Brett wrote:Basically satellite number and position is one of the key aspect of communication reliability and coverage. Tassie being low on the horizon means communication shadows happen with geostationary satellites which means north/south valleys not so bad as east/west valleys. The further south the longer the communication shadow caste by mountains and valleys. Also north/south shadows depend on where the satellite is (extreme east or west) so Tassie could be at the extreme edge thus getting dramatically reduce coverage compared to someone park immediately under the satellite.
OK! Now I know you're confused. Very quickly, there are three main types of satellites. Low Earth Orbit (LEO), Medium Earth Orbit (MEO) and Geosynchronous Earth Orbit (GEO). GEO sats must be equatorial and hence the problems you've mentioned. {There is also and elliptical (high) orbit called HEO that some tv sats use but that's another story.}
Of the three technologies (SPOT, GPS & PLB) only the PLB network (COSPAS-SARSAT) uses GEO satellites (4 GEOSARs & 5 LEOSARs). GPS are in MEO, with a mesh network that allows multiple satellites to be visible at most times from any spot on earth. See here -
http://en.wikipedia.org/wiki/File:ConstellationGPS.gif. SPOT uses Globalstar in LEO - meaning the satellites pass close by Tasmania. Because of the way that LEO orbits work, sometimes a satellite will pass directly over Tasmania, and sometimes they will be varying degrees to the east or west with only partial footprint coverage. Travelling at 26,858 km/h with a footprint of 4500 km they don't take long to pass. As one goes the next one comes.
Brett wrote:Ok the best is the network with more satellites overhead for communications thus we have to surrender the geostationary positioning ones and that is where number of satellites and how they orbit becomes critical. ...However, the signal going up works better the more satellites...
Just because a network has more satellites does not mean they are all above Tasmania or all visible to Tasmania, and they are not all used simultaneously in one conversation. And more satellites in a constellation does not equate to a better cover over Tasmania. LEO satellites are spread all over the earth surface, and only one or two are visible at any given point in time - there needs to be some overlap. (e.g
Globalstar and
Iridium). You can see
live tracking of the Iridium birds. Open all the links in separate tabs and you will see how many are close to Tasmania right now.
LEO sats are best for voice communication because they are closer to the earth and the communication is quicker. GEO sats introduce a minimum of 1/2 second latency into two-way communications (which is undesirable). If I extend the conversation to Sat phones, then the Thuraya constellaton is three GEO sats (only one visible to Tasmania) and as mentioned earlier the Iridium constellation is 66 sats in LEO.
Things get better in the future. Both
Globalstar and
Iridium have announced constellation upgrades. Globalstar recently secured $738 million to complete its upgrade by 2010 and SPOT is a big part of their future plans. Iridium plans to upgrade between 2014 and 2020 and this upgrade is said to include the worlds first ubiquitous 1 Mbps wireless network. A project called Distress Alerting Satellite System (DASS) by NASA aims to put search and rescue (SAR) processors aboard the GPS satellite constellation and the Galileo positioning system (GNSS) constellation used in Europe, so 406 MHz PLBs will eventually be in MEO too.