Satgen456  Satellites in Drag   by GM4IHJ      (BID SGEN456)  20Dec97

Back in the 1970s , Project Oscar supplied a superb printed calendar.
For each day of the year it listed the Equator Crossing (EQX) times
and longitudes for Oscar 7. Would be users got out their Oscar Locator
and set up the orbit track on the designated longitude on the equator 
for the next satellite pass, and counted up the minute intervals to
where the track entered (AOS) and left (LOS) their station footprint.

This system worked well until we discovered an unexpected Earth Sun
relationship. The first cracks appeared in 1977 when by the end of the
year AO7 was running 3 or 4 minutes ahead of the calendar. By mid 1978
the situation was getting rapidly worse with an 8 minute lead on the
calendar. Fortunately relief was at hand . As first the Ohio
Scientific, then by 1981 the famous ZX81, TRS80, and Commodore Pet
computers began to appear on some ham shack benches.

For a while these "small RAM" computers used software targeted at EQX
times . You entered time and longitude, and got an in range read out.
But the approach of P3A heralded a new era as W3IWI introduced the
first useful tracking program based on Keplerian orbital elements, and
most folks soon forgot all about Oscar Locators and EQX times.

We are just begining to see a rerun of the 1978 events. Upper
Atmosphere drag which caused the 1978 problem is back, and the lower
altitude satellites like Mir are begining to show the first effects.
The lower the satellite Perigee height , the greater the effect. The
culprit is the Sun. The upper atmosphere is getting denser as Solar
activity increases, as we start to see Solar Flux rising heralding the
next Solar maximum in perhaps 2 perhaps 3 years time.

Much of the upper atmosphere of the Earth is sun stuff. Regularly
refreshed by the Sun but drifting off into space almost as fast as it
arrives. However as the Sun gets more active input begins to exceed
output and the atmospheric density at the top of the atmosphere
increases markedly. So as we spend the next few years advancing
towards Solar cycle maximum with 10m already begining to open, 6m
probably getting into the act next year along, with DX Low VHF TV
around 48 MHz, and Paging Signals and US Road Crews and Linemen in the
35 to 46 Mhz region. We can expect the atmospheric drag on our
satellites with perigees below 2000 kms height to increase markedly.

The effect will be at its worst for the very low space station orbits
, it will effect Pacsats and Uosats by next year and by 1999 , even
the relatively high RS15, and Fuji orbiters will be feeling the
effects. So be prepared to start having to update the Keplerian
elements in your satellite software much more regularly than you have
been doing up until now . Do not rely on the so called Drag
corrections, they are a guess at best. Noting that, as the upper
atmosphere gets really thick, it does so spasmodically. Your satellite
can cruise uneventfully for a week or more then suddenly appear to hit
a brick wall which causes it to lose orbital velocity, drop to a lower
height and therefore appear earlier than expected.