Flash Period Measurements

The aim of these measurements is to measure the flash period of the satellite.

You follow the object for some time in order to familiarize yourself with the flash pattern. Sometimes this pattern and the flash period itself change during the pass, due to changing geometrical conditions with respect to the sun and the observer. This is called the synodic effect. Even experienced observers occasionally get confused when this happens.

Now that you're familiar with the pattern, start the stopwatch at the time of a flash (or at a minimum if it is more pronounced), and start counting with 'zero'. Stop the stopwatch at the last flash you want to observe, e.g. at count 'twenty' (twenty-one flashes = twenty periods). If you start the count at 'one', you're counting flashes not periods! Write down the total time on the stopwatch. Dividing the total time by the number of periods gives you the flash period.

You should always try to count as many periods as possible during one pass. For flash periods smaller than 5 seconds, you should try to count fifty to one hundred periods for one measurement. This improves the accuracy of the flash period substantially. If your stopwatch allows a split timing, make a split timing somewhere in the middle of the pass. This measurement can act as a check for the final measurement or as backup in case the final measurement fails. The latter can happen when the sattelite disappears in the Earth's shadow or behind an obstacle, or when the flash pattern changes drastically. If you have a stopwatch that allows for multiple timings (fifty to one hundred is typical), you could time each flash you see and then later analyze the data to derive an average period. This can also be done by recording your voice and a time signal (from a shortwave radio tuned to a time signal station) with a tape recorder. For complex variations the latter method is probably the best.

Next you should try to estimate how accurate the total time measured was. If the flash period is short, or the flashes are distinct, the timing accuracy may approach the precision of your stopwatch. If it is long, with indistinct flashes, the accuracy may not be better than several seconds. The estimated accuracy depends on the flash pattern, your reaction time, visibility of the flashes, etc... If you have noticed you were a little late in reacting to a flash, you should adjust your estimated accuracy appropriately. The minimum reaction time of a human is 0.1 to 0.2 s. Accuracies below 0.1 s are not acceptable unless you are superhuman.

It is quite possible that the synodical effect causes the difference between the rotation and flash period to be larger than the observing accuracy. It is however still useful to estimate the accuracy of the total duration since it is not possible to predict how large the synodic effect will be during any given pass. So, it may still be that your measuring accuracy is larger than the loss of precision due to the synodic effect.

E.g. suppose you've estimated the measuring accuracy to be 0.2 s and that the maximum possible synodic effect is 0.5 s. If the synodic effect was only 0.1 s for the geometry at hand, it is useful to know that period is still not more accurate than 0.2 s, due to the observering accuracy.

More experienced observers also estimate the brightness of the flashes and determine the flash pattern.

PPAS Format

The observation is noted and documented on a special form or entered directly into the computer-database of flash period observations. We use a standard format, the PPAS format. The PPAS is our computerized database of Photometric Periods of Artificial Satellites, maintained by Mike McCants. In this format each line is divided in columns and every column always contains the same data. Normally each line contains 80 columns (or characters) The format of the observation is (in column numbers):

01-08

Cospar-code of the satellite in the format yy-nnncc. yy is the year of launch, nnn is the number of the launch (only contains significant numbers and is right justified), cc is the piece of the launch (contains non-numeric characters). e.g. '86- 39 B' was launched in 1986 as the 39 th launch and it was a rocket (B). Normally an 'A' represents a payload, while everything starting with 'C','D',... is usually debris. An important exception are the Russian C-1 rockets which can deliver eight payloads into orbit in one launch. They always have 'J' as extension.

10-17

Date of observation in the format yy-mm-dd. Here all figures are given (even non-significant numbers). e.g. '76-03-01' is March 1 in 1976.

19-28

Time of the end of the observation in the format hh:mm:ss.t. All times are given in Universal Time (UT). Hours (hh) are measured from 0 to 23 h. Minutes from 0 to 59 minutes, while seconds can be given up to one tenth of a second. Depending on the accuracy with which the time was measured, the time can be incomplete. An alternative format is hh:mm.t. This gives hours, minutes and tenths of a minute. Some observers use this alternative format to show their accuracy is worse than 1 second.

30-32

An abbreviation of the name of the observer. The abbreviation used may differ from the initials to avoid duplicate identifications. The code is given to the observer by the Belgian Working Group Satellites.

34-38

Total time in seconds and tenths of a second which passed during the measurement of the flash periods, in the format sss.t . It should be given as a backup to estimate the effect of a wrong count of periods and to check the given period (see below).

40-42

Accuracy in seconds and tenths of a second on the total time if the total time is given. If the total time field is left blank ( which is not advised), the accuracy should relate to the period. In this case accuracies (on the period) below 0.1 s can be entered as '.nn'. This means the accuracy (on the period) is 0.nn seconds.

44-46

The number of periods counted. The total time divided by the number of periods gives the flash period.

48-53

Flash period in seconds and tenths, hundredths (or even thousandths) of a second in the format sss.tht . The 'decimal point' is always found at position 50 unless the period is larger than 99.999 seconds. The number of digits given should be related to the estimated accuracy. Usually larger periods have lower accuracy (i.e. they are less accurate!). The number of digits depends both on the estimated accuracy and on the number of periods counted. E.g. the total duration is 214.5 s, the estimated accuracy on the total duration is 0.5 s, the number of periods counted is 50, then the accuracy on the flash period is . In this case the flash period can be given with two significant digits after the comma (4.29). If the accuracy on the total duration is 2.0 s, then the accuracy of the flash period is . So you can write the flash period with only 1 significant digit after the comma (4.3).

This field is left blank if no period has been measured or if (more likely) the object did not show any variation in brightness. In this last case, the object is 'STEADY'.

55-80

Remarks on the flash-pattern or on other aspects of the passage. A list of abbreviations which are commonly used is given in Table 3. Normally all remarks should be in lowercase, except for 'S' (steady, which always comes on position 55 if applicable) and some other abbreviations. All remarks on one observation are divided by commas and a space.

To describe the peculiarities of the flash-pattern (without drawing figures), the symbols in Table 1 are put into groups, depending on the pattern observed.

Table 1 : Symbols for flash pattern description

Some examples of flash patterns can be found in Table 2.

Table 2: Examples of flash pattern description

The format for describing the satellite's magnitude is:

mag +M.M ->m.m

E.g. mag +4->8.5

E.g. mag +5.5->inv

A few examples of PPAS entries may clarify the above.

90- 23 B 91-03-09 23:34:31.2 DWB 121.5 0.5  20  6.08  F'F'->f'Ff', mag +5->8

David W. Bishop observed 90- 23 B on March 9, 1991 at 23h34m31.2s UT with a flash period of 6.08 s. He counted 20 periods during a time interval of 121.5 s. His estimated accuracy was 0.5 s. Dividing 0.5 by 20 we get 0.025 s. This means the flash period can only be accurate up to 0.02 s, hence two digits after the comma.

The magnitude varied between 5 and 8, and the flash pattern changed during the pass. At first it was two primary flashes (F) on which was measured ('). This gradually ('->') changed into a pattern with one primary maximum (F) and two secondary maxima (f). So, the previously primary maxima were gradually dominated by a new primary maximum. David decided to stick with the maxima originally chosen, a wise choice.

90- 23 B 93-11-04 01:31      MM  147.8 0.1 110  1.343 F'F', ssm, mag +5->inv

Mike McCants observed the same object (90- 23 B) on November 4, 1993 at 1h31m UTC with a flash period of 1.343 s. He counted 110 periods during a time interval of 147.8 s. His estimated accuracy was 0.1 s (the best possible, the flashes were very sharp). Dividing 0.1 by 110 we get 0.001 s. This means the flash period can be accurate up to one thousandth of a second!

The magnitude of the satellite was +5 at maximum. It was invisible during the minima. The flash pattern was relatively simple with two primary flashes (F) on which Mike measured ('). There were some secondary maxima every now and then.

Table 3 : Abbreviations in the PPAS remarks column

Next we can consider the relationship between the rotation and flash period.