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. There are two methods used to obtain TH 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 follow.
True Bearing (TB) Method:
- Determine GMT and body to be observed.
- Extract GHA from the Air Almanac.
- Apply exact longitude, at the time of the shot, to GHA to obtain exact LHA.
- 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]
- Set Zn in the azimuth counter window with the azimuth crank, and set Hc in the altitude counter window with the altitude control knob.
- 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.
Inverse Relative Bearing (IRB) Method:
- Set 000° in the azimuth counter window with the azimuth crank. [Figure 12-14]
- Collimate the body. At the desired time, read the IRB under the vertical crosshair in the field of vision.
- Compute Zn of the celestial body and use the formula: TH = Zn + IRB