5.111 The carrier frequencies 2 182 kHz, 3 023 kHz, 5 680 kHz, 8 364 kHz and the frequencies 121.5 MHz, 156.525 MHz, 156.8 MHz and 243 MHz may also be used, in accordance with the procedures in force for terrestrial radiocommunication services, for search and rescue operations concerning manned space vehicles. The conditions for the use of the frequencies are prescribed in Article 31.
The same applies to the frequencies 10 003 kHz, 14 993 kHz and 19 993 kHz, but in each of these cases emissions must be confined in a band of ± 3 kHz about the frequency. (WRC 07)
US1 The bands 2501-2502 kHz, 5003-5005 kHz, 10 003-10 005 kHz, 15 005-15 010 kHz, 19 990-19 995 kHz, 20 005-20 010 kHz, and 25 005-25 010 kHz are also allocated to the space research service on a secondary basis for Federal use. In the event of interference to the reception of the standard frequency and time broadcasts, these space research transmissions are subject to immediate temporary or permanent shutdown.
US340 The band 2-30 MHz is available on a non-interference basis to Federal and non-Federal maritime and aeronautical stations for the purposes of measuring the quality of reception on radio channels. See 47 CFR 87.149 for the list of protected frequencies and bands within this frequency range. Actual communications shall be limited to those frequencies specifically allocated to the maritime mobile and aeronautical mobile services.

Wiki entries

Access Broadband over Power Line (Access BPL)

Current Use 17.97–21.924 MHz Access BPL

According to the FCC's definition, Access BPL is "a carrier current system installed and operated on an electric utility service as an unintentional radiator that sends radio frequency energy on frequencies between 1.705 MHz and 80 MHz over medium voltage lines or over low voltage lines to provide broadband communications and is located on the supply side of the utility service’s points of interconnection with customer premises. Access BPL does not include power line carrier systems as defined in § 15.3(t) or In-House BPL as defined in § 15.3(gg)."

Low voltage lines are defined as lines carrying, for example, 240/120 volts from a distribution transformer to a customer's premises. Medium voltage lines carry between 1,000 and 40,000 volts from a power substation to neighborhoods, and may be overhead or underground.

Access BPL is an unlicensed service operated under Subpart G of Part 15 of the FCC's rules. Radiated emission limits from medium voltage lines in the 1.705-30 MHz range must not exceed the limits in 47 CFR 15.209, and those from 30-80 MHz must not exceed the limits in 15.109(b). Systems that operate on low voltage lines must comply with 15.109(a) limits across the entire 1.705-80 MHz range.

When Access BPL systems must use notch filters to protect licensed services, the notch must be at least 25 dB below applicable Part 15 limits in the 1.705-30 MHz band, and at least 10 dB below the limits in the 30-80 MHz band.

There are 12 sub-bands in which Access BPL systems are not allowed to operate anywhere to protect aeronautical (land) stations and aircraft receivers. Those excluded bands are listed in 15.615(f), and are excluded in the list of Access BPL bands in this entry.

Access BPL systems are not allowed to operate within the sub-band 2173.5-2190.5 kHz within 1 km of coast station facilities listed in 47 CFR 15.615(f)(2)(i), to protect the internationally-recognized standard maritime calling frequency of 2182 kHz.

No Access BPL emissions are allowed in the 73.0-74.6 MHz radio astronomy band on overhead medium voltage lines within 65 km of the Very Large Array radio telescope in New Mexico (34 04 43.5N, 107 37 03.82W), or within 47 km of the VLA on underground power lines or overhead low voltage lines.

Access BPL operators must consult at least 30 days in advance when deploying systems near various FCC field offices, aeronautical and maritime stations, radar systems, radio astronomy stations, and research areas. Details are in 47 CFR 15.615(f)(3).