This radio is the "little brother" to the well-known WAAS-enabled Garmin GNS-430W. It has all the features of the GNS-430W except the VOR/LOC/Glideslope Receiver. In exchange for this trade-off, it is significantly less expensive.
The GNC-420W is a newer and much-improved version of the original GNC-420. There are a number of improvements in the GNC-420W, but the primary ones are WAAS Approach Capability and Terrain Information.
WAAS Approach Capability - WAAS, or Wide Area Augmentation System, is a fundamental improvement to GPS accuracy. By adding a series of ground stations and geostationary satellites, WAAS GPS is typically 5 times more accurate than "regular" GPS. This means that it is now possible to fly a complete "Virtual Glideslope" approach into an airport with no local ground-based navigation aids whatsoever. A GNC-420W makes the following approaches available:
LNAV (Lateral Navigation): This is a new name for non-precision GPS approaches as we have known them. Like all non-precision approaches, this has a minimum descent altitude (MDA), based on obstructions and terrain.
LNAV/VNAV (Lateral Navigation / Vertical Navigation): This class of approach provides Glideslope vertical guidance with a decision altitude (DA) typically 350 feet AGL.
LPV (Localizer Performance with Vertical guidance): This is the most precise of the WAAS GPS approaches, the GPS equivalent of a Cat I ILS approach. It also offers Glideslope vertical guidance, but the DA is lower, typically from 200-250 feet AGL.
There are thousands of FAA-approved LPV WAAS approaches, with more being added regularly. The FAA's stated goal is to have a WAAS approach available for every public runway at least 3,200 feet long. To see if the airports you use offer WAAS capability, please click here for the FAA's information about GPS/WAAS Approaches.
Terrain Information - With the GNC-420W, the terrain database is standard (for improved awareness and safety), and it provides this information with greater resolution and precision than the GNC-420. (It is possible to add terrain information to a GNC-420, but this is an expensive option.)
Other GNC-420W improvements over the GNC-420 include:
Faster position calculations and screen updates (5x per second)
Autopilot roll steering
Parallel track calculation
Dead reckoning (This is a backup if GPS signal is lost)
More informative status pages
Better, faster communication between dual 400-series / 500-series installations
Configuration for GDL-69 / GDL-69A XM Receivers (if installed)
Simplified installation paperwork
The Garmin GNC-420W may be used with any of these Indicators (the Garmin units are the official choices):
Garmin GI-106A GPS/VOR/LOC/Glideslope Indicator
Garmin GI-106B GPS/VOR/LOC/Glideslope Indicator
Mid-Continent MD200-306 GPS/VOR/LOC/Glideslope Indicator
Mid-Continent MD200-706 GPS/VOR/LOC/Glideslope Indicator
Collins IND-351A GPS/VOR/LOC/Glideslope Indicator
King KI-206 GPS/VOR/LOC/Glideslope Indicator
King KI-209A GPS/VOR/LOC/Glideslope Indicator
King KNI-520 GPS/VOR/LOC/Glideslope Indicator
King KI-525/525A HSI (part of a KCS-55/55A system)
Additionally, the GNC-420 offers 8.33 kHz COMM tuning and transmits with 10 watts output.