By Jeffrey Krauss, satellite skywatcher and President of Telecommunications and Technology Policy

First, a review. Geostationary satellites ("GEO") orbit the Earth in a circular orbit at an altitude of 36,000 km, with an orbital period exactly equal to the 24-hour rotational period of the Earth, so they appear to be stationary as the Earth rotates. (What I said in my July column about balancing of gravitational forces was wrong; the satellite's centrifugal force balances the Earth's gravity.) On the other hand, LEO satellites can have stable orbits that are elliptical rather than circular, or circular orbits at distances less than 36,000 km.

C-band satellites operate at 6 GHz (uplink) and 4 GHz (downlink). Ku-band satellites operate at 14 GHz (uplink) and 12 GHz (downlink). Ka-band, the newest, will operate at 28 GHz (uplink) and 18 GHz (downlink).

Motorola's Ka-band LEO proposal

I reported on Teledesic 's LEO fixed satellite system, with hundreds of orbiting satellites communicating with fixed earth stations on the ground. (Satellite systems like Teledesic and Hughes Galaxy that work with fixed earth stations are called fixed satellite systems—"FSS"—while those that work with mobile earth stations, like Motorola Iridium, are called mobile satellite systems—"MSS." Both FSS and MSS systems can use either LEO or GEO satellites.) The specific frequencies within the Ka-band for this LEO network are slightly different than the frequencies for the Ka-band GEO satellites, because it is difficult for LEO and GEO satellites to share the same frequencies without interference. (It's not impossible, but the proposals for avoiding interference are too complicated to describe in this column.)

Until recently, the Teledesic system was the only satellite system slated to use the portion of Ka-band set aside throughout the world for LEO fixed satellite systems. But then Motorola rained on Teledesic's parade by filing an application for a LEO FSS network called Celestri. Celestri consists of 63 satellites that circle the Earth at an altitude of 1,400 km, compared with Teledesic's network of 840 satellites that orbit at an altitude of 700 km. Meanwhile, Teledesic has made a deal with Boeing for launch services and will redesign its network, probably by cutting the number of satellites to less than 300.

The challenge here will be for both of these networks to operate in the same frequencies without interference. Satellites and earth stations may need to be shut down momentarily in order to avoid interference to the other network. Each network control center will need precise knowledge of the locations and movements of all the satellites in the band.

Video at Ka-band

Meanwhile, Microspace (a subsidiary of Capitol Broadcasting, owner of KRAL-TV Raleigh and the Durham Bulls baseball team), has revealed plans for using a Ka-band FSS satellite to distribute TV stations as well as cable programming. Recall that earlier this year, Rupert Murdoch and Charlie Ergen announced a similar plan using the Echostar broadcast satellite system, but that evaporated when the Murdoch-Ergen deal collapsed.

At that time, there were not only technology questions but also legal questions, because of the copyright laws. Cable TV systems have the blanket right to retransmit the copyrighted programming carried on TV stations because of a "compulsory copyright license." (This goes beyond "must-carry" and "retransmission consent" regulations.) But satellite systems do not have the automatic right to retransmit copyrighted TV programming.

Here are some technical details about the Microspace plan. There are 211 designated market areas ("DMAs") for TV stations in the country, and they plan to use 61 spot beams on the satellite to cover them. There will be three beam sizes. The receive dishes will be 24 inches in diameter. For those who worry about rain attenuation on the 18 GHz Ka-band downlink, the system will be designed for 99.7 percent availability (average of 4.5 minutes of rain outage per day).

The receive dish size is controlled by interference considerations. Antenna beamwidth is related to antenna size; the smaller the antenna, the wider its beamwidth, and the more likely it will receive interference from a satellite in an adjacent orbital slot. These Ka-band FSS satellites will be spaced 2 degrees apart, just like the Ku-band FSS satellite used by Primestar. But antenna gain and directivity increase as the frequency increases, so a 24-inch dish at 18 GHz performs about the same as a 36-inch dish at 12 GHz. In contrast, the DirecTV and Echostar broadcast satellites are spaced at 9 degrees, so they can use smaller antennas.

I was told that Microspace is negotiating with Hughes, GE Americom and Loral, each of which has received Ka-band orbital slot assignments. I took a look at the GE satellite application, filed in 1995, to see whether 61 spot beams and 24-inch dishes are plausible. It calls for 44 spot beams over the country, each covering an area 300 miles in diameter. So 61 spot beams would not be unreasonable. Earth station antennas would be as small as 30 inches, but that was for transmit-receive earth stations. Receive-only stations could be smaller. Yes, a 24-inch dish seems plausible.

So it appears that the big challenge to Microspace would be legal, not technical, changing the copyright laws and perhaps other laws as well. I would expect the cable industry to fight these changes, because a satellite system that carries broadcast stations as well as cable programming would be a serious competitive threat.