Iridium Satellites
Other Iridium Phones:
Derivation of geostationary altitude
In any circular orbit, the centripetal acceleration required to maintain the orbit is provided by the gravitational force on the satellite. To calculate the geostationary orbit altitude, one begins with this equivalence, and uses the fact that the orbital period is one sidereal day.
This is one of 66 low-earth orbiting (LEO) satellites comprising the Iridium satellite system. The original company went bankrupt so the US Department of Defense assisted in making sure the system stayed in operation.
We note that the mass of the satellite m appears on both sides — geostationary orbit is independent of the mass of the satellite. So calculating the altitude simplifies into calculating the point where the magnitudes of the centripetal acceleration required for orbital motion and the gravitational acceleration provided by Earth's gravity are equal.
The centripetal acceleration's magnitude is:
where ? is the angular speed, and r is the orbital radius as measured from the Earth's center of mass.
The magnitude of the gravitational acceleration is:
where M is the mass of Earth, 5.9736 × 1024 kg, and G is the gravitational constant, 6.67428 ± 0.00067 × 10-11 m3 kg-1 s-2.
Equating the two accelerations gives:
The product GM is known with much greater accuracy than either factor; it is known as the geocentric gravitational constant µ = 398,600.4418 ± 0.0008 km3 s-2:
The angular speed ? is found by dividing the angle travelled in one revolution (360° = 2p rad) by the orbital period (the time it takes to make one full revolution: one sidereal day, or 86,164.09054 seconds).[3] This gives:
The resulting orbital radius is 42,164 kilometres (26,199 mi). Subtracting the Earth's equatorial radius, 6,378 kilometres (3,963 mi), gives the altitude of 35,786 kilometres (22,236 mi). Orbital speed (how fast the satellite is moving through space) is calculated by multiplying the angular speed by the orbital radius:
Orbite alocation
Satellites in geostationary orbit must all occupy a single ring above the equator. The requirement to space these satellites apart means that there are a limited number of orbital "slots" available, thus only a limited number of satellites can be placed in geostationary orbit. This has led to conflict between different countries wishing access to the same orbital slots (countries at the same longitude but differing latitudes). These disputes are addressed through the International Telecommunication Union's allocation mechanism.[5] Countries located at the Earth's equator have also asserted their legal claim to control the use of space above their territory.[6] Since the Clarke Orbit is about 265,000 km (165,000 mi) long, countries and territories in less-populated parts of the world have been allocated slots already, even though they aren't used, yet. The problem presently lies over densely-populated areas such as the Americas and Europe/Africa, and above the middles of the three equatorial oceans.
Impact on signals
Satellites in geostationary orbits are far enough away from Earth that communication latency becomes very high — about a quarter of a second for a one-way trip from a ground based transmitter to a geostationary satellite and back, and close to half a second for round-trip end-to-end communication.
For example, for ground stations at latitudes of f=±45° on the same meridian as the satellite, the one-way delay can be computed by using the cosine rule, given the above derived geostationary orbital radius r, the Earth's radius R and the speed of light c, as
This presents problems for latency-sensitive applications such as voice communication or online gaming.
Iridium - Motorola 9500 satellite mobile phone is the original Iridium
phone and the first wireless phone in history to provide total global coverage.
Satellite Transmission Signal latency White Papers
showing details of an exact replica of an Iridium satellite entirely built from spare parts. Most of the structure seems to be made of honeycomb material covered with a layer of reflecting metal. So almost all flat surfaces of the satellite can produce flares. But, for now, only operational satellites can have flares predicted that come from the Main Mission Antennas (MMA) or the solar panels. Triangular body of the satellite. We can easily notice the three MMAs with their white "beta cloth" covered upper surface. The battery compartment is in the upper section of the body but the cover is not visible since it is located on the surface facing the ceiling. This cover is highly reflective and tilted 5 to 10 degrees toward the ground. It's not a flat plate all the way, it has three angled surfaces unlike the other two sides. The white truss at the bottom is not part of the satellite. One of two solar panels. Because of their size, 8 meters total length, both could not be seen well on the same picture. See close-up in picture below. Bottom view of the satellite showing well the MMA arrangement placed at 120 degrees apart. The four bronze dishes are the Gateway Antennas used to send the signals to the ground processing stations. Nice view of the satellite base showing all the antenna types including the three Crosslink Antennas, the flat plates about 30 cm in size, used to relay the signals between satellites of the constellation. Notice the white-sided square blocks on the MMAs. These are the receiving modules that collect signals from Iridium portable phones. Simulated flare on a MMA using the camera flash Close-up of the orientation mechanism for the solar arrays, again with a direct reflection of the camera flash. Notice the different cell colors. Details of the Gateway Antennas. Notice the surface texture on the MMA. One can imagine what a flare would be if it was a perfect mirror