By : Captain Rod & Susie Stebbins
This article is produced in cooperation with Magellan Systems Corporation,
960 Overland Court, San Dimas, CA 91773. Without this collaboration, this
article might have gone on forever to try to explain the magic of satellite
positioning, encrypting reasoning and communications. Their significant
contributions in this venture are greatly appreciated.
The Owners :
T he Global Positioning System (GPS) is a constellation of 24 satellites
that orbit the earth twice a day, transmitting precise time and positioning
information to anywhere on the globe, 24 hours a day. The system was designed
and deployed by the U S Department of Defense to provide continuous, worldwide
position and navigation data for the use of the United States and allied
Use of the System:
G PS's broad commercial applications were recognized early in the system's
development, and it was decided to allow free access to GPS signals with
certain constraints applied.
How it Works:
E ach GPS satellite broadcasts two signals, PPS (Precise Positioning Service)
and SPS (Standard Positioning Service). The PPS signal is an encrypted military-access
code. The SPS signal is an unencrypted, spread-spectrum signal broadcast
at 1575.42 MHz. Unlike signals from land-based navigation systems, the SPS
signal is virtually resistant to multipath and night-time interference, and
is unaffected by weather and electrical noise.
The SPS signal contains two types of orbit data, almanac and ephemeris.
Almanac data contains the health and approximate location of every satellite
in the system. A GPS receiver collects almanac data from any available satellite,
then uses it to locate the satellites that should be visible at the receiver's
location. Ephemeris data contains the precise orbital parameters of a specific
The GPS receivers listen to signals from either three or four satellites
at a time and triangulate a position fix using the interval between the transmission
and reception of the satellite signal. Any given receiver tracks more satellites
than are actually needed for a position fix. The reason for this is that
if one satellite becomes unavailable, the receiver knows exactly where to
find the best possible replacement. Three satellites are required for two
dimension positioning. Two dimension positioning reports position only. Four
satellites are required for three-dimension positioning, that is to say position
I n general, an SPS receiver can provide position information with an error
of less than 25 meters, and velocity information with an error of less that
5 meters per second. It is good to mention here that most "Chartmate" types
are accurate only to 15 meters. It is because the system is so accurate,
the US Government has activated what is known as Selective Availability (SA)
to maintain optimum military effectiveness. Selective availability inserts
random errors into the timing and ephemeris information broadcast by the
satellites, which reduces GPS SPS code accuracy to between 25 and 100 meters.
(Editor's Note: Selective Availability was turned off by the government,
but could be reinstated at any time without notice for national security.)
For many applications, 100 meter accuracy is more than acceptable. For applications
that require much greater accuracy, the effects of SA and environmentally
produced errors can be overcome by using a technique called Differential
GPS, which increases overall accuracy.
USCG and WAASDGPS
The following information was provided
to Weak Industries, Inc., via Sigma Marine Supply, Tarpon Springs,
Florida in response to questions posed to the United States Coast
Guard in May 2001. We greatly appreciate Sigma's permission to reprint
this vital information.
"FW Your questions on WAASDGPS"
"On August 24, 2000, the Federal Aviation
Administration (FAA) announced that their space-based, L-Band Wide
Area Augmentation System (WAAS) became available for use by "some aviation
and al non-aviation" users. The FAA announcement has prompted numerous
inquiries to the Coast Guard regarding the maritime use of WAAS and
the status of the Coast Guard DGPS system. The following Questions
and Answers are directed at helping to clarify the status of these
two systems for the mariner.
1) Why did the Government design and build two
different GPS augmentation systems?
a. The 1994 National Telecommunications and Information Administration
(NTIA) Technical Report to DOT on a National Approach to Augmented
GPS Services studied the necessity of expanded government efforts
in providing DGPS services. Its goal was to recommend the optimum
integrated system to meet aviation and terrestrial navigation needs.
A variety of systems we being proposed at the time. The study concluded
that a combination of two systems, the FAA's Wide/Local Area Augmentation
System (WAAS/LAAS and the USCG's DGPS system, was the optimum mix.
This integrated system, consisting of the L-band line-of-sight WAAS
for aviation users, and the terrain-following medium frequency DGPS
for maritime and terrestrial users, meets the vast majority of the
nation's precise navigation and positioning needs.
2) Is WAAS currently certified for maritime navigation?
a. NO. WAAS is not yet fully operational and is currently in a
testing status, undergoing further development. It is not certified
for use as a safety of life navigation system in the maritime navigation
environment. WAAS may be used, with caution, in the maritime environment
to improve overall situational awareness, but it should not be relied
upon for safety-critical maritime navigation. The Maritime DGPS Service,
on the other hand, is fully operational and meets all the standards
for the harbor entrance and approach phases of navigation.
3) After WAAS reaches initial operating capability (IOC)
in a few years, will it be suitable for maritime navigation?
a. WAAS is not optimized for surface (maritime and terrestrial)
use, rather, it is designed primarily for aviation use. It is intended
to eventually support aeronautical enroute through precision approach
air navigation. The current WAAS test signals are transmitted by
two-geo-stationary satellites on a line-of-sight, L-band radio frequency.
This means that if anything obstructs the view of the portion of
the sky where the satellite is, the WAAS signal will be blocked.
Since geo-stationary satellites are positioned over the equator,
the farther north users are, the lower the geo-stationary satellites
are in the sky - increasing the likelihood of an obstruction. In
contrast, the medium frequency (285-325 kHz) radio beacon-based Maritime
DGPS Service is optimized for surface (maritime and terrestrial)
applications because it's ground wave signals "hug the earth" and
wrap around objects. This means that the Coast Guard DGPS system
is well suited for the maritime environment (down in the "ground
clutter") where a geo-stationary satellite can be clocked by terrain,
harbor equipment and other man-made and natural objects.
4) Can the Coast Guard's DGPS system be used by aviation?
a. That's up to the FAA. However, the Coast Guard's system was
designed with the surface (maritime and terrestrial) user in mind.
It was neither designed not intended to meet aviation requirements.
Although aviation users could potentially get some modest benefit
from the Coast Guard's DGPS for applications such as surface traffic
management at airports of General Aviation, it could not attain the
type and level of aeronautical service which WAAS and LAAS are designed,
without significant re-engineering.
5) Is the Coast Guard DGPS system a "transient technology" that
is here today but will be gone tomorrow?
a. NO. DGPS has already been adopted globally as an international
maritime standard established by the 1994 International Telecommunications
Union document ITU-R-M.823. It meets IMO Resolution A 815(19) standards
for navigation in harbor entrances and approaches. Over 40 nations
have fully embraced this robust technology and are implementing DGPS
services identical to our own.
6) Which system is more accurate, WAAS or DGPS?
a. On the average, WAAS and DGPS accuracy are virtually the same,
although DGPS accuracy is better when the user is near a DGPS transmitting
site. The WAAS architecture is designed to provide uniform seven
(7) meter accuracy (95% of the time) regardless of the location of
the receiver - within the WAAS service area. DGPS is designed to
provide better that ten (10) meter navigation service (95% of the
time), but typically provides better than one (1) meter horizontal
positioning accuracy (95% of the time) when the user is less than
one hundred (100) nautical miles from the DGPS transmitting site.
Accuracy degrades at a rate of approximately 1 meter per hundred
nautical miles as the user moves away from the transmitting site.
A total of 56 maritime DGPS sites provide coastal coverage of the
continental United States, the Great Lakes, Puerto Rice, portions
of Alaska and Hawaii, and portions of the Mississippi River Basin.
Conclusions : Once WAAS becomes
fully operational, the combination of Coast Guard and FAA systems
is expected to provide a robust, complimentary service to all modes
of transportation. We look forward to the day that industry provides
the public with a fully integrated receiver, one that uses all available
radio navigation systems to provide unprecedented accuracy, integrity,
Despite the differences between DGPS and WAAS, it should
always be kept in mind that both services ultimately rely upon a
single navigation system - GPS - which is vulnerable to interruption
at any time. This lends additional credence to the recommended practice
of using all available means of navigation and not relying upon any
single system. Remember, prudent mariners will always keep looking
out the window!"
D ifferential GPS (DGPS) uses a GPS receiver at a fixed point whose position
is known with submeter accuracy. This is the control unit. The receiver collects
data from all visible satellites and computes predicted satellite ranges,
which are compared with actual ranges. The difference is the satellite range
error, which is then converted to correction signals for use by a roving
receiver. The roving receiver would be to one on the system users boat.
It is assumed that this correction will be the same for other GPS receivers
that in the same area and are using the same satellites for positioning.
If the correction is communicated to other receivers in the area, usually
by a beacon on the same site, the range error can be removed from satellite
signals and precise fixes calculated by these receivers. It should be noted
that not all data errors can be corrected in this way. Errors that are caused
by receiver noise (which is inherent in any GPS receiver) and multipath problems
cannot be eliminated with differential equipment. Multipath errors occur
when the receiver's antenna "sees" the reflections of signals that have bounced
off of surrounding objects.
Using DGPS to eliminate the effects of correctable errors requires that
the user's GPS receiver be connected to a compatible Differential Beacon
Receiver (DBR) and be within range of the broadcasting beacon. The DBR accepts
and demodulates the broadcast corrections, which are then relayed to the
GPS receiver. The GPS receiver applies the corrections to the navigation
data it uses to compute a position solution, and then displays differentially
corrected data. Care must be taken to ensure that the DGPS receiver and the
GPS receiver are compatible for this procedure to be successful.
Monitoring and Controlling GPS:
T he Global Positioning System is monitored and controlled by the US Air
Force, which is responsible for updating and maintaining exact satellite
position and signal data accuracy. It is also responsible for performing
maintenance on the satellites, which may require a given satellite be taken "off-line." Since
the system is subject to periodic updates and changes, the almanac data broadcast
by the satellites is current only for a limited time, generally about six
GPS Information Sources:
T he needs of the worldwide civil GPS user community are served by the Civil
GPS Information Center (GPSIC) located in Virginia. The GPSIC is operated
and maintained by the United States Coast Guard for the US Department of
Transportation. Its primary function is to provide information on the Global
Positioning System and satellite status and to serve as a point of contact.
The GPSIC has general GPS literature available free upon request. The Center
also maintains up-to-date almanac data and Operational Advisory Broadcasts
containing current constellation status and planned satellite outages.
There are three ways to quickly obtain current advisories and almanac
data from the GPSIC and they are:
- Recorded phone message can be obtained at (703) 313-5905.
- Computer bulletin board is located at (703) 313-5910.
- Live information is available by calling (703) 313-5900.
There are other sources for GPS information, ranging from free, governmentally
produced literature to purchased professional literature and seminars. The
geography department of your local college or the local office of the National
Geodetic Survey may also be able to offer assistance.
A map datum is a mathematical description of the earth or a part of the
earth. It is used to correctly assign real-world coordinates to points on
a map or a chart. Because the earth has a very irregular shape, taking accurate
measurements of doing calculations on the earth's true surface is very difficult
and complicated. A mathematically regular shape is much easier to deal with,
if the shape accurately represents the earth's true shape. The most representative
shape is an ellipsoid.
A map datum is a mathematical description of the earth or a part of the
earth, and is based on the ellipsoid or the arc of an ellipsoid that most
closely represents the area being described. In addition, the datum is centered
at a specific location know as the datum origin. A datum may describe a small
part of the earth, such as WGS84, depending on which ellipsoid or ellipsoidal
arc is selected and where the datum origin is. Since datums use different
ellipsoids and have different origins, the Latitude and Longitude coordinates
of the same position differs from one datum to another. The difference may
be slight or great, depending on the datum involved, but will affect the
apparent accuracy of the positioning information provided by a GPS receiver.
Most GPS's and Chartmate type equipment use the WGS84 datum, which is the
model of the earth that is the closest possible average of the planet as
a whole. Which datum your charts are based on is usually found in the chart's
legend. Occasionally, electronic charts do not include this information,
which means that position coordinates determined with the Chartmate type
equipment may not appear to agree with coordinates determined from a printed
chart. When the variations are large it will be necessary to insert correction
factors into the equipment. These correction factors will then be applied
to position fixes before they are displayed.
I t is important to remember and understand that aside from all of great
help one gets from all of the electronic navigational equipment, it is still
the final responsibility of the boat driver to know where he or she really
is and where it is that he or she is going and how to get there. It is important
to navigate with the assumptions that the electrical system on any boat may
just decide to take a vacation too. There is little reason to rely on bargain
equipment - check out what is available and purchase as much for features
and brand reputation as for cost of the equipment. A few extra bucks will
buy the best and the best does a better job.
Just for the record, a meter is just a bit longer than a yard which is three
(3) feet. The following will allow you to calculate the meter explanations
above to better understand what you might expect to encounter if your installation
is sterile, properly isolated, grounded and your antenna is not impeded with
other electronic interference (other equipment on the same boat and in the
proximity of the DGPS and GPS antennas.
- One Meter contains = 39.37 Inches.
- One Meter contains = 3.281 Feet.
- One Meter contains = 1.094 Yards.
For sources of GPS and Loran-C coordinates for fishing, diving, wrecks and
reefs go back to the Southern
©Rod and Susie Stebbins, Weak Industries, Inc.