Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 255

desired. If variable, then the J / N meter (described in the following section) and the
CW signal detector in Figure 6.1 are used to make the adjustments dynamically. The
range of T % is adjusted typically between 5% and 15% and N between 4 and 16,
depending on the presence of the CW and the level of CW interference as indicated
by these built-in monitors. The best CW performance at the highest J / N is achieved
when T % is 5 and N is 16. There is less signal degradation at the lowest J / N condi-
tions if T % is 15% and N is 4. The specific design requires tuning (see footnote 2).
6.2.2.4 Receiver Jamming Situational Awareness— J / N Meter
Figure 6.1 illustrates the circuit simplicity of adding a J / N meter to a receiver design
containing an AGC amplifier that is controlled by a digital feedback loop. To
achieve this simplicity, the addition of the J / N meter must be part of the original
receiver AGC design plan. Since the addition of a J / N meter is seldom preplanned,
there have been numerous papers published on interference measurement tech-
niques performed within the tracking loops or the navigation solution of a receiver.
The main problem with these concepts is that the receiver must be tracking to mea-
sure jamming. It is important to have situational awareness about jamming during
the search modes and adapt the search strategy accordingly, then adapt the tracking
strategy accordingly if tracking is possible. It is also important to have situational
awareness when the receiver fails to navigate. In this case, a situational display can
inform the user that the jamming level exceeds the search or tracking capability of
the receiver. Only an AGC J / N meter can perform this function because the J / N
meter continues to operate even if the search and tracking functions are incapable of
operating at the measured jamming levels. The bottom line is that measuring the
control voltage levels to the AGC to determine jamming is superior to any other
method, but the provision for this measurement must be preplanned in the original
AGC design. For military GPS receivers, this means that the situational awareness
requirement must be specified.
As demonstrated for the simple case of detecting CW interference in Figure 6.1,
the digital IF signal statistics can be analyzed and characterized for other classes of
interference. So, not only can the AGC and ADC areas of a receiver provide situa-
tional awareness with respect to the jamming level that is actually being experienced
by the receiver regardless of whether the receiver is capable of operating at that jam-
ming level, but also the jamming type can be identified if these designs are
preplanned.
The principle of operation of the J / N meter in Figure 6.1 is as follows. In the
absence of interference, the signals are buried in thermal noise. Under this condi-
tion, the AGC adjusts the RMS signal amplitude for optimal ADC performance
based on thermal noise. If the AGC control voltage changes from this level, that
change is an indication that the noise level has increased. This increase can only be
caused by interference, and the measure of this interference is the change in the AGC
control voltage. As depicted in Figure 6.1, the AGC gain is reduced in the presence
of increased interference. Because of the logarithmic nature of the AGC control
voltage, J / N is equal to the change in AGC control voltage times a scale factor.
Obviously, J / N is unity (0 dB) in the presence of thermal noise. The digital design in
Figure 6.1 that ultimately provides the analog control voltage to the AGC can be a