GPS – where is it taking you?
The three major segments of GPS - Space, Control, and User
The Space Segment
An original constellation of 24 satellites in six orbital planes (four in each plane) are used to send
coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute
position, velocity and time. The satellites are spaced 60 degrees apart and are positioned at an
altitude of 20,200 km (12,552 miles) with a 55-degree inclination. In addition to the 24 satellites
in the constellation, three additional satellites are in orbit and will eventually replace older space
vehicles. (Dana, Peter H., 2001)
Control
The Control segment consists of five Monitor Stations (located in Hawaii, Kwajalein, Ascension
Island, Diego Garcia, and Colorado Springs), three Ground Antennas, (located at Ascension
Island, Diego Garcia, and Kwajalein), and a Master Control Station (MCS) located at Schriever
AFB in Colorado. The monitor stations passively track all satellites in view, accumulating
ranging data. This data is processed at the MCS and incorporated into satellite orbital models.
The updated orbital information, also called ephemeris data, is then transmitted to each satellite
via the Ground Antennas and is sent with each satellite’s navigation message. (NASA – Jet
Propulsion Laboratory, 2001)
User Segment
The GPS User Segment consists of the GPS receivers and us, the user community. The 24
satellites in their respective orbits provide the user with five to eight satellites visible any where
on the earth to receive data. Satellite signal arrival times from at least four satellites are
processed to estimate four quantities, position in three dimensions (X, Y, and Z) and GPS time
(T). The receiver then computes position dimensions in Earth-Centered, Earth-Fixed X, Y, Z
(ECEF XYZ) coordinates. (Dana, Peter H., 2001)
The computations are based on simple principles of velocity x travel time = distance, which is
somewhat like the old school math problem “If a train leaves Chicago traveling at speed of 30
miles an hour and travels for two hours, how far did it go?” (Trimble Navigation Limited, 2001)
The signals are then processed, not unlike triangulation. One factor that complicates the situation
is that the signals are traveling at the speed of light. So the difference between the arrival times
of the signals is minute. Since the arrival times of the satellite signals are such a critical factor in calculating a position, each satellite is equipped with four atomic clocks, two cesium and two
rubidium. Satellite clocks are monitored by Ground Control Stations and occasionally reset to
maintain time to within one-millisecond of GPS time. This information is then transmitted to
each satellite via the Ground Antennas and is sent with each satellite’s navigation message along
with its ephemeris data. (Dana, Peter H., 2001)
The Signal
The satellites transmit information via two radio waves that can be picked up by GPS receivers.
Each radio wave is modulated so that it can carry specific information. The modulated signal
resembles random electrical noise, but since the signal is not random but coded (and therefore
follows a pattern), it is referred to as a pseudorandom code. The radio waves, which carry the
pseudorandom codes, are distinguished by the designations of L1 and L2, and each carries
different information in its modulated code. L1 carries the Coarse Acquisition (C/A) Code used
by the civilian sector (free of charge) and is also modulated to carry the Navigation Message and
other satellite system parameter information. L2 carries the Precise Code (P-Code) used by the
military. The P-Code is encrypted and can only be received by specific receivers equipped with
key codes used to decipher the signal. (Dana, Peter H., 2001)
The pseudorandom code for each satellite is distinct, which makes it easy for GPS receivers to
distinguish between one satellite and another. In this way GPS receivers can tell exactly which
satellites make up a given configuration. This is important since the signals are very weak. So a
GPS receiver identifies one signal and, using built in almanacs, actually searches for signals from
the other satellites it thinks should be in the configuration. Once it has identified all of the
satellites in the configuration it then begins tracking their signals.
The GPS receiver then mimics or mirrors the pseudorandom code for each of the satellites and
compares the differences between its own code and the one received. It is able to do this because
it knows the fluctuations in the pseudorandom code. It then matches up one known point in the
signal received and its own and begins to make calculations.
To illustrate how this works, imagine that a satellite was playing Iron Butterfly’s “In-A-Gadda-Da-
Vida” from space. At the exact same time, you are sitting in a lounge chair also listening to
“In-A-Gadda-Da-Vida”. As you listened to the version you are playing against the one from
space, you would notice that the version from space was delayed slightly. This is because it takes
some time to travel the distance from the satellite in space to your lazy-boy. To determine the
distance, you could slow your version to match the one from space (which is hard to do on a
45rpm) until they were synchronized. Since the time shift between the two versions of “In-A-Gadda-
Da-Vida” is equal to the travel time of the satellites version, we simply take the time shift
between versions and multiply it by the speed of light and presto, we determine the distance
traveled! (Trimble Navigation Limited, 2001) This same calculation is then made for each signal
received and used to pinpoint a location.
A word about Accuracy
Several factors will affect the accuracy of your readings. Visibility or line of sight is crucial,
since at least four satellites are needed to accurately locate your position. Buildings, mountains and even tree canopy can affect how many satellites you are “seeing” and may prevent you from
“seeing” enough satellites to use in deciphering your location.
Since the pseudorandom signal sent by satellites resembles electrical noise, receivers at times
actually have trouble distinguishing the signal from “true noise” in space caused by solar flares
or other naturally occurring events. Good receivers are better equipped to decipher the noise and
filter it out. Nevertheless it is a distorting factor. (Dana, Peter H., 2001)
An event known as multi-pathing may give you false readings via signal reflection. Multi-pathing
occurs when a nearby object or surface is reflecting or bouncing the satellite signal to
your receiver, making it think that it is the true line of sight reading. The reflected signal is
received and is computed as a real signal and causes an effect similar to the ghosting of your TV
screen. This occurrence is not likely to happen on the open sea but may be experienced in city
locations where there are many surfaces. Multi-pathing can be hard to detect or even avoid, so
good receivers are equipped to try and detect and then reject the reflected signal when multi-pathing
occurs.
Variances in the atmosphere may also cause distortions in your readings. Changes in
temperature, pressure, and humidity affect the troposphere or the lower part of the atmosphere,
which can delay readings. Imagine a glass of water with a spoon sitting in it. The portion of the
spoon in the water appears distorted in relation to the portion out of the water. This is because
light is slowed ever so slightly as it travels through the water distorting the image. This same
effect occurs as the signal sent by a satellite is slowed as it passes through the water vapor in the
air. In addition, delays can occur in the ionosphere, which consists of charged air particles 50 to
500 km in the atmosphere. These delays, although slight, are significant enough to effect
calculating a good fixed coordinate.
A term that you may hear in reference to GPS accuracy is GDOP or Geometric Precision of
Dilution. GDOP is made up of other components such as PDOP (Position Dilution of Precision),
HDOP (Horizontal Dilution of Precision), VDOP (Vertical Dilution of Precision), and TDOP
(Time Dilution of Precision). Even though each of these can be calculated independently, they all
make for a good or bad GDOP reading. (Dana, Peter H., 2001)
To illustrate, you may be receiving signals from four satellites that all happen to be right on top
of you or all in a straight line in front of you on the horizon. (This configuration is not possible
but is used as an example to exaggerate the point). Therefore, even though you have four
readings, the configuration of the satellites does not allow enough variance between the angles of
the readings to obtain a good PDOP, which would require a better spread in the satellite
configuration. Good receivers will automatically pick out the best satellites from a given
constellation makeup to give you the best PDOP, which will make for a good GDOP.
Another factor that cannot be overlooked is Dick Clark’s Law of Goofs and Blunders. This law
states that not even GPS is immune to the occasional software glitch, hardware malfunction or
good ole’ operator error, which might make for good video clips but bollixes your GPS data.
In a perfect world, coordinates can be fixed from just three satellites. This would give us all the
needed data to calculate accurately any given position on the earth. As we have just discussed, however, there are many factors that can delay, distort or mirror signals received giving our GPS
units fits trying to filter out erroneous data. That is why signals from four satellites are used. The
extra measurement helps our receivers to verify against the fourth signal how well it is
computing our position. This increases our chances of obtaining an accurate measurement in
fixing our position.
Differential GPS
The purpose of Differential GPS is to correct errors that may creep in due to numerous factors. It
accomplishes this by taking satellite readings at a known fixed location. It then takes where the
satellite tells it that it is located and compares that against where it knows it is located and
computes an error calculation. That data is then passed onto the roving (or differential) receiver
and used to correct for errors. This type of DGPS is known as “real time” DGPS and requires
that the receiver be outfitted to receive and process this information. How well this process
works is dependent on the quality of the receivers and the distance between the two points. The
range can be anywhere between 30 to 200 kms. The concept is predicated on the fact that (to the
degree possible) the atmospheric conditions are alike at each location.
In instances where “real time” is not a critical factor, the fixed location readings are collected
and processed by computer against the points taken by the roving receiver at a later time. This is
known as “post-processing” and can only be done if the fixed location is taking readings at the
exact time that the rover is also taking readings. If a field technician is taking readings and the
fixed station happens to be down during that time, all of the readings taken will be useless since
there is no fixed location information to be processed against the collected points.
