####### ENGINEERING THE WORLD'S LARGEST

####### DGPS NETWORK

**D. B. Wolfe, C. L. Judy, E. J. Haukkala, D. J. Godfrey**

(United States Coast Guard Command and Control Engineering Center,
Portsmouth, VA, USA)

**Abstract**

**Key words:** US Coast Guard, DGPS, nationwide network

This paper describes the United States Coast Guard's efforts to
implement the world's largest ground-based Global Positioning System
(GPS) augmentation service, Nationwide DGPS (NDGPS). NDGPS is being
implemented to meet all surface transportation navigation requirements
for the US. These applications demand a high level of robustness and
reliability. This paper describes how the Coast Guard is meeting these
challenges through system design, equipment configuration and command
and control functions. This includes a thorough description of how the
Coast Guard ensures the integrity of the system and broadcast
corrections on a continuous basis. We also give some direct experience
in the challenges of "beating swords into plowshares" through the
conversion of the United States Air Force (USAF) Ground Wave Emergency
Network (GWEN) into the NDGPS inland infrastructure.

#### Introduction

Integrating an obsolete radiobeacon system with an outdated USAF ground
wave network coupled with state of the art GPS reference station,
integrity monitoring, networking and leading edge computer equipment to
build a nationwide Differential GPS system is a monumental task faced by
US Coast Guard engineers. To meet not only the traditional maritime
requirements shared by other nations but to cover all of a nation twice
over and provide service for non-traditional DGPS applications as well
adds to the complexity.

Differential Global Positioning Service (DGPS) is a land-based system
that receives and processes signals from orbiting GPS satellites,
calculates corrections from known positions and broadcasts these
corrections via a Medium Frequency (MF) Transmitter to DGPS users in the
Broadcast Site's coverage area.

The United States Department of Transportation (DOT) is coordinating the
implementation of a network of DGPS broadcast sites across the
continental United States, Alaska, Hawaii and Puerto Rico. Several
Federal and state agencies, including the Federal Railroad
Administration (FRA), Federal Highway Administration (FHWA) and the
United States Coast Guard (USCG), are involved in the effort to install
the NDGPS Broadcast Sites. When completed, this nationwide broadcast
network will consist of over 126 sites and provide a standardized signal
for DGPS service throughout the United States. Planned uses of the NDGPS
network include positive train control, precision farming, smart
vehicles, snow plow management, accurate waterway dredging and improved
emergency response, an expansion of traditional uses which include
harbor/harbor approach navigation, vessel tracking and buoy positioning.

The US NDGPS network design is based on the USCG's DGPS maritime service
that began initial operation in 1996 with service coverage of major
harbors and the nation's coasts. A major component of the NDGPS
implementation plan is the reuse of USAF GWEN facilities as DGPS
Broadcast Sites. This reuse of equipment, either in its current location
or moved to new sites, reduces the cost of implementing NDGPS coverage.

####  System Requirements

The primary purpose of the marine DGPS network is to provide reliable
position accuracy of better than 10 meters (2 drms) when navigating in
harbor and harbor approach areas (up to 20 miles offshore) of the
continental US, Puerto Rico, and selected portions of Alaska and Hawaii
\[1\].

The initial requirement is for NDGPS to provide single coverage
throughout the continental United States. The ultimate goal is to
provide dual terrestrial coverage throughout the contiguous 48 states
and single coverage in Alaska, Hawaii and Puerto Rico.

Each Broadcast Site is required to be on-air and monitored real-time by
a centralized Control Station 99.7% of the time. Coupled with the
expected double coverage planned throughout the continental US, this
equates to a signal availability of 99.9%. Availability represents the
percentage of time that the DGPS signal is usable.

The USCG DGPS system provides the user with broadcast messages as
defined by the Radio Technical

Commission for Maritime Services \[2\] and utilizes Reference Station
Integrity Monitor (RSIM) \[3\] messages for intra-system communication.

In order to verify site/system availability, each Broadcast Site sends
RSIM Messages 15 (MF broadcast signal strength, signal to noise ratio
and message error ratio) and 18 (position error) from the online
Integrity Monitor and RSIM Message 24 (transmitter operational status)
from the transmitter to its parent Control Station through a real-time
X.25 communications network connection. USCG has a requirement to
monitor and store these messages. With the expected termination of GPS
Selective Availability (S/A) and the proposed additional civil
frequencies in future GPS satellites, requirements of the US DGPS system
are expected to change drastically.

**Broadcast Site**

As of February 2000 the Coast Guard has 62 operational DGPS broadcast
sites throughout the United States. The original maritime DGPS network
sites are located along US coasts and the Great Lakes. An agreement with
US Army Corps of Engineers (ACOE) \[4\] expanded this maritime network
to include major inland waterways such as the Mississippi River.
Initially the USCG placed DGPS equipment at pre-existing marine
radiobeacon sites and modulated the MF signal broadcast at these sites.
As the use of DGPS expanded and the requirement for a nationwide system
developed, the DOT decided to model the NDGPS system on the existing
USCG maritime system. Currently, nine sites have been built in the
interior part of the United States as part of the nationwide expansion.

Each of the DGPS broadcast sites has the same basic design -- an
equipment rack with the electronic equipment and MF transmitter located
within a special shelter or building and reference masts, antenna
coupler and broadcast antenna located near the shelter. Figure 1 shows
the standard configuration of the Maritime DGPS Broadcast equipment.

![](media/image1.wmf){width="6.320833333333334in"
height="4.420833333333333in"}

**FIGURE 1: EQUIPMENT LAYOUT AT A MARITIME DGPS SITE**

The USCG uses two ASHTECH, Inc. dual-frequency, 12 channel Z-12R^TM^
DGPS Reference Stations. Each Reference Station consists of a GPS
reference receiver and a minimum-shift keying (MSK) modulator. The
Reference Station computes corrections for GPS satellite signals and
outputs these corrections in an MSK format to an MF transmitter at an
authorized frequency. The authorized US frequency ranges from 285 -- 325
kHz in 500 Hz increments, but the USCG's system currently uses 1 kHz
increments.

Two TRIMBLE NAVIGATION, Ltd. 12 channel, L1 frequency 4000IM^TM^ MSK
Integrity Monitors provide the required integrity for the differential
corrections broadcast. As the name implies, the Integrity Monitor
monitors the integrity, or accuracy, of the DGPS Broadcast information
and provides the Reference Station and Control Station feedback on this
accuracy. Both Integrity Monitors monitor the MSK signal from the
broadcast antenna, but each Integrity Monitor only provides feedback to
its respective Reference Station, i.e. Integrity Monitor A is directly
wired to Reference Station A. The Integrity Monitor identifies the
corrections from the individual Reference Station by the Reference
Station ID number encoded into the broadcast.

All USCG Broadcast Sites are unmanned and are not usually located near a
Coast Guard facility; therefore the Coast Guard's design incorporates
redundant Reference Stations and Integrity Monitors. Although both
Reference Stations provide signal inputs to an MF transmitter, only one
set of corrections is broadcast at a time. If the primary Reference
Station fails, the MF transmitter will automatically switch to the
secondary Reference Station and continue to broadcast corrections
allowing the faulty Reference Station to be repaired or replaced. Since
each Integrity Monitor is directly connected to its dedicated Reference
Station, the Control Station must take action to switch to the secondary
Integrity Monitor if the primary Integrity Monitor fails.

In April 1997 the Coast Guard entered into a contract with Southern
Avionics Corporation (SAC) to replace all of the existing radiobeacon MF
transmitters and antenna couplers. The SAC transmitter has two
transmitter drawers (Side A and Side B). The Reference Station's MSK
Output is directly connected to the respective sides, for example
Reference Station A is connected to Transmitter Side A.

An RSIM Control Drawer monitors operation of the on-line transmitter
side and automatically switches to the stand-by transmitter side if the
Radio Frequency (RF) output power falls below a specified threshold or
during other user defined fault conditions. The RSIM Control Drawer
allows for remote control and monitoring of the transmitter by the
Control Stations via standard RSIM messages. The RSIM Control Drawer
also allows for the monitoring of the Broadcast Site environmental and
control alarms (humidity, fire, intrusion, power loss, generator status,
etc.).

The SAC transmitter carrier operating power is adjustable from 50 to
1000 Watts, with most of the USCG DGPS sites falling between 250 and
1000 Watts. Additional functionality includes the ability to operate up
to 22 hours at 85% of maximum power level (850 Watts) on DC back-up
power using a bank of twelve 12-volt, 200 amp-hour batteries and the
ability to re-charge these batteries when AC power is restored.

The Reference Stations' GPS antennas and the Integrity Monitors' GPS and
MSK antennas are located on two reference masts located at the site. The
location for these masts is carefully reviewed prior to installation to
ensure that multipath and masking problems are minimized. To reduce the
effects of multipath the masts are placed at least 75 feet apart. To
further reduce the effects of multipath one mast has GPS antennas for
Reference Station A and Integrity Monitor B while the other mast has the
GPS antennas for Reference Station B and Integrity Monitor A.

There are four different types of MF broadcast antennas used at US DGPS
broadcast sites. The first is a VALCOM 74-foot whip antenna with a
loaded coil and top-loading element (TLE). This antenna was originally
used at marine radiobeacon sites and has been replaced wherever possible
because of its inefficiency (less than 3%) and tendency to break down at
power levels greater than 500 Watts in high salt environments. The
VALCOM whip is in use only at sites where there is not enough property
or where historical concerns (e.g., the broadcast site is co-located
with a historic lighthouse) prevent the installation of a larger and
more efficient antenna.

The second and most commonly used antenna type is a guyed ROHN tower in
90-foot, 120-foot and 150-foot heights, each with three TLEs. The
efficiencies of these towers range from 6-8% for the 90-foot tower to
20% for the 150-foot tower with bandwidths of approximately 3 kHz.

The third antenna type is the Longwire and is most common at sites along
the Mississippi River. The efficiency of this antenna type is
approximately 13%.

USCG RF engineers are currently researching efficiency and bandwidth
characteristics of these three antennas in an attempt to improve system
performance.

The remaining antenna type is located at converted USAF GWEN sites. This
antenna is 299 feet tall and has twelve TLEs. With an efficiency of
approximately 55% and a bandwidth between 30 and 80 kHz, it is
significantly better than any other antenna currently in the Coast
Guard's inventory. The GWEN antenna efficiency translates into an
expanded coverage area at the same power, reducing the number of
required sites, an important factor in designing a system to ensure
nationwide coverage. Consequently, the GWEN antenna design has been
selected as the standard for all future NDGPS sites. Figure 2 shows the
standard configuration of the GWEN DGPS Broadcast equipment.

In addition to re-use of the larger highly efficient antenna, the USCG
is evaluating the re-use of the GWEN RCA MSK-5SS 5,000 Watt transmitter.
The RCA MSK-5SS was originally a continuous wave 5,000 Watt AM
transmitter that the Air Force modified for Low Frequency Pulse
Operations. The Coast Guard has converted the RCA back into continuous
wave operations in the 285 -325 kHz band and removed two of the Power
Amplifier (PA) cards for a maximum output power of 2,500 Watts. The two
removed PA cards are being used as spares since the age of the RCA
transmitters has created a scarcity of spare parts.

Problems arose when trying to use the standard maritime SAC automatic
tuning unit (ATU) with the GWEN antenna for DGPS broadcasts. The GWEN
antenna was designed for operating frequencies of 150 to 175 kHz. A side
effect is that it is resonant near the DGPS operating frequency of 285
kHz to 325 kHz. Thus, the antenna

![](media/image2.wmf){width="6.522916666666666in"
height="4.831944444444445in"}

**FIGURE 2: EQUIPMENT LAYOUT AT A GWEN SITE**

is either outside the tuning range of the SAC ATU or just within tuning
range but does not allow for the full range of the variable tuning
circuit. During the first few site conversions when the TLEs had already
been installed, Coast Guard RF engineers determined that installing a
matching network between the ATU and the antenna was the most
cost-effective way to solve this problem. At new installations the
engineers also redesigned the TLE lengths enabling the SAC ATU to use
its entire tuning range, eliminating the need for the matching network.

Additional modifications were needed to bring the RCA transmitter into
compliance with DGPS requirements including the addition of a switch to
allow for the transfer between Reference Stations and the creation of a
separate RSIM control draw to perform the same functions as the SAC.
During the modification process, each RCA transmitter undergoes a
two-week refurbishment prior to installation at the NDGPS site. This is
expected to extend its operational life by ten years.

A GWEN Site configuration also includes fenced shelters for emergency
power generator and transmitter equipment. The 25,000-Watt generator
with fuel for up to eleven days of operation provides site backup power
capability in the event of commercial power failure.

#### Expansion Efforts

#### 

One of the Coast Guard's original reasons for providing a DGPS
correction was to enhance the ability of Coast Guard Buoy Tenders to set
and service Aids to Navigation. Positioning buoys using the DGPS signal
has resulted in considerable operating cost and time efficiencies and
improved accuracy when compared to the previous method of using
horizontal sextant angles. As more DGPS Broadcast Sites are installed,
more users rely on this highly accurate positioning system and the
demand for even more sites continues.

In 1996, the President assigned DOT as the responsible agency for all
civilian GPS matters and to implement a national GPS augmentation for
terrestrial transportation \[5\]. In January 1997, DOT formed the DGPS
Policy and Implementation Team (PIT) to develop a nationwide
differential system. Federal agencies that are members of the PIT
include the Office of the Secretary of Transportation, Federal Highway
Administration, Federal Railroad Administration and the US Coast Guard.
After reviewing several options, the conversion of USAF GWEN sites into
DGPS sites based upon the USCG standard design was determined to be the
most efficient and cost effective method of providing nationwide
differential coverage. After a successful GWEN to DGPS conversion site
was established and tested at Appleton, Washington \[6\], the decision
was made to expand the Coast Guard's DGPS network into a nationwide
system.

GWEN was an emergency communications network within the continental
United States built for the command and control of US strategic forces.
It consists of a highly redundant network of sites hardened to withstand
electromagnetic pulse and operated via Low Frequency ground wave
signals. In the mid 1990's the NDGPS PIT learned of the USAF plans to
decommission the GWEN system and received authorization to take control
of the sites from the Air Force for use in the nationwide expansion.
This program represents a large defense conversion to civil use and
provides opportunities to utilize unneeded Air Force assets to reduce
the cost of providing NDGPS coverage. Figure 3 shows the typical layout
of a GWEN site.

![](media/image3.wmf){width="6.320833333333334in"
height="3.938888888888889in"}

**FIGURE 3. TYPICAL LAYOUT OF A GWEN SITE.**

Each member of the NDGPS team is an important player in the success of
the NDGPS expansion. The FRA is the sponsor of the NDGPS program, while
the FHWA receives and redistributes all NDGPS funds appropriated by the
US Congress. The FHWA also prepares the environmental impact statements
for all new site construction. Because of its experience with the
implementation of the maritime network, the USCG is tasked with the
design, installation and operational control of each site. Local and
state government agencies help with site selection, ensuring all local
laws and regulations are met prior to the installation of new sites.

**Site Selection**

Approximately 45 of the current GWEN sites will be converted to NDGPS
use at their current locations. If selected, the RCA transmitter with
its increased output power coupled to a highly efficient antenna should
result in service to a larger coverage area when compared to maritime
DGPS sites, requiring fewer sites to be built. Several of these sites
are located in the vicinity of maritime sites in the maritime network
and once on-air will replace these lower power sites. This will improve
overall performance (larger coverage area, stronger signal, less signal
fluctuations due to weather) of the current maritime system and reduce
operating and maintenance costs.

The remaining sites required to complete the NDGPS system will be
constructed using relocated GWEN equipment or entirely new antennas and
transmitters based upon the GWEN design. Exactly how many sites are
needed has not been determined. Selection of sites is based on public
safety requirements that require dual coverage. After this, sites will
be built to fill in appropriate coverage gaps and as required by the
expanding user base until complete nationwide coverage is achieved.

Once the PIT has decided on a location for a new NDGPS site, usually a
specific town or city, a standard site selection guide is sent to a
local State or Federal agency representative to be used to help locate
possible sites. The local representative looks for potential sites
within a 30-50-mile radius of the location based upon criteria outlined
in the selection guide. The selection guide includes criteria on the
required property size (11.2 acres), environmental concerns, the
availability of power and telephone service and the presence of any tall
objects that could mask the GPS antennas. Upon completion of this stage
of the review, the number of potential sites has usually been narrowed
down to one or two and the results are sent to USCG Command and Control
Engineering Center (C2CEN). C2CEN sends a representative to visit these
few sites for a final survey before deciding where the site will be
installed.

Despite these procedures, the selected site(s) often has one or two
associated problems. For example, at a planned site in Ohio a 600-foot
metal bridge is located only a few hundred feet away from the broadcast
antenna. Even using computer modeling, the effects to the coverage area
will not be known until after installation is completed. At the
Annapolis, Maryland GWEN site, a problem arose because of an 855-foot
tall television tower located 980 feet to the southwest of the site. The
television tower partially turned the broadcast antenna into a
directional array. The predicted radiation pattern had a significant
gain to the northeast of the site and at certain frequencies in the DGPS
spectrum, there were several orthogonal dB losses when compared to the
main lobe. In California, the Essex NDGPS site is also the home to an
endangered tortoise. Consequently, special training has to be given to
the maintenance crews to limit the impact to the tortoise and its
habitat.

The initial determination of NDGPS site placement has been based upon
the signal strength predictions using Millington's method. An analysis
conducted by the United States Coast Guard Academy (USCGA) \[7\] shows a
marked difference between these predicted signal strengths and observed
signal strengths especially over rough terrain encountered in the
Western US. The USCGA is in the process of developing a new model based
upon the terrain's slope over which the signal must travel. This new
method appears to have a direct correlation in the differences observed
between the predicted and actual signal strengths. This effort, in
addition to readings taken from operational sites, will help to ensure
that complete coverage of the US is achieved.

Another concern in site selection is the effects of skywave interference
on the DGPS signal. There has been anecdotal evidence of distant DGPS
sites interfering with users' reception of the signal from a local site
\[8\]. As the number of sites increases the probability of receiving
destructive interference from another site also increases. C2CEN and the
USCGA are beginning to study these affects and determine ways to
mitigate the problem. One potential result of this study is the that the
USCG may assign frequencies at 500 Hz increments as opposed to the 1 kHz
increments currently used.

The largest area requiring construction of new sites is in Alaska where
there are no GWEN sites. The estimated cost of installing an NDGPS site
in Alaska is in excess of \$1 million (US) per site. With 15 sites
planned in Alaska, the PIT is searching for ways to reduce this cost.
One idea is to broadcast the DGPS signal using a diplexer onto a
pre-existing broadcast antenna, such as the LORAN-C stations positioned
throughout the US. With an expected cost saving of over \$500,000 (US)
per site, this implementation would drastically reduce the price to
achieve DGPS coverage in Alaska. The engineering aspects of this
proposal are discussed later in the paper.

#### Communications Network

All communications between Broadcast Site equipment (Reference Station,
Integrity Monitor and Transmitter) and the Control Station is performed
via an X.25 Packet Switching Network, a Router and Data Servicing Unit
(DSU). Communications is established from the Control Station
application using dedicated virtual circuits with each piece of
equipment. These virtual circuits are opened and remain open until
communications with the Broadcast Site is explicitly terminated. Each
Broadcast Site has a 9.6 kbps line providing service to the X.25
network. Each Control Station requires a single 56 kbps line; a second
line is installed to ensure redundancy. The Control Station application
initiates the calls after which two-way communication can occur. System
implementation enforces the restriction that no more than one Control
Station can establish/maintain a connection with a given Broadcast Site.

The Control Station router performs protocol translation making the
application network-independent. The Control Station has a fast-Ethernet
network, permitting interconnectivity between Server and Client
machines. A 100 Base T connection provides fast updates between
redundant server machines.

**Control Station**

System requirements specify that NDGPS Broadcast Sites be monitored and
controlled on a continual basis from a central location and that these
Control Stations have the capability to simultaneously monitor and
control at least 200 sites. The control functions afford watchstanders
the capability to change site parameters and disable sites, i.e., turn
off corrections, as circumstances warrant. There are two operational
Control Stations, USCG Navigation Center East (Alexandria, Virginia) and
USCG Navigation Center West (Petaluma, California), and one Engineering
site, USCG C2CEN (Portsmouth, Virginia). Each control suite has the
capability to monitor and control the entire system. Additionally, the
Control Station application, Nationwide Control Station (NCS), must
provide online data storage for at least one year. Figure 4 shows the
relationship between a Control Station and a Broadcast Site.

NCS has been designed adhering to Object Oriented design principles. To
implement the greatest degree of flexibility, the application
incorporates data-driven dynamic design, maximizing the use of a
relational database to ensure data integrity and robust processing

![](media/image4.wmf)

**FIGURE 4. DGPS CONTROL STATION AND BROADCAST SITE CONFIGURATION**

Implementing client-server architecture, NCS processing is easily
allocated into the primary areas of its driving requirements to monitor
and control. The Server portion of the application performs system
Monitoring including all network communications and data storage. The
Client, on the other hand, performs the Control functions, providing a
user interface for watchstander-initiated changes and System Status and
Information display.

This design minimizes and/or eliminates single points of failure and
provides fault tolerant data storage. Features of note include mirrored
servers, redundant power supplies and processors within each server,
redundant SCSI buses, controllers and cables, and finally, Redundant
Array of Inexpensive Disks (RAID) disk configuration.

An independent data warehouse agent running on the Backup Server to
handle administrative/data-analysis tasks removes processor intensive
operations from the Primary Server, minimizing operational impact on the
system.

**Engineering Efforts**

USCG C2CEN has several projects underway to improve Broadcast Site
performance. The first is an in-depth analysis of the entire RF network
to identify ways to improve system performance and decrease the amount
of off-air time. The maritime DGPS network is experiencing systemic
problems with power fluctuations and high levels of reflected power
automatically shutting down the transmitters. These problems are most
likely caused by variations in the weather changing the antenna
characteristics faster than the ATU can adjust.

The underlying problem is that antennas used in the maritime DGPS
network are shorter electrically than ideal quarter wavelength antennas.
These electrically short towers have higher currents and voltages and
are more susceptible to corona effects, arc-overs and weather
variations. The decision to use these antennas was an economical one,
but they have created problems in meeting availability requirements.
Numerous improvements to the system have already been made -- improved
ground planes, insulators, and addition of corona rings. The RF study
will look at these and other areas in an effort to improve system-wide
on-air availability

Another project will install an Autonomous Controller and Data Logger
(ACDL) at each Broadcast Site. In the event of a communications loss
with the Control Station, this commercial ACDL software program and
computer will capture and log all USCG required RSIM messages,
automatically respond to basic alarms and attempt to establish
communications with the Control Station using an alternate network such
as a dial-up modem. When the network connection is reestablished, the
ACDL will send all data logged during the outage to the Control Station.
The Coast Guard counts any time when there is a loss of communications
between the Broadcast Site and Control Station against Broadcast Site
availability since there is no logged data to show how the site
performed during that time. Implementing ACDL will provide the
capability to compute availability using the data captured during the
communications loss. This will give a more accurate picture of Broadcast
Site performance than is currently possible.

C2CEN is in the process of creating a portable DGPS site. There are
several reasons why a broadcast site can be off air for an extended
period of time. Damage from severe weather, an earthquake or even
extended maintenance can cause a site to fail to meet the 99.7%
availability requirement (approximately 2 hours per month). In these
cases, a portable DGPS site could be set up at or near the off-air site
to provide a temporary signal until the main site is brought back
on-line.

The proposed portable DGPS site would include a smaller suite of
equipment (Reference Station, Integrity Monitor, communications
equipment and GPS/MSK antennas), a small transmitter, ATU, a portable MF
broadcast antenna and a generator. All of this equipment would be
packaged together for easy transportation and set-up and stored so that
it could be delivered anywhere in the US within 24 hours. The portable
site may not provide the same coverage as the standard site but could
provide DGPS corrections in an area where and when it is most needed.

Diplexing two separate signals onto a single antenna is a technique
commonly used in broadcast engineering. It has been successfully
implemented at the Savannah, Georgia DGPS site where a DGPS signal (319
kHz at 900 Watts) and a weather data and mariner information broadcast
signal, called NAVTEX (518 kHz at 2500 Watts), are both radiated from
the same antenna. An initial study on the feasibility of diplexing a
DGPS signal onto a LORAN tower is underway. The typical design of LORAN
Towers, 625 feet tall with twelve TLEs, would provide an estimated
efficiency of over 70% at the DGPS frequencies. The location of the
existing LORAN Stations in Alaska and throughout the continental US
would decrease the required number of NDGPS sites to achieve the desired
coverage, significantly reducing the cost. The enormous differences
between the LORAN and DGPS signal powers (LORAN is a pulse-wave
typically operating at over 400K Watts peak power and DGPS is a
continuous wave signal operating at 1K Watt ) might make the application
impossible. Since both are safety of life navigation signals, there must
be assurances that neither signal adversely affects the other.

**Other Applications**

Other US government agencies use the equipment at DGPS Broadcast Sites
\[9\]. The National Geodetic Survey (NGS) uses a connection to both
Reference Stations to poll data as part of NGS's Continuously Operating
Reference Stations (CORS). This provides GPS carrier phase and code
range measurements in support of 3-dimensional positioning activities
throughout the United States. NGS posts the data on its Internet
web-site ([www.ngs.noaa.gov/CORS/)]{.underline} to enable
post-processing of GPS data, providing positioning accuracies that
approach a few centimeters both horizontally and vertically. This data
is also used by the Coast Guard to adjust its own reference positions
thereby improving its differential corrections accuracy.

The National Oceanographic and Atmospheric Association (NOAA) also
utilizes Broadcast Site resources to augment its weather forecasting
capabilities. NOAA has mounted the GPS Surface Observing System (GSOS)
instrument package on one of the reference masts at each site. GSOS
contains equipment to measure local pressure and temperature and a GPS
antenna to measure the time delays between the L1 and L2 frequency
reception to determine the water vapor present in the air. The data is
transmitted to NOAA Forecast Laboratories where it is evaluated by the
GPS Integrated Precipitable Water Vapor Observation System to improve
weather forecasts.

#### 

#### Conclusion

The successful effort to configure legacy systems acquired by DOT and
the USCG with DGPS reference and integrity equipment and modern
computer, networking, and interface systems has shown that building a
large cost effective NDGPS network is feasible. It also demonstrates the
capability to achieve NDGPS coverage goals while maintaining the
integrity and accuracy users expect from the traditional USCG maritime
service.

Several challenges remain before the system can attain full operational
capability scheduled for 2003 \[10\]. Aside from political issues
dealing with US Congressional funding, these unique challenges include
improvements in the existing GPS infrastructure, environmental issues
associated with constructing new sites, and the sliding scale of
technology. While meeting the stringent operational requirements and
supplying the advertised DGPS MF signal, the USCG must implement a
nationwide DGPS system that will be functional and supportable. At the
same time, it must seamlessly integrate planned GPS improvements, the
removal of S/A, and the evolving requirements for improved accuracy for
the complete life cycle of the system.

As the user base increases with the advent of new applications and
technology, C2CEN, USCG and DOT will continue to improve the NDGPS
system to meet constantly changing public needs. Users of the DOT
supplied DGPS MF correction signal can be confident of the integrity and
accuracy of their DGPS position knowing the engineering successes that
have led to this expanding US Nationwide Differential Positioning
System.

#### 

#### References

1\. **DOT (U.S. Department of Transportation) and USCG (United Stated
Coast Guard),** Broadcast Standard for the USCG DGPS Navigation Service,
Technical Report, USCG, COMDTINST M16577.1, 1993.

2\. **Radio Technical Commission for Maritime Services,** RTCM
Recommended Standards for Differential GNSS (Global Navigation Satellite
Systems) Services, RTCM Special Committee No. 104, January, 1998.

3\. **Radio Technical Commission for Maritime Services**, RTCM
Recommended Standards for Differential NAVSTAR GPS Reference Stations
and Integrity Monitors (RSIM), RTCM Special Committee No. 104, August,
1996.

4\. Memorandum of Agreement of February 23, 1994 between the USACE and
the USCG for the establishment of DGPS Services.

5\. Presidential Decision Directive NSTC-6 of March 28, 1996

6\. **Allen, L**., Nationwide DGPS, Proceedings of the 10^th^
International Technical Meeting of The Satellite Division of The
Institute of Navigation, ION GPS-97, pg. 675, 1997.

7.*.* **Peterson, B., Bruckner, D., Danish, M., Morris, P***.,* Analysis
of Terrain Effects on DGPS and LORAN Signals, February 2000.

8\. **Mangs, G., Mittal, S., Stansell, T**., Worldwide Beacon DGPS
Status and Operational Issues*,* Leica Geosystems, Torrance, May 1999.

9\. Memorandum of Agreement of May 18, 1994 between the National Oceanic
and Atmospheric Administration, Coast and Geodetic Survey and the USCG
for cooperation in the establishment of the USCG DGPS Service.

10\. **DOT (U.S. Department of Transportation) and DOD (U.S. Department
of Defense),** 1999 Federal Radionavigation Plan, December, 1999.