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roadways and waterways as well as the more prominent urban returns. Cultural returns build up along
such byways; therefore, discrepancies between the chart (which could be years old) and the PPI display
can be more successfully analyzed.
7.18.3. Prior to fixing, take care to adjust gain, antenna tilt, and heading marker. If you use a mechanical
cursor, ensure its center is aligned with the sweep origin, or risk parallax error. Do not accept a return on
the scope as the chosen target unless you've verified it using surrounding returns. Work from chart to
scope. If your desired target does not show but you see a return you think you recognize, go back to the
chart and verify it before fixing from it.
7.18.4. When obtaining fix readings, remember to compensate for inherent scope errors. If the fix is a
multirange or multibearing type, choose the targets to provide the optimum cut. When using multiple
targets, read the returns that are changing their values the fastest closest to fix time. (With multirange, a
target off the nose changes range faster than the one off the wing.)
AFPAM11-216 1 MARCH 2001 189
7.19. Slant Range. Once you identify a return, use it to fix the position of the aircraft by measuring its
bearing and distance from a known geographical point. Of particular significance in any discussion of
radar ranging is the subject of slant range versus ground range (Figure 7.l5). Slant range is the straightline
distance between the aircraft and the target, while ground range is the range between the point
directly below the aircraft and the target.
Figure 7.15. Slant Range Compared to Ground Range.
7.19.1. To fix the position of the aircraft, the navigator is interested in the ground range from the fixing
point, yet the fixed range markers give slant range. The trick is to determine the critical range below
which the navigator must convert slant range to ground range to keep fixes accurate. This range may be
determined by a simple formula:
Critical slant range = Absolute Altitude (in K)-5
7.19.2. Slant range can be converted to ground range, using the latitude and longitude lines of a chart if
the slant range table is not available. Set dividers at the slant range distance to the target. Place one point
of the dividers at the equivalent (in NM) of the aircraft's altitude on the longitude line. Set the other
point where it meets a nearby latitude line. Without moving point, reset the first point along the latitude
line at the intersection of the latitude and longitude lines. The distance is the ground range in NM
(Figure 7.16). Slant range correction charts are provided in Figures 7.17.
7.20. Side Lobe Interference. Side lobes are small extra fields of energy separate from the main beam,
and are an inherent flaw in any radar. These side lobes are rarely strong enough to generate a return.
However, when a large or very reflective target comes into this field or when the transmitter power
increases the size of the lobes, multiple shadow returns may appear on the PPI. Curved strobes
originating at the center of the radarscope are also caused by the side lobes of the radar receiving energy
from your radar or others in the same frequency range. Solutions to this problem include reducing the
gain or changing transmitter frequencies.
190 AFPAM11-216 1 MARCH 2001
Figure 7.16. Slant Range From Chart.
Figure 7.17. Slant Range Correction Chart.
7.21. Target-Timing Wind. This is a technique for obtaining a wind by using radar targets to provide
track and GS of an aircraft. The MB-4 computer solution for wind requires true heading (TH), true
airspeed (TAS), drift angle (DA), and GS. The first two can be derived from basic aircraft instruments
(indicated airspeed [IAS] and compass). The other two require a target which can be tracked for about 4
minutes and which is preferably within 20 degrees of the radar heading marker. The identity of the target
is irrelevant, but it should not be too big to make range and bearing determination vague, or so small that
it will disappear. Choose a target that has just appeared on the scope and read its range and bearing.
AFPAM11-216 1 MARCH 2001 191
Also, start a stopwatch or note the minute and seconds on a clock so elapsed time can be measured. At
least two ranges and bearings should be taken over a distance of 20 to 25 NM. One technique is to fix at
the 40, 30, and 20 NM range marks to space the fixes evenly. At the last observation, stop the watch and
determine the elapsed time. On the windface grid of the MB-4, place the grommet over the center mark
of the top reference line. Turn the compass rose to the azimuth of the first fix. Using your own values for
each of the horizontal grid lines, plot a point representing the range of the first fix (going down). Then,
　
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