Observatory project

Introduction

In this project, you will visit the University of Maryland observatory on Metzerott Road a couple miles from campus, in order to observe satellites. After you have taken observations with a calibrated telescope, you will use orbit determination methods learned in this class to find their orbital element sets.

As you know, satellites are only visible in terminator (see Figure 11-19 in the text), because they do not generate light on their own. That is, the observer should be in darkness, and the satellite in sunlight. These time windows are a couple hours after sunset and a couple hours before sunrise. The observatory staff are as unenthusiastic about pre-dawn observations as you are, so we will confine ourselves to evening visits. Thus, you will want to get to the observatory about sunset on the day(s) of your observations. By two hours after sunset, your observation will be done, and you can clear out to make way for the astronomy students.

The telescope you will probably be using is a 14" Celestron Schmidt Cassegrain with a motor drive. Actually, you will not be sighting through the main scope, but through the finder scope. It has a 123 arcminute (about 2°) field of view, and cross-hairs in the sight. The drive includes a sidereal tracker that compensates for the earth's rotation in pointing at a fixed right ascension and declination.

You should plan on two visits to the observatory. The first visit may well not produce any useful data, as you learn the preparation necessary, equipment, and coordination of your group. You should strive for at least five good data points from each of two satellites. If you do not achieve this on the first visit, you should return to the observatory for a second visit.

Arranging a visit

You should arrange to make your first visit by the date assigned to your group. Your observatory contact is Elizabeth Warner 301-405-6555. Ms. Warner has numerous duties in her job, of which helping our class is not even one. Nevertheless, she has graciously agreed to help us out. Therefore, please make her job as easy as possible.

Have one person in your group who is easily reached act as a scheduler to arrange observing times. That person should find out from all the members of the group what evenings they are available. When you have selected several days, call Ms. Warner, identify yourself as a ENAE student observing satellites, and find out which is the best day for her. You will not be able to observe the 5th or 20th of any month, as these days are reserved for the observatory open house. When everyone is in agreement on the day, you should do visibility predictions for that evening.

Visibility predictions

The Heaven's Above web site will give you satellite visibility predictions and has a lot of information, but getting exactly what you need takes a little practice.

1. Visiting Heaven's Above page for College Park will automatically enter the observatory location (latitude: 38° 58' 56" N, longitude: 76° 56' 14" W). Note that this is downtown College Park, not where the observatory is located.
2. Look under "Satellites" and "Daily predictions" for the list of brightest satellites (up to 4th magnitude) for the day of your visit. Pick out at least three satellites from this list whose times in view do not overlap significantly. Some days you will not have a very good selection; you might consider rescheduling your observation if this is the case.
3. For a particular satellite, as you click on the time of "Max. altitude," you will get a sky map with the satellite's trajectory. Pan (move off the side) the chart by clicking outside the border; zoom (get more detail) by clicking on the point of interest.
4. For each satellite you have picked,
   1. Make a table giving predicted times, right ascension, and declination. This table should be at one minute intervals for at least five points. For maximum accuracy, pick times for which a tick mark is given in the sky chart, and click on that tick mark on the maximum zoomed chart to center it. Then the right ascension and declination can be read at the bottom of the chart. Note that the telescope takes right ascension in hours, minutes and seconds, and declination in degrees, minutes and seconds.
   2. Click on the name of the satellite. On the page that comes up, note the name, USSPACECOM catalog number, and International Designation Code; these should be included in your report.
   3. Then click on "Orbit." On this page there will be maps and numerical information. Save the orbital element information, including the two line elements; you will want to refer to it later.

You will need to repeat this procedure for every day that you visit; the satellite visibility information changes from day to day.

The day of the visit

• Check the observatory weather forecast or the National Weather Service forecast, particularly the cloud cover image, for clear skies after sunset.
• Call Ms. Warner to confirm your visit; if weather is uncertain, consult with her about observing conditions.
• If there is any reason you cannot make it at the appointed time, call Ms. Warner as soon as you know and cancel your visit.
• Synchronize an accurate (quartz digital) timepiece to the Official time (UTC), Naval Observatory clock 202-762-1401, or better, bring a GPS receiver when you observe, which will give both the precise time and observatory latitude and longitude. Heaven's Above gives local time, but you should use UTC in your calculations; the USNO gives both times.
• Record/print out the relevant information from the heavens-above web site, if you have not already done so. Do not write anything in red; it is invisible under the red observatory lighting.
• Plan on arriving before sunset so that you have time to setup and practice moving the scope.
• Dress warmly You will be out of doors, at night, under a clear sky, for over an hour, in autumn. Even if the day is warm, it can be chilly under these circumstances.

Observing

• Observatory staff When you visit, Ms. Warner or her staff will help you learn how to operate the equipment, but they are not TAs and are not familiar with the problem of sighting satellites and determining their orbit.
• Division of labor One person in the group should do keypad entry to aim the telescope, one person should sight through the scope, and one person should record information. The fourth person can hold the flashlight and provide general assistance. If time allows, you can switch roles between satellites to spread the fun. You should practice your signals, because satellites move fast and there is not much time to waste on miscommunication.
• Aiming the telescope To aim the scope, enter the right ascension and declination in advance of the predicted arrival time at that point. When the last digit is entered, the scope will move to that location. Thus, as soon as the scope is in the right place, the coordinates of the next observation can be entered except for the last digit. When it's time to move again, just enter the last digit, and the scope will start moving
• Telescope motion Because the keypad person controls when the scope starts moving and the observer may be looking through the scope, it is important that you coordinate signals. Prior to motion, the observer should get down from the ladder and move it out of the way, then give his/her OK for the keypad person to enter the last digit. If something/someone is in the way, the observer or the scope can be damaged. When changing azimuth so that the meridian line is crossed on the North, the telescope must go the long way to keep the cables from wrapping up, and it will take at least 45 seconds to complete this motion. This may affect your ability to take an observation on time.
• Blind spots There are tall trees to the North from azimuth about 0° to about 20°, which makes it difficult to sight satellites below an elevation of about 45°.
• Sighting the satellite Assuming you have aimed the telescope correctly, near the appointed time you should see your satellite come into view as a distinct glint of light moving against the background stars. When it comes as close to the center of the crosshairs as it's going to, the observer should yell out a prearranged signal, and the recorder record the time as shown on the calibrated timepiece. Then note how close to the intersection of the crosshairs it came at the closest points, as a fraction of the whole radius of the field of view, and record that.
• If you don't see the satellite It happens; it could be because the coordinates were entered wrong or a cloud got in the way. If you miss one or two observations on a satellite, don't worry; that's why you recorded at least five times of observation. If you do not see the satellite at all, it could be because you got the wrong information off the web site, or the brightness was not as predicted. That is why you are looking for several different satellites.

After the visit

The group should turn in an interim report with the following:

1. The name of the group, and the names of its members.
2. A list of satellites with catalog (SDC) number and international designator.
3. For each satellite, the date and approximation time of observation, and a table of precise observation times, right ascension, declination, and estimate of miss from crosshairs.

If you didn't get enough data, or the data is low quality, it may be necessary to return another evening. Before you schedule another visit however, check with me. In some cases it will be possible to get by with what you have.

Analysis

• You should apply these techniques on all the satellites which you were able to observe the minimum number of observations.
• You should be have at least one satellite with more than the minimum observations. For these cases, try the statistical orbit determination discussed in class.
• If you do not have at least one overdetermined satellite, you will need to return to the observatory to observe a new satellite.
• You should use Cartesian vector to estimate the state; orbital elements are not very good. When you have obtained the final state, you may convert the Cartesian result to orbital elements.
• To compare your results with the "official" elements for the satellite, either use the elements obtained from the Heaven's above page at the time of observation or use space-track with the appropriate satellite ID number.
• Compare your results with published information about the satellites. How should the orbit be represented so that the comparison looks reasonable? Do not be discouraged if your determined orbits are not very good; it is hard to do orbit determination and harder to get accurate results with only a few observations in a single pass!

The Report

I will post information on the report in a separate web page.

Deadlines

Here is a summary of deadlines for this project: