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1. Introduction


Voyager Interstellar Mission (VIM) is an extension of the Voyager primary mission that was completed in 1989 with the close flyby of Neptune by the Voyager 2 spacecraft. Voyager 1 completed its planned close flybys of the Jupiter and Saturn planetary systems while Voyager 2, in addition to its own close flybys of Jupiter and Saturn, completed close flybys of the remaining two gas giants, Uranus and Neptune. The mission objective of the VIM is to obtain useful interplanetary, and possibly interstellar, fields, particles, and waves (FPW) science data until year 2020 and beyond when the spacecraft's ability to generate adequate electrical power for continued science instrument operation will come to an end.


In order to capitalize on this lengthy data acquisition potential, it is imperative that the spacecraft have a continuing sequence of instructions for acquiring the desired science data, and that the spacecraft High Gain Antenna (HGA) boresight remain pointed at the Earth for continuous data transmission. Because of the long mission duration and the likelihood of periodic spacecraft anomalies, it is also advantageous to continue the use of the on-board fault protection capability for automated responses to specific subsystem anomalies, and to provide an on board sequence to continue spacecraft operation in the specific event of the future loss of command reception capability. All of these factors are considered in the VIM operating plan. The following sections describe the overall spacecraft sequencing strategy used for VIM operations, the interaction of the sequence elements with the on-board Fault Protection Algorithms (FPAs), and the planned modifications to the spacecraft configuration in response to the continual reduction in electrical power availability throughout the remainder of the mission.


2. Sequencing Strategy
A spacecraft sequence is the set of instructions that are stored in an on-board computer memory and control the operation of the engineering subsystems and science instruments for the purpose of gathering and transmitting science data to the ground stations. Typically, a spacecraft sequence is designed to function for a specific period of time. Some of the key factors that determine the duration of a spacecraft sequence are:

  • size of the on-board memory;
  • characteristics of the science observations; a series of one of a kind type of observations, or a series of observations that are repetitive for an extended period of time;
  • ability to store HGA pointing information on-board the spacecraft.

With the Voyager spacecraft being mid-1970's technology, the available on-board memory for storing spacecraft sequence information is very limited by today's standards. A total of about 1500 18-bit words are available between the two Computer Command Subsystem (CCS) memories for storing sequence instructions and HGA pointing information. This limited memory space can be a significant factor if the sequence is composed of a series of one of a kind activities. Fortunately, the science observations being made during VIM are very repetitive in nature. This allows a science observation or instrument calibration to be repeated (cyclicized), essentially for the duration of the mission, at practically the same memory word cost as a single observation or calibration. This implementation of events as a cyclic sequence that repeats an observation or calibration on a regular basis is key to the VIM sequencing strategy. Another key factor is the ability to store HGA pointing information out to the end of mission in the CCS memory. This not only simplifies the routine sequence generation support by not requiring HGA pointing information to be included in individual sequences transmitted to the spacecraft, but also enables the ability to provide science data even if command capability to the spacecraft is lost. These two sequence capabilities are referred to as the "baseline sequence."


Augmentation of the baseline sequence with non-repetitive science or engineering events use either an "overlay sequence," or a "mini-sequence." The difference between these two types of augmentation sequences is that the overlay sequence operates for a fixed interval of time, and contains all of the baseline sequence augmentations for that time interval. The mini-sequence is focused on accomplishing a single augmentation need and is not a regularly scheduled activity but is done on an as needed basis. Both types of sequences are developed and transmitted from the ground. It was envisioned that there would be one overlay sequence in every three months and it was practiced in early VIM. However, due to the difficulty in predicting and obtaining the DSN resources, an overlay load has been split into two sequences in recent years. Mini-sequences are infrequent and mainly used for anomaly investigation and resolution.


The baseline sequence, overlay sequence, and mini-sequence provide the basic operational sequence elements for normal operations while command capability with the spacecraft is available. In the event command capability is lost, another sequence element, the "Backup Mission Load (BML)," provides the mechanism for continued spacecraft data acquisition without further ground interaction. A BML is stored on-board each spacecraft and contains the necessary instructions to modify the on-going baseline sequence to maintain the continued return of basic FPW data. The BML also terminates the use of the gyros and Digital Tape Recorder (DTR) utilization when the available electrical power will no longer support their operations. Another important function included in BML is resetting the hourly counter in the CCS about every 13.5 years so the spacecraft can continue to have the sequencing capabilities.


3. Sequence Interaction With On Board Fault Protection
The FPAs on the Voyager spacecraft are designed to recover the spacecraft from specific potential mission-catastrophic failures. They are implemented primarily in the CCS, while some are in the Attitude and Articulation Control Subsystem (AACS). The FPAs in the CCS are invoked by interrupts received from external sources, and followed by preprogrammed responses.

The five FPAs that currently reside in the CCS are:

  • AACS Power Code Processing (AACSIN) - responds to AACS anomalies by processing Power Codes (PCs) received from the AACS.
  • Command Loss (CMDLOS) - provides a means for the spacecraft to automatically respond to an on-board failure resulting in the inability to receive ground commands.
  • Radio Frequency Power Loss (RFLOSS) - provides the spacecraft a means of automatically recovering from a failure of an S-band or X-band exciter or transmitter.
  • CCS Error (ERROR) - provides the capability to respond to certain anomalous CCS hardware and software conditions.
  • Power Check (PWRCHK) - provides a means for the spacecraft to automatically configure itself to a safe, low-power operating mode following a power subsystem undervoltage condition or a CCS tolerance detector trip.

If an AACSIN, CMDLOS, or RFLOSS entry occurs, the response will be integrated into on-going sequence and the commands from an FPA response and the regular sequencing activities will be interleaved. For an ERROR or PWRCHK entry, all the on-going activities will be terminated. However, some of the critical activities including the baseline sequence will be automatically restarted by a special feature called "rollback" although there will be some data loss depending on when a PWRCHK or ERROR entry occurs.


4. Modifications In Response to Electrical Power Limitations
Radioisotope Thermoelectric Generators (RTGs) provide electrical power to the Voyager spacecraft. Due to the radioactive decay of the Plutonium fuel source, the electrical power provided by the RTGs is continually declining. The current rate of decay is approximately 4.2 watts per year. Because of the continual decline in the amount of power that is available, it is necessary to periodically reduce power consumption in order to maintain an adequate power margin. This is accomplished by turning off spacecraft power loads.


VOYAGER 1
The following lists those loads that have been turned off in VIM and the year:

  • IRIS Flash-off Heater OFF (+31.8 W) - 1990
  • WA Camera OFF (+16.8 W) - 1990
  • NA Camera OFF (+18.0 W) 1990
  • PPS Supplemental Heater OFF (+2.8 W) - 1995
  • NA Optics Heater OFF (+2.6 W) - 1995
  • IRIS Standby A Supply OFF (+7.2 W) - 1995
  • WA Vidicon Heater OFF (+5.5 W) - 1998
  • NA Vidicon Heater OFF (+5.5 W) - 1998
  • IRIS Science Instrument OFF (+6.6 W) - 1998
  • WA Electronics Replacement Heater OFF (+10.5 W) - 2002
  • Azimuth Actuator Supplemental Heater OFF (+3.5 W) - 2003
  • Azimuth Coil Heater OFF (+4.4 W) - 2003
  • Scan Platform Slewing Power OFF (+2.4 W) - 2003
  • NA Electronics Replacement Heater OFF (+10.5 W) - 2005
  • Pyro Instrumentation Power OFF (+2.4 W) - 2007
  • PLS Science Instrument OFF (+4.2 W) - 2007
  • PLS Replacement Heater OFF (+4.3 W) - 2007
  • PRA Science Instrument OFF (+6.6W) -2008
  • IRIS Replacement Heater OFF (+7.8 W) - 2011

The following lists those loads that are planned to be turned off:

  • Terminate UVS operations - 2013; TBD. IRIS Replacement Heater which had been left on to keep the scan platform warm for UVS was turned off in 2011. The UVS instrument is still providing useful data even though its temperature is far below the design limit. It will be necessary to turn off one or more of the remaining three loads at the end of 2013 as the power output further decreases.

* UVS Power OFF (+2.4 W)
* UVS Replacement Heater OFF (+2.4 W)
* Scan Platform Supplemental Heater (+6.0 W)

  • Termination of DTR operations (+2.2 W for DTR turnaround; 5.8 W is needed for DTR turnaround but 3.6 W is shared with gyros turn on transient and maneuver) - 2014; TBD. This power load reduction step is currently sequenced to occur on DOY 238, 2014; however, it can be delayed if the DSN resources for playback and power margin are available.
  • Discontinue gyro operations (+14.4 W steady state, +3.6 W turn on transient and maneuver) - 2015; TBD. This power load reduction step is currently sequenced to occur on DOY 350, 2015 but could be changed if the RTG output is better than predicted.

Further responses to decreasing electrical power, beginning in 2020, will consist of either turning instruments off sequentially or turning instruments off and on in a power sharing mode to maintain an adequate power margin.


VOYAGER 2
The following lists those loads that have been turned off in VIM and the year:

  • PPS Science Instrument (+1.2 W) - 1991
  • NA Optics Heater OFF (+2.6 W) - 1994
  • WA Vidicon Heater OFF (+5.5 W) - 1996
  • NA Vidicon Heater OFF (+5.5 W) - 1996
  • The following Scan Platform loads (43.9 W) were turned off and the UVS mission was terminated - 1998

* WA Electronics Replacement Heater OFF (+10.5 W)
* IRIS Replacement Heater OFF (+7.8 W)
* NA Electronics Replacement Heater OFF (+10.5 W)
* Azimuth Actuator Supplemental Heater OFF (+3.5 W)
* UVS Science Instrument OFF (+2.4 W)
* UVS Replacement Heater OFF (+2.4 W)
* Azimuth Coil Heater OFF (+4.4 W)
* Scan Platform Slewing Power OFF (+2.4 W)

  • Pyro Instrumentation OFF (+2.4 W) - 2006
  • TCM Chamber Press Transducers OFF (+1.9W) - 2006
  • IRIS Science Instrument OFF (+6.6 W) - 2007
  • PRA Science Instrument (+6.6W) - 2008
  • Termination of DTR operations (+2.2 W for DTR turnaround; 5.8 W is needed for DTR turnaround but 3.6 W is shared with gyros turn on transient and maneuver) - 2007; DTR remains on for preventing hydrazine lines from freezing
  • AP Branch 2 Backup Heater OFF (+11.8 W) - 2011

    The following lists those loads that are planned to be turned off:
  • Discontinue gyro operations (+14.4 W steady state, +3.6 W turn on transient and maneuver) - 2014; TBD. This power load reduction step is currently sequenced to occur on DOY 120, 2014 but could be changed if the RTG output is better than predicted.

Further responses to decreasing electrical power, beginning in 2019, will consist of either turning instruments off sequentially or turning instruments off and on in a power sharing mode to maintain an adequate power margin.

 
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