Condign volumes 1 to 3

Read from the scan by GLM-OCR; expect the odd misread word.

# UNCLASSIFIED UK RESTRICTED

8. Certain conditions apply for viewing to occur, and which set the conditions (or eliminate!) the possibility of a satellite being mistaken for a UAV.

- The sun must be at least $ 1 0^{\circ} $ below the horizon (giving a shadow height of at least 100Km).

- The satellite must be higher than the shadow.

- Faint satellites can only be seen when the shadow height is >100Km but less than the satellite's height, (which may be as low as 200Km).

9. Earth's Shadow Evening observations of low satellites are usually possible only during the hour or two while the shadow is climbing from 100Km to the satellite's height. The sequence is reversed in the morning. Because of earth's shadow, a low satellite can only be normally observed from 2 narrow bands of latitude (one in each hemisphere). Figure 4 illustrates the limitations during northern summer for a satellite in near-polar orbit with the sun in near orbital plane. The earth's shadow is shown with a sharp edge, but in practice this is fuzzy. The height of earth's shadow is absolutely fundamental for satellite observation, for example, in the Figure (at point P) the shadow height PS is approximately equal to earth's radius (~6000Km), so the only satellites visible overhead at point P would be more than 6000Km high and probably faint and difficult to see. In UAP evaluation most events which catch the eye and might be reported are likely to be bright.

10. Earth's shadow height varies with time of year. Figure 5 shows the variation at $ 5 0^{\circ} \mathrm{N} $ , local time. For viewers near London, Time = GMT. For every degree of longitude west of Greenwich four minutes must be subtracted. This chart can be used two ways; it shows the height of the evening shadow:

- Working from date (e.g. 2 March) at latitude $ 5 0^{\circ} \mathrm{N} $ sunset is $ \sim 1 7. 4 5 $ hrs, twilight lasts nearly one hour - hence the sky will not be dark enough to view satellites until $ \sim 1 8. 4 0 $ hrs, when the shadow height is100Km. By 19.10hrs the shadow has reached 200Km altitude. After that no

satellite lower than 200Km height can be seen overhead, though they could still be observed in the West where the shadow height is still lower. By 21.00 hrs the shadow height exceeds 1000Km and many brighter satellites at these altitudes can be seen.

- Alternatively, if the satellite height is known the chart can be used in reverse. This is a less likely scenario in trying to eliminate mis-reported UAV reports.

11. It is observed that (using the 100 and 400 curves at Figure 5) at $ 5 0^{\circ} \mathrm{N} $ on 1 January, the period available for satellite viewing is only from 17.12-18.18hrs. By April this has lengthened to about 1 hour and from approximately 20 May to 20 July a satellite could be seen at any time of the night. In June any satellite higher than 300Km is always above the earth's shadow at latitudes greater than $ 5 0^{\circ} \mathrm{N} $ . These are all important factors in correlation with potential UAP windows of observation. The observing period is actually slightly more than Figure 5 might suggest (since this is plotted for a satellite overhead). Clearly the satellite might be at some other elevation angle. In the dark eastern sky it is sometimes possible to observe before the end of twilight. For a satellite at 400Km altitude at longitudes up to $ 1 0^{\circ} $ to the west of the observer it is possible to observe up to 40 minutes longer than the figure indicates.

12. A rule of thumb (for $ 5 0^{\circ} \mathrm{N} $ ) is that over the greater part of the year (excluding May, June, July) the possible observation time for a faint satellite ( $ \sim 2 0 0 \mathrm{K m} $ altitude) is one hour and two hours if its height is doubled. As an example (with reference to Figure 5) the observer sees a satellite at 21.30hrs on 29 August the shadow height on the diagram is 500Km. Hence the satellite will be in shadow and visible unless its height exceeds 500Km.

12. Repeat Observations Observation of a satellite on successive evenings is subject to longitude changes at which it crosses the latitude of the observer. A satellite appears to be $ \sim1^{\circ} $ （0.986 $ ^{\circ} $ ）further west on successive nights due to the $ 360.986^{\circ} $ rotation in 24 hours because the earth spins through $ 360^{\circ} $ 23hrs 56 minutes and spends