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Chapter 11 - Radio Navigation: The Frequency Flyer Program
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How GPS Works Inside
The little GPS receiver in your panel (or hand) is a rather remarkable device.
It’s doing all the things you wished you’d done in math class, such as getting
the right answer, and it’s doing them a lot quicker than you and I ever did.
Ranging and calculation of position from a group of satellites in space,
which act as precise reference points, is the basis of operation for GPS oper-
ation. GPS receivers use data from a minimum of four satellites above some-
thing known as the mask angle, as shown to the right (the lowest angle above
the horizon at which the GPS unit can use a satellite). The GPS takes complex
information from four or more satellites and in a virtual instant calculates
where it and you are. Try doing that in your head, whiz kid.
GPS receivers measure what we might best call a “working distance”
from a satellite, using the travel time of a radio signal (even though no actu-
al radio may be involved). Because the clock in the airborne receiver is not
nearly as accurate as the atomic clock carried on each satellite, the
elapsed time for the signal to reach the receiver is not known with perfect
accuracy. However, the answer is equally incorrect for each satellite being
observed, which is what enables the receiver to eventually calculate the
correct answer.
Multiplying the approximated elapsed time by the speed of light (the rate at
which the signal traveled from satellite to receiver) yields the working distance
also known as the pseudorange.
In addition to knowing the pseudorange to a satellite, your little GPS receiver
needs to know the satellite’s exact position in space, which is known as its ephemeris.
Sounds temporary, doesn’t it? Each satellite transmits information about its exact orbital location, which the GPS receiver uses to
precisely establish the position of the satellite.
Using the calculated pseudorange and position information supplied by the satellite, the GPS receiver/processor mathematically
determines its position by calculating the one precise spot (accurate to within about 60 feet) at which a mathematically-construct-
ed sphere (actually, a hyperboloid, for those who really liked math class) around each of the satellites’ intersects. This sphere has
a radius equal to the time delay between the satellite and the receiver multiplied by the speed of light, and you need a minimum of
four hyperboloids to mathematically arrive at one unique point in space.Yes, really. And frequently, too. While doing that calcula-
tion once would probably take you all day (or the rest of your life) with pencil and paper, your GPS unit repeats it many times
every minute and it never gets tired or wants to go to recess.
While it’s at it, and to keep from getting bored while onboard, the GPS receiver computes navigational values (e.g., distance and
bearing to a waypoint, groundspeed, crosstrack error, dinner time, etc.) by using the aircraft’s known latitude/longitude and refer-
encing these to a database built into the receiver.

Let’s say, for instance, that you wanted to GOING DIRECT WITH GPS
go to WACKO intersection, as shown in
Figure 32, position A. Select the letters
WACKO in the GPS’s window, and push
the appropriate buttons. Now you legiti-
mately have the means to go WACKO. If
your GPS has a moving map display, you
can see your track to WACKO (or, your
tracko to wacko) along with other informa-
tion such as groundspeed, track, crosstrack
error (more on this in a bit) and so on
(Figures 32B and C).
GPS is only one form of area navigation.
For instance, airliners have LNAV (lateral
navigation) equipment. Airliners also have
flight management systems (FMS), VLF/
Omega, and the others we’ve already dis-
cussed. GPS is the most common form in use
by general aviation IFR pilots. So it’s the one
Fig. 32
we’ll spend a lot of time discussing. From now
on, if you see the term RNAV (GPS), it means Going direct to an intersection is a fairly simple process with
area navigation based strictly on GPS. many GPS units, often involving no more than a few steps.