By Keith Thomassen
First published in Flight Levels Online, Summer 2013 issue
Perhaps you’ve noted the differences between course directions as given on airway maps and those courses as listed in your GPS flight plan. For example, V195 between FOT and YAGER has a 091degree course listed on the chart in Fig 1, but the course from a GNS 530 simulator with a very old database shows 094 degrees. Using a current database on my GNS 480 the course is given as 093 degrees, so this is database dependent. Similarly, V494 leaves Fortuna (FOT) on the 151 degree course, but a current database lists it as 153 degrees. So why do courses listed on a GPS flight plan often differ from that on a chart?
Figure 1. The 091 degree course from FOT to YAGER is different than shown on a GPS plan.
First, a course between two waypoints, representing fixed points on the ground, has a course relative to true north (as defined by the spin axis of the earth) that does not change. However, its magnetic course (as defined by the magnetic axis of the earth) is continually changing because local magnetic declination –– the difference between true and magnetic north –– is continually changing. The magnetic north pole has been moving south since the Ice Age, but since 2005 has also started moving eastward. Declinations are increasing in the west and decreasing in the east as the pole moves.
How then can a course defined as a radial from a VOR represent a fixed line on the ground –– the airway, for example? The magnetic direction from that VOR is continually changing! To explore this further let’s make sure you know how VORs work and are calibrated.
A VOR station sends two modulated signals, one omnidirectional and another rotating so that it sweeps through 360 degrees at 30 Hz. The phase of the rotating signal, relative to the omnidirectional signal, sweeps from 0 through 360 degrees so your receiver need only detect the phase difference to know what radial it is on. For example, if your receiver determines that the swept signal is 120 degrees out of phase with the omnidirectional signal, you are 120 degrees from the reference direction for that VOR.
This reference direction is set to the local magnetic declination when calibrated. In that direction the phase difference is zero. That calibrated declination is called the station declination. So you only need to set your OBS to 091 degrees and you will track the airway (provided the chart direction is changed when the station declination is determined). You are tracking a course that is 091 degrees phase difference relative to the station declination, and that is where the airway is located. This is still true as time goes on and the local declination increases and the airway magnetic direction decreases. (The declination plus the airway direction add to a constant value, the true north direction of the airway.)
So VOR receivers work fine, but how do GPS receivers deal with this drift in declination? First, you might appreciate how much the magnetic declination changes by noting that the location of the North Pole is currently changing about 37 miles each year, faster than any time in history. So changes in declination are significant, as you may have noticed when runway directions periodically get changed and new numbers are painted at the end.
Now, to understand GPS courses, there are rules in the RTCA document for certifying GPS receivers (DO-229-C) that deal with what magnetic variation the GPS receiver must use and have in its database. Your GPS determines the course between two points relative to true north very accurately, so the whole issue here is what variation it uses to convert those courses to magnetic.
Three rules apply magnetic variation given in DO-229C. First, if a flight leg is part of a database terminal area procedure, the state where it is located specifies the variation and that is what must be used. Second, if not part of a procedure and the active fix is a VOR, the published station declination must be used. Otherwise, the variation shall be defined by the system using an internal model.
Specifically, “the navigation system shall have the capability of assigning a magnetic variation at any location within the region that flight operations may be conducted using Magnetic North for reference. The assigned magnetic variation shall be within two degrees of the value determined at the same location and time by an internationally recognized magnetic model that is valid for the time of computation.”
With these rules you can figure out what is going on for a given situation. For example, if the magnetic declination was 15 degrees E when the FOT station was calibrated, the airway would be on a 106 degree (true) course. The actual declination is now a little higher so the magnetic course is correspondingly less, but you don’t know what that difference is. You only know your GPS tells you to fly 093 degrees, and it arrived at that number by calculating the true course from FOT to YAGER and subtracting the station declination at the time of calibration. So you are subtracting less than you should because the true course hasn’t changed. You will get a higher number than 091 degrees, even though the airway is now located on magnetic course less than 091 degrees.
Despite that, your autopilot will take you to YAGER. It’s just telling you the wrong course!
Keith Thomassen is a pilot, instructor, and educator who specializes in teaching pilots how to get the most out of their GPS units. He has written several manuals on this and other subjects. For more see http://www.avionicswest.com/default.html
