isgoed said:
And then again, I presume that the data rate of phones (56k) or UMTS for that matter (380k) are way higher than GPS. Correct me if I wrong on that, because I don't know the data rate of GPS.
I don't know anything about GPS, but I presume they do operate on another frequency than telephones, so this alone would make them uncomparable. And GPS as it is has many satelites in LOW ORBIT, unlike the satelite phones which are in Geo Synchronis Orbit.
I keep my original statement that: It would NEVER be possible in this universe to have a telephone conversation over a satelite at 35800m with an antenna no bigger than 5cm.
Actually, the GPS satellites are in higher orbits (20,200km) than those used by either Iridium (780km) or Globalstar (1414km). All use "constellations" of dozens of satellites in low earth orbit in order to reduce the transmission power requirements (read, battery size and capacity for a telephone, and solar panel array size and launch costs for a satellite), and in the case of telephony, transmission lag (light speed delay) time. Calls are handed off from one satellite to another as they pass within line of site of the caller's position; you can think of it as mobile cell basestations (in orbit) travelling past a fixed user, rather than a mobile user travelling past fixed cell basestations. In fact, Iridium's name came from the system originally being designed to use 77 (the atomic number of iridium) satellites, subsequently reduced to 66 to reduce the cost.
GPS operates on the 2.4GHz band. Out-of-cal Bluetooth and 802.11 transmitters, and microwave ovens are a noise problem, as are steel-framed buildings (signal fading). This is not significantly higher than the 1.9GHz PCS band used by cell phones. Because GPS is a CDMA-based system very similar to that used by most North American and South Korean cellular carriers, CDMA phones frequently try to use a single tri-band (800MHz, 1.9GHz, 2.4GHz) antenna to pick up and partially process GPS signals for emergency location, though the compromises in antenna efficiency sometimes call for a separate antenna tuned specifically for GPS.
Power has everything to do with how far a signal will radiate before it drops below the level of noise (and thus, detection) due to the inverse square law. It has
nothing to do with how long a dipole antenna needs to be to transmit the signal efficiently. An antenna longer than it needs to be is actually less efficient than a correctly sized, half-wave antenna. A dish can increase the gain of a received signal by intercepting more of it and focusing it at the actual dipole in the receiver horn at the expense of making the antenna far more directional. Ditto for transmitting with a dish, using a phased antenna array, or a comb filter. Use a powerful enough transmitter and you won't need any of those things in either the satellite or the mobile unit. You'll waste a HUGE amount of energy radiating the signal in almost every other direction (this is where dishes and arrays come into play, to direct the signal) and heating up the antenna due to the inevitable inefficiencies in coupling -- but you still won't need a longer antenna. And it just so happens that 5cm is the antenna length for a 10cm radio wave -- which falls smack in the super high frequency band used for space communications, and only slightly higher (~3GHz) than that used for GPS (2.4GHz).
You were saying?
Please go through the lectures
here and this
note. Some of us actually do (or work alongside those who do) this stuff for a living.
