1. Errors & Error Correction in GPS Satellite
The Most Common Causes of GPS Error Are:
- Incorrect placement of satellites / satellite geometry.
- Atmospheric conditions in the ionosphere and the
troposphere that may affect how the rays travel
between satellite and receiver.
- Atomic clock error - this is the clock built within
the actual satellite. It is adjusted for some margin
of error, but if the clock is too far off, even by a
nano-second, this can translate into GPS errors
in distance on the ground of up to several feet.
- Large buildings or topographic interference resulting
in refraction errors (the signal is blocked or bounces back).
This results in a "multi-path" errors causing two signals.
- Angle at which the satellites are placed to each other:
Ideally, satellites should be placed equidistant and at
a 90 degree angle for the best communication. Minor
deviations can result in large problems.
- Gravitational shift - satellites affected by the earth's gravity,
according to the theory of general relativity: these errors
are adjusted using the Lorentz theory.
- Solar flares - these are eruptions on the surface of the sun,
areas that are highly magnetized that wreak havoc on GPS.
We know how GPS works and how well it can work, but what can go
wrong and why? There are myriad factors when considering how well
a GPS signal is transmitted and received. It helps to keep in mind that
the GPS signal is really not so different from any other wave that
travels through the air (say, the speed of light for example). Knowing
this and understanding this, we know that radio waves can meet with
interference, and likewise, light-waves. Therefore, we know that the
same rules apply to GPS but are a little more complex because we
want to fix a large piece of machinery (man-made) which we are

2. bending to our will (or trying to), which is not that simple.
If a light-ray is blocked or bent, it's a little more simple, we can most
often remove the source blocking the ray or create an additional light
source. It may help to think of a GPS signal traveling through the
ionosphere and troposphere much the way a ray of light travels (and
we know too that GPS signals travel at the speed of light). Well, it can
meet with much of the same interference; sources that block the wave
and prevent it from meeting the intended receiver. More, GPS rays are
subject to gravitational pull, solar flares - these can wreak havoc on
the system. But before we even get to those sources of interference,
the most important aspect of GPS efficacy is the exact placement of
the satellites in the firmament. This is essential for proper and
accurate functioning.
How does a GPS signal work? Simplified, one GPS satellite sends a
signal from a ground location which is called the unknown point of
origin. This signal is then relayed to one or more GPS satellite(s) in
orbit. The information is then computed based on various factors:
a. The time of the signal (when it was sent, when it was received -
these are exact measurements).
b. Once the receiver satellite knows the exact time the signal was
sent, that time is then multiplied by the Speed of Light (satellite signals
travel at the speed of light, 186,000 per second). The answer to the
equation is the distance.
For GPS to work accurately, certain variables must be known: when
the signal left the first receiver and b., when it was picked up by the
second receiver. Any interference in between this process can cause
GPS error or failure.
So what can really go wrong? Well for one, the first premise, we know
is that the speed of light (note: the signal for GPS which is
approximately the same value) are only constant in a vacuum and we
are not operating in a vacuum. Instead, we are dealing with constant
variables. The equation for correct GPS is as follows; The time the

3. first signal leaves the GPS transmitter, the satellite position at the time
of transmission (reception), multiplied by the speed of light (186,000
miles per second). However, it's not quite that simple: GPS first sorts
out a "pseudo-range" which is an approximation of the distance from
satellite to receiver. This in turn defines a certain sphere (up for three
or four satellites can be used to determine one position). With this
information, knowing the speed of light and accounting for margin of
error, the GPS transmits back a signal.
But there are many things that can interfere with a GPS satellite
signal. Just as light itself can be refracted, scattered, altered and
sometimes even obscured, so it is with a GPS signal. With GPS, we
have to make adjustments for atmospheric conditions as the signal
travels through the ionosphere and troposphere.
Sometimes, during the signal's journey, the signal is refracted (again,
the way light can be refracted by a tall building or a boulder - many
things can cause this refraction) - even weather system could cause
some inaccuracy in GPS or humidity. More troublesome however are
sunspots (again, Galileo first noted these), which are highly
magnetized can create sun-flares that cause interference making it
difficult to get an accurate GPS read.
Other errors relate more to the actual GPS satellite and its inner-workings/
mechanics. For example, even a minor variation in the
atomic clock (each satellite must have a clock to function properly to
relay time in the necessary equation), can result in quite a large error.
How? A seemingly minor clock error of, say, a single nanosecond
translates into a distance between one and three metres on the
ground; that's a significant margin of error.
Because GPS is essentially a wavelength (just as light and radio are),
the signal can be blocked by large buildings (often a problem in high-density
urban areas where there are large structures that may
interfere with the signal. More, the signal may encounter another
reflective surface before it reaches the partner satellite's antenna and
bounce off of that surface. When this happens, we see what is known
as a multi-path error. Roughly translated this means that there are two

4. signal responses when there ought only be one (a direct line between
receiver and satellite). When a third object is introduced, it creates
another line (the second line). When both signals (lines) are relayed at
the same time then we have "multi-path error" which looks like an
overlay of two images (one correct, the other a sort of "ghost image") -
a duality.
Most of how GPS operates comes down to geometry and physics (if
you thought geometry was not important, think again.) GPS relies
heavily on geometry and exact placement of the satellites in our
firmament. A satellite tipped at the wrong angle will cause many
errors. Of utmost importance for proper GPS functioning is the exact
layout of the whole network of satellites.
Imagine a web or cage of satellites that surround the globe and are in
orbit, each relaying signals. How far these satellites are spaced apart
from each other is critical (this is called "satellite geometry"). The
satellites need to be evenly distributed over the network. The wider
the angle between satellites, the better the result will be. Distribution
of precision by satellites or satellites angled incorrectly will relay a
poor signal or an incorrect signal. For the best coverage, we need
even coverage (again, think evenly-spaced network) and with the
specific angle that has been proven to work best (generally a 90
degree position). When the satellites are incorrectly placed in their
orbit, scientists call this "Dilution of Precision". Re-positioning the
satellites (redistributing them evenly) is the best solution, however
there are mathematical models that help sort out the margin of error
and the satellite then makes the necessary adjustments, generally
related to its atomic clock.
Finally, there is one last thing to consider when looking at GPS margin
of error and that is the Earth itself which, depending on where the
satellite is (closer or farther away), will create a notable gravitational
shift which will affect time (the single biggest factor in accuracy). A
clock closer to a large object will be slower than a clock farther away
due to the theory of general relativity. This means that GPS satellites
in orbit (and their atomic clocks, which is really what we are looking
at), will be faster than those that are closer to the earth. There is a

5. calculation that can be made for the adjustment that is based on the
Lorentz transformation which in part factors in the fact that a satellites
orbit is elliptical (not circular), which changes the equation.
In Summary
As GPS continues to develop, both within the States and worldwide
(as well as with increasing worldwide cooperation), it is likely that
these errors will become fewer. Some however, are bound to remain:
the sun will remain as will solar flares, obstacles of refraction will
remain and so forth. Despite all of this however, the most remarkable
thing at all is that GPS works at all when one considers what could go
wrong (and often does) and just how far we have come in correcting
and adjusting for those margins of error. Physicists and scientists of
the past (including Einstein, Lorentz, Galileo, among many others)
helped set our man-made "stars" in motion. So what has changed?
We're still navigating by the skies, only our skies now have a little help
from mankind.
- 30 -

6. calculation that can be made for the adjustment that is based on the
Lorentz transformation which in part factors in the fact that a satellites
orbit is elliptical (not circular), which changes the equation.
In Summary
As GPS continues to develop, both within the States and worldwide
(as well as with increasing worldwide cooperation), it is likely that
these errors will become fewer. Some however, are bound to remain:
the sun will remain as will solar flares, obstacles of refraction will
remain and so forth. Despite all of this however, the most remarkable
thing at all is that GPS works at all when one considers what could go
wrong (and often does) and just how far we have come in correcting
and adjusting for those margins of error. Physicists and scientists of
the past (including Einstein, Lorentz, Galileo, among many others)
helped set our man-made "stars" in motion. So what has changed?
We're still navigating by the skies, only our skies now have a little help
from mankind.
- 30 -