SECTION 5: DETECTING SOPHISTICATED TRANSMITTERS
REI OSC-5000E 84
The main concept is to recognize the difference in the spectrum traces and evaluate each
difference manually.
Furthermore, this particular display is showing the frequency band from 5MHz to
1505MHz, and the frequency spectrum may become much busier in a larger city. In this
case, you can simply zoom into each portion of the spectrum and analyze the areas as
needed. It should also be noted that the OSCOR Peak Display mode stores frequency
information at 50kHz resolution throughout the frequency band. Therefore, when the
display is zoomed in to view a signal, the Peak display graph may appear to have coarse
steps, but the main evidence of transmission will be maintained.
SOME SOPHISTICATED TRANSMITTER TYPES
Sophisticated signals are not always logged by the OSCOR in automatic mode due to
the nature of their transmission frequency, however evidence of these types of
sophisticated transmitters can be detected using the OSCOR’s manual sweep displays.
Spread Spectrum
Spread Spectrum technology spreads the transmitted energy over a wide frequency
spectrum, making the transmitter more elusive. Due to the method of digital encoding in
the spread spectrum signal, there may appear to be multiple low-level signals that are
frequency hopping.
Burst Transmitters
Burst Transmitters are devices that store audio information for a period of time, and then
in a single burst radiate out all of the information in a period fashion. These devices are
digitally modulated and typically very wide bandwidth depending on the burst period and
duty cycle. The burst period depends on the design of the device, but could be every few
milliseconds to many minutes. However, it is possible to have a burst transmitter that only
transmits every few hours. This type would be extremely difficult to detect because of the
long period of time with no transmission. In this case, an NLJD is recommended.
The OSCOR can display the difference between the peak display and the real-time
display. By displaying the peak minus real-time display, all continuous wave signals will
be subtracted from the display, and the remaining graph will simply display the evidence
from intermittent transmitters that have radiated but have ceased to transmit.
Frequency-hopping
Frequency-hopping bugs are designed to transmit on one frequency for a very short
period of time (5 to 100 milliseconds) and then change frequency to an apparently
random new frequency. These frequency hoppers will hop to several different frequency
channels within a well-defined bandwidth. A frequency hopper is typically a digitally
modulated signal, but can be analog.
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