Navigation - Charts, Coordinates, GNSS and Lost Link
Charts and coordinates
Use the VNC (1:500,000) for airspace and navigation, the VTA (1:250,000) for terminal detail and the CFS for aerodrome data. Positions are given as latitude/longitude in degrees, minutes and seconds; in Canada latitude is north and longitude is west. One minute of latitude is approximately one nautical mile (1.852 km). Know how to convert between DMS and decimal degrees, and be aware that coordinates taken from different sources may sit on different datums (WGS84 versus NAD83) - a difference that matters when you are measuring distance from a control zone boundary.
GNSS use
A 3D fix needs at least four satellites; integrity monitoring (RAIM) needs more. Expect degraded accuracy from poor satellite geometry (high PDOP), multipath near buildings, ionospheric disturbance, and masking by trees or terrain. Crucially, GPS altitude is referenced to the ellipsoid, not mean sea level, so the number on the display is not your AGL height - use the aircraft's barometric or downward sensor for low-level altitude control.
Lost link and position reporting
Lost-link behaviour must be pre-programmed and briefed: hold, return to home at a set altitude, or land. The return altitude must clear every obstacle in the operating area, and the aircraft must not depend on GPS to execute it. Position reports should give a fix, altitude and intentions in plain, standard words.
Traps
- Assuming GPS altitude equals AGL.
- Setting RTH below nearby obstacles.
- Treating a lost GPS signal as a lost control link (the responses differ).
Rule of thumb and worked example
400 ft is about 122 m; 30 m is about 100 ft; 1 NM is about 1.85 km. Convert 45°30'30"N, 73°35'00"W: 45 + 30/60 + 30/3600 = 45.508°N and 73 + 35/60 = 73.583°W.