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     1. Direct vision (contact conditions)
     2. Electronic instruments

Electronic transmission of intelligence in turn may be classified into two categories:
   1. Through direct appeal to the senses, as by voice communication
   2. Through instrumental indications requiring interpretation
   [[and here is a handwritten note]] 3. For use of automatic devices. [[/and here is a handwritten note]]

The second category depends in general upon some characteristic of the signal
   1. Of a single [[strikethrough]]singla[[/strikethrough]] signal
   II a. Amplitude
      b. Frequency
      c. Phase
      d. Polarization
      e. Direction
   2. Of two or more signals
      a. Relative amplitude
      b. Relative frequency
      c. Relative phase
      d. Relative polarization
      e. Relative direction
   3. Form of modulation, including relative time of arrival and spacing of pulses.

Whatever channel is employed, the information must be presented to the pilot upon reaching the aircraft. A possible classification of presentation devices is the following.
   1. Warning devices - signal lamp, flag indicator, horn etc.
   2. Deviation devices - left-right pointers, A-N coding, etc.
   3. Quantitative indicators