Radar System Engineering

SEC. 17.3] SCANNER-DA TA TRANSMISSION 683
variety. Complications introduced by multiple sets will be described
later.
17.3. General Methods of Scanner-data Transmission.-Simultaneous
transmission of video signals and range and angle markers is relatively
simple. Marker signals need only be mixed with the video
signals; no separation is performed at the receiver. Trigger pulses can
also be mixed with the video since there is no conflict in time, but some
method of separating them at the receiving station must be provided.
If the pulses are transmitted at a higher power level than the video
signals, the difference in voltage amplitude can be used as a criterion.
This high-level transmission of pulses is usually done with amplitudemodulated
r-f equipment, since the brevity of the pulses allows high pulse
powers, with attendant signal-to-interference gain, to be obtained cheaply
(Sees. 17”11, 17.12), but it is not feasible with frequency-modulated
equipment. The alternative is to separate the signals in time, the video
signals being excluded from the transmitter during an interval prior to
and including the pulse. The trigger channel at the receiving station is
blocked by an electronic switch except when the pulse is expected.
.Means for accomplishing both of these separation methods will be
described later.
Relaying the scanner data is much more complex. It is not feasible
to transmit the numerical value of an angle by a proportional amplitude or
frequency modulation of an r-f carrier. It is possible to devise methods
whereby certain functions (e.g., the sine and cosine) of the scan angle can
be used to modulate two or more subcarriers’ directly by means of slowly
varying voltages; this has been done in the laboratory. It has been
found far more effective to convey the information through the medium
of periodic signals whose frequencies or repetition rates are several times
those involved in the scanning and whose characteristics are in some way
descriptive of it. Since scanning frequencies lie in the interval from
zero to a few cycles per second, the scanner data signals usually have
periodicities in the audio range.
The signals mentioned above can take either of two forms.
1. Continuous a-f signals can be used, the information being carried
in terms of the amplitude, the frequency, or the phase with respect
to a fixed reference. Up to the present time this technique has
been very little used.
2. Pulse-timing techniques can be applied. Data can be transferred
in terms of the frequency of a single train of pulses, the degree of
I A subcarrier is a sine wave, usually in the audio- or video-frequency range, which,
after it has hem modulsted by the information-bearing signals, is used to modulate an
r-f, carrier,