Celestial navigation explained - page 4

Celestial mechanics - a blueprint

The hatched triangle on the top of the celestial sphere is the one we will use to solve the celestial navigation problem.

The three sides of the triangle are:

col, the co-latitude (90° minus latitude);
z, the zenith distance (90° minus altitude h);
Delta, the polar distance (90° minus declination delta).

The angle of the triangle opposite to the side z is called the polar angle P (180°W to 180°E). This angle is also the angular distance Q-D at the celestial equator:
Here: PE = gw - GHA*

This is a spherical triangle, not a plane triangle. 

celestial navigation triangle

While you may remember the formulas to solve a plane triangle in geometry, the formula for spherical geometry may not be quite so memorable?

As a recap, the triangle in plane geometry and the cosinus rule for the spherical geometry in the general case:
plane and spherical triangles

The application of the general case to our problem:

cosinus rule for spherical triangle

We found a mathematical relation between what we know (delta, GHA, h) and what we are looking for (latitude, longitude).

 With two observations, we get a system of 2 equations with 2 unknowns that we are able to solve.
The celestial navigation problem is now resolved.

Of course, this resolution is not really simple by hand but for a computer the process is quite straightforward: it solves the system of equations by an iterative method using the estimated latitude and longitude as starting values.

 With more than 2 observations, it’s even possible to improve the traditional method and to perform a statistical analysis:

  • to give a certain weight to each observation according to its reliability in the normal law model;
  • to compute and eliminate the possible systematic error of the observer;
  • to correct the assumed course and speed if enough observations are provided (exactly the same way the GPS is able to give the course and speed of the vessel if enough satellites are visible).

To do this, a program like ASNAv is using the least-square method with iterative weighting adjustment by the Biweight function on a system of equations given by the differential correction method. Each equation i looks like:

ASNAv equations For a normal human being, solving such a celestial navigation equations system will take hours !

To check manually the results of celestial navigation programs, we can use the traditional method of the Lines of Position (LOPs).



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