54 STAT.] INTER-AMERICAN-RADIO COMMUNICATIONS-DEC. 13 , 1937 (b) Full use shall be made of the list of abbreviations appearing in Appendix 9 to the General Radio Regulations annexed to the Inter- national Telecommunications Convention, Madrid, 1932. Plain language shall not be used if an abbreviation will suffice. Service indications are as follows: P-Priority, for messages which are to be sent immediately regardless of the number of other messages on file. No service indication, messages that are to be transmitted in the order of receipt. (c) The message shall contain the preamble, text, and signature, as follows: (1) Preamble: The preamble of the message shall consist of the following: The serial number preceded by the letters NR; service indications as appropriate; check (this is the group count accord- ing to standard cable count system, the letters "CK" followed by numerals indicating the number of words contained in the text of the message); office and country of origin (not abbreviated); day of month and month; hour of filing; address. (2) Text: The text may be in either plain language or code. (3) Signature: The signature shall include the name and title of the person originating the message. SECTION 11. RADIO AIDS TO Am NAVIGATION STANDARDS OF FIELD STRENGTH AND INTERFERENCE RATIOS 2531 49 Stat. 2619. 1. WHEREAS the Inter-American Radio Conference has carefully na^ioad^is. considered the various resolutions of the September 1937 Inter- American Technical Aviation Conference of Lima, and in particular resolutions XIV, XVII and XVIII which were referred to this Con- ference and 2. Considering a) the great importance of radio aids to air navigation, the phe- nomenal growth of air traffic and the further expansion which will certainly take place in the future; b) the exacting requirements of such radio aids to air navigation with respect to stability of emissions as affected by multiple path transmission which can be minimized to the greatest extent by selection of frequencies least subject to sky wave effects; c) the extremely limited range of frequencies which possesses the necessary propagation characteristics as mentioned in (b) above; d) the absolute dependence of aircraft in flight upon radio for navigational guidance and communication; e) the vast number of aircraft throughout the world that now use, and must continue to share the limited number of frequencies suitable for aids to air navigation, therefore making the strictest economy of use necessary in order that all may be accommodated with a minimum of interference;
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