GPS/DGPS/WAAS - Updated April 2009
by Chuck Husick
The GPS satellites
transmit two position information messages. The most precise, the "P"
code, is encrypted and is normally available only to the military and
other specially authorized users. Civilian GPS sets use the "C"
code information. Until the spring of 2000, the Department of Defense,
the operator of the GPS system, intentionally degraded the accuracy of
the "C" code information. This degradation process was called
Selective Availability (SA) and was intended to deny precise position
data to unfriendly users. The DOD has discontinued the use of SA, to the
delight of millions of GPS users. Today, the typical user will find that
the position of a fixed terrestrial reference point will vary by not more
than about 50 feet 95% of the time. Depending on atmospheric conditions,
the observed position variation may be less than a few feet. This level
of accuracy is sufficient for most, but not all navigation purposes.
When in use, SA created
serious navigation accuracy problems for vessels navigating in near shore
waters, especially near and in major port areas. Safety of navigation
required a means to eliminate the intentional inaccuracy created by the
DOD with SA. In response to this need, the Coast Guard established a coastwise
network of radio transmitters from which they broadcast satellite error
correction messages. The Coast Guard was able to avoid the cost and time
delay of building new transmission stations by converting existing low
frequency radio beacon stations for the accuracy improvement project.
The system is called Differential GPS (DGPS). Receivers at each DGPS site
monitor all GPS satellites in view. By comparing its precisely known position
with the information received from each satellite the DGPS equipment can
compute the corrections necessary to eliminate virtually all of the error
inherent in the signal received from each individual satellite. The error
corrections for each satellite visible from the DGPS station are assembled
into a correction message that is transmitted on a low frequency (200-500
kHz) to all DGPS receivers within range, usually about 50 - 100 miles.
DGPS navigators contain
two separate receivers, one for the signals from the satellites, the other
for the low frequency correction signal. Although DGPS transmitters are
normally some hundreds of miles apart it is necessary to ensure that they
do not interfere with one another. For this reason, the DGPS transmitters
operate on different frequencies within the low frequency beacon band.
Most of the DGPS receivers can automatically search for and tune to the
strongest signal, eliminating the need for the operator to make this selection.
However, many DGPS systems allow the user to manually select the desired
frequency, a capability of value in atmospheric conditions that effect
signal propagation on the low frequency bands.
Reception of the low
frequency DGPS signals requires a separate antenna from that used to receive
the gigahertz GPS signal. Two types of low frequency antennas are used,
an "E" field antenna resembling a short whip or an "H"
field antenna that is usually built into the dome-like GPS antenna. The
base of the "E" field antenna must be connected to the vessel's
ground system. The "H" field antenna is usually a bit more expensive,
however it does not require a separate ground wire, making it somewhat
easier to install. The "H" field antenna may be somewhat more
immune to the static electricity interference caused by nearby lightning
The elimination of
SA may make it unnecessary for some mariners to install differential capable
GPS equipment. However, DGPS provides a means for informing GPS receivers
of an "unhealthy" satellite that is transmitting erroneous information
and has not yet been corrected or turned off by a ground based control
station. If you routinely navigate in areas where precise position location
is important DGPS is well worth the added cost. DGPS provides superior
accuracy, with occasional maximum errors of less than ± 15 feet
95% of the time and typical errors less than ±8 feet. The use of
DGPS also provides noticeably improved stability in both the speed over
ground and course data displays.
GPS use is not limited
to marine navigation. The aviation community is a major user of GPS and
will, according to current planning eventually become almost totally dependent
on the GPS system. The need for a high degree of air navigation precision
and reliability has led to the development of an aviation version of DGPS,
the Wide Area Augmentation System (WAAS). In the U.S., WAAS consists
of approximately 25 ground reference stations that monitor and collect data from the overflying satellites, compare the received position information to known ground positions, create a correction message and relay the message to two master stations located near the East and West coasts for relay to two geostationary satellites. The correction messages from these satellites can be received by WAAS capable marine GPS receivers and used to improve position reporting accuracy. Other countries, including Japan and those in Europe have built similar systems that can be used by mariners.
Position accuracy is typically within 15 meters for DGPS and 3-5 meters for WAAS aided positions.
correction transmissions from the satellites are at the same frequency
as the GPS satellite transmissions, eliminating the need for a separate
receiving antenna for the differential correction signals. Aviation DGPS
systems are programmed to accept these data and apply the corrections
to the satellite signals they are using for navigation. The end result
will be closely similar to what is achieved with marine DGPS. Many marine
GPS units are now able to utilize WAAS differential correction data. It
seems likely that WAAS capability will become part of the basic GPS system.
A further aviation spurred refinement of DGPS, Local Area Surveillance
System (LASS) will provide correction signals for airborne GPS used to
make precision instrument landings. LASS accuracy is on the order of a
foot or two, and includes instant warning of any system malfunction. Unless
you are planning on running your boat up the centerline of a flooded runway
this system appears to have no marine application.