You will need the parameters of the shuttles orbit before you can predict the rise and set times of the shuttle as well as to steer your antennas. These are called the Keplerian Elements, or Keps for short. The Keplerian elements are a set of numbers which describe the Shuttle's orbit. Specifically, the Keplerian elements describe an ellipse in the celestial reference frame centered on the on the earth. They describe the motion of the Shuttle as it traces out an elliptical path, where one of its focii of this ellipse is the center of the earth's mass. The Space Shuttle will typically perform many maneuvers during its mission. Orbit changes to meet up with a satellite for retrieval or on-orbit repair, or to achieve a specific orbit for releasing a satellite will cause significant changes to the orbital parameters.
Anytime the Orbital Maneuvering System (OMS) is used, it affects the orbit of the shuttle so significantly that the Keplerian elements used up to that time are useless for determining the rise and set times for any time after the OMS burn. Minor changes in the orbit will result from numerous other activities conducted during the mission, the effects of which will accumulate over time and also render the current Keplerian element set useless. These include Reaction Control System (RCS) thruster firings to reorient the shuttle or make minor changes in orbital velocity, satellite deployments and retrievals, waste water dumps, and crew movement within the Shuttle. The effect of all this on your team is that the actual contact time will never occur exactly when it was predicted to occur prior to launch and the Keplerian element set will require frequent updates during the course of the mission if your satellite tracking software is to accurately determine the rise and set time, as well as the track the shuttle will take across your sky during your contact opportunity. NASA uses a variety of sophisticated methods for maintaining an exact model of the shuttle's orbit, position, and attitude (orientation in space) at all times.
The SAREX Flight Operations Team at in the Customer Support Room (CSR) at Mission Control constantly monitors this orbit and issues a new set of Keplerian elements each time the Shuttle's orbit causes the rise and set times to change by more than a couple of seconds. A Keplerian element set issued during the STS-60 Shuttle mission is shown in Figure 6. The first two elements, Satellite and Catalog Number identify the spacecraft. In this example, the Satellite is named STS-60 and its entry into the catalog of orbiting objects is 22,977. It is a formal ID number assigned by NASA. The next two entries identify the element set itself. Epoch time is the time for which these elements were computed. The orbit described by this set is the orbit that existed at the Epoch Time. This time is specified in a decimal format. In this example, the epoch time is day 35 of 1994 (94035) or February 4, 1994. The time is 0.13981770 * 24 hours. This is 3.356 or 3 AM plus 0.356 * 60 minutes, or 21.337 minutes. The seconds are 0.337 * 60 or 20.249 seconds. So the time of this epoch is February 4, 1994 at 03:21:20.249. The next entry, Element Set, is the serial number of the element set. It is used to identify the element set and distinguish it from the others issued during the mission. All element sets will be referred to by this number. The phrase used for this example would be something like "kep set three" or "element set three". The next six entries are the six key orbital elements. They describe the orbit that the Shuttle was in at the epoch time.
The inclination describes the orientation of the Shuttle's orbital plane with respect to the earth's equator. This number, for Shuttle missions, will never be lower than 28.5, the latitude of KSC. It also specifies the farthest latitude north and south that the shuttle's ground track will go. The higher this number, the farther north and south of the equator the shuttle will travel. High inclination orbits allow for more direct contacts as the shuttle will be visible to most of the United States and inhabited world. Mission STS-60's orbit was inclined at 56.9857 degrees, meaning that stations as far north as Alaska and as far south as Antarctica.
The Right Ascension of the Ascending Node (RA of Node or RAAN) specifies the orientation of the Shuttle's orbital plane with respect to the "fixed" stars. For navigational use, the stars are assumed to exist in a fixed sphere, and the direction to each star is measured in degrees from a fixed location in space. The RAAN therefore fixes the ellipse of the shuttle's orbit in the universe at the epoch time.
Johnas Kepler discovered, in the 17th century, that all orbiting bodies actually move in an ellipse. A circle is a special form of an ellipse. An ellipse is formed from two focal points, and the line which intersects the ellipse, passes through both focal points and then intersects the ellipse on the other side is called the major axis. This is the long part of the ellipse. The length of the major axis is measured from one side of the ellipse to the other. A line that bisects (divides equally in half) the major axis and touches both sides of the ellipse is called the minor axis. The eccentricity is a number, based on the lengths of the major and minor axes which describes the shape of the ellipse. The closer to zero, the more circular and less elongated the ellipse appears.
The argument of perigee describes where the perigee, or lowest altitude, of the Shuttle is located in the orbital plane with respect to the Earth. Arguments of perigee between 180 and 360 degrees describe an orbit where the perigee is over the southern hemisphere. The apogee is then located on the exact other side of the orbit, over the northern hemisphere.
The mean anomaly locates the Shuttle on its orbit at the epoch time. A mean anomaly of zero indicates that the Shuttle is at the perigee while a mean anomaly of 180 indicates that the shuttle is located at the apogee. A mean anomaly between 0 and 180 indicates that the shuttle was between perigee and apogee at the epoch time, heading up, away from the surface of the earth towards it apogee. A mean anomaly between 180 and 360 indicates that the shuttle was located between its apogee and perigee, moving along its orbit down towards the surface of the Earth towards its perigee.
The Mean Motion specifies the number of orbits, or revolutions (`revs" for short), the Shuttle will make in a 24-hour day. Since the number of orbits per day is mathematically related directly to other orbital parameters, this figure indirectly provides a number of other facts about the Shuttle's orbit as well, such as the size of the orbital ellipse.
The Decay Rate is a value that compensates for the effects of the frictional drag produced by the movement of the Shuttle through the earth's atmosphere. While the Shuttle is in space and above nearly all of the Earth's atmosphere, some traces of the air remains at the altitude at which the shuttle flies. Atmospheric drag can be the largest natural change to the shuttle's orbit and is measurable for all satellites below 800 kilometers (500 miles). Orbits for the Shuttle vary from mission to mission and usually range between 290 kilometers (180 miles) to 400 kilometers (250 miles). It is very difficult to predict the atmospheric drag because it depends on the density of the upper atmosphere, which is significantly affected by past, present, and future levels of solar activity. Atmospheric drag tends to be the highest for satellites having a high ratio of the surface area to satellite mass in lower altitude orbits. An analogy of this is Galileo's classic experiment where a balloon and a steel ball are dropped at the same time. The balloon will take longer to fall due to the atmospheric friction effects, even though both may have the same surface area. A famous version of this experiment was conducted during an Apollo lunar landing, where the astronaut on the moon dropped a hammer and a feather. Since there was no atmosphere on the moon, both objects struck the ground at the exact same time. The Shuttle is an irregularly shaped object whose cross-sectional area (with respect to the direction of travel) changes every time the Shuttle reorients itself through RCS firings in orbit. The Shuttle's mass changes during flight due to numerous events, such as satellite deployments and retrievals, fuel expended, and waste water dumps. Therefore, the Decay Rate is typically derived through observations (empirically) and is applied to the orbital parameters to cause the tracking programs to match known observations.
The Epoch Rev is the orbit number that the Shuttle was on at the Epoch Time. This number is important because you will be told what orbit on which your contact will occur. A discrepancy in the way most tracking programs number orbits and the way NASA numbers them has sometimes produced results where the orbit number was one off from the orbit you are given. Most amateur radio tracking programs increment the orbit number when the satellite passes perigee. NASA increments the orbit number when the shuttle crosses the equator, heading from the southern hemisphere to the northern hemisphere. The element sets now produced by the SAREX Operations Team takes this discrepancy into account so that the orbit number produced by tracking programs which count orbits from perigee agrees with the NASA orbit number, and thus the information you are given about your contact schedule.
There are a number of tracking programs available for personal computer systems. They vary in features and capability, as well as the system on which they run. Some programs include the ability to drive rotors through interfaces in your computer, such as the Kansas City Tracker. These programs are available from AMSAT for a modest fee. AMSAT offers programs fro IBM-PC/compatibles, Macintosh, Commodore C- 64, C-128, and AMIGA, Apple II, Tandy Color Computer 2 and 3, TRS 80 Model 4, ATARI 8-bit and ST systems, as well as a program for HP-41 programmable calculators. Needless to say, the HP-41 calculator cannot be used to drive rotors, but will produce real-time azimuths and elevations and could be useful as a backup. For more information and details on how to obtain a program, contact AMSAT at the address in Appendix A of this guide. A summary of each program can be found in Reference 5.
You will be provided with a set of elements at the time you are notified of selection. These elements are called the "pre-flight elements" are should be used for scheduling purposes only. They are derived from the planned orbit in the mission plan based on a specified launch date. If, for any reason, the launch date changes, the pre-flight element set becomes obsolete. You can estimate the change for your contact date and time if you know how long the launch delay will be. An approximation can be obtained by simply adding the launch delay length to your contact time. Again, this provides a rough estimate for scheduling purposes. Your Technical Coordinator will see that you get Keplerian element set updates prior to launch. Shortly after launch, a set will be issued that is derived from NASA's tracking data and will take into account any changes from the pre-flight launch plans. These are called ascent anomalies. If the Shuttle has to hold in its launch countdown by an hour or so, this also invalidates the pre-launch orbital elements and you need to wait for the new set to come out after the Shuttle is in orbit. Again, you can approximate the effects of the hold on your schedule by adding the delay duration to your contact time. As stated in previously, the Shuttle's orbit is subject to constant change and you will need to obtain updates to your Keplerian elements throughout the mission. There are several ways to do this. If you have an email address, furnish this information to your Technical Coordinator and they will see that you are added to the automatic distribution list. If you have a FAX machine, you can be added to the FAX distribution list. Again, furnish your FAX number to your Technical Coordinator. Consider where you will be receiving your element set updates. If you are receiving them at work, and your contact is on a Sunday, this will mean a trip to the office before the contact. It is best to confirm that your Keplerian elements are current with Mission Control just prior to your contact. When the CSR team calls you one- half hour before your contact, make it your first order of business to confirm your elements or else obtain the latest set in plenty of time to enter them into your computer and check them against the CSR's figures.