WAAS approach plate channel numbers


New Member
Originally this was going to be a question, but I found the answer and edited my post to reflect the answers I'd gathered. In researching this on the internet I found quite a few instances of people teaching things about GPS in general that were inaccurate. I am posting this in hopes of clearing some things up.

I haven't been on this forum in probably over a year because I have changed careers and am now in agriculture rather than aviation. Believe it or not, agriculture nowadays has benefited greatly from the GPS technologies that are available today. My understanding of GPS and the technologies related to increasing its accuracy have increased greatly since leaving the aviation world due to this fact. A friend of mine who is still in the aviation industry indicated that WAAS approaches required the user to input a channel number somewhere in order to receive WAAS accuracy.

This didn't ring true from my experiences in agriculture, but when your experience is primarily with traditional nav aids it would seem reasonable that if there is some channel number on the approach plate, you probably need to tune it in so that you can access the information from that navigation facility. This is NOT the case for WAAS approaches. The channel number in the upper left hand corner of a WAAS approach is just a number that is associated with that particular approach plate. If you don't believe me, my reference for this claim is AIM 1-1-20 d 6 which states:

[FONT=&quot] A new method has been added for selecting the final approach segment of an instrument approach. Along with the current method used by most receivers using menus where the pilot selects the airport, the runway, the specific approach procedure and finally the IAF, there is also a channel number selection method. The pilot enters a unique 5-digit number provided on the approach chart, and the receiver recalls the matching final approach segment from the aircraft database. A list of information including the available IAFs is displayed and the pilot selects the appropriate IAF. The pilot should confirm that the correct final approach segment was loaded by cross checking the Approach ID, which is also provided on the approach chart.[/FONT]
If anyone has any ideas as to why "they" decided to call the number that associates a chart reference number with the word channel, please let me know. To me this seems like it is encouraging a misinterpretation of what the number is actually for.

Every pilot should understand how any navigation system that they utilize works. An understanding of how the system works prevents false assumptions based on misconceptions. False assumptions about how something works can lead to accidents. Everyone needs to know what to expect from their equipment and why so that when it does something unexpected you can have an idea of how serious it is and what your best course of action is.

It seems to me that a lot of the information about how WAAS works is pretty dry. It's difficult to want to understand it because of the tedious language with which it is described. For anyone who finds themselves in that situation, I'll give you my simplified description that starts with the ag equipment I use.

Farmers interested in precision agriculture need much more accuracy than pilots need for various reasons that I won't get into. We're in the quest for sub inch accuracy rather than sub meter accuracy. All ag GPS equipment is WAAS equipped. This gets us to the sub meter level which still isn't good enough. The solution we have is that we set a WAAS equipped GPS receiver in a fixed location (in other words we mount it permanently to a cement fixture). We survey this position so that we know exactly what the true latitude, longitude and altitude is. We then program this latitude, longitude and altitude into the receiver so that it knows what its position should be.

When the receiver is running it continuously calculates its position. Let's say that the position that it calculates is 2 feet to the west of where the survey showed it to be. This receiver is equipped with a transmitter that sends a signal to other GPS receivers in the area that are mounted in tractors and other equipment. It says "hey, the signal is 2 feet too far to the west so you're 2 feet too far to the west". So all the receivers that are in the local area (less than 6 miles away) change their information 2 feet east to compensate. The atmospheric distortions that cause inaccuracies in GPS signals are different in different locations so this only works for relatively small areas if you're wanting sub inch accuracy. That's why we can only use this within 6 miles of the fixed receiver.

That's essentially how WAAS works. Of course WAAS doesn't start with a WAAS receiver like the ag situation does (it's creating the WAAS correction, not using the WAAS correction). Each fixed station receives a normal GPS signal. There are also a lot more fixed receivers that are spread out throughout the United States and Canada and they are all networked together. There are several locations that gather all this data about how the atmosphere is affecting GPS accuracy in each of the fixed locations. Error corrections are calculated and are sent to satellites that are always visible. These satellites send that correction information out. WAAS enabled receivers can utilize this information to provide aircraft with sub meter navigation capabilities in the horizontal plane and less than 1.5 meters typically in the vertical plane.