A priest recently mentioned this GPS thing to me, the God Positioning System.  While he wasn’t referring to the constellation of satellites, that seems appropriate for GPS since, for most people, the technology seems like some form of a divine miracle that provides you with your exact position on earth (or above it) at any point in time.  The reality is a little more scientific.  Here is a VERY brief explanation of the technology. 

GPS Overview:
Started in 1973 as a way to provide better navigational coverage, the Global Positioning System was developed by the US DoD to overcome limitations of previous navigation systems.  Have you ever seen the movie Hidden Figures?  The movie follows several important woman who worked as mathematicians during the space race.  One of those, Gladys West, was instrumental in the development of GPS.  Yes, the father of GPS was in fact, the mother.
Originally, the specification called for a minimum of 24 satellites with 4 satellites in each of the six orbital planes.  This is the typical number that is presented to aviation students when discussing GPS.  Over the years, that number has increased and the current system is organized in a 27 satellite configuration.  As of this writing, there are 31 satellites in orbit and another in “checkout.”  The original system became fully operational in 1995, although not without substantial limitations.  Some of you may remember when GPS seemed like a good idea but it was still pretty far off.  This was due to what was called Selective Availability.  It was meant to degrade the signal for civilians and provide only the military with the accuracy that we have come to know today.  As with most things in aviation, it took a tragic accident before GPS was determined to be a technology that should be available to everyone “for the common good,” to quote President Reagan.  In 1983, a Korean Air Lines 747 was shot down in Russian airspace because of an issue between their INS (Inertial Navigation System) and autopilot.  While it was released for civilian use after that, it wasn’t until 2000 that President Clinton signed a policy effectively removing Selective Availability from the GPS.  

So how does it work?
The basic theory of GPS is the use of time between the satellite time transmission and the reception of that signal by your GPS receiver.  Radio waves travel at a constant speed so the receiver simply does some simple mathematical equations to determine your position.  2+2=4 right?  The fundamental equation is actually quite simple, distance = rate x time.  GPS satellites broadcast time based on a HIGHLY accurate atomic clock at the speed of light.  Your GPS receiver will then receive that signal at some point in time.  That travel time is used in the calculation.  In order to get your 3D position, a minimum of 4 satellites is required.  Your GPS receiver is constantly listening for these signals in the 1,200 or 1,500 MHz frequency and making the appropriate calculations to determine your position on Earth.  Simple right?  Now that we have a very basic understanding of GPS, let’s look at the concept of WAAS and how that works in our aviation world.

WAAS Overview:
Present day GPS is far more advanced than the early days of the technology.  Some very smart people determined that we could utilize GPS in order to allow very precise location guidance in aircraft to fly instrument approach procedures.  In order to do that though, we need to be able to make corrections to the satellite signals.  In order to do this, there are a series of GPS receivers around North America whose sole purpose is to, well receive GPS signals from the various satellites.  Think of it like this:  A Garmin 430 sitting in a building somewhere just receiving signals.  These stations then take this collected information and send it off to a Wide-area Master Station, or WMS, to process the corrections.  This is done through standard terrestrially-based communication mediums.  Those master stations make the appropriate corrections and send those off to the uplink stations that then send corrected signal to the WAAS Satellites.
How does that corrected GPS information get back to you then?  An excellent question that many pilots incorrectly understand.  There are currently 3 satellites in geostationary orbit above the earth that receive these corrected signals.  These satellites then transmit the corrected signal data back down to your WAAS receiver to display corrected data.  All done in a matter of seconds, and all without any user intervention.
Practical Application:
Now that you have some basic knowledge about GPS, let’s put this into a scenario that you can apply to your next flight.  You are flying from San Diego to Monterey, CA.  The ILS is out of service so you decide that, since you have a Garmin 650 WAAS receiver, you will plan to fly the RNAV 10R approach since the winds favor that runway.  What minimums do you plan for?  Many pilots will look at the approach chart and determine that the LPV, since it has the lowest minima, is the way to go and brief it accordingly.  How do you know if you can fly that approach though?  Your Garmin 650 is continuously receiving signals from the WAAS satellites and if it is determined that the integrity of the signal will be compromised, the approach will degrade to the next lower minima (LNAV in this case) and you will see an annunciation on the unit.  This happens approximately 1 minute from the final approach fix.

GPS is an amazing technology that will continue to be improved upon for the foreseeable future. It is important that as pilots, we understand the underlying technology not just because an examiner may ask it someday, but because our knowledge of these systems can directly affect the outcome of our flight.

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