Week 6 - Communications + Attitude Determination & Control

Submitted by j.akagi on Tue, 01/26/2010 - 11:09am

Communications (COMM) subsystem was covered first. Students learned the primary function of this subsystem as well as components that are typically found within it. A variety of COMM-related concepts were then covered in slight detail including antennas, signal modulation (such as frequency and amplitude modulation, i.e. FM/AM), signal amplification,and transponders. Analogies were used for each of these concepts for relativity purposes.

Attitude Determination & Control Subsystem (ADCS) was then covered in a similar fashion as COMM. Students were educated on the two main constituents in the ADCS and their functionalities, as well as the relevance of positional stability (with several different missions used as examples). Session concluded with students briefly discussing ways to manipulate these constituents to accomplish the objective of the ADCS.

 

 
 
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Critical Thinking Problem Set 6.doc42 KB
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05 Apr10:22

Late answers to problem 3 and 4

By brownmk

3. The six degrees of freedom are movement in the x, y, and z directions as well as rotation about the x, y, and z axes. There are many forces that act on the satellite that affect its movements, the most prominent being gravity and collision.

4. Gravity is used to keep the satellite in orbit and can even be used in conjunction with on-board thrusters to keep the satellite in geosyncronous orbit.

It's a lot harder to use collisionary forces to your advantage because you have to be able to track the path of not only your satellite (which is easy) but that of the thing you want to collide with as well. But assuming you have something you want to collide with in order to correct your satellites pathing, you might want thrusters to stabilize the satellite as well as a gyro and flywheel in order to correct its attitude.

25 Mar14:12

Wei-Hann's Group: John

By jfurumo

Problem 3) 

A satellite can be considered to be a rigid body moving in a 3 dimensional Cartesian space. In this way, it would have a body frame of reference moving within a stationary inertial frame of reference. This body frame would have a set of 3 orthogonal axes, most often denoted X, Y, and Z. In order to describe translational and rotational motion, the position and orientation of the body frame could be related to the inertial frame. In this frame, the body could move translationally, that is, forward or backwards, along any one of the three axes or in a direction that is a combination of the three, described by a vector. The body could also rotate around one of the axes, or any combination of the three. This would be described by a torque or angular momentum. In this way, there are six degrees of freedom, or six different ways in which the spacecraft can move. Therefore, the six degrees include translation in any or each of the 3 principle directions described by the coordinate axes, and rotations around these same axes in either direction.
 
External forces acting on a satellite could cause any combination of these movements. They could cause strictly translational motion, pushing the satellite in a certain direction. Most likely, an external force would cause a combination of movements to occur, namely a translation and a rotation. External forces acting on a satellite would likely create a moment around one of the principal axes, causing it to rotate one way or another. Some examples of external forces acting on a satellite are gravitational and magnetic forces from the Earth or any other celestial body. Additionally, solar wind can create a force that will be enough to affect certain spacecraft, namely solar sails.
 
Problem 4) 
Gravity is the most obvious external force acting on a satellite, acting radially inward towards any body of significant mass, such as the Earth or Sun. This is how orbits are formed. Gravity is used to keep a satellite circling the Earth at a given speed and altitude. Therefore, it is vital that calculations be done to find proper launch conditions that will establish the proper orbit for a given satellite. While satellite orbits gradually decay with time, moving closer to the planet or star, thrusters can be used to adjust orbit, thereby maintaining a satellite in a proper orbit to execute its mission.
 
 Magnetic forces from the Earth’s magnetic field are also very important for satellite design. This magnetic field has been very precisely measured in terms of strength and orientation. Any satellite in orbit can make use of this field by containing an apparatus that responds to a magnetic field, such as an electromagnet (solenoid or torque rod). The magnetic field created by such an apparatus would respond to the field of the Earth, causing a force in the satellite that would align it accordingly. This knowledge can be put to good use to design an ADCS subsystem that uses the Earth’s magnetic field to orient itself in space. Torque rods can be oriented along the principal axes of the spacecraft, thereby allowing complete orientation capability based on the earth’s magnetic field. The EyasSat kit has two incorporated solenoids for this purpose, also known as torque rods. It also has a torque wheel apparatus that creates greater magnetic torque for alignment along its third axis.
25 Mar13:46

Wei-Hann's Group: John and Dana

By jfurumo

 

Problem 1) Using what you learned about COMM and ADCS subsystems, why do you think that commands used to control satellites are generally comprised of few characters?
 
The prospect of communicating with a satellite in earth orbit, while having been mastered, is still a daunting task. It requires precise knowledge of a satellites location and orientation. For most operations, it requires a satellite’s antenna or receiver to be pointed directly at the ground station, and vice versa. Even then, there is usually a short window of opportunity in which to transmit and receive data. Given the large distances in between satellite and ground station, transmit times can actually come into account even though the signals travel at nearly the speed of light. In order to maximize productivity, commands are usually kept short so that minimal time is spent by the satellite decoding and validating commands. This is especially true for commands of vital importance, such as real-time information about location such as GPS applications. Satellites used for relaying ground communications must also relay this information in a prudent manner.
 
Problem 2) Given that there are presently over 2300 satellites in Earth orbit, and that many of these utilize similar or identical frequencies to communicate, how would an engineer design a communications subsystem to deal with this problem?
 
There may be an almost infinite number of frequencies in the electromagnetic spectrum, but modern communications technology can only utilize a small range of these. Radio waves from 3 MHz up to 30 GHz are the common range used. Below this range radio waves will not have enough energy to penetrate the earth’s atmosphere and will be reflected back to the surface. In order to maintain communication links with many different satellites, one obvious method is to transmit at a different frequency to each individual satellite. This may seem logical but it might not be practical. Many frequencies in this range are utilized for ground communication applications such as cellular telephones and wireless internet.
 
The reality is that a satellite may use a frequency that many others use simultaneously. In order to avoid confusion or sending/receiving the wrong message, an engineer should design a COMM system that verifies signals that it receives, much the same way a CDH verifies signals it receives from other subsystems. Signals could be sent encrypting data that would identify the sender. In this way, a satellite would operate on a given frequency but only accept messages that come from the proper source. When decoded, a message should start with a few token characters that the satellite would recognize as the correct source.
24 Mar11:40

Elizabeth Viernes' Critical Thinking Response

By Elizabeth

3) The six degrees of freedom for a satellite are

1) Moving up and down
2) Moving left and right
3) Moving forward and backward
4) Tilting side to side (roll)
5) Tilting forward and backward (pitch)
6) Turning left and right (yaw)

External forces would orient a satellite in different directions, making it harder for the satellite to stay in a particular position. Two external forces would be electromagnetic waves and gravity.

4) To utilize electromagnetic waves, the satellite can have torque rods or coils that use current to generate a magnetic field. This field will attract to the Earth's magnetic field, and we can use that to our advantage to orient the satellite in the direction of the field. For gravity, we can use a gravity gradient so that if the rod is long enough, the Earth's gravity will pull onto the heavier side of the satellite and keep the satellite pointing towards Earth.

24 Mar02:05

Wei-hann's Group: Dana

By danarose

 3) The six degrees of freedom include movement up, down, left, right, forward, and backward.  This describes movement along three perpendicular axes, namely x, y, and z.  External forces that act upon a satellite module include earths gravitational and magnetic fields, and thrusters within the satellite.

4) In the design of the satellites attitude control system, I could use gravity to my advantage by designing on end of the satellite to be more massive causing it to orient itself toward earth.  That way any devices, radios, or cameras that require to be facing earth, can be placed in this location.  We would still require stabilization along the other 2 axes, and could use earths’ magnetic field to our advantage by applying a charge to the satellite creating a dipole effect that will cause the satellite to align itself with the present magnetic field.  Installing thrusters along the axes of rotation within the satellite would enable the satellite to autonomously adjust its position using sensor readings from horizon sensors, sun sensors, gyroscopes, and magnetometers.  The magnetometer can also be used in accordance with a charger to achieve stabilization within a magnet field.  

18 Mar03:52

Philip Truong's Answers to Questions 3 and 4

By philipq

3) The six degrees of freedom is the ability to move up/down, forward/backward, and left/right; and the ability to rotate along the three-dimensional Cartesian axes. External forces may act on the satellite module by pushing or pulling it along one or more of these axes to affect its movement. One of these external forces is gravity which pulls the more massive side of the satellite toward the center of the Earth. Another external force is the Earth's magnetic field, which may attract or repel charged parts of the satellite that may act on the satellite depending on the location and charge of the satellite in respect to the magnetic field. Also, electromagnetic waves (and therefore solar winds) or gas particles may push satellites away due to their momentum.

4) In order to take advantage of the external forces acting on the satellite, I can increase the mass of the side of the satellite that needs to point at Earth (e.g., a camera) and make the satellite's body longer perpendicular to the Earth in it's desired position, so that gravity will automatically orient the satellite's body toward the Earth. The gravitational orientation could even be used further through mechanical parts inside the satellite that can redistribute mass (though this may make the satellite less durable). Also, thrusters would be needed to keep the satellite from rotating sideways once it points in the correct direction. To take advantage of the magnetic field acting on the satellite, I could charge different sides of the satellite to cause attraction or repulsion to propel the satellite and orient the satellite along the direction of the magnetic field. This would require the use of a magnetometer to determine the direction of the magnetic field, a charger to cause an internal magnetic field, internal protection against magnetic fields, and thrusters to prevent rotation sideways once the satellite points in the correct direction. You could also move a powerful permanent magnet around, though you wouldn't be able to turn it off. Finally, solar sails could be used to propel or counter-propel the satellite using the pressure from electromagnetic waves, and in a similar manner propulsion and counter-propulsion can be achieved through the use of pressure from gas particles (both have fairly predictable directions of momentum so, if you adjust the sails correctly, you could easily use the pressure to stabilize the satellite's attitude). For all of the designs, you would also have to be able to keep track of the attitude of the satellite using gyroscopes, horizon sensors, sun sensors, magnetometers, etc. in order to know what counter-movements are needed to stabilize the attitude of the satellite in the six degrees of freedom.

18 Mar03:33

James' Group: Philip, Pamela, Elizabeth

By philipq

1) The commands used in control satellites are only a few characters in length so that there are minimal errors when sending and receiving commands. It is easier to decode and distribute, or debug a program when errors occur.

2) The communications module can be programmed to use a name (similar to how your computer uses an IP address) to address you satellite and tell it to start receiving data. Due to the fact that a satellite could pick up any signal, you may have to attach the name to the same string of bits as the information you are going to send every time you send a string of data, and you would have to have some type of acknowledgment to check if the information sent is correct.
 

17 Mar22:42

Pamela Toshi's answers to questions 3 & 4

By ptoshi

3) The six degrees of freedom denote the three-dimensional movement of the satellite in space. These are foreward/backward, left/right, up/down, and rotation about the axes. Gravity is one of the many forces that acts on the satellite in space.

4) I would design a satellite that used gravity gradient stabilization so that it would remain in a fixed orientation. Reaction wheels can be used if the satellite needs to be rotated slightly. Thrusters and solar sails can help with altitude adjustments.

15 Mar22:03

Alex Gao's Answers to Questions 3 & 4

By Alex Gao

 3) The six degrees of freedom are the directions/ways an object can move in three dimensions, which are: forward/backward (surging), up/down (heaving), left/right (surging), pitching, yawing, and rolling. There are hundreds of external forces that can act on a satellite module to affect its movement, some of which are gravity and forces from photons and thrusters.

4) I could design a satellite to use a gravity gradient stabilizer in order to stabilize a satellite using rotational dynamics.  This set-up would have relatively low power requirements.  A solar sail can use the force from photons striking the reflective sail in order to align itself in the desired attitude.  It would require the implementation of a system that can deploy and control the sail.  The sail itself would not require any power, but orienting it in order to orient the satellite will require some power.  Thrusters could be placed at strategic locations around the satellite in order to “spin” the satellite into a desired attitude.  A source of fuel would be required, and would be limited/non-renewable.  

15 Mar22:02

John's Group Question 1 & 2

By Alex Gao

 John's Group: Matt, Alex

Problem 1: Why are brief commands preferable to longer commands when transmitting commands to satellites?

1) Commands used to control satellites need to be short because it minimizes the amount of information sent.  A longer message is unnecessary because a lot of the transmitted data is superfluous.  Longer transmitted messages might also be degraded so severely by the time it reaches the satellite that the satellite might only receive part of the originally intended command and misinterpret the command.  Short messages reduce the risk of errors and waste of resources.

Problem 2: What are some ways with overcoming the problem of crowded airwaves when communicating with satellites?  i.e. How can you make sure your satellite will receive only the commands you send and not commands intended for other satellites? 

2) One way to overcome the problem of the abundance of satellites using same or similar frequencies is to transmit commands in a unique format.  A unique format could allow a satellite to disregard all other commands it might receive and treat it as “junk.”  Given the short length of most commands, however, this could be quite hard to implement.  Minimizing the dependence on communication with ground station by implementing more autonomy could also help alleviate the problem.