Written by Gary Harpster, Duncan Aviation Avionics Sales Rep.
How does WAAS work?
WAAS uses the existing GPS satellites that are currently in orbit for the geometric aspect of the equation, a basic triangulation from multiple satellites to your position in three dimensional space. The problem is, in using raw GPS signals, is there are various forms of interference (atmospheric disturbances, satellite position error ..etc.) that play a part in the received signal. WAAS Receivers use a very precise timing signal to filter out this error and give you a higher degree of position accuracy, so much so, that the defined vertical accuracy is more precise that your altimeter in unusual temperature (both hot or cold) conditions.
What do all those new acronyms (HAL, VAL, HPL, VPL, LPV) really mean?
These terms really are not something you as a pilot need to be concerned with, these are acronyms the manufacture wants to make sure their box meets during certification. When a manufacturer designs a WAAS receiver they test their components to meet these specifications. As a pilot, you're mostly concerned with the proper WAAS approach plate and the LPV decision altitude (DA)
HAL - Horizontal Alert Limits
VPL - Vertical Alert Limits
HPL - Horizontal Protection Limit
VPL - Vertical Protection Limit
LPV - Localizer Performance with Vertical guidance
What new rules apply to WAAS users?
No new rules really, the FAA recently published there are now over 2,200 WAAS LPV approaches and that Europe is in the process of adding WAAS approaches over there.
How are WAAS-based approaches constructed
Before any airport is granted a WAAS approach, it must meet the design criteria called out in the FAA Advisory Circular 150/5300-13. Some of the requirements are the amount of space they consider the PFOZ Precision Obstacle Free Zone, Runway Edge Lighting, Parallel Taxiway and a Vertically Guided Airport Airspace Analysis Survey. As you can imagine the airport has to be surveyed to make sure there are no obstacles in your flight path and the decent rate can not be anymore that a typical ILS approach. You will not find a WAAS approach into a valley, where the airport is surrounded by obstacles that could potentially block your GPS signal as you near your minimums.
What are the similarities and differences between WAAS-based approaches and ILS, VOR, or non-augmented GPS?
A WAAS with LPV approach is very similar to an ILS approach, as the aircraft receives both lateral and vertical guidance just like an ILS with the same or better precision. The main difference is ILS signals are generated from transmitters located on the ground and aimed at the designed flight path, whereas a WAAS with LPV approach is getting the signals from satellites in orbit. VOR approaches provide lateral guidance from signals generated by transmitter on the ground. VOR approaches are commonly referred to by pilots as "dive and drive" approaches, this stems from the fact that during this approach you follow a lateral signal for a certain length of time, then when you cross a predefined waypoint calculated by another external reference point you are allowed to descend again for a period of time. This method continues until you are at a minimum decent allowance where you are then required to make visual identification of the airport for landing.