SB-11 – VOR/DME Navigation for Aviation

Description:

Calculate an aircraft's current geographical latitude and longitude (sometimes also called a fix) from the known position of a VOR/DME station, the station's magnetic declination, the aircraft's magnetic radial FROM that station, and its DME distance. Optionally, enter the aircraft's height above the station to correct the DME slant range to horizontal distance.

Disclaimer:

This program is designed for experimental navigation calculations over distances and at altitudes typical of general aviation (GA). It uses an approximate planar model of the Earth's surface, with corrections for the convergence of meridians towards the poles, rather than full spherical or ellipsoidal geometry.

This approximation introduces systematic position errors that can become substantial over long distances. The magnitude of these errors depends on the geographical configuration and generally increases with distance. Consequently, the program is not suitable for the extended navigation distances typical of high-altitude airline operations.

Operating conditions:

• The calculator must use 2nd Deg mode, not 2nd Rad or 2nd Grad. The enabling/initialization functions D, 2nd D' and 2nd E' enforce 2nd Deg mode.
• Southern latitudes and western longitudes are negative.
• Magnetic declination is positive east and negative west.
• Distances are entered in nautical miles and height differences in feet.
• Enter the aircraft's vertical height relative to the DME station antenna, in feet. If using above mean sea level (AMSL) altitudes, subtract the station antenna elevation.
  If no height difference is entered, the measured slant range is used as an approximation of the horizontal distance.
• If the absolute height difference (in feet, converted to nautical miles) is greater than the measured slant range (in nautical miles), which is physically impossible in reality (or a severe measurement error), the program will stop with an error (blinking "9.9999999 99" in the display), and t is set to zero.
• Enter the magnetic radial FROM the station, not merely the OBS course selected on the receiver (unless the CDI is centered with a FROM indication).
• Before first use, press 2nd E' to initialize the program.

Usage:

To initialize for a new VOR/DME station, press 2nd E'. Enter the station data, followed by the aircraft's radial, DME distance and optionally height:

Press E to calculate the aircraft's current position. The aircraft's latitude appears on the display; its longitude is placed in the test register t. After calculation, 2nd A', 2nd B', 2nd C', A, B and C retrieve their stored values instead of accepting new ones.

To change the radial, DME or height without clearing their current values, press D; then enter only the values you want to change. To clear all three measurement inputs, press 2nd D'. To change to a different VOR/DME station and clear all station and measurement inputs, press 2nd E'.

Function keys:

Subroutines:

Output:

Good to know:

The program does not interrupt ongoing calculations. Its functions and subroutines do not use CLR or = internally, so ongoing arithmetic operations are not prematurely cleared or evaluated.

The program uses a planar approximation to calculate latitude and longitude. If the optional height is supplied, it first converts the DME slant range to horizontal distance, which can increase accuracy.

Registers used:

Glossary: