Three Segments of the Global Positioning System
The
Global Positioning System is comprised of three segments: the Control Segment,
Space Segment and User Segment.
Control Segment
The
Master Control Station, or MCS (also known as the Consolidated Satellite
Operations Center) is located at the US Air Force Space Command Center at
Schriever Air Force Base (formerly Falcon AFB) in Colorado Springs, Colorado.
The MCS responsible for satellite control and overall system operations. The
Control segment is made up of a Master Control Station (MCS), four monitor
stations, and three ground antennas (plus a reserve antenna at Cape Canaveral
used primarily for pre-launch satellite testing) used to uplink data to the
satellites. Monitor Stations continuously receive GPS satellite transmissions,
and relay this information in real time to the Master Control Station in
Colorado. The user segment also receives these same transmissions.
Monitor stations (MS) are located at Schriever Air Force
Base, Hawaii, Kwajalein Atoll, and Diego Garcia, and Ascension islands. These
stations are unmanned remote sensors that passively collect raw satellite
signal data and re-transmit it in real time to the MCS for evaluation. Monitor
stations basically function as very precise radio receivers, tracking each
satellite as it comes into sky view. Ground antennas are remotely controlled by
the MCS. They are also located at Ascension, Diego Garcia, Kwajalein Atoll, as
well as Cape Canaveral, Florida. Ground antennas transmit data and commands
from the Master Control Station to GPS satellites. The MCS uplinks data to GPS
satellites, which includes:
-Clock-correction
factors for each satellite; necessary to insure that all satellites are
operating at the same precise time (known as “GPS Time”).
-Atmospheric
data (to help correct most of the distortion caused by the GPS satellite
signals passing through the ionosphere layer of the atmosphere).
-Almanac,
which is a log of all GPS satellite positions and health, and allows a GPS
receiver to identify which satellites are in its hemisphere, and at what times.
An almanac is like a schedule telling a
GPS receiver when and where satellites will be overhead. Transmitted
continuously by all satellites, the almanac allows GPS receivers to choose the best
satellite signals to use to determine position. The almanac is automatically
downloaded from satellites whenever a receiver is collecting a GPS signal. An
almanac can also be downloaded from a computer, a base station or other
archived almanac.
-Ephemeris
data is unique to each satellite, and provides highly accurate satellite
position (orbit) information for that GPS satellite alone. It does not include
information about the GPS constellation as a whole. Ephemeris information is
also transmitted as a part of each satellite’s time signal.
By
using the information from the GPS satellite constellation almanac in
conjunction with the ephemeris data from each satellite, the position of a GPS
satellite can be very precisely determined for a given time.
Space Segment
The
Space Segment is an earth-orbiting constellation of 24 active and five spare
GPS satellites circling the earth in six orbital planes. Each satellite is
oriented at an angle of 55 degrees to the equator. The nominal circular orbit
is 20,200-kilometer (10,900 nautical miles) altitude. Each satellite completes
one earth orbit every twelve hours (two orbits every 24 hours). That's an
orbital speed of about 1.8 miles per second, and each satellite travels from
horizon to horizon in about 2 hours.
Each
satellite has a design life of approximately 10 years, weighs about 2,000
pounds, and is approximately 17 feet across with its solar panels extended.
Older satellites (designated Block II/IIA) still functioning are equipped with
2 cesium, and 2 rubidium atomic clocks. Newer satellites (Block IIR) are
equipped with rubidium atomic clocks. All satellites also contain 3
nickel-cadmium batteries for backup power when a satellite is in earth eclipse
(out of view of the sun).
Each satellite transmits as part of its signal to ground
stations and GPS receivers the following information:
-Coded
ranging signals (radio transmission time signals that allow a GPS receiver to
triangulate its position).
-Ephemeris
position information (a message transmitted every 30 seconds containing precise
information on the location of the satellite in space).
-Atmospheric
data (information to help correct interference of the signal as it travels
through the earth’s atmosphere).
-Clock
correction information defining the precise time of satellite signal
transmission (in GPS Time), and a correction parameter to convert GPS Time to
Universal Coordinated Time (UTC).
-An
almanac containing information on the GPS constellation, which includes
location and health of all the satellites. Whenever a GPS receiver is receiving
a satellite signal it is automatically downloading an almanac. This almanac is
stored in the receiver’s memory for future use. The stored almanac allows a
receiver to more quickly acquire GPS satellite signals because it already knows
the general location, and other information, about the satellites in the
constellation. However, if a GPS receiver is left turned off for several
months, or is moved more than 300 miles while turned off, the stored almanac
may not be of any use to the receiver when it is turned on. A new almanac will
be need to be downloaded for the receiver to function properly.
The Four Basic
Functions of the GPS
The
primary functions of the GPS fall into four categories:
1)
Position and waypoint coordinates: A GPS receiver can provide position or
waypoint information for its current location or any remote location on earth,
and display that information in a variety of coordinates.
2) The
distance and direction between a receiver’s position and a stored waypoint, or
between two remote waypoints.
3)
Velocity reports: Real time distance to any waypoint; tracking to a waypoint;
heading (direction of travel); current speed; estimated time of arrival to a
waypoint, course over ground, etc.
4) Accurate
time measurement: GPS has become the universal timepiece, allowing any two
receiver clocks (as well as any two clocks or watches) to be precisely
synchronized anywhere in the world. The
Global Positioning System operates using “GPS Time,” which varies slightly from
Universal Coordinated Time (UTC). A GPS
receiver corrects GPS Time anomaly to match UTC time (also known as “Zulu Time”
or “Greenwich Time”), which is then offset by local time zone entered into the receiver
by the user.
How a Receiver Determines
Its Position
Traveling
at the speed of light, each satellite PRN signal takes a brief, but measurable
amount of time to reach a GPS receiver. The difference between when the signal
is sent and the time it is received, multiplied by the speed of light, enables
a GPS receiver to accurately calculate the distance between it and each
satellite, provided that several factors are met.
Those
factors are:
Good
satellite signal lock by the GPS receiver (already covered)
A
minimum of four satellite signals (discussed next)
Good
satellite geometry (discussed later)
When a
GPS receiver is turned on it immediately begins searching the sky for satellite
signals. If the receiver already has a current almanac (such as one acquired on
a previous outing), it speeds up the process of locating the first satellite
signal. Eventually it locates and acquires its first signal. Reading this
signal the receiver collects the Navigation Message. If the receiver does not
have a current almanac, it must collect a new almanac, which will take about
12-13 minutes after the first satellite signal is acquired. The almanac is
automatically updated during normal use.
In the
above graphic, the GPS receiver has calculated a rough location that places it
somewhere on the three dimensional sphere, which is actually thousands of miles
in diameter. All the receiver can really do at this point is collect system
data and search for more satellite signals.
How a Receiver
Determines Its Position (cont.)
For
most receivers three satellites can only provide a two-dimensional (2D)
position. Without manually entering the receiver’s exact elevation (most
GPS receivers don’t allow elevation to be entered manually), the rendered 2D
position may be off by several kilometers on the ground. If the exact elevation
of the GPS receiver is known, entering that elevation into a receiver with this
capability replaces the need for a fourth satellite signal to allow a receiver
to triangulate a precise position. The receiver essentially uses elevation in
lieu of a fourth satellite, and makes the appropriate adjustments to
trilaterate a reasonably good 3D position.
But
without manual elevation correction most GPS receivers must rely on a fourth
satellite to provide the final clock correction information necessary to
calculate a 3D position. Until a fourth satellite signal is acquired the
receiver will not be able to determine x and y horizontal, and z vertical
positioning (a true 3D position). This is because the fourth satellite signal
is used by the receiver not to provide more position data, but, rather, the
final time correction factor in its ranging calculations.
As a
rule, 2D positions should always be avoided whenever possible. Use 2D
positioning only when a 3D position is not possible, but be aware of the
horizontal error inherent in any 2D position. The inability of a GPS receiver
to triangulate a 3D position may be due to a variety of factors, including user
error, poor satellite geometry, and harsh landscape conditions (tall buildings,
canyons, and dense tree cover among others). As will be shown later in the
course, all GPS receivers provide some means for informing the user which mode
they are operating in. It’s up to the user to be aware of the errors associated
with 2D positioning.
How a receiver determines its position (cont.)
For a
GPS receiver to achieve three-dimensional (3D) positioning it needs to
acquire four or more satellite signals. A 3D position is comprised of X and Y
(horizontal), Z (vertical) positions, and precise time (not varying more than a
few hundred nanoseconds). The receiver’s processor uses the fourth satellite
pseudo-range as a timing cross check to estimate the discrepancy in its own
ranging measurements and calculate the amount of time offset needed to bring
its own clock in line with GPS Time (recall the radio station and record player
simultaneously playing the same song). Since any offset from GPS Time will
affect all its measurements, the receiver uses a few simple algebraic
calculations to come up with a single correction factor that it can add or
subtract from all its timing measurements that will cause all the satellite
spheres to intersect at a single point (x, y, and z).
That
time correction synchronizes the receiver's clock with GPS Time. Now the
receiver essentially has atomic clock accuracy with the time correction factor
needed to achieve precise 3D positioning. The pseudo-ranges calculated by the
GPS receiver will correspond to the four pseudo-range spheres surrounding the
satellites, causing the four spheres to intersect at precisely the receiver’s
location (the dot in the diagram).
Selective Availability (Anti-Spoofing)
Selective
Availability (S/A) was the intentional degradation (referred to as “dithering”)
of the Standard Positioning Service (SPS) signals by a time varying bias.
Selective Availability is controlled by the Department of Defense to limit
accuracy for non U. S. military and approved users. The potential accuracy of
the coarse acquisition (C/A) code at around 30 meters was reduced by Selective
Availability up to 100 meters. In May, 2000, the Pentagon set Selective
Availability to zero. The Pentagon did not turn S/A off, but rather merely
reduced the amount of signal dithering to zero meters, effectively eliminating
intentional position errors for Standard Positioning Service users.
Sources of Signal
Interference (cont.)
Selective Availability (see previous slide).
Control Segment blunders due to computer glitches or
human error can cause position errors from several meters to hundreds of
kilometers. Checks and balances by the Air Force Space Command virtually
eliminates any blunders in the Control and Space segments of the Global
Positioning System.
User mistakes account for most GPS errors on the
ground. Incorrect datum and typographic errors when inputting coordinates into
a GPS receiver can result in errors up to many kilometers. Unknowingly relying
on a 2D position instead of a 3D position can also result in substantial errors
on the ground. A GPS receiver has no way to identify and correct user mistakes.
Even the human body can cause signal interference.
Holding a GPS receiver close to the body can block some satellite signals and
hinder accurate positioning. If a GPS receiver must be hand held without
benefit of an external antenna, facing to the south can help to alleviate
signal blockage caused by the body because the majority of GPS satellites are
oriented more in the earth's southern hemisphere.
Errors in GPS are cumulative, and are
compounded by position dilution of precision (PDOP) (covered later). It is the
user’s responsibility to insure the accuracy of the data being collected with
the GPS.
Ideal Satellite
Geometry
Satellite
geometry refers to the positions of satellites relative to each other in space.
Dilution of Precision (DOP) is an indicator of the quality of a GPS receiver’s
triangulated position relative to the quality of the geometric positions of the
satellites whose signals the receiver is using. GPS receivers get satellite
position information from the ephemeris message sent as part of the data stream
from each satellite.
Dilution
of precision uses numerical values to represent the quality of satellite
geometry, from 1 to over 100. The lower the number, the better the accuracy of
position fixes. Some high-end GPS receivers (such as Trimble data loggers) have
a default PDOP setting of around 8, and the value can be changed to meet the
needs of the user. Garmin receivers do not allow PDOP manipulation by the user,
nor do they provide a PDOP value. Instead they use estimated position error
(EPE) value in feet or meters, which provides an estimate of the amount of
horizontal error caused by poor satellite geometry.
The
outer ring of the circle in the above diagram represents the earth’s horizon.
The center of the cross hair represents the sky directly above the GPS
receiver. The satellite configuration shown is considered optimal for providing
the best 3D positioning because any horizontal error from one direction will be
offset by the opposing satellites. The fourth satellite directly overhead
improves vertical accuracy.
Poor Satellite
Geometry
(Note: To properly view the animation in this diagram, use
Slide Show feature of PowerPoint.)
The
locations of satellites in relation to each other in space at any given time
can affect the quality of a GPS receiver’s position fix. Spaced low on the
horizon, with no satellite directly above the receiver, can result in high
PDOP. Similarly, if all satellites acquired by a receiver are bunched closely
together in one quadrant of the sky can also result in poor triangulation
measurements (and a high PDOP). Topography on the ground also affects satellite
geometry. A receiver inside a vehicle, near tall buildings, under dense canopy,
or in mountainous terrain can be affected by blocked signals. GPS receivers
require clear line of sight to every satellite being acquired.
The
above diagram is a PowerPoint animation. Each part of the animation corresponds
to the following sets:
Satellite set 1: This satellite configuration results
in poor PDOP and HDOP, but good VDOP. This is an example of a poor satellite
configuration for achieving a precise position.
Satellite set 2: This satellite configuration
represents poor PDOP and VDOP, but good HDOP. It’s important to remember that
satellite geometry that is poor for one kind of DOP can actually reduce another
kind of DOP. If you need the best horizontal measurements, but don’t care about
vertical accuracy, then this example is an acceptable satellite configuration.
Satellite set 3: This satellite configuration
represents poor PDOP, VDOP, and HDOP. This is another example of a poor
satellite configuration.
How Good is WAAS?
The
Wide Area Augmentation System (WAAS) dramatically improves existing GPS
technology for positional accuracy (in the United States and portions of Canada
and Mexico). Under ideal conditions, with Selective Availability set to zero,
horizontal accuracy with GPS can be fifteen meters or less. Under the same
conditions with good WAAS signal acquisition that horizontal accuracy can be
reduced to as low as three meters or less on the ground.
Bear in
mind that many factors dictate the level of accuracy that can be achieved by
any GPS receiver on the ground. Among these factors include errors in the GPS,
multipath interference, atmospheric errors, closed canopy or other signal
blockers, and human error. Combined, these errors can degrade positional
accuracy to 100 meters or more. For WAAS, two downsides are its reduced
capability under heavy canopy (trees, canyons, etc.), and its limitation to
mostly the contiguous U.S. In fact, some studies have shown that WAAS signals
are degraded the further north from the 35 parallel one goes, reducing WAAS
reliability in northern latitudes.
