GNC

Guidance, Navigation, and Control (GNC) refers to the systems on a spacecraft that give it the capability to know where it is, which way it is facing, and how to get to where we want it to go.

Spacecraft Mass Properties

In this section, you must specify the dry mass (spacecraft mass without fuel) and dry inertia matrix for the Wildfire spacecraft. These values affect the ease with which the spacecraft can be moved and rotated.

Define the mass properties

Input these values:

• Dry mass: 1000 kg
• Dry inertia matrix:

  270	0	0
  0	420	0
  0	0	520

Fuel Tanks

Fuel tanks are used to store fuel and are aggregated together with fuel reservoirs.

Create a fuel tank

Add the following fuel tank:

Fuel Tank Name	Fuel Capacity (kg)	Initial Fuel Mass (kg)	Priority	Tank Shape	Cylinder Length (m)	Cap Diameter (m)	Length Orientation Vector	Tank Centroid Location [X, Y, Z]
Fuel Tank	35	35	1	Spherocylinder	0.3	0.3	+X	[0, 0, 0]

Fuel reservoirs

Fuel reservoirs are abstract fuel collections comprised of fuel tanks. Thrusters pull from fuel reservoirs, which in turn pull from one or more fuel tanks.

Create a fuel reservoir

Add the following fuel reservoir:

Reservoir Name	Fuel Tanks
Fuel Reservoir	Fuel Tank

Sensors

Sensors allow your spacecraft to collect information that it uses to detect where it is and which way it is facing.

Create a sensor

Add these sensors:

Sensor Name	Sensor Type	Direction Vector	1σ Errors	Sensor Field of View
Star Tracker	Optical Attitude Sensor		Cross-Axis: 0.0015° Boresight: 0.01°	Star Tracker FoV
Gyro	Angular Velocity Sensor		Per-Axis: 0.0004 rad/s
Magnetometer	Direction Sensor	Magnetic Field	Angle: 1°
GPS	Position Sensor		0.01 km

Actuators

Actuators give your spacecraft the ability to move and rotate itself.

Create an actuator

Before you add any actuators, you will need to momentarily skip ahead to add the power controller and bus regulators so that you can use them as power sources. Complete the Power Processing step of the Power section and then return here.

Reaction Wheels

Reaction wheels are the most commonly used actuators to control a spacecraft's orientation. They work by speeding up or slowing down a spinning disk in the reaction wheel housing, which in turn rotates the spacecraft.

To add a reaction wheel, select Reaction Wheel in the Actuator Type dropdown when creating an actuator. Add the following reaction wheels:

Actuator Name	Inertia (kg·m2)	Rated Torque (N·m)	Rated Angular Momentum (kg·m2/s)	Efficiency	Torque Body Frame Vector	Power Source
RW-X	0.02	0.25	8	90%	+X	28 V
RW-Y	0.02	0.25	8	90%	+Y	28 V
RW-Z	0.02	0.25	8	90%	+Z	28 V

Magnetorquers

Magnetorquers are another type of commonly used actuator. They work as electromagnets that use Earth's gravitational field to produce a very small force to rotate the spacecraft. They are often coupled with reaction wheels to help prevent the reaction wheels from becoming slowly overwhelmed.

To add a magnetorquer, select Magnetorquer in the Actuator Type dropdown when creating an actuator. Add the following magnetorquers:

Actuator Name	Rated Magnetic Moment (A·m2)	Power at Rated Magnetic Moment (W)	Torque Body Frame Vector	Power Source
MT-X	10	2	+X	12 V
MT-Y	10	2	+Y	12 V
MT-Z	10	2	+Z	12 V

Thrusters

Thrusters are used to change the velocity of a spacecraft and are often intended for use in changing the spacecraft's orbit. Wildfire will use its thruster to raise its orbit at the beginning of its simulation.

To add a thruster, select Thruster in the Actuator Type dropdown when creating an actuator. Add the following thruster:

Actuator Name	Specific Impulse (s)	Minimum Thrust (N)	Maximum Thrust (N)	Thruster Location [X, Y, Z]	Thruster Orientation	Fuel Reservoir	Power Source
Thruster	225	50	125	[0.5, 0, 0]	+X	Fuel Reservoir	Power Processor

Algorithms

Algorithms are used to determine the specific behavior of sensor measurement outputs and actuator inputs.

Create an algorithm

Add the following algorithms:

Algorithm Name	Algorithm Type	Algorithm Gains	Sensors/Actuators
Attitude Determination	MEKF		Gyro, Star Tracker
Orbit Determination	GPS Direct		GPS
Attitude Control	Sliding Mode	Gain K: 0.1 Gain G: 0.015 Gain C: 0.03 Epsilon: 0.07	RW-X, RW-Y, RW-Z, MT-X, MT-Y, MT-Z
Nominal Thrust	Static Thrust Control	Thrust: 100 N	Thruster

Pointing Modes

Pointing modes determine which direction your agent will try to point in a simulation. Each operational mode has an associated pointing mode, which in turn has associated algorithms. There are three main types of pointing modes:

• Passive: the agent will not use actuators to actively control its orientation
• Direction Lock: the agent will use actuators to try to align one of its body frame vectors with a reference vector
• Max Secondary Alignment: the agent will use actuators to try to align one of its body frame vectors with a reference vector and will simultaneously try to maximize the alignment between a secondary body frame vector and reference vector pair

Create a pointing mode

For each pointing mode, associate the algorithms you created above with the orbit determination, attitude determination, and attitude control algorithms slots. Only associate the thrust control algorithm with the Burn Pointing pointing mode. Add the following pointing modes:

Pointing Mode Name	Pointing Mode Type	Locked Body Frame Vector & Pointing Direction	Max Aligned Body Frame Vector & Pointing Direction
Burn Pointing	Direction Lock	-X & Ram
Crosslink Pointing	Direction Lock	Crosslink Boresight & LaserComm Relays
Forward Pointing	Max Secondary Alignment	-X & Ram	+Z & Sun
Nadir Idle Pointing	Max Secondary Alignment	-Z & Nadir	-X & Sun
Imaging Pointing	Max Secondary Alignment	-Z & Wildfires	-X & Sun