1、Lessons Learned Entry: 1765Lesson Info:a71 Lesson Number: 1765a71 Lesson Date: 2006-10-06a71 Submitting Organization: JPLa71 Submitted by: Francis Taylora71 POC Name: David Bella71 POC Email: David.J.Belljpl.nasa.gova71 POC Phone: 818-354-8041Subject: Managing Rover-Orbiter Relay Link Prediction Var
2、iability Abstract: The difference between the predicted versus achieved data volume returned by the Mars Exploration Rover relay link impacted the daily planning of rover driving and science data collection. This problem can be alleviated by refining the operations and science data return planning p
3、rocess. This should reflect a priority scheme based on (1) a minimum volume requirement (30 Mb for MER) and (2) a daily assumption of achieving a data volume level of one sigma (1 standard deviation) less than the predicted volume. Description of Driving Event: By relaying data to Earth through one
4、of the spacecraft orbiting Mars, the two Mars Exploration Rovers (MER) were able to transmit science data at a higher rate than by direct links from the Martian surface to the Deep Space Network (DSN). However, these surface-to-orbiter relay links (via the UHF frequency band) are known to be less pr
5、edictable in total data volume as compared to direct-to-Earth links (via X-band transmission). The data volume received during orbiter overflights has exhibited nominal variations of 75 to 125 percent of the volume predicted, and extremes of 50 to 150 percent of the predicted volume are occasionally
6、 experienced. Reference (1) is a very recent statistical study that for the first time documents MER data volume variations over 186 recent overflights. The study found that 11 overflights achieved less than 75% of the predicted volume and 78 passes exceeded 125% of predicted. Additional statistical
7、 studies are underway. Tactics such as changing a rover?s heading to facilitate communications after a day of driving and science activities have sometimes been necessary to attain an MER data return goal of 30 megabits per rover per Provided by IHSNot for ResaleNo reproduction or networking permitt
8、ed without license from IHS-,-,-“sol“ (Martian day). Because there are multiple contributors to relay link variability, it is hard to identify and quantify the factors that contribute to the difference between the predicted and received data volume on a specific overflight. This is true even after e
9、ngineering analysis of available rover and orbiter telemetered data, leaving mission planners with uncertain expectations as to actual performance. The impact on mission operations has been an inconsistent flow of relay link data during Mars surface operations. This has caused the following problems
10、 for the MER science team (Science Operations Working Group): 1. When a UHF data relay pass yields a data volume that is less than 30 megabits, the science team may lack sufficient information (i.e., pictures of the rover?s surroundings) on the current sol?s drive to safely plan the next day?s drive
11、.2. In contrast, larger than anticipated data volumes may represent a lost opportunity for the science team. Had they known, the team would have planned for more activity by rover instruments or higher image resolutions so that the extra telemetry capacity would not have been wasted. (See Reference
12、(2).)The inability to consistently and accurately predict actual relay pass data volume is not an MER design or operational anomaly. It is an anticipated result of a telecommunications system constrained by the spacecraft mass and configuration, project budget, development and testing schedule, and
13、operational factors, including: 1. The UHF antenna pattern on MER is highly variable and not completely understood, even after extensive preflight mockup tests and post landing data collection.2. The UHF antenna pattern is further modified by local geologic surroundings which change the multipath si
14、gnal phase and signal power. Geologic changes variables include proximity to a hill, proximity to a crater, proximity to a large rock, and changes in the electromagnetic properties of the underlying soil/rock as the rover changes location.3. Rover tilt impacts antenna pattern. Predicted rover tilt i
15、s only known to a few degrees, and the relationship between tilt and antenna gain pattern is not modeled.4. Rover yaw angle is set to one of 20 steps, with the data volume computed based on the gain value for the step. Thus there is uncertainty in the gain pattern and in the resulting data volume du
16、e to this quantization. In addition, the gain pattern may be further changed by deployment or retraction of a rover appendage.5. The Mars orbiter UHF antenna gain pattern is also variable based on the pointing angle to the rover. It may further change with variations in orbiter solar panel orientati
17、on.The Mission Operations Team has limited insight into which of these design and operational factors apply, and the extent to which they apply, on a given relay pass. Constrained by mass, cost, and schedule, the MER lander project and the Mars orbiter projects elected to use omni directional UHF an
18、tennas mounted in the midst of a crowded science deck. This Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-resulted in highly variable and unpredictable antenna gain patterns (Figure 1). Due to the same program constraints, testing of the antenna pa
19、tterns was limited to model mockups which have limited fidelity. Thus much of the UHF link performance variability and uncertainty resulted from engineering and programmatic decisions with known consequences. (a) Drawing of an MER-like rover with a monopole UHF antenna with a radiation pattern havin
20、g many lobes and nulls. A meter indicating link predictability to an orbiter overhead displays a moderate reading.(a) Drawing of an MER-like rover with a monopole UHF antenna with a radiation pattern having many lobes and nulls. A “meter“ indicating link predictability to an orbiter overhead display
21、s a moderate reading. (b) Same as Drawing (a), except the MER-like rover is tilted and overhung by a crater lip. The meter reads at the low end of the scale.(b) Same as Drawing (a), except the MER-like rover is tilted and overhung by a crater lip. The meter reads at the low end of the scale. (c) Sam
22、e as Drawing (b) with tilt and crater, but now a hypothetical future rover sports a high gain, steered UHF antenna that aims a single lobe toward the orbiter. The meter reads at the high end of the scale.(c) Same as Drawing (b) with tilt and crater, but now a hypothetical future rover sports a high
23、gain, steered UHF antenna that aims a single lobe toward the orbiter. The meter reads at the high end of the scale. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Figure 1. Effect of antenna design on link volume predictabilityThe majority of MER ov
24、erflights have produced data volumes reasonably close to predictions. UHF data has totaled about 90 percent of the science data returned from the rovers. Use of the Proximity-1 protocol has assured that virtually all data actually returned is error free (Reference (3). References: 1. Andrea G. Thoma
25、s, Mars Exploration Rover UHF Data Return Statistics, JPL Memorandum No. 337H-06-001, September 21, 2006. https:/mars03-lib.jpl.nasa.gov/docushare/dsweb/View/Collection-263752. “Provide In-Flight Capability to Modify Mission Plans During All Operations,“ Lessons Learned No. 1480, NASA Engineering Ne
26、twork, June 21, 2004.3. “Mars Exploration Rover Telecommunications“ (Article 10, Deep Space Communications telemetry rates; data rates; relay passes; telecommunications performance; link performance; antenna performance; telecommunications accuracy; UHF design; UHF data return; UHF data volumes; err
27、or detection; error correction; operations phase; mission operations; operations design; occlusion; antenna pointing angle; antenna field of view; antenna field-of-view; antenna FOV; link margin; relay link planning Lesson(s) Learned: Constrained by mass, cost, and schedule, the Mars orbiter and Mar
28、s rover UHF antenna system designs and test programs have exhibited sol-to-sol variations between predicted and achieved link performance. Though not unexpected, actual data return uncertainty has necessitated modifications to the rover mission planning processes for operations supported by the orbi
29、ters. The difference between the predicted versus achieved data volume returned by the MER UHF data link has sometimes impacted the daily planning of rover driving and science data collection. Although for a small number of sols this resulted in a data return less than the maximum achievable, the gr
30、eatly extended MER surface mission duration has returned science data that has vastly exceeded requirements and the most optimistic early expectations. Recommendation(s): Develop a data return planning process based on analyses of link performance that anticipates and plans for the unpredictable cha
31、racteristics of UHF omni links. Also, be prepared to update the Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-process as a result of subsystem and spacecraft development, test, and flight operations experience. Data return planning should begin wit
32、h an estimate or model of the predicted data volume statistics which may feature a mean value and variance that change over time. With an assumption of achieving a data volume level for the sol of one sigma (1 standard deviation) less than the predicted volume, prioritize the data return for (1) mis
33、sion operations and (2) high priority science data. Lower priority data will then be relegated to any remaining data volume actually achieved during the sol. For future mission, reassess the predicted data model as the design and test of antennas, mission planning tools, and procedures evolve.Eviden
34、ce of Recurrence Control Effectiveness: JPL opened Preventive Action Notice (PAN) No. 1475 on July 19, 2006 to initiate and document appropriate Laboratory-wide action on the above recommendations.Documents Related to Lesson: N/AMission Directorate(s): a71 ScienceAdditional Key Phrase(s): a71 Progra
35、m Management.a71 Program Management.Science integrationa71 Missions and Systems Requirements Definition.a71 Missions and Systems Requirements Definition.Vehicle conceptsa71 Systems Engineering and Analysis.a71 Systems Engineering and Analysis.Engineering design and project processes and standardsa71
36、 Systems Engineering and Analysis.Human factors planninga71 Systems Engineering and Analysis.Level II/III requirements definitiona71 Systems Engineering and Analysis.Mission and systems trade studiesa71 Systems Engineering and Analysis.Mission definition and planninga71 Systems Engineering and Analy
37、sis.Planning of requirements verification processesa71 Systems Engineering and Analysis.Systems analysis - cost analysisa71 Engineering Design (Phase C/D).a71 Engineering Design (Phase C/D).Spacecraft and Spacecraft Instrumentsa71 Mission Operations and Ground Support Systems.a71 Mission Operations
38、and Ground Support Systems.Ground support systemsa71 Mission Operations and Ground Support Systems.Mission control Planninga71 Mission Operations and Ground Support Systems.Mission operations systemsProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71
39、 Mission Operations and Ground Support Systems.Planetary Operationsa71 Safety and Mission Assurance.a71 Safety and Mission Assurance.Early requirements and standards definitiona71 Additional Categories.a71 Additional Categories.Communication Systemsa71 Additional Categories.Flight Equipmenta71 Addit
40、ional Categories.Flight Operationsa71 Additional Categories.Ground Equipmenta71 Additional Categories.Ground Operationsa71 Additional Categories.Hardwarea71 Additional Categories.Payloadsa71 Additional Categories.Risk Management/Assessmenta71 Additional Categories.Spacecrafta71 Additional Categories.StandardAdditional Info: a71 Project: Mars Exploration Rover (MER)a71 Year of Occurrence: 2005Approval Info: a71 Approval Date: 2007-02-01a71 Approval Name: ghendersona71 Approval Organization: HQProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
