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positive. If the dot is on the zero line, the motion is negative. When solving for motions using grid, all
directions must be grid directions!
EXAMPLE: Given the following information, find the combined 1-minute motion adjustment.
Assumed Latitude 45o 10' N
True Track 270o
GS 240 knots
True Zn 171o
Answer +1'
12.11. Combinations of Sun, Moon, and Venus. The moon or Venus are often visible during daylight
hours and can be used to obtain an LOP. Always consider fixing using these bodies during daylight
celestial flights. When planning the flight, use the sky diagrams in the Air Almanac to determine the
availability of the moon and Venus. If the bodies are available, they can be readily found by accurately
precomputing their altitudes and azimuths.
12.11.1. When looking for Venus, take all the filters out of the sextant and point it at the precise location
of the planet. A bright, small pinpoint of light will be visible but hard to detect, unless sky conditions
268 AFPAM11-216 1 MARCH 2001
and separation from the sun are ideal. With practice, acquisition should become easier and you will be
familiar with those conditions conducive to successfully making a Venus shot.
12.11.2. During the day when the sun is high, the moon or Venus, if they are available, can be used to
obtain compass deviation checks. In polar regions during periods of continuous twilight, the moon and
Venus will be available if their declination (Dec) is the same name as the latitude.
12.12. Duration of Light. Sunrise and sunset at sea level and at altitude, moonrise and moonset and
semiduration graphs will not be discussed in detail in this chapter. It is imperative; however, to preplan
for any mission where twilight occurs during the course of the flight, especially at the higher latitudes
where twilight extends over longer periods of time. An excellent discussion, with appropriate examples,
is provided in the Air Almanac and should be sufficient for those missions requiring detailed planning.
Section 12D— True Heading Celestial Observation
12.13. Basics. The periscopic sextant, in addition to measuring celestial altitudes, can be used to
determine true headings (TH) and true bearings (TB). Any celestial body, whose azimuth can be
computed, can be used to obtain a TH. Except for Polaris, the appropriate volume of Pub. No. 249 is
entered to obtain Zn (true bearing). In the case of Polaris, the Air Almanac has an azimuth of Polaris
table. It does not require information from the Pub. No. 249 tables. The two methods used to obtain THs
with the periscopic sextant. The TB method requires precomputation of Zn. Postcomputation of Zn is
possible with the inverse relative bearing (IRB) method. The procedures are as follows:
12.14. True Bearing (TB) Method:
12.14.1. Determine GMT and body to be observed.
12.14.2. Extract GHA from the Air Almanac.
12.14.3. Apply exact longitude, at the time of the shot, to GHA to obtain exact LHA.
12.14.4. Enter appropriate Pub. No. 249. table with exact LHA, latitude, and Dec. Interpolate if
necessary and extract Zn and Hc (Figure 12.12). If Polaris is used, obtain the azimuth from the Azimuth
of Polaris table in the Air Almanac and use your latitude instead of Hc (Figure 12.13).
12.14.5. Set Zn in the azimuth counter window with the azimuth crank and set Hc in the altitude counter
window with the altitude control knob.
12.14.6. Collimate the body at the precomputed time and read the TH of the aircraft under the vertical
crosshair in the field of vision. If you are using precomputation techniques, a TH is available every time
an altitude observation is made.
NOTE: Shot must be taken at precomp time.
AFPAM11-216 1 MARCH 2001 269
Figure 12.12. True Bearing Method (Except Polaris).
270 AFPAM11-216 1 MARCH 2001
Figure 12.13. True Bearing Method (Including Polaris).
12.15. Inverse Relative Bearing (IRB) Method:
12.15.1. Set 000o in the azimuth counter window with the azimuth crank (Figure 12.14).
AFPAM11-216 1 MARCH 2001 271
Figure 12.14. Inverse Relative Bearing Method.
12.15.2. Collimate the body. At the desired time, read the IRB under the vertical crosshair in the field of
vision.
12.15.3. Compute Zn of the celestial body and use the formula:
TH = Zn + IRB
Section 12E— Celestial Navigation in High Latitudes
12.16. Basics. Celestial navigation in polar regions is of primary importance because (1) it constitutes a
primary method of determining position other than by DR and (2) it provides a reliable means of
establishing direction over much of the polar regions. The magnetic compass and directional gyro (DG)
are useful in polar regions, but they require an independent check, which can be provided by a celestial
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