Some time after February 17 2009 -- see the coda at bottom regarding the newly implemented "analog searchlight" program -- all full-power TV broadcasting in the U.S. will be digital (DTV). Full-power analog TV broadcasting will cease although some low-power analog broadcasting will persist. This change and the associated reallocation of the spectrum below 806 MHz will affect astronomical observing, not all for the worse, at least initially.
The upper limit of full-power DTV broadcasting will be 698 MHz, channel 51. The spectrum between 698 and 806 MHz presently occupied by TV channels 52-69 will be reallocated to applications such as wireless broadband internet services, cellular telephone services, and a nationwide public safety broadband network. In the very near future, the spectrum band between 698 and 806 MHz will be largely empty as the winners of recent spectrum auctions begin to roll out their service. In the more distant future, the 698-806 MHz band will be used at a level and in ways comparable to the present cell phone bands, i.e., much more heavily and more ubiquitously than was previously the case for analog TV broadcasting.
The temporary absence of signals in the 698-806 MHz band presents a gradually-closing window of opportunity for radio observing. Also, the FCC has not yet designed a working model for the bands at 758-763 and 788-793 MHz which will eventually be dedicated to the national broadband network for emergency workers (the so-called Block D). This may be one of the last bands to fill in but it will probably impinge on radio astronomy use everywhere, eventually, including within the Quiet Zone.
Channels 2-51 will continue to be occupied by TV broadcasting, in the same 6 MHz wide channels as before, and these 50 stations will have to accommodate the industry which previously occupied channels 2-69. A DTV signal fills its allocated band uniformly, as opposed to analog signals that are borne by narrower carriers within their band. The mean level of a DTV signal is generally somewhat below the peak level of an analog TV carrier. A convincing comparison of spurious emission levels beyond the allocated band edges remains to be demonstrated.
For a given market and channel, DTV signals generally use slightly lower power to achieve the same service level but the actual situation will be determined by which channels are occupied. A station whose signal moves from a lower-numbered analog channel to a higher-numbered digital channel may use higher power, roughly corresponding to the lower effective collecting area of isotropic antennas at higher frequencies.
Overall occupation of the spectrum below 698 MHz should decline on 17 February 2009 because stations are already broadcasting their DTV signal and many are broadcasting their old analog signal as well, which will go dark. This represents a reversal of the gradual infill of the TV bands which has occurred as DTV began to proliferate and stations replicated their analog broadcasts. Did radio astronomy notice as the TV bands filled up? Well, the situation is reversing itself somewhat.
The FCC has also decided to allow local unlicensed non-broadcast-TV uses of DTV bands whenever a DTV channel is locally unavailable. Unused spectrum is generally known as "white space" and apparatus for utilizing the white spaces is known as white space devices or (at the FCC in the present context) TV Band devices. The FCC intent is to recreate the 2.4 GHz ISM bands used for WiFi etc., but throughout the lower-frequency DTV bands where signals propagate more easily. There are some additional details concerning which devices may use which channels, depending on whether the devices are fixed or mobile.
The FCC has issued rules which forbid the operation of TV Band devices within 2.4 km of existing radio telescopes, or in the protected band at 608-614 MHz (TV channel 37 shared with wireless medical telemetry) but without clarifying what it means to be within 2.4 km of, say, the Very Large Array, which is characterized in FCC documents by a point. The origin of the 2.4 km separation distance is obscure.
TV band devices will initially determine local spectrum availability using GPS and internet access to a database of DTV channel service contours; both capabilities will be needed before a TV Band device may broadcast. However, the FCC also intends eventually to license devices that merely sample the spectrum to determine availability. How such devices would know they are in the vicinity of a radio telescope is undefined.
The expected result here is that the spectrum below 698 MHz will be somewhat emptier of broadcast TV stations for quite some time, but will gradually fill in as use by unlicensed TV Band devices proliferates.
At the behest of Congress, the FCC will allow full-power analog TV stations to remain on the air for at least 30 days after the scheduled 17 February 2009 DTV transition. Stations whose analog signals will not interfere with DTV broadcasts are being encouraged to remain on the air broadcasting public service information to assist whatever remnant of the TV-viewing public is surprised by the DTV transition and would lose access to the public safety information provided by watching TV. This program is known as the "analog searchlight" and it implements the recent "Short-term Analog Flash and Emergency Readiness Act."