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About
GPS
and Electronic Charts
How GPS Works - DGPS - Chart Datum - Converting
Chart Datum
The
selections below are protected by copyright and are not to
be reproduced or distributed without permission of the copyright
holder.
How GPS Works
The Global
Positioning System (GPS) is a world-wide 24 hour
navigation positioning system operated by the US
Department of Defence. It consists of a Ground Control
Segment, a Space Segment and a User Equipment Segment.
The User equipment segment is what is commonly known as a
GPS receiver.
24 earth-orbiting satellites in six different orbits
form the Space Segment . (There are also 3 or 4
operational spares in orbit at any one time.) Each
satellite circles 10,900 nautical miles above the earth
in orbits inclined at an angle of 55 degrees to the
equator. Each satellite transmits precision timing
signals (derived from onboard atomic clocks) on two
frequencies, L1 and L2. A separate channel on each
frequency is dedicated to each satellite.
The navigation messages broadcast on the L1 frequency
contain two codes, one for civilian use, and another
encrypted code for military use. The L2 broadcast
contains a second set of navigational messages, which
when combined with the encrypted code in the L1
frequency, can resolve positions to less than 20 meters.
Known as the Precise Positioning Service (PPS), this
service is available only to the US military, and its
allies.
The non-encrypted codes in the L1 frequency, (available
to civilian users), provide the Standard Positioning
Service (SPS). When GPS was in its initial testing phases
it was found that this service provided position fixes
that were far more accurate than was originally intended,
so SPS accuracy was intentionally degraded by the
introduction of random errors in the timing signal--reducing
the position fixing accuracy of GPS to 100 meters 98% of
the time. This intentional degradation of the timing
signal was known as Selective Availability
(SA), and constituted over half the total GPS error prior
to May 1, 2000. (The satellite clock need only be "dithered"
by a few millionths of a second to create the desired
effect. That is why, in spite of SA, GPS time is the most
accurate clock you will have on board your vessel.)
However, recognizing the importance of GPS to the
civilian economy, the United States Government removed
Selective Availability on May 1, 2000. Now the single
largest contributor to GPS error is interference with the
broadcast signals caused by the ionosphere (a shell of
electrically charged particles that surrounds the earth.)
Now a GPS position is expected to be accurate within 20
meters.
Each satellite also broadcasts "Almanac"
and "Ephemeris" messages. Your
earthbound GPS receiver uses the almanac
to determine which satellites are above the horizon and
what channels they are broadcasting on. The receiver then
locks on to the most appropriate satellites for fixing a
position.
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Given the exact time the navigation message was
broadcast, and knowing the time it was received, the GPS
receiver determines the amount of time it takes for the
coded signal to travel from the satellite to your antenna.
From there it is a simple computation to determine the actual
distance between the satellite and your GPS
antenna.
From this point, the GPS receiver calculates a
position in the same way as a human navigator using radar
ranges. The ephemeris message tells the
receiver the exact location of the satellite when the
message was broadcast, and since the receiver now knows
the distance to the satellite, it calculates that it must
be on the surface of an imaginary sphere, centered on the
satellite. Where that sphere intersects with the surface
of the earth, a Circle of Position ( COP) is formed.
From two satellites the
receiver calculates two COP's, which cross at two
possible positions. To determine which position is the
correct one, a third satellite range is needed. Thus, for
a receiver at sea level, a minimum of three satellites
are needed to determine a two-dimensional position. For
aircraft, and vehicles on land, which operate above sea
level, a fourth satellite is needed to determine a three-dimensional
position (including altitude).
Satellite timing
signals are subject to small errors, so each orbiting
satellite is closely monitored from five sites around the
world (The Ground Control Segment). The main control
facility at Falcon Air Force Base, Colorado, makes minor
adjustments to keep the system within its prescribed
limits of accuracy--(20 meters).
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From
GPS--Instant Navigation 2nd Edition
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DGPS
Note:
In 2022 Differential GPS was discontinued in Canada,
the United States, the UK and Ireland
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Chart
Datum
From GPS--Instant
Navigation, Second Edition, Chapter
3, by Kevin Monahan and Don Douglass
Not only may charts be subject to small
errors in the location of certain geographic features
such as shorelines, isolated rocks, shoals and the like,
but they may also contain large systematic errors due to
inaccurate determination of latitude and longitude.
As an example, certain islands in the
South Pacific Ocean actually lie many miles from
their charted positions because their discoverers made
errors in computing latitude and longitude—especially
longitude. When you consider that accurate determination
of longitude depended on the ship’s captain
accurately tracking the error of his chronometer for two
years or more, it is wonderful that we can make any sense
at all out of these older charts.
In the Strait of Magellan, one of the
authors noted discrepancies of 0.7 Nm to 1.1 Nm between
the charts and his GPS position. On British Admiralty
charts the largest known difference is 9 miles.
These errors developed because there
was no single reference point to which charted features
could be compared. For instance, when charting a South
Seas lagoon, a nineteenth century surveyor could only
reference the chart to a datum point somewhere on the
shore of the lagoon; he could not accurately position
that datum point in relation to the surface of the earth.
What he needed was a single datum point somewhere on the
earth’s surface to which he could reference his
charts. Although such a datum point did exist (the Royal
Observatory at Greenwich, England), it was too far away
to be used as an accurate reference for the chart, so the
surveyor had to make do with a local datum.
Since that time, cartographers and
hydrographers have continued to attempt to define local
datums that are relevant over larger and larger areas. In
order to do so, they have used measurement systems that
maintain accuracy over greater distances but, when
crossing bodies of water, these systems break down
because there is no fixed ground in which to set survey
stakes. Consequently until the arrival of satellite
positioning, map datums were limited to continental-sized
areas.
An additional problem arises when a
hydrographer attempts to extend a datum over a large area.
The earth is not a true sphere; it is flattened at the
poles and has lumps, bumps, and depressions. The surface
of the ocean, likewise, does not follow an ideal
spherical shape. Local differences in the force of
gravity cause parts of the world’s oceans to be more
than one hundred meters higher or lower than others,
refuting the idea that there is a perfect "sea level"
to which heights can be referred.
Hydrographers and cartographers must
make assumptions about the shape of the earth when they
draw their maps so, in 1927, North American hydrographers
began to use a standard set of assumptions about the
earth’s shape which applied to all North American
charts. This standard model is known as a "geodetic
system," "chart datum," or "horizontal
datum." In North America it is known as North
American Datum 1927 or NAD27 (not to be confused with
tidal datum which is the zero tide level). In other parts
of the world, surveyors have established different datums.
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Since 1927, cartographers have learned
more about the shape of the earth, and in 1983, using
satellite telemetry data they established a new datum (NAD83)
in North America. Among its many features, it makes
allowance for changes in their understanding of the shape
of the earth’s surface that introduced errors of up
to 200 meters in NAD27. When charts were drawn to the new
datum, cartographers discovered that the positions of
geographic features on charts drawn to the older datum
could not be reconciled with their positions on new
charts—the lines of latitude and longitude on the
older charts were in the wrong places. In many areas of
the continent, these differences are minimal—just a
few meters—but in other areas the difference between
NAD27 and NAD83 is over 200 meters (more than 0.10 Nm).
Displacement of position due to datum differences is
called datum shift.
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These discrepancies
didn't matter when navigation systems were less exact,
but with the advent of precise positioning, cartographers
began to take these datum shifts seriously. Notices
suddenly appeared on charts indicating the datum to which
the chart was drawn, as well as the degree of correction
to be applied to older charts to reconcile them with the
new navigation systems. Now that worldwide satellite
positioning is available, GPS uses a truly universal
chart datum—World Geodetic Survey 1984 (WGS84). In
North America, WGS84 is equivalent to NAD83.Boaters operating in northern British Columbia
and southeast Alaska may find discrepancies of up to 200
meters. On older charts of Dover Strait, charted to the
1936 Ordinance Survey of Great Britain (OSGB), the shift
is approximately 140 meters. Unless you resolve these
differences, your GPS receiver will lose some of its
usefulness.
Converting
from One Chart Datum to Another
Since many parts of the world are charted using
the old datums, hydrographic agencies are now adding
notices on older charts indicating the horizontal datum
and the required corrections to be applied to a GPS
position before you plot it on a chart. When new surveys
are conducted and a new chart issued, it is drawn to WGS84,
and a datum note is incorporated. If you are using an electronic chart
system, the chart itself should already have been registered to the
WGS84 datum, even raster charts that carry a horizontal datum notice
that they were drawn to NAD27!!
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Before you plot GPS-derived WGS84 co-ordinates
on a paper chart drawn to a local datum, you will have to apply
corrections either to your plot or to the GPS receiver
itself. If you decide to correct the co-ordinates
manually, you will find it a cumbersome process. If you
want to lift positions off a chart not drawn to WGS84 or
an equivalent datum, and you wish to use the co-ordinates
with a GPS set to WGS84, you will have to perform the
conversion in reverse. If no horizontal datum is noted,
you may be able to find the corrections in the issuing
agency’s Notices to Mariners. But you should be aware that there
are some charts that have been drawn to an unknown datum. When these
charts are rendered into raster electronic charts, they may not display
positions correctly.
To
the right is a screen-capture of a series of tracklines displayed by an
electronic chart system against a chart drawn to an unknown datum.
Clearly the chart is in error, and the vessel did not pass over land. Someone would have noticed!!
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Manufacturers of GPS receivers are
aware of chart datum problems and have gone to great
lengths to include corrections for over 100 datums in
their software. (See Appendix I of GPS--Instant Navigation for a list of 151
datums,
of which approximately 100 are used on nautical charts.) Simply select the correct datum when
working with any particular chart and the receiver will
then display lat/long co-ordinates consistent with the
charted position. This is the method most skippers use,
and it certainly simplifies things.
If you want to make the conversions
with your GPS, you must ensure that the chart datum being
used by the GPS at any one time matches that of the chart
you are using. This means that you must check every chart
you use and, if necessary, reset the GPS every time you
move from one chart to another. The value of your GPS as
a precision navigation tool will be seriously compromised
if you fail to do so.
Remember: Corrections used on one chart
may not be the same as those for a chart of another
nearby area drawn on the same datum. However, modern
receivers include most of the common datums in use
worldwide—some software packages contain over 100
choices.
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