Guidance and Navigation

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Guidance and Navigation

• AERSP 401A

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Definitions

• Navigation Orbit Determination determining
  satellites position and velocity (or orbital
  elements) as a function of time
• Real Time Orbit Determination estimates where the
  satellite is at the present time
• Definitive Orbit Determination provides the best
  estimate of the satellite orbit at some earlier
  time
• Ephemeris is a tabular listing of the state
  vector of the satellite as a function of time
• Orbit Propagation refers to integrating the
  equations of motion to determine where the
  satellite is at some other time

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Definitions

• Guidance Orbit Control adjusting the orbit to
  meet some predetermined conditions
• Orbit Maintenance refers to maintaining the orbit
  elements but not the timing of when the satellite
  is at any location in its orbit
• Altitude maintenance uses thruster firings to
  overcome drag
• Stationkeeping refers to maintaining the
  satellite within a predefined box
• Geosynchronous Stationkeeping maintains the
  satellite in a box over one place on the Earth
• Constellation Maintenance maintains the satellite
  in a moving box defined relative to other
  satellites in the constellation

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Process for Defining the Guidance and Navigation
Subsystem
Step Principal Issues Where Discussed
1. Define navigation and orbit-related top-level functions and requirements Mapping and pointing Scheduling Constellation or orbit maintenance Rendezvous or destination requirements 1.4, 2.1, 4.2, 7.1
2. Do pointing and mapping trades to determine preliminary navigation (position) accuracy requirements What payload functions will the navigation data be used for? Payload data processing (mapping) Payload pointing 5.4
3. Determine whether orbit control or maintenance is needed Geosynchronous stationkeeping Constellation stationkeeping Altitude maintenance Maintaining orbital elements Mid-course corrections Chapter 7, Section 11.7.3
4. If yes, do trade on autonomous vs. ground-based orbit control Is reduced operations cost and risk worth introducing a nontraditional approach? 2.1.2, 11.7.1, 11.7.3
5. Determine where navigation data is needed Is it needed only at ground station for mission planning and data evaluation? Is it needed on board (orbit maintenance, Sun vector determination, payload pointing, target selection)? Is navigation (or target location) data needed by several end users who may get information directly from the spacecraft? 2.1.1
6. Do autonomous vs. ground-based navigation trade Does reduced operations cost and risk justify a nontraditional approach? Is there a need for real-time navigation data? 2.1.1, 11.7.1
7. Select navigation method See section 11.7.2 for main options 11.7.2
8. Define GN system requirements Top-level requirements should be in terms of what is needed (mapping, pointing, constellation maintenance, level of autonomy) not how the mission is done 11.7.1, 11.7.4
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Alternative Navigation Methods
System Basis Status Determines 3? Accuracy Operating Range Comments
GPS Network of navigation satellites Operational Orbit 15 m 100 m in LEO LEO only Semi-autonomous
MANS Observations of Earth, Sun, and Moon Flight tested in 1993 Orbit, attitude, ground look point, Sun direction 100 m - 400 m in LEO (using only Earth, Sun and Moon LEO to GEO, lunar and planetary orbits Can use other instrumentation (GPS receiver, star sensor, IMUs to improve accuracy
Space Sextant Angle between stars and Moons limb Flight tested Orbit and attitude 250 m LEO to GEO Not being actively marketed for space at the present time
Stellar Refraction Refraction of starlight passing through the atmosphere Proposed, some ground tests done Orbit and attitude 150 m 1 km Principally LEO Could use attitude sensor data
Landmark Tracking Angular measurements of landmarks Proposed, observability conditions uncertain Orbit and attitude Several kilometers Principally LEO Could, in principle , use observation payload data
Satellite Crosslinks Range and range rate or angle measurements to other satellites in a constellation Proposed may be used on communications constellations Orbit Theoretically, as good as 50 m Principally LEO Operation with less than full constellation problematic no absolute position reference
Earth and Star Sensing Observe direction and distance to Earth in inertial frame Proposed Orbit and attitude 100 m-400 m in LEO LEO to GEO, planetary orbits Similar to MANS with higher accuracy and availability
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Alternative Navigation Methods Advantages and
Disadvantages
System Advantages Disadvantages
Ground tracking Traditional approach Methods and tools well established Accuracy depends on ground-station coverage Can be operations intensive
TDRS tracking Standard method for NASA spacecraft High accuracy Same hardware for tracking and data links Not autonomous Available mostly for NASA missions Requires TDRS tracking antenna Usable for Earth orbiting spacecraft only
GPS/ GLONASS High accuracy Provides time signal as well as position Semi-autonomous Depends on long-term maintenance and structure of GOS Orbit only Must initialize some units
MANS Fully autonomous Uses attitude-sensing hardware Provides orbit, attitude, ground look-point, and direction to Sun First flight test in 1993 Initialization and convergence speeds depend on geometry
Space Sextant Could be fully autonomous Flight tested prototype only not a current production product Relatively heavy and high power
Stellar Refraction Could be fully autonomous Uses attitude-sending hardware Still in concept and test stage
Landmark Tracking Can use data from observation payload sensor Still in concept stage Landmark identification may be difficult May have geometrical singularities
Satellite Crosslinks Can use crosslink hardware already on the spacecraft for other purposes Unique to each constellation No absolute position reference Potential problems with system deployment and spacecraft failures
Earth and Star Sensing Earth and stars available nearly continuously in vicinity of Earth Cost and complexity of star sensors Potential difficulty identifying stars
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Orbit Control

• The cheapest orbit control system is none at all
• May be able to eliminate propulsion system
  entirely
• Once separated from the upper stage or launch
  vehicle, no further orbit control is possible
• This is the case for most small satellites

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Orbit Control

• Orbit control is needed when any of the following
  is required
• Targeting to achieve an end orbit or position
  (flight to Mars)
• To overcome secular perturbations (altitude
  maintenance, geosynchronous stationkeeping)
• To maintain relative orientations (constellation
  maintenance)

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Orbit Control

• Two types of constellation maintenance
• Relative stationkeeping maintains relative
  position between satellites, but allows whole
  constellation to decay (or drift)
• Absolute stationkeeping maintains position within
  a predefined box
• For long-term constellation control, there are
  several advantages to absolute stationkeeping and
  no disadvantages

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Constellation Stationkeeping
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Ground-Based vs. Autonomous Orbit Control

• Traditionally, orbit maintenance and control is
  implemented from the ground
• Thruster commands may be stored on board for
  later execution
• In the past, there was no realistic alternative
• Autonomous navigation systems make autonomous
  orbit control possible, economical, and safe
  but still non-traditional

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Ground-Based vs. Autonomous Orbit Control

• Orbit and attitude control are analogous with
  several important differences
• Attitude control must be continuous to avoid a
  major system upset, with the possible risk to the
  spacecraft and the mission
• Orbit control is inherently fail-safe nothing
  bad happens immediately if it is not done
• Frequency of control
• Attitude control typically 1 to 10 Hz,
• Orbit control 10-4 to 10-5 Hz
• Less computational burden, more time to react if
  things dont work right

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Things to Look For

• Avoid double booking look for joint
  implementation of orbit and attitude
  determination and control when optimizing system
  performance
• Reasonable initial design will incorporate all
  functions into a single processor, although there
  may be other reasons to distribute
• Overall objective is to minimize the cost and
  risk of determining and controlling the orbit and
  attitude for the ENTIRE MISSION!!!!