GNSS - Frequently Asked Questions - GPS
What is GPS?
is GPS used?
the status of the GPS?
- What are the service levels provided by GPS?
- What is Selective Availability (SA)?
- Why was SA necessary?
is the status of Selective Availability (SA)?
- Will SA ever be turned back on?
can civil users depend on a system controlled by the U.S. military?
- How many GPS satellites are there at a given time in the GPS constellation?
kind of orbits are the GPS satellites in?
do GPS accuracy and integrity compare to that of existing ground-based
navigation systems such as VOR/DME?
- Are there plans to increase the capabilities
vulnerable are GPS satellites to jamming and interference?
concerns are there regarding Radio Frequency Interference (RFI)?
the basic GPS signal sufficient to meet all the needs
augmentations to the basic GPS service is the FAA working on
- What is DGPS (Differential GPS)?
|Q. What is GPS?
GPS consists of three segments - the satellite constellation,
ground control network, and user equipment. The satellite constellation
comprises satellites in low earth orbit that provide the ranging
signals and navigation data messages to the user equipment.
The ground control network tracks and maintains the satellite
constellation by monitoring satellite health and signal integrity
and maintaining the satellite orbital configuration. Furthermore,
the ground control network also updates the satellite clock
corrections and ephemerides as well as numerous other parameters
essential to determining user position, velocity and time (PVT).
The user equipment receives signals from the satellite constellation
and computes user PVT. More details on each of the aforementioned
GPS segments are provided below.
GPS Satellite Constellation:
The baseline satellite constellation consists of 24 satellites
positioned in six earth-centered orbital planes with four operation
satellites and a spare satellite slot in each orbital plane.
The system can support a constellation of up to thirty satellites
in orbit. The orbital period of a GPS satellite is one-half
of a sidereal day or 11 hours 58 minutes. The orbits are nearly
circular and equally spaced about the equator at a 60-degree
separation with an inclination of 55 degrees relative to the
equator. The orbital radius (i.e. distance from the center
of mass of the earth to the satellite) is approximately 26,600
With the baseline satellite constellation, users with a clear
view of the sky have a minimum of four satellites in view.
It’s more likely that a user would see six to eight satellites.
The satellites broadcast ranging signals and navigation data
allowing users to measure their pseudoranges in order to estimate
their position, velocity and time, in a passive, listen-only
Ground Control Network:
At the heart of the Ground Control Network is the Master
Control Station (MCS) located at the Schriever (formerly named
Falcon) Air Force Base near Colorado Springs , Colorado . The
MCS operates the system and provides command and control functions
for the satellite constellation.
The satellites in orbit are continuously tracked from six
USAF monitor stations spread around the globe in longitude:
Ascension Island , Diego Garcia, Kwajalein , Hawaii , Cape
Canaveral and Colorado Springs . The monitor stations form
the data collection component of the control network. A monitor
station continuously makes pseudorange measurements to each
satellite in view. There are two cesium clocks referenced to
GPS system time in each monitor station. Pseudorange measurements
made to each satellite in view by the monitor station receiver
are used to update the master control station’s precise
estimate of each satellite’s position in orbit.
The user equipment, often referred to as “GPS receivers”,
captures and processes L-band signals from the satellites in
view for the computation of user position, velocity and time.
How is GPS used?
receivers collect signals from satellites in view. They display
the user's position, velocity, and time, as needed for their
marine, terrestrial, or aeronautical applications. Some display
additional data, such as distance and bearing to selected waypoints
or digital charts.
The GPS concept of operation is based upon satellite ranging.
Users determine their position by measuring their distance
from the group of satellites in space. The satellites act as
precise reference points.
Each GPS satellite transmits an accurate position and time
signal. The user's receiver measures the time delay for the
signal to reach the receiver, which is the direct measure of
the apparent range (called a "pseudorange") to the
satellite. Measurements collected simultaneously from four
satellites are processed to solve for the three dimensions
of position (latitude, longitude, and altitude) and time. Position
measurements are in the worldwide WGS-84 geodetic reference
system, and time is with respect to a worldwide common U.S.
Naval Observatory Time (USNO) reference.
Who uses GPS?
A. GPS is used to support land, sea,
and airborne navigation, surveying, geophysical exploration, mapping
and geodesy, vehicle location systems, farming, transportation
systems, and a wide variety of other additional applications. Telecommunication
infrastructure applications include network timing and enhanced
911 for cellular users. Global delivery of precise and common time
to fixed and mobile users is one of the most important, but least
appreciated functions of GPS.
What's the status of the GPS?
A. The Global Positioning System
reached Full Operational Capability (FOC) July 17, 1995. Per
U.S. Policy and Law, the GPS Standard Positioning Service is
available to civil users worldwide for their peaceful transportation,
scientific, and other uses free of direct user charges.
What are the service levels provided by GPS?
A. GPS provides two levels of service:
Standard Positioning Service (SPS) for general civil use; and
- a Precise Positioning Service (PPS) primarily intended
for use by the Department of Defense and U.S. allies.
There are no restrictions on SPS usage and is available to
users worldwide. With Selective Availability (SA) , SPS provides
predictable accuracies of 100m (2drms, 95%) in the horizontal
plane and 156m (95%) in the vertical plane. UTC (USNO) time dissemination
accuracy is within 340 nanoseconds (95%) referenced to the time
kept at the U.S. Naval Observatory. These accuracies reflect
the last signal specification in the Federal Radio Navigation
Plan which is in the process of being revised to reflect the
accuracy obtained following the deactivation of Selective Availability.
Without SA, SPS accuracy would be of the order of 25m (2 drms,
95%) in the horizontal plane and 43m (95%) in the vertical plane.
PPS provides a predictable accuracy of at least 22m (2drms,
95%) in the horizontal plane and 27.7m (95%) in the vertical
plane. PPS provides UTC (USNO) time transfer accuracy within
200 nanoseconds (95%) referenced to the time kept at the U.S.
PPS is primarily intended for military and select government
agency users. Civilian use is permitted but only upon special
U.S. Department of Defense approval.
What is Selective Availability (SA)?
A. SA was a technique
implemented by the DOD to intentionally degrade a user’s
navigation solution. The single largest source of error for SPS
users was SA. The net result of SA was about a five-fold increase
in positioning error. DOD achieved signal degradation by altering
(also known as dithering) the satellite clock. Another means
designed by DOD to degrade GPS performance was to broadcast less
accurate ephemeris parameters.
The DOD-authorized users were able to undo SA. However, due
to the fact that SA is spatially correlated, civil users were
able to eliminate SA through the implementation of Differential
GPS (DGPS), albeit an additional expense on the part of the users.
Why was SA Necessary?
A. SA was used to protect the security interests of the U.S. and its allies by globally denying the full accuracy of the civil system to potential adversaries.
What is the status of Selective Availability (SA)?
A. By order of the President of the United States, the use of Selective Availability was discontinued on May 1, 2000.
Will SA ever be turned back on?
A. It is not the intent of the U.S. to ever use SA again. To ensure that potential adversaries to do not use GPS, the military is dedicated to the development and deployment of regional denial capabilities in lieu of global degradation through SA.
How can civil users depend on a system controlled by the U.S. military?
GPS is owned and operated by
the U.S. Government as a national resource. DOD is the "steward" of
GPS, and as such, is responsible to operate the system in accordance
with the signal specification. The National Space-Based Positioning,
Navigation, and Timing (PNT) Executive Committee was established
by Presidential directive in 2004 to advise and coordinate federal
departments and agencies on matters concerning the Global Positioning
System (GPS) and related systems. This Committee replaced the
Interagency GPS Executive Board (IGEB), which oversaw GPS policy
matters from 1996 to 2004. The Executive Committee is chaired
jointly by the Deputy Secretaries of Defense and Transportation.
Its membership includes equivalent-level officials from the Departments
of State, Commerce, and Homeland Security, the Joint Chiefs of
Staff, and NASA. Components of the Executive Office of the President
participate as observers to the Executive Committee, and the
FCC Chairman participates as a liaison.
DOD is required by law to "maintain a Standard Positioning
Service (SPS) (as defined in the Federal Radionavigation Plan
and the Standard Positioning Service Signal Specification)
that will be available on a continuous, worldwide basis," and, "develop
measures to prevent hostile use of GPS and its augmentations
without unduly disrupting or degrading civilian uses." These
strict requirements and current augmentation systems should
actually make DOD use of the system transparent to the civil
user. (Note: There will, necessarily, be localized testing
of the system by military and development teams but the testing
will fall under strict notification guidelines of safety-of-life
users such as Coast Guard and FAA).
U.S. transportation, public safety, economic, scientific,
timing, and other users rely on GPS extensively. In aviation
and maritime transportation, GPS is used for "safety of
life" navigation and it is a critical system for these
applications. DOD is the steward of the system, responsible
to maintain the signal specification; the PNT provides management
oversight to assure that civil and military needs are properly
How many GPS satellites are there at a given time in the GPS constellation?
A. The exact number of satellites
operating at any one particular time varies depending on the number
of satellite outages and operational spares in orbit. For current
status of the GPS constellation, please visit http://tycho.usno.navy.mil/gpscurr.html.
What kind of orbits are the GPS satellites in?
The GPS satellites operate in
circular 10,900nm (20,200km) 12-hour orbits at an inclination
of 55 degrees. They are not in geo-stationary orbit.
How do GPS accuracy and integrity compare to that of existing ground-based
navigation systems such as VOR/DME?
A. The basic GPS signal is accurate
on a worst-case basis to within approximately 100 meters lateral
and 140 meters vertical everywhere on earth. GPS, as provided
to civil users, appears to be just as accurate as the most accurate service
being provided by the VOR/DME, i.e., non-precision approaches. It should be
noted that VOR accuracy degrades as you move farther away from the navigation
aid. GPS accuracy is space-based, and thus not constrained by ground equipment.
The basic GPS signal is not as accurate as the existing ILSs; however, augmented
by WAAS and GBAS, GPS will be able to supply a precision approach capability
(CAT-I with WAAS and progressing to CAT-II/III with GBAS).
Are there plans to increase the capabilities of GPS?
A. Yes, one of the main components
of GPS modernization is the addition of two new navigation
signals for civil use. These signals will be in addition to
the existing civilian service broadcast at 1575.42 MHz (L1).
The first of these new signals will be a new civil code, called
L2C, which will be added on the existing L2 carrier, located
at 1227.60 MHz. It will be available for general use
in non-safety critical applications. The Block IIR-M satellite,
the first to add his capability was launched September 25,
A third civil signal, located at 1176.45 MHz (L5), will be
provided initially on GPS Block IIF satellites beginning in
2007, and continuing with the Block III satellites scheduled
for launch beginning in 2012. This new L5 signal is protected
worldwide for aeronautical radionavigation use, and will support
aviation safety-of-life applications. The addition of L5 will
make GPS a more robust radionavigation service for many aviation
applications, as well as all ground-based users (maritime,
railways, surface, shipping, agriculture, recreation, etc.)
At the current GPS satellite replenishment rate, all three
civil signals (L1-C/A, L2C, and L5) will be available for initial
operational capability by 2012, and for full operational capability
by approximately 2015. For more information on GPS modernization
activities, please visit our GPS
Modernization page and http://pnt.gov.
How vulnerable are GPS satellites to jamming and interference?
A. GPS satellite signals, like
any other navigation signals, are subject to some form of interference.
The FAA is actively working with the U.S. Department of Defense
and other U.S. Government Agencies to detect and mitigate these
effects and make sure that the GPS and any related augmentation systems are
available for safe aviation operations. As with all navigation aids, interference,
whether intentional or unintentional, is always a concern. A number of methods
for minimizing interference have been identified and tested and others are
being investigated. The FAA is also working to make sure augmentation systems
detect and mitigate these effects.
What concerns are there regarding Radio Frequency Interference (RFI)?
A. As with all navigation aids,
Radio Frequency Interference (RFI), unintentional or intentional,
is always a concern. The FAA is evaluating several GPS interference
detection systems, which will determine the direction and source of the GPS
interference. The FAA is also working with DOD and other agencies to make sure
that GPS augmentation systems detect and mitigate the effects of interference.
Is the basic GPS signal sufficient to meet all the needs of civil
A.This is not a simple yes/no answer. The answer is that it depends on the service
requirements of each user or aviation authority. For many countries, GPS supplies
a better capability than the existing ground-based systems or lack thereof. Yet
for other countries with large infrastructures, the GPS signal does not meet
the accuracy, integrity, availability, and continuity requirements critical to
safety of flight. Enhancements to the Global Positioning System (GPS) such as
the Wide Area Augmentation System (WAAS) and Ground Based Augmentation System (GBAS)
provide the necessary corrections for meeting safety-of-life flight requirements.
What augmentations to the basic GPS service is the FAA working on
A. The FAA is developing
the Wide Area Augmentation System (WAAS) and the Ground Based Augmentation
System (GBAS). Both augmentation systems focus on the same concerns:
integrity, availability, and accuracy.
Integrity is the ability to alert users, within
a prescribed number of seconds (depending on the type of flight
operation), when GPS should not be used for navigation. Availability
is needed to assure users that the basic GPS civil service is
accessible nearly 100% of the time. Accuracy enhancements are
necessary to conduct precision approach and terminal navigation
The WAAS will cover the Continental U.S. and
provide a navigation signal capable of supporting navigation from
enroute through Category I precision approach. GBAS will cover
approximately a 30-mile radius and will provide up to a Category
III precision approach. WAAS and GBAS will work together to provide
users a navigation capability for all phases of flight.
What is Differential GPS (DGPS)?
DGPS achieves enhanced
accuracy since the reference and user receivers both experience
common errors that can be removed by the user. Position errors
less than 10 meters are typically realized.
In the basic form of DGPS, the position of a reference receiver
at a monitoring or reference station is surveyed in, that is,
its position is known accurately. The user receiver should
be no more than about 300 miles away from the reference receiver
which makes pseudorange measurements, just as any user receiver
would. However, because the reference receiver knows its position
accurately, it can determine “biases” in its pseudorange
measurements. For each satellite in view of the reference receiver,
these biases are computed by differencing the pseudorange measurement
and the satellite-to-reference receiver geometric range. These
biases incurred in the pseudorange measurement process include
errors arising from ionospheric delay, tropospheric delay,
and satellite clock offset from GPS time. For
real-time applications, the reference station transmits these
biases, called differential corrections, to all users in the
coverage area of the reference station. Users incorporate these
corrections to improve the accuracy of their position solution.
For the basic local area DGPS (LADGPS) the position solutions
of users further away from the reference station are less accurate
than those closer to the monitoring station because pseudorange
measurement errors tend to be spatially correlated. This loss
of accuracy due to spatial decorrelation can be improved with
more sophisticated techniques that fall under the heading of
wide area DGPS (WADGPS) such as WAAS.