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ObsReduce is an MS Windows program that reduces observations of satellites relative to the backgroun

The date and time of observations are entered in the second row of input boxes from the top of the program's window. ALWAYS enter the date and time as UTC (aka GMT), NEVER your local date and time.
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Use the option buttons at far left to specify the mode of date and time entry: absolute or elapsed.
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<B>D.5.1 Absolute Time Mode</B>
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Absolute time is as read directly from a clock. In this mode, the date input boxes are labelled "Obs Date", and the time input boxes are labelled "Obs Time UTC."
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<B>D.5.2 Elapsed Time Mode</B>
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Elapsed time is as read from a stopwatch. This is intended for use by observers who use what is commonly called a multiple split-time stopwatch. Each time an observer times a satellite position, the stopwatch records the elapsed time of the event as a split-time. Typically, such stopwatches record the split-times in memory, for later readout.
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In Elapsed time mode, the date input boxes are labelled "Base Date", and there are two sets of time input boxes, labelled "Base Time UTC" and "Elapsed Time". There is also a box for the rate of drift of the stopwatch.
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Enter the date and time that the stop watch was started, into the Base Date and Base Time input boxes. Enter the split-time of the observation into the Elapsed Time input box. The program internally computes the absolute date and time of the observation, for use in the observation report.
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Also, since changing the date and time of an observation re-sets the date and time of the centre of the field of view, you can temporarily view the computed absolute date and time in the <a href="#Centre">FOV Centre Frame</a>.
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In elapsed time mode, the program provides a permanent record of the Base Date, Base Time, Elapsed Time and Drift Rate, by writing them to the details file, as shown in Section <a href="#Obsfiles">D.18.1 Observation Files</a>.
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<B>D.5.2.2 Adjust for Stopwatch Drift</B>
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If your stopwatch drifts significantly during your observation, then you should enter its rate of drift into the box labelled "Drift s/h". Units are in seconds per hour. Compute the rate of drift as follows:
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drift = 3600 * [(e2 - e1) - (t2 - t1)] / (t2 - t1)

where: drift = rate of drift, seconds per hour
          t1 = actual UTC #1, hours
          e1 = elapsed time at t1, hours
          t2 = actual UTC #2, hours
          e2 = elapsed time at t2, hours
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<B>D.5.3 Switching Between Modes</B>
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You may switch between time modes at any time. When you switch from elapsed to absolute mode, the program remembers the Base Date, Base Time. and Drift Rate. Upon your return to elapsed time mode, those values are restored.
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<B>D.5.4 Timing Accuracy</B>
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Specify the time accuracy in the input text box labelled "Accy". Please note that the default time accuracy may be changed by editing line 5 of ObsReduce.ini.
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<B><a name="Early/Late">D.6 &nbsp;Satellite Early (-) Late (+) Test Box</B>
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If you know how early or late the satellite was, enter the number of seconds here. If the object was early, enter the value as negative; otherwise, as positive (the plus sign may be omitted).
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<B><a name="Plot">D.7 &nbsp;Plot Satellite Button</B>
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The Plot Satellite button reads the satellite's orbital elements and plots its predicted track and star background, at the date and time shown in the FOV Centre frame.
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The satellite's track is plotted as a solid line, with a filled circle at one end, to indicate the direction of motion.
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The colour of the track denote's the satellite's illumination; blue denotes an object fully in eclipse; red denotes an object in the penumbra of Earth's shadow; yellow denotes an object totally outside Earth's shadow, i.e. in sunlight.
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<B><a name="Elements">D.7.1 &nbsp;Reading Orbital Elements from a File</B>
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For efficiency, Plot Satellite reads the orbital elements file only if a new <a href="#Desig">Desig</a> or <a href="#SSN">SSN</a> has been entered since the last time it was pressed, or the time/date stamp of the orbital elements file has changed since the last time it was pressed.
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The latter feature enables editing the file or replacing it with another of the same name, without having to shutdown and restart ObsReduce. This simplifies switching among several elsets of the same object.
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ObsReduce uses the first elset that matches the entered Desig and/or SSN. To make it skip an elset, edit one of its two lines to insert any character at the start of the line, for example a semicolon. It will no longer look like an elset to ObsReduce, so it will skip past it. In this example, the first elset will be skipped:
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;1 24680U 96072A   03160.08334785  .00031614  00000-0  35000-3 0    01
2 24680  97.8269 223.2766 0505000  31.8143 331.2548 14.81313481    01
1 24680U 96072A   03161.09651195  .00031057  00000-0  33552-3 0    01
2 24680  97.8181 224.2537 0506798  28.7815 334.0379 14.81377728    04
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To quickly compare plots of the same object switching among multiple elsets, open the elements file in a text editor, edit-out the ones to be skipped, save the file, and press Plot Satellite.
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When working with elements, it is not necessary to enter both the Desig and the SSN. ObsReduce will find the value of the missing entry in the elset, and automatically fill in its empty text box.
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If you have filled in both the Desig and SSN boxes, then upon reading the elements, ObsReduce will report an inconsistency if it finds one. For example, if it finds an elset with the same Desig that you entered, but a different SSN, it will require you to resolve the discrepancy before allowing you to proceed with plotting the satellite's track. This is intended to promote accuracy of the observation reports.
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On occasion, you may find yourself in disagreement with the Desig and/or SSN of an elset. Instead of your having to adopt the elset's values, or edit the elset to match yours, you can assign an alias to the elset. For example, here is an estimated pre-launch elset of the object that became Desig 95034A / SSN 23609:
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1 72001U          95191.53601852  .00000000  00000-0  00000-0 0    08
2 72001  55.0600  16.1740 0000320 299.9680 127.5380 16.32783000    01
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If you observed the object, and wish report it as 95034A / 23609, but use the pre-launch elset, insert an alias line into the file, just ahead of the elset, and save the file:
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*alias* 95034A 23609
1 72001U          95191.53601852  .00000000  00000-0  00000-0 0    08
2 72001  55.0600  16.1740 0000320 299.9680 127.5380 16.32783000    01
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Aliases are also useful in cases of disagreement among analysts over the correct designation  for an object.
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<B><a name="Select">D.8 &nbsp;Select Reference Stars</B>
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Depending upon the geometry of the observation, one, two or three reference stars may have been used, which must be selected in the simulated FOV. The first star selected is called A, the second, B, the third C.
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If the satellite crossed the imaginary line joining two stars, either between them or outside them, then select them as stars A and B.
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If the satellite formed a right-angled triangle with two stars, then select them as stars A and B.
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If the satellite appulsed (passed very near to) a star, then up to three stars may have been involved in the observation. The appulsed star must be selected first, and it will be denoted as A. If you judged the miss-distance without reference to any other stars, then no additional stars should be selected.
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If you judged the miss-distance as the fraction of the distance between the appulsed star, A, and another star in the FOV, then select that star as B.
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If you judged the miss-distance as the fraction of the distance between two other stars in the FOV, then select them as stars B and C.
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In the case of a camera observation of a satellite located to the right or left of two reference stars, select them as stars A and B.
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In the case of a camera observation of a satellite and three reference stars, select them as  stars A, B and C.
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Moving the mouse cursor over a star in the displayed FOV (field of view) temporarily changes its colour from white to green, and displays its coordinates and magnitude in the box below the FOV.
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To select Star A, left-click the mouse when it is over a star. When you move the cursor away, its colour will change to yellow, and a yellow upper case letter A appears below the FOV.
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The next star selected becomes Star B, denoted by its change to the colour red, and the appearance of a red upper case letter B in below the FOV.
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The third star selected becomes Star C, denoted by its change to the colour cyan, and the appearance of a cyan upper case letter C in below the FOV.
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All stars may be cleared at any time by left-clicking anywhere in the FOV where there is no star. Note that this will clear the results of the reduced observation, if any.
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<B><a name="Geometry">D.9 &nbsp;Geometry and Position Frames</B>
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Geometry refers to the geometrical pattern formed by the satellite and reference stars at the moment of your observation. Position refers to the satellite's location relative the reference stars.
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<B><a name="Traditional">D.9.1 &nbsp;Traditional Geometries</B>
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ObsReduce supports three traditional observational geometries: satellite in-line with reference stars, satellite forming right-angled triangle with reference stars, and appulses (satellite passing near a single star). They are used primarily by observers who rely on their own memory to record observations. Observers who use cameras may also use these geometries when they arise, but are likely to prefer the special geometries described in <a href="#Camera">Section D.9.2 Camera Geometries</a>.
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<B>D.9.1.1 &nbsp;Satellite on Line AB</B>
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If the satellite crossed the imaginary line joining star A and B, either between or outside the stars, then in the Geometry frame select the "On line AB" option, and in the Position frame, enter the fraction AS/AB, (the distance from Star A to the satellite, expressed as a fraction of the distance from Star A to B) into the box labelled "Frac AS/AB". If the satellite passed between the stars, then this value will be less than 1; if it passed outside them, it will be greater than 1.
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<B>D.9.1.2 &nbsp;Right-Angled Triangle</B>
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If the satellite formed a right-angled triangle with Stars A and B, A-B-S, then if it passed to the right of Star B, select option "Right of B"; if it passed to the left of Star B, select option "Left of B". In the Position Frame, enter the fraction BS/AB, i.e. the distance from Star B to the satellite, expressed as a fraction of the distance from Star A to B.
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<B><a name="Appulse">D.9.1.3 &nbsp;Appulse</B>
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If the satellite appulsed (passed very near to) a star, then in the Geometry frame, select the "Appulse of A" option. Two input fields will appear in the Position frame, the first of which is the position-angle, labelled as "Pos Angle"
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In ObsReduce, the position-angle is defined as the angle of the radius from the appulsed star to the satellite, at the moment of appulse, measured in units of degrees clockwise from the top of the FOV (field of view).
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If you use ObsReduce to plot a satellite's track using its orbital elements, then it automatically computes an accurate estimate of its position angle relative your selected Star A, and enters the value in the "Pos Angle" box.
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Frequently, satellites are observed to appulse stars on the side opposite that predicted by the latest orbital elements, in which case ObsReduce's estimate of the position angle will be wrong by 180 deg. To correct that, press the button labelled "+180", located to the right of Pos Angle input box.
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If you prefer to use your own value of position angle, enter it into the Pos Angle input box.
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If you prefer never to use ObsReduce's predicted position angle, you can turn it off by editing line 35 of the ObsReduce.ini file. Please note that if your ObsReduce.ini file came with a version of ObsReduce earlier than 1.1, then you must run ObsReduce 1.1 once to create the entry at line 35.
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The second position data entry field is used either to specify or help compute the miss-distance, i.e. the angular separation between the satellite and the star at the moment of appulse.
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If you estimated the miss-distance without reference to any stars, i.e. selected only the appulsed star, A, then the input box below the Pos Angle box will be labelled "Miss Dist", and you should enter your estimate there.