Line A
From The RadioReference Wiki
A line spanning from Washington State to Maine which approximates the border between the United States and Canada. The exact position of the line with respect to the border varies, however in most places it is about 75 miles/ 120 km from the border. The line was established in the Terrestrial Radiocommunication Agreements and Arrangements treaty of 1965, titled "Telecommunication: Coordination and Use of Radio Frequencies Above 30 Megacycles per Second."
Applications from stations which operate in Part 90 (business and public safety) are sent by the FCC to Industry Canada for review against their Technical and Administrative Frequency List (TAFL) database records. If the proposed application's -148 dBW signal signal (for VHF, -146 dBW for UHF) touches a Canadian licensee, a "NOTICE OF RETURN FOR CANADIAN OBJECTIONS" is generated. Although the treaty does not specify any particular vetting methodology, the algorithm used to determine this return is roughly based on Longley-Rice F(50,10) at -118 dBm for VHF and -116 dBm for UHF (which is equal to -148 dBW/-146 dBW) at the height of the listed receiver antenna. If the objection references an area of operation, the area usually refers to signal incursion into a 50 km radius from the point coordinate referenced. The typical model which approximates this incursion uses a 3m antenna elevation for the propagation model. Any incursion at the receiver antenna or within the area of operation specified, even if it is compliant to what would be accepted U.S. frequency coordination practices, is grounds for return.
The Canadian objection letter usually states that harmful interference is anticipated ("HIA") and provides specific information against which the proposed system may be disputed. The dispute might be settled by providing a detailed engineering study indicating the signal does not impinge on Canadian operations. The engineering study may, however, indicate that the proposed system will impinge on Canadian operations, thus requiring the selection of another frequency, reduction in power, selection of antenna patterns which "protect" (or harshly notch radiation in the direction of) the Canadian station noted, or reduction in height above average terrain. On-air testing may be arranged if both parties remain at an impasse, but only after other methods have been exhausted.
Line A is complemented by Line B, which is a similar line within Canada. Stations south of Line B must coordinate with the United States, as well.
A Line C exists in Alaska and a complementary Line D exists in Canada bordering Alaska. Due to sparse population, Line C issues present less frequently.
The Industry Canada TAFL database contains information which has been protected for Canadian national security reasons. This information is unavailable through Industry Canada's search features. It is, therefore, possible to receive a Canadian objection with no specific information provided by the FCC, or on a frequency that otherwise appears to be "clear" between Lines A and B. These objections are based on harmful interference anticipated to the protected operations. Depending on province, the amount of redacted information may be between 20 - 35% of the database content, making system implementation north of Line A extremely difficult due to the element of chance and uncertainty of available resources. If a frequency north of Line A seems clear, it's probably clear for a reason.
If all attempts to justify implementation fail Industry Canada review and appeal, the potential licensee may accept secondary status and a power level not greater than 5 Watts output/5 Watts effective radiated power. Often, this power level is unsatisfactory for the proposed use. It is wise to not procure VHF or UHF systems, particularly those of significant expense, without first insuring that uncontested resources are available within the required area. Given the element of uncertainty from redacted data, a salesperson typically cannot make those assurances without going through the application process and receiving official responses first. Caveat emptor.