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65 Cards in this Set
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What is being defined? -An imaginary line drawn from the center of a satellite to the center of the Earth at a point called the sub-satellite point |
Ground Track |
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What is orbit is being defined? -The simplest example of a ground track -Satellite moves at a constant speed above the equator and completes one full orbit per sidereal day --Tracks look like a dot on the equator |
Geostationary Orbit |
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How does a size of the semi-major axis effect ground track? |
1. Orbits with a smaller semi-major axis have a faster circular velocity 2. As a satellite moves faster, it covers more ground and the ground track appears to be stretched out |
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True or False: Geosynchronous ground tracks will repeat every orbit? |
True |
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A semi-synchronous ground track will repeat every (blank) orbits |
Two |
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Explain why a more eccentric orbit will appear to be less 'symmetrical' than a less eccentric orbit in terms of ground track? |
This is due to the increasing speed differences between apogee and perogee. As the orbit gets more eccentric, speeds at apogee slow way down, creating a 'scrunched' look for the ground track, and the increased speed at perogee drives the satellite to cover more ground in less time, thus stretching out the ground track |
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What COE determines the highest latitude crossed by a ground track? |
Inclination |
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True or False: A ground track with an inclination greater than 90 degrees (retrograde orbit) will move to the West |
True |
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What happens to the ground track of a geostationary satellite if we increase the inclination of its orbit? |
It draws a figure-8 |
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These results in the ground track happen due to change in what? -Different forms of asymmetry -causes satellite to dwell more over certain areas -May have unexpected results |
Changing the Argument of Perogee |
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What is being defined? -The portion of the Earth's surface that is geometrically visible from the satellite's position in space; also known as the access area -Increases with satellite altitude -Satellite may not be able to provide coverage of that entire area at one time, depending on payload design |
Field of Regard |
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What is being defined? -The length of time a satellite has line of site to a point on the Earth -Important for determining how long a satellite can provide coverage of an area before it goes out of view |
Dwell Time |
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What is being defined? -The length of time between successive passes of a satellite over a target area -Determines how frequently a target can be covered |
Revisit Period |
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True or False: A rocket can launch directly into an orbit with a lower inclination than the launch site's latitude |
False |
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How many direct launch opportunities do you have per day in the following scenerios: 1. Launch site latitude=inclination 2. Launch site latitude < inclination 3. Launch site latitude > inclination |
1. one per day 2. two per day 3. zero chance |
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List launch site considerations |
-Fuel -Earth's rotation on a prograde orbit (gives extra boost) --ideal launch location for prograde orbit is at the equator -Earth's rotation on a retrograde orbit --ideal launch location at higher latitudes, but not at 90 degree -Launch into a parking orbit |
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Effects of drag on satellites in LEO: -Large satellites v smaller -Satellites with larger surface area |
-More massive satellites tend to have longer orbital lifetimes -Satellites with larger surface area tend to have shorter orbit lifetime |
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Why would we want multiple satellites in a constellation around Earth? |
To provide full Earth coverage for things such as communications and GPS |
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What is being defined? -the process of identifying the orbital parameters of an object in space |
Orbital Determination - enables prediction of the object's trajectory |
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Finish this sentence: Ultimately, we would like to express a satellite's orbit in terms of... |
the six Classical Orbital Elements |
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What is being described? -Measurements of a satellite's position and/or vector are called... |
metric observations |
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Why do we use the Earth-Centered Inertial frame of reference? |
To plot sensor reports of a satellite's location on a common frame Ex. Terrestrial sensors reference a satellite's location from themselves and space-base sensors reference satellite's from themselves causing a conflict for frame of reference |
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What two factors contribute the most to the uncertainty of orbit location? |
-Imperfect measurements (equipment limitations) -Incomplete modeling of perturbing forces Results in the need to use Estimation Theory to get the best 'guess' of a satellite's ortbit location |
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Estimation Theory is used to predict a satellite's orbit location. What are the two considerations needed to utilize estimation theory? |
1. Statistics of measurement errors 2. Data fitting |
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What two factors contribute to measurement errors? |
1. Bias 2. Standard Deviation (precision) |
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What is being described? -The difference btw the mean measured value and the true value -Once ID'd, it can be corrected for -Accuracy |
Bias |
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What is being described? -A quantitative measure of the consistency of the measurements obtained by an instrument -Precision |
Standard Deviation |
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What is being described? -Determines bias and standard deviation of a sensor -compiles repeated measurements of a known quantity to create a histogram |
Calibration |
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What does Standard Deviation give us? |
It demonstrates how trustworthy the data should be |
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True or False: Once the performance of a sensor has been calibrated, the bias can be subtracted from subsequent measurements. |
True |
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What is being described? -Individual measurements of a satellite's position at different points in time can be combined, as before, to create a picture of the satellite's trajectory |
Data Fitting |
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What contributes to modelling errors for orbit locations? |
-Perturbing forces that were ignored in the restricted two-body problem -Gravitational perturbations -Drag and Solar pressure, which cannot be reliably predicted |
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What is the standard model that the AF uses to predict orbits of objects that are in LEO? |
Simplified General Perturbations-4 -Includes two-body dynamics -Models J2 effects -Attempts to account for atmospheric drag |
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What is being described? -The AF publishes orbit estimates -Contain all six COEs -Contains est. ballistic characteristics of satellite -Does not contain any info. about the reliability of the estimate |
TLE - Two-Line Element Sets |
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True or False: Once the initial orbit estimate has been obtained, the orbital parameters can be propagated forward to any desired time using the dynamics model |
True, however, orbit predictions become obsolete very quickly |
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What three factors can be used to manage measurement uncertainty? |
1. Collect New Observations 2. Use a more accurate dynamics model other than SGP-4 3. Use more reliable data to being with |
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What is being defined? -Any action taken to control or alter the motion of an orbiting satellite |
Orbital Maneuver -altering trajectory REQUIRES a change in velocity |
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What are three reasons why we would want to maneuver a satellite? |
1. Orbit Insertion 2. Station Change 3. Station Keeping |
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What is being defined? -The attempt to launch a satellite into orbit |
Orbit Insertion -May need to utilize a lower altitude parking orbit and transfer orbit |
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What is being defined? -A satellite may be moved due to changing mission requirements -May move from one orbital plane to another within a constellation -May move to a different position with the same orbital plane |
Station Change |
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What is being defined? -Orbital perturbations cause satellites to drift from their assigned orbits -Small propulsive maneuvers may be needed at regular intervals to keep the satellite in its desired position |
Station Keeping |
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What COEs are altered during In-Plane Maneuvers? |
-Semi-major axis -Eccentricity -Argument of perigee |
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What three types of maneuvers can be used to achieve the same overall change in semi-major axis? |
1. Hohmann transfer 2. Fast transfer 3. Continuous Thrust transfer |
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What is being described? -Requires two burns in the velocity direction ----location of first burn becomes perigee of transfer orbit -Method to move btw two circular orbits with minimum change in velocity requirement -Uses transfer orbit tangential to the initial and final orbit |
Hohmann Transfer |
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What is being described? -Arrives at the desired orbit sooner at the expense of additional change of Velocity |
Fast Transfer |
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What is being described? -A continuously burning, low-thrust propulsion system which can provide a more fuel-efficient way to achieve an orbit transfer |
Continuous Thrust Transfer |
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What is being described? -Used to change location of perigee within the orbit plane -Requires only one burn when performed at the intersection of the original orbit and the desired orbit |
Line of Apsides Rotation |
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What COEs are altered during Out-of-Plane Maneuvers? |
-Inclination -RAAN |
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What is being described? -Changing orbit plane and semi-major axis of the orbit ----Performing an inclination change followed by a Hohmann transfer would require three separate burns ----It is possible to use only two burns by accomplishing the transfer burn at the same time as the plan-change |
Combined Plane Change Maneuver |
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What is being described? -Spacecraft Performing the Rendezvouse |
Chase object (interceptor) |
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What is being described? -Object or orbital slot with which the chase spacecraft is attempting to rendezvous |
Target |
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What is being described? - Location in space where the interceptor will meet the target |
Rendezvous Point |
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What is being described? - Angular measurement from the target to the rendezvous point |
Lead Angle (b/c we are 'leading' the tgt) |
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What is being described? - Angular measurement between the interceptor and the target |
Phase Angle |
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What is being described? -A type of rendezvous where meeting at the rendezvous point when both the target and the chase vehicle are already within the same orbital plane |
Coplanar Rendezvous |
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What is being described? -Meeting at a target point when both target and chase are located in the same orbit -Two ways to catch up to a satellite: either by slowing down or speeding up |
Co-Orbital Rendezvous |
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What is being described? -Final stage of a rendezvous maneuver, when two satellites are in close proximity to one another -Formation flying in space -May be used to dock with another satellite or move around it |
Proximity Operations |
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What is Along-track, Cross-track, Radial (ACR) coordinate system? |
An orbital reference frame centered on the target satellite - the position and velocity of the chase vehicle will be expressed in relation to the target Along Track - pointing ahead of the target satellite Cross Track - pointing to the left of the target satellite Radial - pointing above the target satellite (away from Earth) |
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(ACR and Relative Motion) -What type of motion does a change in semi-major axis produce? |
Linear Drift |
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(ACR and Relative Motion) -What type of motion does a change in eccentricity produce? |
Natural Motion Circumnavigation (NMC) -at apogee, the chase satellite will be above target and appearing to move backwards -as the chase satellite approaches perigee, it will move below the target while appearing to move forward -(in a nut shell, the chase satellite looks to be porpoising in front of the target satellite, while it goes through a series of speed-ups and slow-downs) |
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(ACR and Relative Motion) What type of motion does a change in inclination produce? |
Out-of-Plane motion (like a tilted sidewinder) |
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(ACR and Relative Motion) What type of motion does a change in semi-major axis and eccentricity produce? |
The combination of linear drift and NMC = OPEN LOOP |
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(ACR and Relative Motion) What type of motion does a change in semi-major axis and inclination produce? |
The combination of linear drift and out of plane motion = HORIZONTAL WAVE |
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(ACR and Relative Motion) What type of motion does a change in Eccentricity and Inclination produce? |
The combination of NMC and out-of-plane motion = NMC CLOSED LOOP that is tilted out of the orbit plane |
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(ACR and Relative Motion) What type of motion does a change in Semi-major axis, Eccentricity, and Inclination produce? |
Corkscrew |
