Transcript Structures and Mechanisms Subsystems

Guidance and Navigation
AERSP 401A
Definitions
• Navigation = Orbit Determination = determining
satellite’s 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
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
Process for Defining the Guidance
and Navigation Subsystem
Step
Principal Issues
Where
Discussed
1. Define navigation and orbitrelated 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. groundbased 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 G&N 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
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
Moon’s 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
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
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
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)
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
Constellation Stationkeeping
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
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 don’t
work right
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!!!!