USCG Navigation Problems

According to Bowditch, star selection for a reliable fix requires bodies spaced approximately 120 degrees apart in azimuth with moderate altitudes between 15° and 65°. This minimizes geometric intersection errors and refraction anomalies during twilight observations.

Observed altitude (Ho) is derived by applying index error (on the arc is subtracted, off the arc is added) and dip correction for height of eye, followed by semi-diameter and refraction adjustments from the Nautical Almanac to find the true center of the body.

Vector addition of vessel velocity and current vector determines the course made good and speed made good. Constructing a current triangle using set and drift yields the true track across the bottom rather than the heading steered.

Local Apparent Noon occurs when the sun crosses the vessel's meridian. The time is calculated by applying the equation of time and longitude correction expressed in time to Greenwich Mean Time of transit, adjusted for the local zone.

Compass error is the algebraic sum of variation and deviation (West is minus). Applying variation and deviation to compass course yields magnetic then true course. Leeway from a southwest wind pushes the vessel to starboard, requiring appropriate angular correction.

Azimuth (Zn) calculation via the intercept method utilizes the celestial triangle solved through assumed latitude, local hour angle, and declination. Standard sight reduction tables (Pub. No. 229 or 249) provide the calculated azimuth angle, converted to true azimuth.