OnOrbit Stationkeeping With Ion Thrusters Telesat Canadas BSS702 Experience

1
On-Orbit Stationkeeping With Ion
ThrustersTelesat Canadas BSS702 Experience

• Track T3 - SpaceOps 2004
• Wednesday, May 19
• Tim Douglas, Christine Kelly, Tony Grisé
• Flight Dynamics Operations
• Telesat Canada

2
References

• Details of Boeing BSS-702 XIPS operations are
  taken from the public
• domain
• David Erzen, Xenon Ion Propulsion System,
  http//kgb.ijs.si/kzagar/fi96/seminarji99/XIPS.do
  c.
• Boeing Satellite Systems, XIPS The Latest Thrust
  in Propulsion Technology, http//www.boeing.com/de
  fense-space/space/bss/factsheets/xips/xips.html.
• John R. Beattie, XIPS Keeps Satellites on Track,
  The Industrial Physicist, vol 4 issue 2, American
  Institute of Physics, June 1998.
• Paul J. Wilbur, Vincent K. Rawlin, J.R. Beattie,
  Ion Thruser Development Trends and Status in the
  United States, vol 14 No. 5, Journal of
  Propulsion and Power, September October, 1998.
  
• Boeing Satellite Systems, Electric Propulsion,
  http//www.boeing.com/ids/edd/ep.html25cm.
• B. Anzel, Stationkeeping the Hughes HS 702
  Satellite with a Xenon Ion Propulsion System,
  IAF-98-A.1.09, 49th International Astronautical
  Congress, Melbourne, Australia, 1998.
• B. Anzel, Controlling a Stationary Orbit Using
  Electric Propulsion, DGLR/AIAA/JSASS 20th
  International Electric Propulsion Conference,
  Garmisch-Partenkirchen, Germany, 1988.

3
Introduction

• Telesat took control of Anik F1 in September
  2001, and in the following two months the two
  XMRadio satellites, XM2 (Rock) and XM1
  (Roll).
• Telesat Canada is one of only two operators of
  the BSS 702 bus to date.
• The move to ion propulsion has brought new
  control strategies, and required enhanced
  estimation techniques and improved monitoring
  processes.
• The result has been very successful.

4
BSS 702 Xenon Ion Propulsion System (XIPS)An
Introduction
5
XIPS Thruster History

• The concept of electric propulsion was introduced
  by Dr. Wernher von Braun in the 1930s.
• The first ion engine was developed in 1961.
  Early engines used Cesium or Mercury - unsuitable
  for commercial operations.
• In the 1980s and 90s ion engines using inert
  Xenon gas were developed.
• The first commercial spacecraft to use Xenon ion
  engines was launched in 1997.

6
XIPS Thruster Simplified Functional Diagram

• XIPS thrusters work by accelerating Xenon ions
  through a series of charged grids.

7
BSS-702 Xenon Ion Propulsion System

• 4 XIPS thrusters (in red) are mounted on gimbals
  on the anti-Earth deck.
• XIPS thrusters provide very low thrust - about 79
  mN, compared to up to 22N for bipropellant
  stationkeeping.
• Efficiency is high - Isp values around 3400 sec
  versus bipropellant Isps of 300 sec.
• On-station, XIPS firing is used for

- Stationkeeping - Momentum Control
- Station Change - Deorbit
8
XIPS ThrustVector Components
Thrust Vector

• XIPS thruster thrust vectors have Normal, Radial
  and Tangential components.
• The nominal thruster alignment points the thrust
  vector through the spacecraft centre of mass
  (CM).

X (Tangential)
a (Elevation)
? (Azimuth)
Z (Radial)
a 50º ? 13º
Y (Normal)
9
XIPS vs. Chemical Propulsion Stationkeeping
Strategies
10
XIPS StationkeepingMultiple Daily Burns

• Ion thrusters provide very low thrust, so
  frequent maneuvers are required.
• In BSS-702 XIPS stationkeeping, four burns are
  executed each day. Each thruster fires once or
  each thruster of a diagonal pair fires twice.
• The four stationkeeping burns are optimized to
  provide the required daily change in inclination,
  drift and eccentricity and dump momentum from the
  momentum wheels

11
Inclination Control(Traditional Method)

• Inclination vector i grows roughly towards 90
  from Aries.
• North-facing thrusters fired at Ascending Node to
  move velocity vector to target plane, moving
  inclination vector to i.

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Inclination Control(Traditional Method)
Mvr 1,2
Mvr 3,4
i
Mvr 3
?
Mvr 1
Mvr 4
Mvr 2

• Periodic maneuvers move the inclination vector
  from the top of the allowable range to the bottom.

13
Inclination Control (XIPS)

• Two North XIPS firings around 90 from Aries and
  two South firings around 270 control
  inclination.
• Multiple daily corrections maintain the orbit
  always close to the target plane.

14
Inclination Control(XIPS)
Inclination deg
6-Month Trend

• Daily XIPS orbit corrections allow very close
  control of inclination, so latitude librations
  are very small.

15
Drift and Eccentricity Control(Traditional
Method)

• One or two maneuvers can be used for drift and
  eccentricity control.
• Eccentricity changes at 90 to the maneuver DV
  (opposite directions), and drift changes at 180
  to the DV (same direction).

16
Drift Eccentricity Control(Traditional Method)
Mean e
Longitude 60-day trend
180-day trend

• Periodic maneuvers maintain eccentricity on a
  sun-synchronous circle to a value which can be
  tolerated in the daily longitude oscillations. A
  longitude drift cycle utilizes the full s/k box.

17
Drift Eccentricity Control(XIPS)

• All four XIPS burns contribute to inclination
  control drift and eccentricity are controlled by
  varying the magnitudes of the burns to provide a
  net in-plane DV.

18
XIPS StationkeepingDrift Eccentricity Control
Mean e
365-days
Right Ascension

• Daily XIPS orbit corrections allow essentially
  zero eccentricity and small longitude excursion
  for a cycle in the order of 0.025.

19
XIPS Stationkeeping Momentum Control

• With chemical propulsion, torques applied during
  maneuvers are often used to drive the satellite
  momentum to a desired state.
• In XIPS stationkeeping, two of the four daily
  burns correct momentum.

20
Orbit Determination for XIPSReal-Time Kalman
Filter
21
OD for XIPSReal-Time Kalman Filter

• The Flight Dynamics System (FDS) includes an
  Extended (non-linear) Kalman Filter.

22
OD for XIPSReal-Time Kalman Filter

• The Kalman Filter processes tracking data as each
  measurement is taken, providing a near real-time
  orbit estimate update.
• This is in contrast to batch processing
  algorithms which fit the best orbit to a set of
  measurements taken over some time interval (hours
  or days).
• It also monitors telemetry for XIPS thruster
  firings, and generates maneuver event records on
  an FDS database file.
• These events are used to propagate the orbit for
  the Kalman Filter, and can be used for
  predictions or analysis purposes.

23
OD for XIPSReal-Time Kalman Filter

• A key advantage of the Kalman filter is that
  Range residuals can be provided immediately to
  the satellite control centre to show that Ranging
  is successful.

24
BSS 702 Kalman FilterState Parameters

• The estimated state parameters for the FDS Kalman
  Filter are
• orbit (position and velocity) vector
• solar radiation force correction
• tracking antenna biases (Az, El and Range for 6
  stations)
• XIPS thruster biases (thrust, gimbal rho angle,
  and gimbal gamma angle for the four thrusters).
• The orbit vector is always estimated. Estimation
  of the other state parameters is optional.

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BSS 702 KF OverviewData Processing

• The filter will process tracking data for up to 6
  tracking stations (normally no more than 2 are
  used).
• Automatic culling of data (at a specified sigma
  level) helps to prevent filter divergence.
• A range bias is estimated for the closest to
  sub-satellite tracking station. For Anik F1 (at
  107.3W), range bias is estimated for Calgary.
• On initial start-up Telesat corroborates tracking
  biases with radar range data from MIT-Lincoln
  Laboratory to set the biases for the tracking
  station which will not be estimated.

26
BSS 702 KF OverviewProcess Noise

• Long-term filter stability is achieved through
  careful use of measurement and process noise in
  the Kalman Filter equations. Process noise must
  be appropriately defined
• Measurement noise (Az, El Range) is observable
  from the standard deviations in a weighted least
  squares orbit determination.
• Process noise is incorporated for velocity, Solar
  radiation force and estimated range biases.
• Process noise for thruster biases is handled
  uniquely in that the standard deviation for these
  biases is increased only at the time of firing.

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BSS 702 KF OverviewKalman Filter Features

• Some features of the FDS Kalman Filter
  application
• The icon visually displays the innovations and
  flags problems by turning red.
• A state estimate is archived daily to a database
  file.
• Estimated FDS Kalman Filter state parameters can
  be plotted
• Tracking data and XIPS burns can be reprocessed
  off-line for analysis or problem solving.
• A state analysis printout can be generated
  (automatically each day or user initiated) with
  the orbit state and standard deviation, tracking
  data statistics and residual plots, and longitude
  and inclination propagations.

28
BSS 702 KF OverviewKalman Filter Stability

• In 2 ½ years of operation, there have been no
  occurrences of filter divergence issues.
• Only very occasional operator intervention has
  been required over this time, usually when a
  telemetry glitch or telemetry data loss has
  corrupted the construction of a XIPS event.
• The FDS Kalman Filter has provided precise,
  highly automated orbit determination and XIPS
  burn event generation.

29
XIPS Stationkeeping
30
XIPS Stationkeeping On-Board Burn Plan

• XIPS burns are planned by the satellites
  on-board computer.
• The on-board computer develops a maneuver plan
  for the day to maintain the orbit and momentum,
  with four XIPS firings.
• Orbit change requirements are uploaded from the
  ground for each day of a stationkeeping cycle
  momentum change requirements are computed
  on-board each day.
• Currently, Telesat uploads a stationkeeping plan
  every 3 weeks.

31
XIPS Stationkeeping Concepts A B Burns

• The 4 station-keeping burns are divided into
  pairs A burns and B burns.
• Each pair uses a diagonal pair of XIPS thrusters
  (N1/S2 or N2/S1).
• A burns control inclination, eccentricity and
  momentum.
• B burns control inclination, eccentricity and
  drift.

32
XIPS Stationkeeping Planning
33
XIPS Planning FDS Software

• The Flight Dynamics System (FDS) stationkeeping
  planning software mimics the behaviour of the
  software in the satellites on-board computer to
  find orbit change requirements that the satellite
  will be able to satisfy for each day of the plan.
  

34
XIPS PlanningFDS Software

• Orbit change requirements are computed for each
  day of the cycle to achieve the specified
  end-of-cycle targets in inclination,
  eccentricity, drift and longitude.
• For each day, a set of momentum change
  requirements is computed, enveloping the values
  expected on-orbit.
• The software submits each set of orbit and
  momentum change requirements to the burn planning
  algorithm. This algorithm is identical to that
  in the satellite on-board computer.
• The burn planning algorithm returns a set of four
  XIPS burns (a burn table) and/or error flags
  indicating failure to achieve the required orbit
  or momentum change.

35
XIPS PlanningFDS Software

• If the algorithm cannot satisfy the orbit change
  and momentum control requirements together, the
  momentum change requirement is reduced by half,
  and a warning message is generated (Degraded
  Table Ready).
• If the reduced momentum change requirement still
  cannot be met, the momentum change requirement is
  eliminated altogether and another error flag
  returned (Twice Degraded Table Ready).
• This parallels the behaviour of the on-board
  software. However, the planning software must
  avoid the failure to generate a burn table. If
  the orbit change cannot be satisfied with no
  momentum change at all, it is reduced in steps of
  10 until it can be met.

36
XIPS PlanningFDS Software

• Unfortunately, experience has shown that other
  variables (i.e. changes in on-board estimated
  parameters from the time of the plan to the
  actual burn day) can result in failure of the
  on-board computer to generate a burn table when
  orbit change requirements are reduced.
• For this reason, any burn plan with reduced orbit
  change requirements is rejected.
• If necessary, the orbit change targets for the
  stationkeeping cycle are modified so that they
  can be satisfied with no reductions.

37
XIPS PlanningProcess Assemble Data

• The following data is gathered to prepare for the
  plan
• State Analysis log and orbit state biases plot
  (from FDS Kalman Filter)
• Momentum dump and Solar Wing power telemetry
• Nominal reconstructed thrust levels for each XIPS
  thruster (from reconstructed event file)
• Previous cycle plan data
• Current XIPS burn table log.

38
XIPS PlanningProcess Compute Plan(s)

• Two instances of the planning software are run in
  parallel with different A-burn thruster sets.
  The best plan is selected, based on
• No orbit change reductions.
• No reduced momentum dumps.
• A-Burn separation angles conducive to good
  momentum control authority.
• Consistent A-Burn and B-Burn durations throughout
  cycle.
• If both plans are good, keep the same set of
  A-burn thrusters as the cycle before.
• Consistent orbit change requirements throughout
  the cycle.

39
XIPS PlanningReview/Approval

• The final plan undergoes a rigorous two-stage
  review process.
• A detailed package is reviewed first by another
  FDO Analyst and then by the FDO Manager,
  consisting of
• Maneuver Message, upload files and summary table
  
• Inclination, longitude and drift predict plots
  for the cycle
• Summary and detailed spool file and plot output
  from the planning software
• State analysis output and state bias plot
• All supporting data used for the plan
• Planning data from previous cycle for comparison.

40
XIPS PlanningUpload Dissemination

• Once the upload plan is approved, the Maneuver
  message and upload files are submitted to the
  Satellite Control Centre.
• Planning data is archived, upload and summary
  data is sent electronically to Satellite
  Engineering and FDO, and eclipse and burn tables
  are copied to the Telesat Intranet for reference.

41
XIPS PlanningSummary Table
ANIK_F1 Planned Burns Summary with Eclipse Times
and FDO Eclipse Padding Times (2002) End
Eclipse Planned ... Planned
Eclipse Start Eclipse Padding
Burn 1 ... Burn 4
Padding Eclipse --------- ---------
------------------- ... -------------------
--------- --------- 2500731 2500836
2491355-1430 N1 ... 2500415-0453 S2
2500540 2500644 2510732 2510836
2501352-1426 N1 ... 2510355-0430 S2
2510536 2510642 2520733 2520836
2511356-1429 N1 ... 2520322-0358 S2
2520536 2520641 2530734 2530836
2521352-1424 N1 ... 2530259-0337 S2
2530536 2530639 2540734 2540840
2531330-1402 N1 ... 2540253-0334 S2
2540532 2540638 2550735 2550841
2541355-1434 N1 ... 2550305-0321 S1
2550533 2550636 2560735 2560841
2551303-1339 N1 ... 2560321-0341 S1
2560533 2560635 2560735 2560845
2561335-1419 N1 ... 2570334-0346 S1
2570529 2570634 2570736 2570841
2571312-1354 N1 ... 2580237-0325 S2
2580529 2580633 2580736 2580841
2581320-1402 N1 ... 2590226-0315 S2
2590529 2590632 indicates momentum
control thruster

• A cycle summary is posted to the Telesat intranet
  for reference a sample is shown above.

42
XIPS Stationkeeping Monitoringand Performance
Evaluation
43
XIPS Monitoring Evaluation

• Monitoring daily operations and stationkeeping
  performance and prompt reaction to anomalous or
  unexpected behaviour is critical to successfully
  operating in the ion thruster environment.
• WHY?

44
XIPS Monitoring Biased Mean Drift Rate

• Radial thrust component moves mean longitude to
  the East. The orbit radius (and hence the mean
  drift rate) is biased to compensate.
• Longitude control is very tight but any
  interruption in XIPS firing can lead to
  stationkeeping limit violations in a few days due
  to the biased drift rate.
• Further, the XIPS low thrust means that any
  corrective action will take time.

45
XIPS Monitoring Biased Mean Drift Rate

• In a typical stationkeeping plan day the mean
  drift rate can vary between XIPS burns on the
  order of 0.01 to 0.02/day.
• In the example, on June 13th the planned mean
  drift varies between 0.007/day and 0.020/day.

Latitude
Longitude
Mean Drift Rate
46
XIPS Monitoring Biased Mean Drift Rate

• If the XIPS burns are interrupted, the mean drift
  rate stays at the intermediate value of
  0.015/day.
• If no action is taken, the satellite leaves the
  stationkeeping box within 3 days.

Latitude
EXAMPLE
Longitude
XIPS Abort
Mean Drift Rate
47
XIPS Monitoring Momentum

• An interruption of XIPS firings can result in
  momentum build up. Alternately, excessive
  momentum build-up can interfere with the daily
  burn plan.
• Manual momentum unloading can be performed to
  reduce excessive momentum build-up.
• Monitoring momentum unloading performance and
  momentum values throughout the cycle is
  necessary.

48
Daily MonitoringEnd of Cycle Longitude

• Having the orbit determination process run
  continuously in real-time has allowed Telesat to
  automate some very useful monitoring tools.
• A program runs daily to plot the predicted end of
  cycle longitude and latest estimated gimbal
  biases.
• This provides a quick and easy check on the
  progress of the cycle as it progresses.
• If observed performance is deviating from
  expected, corrective action can be made before
  the effects are severe.

49
Daily MonitoringEnd of Cycle Longitude
Predicted End of Cycle Longitude

• The planned end of cycle target was for the
  center of the s/k box.
• As the cycle progresses we observe the predicted
  end-of-cycle longitude will be lower than the
  original target.
• If the predicted deviation warrants it, a revised
  plan can be uploaded.

N1 Rho/Gamma
N2 Rho/Gamma
S1 Rho/Gamma
S2 Rho/Gamma
50
Daily MonitoringBurn Plan Execution

• A program runs twice daily to retrieve and
  summarize telemetered burn information, first
  when the burn plan is computed and again when all
  the burns have been executed.
• The text file created is printed and also posted
  to a the Telesat intranet, allowing easy access
  by Flight Dynamics, Satellite Engineering and the
  Satellite Control Center.
• Failure to compute a burn plan or discrepancies
  between planned burns and executed burns are
  flagged in the file for quick inspection.

51
Daily MonitoringBurn Plan Execution

• ANIK_F1 Burn Table Summary updated at
  2002006110707000 UTC
• XIPS Scheduler Status Current Burn Table
  Retrieve
• XPS Retrieve Buffer Select 1
• XPS Retrieve Index - Current 3
• - Last Written 14
• Thr Start Epoch Julian
  Day Planned Duration Actual Duration
• --- -------------------------
  ------------ -----------------
  -----------------
• Burn Table 2 Received
• 2002005061032664 UTC 734
  65432.66
• Burn 1 S1 2002005185257020 UTC 735
  24777.02 5254.76 012734 5244.87 012724
• Burn 2 S2 2002006043137371 UTC 735
  59497.37 833.86 001353 830.84 001350
• Burn 3 N2 2002006051614895 UTC 735
  62174.89 5880.70 013800 5869.72 013749
• Burn 4 N1 2002006082907875 UTC 735
  73747.87 735.26 001215 732.24 001212

52
Tri-Weekly MonitoringPredict Plots

• 3 times weekly a predict plot of inclination,
  longitude, and mean drift rate is automatically
  generated for the next 10 days using planned XIPS
  burn events.
• Progression of cycle propagation is checked
  against the plan and the stationkeeping limits.

Inclination
Longitude
Mean Drift Rate
53
Real-Time MonitoringStripchart Monitors

• A software strip-chart program allows for
  constant, real-time monitoring and alarming on
  any telemetry data.
• This tool is used extensively and on a continuous
  basis by the Satellite Control Center, Satellite
  Engineering, and Flight Dynamics Operations to
  monitor the spacecraft status and health.
• Graphical presentation of telemetry data is much
  easier to review, trend, analyse, and respond to.
• Audible and visual alarming is configured for
  critical parameters.
• The tool is also accessible off-site to key
  personnel for monitoring or anomaly and emergency
  response.

54
Real-Time MonitoringStripchart Monitors

• This page shows a variety of momentum and XIPS
  data over a two-day period.

55
Real-Time MonitoringStripchart Monitors

• Data can also be plotted in X vs. Y format in
  this case Yaw vs. Roll body momentum.

56
Daily/Weekly MonitoringStationkeeping Telemetry

• At the end of each Plan Day plots are
  automatically generated to summarize key
  attitude, momentum and XIPS burn telemetry.
• A weekly summary plot is also produced which
  assists in trend evaluation.
• These plots provide the analyst with a good
  overview of the control system status and help to
  highlight any changes to performance as early as
  possible.
• A hardcopy history is kept.

57
Daily MonitoringStationkeeping Telemetry
Momentum
XIPS
58
Weekly MonitoringStationkeeping Telemetry
Weekly Summary
59
XIPS StationkeepingTelesat Operational Summary

• Telesats BSS 702s operate with 3 week upload
  cycles.
• Stationkeeping control is much tighter with XIPS,
  typically better than 0.025 in latitude and
  longitude.
• Multiple daily XIPS firings require additional
  monitoring of both the orbit and momentum.
• Proper tuning of the Kalman filter for orbit
  and XIPS bias estimation makes the day-to-day BSS
  702 operations a very automated process requiring
  minimum man-power.

60
XIPS StationkeepingConclusions

• The advantages of XIPS operations are
  significant reduced propellant consumption,
  extended mission life, and very close control of
  the orbit.
• With the automation enabled by the Kalman filter,
  the workload for stationkeeping analysts consists
  largely of monitoring of the process and
  stationkeeping planning. While cycle planning is
  time-consuming, the overall workload is similar
  to chemical satellite stationkeeping.
• For the most part Telesats experience with XIPS
  stationkeeping operations has been extremely
  favourable, and we look forward to taking control
  of two more BSS 702 satellites in the next year.

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fin