REG NASA-LLIS-1843--2008 Lessons Learned - Science Data Downlink Process Must Address Constraints Stemming from Fixed DSN Assets.pdf
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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
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