======================================================================== Visual Satellite Observing F A Q Chapter-07 Satellite Observations: More Than Just Looking ======================================================================== ---- 7.0 Satellite Observations: More Than Just Looking There is definitely a lot of satisfaction involved in predicting where and when a satellite will appear and then to actually observe it. But soon one asks oneself whether it is possible to do more with these satellites than just looking at them. The answer is yes and it is twofold. ---- 7.1 Positional Measurements First, an observer can make positional measurements, i.e. measure the position of the satellite with respect to the stars at a certain time. Using these measurements, it is possible to determine deviations from the predicted track. These deviations can be due to atmospheric drag (i.e., friction with the air molecules that are still present even at orbital heights), pressure from solar radiation, orbital maneuvers, or gravitational resonances. Using positional observations from all over the world, scientists are able to determine several things: * The density of the atmosphere, which is highly variable and dependent on solar activity. At high solar activity, the temperature and density of the upper atmosphere both increase significantly. This results in greater atmospheric drag on satellites. Observing satellites that are close to reentry is especially useful for density determinations. * The rotation of the atmosphere, which is dependent on height. This differential rotation influences the satellite's orbit. * The pressure of the solar radiation. Satellites with low mass to surface ratios, such as balloon satellites, are very sensitive to this otherwise negligible force. * The gravitational field of the Earth, which is latitude and longitude dependent. The latitude dependency usually averages out over several orbits (with a few exceptions). However, the longitudinal dependency doesn't average out. Satellites which travel an integer number of periods per day or per two days (e.g. 14 or 29) resonate with the Earth's gravitational harmonic potentials of the same order (e.g., 14th or 29th harmonics), which causes serious disturbances of their orbits. This is, by far, the most important application of positional measurements of satellites. Very accurate values for the Earth's gravitational field have been determined in this way. In the early days of the space age, very few tracking stations were available for obtaining accurate positional measurements. Amateur observers were of great help then to professionals in obtaining accurate satellite position measurements. Later, more sophisticated radar equipment became readily available and was positioned all over the world to get better coverage of the orbits. Due to this evolution, measurements by amateurs became less desired. Over the past twenty years, support for amateur networks by professional research institutes (such as providing predictions, et cetera) has gradually, almost entirely, disappeared. Nevertheless, research has not completely dwindled and amateur positional measurements remain useful. Observations of the more interesting objects are not necessarily available in abundance, which is one reason visual observations are valued. Another reason is that visual observations are of help in dealing with the biases that might be present in more accurate data. And visual observations are quite adequate and appropriate for high drag objects. During the last several years, a unique application of positional measurements has become more popular, i.e., the determination of the orbital elements of USA spy satellites. Orbital elements are required to predict the passage of satellites. USSPACECOM distributes orbital elements of virtually all satellites, except for secret USA military satellites, such as the KH (KeyHole) and NOSS satellites (three satellites that move together in tight formation). A rather informal worldwide network of persistent amateurs has specialized in tracking these objects and calculating the orbital elements themselves. This celestial detective work certainly gives an additional stimulus to positional measurements. ---- 7.1.1 Required Equipment To make accurate positional measurements of satellites, certain equipment and skills are needed: * A regular pair of binoculars like a 7 x 50 (which magnifies sevenfold and has an objective diameter of 50 mm). * A stopwatch accurate to within 0.1 seconds. * A star atlas (and optionally a star catalog). * Predictions of where and when satellites will make passes visible from your location. * A short wave radio tuned to a time signal station (such as WWV in the USA) or alternatively a clock tuned to one of those radio stations. * For faint objects and very accurate positional measurements, a pair of 11x80 binoculars can be desirable. ---- 7.1.2 Technique The aim of these measurements is to measure the position of the satellite at a certain time. One might think that the most accurate way to estimate a satellite's position is to note when the satellite passes in front of a star. That is not necessarily true when the satellite and the star are not of comparable brightness. If a bright satellite passes "in front of" a faint star, the star will disappear in the glare of the satellite, thus making it difficult to determine the exact time of occultation and, hence, difficult to obtain an accurate measurement. The opposite problem, with the same effect, occurs when a faint satellite passes in front of a bright star. The easiest way to determine the position of a satellite is to note when it passes between two stars that are not too far apart, say, at most half a degree apart. The two stars should not be too far apart from one another, since the accuracy of the satellite's position should be a few arcminutes. (The moon's diameter is about half a degree or 30 arcminutes.) The most accurate measurements are possible when the path of the satellite is perpendicular to the line connecting the two stars. At the instant that the observer sees the satellite intersecting the imaginary line joining the two stars, (s)he should (a) start the stopwatch and (b) mentally note the approximate position of the satellite on that line, relative to the two reference stars, for a later interpolation step. The stopwatch should be stopped some time later when the next reference "tick" mark is heard on a radio tuned to a time signal station. (Such "ticks" are generally broadcast at the start of each full minute.) Subtracting the recorded duration from the reference time gives the time that the satellite passed between the two stars. Universal Time (UT) (also called Greenwich Mean Time or GMT) should always be used in reporting these times. The time should be recorded with the highest possible accuracy. Since the human reaction time is not better than 0.1 to 0.2 seconds, the observer should strive to achieve 0.1 to 0.2 seconds accuracy. After the time has been determined, the observer should next look up the two reference stars in the star atlas. Note that a more accurate star atlas is needed to process positional measurements than for drawing predictions. The star atlas should have sufficient scaling to allow for an estimating accuracy of 1 or 2 arcminutes. The satellite's position at the time of measurement should then be plotted on the star atlas. For example, if "1" is the total length of the line joining the two stars, and if the observer had estimated that the satellite passed at "0.4" from one star, the satellite's position in the star atlas can be easily plotted using a ruler. At this point, it becomes possible to estimate the Right Ascension and Declination coordinates of the satellite's position from the plotted position in the star atlas. Alternatively, the observer can look up the position of the two reference stars in a star catalog and, using the previously estimated position of the satellite relative to those two stars, calculate the satellite's position by linearly interpolating between the known Right Ascension and Declination coordinates of the two reference stars. The accuracy of a position does not depend only on the way the position is measured, but also on the angular velocity of the satellite. A low earth orbit satellite (altitude less than 500 km) moves across the observer's sky with an angular velocity of between 0.5 and 1 degree per second. During the time interval of 0.1 second, our timing precision, such a satellite will move between 3 and 6 arcminutes, which will ultimately be the best possible positional accuracy. Naturally, for objects at higher altitudes, the angular velocity will be smaller and the positional accuracy will accordingly be better. ---- 7.1.3 Reporting Positional Measurements One observation should contain the following: * Satellite Code. There are actually two formats for this. The NORAD number (like #16619) is used by many American observers. The COSPAR international code (like 86- 17A) is preferred by the Royal Greenwich Observatory. The COSPAR code, YY-NNNL, gives the year (YY) of launch, the number of the launch that year (NNN) and the letter (L) indicating the element of that launch (from A onwards). For example, the primary payload may be designated as element A, a secondary payload as element B, and the accompanying rocket as element C. * Time in UT, up to 0.1 second accuracy. * Position in Right Ascension and Declination. * Estimated accuracy in time and position. * The magnitude (optional). * The observer's exact location (longitude, latitude and height above mean sea level). * The equipment used. The observer's location should be measured with 0.1 minute positional accuracy using geodetic maps. The Royal Greenwich Observatory (RGO) invites observers to send in positional observations of satellites on their priority list. The priority list contains satellites whose orbits are interesting for researchers. The data is used to determine orbits. The orbits are analyzed and the analyses are published in scientific journals. For observing forms, the priority list and more information you can contact: Phil Gibbs, Satellite Laser Ranging Team (SLR), Herstmonceux Castle, Hailsham, East Sussex, BN27 1RP England. The priority list is also regularly published on the Internet mailing list SeeSat-L. The British Astronomical Association (BAA) also coordinates positional measurements in conjunction with the RGO. The BAA can be contacted at: British Astronomical Association, Burlington House, Piccadilly, London, W1V 9AG England. Orbital elements for secret objects like Lacrosse are not available from NASA. An informal network of amateurs has specialized in keeping track of secret USA satellites. Regular accurate positional measurements are needed for this kind of detective work. Most of the observers and analysts involved in this work are active on the Internet mailing list SeeSat-L. Observations of secret USA satellites are welcomed on SeeSat-L. Make sure the observation contains all necessary information (see above). ---- 7.2 Flash Period Timings The aim of these measurements is to measure the average flash period of the satellite during one pass. These measurements are collected and analyzed by the Belgian Working Group on Satellites (BWGS), which maintains a database of Photometric Periods of Artificial Satellites (PPAS). The analysis of PPAS data sheds light on the temporal evolution of the rotational state of satellites, which in turn can provide information about the forces and torques acting on satellites. ---- 7.2.1 Required Equipment * A regular pair of binoculars like a 7 x 50 (which magnifies sevenfold and has an objective diameter of 50 mm). * A stopwatch that is accurate to within 0.1 second. * Predictions of where and when satellites will pass. * [optional] A portable battery powered tape recorder with fresh batteries can be useful for recording observer commentary and for later flash period reduction. It is not a necessity. Most observers don't use it. ---- 7.2.2 Technique Once the object has been found, it is best to follow it for some time in order to familiarize oneself with the flash pattern. Sometimes this pattern and the flash period itself change during the pass, due to changing illumination conditions. This is called the synodic effect. Even experienced observers occasionally get confused when the synodic effect is large. Once the observer has become accustomed with the satellite's flash pattern, the measurements should be started. ---- 7.2.2.1 Using An Old-Fashioned Stopwatch If the observer uses an old-fashioned type of stopwatch that does not allow, or allows only one, split timing, the following procedure should be used. The observer should start the stopwatch at the time of a flash (or at a minimum if it is more pronounced), and start counting with "zero". Each subsequent flash is counted, while the stopwatch is running. Finally, the stopwatch is stopped at the last flash that the observer wants to, or can, observe. Since the count was begun at "zero", the final count is the total number of flash periods that occurred during the interval measured by the stopwatch. If the observer started the flash count at "one" instead of "zero", the final count must be reduced by "one", in order to obtain the number of flash periods. (Remember, the number of flash periods, i.e., the number of sub-intervals between flashes, is one less than the number of flashes!) Next, the observer reads the total time on the stopwatch and writes it down. Dividing the total time by the number of periods gives the flash period. The observer should always try to count as many periods as possible during one pass. For flash periods smaller than 5 seconds, the observer should try to count fifty to one hundred periods for one measurement. This substantially improves the accuracy of the flash period measurement. If the stopwatch allows a split timing, the observer could 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 satellite disappears in the Earth's shadow or behind an obstacle, or when the flash pattern changes drastically. ---- 7.2.2.2 Using A Multiple-Memory Stopwatch Using a stopwatch that allows for multiple timings (fifty to one hundred is typical), it is possible to time each flash. The data can then be analyzed later to derive an average period, using the same procedure described above in section 7.2.2.1, i.e. by chossing a pair of flashes and counting the number of periods between them. By picking several pairs of flashes and calculating the average flash flash period between them, it is possible to detect errors the observer made, correct for them and pick out the best "average flash period" of the pass. Making multiple timings can also be done by recording 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, since it allows for comments about the flash pattern to be recorded as well. The data thus obtained provides information about the size of (variation of) the synodic effect and can also be used, under certain conditions, to try to determine the direction of the rotation axis of the flashing satellite. This is discussed in section 7.3 about the DRA (Determination of Rotation Axis) project. ---- 7.2.2.3 Estimating The Accuracy Of The Measurement Having obtained an estimate for the average flash period during the pass (using one of the methods described in the preceding two sections), the observer should try to estimate the accuracy, with which the total time (on the stopwatch or from the analysis of multiple timings) was measured. If the flash period is short, or if the flashes are distinct, the timing accuracy may approach the precision of the stopwatch. If the flash period is long, with indistinct flashes, the accuracy may not be better than several seconds. The estimated accuracy depends on the flash pattern, observer's reaction time, visibility of the flashes, and so on. If the observer noticed that (s)he was a little late in reacting to a flash, then the estimated accuracy should be adjusted accordingly. The minimum reaction time of a human is 0.1 to 0.2 second. Accuracies better than 0.1 second are not acceptable unless the observer is superhuman. Note that the accuracy of the total time can also be estimated from the analysis of multiple timings. The observer should choose an interval between two of the many flashes that were measured and calculate an average flash period from this pair of timings. Next the observer should do this for several other widely spaced pairs of flashes to obtain several estimates of average flash period. For each of these average flash period estimates, the observer should then compute a total time (by multiplying the corresponding flash period by the total number of flash periods). The slight differences between these total time estimates are a measure of the accuracy of the total time. It is quite possible that the synodic effect will cause the difference between the rotation and flash period to be larger than the observing accuracy. However, it is 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 the measuring accuracy is worse than the loss of accuracy due to the synodic effect. For example, suppose the measuring accuracy is estimated to be 0.4 second and that the maximum possible synodic effect is 0.5 second. If the synodic effect was only 0.1 second for the geometry during a pass, it is useful to know that the flash period is still no more accurate than 0.2 second, due to observing accuracy. ---- 7.2.3 Reporting Flash Period Measurements The flash period observation should be noted and documented on a special form or entered directly into the PPAS computer database of flash period observations. PPAS is an acronym for Photometric Periods of Artificial Satellites. More experienced observers also estimate the brightness of the flashes, determine the flash pattern, and report these observations, as well. ---- 7.2.3.1 PPAS Format The format described in this section is the standard format used for entry of a flash period observation into the PPAS database. The PPAS line format is column-sensitive. Each line is divided into column ranges (or fields), and each field always contains the same data. Normally, each PPAS line contains 80 columns (or characters). In the following, each field of a PPAS line is described as a column range and the contents of the data to be put in that field. 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 for that year (only contains significant numbers and is right justified) cc designates the piece of the launch (contains non-numeric characters and is right justified) For example, object '86- 39 B' was launched in 1986. It was the 39th launch in 1986, and it was a rocket (B). Usually, an 'A' represents a payload, while everything starting with 'C', 'D', et cetera is usually debris. Important exceptions to this scheme are the Russian C-1 rockets, which can deliver eight payloads into orbit in one launch. They always have 'J' as extension. The COSPAR code is usually included in some way in the orbital elements used to predict the pass. 09 blank 10-17 Date of observation in the format yy-mm-dd. yy is the year mm is the month dd is the day Here all figures are to be given (even non-significant numbers). For example, '76-03-01' is March 1 in 1976. Note that the date given here should correspond to the time given in the next field in Universal Time (UT). 18 blank 19-28 Time of the END of the observation, in Universal Time (UT), in the format hh:mm:ss.t. hh is the hour, measured from 00 to 23 mm is the minute, measured from 00 to 59 ss is the second, measured from 00 to 59 t is the tenth of the second, measured from 0 to 9 Depending on the accuracy with which time was measured, the time can be incomplete. An alternative format is hh:mm.t. This format gives hour, minute, and tenth of the minute. Some observers use this alternative format to indicate that their accuracy is worse than 1 second. 29 blank 30-32 An abbreviation of the observer's name. The code used may differ from the observer's initials to avoid duplicate identifications. The code is given to the observer by the Belgian Working Group on Satellites (BWGS). 33 blank 34-38 The 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). 39 blank 40-42 Accuracy in seconds and tenths of a second on the total time if the total time is given, in the format s.t . If the total time field is left blank (which is not advised), the accuracy should relate to the period. In that 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. 43 blank 44-46 The number of periods counted, right justified in the field. The total time divided by the number of periods gives the flash period. 47 blank 48-53 Flash period in seconds and tenths, hundredths (or even thousandths) of a second in the format sss.tht . The 'decimal point' should always appear in column 50 unless the period is larger than 99.999 seconds. The number of digits to the right of the decimal point should always be consistent with the estimated accuracy. (Note that larger periods are usually less accurate!) The number of digits that may be reported depends both on the estimated accuracy and on the number of periods. For example, if the total duration is 214.5 seconds, the estimated accuracy on the total duration is 0.5 seconds, and the number of periods counted is 50, then the accuracy on the flash period is 0.5 / 50 or 0.01 second. In this case, the flash period can be reported with two digits to the right of the decimal point (i.e., as 4.29 seconds). If the accuracy on the total duration is only 2.5 seconds, then the accuracy of the flash period is 0.05 second, so the flash period can be written with only one digit to the right of the decimal point (i.e., as 4.3 seconds). 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 said to be "steady". 54 blank 55-80 Remarks on the flash pattern or on other aspects of the passage. A list of abbreviations which are commonly used is given below. Normally, all remarks should be written in lowercase, except for 'S' (steady) which should always be in column 55 if applicable, and a few others. Each of the remarks about one observation should be separated by a comma and a space. The format for describing the satellite's magnitude is: mag +M.M->m.m where M.M is the maximum magnitude and m.m is the minimum magnitude, for example, mag +4->8.5 . The '+' is omitted for the minimum magnitude. When the minimum is invisible, this is indicated with 'inv', for example, mag +5.5->inv . Some observers only mention the maximum magnitude, for example, mag +5 . The following abbreviations are used in the PPAS remarks field: ? The photometric period is uncertain, or the remark that accompanies this question mark is uncertain. a The photometric period is an approximation. amp Amplitude. b The observation has been made with binoculars. This applies to most measurements, but is sometimes mentioned explicitly. dec Decreasing in brightness, sometimes mentioned with 'sm'. df Two flashes in one period (double flash). dif Different (maxima). dm Double maxima: two close maxima in one period. dp Double period: the period measured has been taken between three (instead of two) similar points in the light curve. dtm The period was difficult to measure. fm Flat maxima: the brightness remained fairly steady during a relatively long time. Because of this, the exact moment of brightness maximum is difficult to define. hp Half period: the period measured was half of the real period. I, irr Irregular brightness pattern, irregularly varying. inc Increasing in brightness. lp Long period. min The period has been measured on the minima in brightness. N? The counted number of maxima is uncertain. occ Occasionally. ph Photographic observation. pm Primary maxima. qm Quadruple maxima: four maxima in one period. R Regularly varying. S The brightness did not (or only very slightly) change, except for the variation caused by the change of phase angle. sa Small amplitude: there is a small difference in brightness between a maximum and a minimum. sec Secondary maxima. sf Short flashes, possibly used as maxima. sm A relative (secondary) maximum occured between two absolute maxima. ssm Some secondary maxima were visible. tm Triple maxima: three maxima in one period. u An observation made with the unaided eye. var Varying (usually used in the phrase 'slowly var'). V Varying. vm The observed maxima varied in brightness during the observation. To describe the peculiarities of the flash-pattern (without drawing figures), the following symbols are put into groups depending on the pattern observed: A Smooth primary (or absolute) maximum. a Like 'A' but secondary or relative. F Sharp (like a flash) primary (or absolute) maximum. f Like 'F' but secondary or relative. M Flat primary (or absolute) maximum. m Like 'M' but secondary or relative. _ or - The underscore or dash indicates a minimum or the absence of an expected maximum. ' The apostrophe usually indicates the maximum on which the count was incremented. Sometimes used with _ or - to indicate that minima were counted. , The comma indicates the location of the minimum which was used to count the periods. Here are some examples: A'A' A regularly varying object with primary maxima only. A'aA' A regularly varying object with secondary maxima. Primary maxima were used to count. A'FA_A' This pattern occurs very frequently in case of Soviet A2-rockets. A maximum is followed by a flash (comparable in brightness), by another maximum, after which the next flash is absent. After this, the pattern is repeated. A'fA_A' The same as the previous example, but the flash is less bright. a'Fa_a' In this case, the flash is definitely brighter than the smooth maxima. M,M A pattern with two flat maxima divided by a sharply defined minimum (which was used to count the periods). F'F' A pattern that has flashes only. The following 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 seconds. He counted 20 periods during a total time interval of 121.5 seconds. His estimated total time accuracy was 0.5 second. Dividing 0.5 by 20 gives 0.025 second. This means the flash period can only be accurate up to 0.02 second, hence, two digits after the decimal point. 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 period counts were taken ('). 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 seconds. He counted 110 periods during a time interval of 147.8 seconds. His estimated accuracy was 0.1 second (the best possible, the flashes were very sharp). Dividing 0.1 by 110 gives 0.001 second, so 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 counted ('). There were some secondary maxima every now and then. ---- 7.2.3.2 How And Where To Report PPAS Observations There are two software programs that allow for user-friendly entry of flash period observations into the PPAS format: * PPAS Input for Windows, v.2: This program, written by Jim Varney (jamesv@softcom.net), takes stopwatch readings of flashing satellite observations and converts them into standard reporting format. Supports both PPAS and DRA (see Section 7.3) formats. For IBM compatible PCs that run under Windows. Can be downloaded from: http://www.quiknet.com/~jvarney/Ppasinp2.zip * SatFlash v6.3: Written by Jean De Weerdt and Bart De Pontieu for IBM compatible PCs that run under DOS. Is multiple purpose PPAS analysis program, with tools for maintenance of PPAS database, graphical analysis, and user-friendly entry of flash period data into PPAS format. Can be downloaded from: http://www.satobs.org/ Flash period observations in the PPAS-format are welcome at the following e-mail address: ppas@satobs.org The BWGS contact-person for flash period observations (for questions, remarks, problems, et cetera) is Kurt Jonckheere (kjonckheere@unicall.be). ---- 7.2.3.3 What To Observe: BWGS Observing Program The Belgian Working Group on Satellites (BWGS) has been active in collecting flash period measurements since 1987. Their priority list contains over 200 satellites of which most are flashing, suspected of flashing, or expected to start flashing. The BWGS maintains the PPAS database which contains over 42,000 observations of more than 1300 different satellites by over 150 different observers. The BWGS maintains a network of 35 flash period observers scattered over the world and is always looking for more observers. The BGWS observing program and the PPAS database are updated monthly. The priority list of satellites of the BWGS can be found at several locations on the Internet: * http://www.satobs.org/ * e-mail: by sending a message with in the SUBJECT-line the words "archive get program/program.*" (without the quotes) to SeeSat-L-Request@satobs.org The BWGS priority list is divided into several categories: b Flashing object for beginners d Difficult to measure S Steady object, potentially flashing in the future ? Flash period is unknown or uncertain - Long flash period (greater than 40 seconds) ! Top-priority object x Object for DRA project regular object The home page of the BWGS is at: http://users.skynet.be/satimage/bwgs/bwgs.htm ---- 7.3 Determination Of Rotation Axis (DRA) Project DRA is a project aimed at determining the direction of the rotation axis vector of tumbling satellites. Tumbling satellites (usually third stages of rockets) usually give off regular flashes. By recording the times of a long series of these flashes, it is possible to determine the direction of the rotation axis vector. ---- 7.3.1 Why Bother Trying To Determine The Rotation Axis Vector Of A Satellite? The (change of) direction of the rotation axis vector gives information about the torques acting on the satellite. These torques provide insight into what makes the rotation period of tumbling satellites change. It can be used to check the theoretical models (for rotation periods that are increasing over time), or it can help development of such models (for the so-called accelerating rockets, whose rotation periods are DEcreasing over time). Why certain rockets exhibit an accelerating flash period is, as of yet, a mystery. ---- 7.3.2 What Data Is Needed For DRA ? The goal is to accurately measure the absolute time of all (primary) flashes during the pass of one of the satellites in the DRA list (see section 7.3.6). There are basically two techniques for measuring the flash times. The first method involves using a stopwatch that can store 50 or more split timings. Each time a flash is seen, the observer records a split by pushing the button. At the end of the pass, the stopwatch should be stopped on an accurate time signal (such as shortwave radio or GPS receiver). The absolute time of the first flash can then be determined by subtracting the total duration of the observation from the absolute time that was recorded at the end. The observer reports the absolute time of the first flash and the elapsed time (since the start time) for all subsequent flashes seen. The second method involves using a tape recorder, on which a radio time signal is recorded during the pass. Each time a flash is observed, the observer shouts or makes some other noise, preferably short. Afterwards, the tape is reviewed and analyzed to determine the absolute times of all flashes. The first method is used by most observers (though there are notable exceptions), presumably because it is less involved. ---- 7.3.3 What Are The Caveats For DRA Timings ? First, the observer should try to keep track of which flashes are primary and stick to timing the same type of flash, even if their character changes later during pass from primary to secondary. Some rockets show different types of flashes, all during one rotation period. It is allowed (even encouraged) to time and report all the flashes, but the observer's report should indicate which ones are of the same type. Some rockets also change overall flash behavior during the pass. This is a guaranteed source of confusion, but it can also be very useful, for determining the direction of the rotation axis vector. The most accurate determinations of rotation axis vector have been made using such observations! Finally, it is encouraged to report secondary flashes as well. Times of secondary flashes can give insight to the rotational geometry, which is important for the analysis. ---- 7.3.4 What (Timing) Accuracy Should Be Endeavored? The observer should try to achieve the best possible timing accuracy. This will largely depend on the type of maximum in the light curve, that is, whether the maximum is flat, roundish, or very sharp. If the satellite is showing very diffuse maxima, this should be reported, since it is important for the analysis. The observer should also try to estimate how accurate the times are. ---- 7.3.5 How Should The DRA Observations Be Reported? DRA observations should be reported in the standard DRA format, so that the data can be fed directly into the analysis software. Here's an example of the DRA format: 95- 58 B 96-12-25 17:36:05.59 LB 52.6333 4.7833 3 45 0 1 2 3 4 5 6 7 8 9 0 0.00 5.15 10.80 16.13 21.30 27.03 32.96 37.86 43.05 49.16 1 54.33 59.68 64.97 m 77.70 81.51 86.65 91.73 97.87 103.83 2 m 113.56 119.47 124.38 130.31 135.75 141.20 146.56 152.39 156.66 3 162.61 167.50 173.13 179.22 184.89 189.83 195.61 200.91 206.50 212.16 4 217.52 222.53 228.12 233.65 239.40 245.65 251.43 Notice that this format is both line and column sensitive. The first line contains: Columns 01-08 COSPAR code of the satellite in the format yy-nnncc. (See section 7.2.2.3 on the PPAS format for details.) Column 09 blank Columns 10-17 Date of observation in the format yy-mm-dd. (See section 7.2.2.3 on the PPAS-format for details.) Column 18 blank Columns 19-29 Time of the BEGINNING of the observation in the format hh:mm:ss.th . All times are given in Universal Time (UT). Hours (hh) are measured from 00 to 23 h, minutes (mm) are measured from 00 to 59 minutes, and seconds (ss.th) can be given up to one-hundredth of a second. A precision of 0.1 second is required. Column 30 blank Columns 31-33 An abbreviation of the observer's name. The code used may differ from the initials to avoid duplicate identifications. The code is given to the observer by the Belgian Working Group on Satellites. Column 34 blank Column 35-42 Observer's latitude in decimal degrees, in the format dd.thtt (degrees, tenths of degrees, hundredths of degrees, thousandths of degrees). Positive values designate north latitude. Negative values designate south latitude. Column 43 blank Columns 44-51 Observer's longitude in decimal degrees, in the format dd.thtt (degrees, tenths of degrees, hundredths of degrees, thousandths of degrees). Positive values designate locations east of the Greenwich meridian. Negative values designate locations west of Greenwich. Column 52 blank Columns 53-56 Observer's height (in meters) above mean sea level. Column 57 blank Columns 58-60 Total number of timings (excluding missing ones). Columns 61-80 blank The second line gives the digit ("one's place") of the flashes counted, measured, and reported in later lines. The third and later lines record the relative times of the flashes observed. These times are in seconds AFTER the absolute time of the BEGINNING of the observation reported on line 1. Up to 10 flash times are reported per line. The format of these lines is as follows: Column 01 Decade ("ten's place") of the flashes reported on this line. Column 02 blank Columns 03-08 Relative time of flash with digit=0. Column 09 blank Columns 10-15 Relative time of flash with digit=1. Column 16 blank Columns 17-22 Relative time of flash with digit=2. Column 23 blank Columns 24-29 Relative time of flash with digit=3. Column 30 blank Columns 31-36 Relative time of flash with digit=4. Column 37 blank Columns 38-43 Relative time of flash with digit=5. Column 44 blank Columns 45-50 Relative time of flash with digit=6. Column 51 blank Columns 52-57 Relative time of flash with digit=7. Column 58 blank Columns 59-64 Relative time of flash with digit=8. Column 65 blank Columns 66-71 Relative time of flash with digit=9. Columns 72-80 blank For example, line 3 (the line starting with 0 in column 01) records the relative times of flashes 0 through 9. Line 4 (the line starting with 1 in column 01) record the relative time of flshes 10 through 19. And so on... Where the observer missed a flash, an 'm' for 'missing' is recorded instead of a time. In the example above, flashes 13 and 20 were not timed. The observation was made by LB (Leo Barhorst) on December 25, 1996 from his location at 52.6333 degrees north, 4.7833 degrees east, 3 meters above sea level (in the Netherlands). He made a total of 45 timings (46+1-2) of the satellite with COSPAR code 95- 58 B. Note that there is a program that much facilitates writing observations into DRA format: PPAS Input for Windows, v.2. This program, written by Jim Varney (jamesv@softcom.net), takes stopwatch readings of flashing satellite observations and converts them into standard reporting format. Both PPAS (see section 7.2) and DRA formats are supported. The program is available for IBM compatible PCs that run under Windows. It can be downloaded from: http://www.quiknet.com/~jvarney/Ppasinp2.zip ---- 7.3.6 What Satellites Should Be Observed? In general, the DRA project coordinators are interested in all flashing satellites with clear-cut flashing behavior, with a flash period roughly between 5 and 20 seconds, and for which the flashes can be timed to at least 0.5 to 1 seconds accuracy. Good candidates include: * Satellites in the BWGS program of flashing satellites that are marked with the top-priority code '!' * Satellites in the BWGS program of flashing satellites that are marked with the priority code 'x' * Satellites for which DRA observations have recently appeared on the SeeSat-L mailing list * Satellites which have been mentioned in the sporadic "DRA-Update" on the SeeSat-L mailing list ---- 7.3.7 How Many Timings Are Needed For The Analysis? The more timings the better! Timing only 5 flashes is much less useful than timing 50 flashes. Also, unless something "peculiar" happened during your pass, the analyst will need measurements of a pass shortly before or after yours. The analysis procedure produces best results if a very long pass is observed, or if information from two or more passes (on about the same day) is included. These kind of observations are called "quasi-simultaneous observations". ---- 7.3.8 Which Kind Of Passes Should Be Observed ? Consecutive passes of the same object (that is, with about 90 minutes in-between) are preferred. If at all possible, try to choose passes that have different observer-sun geometry. This means that the observer should try to observe the object under different illumination conditions. For example, avoid always observing the south-to-east pass, but instead try to observe the west-to-north pass also. Another example: avoid always observing the zenith pass, but instead try to observe the pass that is only 40 degrees above the horizon, as well. This approach will maximize the accuracy that can be obtained during analysis. ---- 7.3.9 Where Should DRA Observations Be Submitted? Send your DRA observations (in standard DRA format) to: ppas@satobs.org When you submit a DRA observation, be sure to include a regular PPAS-format observation as well (see section 7.2). ---- 7.3.10 How Will The Observations Be Used? Observations will be used only for scientific purposes. At regular intervals, the observations are analyzed. Usually, observations of the same satellite at approximately the same time (from all observers) are combined. In the current phase of the DRA project, we are still attempting to validate the usefulness of the method. Work is underway on a publication for a refereed scientific journal using the results obtained so far. Regular updates have and will appear on the SeeSat-L mailing list and the home page of the BWGS at: http://users.skynet.be/satimage/bwgs/bwgs.htm In any case, the observers will get regular feedback through the SeeSat-L mailing list, as well. All observers will be acknowleged in the scientific paper(s) that may result from the DRA project. ======================================================================== This FAQ was written by members of the SeeSat-L mailing list, which is devoted to visual satellite observation. Members of this group also maintain a World Wide Web site. The home page can be found at the URL: http://www.satobs.org/ The information on the VSOHP web site is much more dynamic than that found in this FAQ. For example, the VSOHP site contains current satellite visibility and decay predictions, as well as information about current and upcoming Space Shuttle missions and Mir dockings. The VSOHP site also contains many images, equations, and data/program files that could not be included in this FAQ while maintaining its plain text form. This FAQ and the VSOHP web site are maintained asynchronously, but an effort is made to synchronize information contents as much as possible. The material in this FAQ chapter was last updated in February 1998. ========================================================================