Condign volumes 1 to 3

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

# UNCLASSIFIED UK RESTRICTED

## VISUAL OBSERVATION OF SATELLITES

1. Orbital Period This could be relevant to UAP studies, as an event (i.e. satellite mistaken for a UAP) could be reported once or more on the same pass or on successive passes. The orbital period (i.e. in this UAP context the time of appearance or reappearance) is shown at Figure 1. Thus if the orbit altitude is changed from 200km to 1500km the orbit time changes from 88 to 116 minutes. Hence, UAP sightings with similar characteristics and at these approximate time intervals could be satellite sightings. $ ^{1} $

2. Inclination This is the angle at which the plane of an orbit cuts the equator. It determines the maximum latitude reached by the satellite and hence its potential to be observed in the UKADR.

3. Re-entry The effect of air drag (even with much reduced density) causes LEO satellites especially to re-enter the atmosphere. This occurs even with elliptic orbits, which suffer higher drag at each perigee. The lifetime of the satellite is a function of its mass and drag and is thus directly proportional to its mass/area ratio. Because the upper air density is variable, precise re-entry timing is difficult to predict. A visual example of re-entry (Skylab 1) is shown at Figure 2. It is important to note that the colours produced can be white, whitish-yellow, with touches of orange, red, green and blue, all colours often reported as UAP events. Re-entry velocity can be close to 8km sec $ ^{-1} $ and the pyrotechnic display ceases when it has descended to $ \sim 3 0 \mathrm{k m} $ altitude, having commenced its glow at 90km altitude. After 30km the remaining fragments fall at a steeper angle. A single calculation shows that a 60km downward path (at 8km per second) lasts only about 7.5 seconds.

4. Satellite Brightness Brightness depends on size, shape, surface finish and phase angle relative to the sun. A spherical satellite (e.g. 40+m in diameter (Echo 2 balloon) or 0.3m dia, at the other extreme, may have all its surface illuminated facing the sun. This occurs if a spherical satellite is in the east with the sun just below the horizon in the West. If the sun is in the west when the

satellite is in the West, only a crescent is illuminated. The texture of satellite surface (i.e. its surface finish) varies. A white or shiny surface is brighter than a black satellite. It may be partially illuminated and hence seen as an inclined disc rather than a sphere.

5. Comparison of Brightness with Stars It is possible that some UAP reports (from those with knowledge of astronomy) may rate the UAP in terms of the 'magnitude' scale defined by reference to standard stars/planets and even compare a UAP brightness with that of a familiar star (e.g. Pole Star). The level 'magnitude 1' is the brightest with 'magnitude 6' roughly the faintest. the Pole Star has a magnitude of 2.1 Sirius; (a regular UAP report), Mars and Salyut-type systems have a magnitude of -1 (i.e. 2.5 times brighter than a star of magnitude 0 such as Vega, itself 2 times brighter than a star of magnitude 1. Figure 3 relates magnitude against satellite diameter, velocity and range from observer. Although this is short-range it can be roughly approximated to orbit height when a satellite is viewed approximately overhead.

6. Cylindrical and Balloon Satellites Brightness is estimated by converting to the brightness of an equivalent sphere from the side-on area of the cylinder (i.e. length x diameter) and then taking the square root to give the equivalent sphere. (e.g. 8 x $ 2=1 6 \mathrm{m}^{2} $ (cylinder side area) = 4m diaspherical equivalent). Clearly the area would be less when viewed end-on. Cylinder area viewed may fluctuate, if they are tumbling, giving the appearance of flashing lights. Balloon satellites are very sensitive to air drag. They may break up in space, producing several reflecting objects.

## CONDITIONS FOR VIEWING SATELLITES

7. This is a key topic for UAP filtering. A satellite can only be seen:

- If illuminated by the sun

If the observer is well in shadow

- If the satellite is against a dark background.