Daytime fixing, using celestial techniques, is rather limited because often only one body, the sun, is visible. Ordinarily, three LOPs cannot be obtained for a fix from one body, because the LOPs plot nearly parallel to each other.
The Sun Heading Shot at High Noon
The azimuth of the sun changes very rapidly when the subpoint of the sun is directly over the longitude of the observer, which is called the time of transit. The LHA at transit time is 360°. This phenomenon is more pronounced at lower latitudes as the subpoint of the sun passes closer to the observer. This makes it extremely difficult to get an accurate celestial heading shot at the transit time. Therefore, if you need a heading shot near the time of transit, you must take extra precaution to get the heading observation exactly at the precomputed fix time. If the moon or Venus is available, consider using these bodies for an accurate celestial heading. If using the sun, you should weigh the increased possibility of an inaccurate heading shot. If the accuracy is questionable, get another heading shot as the sun’s rate of azimuth change slows enough to allow a more accurate shot.
The intercept method is normally used in obtaining a noon day fix. If the sun passes close to the observer’s position, within about 4°, the subpoint method of plotting the fix may be used. This method differs from normal procedures in that three different precomps for three different times are computed. Because of the rapid change of the sun’s azimuth at or near transit, this variation is necessary. The procedure is:
- Determine the time of transit.
- Select the LHA before and after transit for which the change in azimuth is 30° or more. Since 1° of LHA is equal to 4 minutes of time, the difference in transit LHA and the new LHA can be converted to time in minutes. Thus, the time preceding and following transit can be determined.
- Plot the DR positions for times determined in 12.7.2. Select the appropriate assumed positions necessary for the computation and plotting of the LOPs. The assumed position for time of transit is also plotted.
- Determine the intercepts and azimuth for each LOP. Plot these data from the respective assumed positions.
- Resolve the LOPs to a common time, preferably that of the transit LOP.
NOTE: At 30° N latitude, the linear speed of the sun is approximately 780 knots. Thus, on westerly headings in highspeed aircraft, the DR distance involved before encountering a 30° change in azimuth is considerable.
When the observer is within approximately 4° of the subpoint of the body, the subpoint method of solution is normally used. This is because the radius of the circle of equal altitude is so small that a straight line does not approximate the arc and a straight line does not give an accurate LOP. The procedure is:
- Plot the subpoints of the body for the time of the observations (using GHA and/or Dec).
- Find the co-altitude of the shots and convert it to NM (90° – Alt × 60 NM).
- Advance the first subpoint and retard the third along the DR track, using best-known track and GS.
- Set the distance found from the co-altitude and strike it off from the resolved subpoints (with a compass or pair of dividers). Do this for each observation.
NOTE: The resulting intersection, or triangle, gives one ontime fix. If the LOPs form a triangle, the aircraft position is probably within the triangle.
The subpoint method is convenient because Pub. No. 249 is not used—only the Air Almanac. This method can also be used with a star near your assumed position and may be necessary if, for some reason, your Volume 1 is unavailable. The stars Dec and GHA are needed to determine if the observer is within 4° of the subpoint. The Air Almanac may be used to find the Dec and sidereal hour angle (SHA) of the star. The SHA of the star is added to the GHA of Aries to find the GHA of the star.
Eliminating Motions with the Bracket Technique
For sun observations, you can eliminate motion calculations by using a shooting schedule of 3 minutes early, on fix time, and 3-minutes late. With this schedule, the 3-minute early and 3-minute late shots have the same magnitude of motion but an opposite sign. Therefore, these motions cancel each other out and do not need to be computed. The on-time shot has no motions. Therefore, the three intercepts can be averaged for a single LOP. At night, shooting the same star 4 minutes early and late, with a different star shot on time, can employ a similar method. In this case, the intercepts for the same star’s 4-minute early or late shots can be averaged. This reduces workload, but only two LOPs are obtained.
DR Computer Modification
Rather than eliminating motions, your DR computer can be modified so both observer and body motions can be computed at one time, without entry into the Pub. No. 249. Make a GS and latitude scale. [Figure 12-8] After constructing these, the DR computer can be modified for quick and accurate computations of 1-minute motion adjustments.
Tape the GS scale (0 through 900) along the centerline of the grid scale. Match zero to zero, 300 to 50, and 600 to 100 as shown in Figure 12-8. Then, tape the latitude scale along the zero grid line so that 90° falls on the centerline and the scale extends to the left as shown. Check the accuracy of your placement: 30° latitude should fall 13 divisions left of centerline. Juggle the scale as necessary to provide the greatest accuracy between 30° and 45°.
To use the modified MB-4 computer for motion adjustments:
- Set true north under the index. If computing for grid, set polar angle (PA) under the index. In the NW and SE hemisphere quadrants, PA equals convergence angle (CA). In the NE and SW quadrants, PA = 360 – CA. Next, place the grommet over the zero grid line. Mark a cross (+) at the assumed latitude. [Figure 12-9]
- Set track (or grid track) under the index and position the slide so the GS is under the grommet. Place a dot on the zero point of the grid scale. [Figure 12-10]
- Place the Zn (or grid Zn) of the body under the index. Position the slide so the cross or the dot, whichever is uppermost, is on the zero line of the grid. [Figure 12-11]
NOTE: The vertical distance between the zero line and the low mark is the combined 1-minute motion. Each line of the grid equals 1 minute of arc (1 mile). If the cross is on the zero line, the motion is 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||45° 10′ N|
Combinations of Sun, Moon, and Venus
The moon or Venus is 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.
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 is visible but hard to detect, unless sky conditions 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.
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 are available if their Dec is the same name as the latitude.
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 flight 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.