Last year when GPS manufacturers were opposing Lightsquared’s deployment of a satellite / terrestrial hybrid data network, the GPS receiver interest group told me their concern was the terrestrial component. The proposed system had land-based transmitters just a few MHz away from the GPS band. GPS receivers need to receive signals in the -130dBm range. It was unclear how much signal in the adjacent band typical GPS receivers could tolerate and how much the dynamic range of future GPS receivers could be improved.
An article in the latest edition of IEEE Wireless Communications explains the reason for putting terrestrial transmitters in a band formerly dedicated to satellites. Mobile satellite services (MSS) are low orbiting satellites that have enough link margin (i.e extra transmit power and receive sensitivity to cover fading as the user moves) to allow them to communicate to handheld devices. In urban areas terrestrial systems, such as the mobile phone networks, have huge advantages.
- Buildings block line-of-sight path to satellites.
- Antennas can be placed at low elevations allowing very small cells and frequency reuse only a few kilometers away.
- Deploying and maintaining terrestrial sites is much less expensive than satellites.
Satellites are better for rural areas because they can cover large areas. They also can be used in disasters that take out the terrestrial infrastructure.
There are high-end transceivers on the market that can use terrestrial and satellite networks, and there are combined service agreements for them. These user to work out which system is better for their location and to manually select between terrestrial or satellite.
There are also satellite radio networks like XM and Sirius, that use terrestrial repeaters to help out in urban areas where line-of-sight is an issue. These one-way repeaters are simple because communication is one-way.
Because of the advantages of having true two-way hybrid systems using satellites and terrestrial sites, in 2004 the FCC began approving ancillary terrestrial components (ATCs) to existing satellite services. The networks will seamlessly hand off between the ATC and MSS. Transceivers that use ATC/MSS networks have not been developed yet. They are not immediately on the horizon because they will face all the difficulties of devices designed for terrestrial sites plus the complexity of satellite transceivers plus the difficulties of integrating the two.
The systems will face the well-known problem of interference between one cell and a nearby cell that reuses the same frequencies and all a new problem of interference between the MSS and ATC components. The satellites cells could be focused down to a minimum of 100 km in diameter. The signal strenght does not drop off that rapidly as you leave the satellite cell, so there needs to be an “exclusion region” buffer zone around the the cell. Complicating cell structure further, the cells boundaries move with the orbit of the satellites. To overcome interference from satellites, terrestrial sites may use beamforming to make their antenna pattern directional toward each user it communicates with. Beamforming is a technique that uses several antennas act as a directional antenna focused on any desired direction. It sounds like something that exists only in MATLAB simulations, but it has been used commercially for years to control the directionality of an array of microphones. It is also option in the 802.11(n) standard. Beamforming technology is mature enough. The devil will be in the details of keeping the system robust against a real-world RF environment with moving users, satellite cells, and interference sources. 
Despite the complexity, hybrid systems will become a fact of life. The MSS components will be slower and used rarely because terrestrial cell sites will be available in all but the most rural areas. Urban customers may be asked to subsidize the system, similarly to how uran customers subsidized rural electricity and telephone service. Like MIMO ten years ago, hybrid MSS/ATC systems sounds too complicated to work. Like MIMO today in a multi-stream 802.11(n) card, in ten years MSS/ATC systems will be a common technology that is mostly used in special cases.
Images are from the IEEE Wireless Communications article: Personal Satellite Communication: Technologies and Challenges.
