By demand: More on WAAS

This is the eighth in a series of articles looking at the impact of the Next Generation Air Transportation System (NextGen) on GA pilots.

First I would like to thank all the readers who have responded to the NextGen series. Feedback is always beneficial in providing a clearer understanding of each article’s content. It also allows us to modify and improve the content by way of specific requests and additional information from you, the reader. We thank you for that.

With that, we have received a fair amount of mail asking to provide more information on WAAS before we dive into ADS-B. WAAS is a more involved GPS system and does deserve more attention than just a mention since it will play a part with ADS-B and NextGen.

First let’s review a little bit on WAAS, which stands for Wide Area Augmentation System.

The WAAS GPS system is unlikely to easily integrate with legacy aircraft. Systems like this also cannot be installed with typical field approval procedures. They have to be installed by a certified facility and get an STC approval. These systems also require annunciators, whether internal to the WAAS system or as an external device. This all may sound way out of our typical GA pilot’s working environment, but by the time NextGen becomes real for the GA pilot, new and less expensive systems should be available. But for right now, these are the necessary building blocks towards the newer NextGen navigation system.

The third word in WAAS — augmentation — simply means that we are expanding or improving the accuracy of the system by determining ionosphere delays, clock drifts or ephemera (a satellite’s integrity and exact location data). This highly accurate GPS system was designed to function on its own, allowing aircraft the ability to perform all phases of flight, including precision approaches, by only using GPS. Accuracy with these new systems has improved from 20 meters down less than 2 meters.

There are three types of Augmented GPS Systems:

  1. ABAS: Aircraft Based Augmentation System;
  2. SBAS: Space Based Augmentation System;
  3. GBAS: Ground Based Augmentation System.

The current GPS space-based systems do not provide the reliability and accuracy required for the next phase of navigation. With traffic expected to double by 2025, a far better system will be mandatory.

NextGen will incorporate the SBAS. The system is made up of:

  • 31 GPS satellites with two additional stationary satellites for data correction retrieval;
  • two geosynchronous stationary satellites;
  • 38 perfectly located ground-based WAAS Reference Stations, which are used to provide perfect satellite correctional data. They will also be used to gather and processes pertinent GPS information that then gets routed directly to two master WAAS stations;
  • Two WAAS Master Control Stations (WMS), which take in all the GPS information and sends it to WAAS GPS receivers in aircraft by way of two geosynchronous satellites (Satellites 135 and 138);
  • Four discrete uplink stations, which transmit signal data up to Satellites 135 and 138;
  • Two centers for operation and control, which process all the data received back from the reference stations.

GPS satellites that orbit around the North American continent send their own signal data down to 38 Wide Area Reference Stations on Earth. These stations are place in strategic locations so that errors within the GPS signal can be detected. This information is then transmitted by terrestrial transmission methods to two master stations in Washington, D.C., and Los Angeles.

These two stations are the main stations where all the WAAS augmentation messages are processed. At these stations all errors that were detected are removed, which substantially increases GPS location accuracy and reliability.

These new augmented messages are sent over the four uplink stations, where they are transmitted up to the two geosynchronous stations satellites. These satellites broadcast the augmented signal to your aircraft. The GPS WAAS receiver processes this data as part of the new estimated position.

Incorporating two WAAS GPS systems allows aircraft to fly LPV approaches. In the past, LNav and VNav were the only GPS type of approach procedures typically used. They do use GPS for lateral navigation, but there is no vertical guidance and will typically take the aircraft down to 400 feet. LNAV/VNAV offers lateral guidance from the GPS or WAAS but relies solely on vertical guidance from a barometric altimeter.

However, with Dual WAAS GPS systems, LPV can be used in a precision approach with just a GPS. LPV approaches can take an aircraft down to a 200- to 250-foot decision height. There are currently about 1,900 runways that support LPV approaches.

While many of us in GA do not have WAAS or Dual WAAS systems, this gives you a pretty good idea as to the expected performance of NextGen and ADS-B (Automatic Dependent Surveillance-Broadcast, which is the cornerstone of NextGen).

A special effort was put forth to keep the details of WAAS somewhat light and straightforward in this article, however it really needed to be brought to the table before we dive into ADS-B.

In some ways ADS-B will be easier to understand and use. Consider for a moment the fact that we are currently using 38 ground reference stations in WAAS, while in ADS-B there will be almost 800 sites scheduled to be deployed upon completion. They will be able to handle far more tasks in supporting ADS-B.


Jeffrey Boccaccio is a private pilot and chief engineer at MatchBox Aeronautical Systems. You can reach him at or


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