IRIDIUM Satellite Telephony
The initial plans for IRIDIUM have been proposed by Motorola.
This cellular satellite system is called Iridium,
after element 77 on the periodic table, because
its original design called for 77 satellites.
Now the design assumes 66 satellites plus 6 spare ones.
Iridium's 66 satellites will be located in eleven nearly polar orbits,
which are tilted 86 degrees, some 670 km above the earth. Cells
will have a radius of about 300 km. Orbit period is about 100 minutes.
Exercise
assuming circular cells of 300 km radius, and an orbit time of 100 minutes,
compute the average time one satellite can maintain a link.
- Assume that the stationary subscriber initially is in the cell center
- Assume that the call starts when the subscriber enters the cell. Average of all possible
linear trajectories through the cell.
The Iridium satellites will not only set up radio
links to handheld terminals and ground
stations, but also to each other. Calls can be handed-over to other satellites.
Because of these satellite-to-satellite crosslinks, the Iridium system
will be able to handle calls to between subscribers
without connecting to any ground stations at all,
except for signalling messages, such as
call set-up, user authentication, billing etc.
Iridium Facts
- Global Wireless, handheld system
- based on 66 low earth orbit satellites
- Telephone, G3 fax and data up to 2400 bit/s
- complementary to terrestrial cellular telephony
- Dual mode operation cellular, satellites reach remote rural areas, while terrestrial
cellular reaches densely populated areas.
- voice services similar to GSM
- satellite power 1.2 kW
- 48 beams per satellite
