1. Introduction

The Voyager Interstellar Mission (VIM) has the potential for obtaining useful interplanetary, and possibly interstellar, fields, particles, and waves (FPW) science data until around the year 2025 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) remain boresighted on 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 sequencing strategy. The following sections describe the overall spacecraft sequencing strategy being used for VIM operations and the four sequencing elements (Baseline Sequence, Overlay Sequence, Mini-Sequence, and Backup Mission Load) used to implement the strategy.

The VIM sequencing strategy for spacecraft operations is significantly different from the strategy used during the primary mission which ended with the Voyager 2 flyby of Neptune in 1989. Only the current VIM strategy is discussed.

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 less than 1300 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 approximately the year 2030 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 a 40 year science data return mission even if command capability to the spacecraft is lost.

As long as command capability exists between the ground and the spacecraft, new spacecraft sequences can be stored on-board the spacecraft, or existing on-board sequences can be modified. If command capability is lost, the spacecraft must continue to operate with the sequence that is already stored on-board. Both of these command capability considerations are part of the VIM sequencing strategy.

The VIM sequencing strategy relies on having stored on-board each spacecraft a continuously executing sequence of repetitive science observations that satisfies the basic VIM heliospheric data acquisition requirements. HGA pointing information (HPOINTS), to keep the HGA pointed at the Earth until approximately the year 2030, are also stored in sequence memory to provide for continual communications capability. 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, currently six months, 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.

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 continuously executing baseline sequence to maintain the continued return of basic FPW data. The BML also terminates both the use of the gyros when the available electrical power will no longer support their operation, and Digital Tape Recorder (DTR) utilization when telecom performance is no longer sufficient to support the minimum DTR playback data rate of 1.4 kbps. The termination of gyro and DTR operations are unconditional events and will occur at the programmed times even if command capability is available. If command capability is available, the times of these events can be updated based on observed actual performance.

All of these sequence elements use pre-defined blocks of commands to accomplish specific spacecraft functions. This is in contrast to the prime mission method of essentially programming each sequence in assembly language to maximize the science data return for each sequence. While the ability to use pre-defined blocks of commands greatly reduces the effort required to generate and validate a sequence of commands, there is an increase in the number of memory words needed to accomplish a given function. Fortunately, the VIM science data acquisition requirements, sequencing strategy, and available CCS memory space support the use of pre-defined blocks of commands.

The remainder of this write-up describes each of the four sequence elements. Also discussed is:

1. the interaction of the sequence elements with the on-board Fault Protection Algorithms (FPAs) that continually monitor selected subsystem performance parameters and contain programmed responses which are initiated in the event of out of-tolerance performance;

2. the planned modifications to the spacecraft configuration in response to the continual reduction in electrical power availability throughout the remainder of the mission.

3. Baseline Sequence Description

The baseline sequence is stored in CCS memory and is the continuously executing set of spacecraft operating instructions that controls the collection and return of the bulk of VIM science data acquired during normal operations (when command capability exists). It also provides the foundation for continuing the return of science data in the event of loss of command capability and the resulting BML entry (described in the following BACKUP MISSION LOAD DESCRIPTION).

Baseline sequence implementation relies on the use of eleven "spacecraft block routines" that are also stored in CCS memory. These routines are the equivalent of software program subroutines and are called by the baseline sequence for execution. These block routines consist of:

BR1 GYRO ON (GYONAB, GYONBC)

BR2 SS-HGA CALIBRATION (ASCAL)

BR3 MAGNETOMETER ROLL MANEUVER (MAGROL)

BR4 GYRO ACTIVITY INTEGRATED (GAIN)

BR5 ATTITUDE CONTROL DEADBAND CONTROL (ADB)

BR6 FDS DATA MODE CONTROL (FDAMO)

BR7 PLASMA WAVE SUBSYSTEM DATA RECORD (PWSREC)

BR8 DTR MAINTENANCE (DTRMA)

BR9 PLASMA WAVE SUBSYSTEM DATA PLAYBACK (PWSPB)

BR10 DTR ACTIVITY INTEGRATED (DAIN)

BR11 PLS/MAG/PESCAL (PMPCAL)

During normal operations, each spacecraft performs the following repetitive science and engineering activities under the control of the baseline sequence:

• collection and transmission of continuous real time cruise science telemetry data, primarily at 160 bps data rate;
  
• recording of one frame of high rate Plasma Wave Subsystem (PWS) data on the DTR each week
  
• playback of six months of recorded high rate PWS data every 6 months;
  execution of a magnetometer calibration roll maneuver (MAGROL) every 3 months;
  
• execution of a HGA/sun sensor calibration maneuver (ASCAL) every 6 months;
  
• perform a PMPCAL once a month. The PMPCAL consists of a combined
• Plasma Subsystem (PLS), Magnetometer Subsystem (MAG), and FPW
• Periodic Engineering and Science Calibration (PESCAL), calibration ;
  
• perform DTR maintenance twice a year;
  
• perform gyro conditioning (on/off) and a CCS timing test every 3 months

In addition, the baseline sequence contains the HGA pointing information to keep the antenna pointed at Earth for communication purposes. Commands are stored on-board to maintain HGA pointing on Voyager 1 until Day-Of-Year (DOY) 342, 2020, and until DOY 204, 2017 on Voyager 2. Work is in progress to update the HGA pointing tables to EOY 2030.

HGA pointing accuracy is assessed every 6 months with the ASCAL maneuver. If necessary, the stored pointing information can be updated, via ground command, with a new pointing file. No updates have been required since the HGA pointing information was initially stored in both spacecraft in 1990 and none are expected in the future based on performance trends to date.

All of the Voyager 1 baseline sequence spacecraft events which require 70m station coverage for capturing the telemetry data on the ground (MAGROL, playback of recorded high rate PWS data, & ASCAL) are scheduled to occur over the Goldstone station complex and the Voyager 2 events over the Canberra complex. These combinations provide the best telecom performance for data return. In order to keep baseline sequence spacecraft events in sync with Deep Space Station (DSS) view periods (requires a near constant sidereal time), spacecraft events are shifted in time at the rate of approximately 4 minutes earlier per day. This adjusts for the difference between a solar day (24 hours) and a sidereal day (23 hours, 56 min, 04.09 sec). As a result of the accumulated effect of this 4 minute per day shift, spacecraft events occur one day earlier for each year of operation. After six consecutive years, the spacecraft events have shifted 6 days earlier in time. At the start of the seventh year, the timing of the events are delayed 7 days and the 4 minute per day shifting begins again. This periodic 7 day timing shift keeps the spacecraft events occurring within a one week calendar time window for the duration of VIM, thus avoiding critical spacecraft (DTR playbacks or attitude maneuvers) from occurring during designated quiet weeks (i. e. Thanksgiving, Christmas/New Years). It should be noted that the described timing shifts are designed to maintain the proper Earth Received Time (ERT) of the spacecraft telemetry data within the desired station view period. This is important since the one-way light time increases approximately 1/2 hour per year of flight.

Once a year, in November for Voyager 1 and September for Voyager 2, the baseline sequence cyclics end and restart with the previously described timing adjustments being made. At this time, any ground commanded modifications to the baseline sequence also take effect.

Because of the continual reduction in the electrical power available to operate the spacecraft, gyro operations have to be terminated at some future time. DTR operations also have to be terminated when the downlink telecom performance will no longer support the minimum DTR playback data rate of 1.4 kbps. Based on expected power and telecom subsystem performance, the dates for the termination of gyro and DTR operations are stored in the long term events table of the BML for each spacecraft. These timed unconditional execute commands are used to terminate the baseline sequence gyro activities (MAGROLs, ASCALs, and gyro conditioning), and to terminate the recording and playback of high rate PWS data. The current dates stored in the long term events table are:

(Current RTG decay trends may delay these events from 1 to 2 years)

Voyager 1

Terminate gyro operations end of 2016

Terminate DTR operations end of 2014

Voyager 2

Terminate gyro operations end of 2014

Terminate DTR operations Done in 2007 (due to an anomaly on the PWS)

4. Overlay Sequence Description

Overlay sequences are used to augment the continuously executing baseline sequence and are prepared on a regularly scheduled basis. At the start of VIM (1990), overlay sequences were of 3 month duration, with a 6 week development period. Considering both spacecraft, with overlay sequence execution staggered 1 1/2 month between spacecraft, there was essentially an overlay sequence being developed at all times. Initial VIM flight team staffing levels supported two Sequence Integration Engineers (SIEs). This allowed one SIE to be developing an overlay sequence while the second SIE could be performing other tasks (real time command requests, documentation updates, software maintenance, etc.). With the 1993 reduction in flight team staffing (reduction to one full time SIE), the duration of the overlay loads was extended to 6 months. The development time remained at 6 weeks providing time for the single SIE to perform the other necessary SIE tasks between overlay sequence development periods.

The intent of the overlay sequence is to provide a mechanism for incorporating non-repetitive science or engineering events into the spacecraft sequence of activities in combination with the baseline sequence. Typical events in overlay sequences have included:

• Ultraviolet Spectrometer Subsystem (UVS) stellar and heliospheric observations;
  
• two additional MAGROLs per year per spacecraft;
  
• additional high rate PWS records and DTR playbacks;
  
• DTR playbacks to recover data when the baseline sequence playback was not captured on the ground;
  
• CCS/Flight Data Subsystem (FDS)/Attitude and Articulation Control Subsystem (AACS) memory readouts;
  
• updates to BML and FPAs;
  
• modifications to the baseline sequence.

The need for the 3 month and then 6 month overlay sequences has been driven primarily by the requirement for acquiring UVS stellar and heliospheric observations. The termination of UVS observations will eliminate the necessity for these regularly scheduled sequence loads. However, there remains a need to continue to provide a capability for augmentation of the baseline sequence for either science data acquisition or spacecraft engineering needs. This augmentation will be accomplished by small individual mini-sequences that perform a specific function and are implemented on an as needed basis.

5. Mini-Sequence Description

Mini-sequences also augment the baseline sequence but are prepared on an as needed basis rather than on a regularly scheduled basis. These sequences are usually intended to perform a single function rather than the multiple functions performed by an overlay sequence. The development time for a mini-sequence varies from a day to a week or so depending on the complexity of the sequence. A common use of a mini-sequence is to sequence a second DTR playback of high rate PWS data if the baseline sequence playback is not captured on the ground. Another common use is in response to a spacecraft anomaly (i. e. the performance of a memory readout, a CCS timing test, or other diagnostic or anomaly resolution effort). When the termination shock is expected to be encountered, a mini-sequence will be used to enable an on-board stored routine that will increase the number of high rate PWS records from once a week to approximately every nine hours. A mini-sequence will also be used to play back the recorded data.

6. Backup Mission Load Description

The BML provides automated on-board protection against the loss of command capability to the spacecraft. Without command capability, the spacecraft must continue to operate with the instructions previously stored in the CCS memory. Key to the automated protection against the loss of command capability is the implementation of a command loss timer on-board each spacecraft. The command loss timer is simply a programmable timer that resets to a programmed duration each time a command is received by the spacecraft. The current duration of the command loss timer is six weeks. When a command is received by the spacecraft, the timer is reset to a six weeks duration and immediately begins counting down towards zero. If the timer reaches zero, as a result of a command not being received by the spacecraft within the programmed six week duration, the command loss timer will have expired and the Command Loss (CMDLOS) routine will be activated which leads to the initiation of the BML.

The implementation of BML-7 (the seventh BML to be loaded on-board Voyager 2), in conjunction with the baseline sequence, provides this automated protection against loss of command capability. BML-7, with some differences in implementation for the two spacecraft, is loaded on-board both Voyager 1 and 2.

BML-7 is designed to meet the following science and mission objectives:

1. Continue to provide baseline sequence FPW data after a loss of command capability, for as long as the spacecraft continues to operate. In order to achieve this, BML-7 modifies the baseline sequence events to limit spacecraft activities (i. e. terminate MAGROLs and ASCALs) and configures the spacecraft FPAs for compatibility with the loss of command reception capability.

2. Continue to provide baseline sequence periodic calibrations of the science instruments (PMPCALs) in order to allow science teams to properly interpret the data.

3. Continue to perform baseline sequence DTR maintenance in support of the PWS high rate data acquisition, until DTR operations are terminated

4. Provide periodic gyro conditioning to maintain gyro integrity for FPA responses. Gyro conditioning is continued until the sequenced termination of gyro operations

5. Include reasonable provisions for maintaining an acceptable telecom posture for receipt of telemetry data via the X-band link.

6. Include additional reasonable fault protection enhancements.

7. Be transparent to the normal VIM sequence activities.

In response to these objectives, the design of BML-7 is organized into three independent tables, the BML Long-Term Events Table, the BML Initialization Table 1 (BMLIT1) and the BML Initialization Table 2 (BMLIT2). BMLIT1 and BMLIT2 are initiated after entry into the CMDLOS routine and contain commands that are only needed in the event command reception capability is lost. The BML Long-Term Events Table is a continuously active time event region in the CCS that executes independently of BML (the events contained in this table will occur whether BML-7 is entered or not).

BML is initiated with the execution of BMLIT1. The BMLIT1 events will be executed two weeks after CMDLOS entry if the CMDLOS timer has not been reset, but not prior to the end of a current overlay sequence plus 24 hours. The former condition indicates that BMLIT1 will not execute if command ability is restored. The latter condition is necessary to maintain the transparency of BML-7 from the normal sequencing activities. It is critical for BML to be independent of an active overlay sequence to avoid the necessity of constraining the contents of the overlay sequence to be compatible with BML-7.

A flag (CCS location 6714B) used in normal sequence design to allow the multiple uplink of a sequence load is also used in BML-7 to check for the end of an overlay sequence. This flag is set to 1 by the successful reception of all command blocks of an overlay sequence transmission. It remains set to 1 during execution of the overlay sequence and is automatically reset to 0 when the next planned uplink window opens at the end of an overlay sequence. This flag constrains the BMLIT1 events to not occur until 24 hours after the flag has been reset to 0 by the completion of the overlay sequence, even if the time from CMDLOS entry has exceeded 2 weeks.

Once initiated, BMLIT1 will:

1. Disable future attitude control maneuvers (ASCALs and MAGROLs). ASCALs will not be needed if the HPOINTS (sun sensor bias commands to update HGA pointing) cannot be reloaded, and MAGROLs may require update of the gyro scale factor, neither of which can be done if spacecraft command capability does not exist.

2. Initiate gyro conditioning. One pair of gyros will be powered on for a one week duration with the gyro fault test enabled once a year.

3. Select the 0.05 degree yaw deadband and the 0.25 degree roll deadband. All the future attitude control deadband will be 0.05/0.05/0.25 degrees, pitch/yaw/roll to maximize telecom performance.

4. Select the 160 bps downlink data rate for cruise data return.

5. Select the Attitude Propulsion (AP) thruster pulse width to the nominal 10 ms duration.

Prior to the start of the A022 sequence (October 16,1995) for Voyager 1 and the start of the B025 sequence (January 15,1996) for Voyager 2, the command loss timer for each spacecraft was set to four weeks. Beginning with sequences A022 and B025, the command loss timer for each spacecraft is set to six weeks. If command reception is not achieved during the duration of the command loss timer following the last successful command reception, entry into the CMDLOS routine automatically occurs. CMDLOS entry initiates a programmed response to try and establish a command reception capability using different combinations of redundant command reception hardware. The reception of a ground command by the spacecraft during CMDLOS execution terminates CMDLOS and resets the CMDLOS timer while maintaining the successful command reception hardware configuration. After CMDLOS entry, the CMDLOS routine is executed two consecutive times. After the second execution of the CMDLOS routine, the routine is permanently disabled and BML will take effect. The BMLIT2 events will occur approximately six months after the initiation of the second, and final, CMDLOS execution if the command loss timer is set for six weeks

Once initiated, BMLIT2 issues commands to power down the scan platform and configure the spacecraft for the long-term BML mission. Specifically, BMLIT2 will:

1. Power down the scan platform. The following electrical loads will be powered off if not already off:

• Imaging Subsystem (ISS) Wide Angle (WA) Vidicon Replacement Heater
  
• ISS WA Electronics Replacement Heater
  
• ISS Narrow Angle (NA) Vidicon Replacement Heater
  
• ISS NA Electronics Replacement Heater
  
• Scan Platform Azimuth Actuator Heater
  
• Azimuth Actuator Coil Heater
  
• Infrared Interferometer Spectrometer and Radiometer Subsystem (IRIS) Replacement Heater
  
• Ultraviolet Spectrometer Subsystem (UVS)

2. Initialize the Low Energy Charged Particle Subsystem (LECP) for data gathering.

3. Initialize the CRS for data gathering.

4. Select data rate of 1.4 kbps for DTR playback of high rate PWS data.

5. Inhibit switch of X-band power level from high to low power after DTR playback of high rate PWS data.

6. Configure the RF subsystem for the BML mission by establishing X-high power for all BML downlink operations if the prime X-band traveling wave tube (TWT) is operating on Voyager 1 and either X-band TWT is operating on Voyager 2. Because of the previous TWT switch on Voyager 1, X-Hi is maintained only if the current prime TWT is operating. If the current prime TWT fails and a switch to the backup TWT is made, X-Lo will be used for telemetry transmission.

7. Modify PWRCHK fault protection algorithm to select X-band TWT to the BML configuration after a PWRCHK entry.

8. Link the AACS Trajectory Correction Maneuver (TCM) thrusters as a second backup to provide pitch/yaw attitude control if the primary and backup thruster branches have failed.

The BML Long-Term Events Table is a separate continuously executing time event region that contains all of the unconditional long-term events for baseline sequence and BML operation. These events have absolute time values associated with them and will occur independent of whether the BML has been initiated. Key spacecraft reconfiguration events contained in this table include the termination of gyro and DTR operations.

7. 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 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)
  
• Command Loss (CMDLOS)
  
• Radio Frequency Power Loss (RFLOSS)
  
• CCS Error (ERROR)
  
• Power Check (PWRCHK)

The main function of the AACSIN routine is responding to AACS anomalies by the processing of Power Codes (PCs) received from the AACS. A PC is a regularly transmitted message from the AACS to the CCS providing status information about the AACS. Some of those PCs relating to a faulty AACS condition include heartbeat failure, celestial reference loss/acquisition, power supply failure, gyro failure, scan slew abort, command parity error, and bad/no echo response. The PCs are either functional or informational: the functional PC is used to generate a command to the power subsystem to initiate the proper response, and the informational PC is used by the CCS to invoke the preprogrammed responses. In most instances, the CCS echoes the PC back to the AACS to confirm receipt.

The purpose of the CMDLOS routine is to provide a means for the spacecraft to automatically respond to an on-board failure resulting in the inability to receive ground commands. Whenever a specified number of hours have elapsed without the CCS receiving a valid command, the CCS assumes a spacecraft failure and attempts to correct that failure by systematically switching to redundant hardware elements until a valid command is received. CMDLOS will be executed four consecutive times if command reception is not successful. After four unsuccessful executions, CMDLOS will be permanently disabled and BML will be activated.

The RFLOSS routine provides the spacecraft a means of automatically recovering from a failure of an X-band exciter or transmitter. The CCS monitors four input signals from the RFS that are associated with a failure of the exciters and transmitters. Whenever one or more of the above signals is input to the CCS, RFLOSS will systematically interrogate the four interrupts and attempt to correct the failure by swapping to a redundant unit.

If an AACSIN, CMDLOS, or RFLOSS entry occurs, the response will be integrated into any on-going sequencing activities which typically includes the baseline load, overlay load, HPOINTS and any unconditional long term events that are contained in the BML Long-Term Events Table. The commands from an FPA response and the regular sequencing activities will be interleaved.

The ERROR routine provides the capability to respond to certain anomalous CCS hardware and software conditions. In the event of a detected anomaly, the routine puts the CCS in a known, quiescent state and waits for ground action. It also stops any on-going sequencing activities including baseline load, overlay load, HPOINTS, and BML Long-Term Events Table. However, if an ERROR entry occurs in the BML mission, the sequencing activities will be restarted by the programmed rollback feature as described below.

The purpose of the PWRCHK is to provide 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 a PWRCHK entry is due to an undervoltage condition, the routine issues commands reducing the spacecraft power load in an attempt to recover from the failure condition and configure the spacecraft to a safe, low-power mode. If the CCS tolerance detector trip occurs indicating that the CCS input power has dropped below a level where the processor can reliably function, the spacecraft assumes that all other loads have experienced power-on-resets and issues the commands to re-enable the essential functions. In addition, due to the long round trip light time (RTLT), reduced Deep Space Network (DSN) coverage and BML vulnerability, a set of commands was designed and implemented on the spacecraft for VIM to configure the science instruments and DTR back to the nominal mission.

Since the PWRCHK is entered via ERROR routine, all the sequencing activities including heartbeat, baseline load, overlay load, HPOINTS and BML Long-Term Event Table will be terminated as in the ERROR entry. However, the baseline load and HPOINTS will be restarted by rollback, and the heartbeat and the BML Long-Term Events Table by PWRCHK.

The rollback is a special feature designed to allow one time event region per processor to be automatically restarted in the event of a PWRCHK entry or an ERROR entry in the BML mission. The rollback keeps track of timing of that particular event so it can be restarted as if there were no interruption. The CCS processor A rollback is used to restart the HPOINT table and the CCS processor B is for the baseline load.

In addition, one location in PWRCHK is reserved for restarting the BML. Unlike the baseline load or HPOINTs, timing for BML Long-Term Events is not kept track of, and that location has to be periodically updated in the BML Long-Term Events Table to point to the next long-term event. As a result, some of BML long-term events may be repeated. However, this will not pose any harm to the spacecraft.

8. 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 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. Modifications planned are listed in the following tables by specific events and times for each spacecraft. The first tables illustrate power events that have occurred to the end of 2008.

Voyager 1 Power Events to end of year 2008

YEAR/DOY	HARDWARE COMPONENT	POWER
1990-017	IRIS FOH OFF	31.80
1990-017	IRIS REPL HTR ON	7.81
1990-045	ISS WA OFF	16.80
1990-045	ISS NA OFF	18.00
1990-176	SS HTR A OFF	1.80
1990-176	SS HTR B ON	2.90
1992-245	S‑BAND EXC ON	2.37
1992-245	USO OFF (failed)	2.66
1995-290	PPS SUPP HTR OFF	2.80
1995-290	UVS REPL HTR ON	2.40
1995-290	ISS NA OPTICS HTR OFF	18.00
1995-290	IRIS STBY A SUPPLY OFF	7.20
1998-154	IRIS OFF	6.60
1998-208	ISSNA VID REPL HTR OFF	5.50
1998-208	ISSWA VID REPL HTR OFF	5.50
2002-087	HYBIC 1 PWR ON (SWAP)	12.00
2002-087	HYBIC 2 PWR OFF	12.00
2002-087	CST 1 ON (SWAP)	2.05
2002-087	CST 2 OFF	2.05
2002-141	SCAN PLAT SUPL HTR ON	6.00
2002-141	ISSWA ELEC REPL HTR OFF	10.50
2003-210	AZ ACT COIL HEAT OFF	4.44
2003-267	AZ ACT HTR OFF	3.52
2005-024	ISSNA ELEC REPL HTR OFF	10.50
2007-032	PSU INSTR OFF	2.40
2007-032	PLS OFF	4.20
2007-130	PLS REPL HTR OFF	4.32
2008-015	PRA OFF	6.60

 

Future Voyager 1 Power Events

~ 2010-300 - Terminate UVS operations - turn-off all Scan Platform loads (18.6 W). Date may change.

• IRIS Replacement Heater OFF (+7.8 W)
  
• Scan Platform Supplemental Heater Off (+6.0 W)
  
• UVS Power OFF (+2.4 W)
  
• UVS Replacement Heater OFF (+2.4 W)
  

2014 - Termination of DTR operations (+5.8 W for DTR turnaround)
This power load reduction step is currently sequenced to occur on DOY 238, 2014 and could be changed if the current performance trends continue.

2016 - Discontinue gyro operations (+14.4 W steady state, +3.6 W turn on transient). This power load reduction step is currently sequenced to occur on DOY 350, 2016.

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

Voyager 2 Power Events to end of year 2008

YEAR/DOY	HARDWARE COMPONENT	POWER
1989-283	ISS WA OFF	16.80
1989-339	ISS NA OFF	18.00
1991-093	PPS OFF	1.20
1994-060	ISS NA OPTICS HTR OFF	2.60
1996-023	ISSNA VID REPL HTR OFF	5.50
1996-023	ISSWA VID REPL HTR OFF	5.50
1998-316	AZ ACT COIL HEAT OFF	4.44
1998-316	AZ ACT HTR OFF	3.52
1998-316	UVS OFF	2.41
1998-316	UVS REPL HTR OFF	2.40
1998-316	ISSWA EL REPL HTR OFF	10.50
1998-316	ISSNA EL REPL HTR OFF	10.50
1998-316	IRIS REPL HTR OFF	7.81
2006-325	TCM PRESSURE XDCRS OFF	2.0
2006-325	PSU INSTR OFF	2.40
2006-341	MAG OBLFM FWD FLPR OFF (ISA)	10.20
2007-032	IRIS OFF	6.60
2007-254	TERMINATE DTR OPERATIONS	5.80
2008-052	PRA OFF	6.60

Future Voyager 2 Power Events

2011 - AP Branch 2 Heater OFF (+11.8 W)
This power reduction step is currently sequenced to occur on DOY 033, 2011.

2014 - Discontinue gyro operations (+14.4 W steady state, +3.6 W turn on transient)
This power load reduction step is currently sequenced to occur on DOY 120, 2014 and could be changed if the current performance trends continue.

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