Satgen260  Satellite Spotter Part 8  by GM4IHJ               19th March 94
BID of this msg is SGEN260  Please use this BID if you retransmit this msg

The time at which you hear a strange signal is a good clue to what it
might be. Please write down the time you hear it and how long you hear it
for, see Pt4 in sgen255 for details . A Low Earth orbit LEO sat at 1000km
altitude will be heard for 15 to 17 minutes if it comes nearly overhead
your station. Mir , Space Shuttle and other 400 km altitude orbiters are
usually heard for 10 minutes or less. Whereas, the slow moving high
altitude elliptical orbiters such as Oscar 13 ,the Russian Molniya and the
GPS Navsats may stay in range for 6 hours or more.
Once the signal disappears, leave your receiver tuned to the same
frequency . If your signal is from a LEO 400 km altitude sat it will have
an orbit period of about 94 minutes . So it could be back again 94 + or -
5 minutes after the time you first heard it. It will not be exactly one 94
minute orbit period later because satellites only reappear at the same
time over the same latitude. So as a LEO satellite approaches your station
horizon each time it comes round it does so at a slightly different
latitude and hence a slightly different separation time from the orbit
period. Higher latitude crossing of your horizon from previous orbit means
slightly later than orbit period. Lower latitude approach means slightly
earlier. Typical separation and times might be    1240ut 96 1416ut 94
1550ut 94 1724ut 92 1856ut 90 2026ut, suggesting an average of about 94
minutes. You can confirm this if you hear another orbit the next day at
say 1213ut 27 minutes earlier . Which says you had X complete orbits in
1440 - 27 = 15 x 94.2 or 16 x 88.3  or 14 x 101 minutes. Clearly only 15 x
94.2 would fit your previous days orbit separations, which suggests your
signal is coming from a satellite at roughly the same orbit height as Mir
400 kms.
Please note that a LEO sat cannot stay up for long with an orbit period of
less than 89 minutes, and, it has to stay below the radiation belts so it
cannot have a period greater than about 125 minutes and have a long life.

For further orbit data you can check whether the sat always passes between
you and the Equator, and therefore has an orbital inclination less than
your station latitude. Or does it pass on the polar side of your station ?
indicating a higher inclination than your station latitude. With Americans
going left hand (western ) side of north pole and Russian ascending right
hand side (east) of the north pole in most cases.

For higher orbiters above the radiation belts at say 24000 km or more ,
you can hear them in range for up to 10 hours or so . So time first heard
is of little use in working out orbit period. But the slowly rising
doppler shift on their signals is a good clue to this type of sat.
Remember LEO sats have fast descending doppler, High Ellipticals have slow
rising doppler except as they flash through their perigee low point.

Satgen 260 marks the 5th Anniversary of this series of bulletins , which
gives GM4IHJ the opportunity to say a very grateful "Thank You" to all
those readers everywhere, whose comments and questions have made the last
5 years so interesting and friendly. Very grateful thanks are also
extended to the Sysops and the Satellite Gateway Operators who have sent
these bulletins around the world each week.
73 de John GM4IHJ @ GB7SAN