GUIDANCE –GNC SENSORS AND NAVIGATION · Ground based orbit

GUIDANCE
NAVIGATION AND CONTROL

·        
The
GNC functions includes guidence algorithms ,navigation filters, controllers,
and possibly actuator managment functions.

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·        
Three
main functions on board a spacecraft

1.      Navigation/where am i: determining
the vehicle’s position and velocity or orbital elements as a function of time(attitude
, angular velocity, orientation).

2.      Guidence/ where to go:defining the
optimal state vectors of the vehicle as a function of time.

3.      Control /how to get there:the process
of deriving the commands to match the current and optimal vehicle state
vectors.

·        
Mainly
2 subsystems

1.      Attitude GNC

2.      Orbit GNC

ATTITUDE
GNC(functions)

1.      Payload pointing

2.      Antennae pointing

3.      Thermal control by changing
orientation

4.      Thrusting in the right direction   

                   ORBIT GNC(FUNCTIONS)

1.     Orbit maintanace

2.     Station keeping

3.     Relative motion control

Attitude GNC, sensors and navigation

·        
Sun sensors, magnetometers,
gyroscopes:

1.      deterministic three-axis attitude
determination
(TRIAD, QUEST ,q-method)

2.      low pass filter applied to the sensor
outputs

·        
Star tracker(uses on board catalogue of stars)

1.      Filter merge sensor measurements with
on board dynamics model

2.      High accurate and autonomous

3.      Do not require measurement of all
states

 

ATTITUDE GNC –GUIDANCE

·        
Way points (desired pointing directions)

·        
Trajectory generation(desires attitude profile)

 

ATTITUDE GNC-ATTITUDE CONTROL AND ACTUATORS

1)    Spin stabilization

i)       
Quaternion
control

ii)      Yaw steering in sun-syncnchronous
orbit

2)    Gravity gradient stabilization

3)    Magnetic  stabilization (magnetorquer)

4)    Zero momentum three axis control

i)       control moment gyros

ii)     reaction wheels

iii)   thrusters

 

ORBIT –GNC  SENSORS AND NAVIGATION

·        
Ground
based orbit determination(lamberts problem)

·        
GPS
receivers

ORBIT GNC – GUIDANCE

·       
Desired
way points

·       
Desired
relative trajectory

 

ORBIT GNC-CONTROL AND
ACTUATORS

·       
Impulsive
maneuvers to instantaneously modify the velocity

i.       
Ion
thrusters

ii.      Chemical thrusters

·       
continous
thrust to track a trajectory through electrical propulsion