas or more antennas. The number of Mars missions has increased, and they consume more DSN resources than Spitzer because of the very low data rates from Mars. Other spacecraft will sometimes r
12、eceive priority over Spitzer for DSN services, e.g., spacecraft in safe mode requiring Spitzer to give up a downlink pass or a portion of a pass. Demand peaks may also occur when spacecraft conduct Entry, Descent, and Landing (EDL), or when spacecraft perform intensive operations such as Phoenixs 3.
13、5 months of Mars surface operations. In addition, all operating Mars missions are simultaneously in view of the same complex and may share a single antenna for downlinks, preventing use by other missions.3. NASA faces a major challenge in maintaining and upgrading the 45-year old DSN facilities. It
14、is difficult to obtain replacements for DSN components such as the special high capacity transformers and circuit breakers. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Figure 1. DSN/NAV Real Time Tracking Data Monitor (http:/rmdc.jpl.nasa.gov/trk
15、mon.html)Lacking significant leverage to solve such endemic DSN resource constraints, the Spitzer project sought post-launch to optimize the Spitzer mission design by: 1. Downlink-by-downlink tracking of predicted data volumes versus actual data volumes and downlink performance. Data volume predicti
16、ons are used to extrapolate from the most recent report of Spitzer onboard data storage and to determine (1) if the Mass Memory Card will be completely filled and (2) how long it will take to clear the onboard data backlog. 2. Early analysis of data volumes. This allows Spitzer to determine when lar
17、ge predicted Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-volumes might compromise single-fault tolerance and how many passes it would take to clear the resulting backlog. Spitzer also adjusts the predicted data volumes to more closely match the o
18、bserved data volumes, an issue discussed in detail in Reference (3). 3. Early deletion of data. Ground controllers typically command the deletion of science data from on-board storage on the pass following the downlink. The Spitzer project has the ground capability to command deletion of at least a
19、portion of the data one pass earlier than normal- during the same pass that the data were downlinked. Spitzer uses this capability to mitigate the effects of large predicted data volumes, and during recovery from downlink anomalies. It frees space early, helps to recover to a single-fault tolerant s
20、tate sooner, and reduces the impact should a second fault occur. 4. On-board free-space checking. Before each science observation is run, the Spitzer spacecraft checks that there is enough free memory space to hold the predicted data volume. This reduces the risk of a skipped science observation due
21、 to insufficient memory space resulting from entry into standby mode. Spitzer practice is to reserve two days memory consumption to provide a margin so that engineering data will not overfill on-board storage.5. Using a second ground antenna for backup downlink. If Spitzer misses a significant porti
22、on of a downlink pass, a second ground antenna provides redundant downlink capability and mitigates the risk of filling on-board memory storage. However, opportunities for this mitigation are now limited because the Spitzer downlink margins require use of a 70-meter antenna. 6. Using a second ground
23、 antenna for backup uplink. If Spitzer misses a significant portion of an uplink, a second ground antenna can also provide redundant uplink capability and mitigate the risk of filling on-board memory storage. Unlike the downlink example above, this mitigation is usually feasible since Spitzer operat
24、ions can uplink with a 34-meter antenna. During each pass, Spitzer uplinks commands to delete data from on-board memory that were received during the previous pass. 7. Reduction of the downlink bit rate by real-time commanding. To mitigate the risk of failure of Spitzers assigned antenna, DSN is som
25、etimes able to provide a smaller antenna as backup. Should the primary antenna fail, then Spitzer can reduce the downlink data rate to receive a portion of the planned downlink rather than miss the entire pass.8. Scheduling antenna arrays. When Spitzer can gain access to arrays of smaller antennas,
26、the project achieves uplink redundancy and a more graceful degradation of downlink resources. 9. Making numerous improvements to the Mission Control and Analysis tool used to build sequences to delete on-board data. Fixing bugs and adding functionality to the ground-based tool have made it easier to
27、 operate and more robust.Many of these measures were accomplished post-launch via on onboard patch of a global variable, or Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-by blind uplink commanding (i.e., when no downlink can be detected). Flight pr
28、ojects can also mitigate DSN availability and throughput limitations through spacecraft design measures, such as adding onboard data storage to accommodate periods when downlink is not available. References: 1. “NASAs Deep Space Network: Current Management Structure is Not Conducive to Effectively M
29、atching Resources with Future Requirements,“ Report No. GAO-06-445, U.S. General Accounting Office, May 22, 2006, http:/www.gao.gov/htext/d06445.html. 2. “Characterization of Spitzer DSN-related Discrepancy Reports (DRs) and Interrupt/Surprise Anomalies (ISAs),“ Spitzer Project, NASA/Caltech Jet Pro
30、pulsion Laboratory, November 29, 2007.3. “Managing Rover-Orbiter Relay Link Prediction Variability,“ NASA Lesson Learned No. 1765, NASA Engineering Network, October 6, 2006.4. “Galileo Scan Platform Anomaly at Ida Encounter,“ NASA Lesson Learned No. 0560, NASA Engineering Network, May 6, 1997.Lesson
31、(s) Learned: Typical Level 1 mission requirements for science data downlink are currently difficult to achieve due to spacecraft faults, operations errors, very high demand for fixed Deep Space Network (DSN) assets, and scheduled/unscheduled DSN downtime. The requirements are negotiated between scie
32、ntists and engineers and, from an engineering perspective, may be based on overly optimistic assumptions about downlink resources. Because the near-term potential for DSN downlink availability and throughput shortfalls is not likely to abate, the spacecraft design and mission design must compensate
33、for DSN resource limitations.Recommendation(s): Recommendations for DSN improvement lie outside the scope of this lesson learned. Flight projects that plan to use DSN telecommunication services must consider factors that may limit downlink availability and data rates. To mitigate the risk that Level
34、 1 requirements for science data downlink will not be achieved: 1. In mission planning, consider the 9 post-launch mitigations listed above that were effectively employed by the Spitzer project.2. Provide ample and reliable onboard data storage to accommodate periods when downlink is not available.E
35、vidence of Recurrence Control Effectiveness: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-JPL has referenced this lesson learned as additional rationale and guidance supporting Paragraph 6.2 (“Engineering Practices: Telecommunication Design“) and
36、Paragraph 6.3.4.3 (“Engineering Practices: Mission Operations- Spacecraft Health and Safety, and Performance Analysis“) in the Jet Propulsion Laboratory standard “Flight Project Practices, Rev. 6,“ JPL DocID 58032, March 6, 2006. In addition, JPL has referenced it supporting Paragraph 4.1.4.3 (“Flig
37、ht System Design: Design Margins During Development- Margins at Design Completion“), Paragraph 4.4.4.4 (“Information System Design: Commanding and Sequencing- Assured Commanding“), Paragraph 6.3.1.2 (“Managed Margins: Flight System Technical Resource Margins- Technical Resource Margins for Flight Op
38、erations Phase“), Paragraph 7.1.7 (“Flight Scenario Design: Resource Margins for Stored Sequence Operations“), and Paragraph 9.5.1 (“Flight System Flight Operations Design: Operating Margins- Adequate Operating Margins“) in the JPL standard “Design, Verification/Validation and Operations Principles
39、for Flight Systems (Design Principles),“ JPL Document D-17868, Rev. 3, December 11, 2006.Documents Related to Lesson: N/AMission Directorate(s): a71 Space Operationsa71 ScienceAdditional Key Phrase(s): a71 Program Management.Configuration and data managementa71 Program Management.Cross Agency coordi
40、nationa71 Program Management.Program planning, development, and managementa71 Program Management.Risk managementa71 Program Management.Science integrationa71 Missions and Systems Requirements Definition.a71 Missions and Systems Requirements Definition.Configuration control and data managementa71 Mis
41、sions and Systems Requirements Definition.Level 0/1 Requirementsa71 Missions and Systems Requirements Definition.Mission concepts and life-cycle planninga71 Systems Engineering and Analysis.Long term sustainability and maintenance planninga71 Systems Engineering and Analysis.Mission and systems trad
42、e studiesa71 Systems Engineering and Analysis.Mission definition and planninga71 Engineering Design (Phase C/D).a71 Engineering Design (Phase C/D).Software Engineeringa71 Engineering Design (Phase C/D).Spacecraft and Spacecraft Instrumentsa71 Mission Operations and Ground Support Systems.Logistics a
43、nd maintenanceProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 Mission Operations and Ground Support Systems.Mission control Planninga71 Mission Operations and Ground Support Systems.Mission operations systemsa71 Safety and Mission Assurance.a71 S
44、afety and Mission Assurance.Early requirements and standards definitiona71 Safety and Mission Assurance.MaintenanceAdditional Info: a71 Project: Spitzer Space TelescopeApproval Info: a71 Approval Date: 2008-05-28a71 Approval Name: mbella71 Approval Organization: HQProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
