Satgen224 Where you are determines What you get by GM4IHJ 10 July 93 BID of this msg is SGEN224 Please use this BID if you retransmit the msg Recent packet exchanges with PS7KM at 5N 35W, have revealed a gap in IHJ's knowledge , in respect of our very different radio propagation , and the way our very different latitudes (56N versus 5S) affect what orbits we see each day. The propagation differences will be discussed in later satgens. Here is a starting discussion about the different orbit patterns we get using as a first example - Space Station Mir 51 deg orbit inclination and, 400 km altitude = 20 degree footprint radius. This list reports aquisition times on the same day at 6 different latitudes , all on the same meridian of longitude. Station Latitude Timing of Mir Acquisitions UTC 80 N no orbits seen 70 N 1635 1808 1944 56 N 1456 1631 1806 1941 2118 40 N 1317 1452 1629 1807 1944 2120 2256 20 N 1140 1312 1450 2125 2259 0 N 0001 0138 1132 1309 2305 Why are there such big differences in who sees what ? The 80N station sees nothing. MIR's inclination of 51 degrees brings it no further north at any time than latitude 51 N. Mir sees a footprint of 20 degrees from its 400 km altitude so it sees stations only to 51 + 20 = 71N The 70N station can just see two or three orbits in succession passing along his southern horizon. The 56N station sees a succession of 5 orbits all passing to his south . The 40N station has the best view of all. 7 orbits in succession , some passing north of his station some passing south . The 20N station cannot see orbits which pass his station at latitudes above 40 degrees ( 20N + 20 degree footprint = 40 N horizon ). So he misses some of the central succession of orbits which pass his longitude at latitudes between 40N and 51N. The 0N station down on the Equator cannot see any Mir orbit passing his longitude at latitudes above 20N , or conversely below 20S. So the station located near the Equator sees two separate groups of orbits spaced roughly 12 hours apart. Please note that the above pattern only applies to MIR, or, the rare Space Shuttle flights which have orbit inclinations of about 57 degrees. Most Space Shuttle flights launch to 28 degree inclination orbits so they never go above 28 N or 28S , and hence they see or are seen only by stations between 28 + 18 (usual shuttle footprint)= 46 north or south. Future satgen bulletins will discuss these Equatorial region only Space Shuttle orbits and, the quite different, Radio Sport orbits which see all the world and are seen from some near polar stations , on every orbit. Do not Forget the Perseids shower meteors due late pm 11 Aug to early am 12 August utc. Some Meteor specialists are expecting a SUPER SHOWER ( more details in satgen due 24 July). 73 de John GM4IHJ @ GB7SAN