The plan display is a sector scan presentation that indicates the range and direction of obstructions projecting above a selected clearance plane. The clearance plane can be manually set at any level from 3,000 feet below the aircraft up to the level of the aircraft. Only those peaks projecting above the clearance plane are displayed; all other returns are inconsequential and are eliminated. The sector scan presentation limits the returns to those ahead of the aircraft. The vertical line represents the ground track of the aircraft, and ranges are determined by range marks.
The profile display, normally received only by the pilot, provides an outline of the terrain 1,500 feet above and below the clearance plane. Elevations of returns are represented vertically; azimuth is represented horizontally. This display gives the operator a look up the valley. The returns seen represent the highest terrain within the selected range. The position of the aircraft is represented by an aircraft symbol on the indicator overlay.
Techniques on Radar Usage
Radars currently in use offer variations of special equipment and capabilities. The following are techniques to use with radar in common situations and with special equipment designed to enhance radar usage. These are basic suggestions that can and should be adapted to specific aircraft.
Techniques in radar fixing change from operator to operator and most provide accurate results. The following are reminders that affect the fix accuracy if not considered.
Radar is an aid to DR. Before any radar return can be accurately identified, the operator should be familiar with a chart of the target area. This chart study relies on knowing the approximate location of the aircraft and, therefore, it is essential to radar fixing that the best possible DR position is ascertained.
In examining the area surrounding the DR on the chart, attention should be given to details like 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 display can be more successfully analyzed.
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 have 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.
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.)
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-l4] Slant range is the straight-line distance between the aircraft and the target, while ground range is the range between the point directly below the aircraft and the target.
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
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 7-15] Slant range correction charts are provided in Figures 7-16.
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 display. 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.
This is a technique for obtaining a wind by using radar targets to provide track and groundspeed (GS) of an aircraft. [Figure 7-17] 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 that 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 disappears. Choose a target that has just appeared on the scope and read its range and bearing. 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, turn the compass rose to the azimuth of the second fix and plot a point (measuring from the top line again) representing the range of the second fix. Repeat for the successive fixes.[Figure 7-18] To solve for the wind, rotate the compass rose so that the three plotted lines are parallel to the vertical grid lines and read the track under the true index of the compass rose. Then, determine the GS by measuring the distance between the first and last plotted points using the grid lines. Using track and TH, find the DA and use the standard MB-4 wind solution.