The Ka-band (18.8-19.3 GHz downstream/28.6-29.1 GHz upstream): Without a doubt, it's the next great frontier for VSATs (very small aperture terminals). Offering substantially more bandwidth than either C- or Ku-band, Ka-band seems a natural for large Internet-style data transmissions: the kind of communications that are being increasingly demanded by businesses and consumers alike.
However, this new spectrum comes with a price tag attached, and that's rain fade. Because it operates at higher frequencies than C- or Ku-band, Ka-band broadcasts are more vulnerable to being washed out by heavy storms in their path. That's why vendors are only talking about 99.5 percent guaranteed up-time for the Ka-band, as opposed to the more than 99.9 percent availability associated with the C- and Ku-bands.
Is this a problem? Well, that's a hot topic of debate in the satellite industry. But in order to understand both sides of the issue, we first need to know more about Ka-band as a satellite transmission medium--and the plans vendors have for it--before asking if Ka-band VSATs are really ready for prime time.
In The Beginning
Before 1993, most people didn't think the Ka-band could be used for satellite transmissions. However, NASA's Advanced Communications Technology Satellite (ACTS), which was launched in September that year, changed their minds. Parked in a geostationary orbit at 100 degrees W, ACTS was the focus of many successful Ka-band transmission experiments. NASA used it to test all kinds of platforms for Ka-band transmissions: not just earth stations, but also aircraft, land vehicles and ocean-faring ships. In addition, the variety of applications tested was stunning. Beyond basic communications, uses such as telemedicine, distance learning, and remote monitoring of electrical utility networks were also put through the wringer on ACTS.
To say the least, ACTS was a huge success. During its 81-month run--more than three times longer than planned--more than 100 experiments were conducted with this satellite. The results were undeniable. NASA not only proved that Ka-band could be a satellite transmission medium, but that satellites could send and receive data at 45 Mbps downstream/1.54 Mbps upstream using only 26-inch earth station antennas.
Of course, this ability came with a few strings attached. First was the rain fade problem. For Ka-band uplink earth stations to cut through heavy rain, they have to be capable of radically boosting their signal output at a moment's notice. According to John Evans, chief technology officer at Comsat Labs, this means that a VSAT earth station usually running at 0.1 watts must jump up to 1.0 watts or more to make itself heard by a Ka-band satellite.
The next consideration is coverage. Thanks to the signal losses caused by rain fade, Ka-band satellites aren't able to blanket an entire continent the way a C-band satellite can. That's why the Ka-band satellites currently under development use a number of focused spotbeams to reach their coverage areas. This allows them to cut through the rain, but only at the cost of more EIRP (effective isotropic radiated power).
The upside of this approach is the spotbeams can be aimed wherever they're needed. The downside is that uplinking information from one spotbeam and then downlinking it via another requires signal switching. This switching has to be handled within the satellite itself, which adds extra complexity to this system. That's not good news, since extra complexity typically spells lower reliability and higher costs in any engineered system. (Another solution is to forego the onboard switching, and have the satellite only speak to stations dedicated to specific beams, with no crossover between transmit channels).
Satellite equipment manufacturers and service providers, however, are not daunted by the challenges of Ka-band. Far from it. In fact, many have already announced ambitious plans for this spectrum.