**Previous message:**Brierley David: "DMB Some obs Sep 3-4"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

There have been a few postings recently about observing accuracy, so here is my contribution. Many (probably most) observers determine satellite positions as a fraction between 2 stars. The error in position is partly due to the accuracy of the star positions, partly due to the error in determining the fraction. In the pre-computer days star positions were measured by ruler on star maps (error maybe 0.02deg with a good map); if you still do this then check the accuracy by measuring a few stars whose true positions are known. These days star catalogs are commonly used, so the error in star positions is very small. The major error is due to the fraction. On 1968 October 16 an experiment was performed in which a simulated satellite passed among a projected star field, and about 30 satellite observers estimated the fraction (which was later measured accurately). Here is a summary of the results collected by Gordon Taylor, all in %; see below for column f2. Test True Mean Standard f2 fraction error deviation 1 55 -1 4 5 2 34 1 6 4 3 157 -10 11 18 4 27 -1 4 4 5 53 -3 5 5 6 203 -4 20 42 7 85 0 4 3 8 -21 3 4 5 9 27 -2 4 4 10 73 1 5 4 11 -59 8 11 19 12 20 0 4 3 Note that tests 3 6 8 11 extrapolate beyond the stars, the others are interpolations. As one might expect, the extrapolations are generally less accurate. Unfortunately none of the tests went close to either star. I believe that close to a star, the error would be approximately proportional to the distance from that star. In 1985 I started interpolating by computer, and I wanted a formula to give me accuracy, depending on fraction and star separation. I chose: error = e+(star separation)*abs(f*(1-f))*k where e and k are constants. The formula is symmetric about f=0.5 (=50%), and gives the desired behaviour near either star (f near 0 or 1), and larger errors for extrapolations (f<0 or f>1). Column f2 above is this function with constant e=0 and k=0.2 (=20%), which is a reasonable fit to the standard deviations except for the 3 biggest extrapolations. Constant e allows for the resolution of your binoculars (plus star map errors if used), and should be small, but not zero. Currently I use a peculiar e=0.02+0.005*(star separation) deg and k=1/12 which gives 3.6% for separation 2deg and f=0.5, (similar to my personal results at the meeting). I probably should use e=0.01deg and k=0.12 which would usually give similar results. Those with higher magnification than my 11* should use smaller e. Note that e is much larger than the nominal resolution of the binoculars, since we are watching moving objects, often variable or faint. Sometimes the accuracy from the formula has to be degraded, for example if the visibility is bad, the satellite very faint, or only briefly visible, or a cat has just jumped onto your lap. You can get some idea of your accuracy by finding 3 stars in a line, quickly estimating the fraction, and later calculating the exact fraction from the star positions. Or estimate the ratio (separation stars A & B)/(separation stars B & C). Or write a PC simulation of the 1968 experiment (I have a QBASIC program for this). Mike Waterman mike.waterman@marconi.com Site Yateley = COSPAR 2115 = 51.3286N 0.7950W 75m. ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html

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