DESIGN PROCESS FOR THE\
UNITED STATES COAST GUARD's\
DIFFERENTIAL GPS NAVIGATION SERVICE

Gene L. Schlechte, LCDR, USCG

DGPS Operations Planning Officer

U.S. Coast Guard Omega Navigation System Center

7323 Telegraph Road

Alexandria, VA 22310-3998

USA

**ABSTRACT**

The United States Coast Guard plans to provide a Differential Global
Positioning System (DGPS) service for harbor and harbor approach
navigation by 1996. DGPS technology is the first to economically offer
geodetic accuracy meeting the Federal planning requirement of eight to
twenty meters for harbor and harbor approach navigation. The DGPS
service coverage area is to include the coastal United States, Great
Lakes, Puerto Rico and most of Alaska and Hawaii. This DGPS service will
be available to the public navigator as an all-weather navigation sensor
to supplement traditional visual, radar and depth sounding techniques.

The design process for the United States Coast Guard's DGPS service
began with efforts to define system operational requirements. The goal
of these requirements was to ensure the same level of user integrity
provided by present Coast Guard electronic navigation aids (Loran-C and
Omega). Refinement of operational requirements by risk analysis of
specific harbor navigation scenarios was then conducted. The final
system architecture evolved to meet the defined requirements under three
traditional constraints:

\~ Current technology,

\~ Present and future economics, and

\~ Maximum flexibility to adapt for future requirements.

The final design step is the development of an operational doctrine to
define DGPS service parameters and the service management
infrastructure.

**DISCLAIMER**

The views expressed herein are those of the author and are not to be
construed as official or reflecting the views of the Commandant or of
the U.S. Coast Guard.

[~1.\ Horizontal\ accuracies\ listed\ in\ this\ paper\ are\ all\ 2drms\ values.\ "2drms"\ means\ twice\ the\ distance\ of\ the\ root\ mean\ square\ error.\ In\ practice,\ any\ position\ fix\ obtained\ using\ the\ given\ system\ has\ a\ 95%\ probability\ of\ having\ a\ radial\ error\ equal\ to\ less\ than\ the\ 2drms\ value\ expressed.~]{.smallcaps}

[~2.\ Minimum\ Shift\ Keying\ (MSK)\ is\ a\ special\ form\ of\ frequency\ modulation.\ MSK\ involves\ utilizing\ the\ smallest\ possible\ frequency\ shift\ of\ the\ carrier\ frequency\ to\ relay\ digital\ information.\ A\ shift\ up\ in\ frequency\ from\ the\ carrier\ relays\ a\ digital\ "1"\ and\ down\ a\ "0".\ In\ practice\ the\ shift\ required\ for\ a\ data\ rate\ of\ 50\ Baud\ is\ a\ 12.5\ Hertz\ frequency\ increase\ or\ decrease\ from\ the\ carrier\ frequency.~]{.smallcaps}

**1.0 BACKGROUND**

The U.S. Coast Guard is mandated by Federal law (14 USC 81) to
implement, maintain, and operate electronic navigation aids that meet
the needs of U.S. Armed forces, maritime commerce, and (if requested by
the Federal Aviation Administration) air commerce. This responsibility
falls under the internationally recognized U.S. Coast Guard mission of
protection of life and property engaged in marine transport. The U.S.
Coast Guard's expertise in enhancing maritime safety through the
utilization of radio (electronic) navigation services dates to 1921 with
the first operational radiobeacons \[1\]. This experience base builds
through the next seven decades with Loran-A, Loran-C, and Omega
services. It should be noted that the U.S. Coast Guard was not directly
involved with the initial research and development of the Loran and
Omega systems. In the case of both of these services the U.S. Department
of Defense (DOD) designed, tested, and implemented the first generation
system. DOD then requested that the U.S. Coast Guard take over the
operation and maintenance of both systems. The U.S. Coast Guard enhanced
the Loran and Omega root designs and developed operational and system
life cycle maintenance doctrine to comply with international maritime
radionavigation standards. Hence, the U.S. Coast Guard's electronic
navigation expertise is well honed in managing the DOD to civilian
transition and the operation of terrestrial navigation systems
originally designed to meet military mission applications.

**1.1 GPS INVOLVEMENT**

In the last two decades, the U.S. Department of Defense (DOD) has led
technology from terrestrial to space-based radionavigation systems,
first with TRANSIT, and then the prototype NAVSTAR Global Positioning
System (GPS). GPS, as with the previous DOD radionavigation systems, was
designed to meet military mission requirements with little consideration
for civilian applications. However, as prototype GPS satellites were
placed in orbit, innovative civil users found economical applications
for the available GPS signals. Industry, perceiving the growing demand,
developed and produced GPS receivers tailored to emerging civil market
applications. DOD requested the U.S. Department of Transportation (DOT)
to assume the lead in civil GPS matters. On 13 February 1989, DOT
assigned the U.S. Coast Guard as the lead agency in providing a civil
GPS interface. The U.S. Coast Guard's assignment to the civil interface
role followed the natural evolution of past U.S. radionavigation
systems. The U.S. Coast Guard had been involved in the investigation of
potential GPS civil use since the late 1970s and was therefore well
prepared for this interface initiative.

**1.2 DIFFERENTIAL REQUIREMENT AND RESEARCH**

During this same period, the U.S. Coast Guard Research and Development
Center had been conducting research and testing of differential
techniques to enhance Loran-C and Omega accuracy. Simply stated, the
differential technique involves installing navigation equipment at a
precisely known location. The equipment receives the Loran or Omega
signal and compares the position solution from the received signal to
its known location. The result of this comparison generates a correction
which is then provided to local users through an independent data link.
The received correction is applied by the user's Loran or Omega
equipment to reduce the system position error, thereby improving the
user's absolute accuracy. The differential effort was driven by the
search for a system with the capability to meet the accuracy requirement
for Harbor/Harbor Approach (HHA) navigation as had been defined in the
Federal Radionavigation Plan (FRP). The FRP identifies that accuracy of
8 to 20 meters (2drms)1 is required for the HHA phase of navigation. The
FRP also states requirements for the Coastal and Ocean Phases for
maritime navigation, which have respectively been satisfied with the
Loran-C and Omega services.

As the DOD development of GPS evolved, and the U.S. Coast Guard gained
knowledge from civil application research, GPS appeared as the natural
progression of differential technology application. The GPS Standard
Positioning Service (SPS) that DOD makes available for civil users
provides predictable accuracy to 100 meters (2drms); roughly a fourfold
improvement on Loran-C predictable accuracy, but not quite as good as
Loran-C repeatable accuracy \[2\]. In fact, GPS military user's Precise
Positioning Service (PPS) accuracy of 21 meters (2drms) is short of the
1992 Federal Radionavigation Plan's HHA requirement. However,
application of differential technology to GPS does promise to provide
the required accuracy improvement. As early as 1983, GPS receiver
manufacturers and the U.S. Department of Transportation, with the U.S.
Coast Guard as a participant, began work to develop a standard
differential GPS correction message format. This effort was coordinated
through the Special Committee (SC) 104 created by the Radio Technical
Commission for Maritime Services (RTCM).

**1.3 DIFFERENTIAL GPS PROOF-OF-CONCEPT**

In 1987 the U.S. Coast Guard Research and Development (R & D) Center
demonstrated that differential corrections broadcast to local user
equipment improved GPS SPS to a predictable accuracy of 10 meters
(2drms) inside the coverage area of the correction broadcast \[3\]. This
U.S. Coast Guard R & D Center work utilized a VHF data link to broadcast
corrections conforming to a draft RTCM SC-104 format at a 50 bits per
second data rate. With the promising capability of DGPS, the HHA
accuracy level requirement was verified by a U.S. Department of
Transportation study of the navigation of vessels over 30,000 dead
weight tons in the restricted waters of the Great Lakes/St. Lawrence
Seaway \[4\].

In 1989, the U.S. Coast Guard modified the existing marine Radiobeacon
located at Montauk Point, New York to broadcast differential corrections
in the RTCM SC-104 format. The Montauk Point field tests demonstrated
that Minimum Shift Keying (MSK) 2 modulation of an existing Radiobeacon
signal was effective in transmission of RTCM SC-104 format corrections.
The MSK modulation technique could be utilized with no adverse effect on
the automatic direction finding receivers of traditional marine
Radiobeacon users. Important to both the U.S. Coast Guard and public;
MSK technology is economical to implement at existing radiobeacons and
within user receivers \[5\]. By January 1990, the RTCM published the
SC-104 format version 2.0 document \[6\]. With a formal U.S. industry
differential GPS correction format and the initial Radiobeacon broadcast
success, Montauk Point began the first continuous public U.S. DGPS
broadcast on August 15, 1990. This transmission marks the beginning of
the U.S. Coast Guard transition from DGPS research and development
towards implementation of a U.S. maritime differential GPS (DGPS)
service.

**1.4 GPS INTEGRITY**

After the highly successful field demonstration of DGPS capability to
meet HHA requirements, concern still remained in a second area - the
recognized civil shortfall of GPS with regards to system integrity. The
DOD monitor and control segment design for GPS can allow a satellite to
transmit erroneous navigation information for up to six hours before
detection and correction or user notification \[7\]. The International
Association of Lighthouse Authorities (IALA) and the International
Maritime Organization (IMO), having kept track of GPS trends and
potential also recognized the need for better integrity than GPS can
provide. DGPS can inherently fill this GPS civil integrity 'gap' with
its continuous monitoring of individual satellite accuracy and virtual
real-time communication link to the navigation service users. Both of
these integral DGPS subsystems are required to generate and broadcast
differential corrections to users. IALA and IMO have also recognized the
potential improvement to navigation safety offered by using a worldwide
standard for HHA radionavigation. IALA has endorsed DGPS and the use of
Medium Frequency (MF) marine radiobeacons as the correction broadcast
medium \[8\].

**1.5 CONCEPTUAL DGPS SERVICE ARCHITECTURE**

With the Federal Radionavigation Plan's stated HHA requirements and the
field results of the R & D Center's Radiobeacon tests, the U.S. Coast
Guard gained U.S. Congressional funding to implement a maritime DGPS
service. A Tentative Operational Requirements (TOR) document was written
and circulated internal to the U.S. Coast Guard in December 1990. The
TOR and the U.S. Coast Guard R & D Center's work established several
DGPS design elements on which to base the DGPS service implementation
effort. These design elements can be separated into desired capabilities
and design constraints, which are summarized in Table 1-1 and Table 1-2
respectively. The conceptual DGPS service architecture founded on the R
& D Center's work is shown in Figure 1-1. The functional elements of the
U.S. Coast Guard DGPS Navigation Service include:

\~ **Reference Station** - Precisely located GPS receiving equipment
with computer to calculate corrections based on comparison of satellite
navigation message to known location.

\~ **Broadcast Site** - A marine Radiobeacon providing correction data
link to DGPS service users.

\~ **Integrity Monitor** - Precisely located MSK Radiobeacon receiver
and GPS receiver capable of applying differential corrections. The
corrected GPS position would be compared to the known position to
determine if the correction broadcast was in tolerance.

\~ **Control Station** - Site for human, centralized control of DGPS
service elements. Also, DGPS service performance data archiving and
processing is accomplished here.

\~ **Communication Network** - Provides connectivity between sites for
passing performance data and control commands.

DGPS user equipment consists of two interfaced receivers with a display;
a Radiobeacon receiver capable of MSK demodulation, and GPS receiver
capable of applying differential corrections from the Radiobeacon
receiver.

+-----------------------------------------------------------------------+
| -   Must be capable of continuous service with system usable at least |
|     99.9% of the time.                                                |
|                                                                       |
| -   Corrections should be provided at a rate sufficient for user      |
|     position fixing to a geodetic accuracy of 10 meters (2drms) or    |
|     better as referenced to WGS-84.                                   |
|                                                                       |
| -   The design should include an automatic independent system to      |
|     continuously monitor and detect system abnormalities and          |
|     failures.                                                         |
|                                                                       |
| -   On detection of abnormality or failure the correction broadcast   |
|     should automatically shutdown.                                    |
|                                                                       |
| -   Sufficient information on system performance to validate accuracy |
|     and integrity shall be automatically archived.                    |
|                                                                       |
| -   Service coverage area should include all critical U.S. waterways  |
|     and extend 20 kilometers seaward.                                 |
+-----------------------------------------------------------------------+

**Table 1-1 Desired DGPS Service Capabilities, December 1990**

+-----------------------------------------------------------------------+
| -   The DGPS broadcast should follow the RTCM SC-104 standard for     |
|     pseudorange corrections.                                          |
|                                                                       |
| -   Existing Radiobeacons would be modified for MSK modulation to     |
|     transmit DGPS corrections at a minimum rate of 50 BPS.            |
|                                                                       |
| -   The design should protect the option of future user fees.         |
|                                                                       |
| -   The design should be kept as flexible as possible to provide ease |
|     of coverage expansion and adding new capabilities to improve      |
|     system performance, or meet new requirements.                     |
|                                                                       |
| -   System lifetime will be at least 25 years.                        |
|                                                                       |
| -   The first Generation system acquisition and deployment cost       |
|     should not exceed 20 million U.S. dollars.                        |
|                                                                       |
| -   Service operation cost, including personnel, should not exceed 4  |
|     million U.S. dollars.                                             |
|                                                                       |
| -   The total service staff for operations, administration, training, |
|     and logistic support should not exceed 60 people.                 |
|                                                                       |
| -   Logistics support is to be a major consideration during system    |
|     design.                                                           |
|                                                                       |
| -   The system should be self-diagnostic with built-in test features. |
|                                                                       |
| -   The Service should be operational in most U.S. coastal areas by 1 |
|     January 1996.                                                     |
+-----------------------------------------------------------------------+

**Table 1-2 DGPS Service Design Constraints, December 1990**

**2.0 DEFINING OPERATIONAL REQUIREMENTS**

The U.S. Coast Guard began the DGPS service implementation phase in 1991
with the formation of a project team. One of the first efforts was to
install three more prototype DGPS Broadcast Sites at existing
radiobeacons. Each site received identical Reference Station equipment.
With the Montauk Point Radiobeacon, the combined broadcasts provided
nearly continuous coverage of the Northeast U.S. coast by June of 1992.
These prototypes are called the Northeast U.S. DGPS Testbed and are
being utilized to gain operational experience. Data is also collected on
actual Testbed equipment reliability, signal coverage, and accuracy
performance. The results are being used, in part, to refine procurement
specifications for Reference Station and Integrity Monitor equipment, as
well as the basis for other engineering decisions.

**2.1 FOUR OPERATIONAL MISSIONS**

It was clear to the Project Team that in addition to gaining field
experience, more detailed DGPS service performance requirements than
provided by the Tentative Operational Requirements and Federal
Radionavigation Plan were needed to support final equipment
specifications. The Tentative Operational Requirements document
identified four missions to be supported by DGPS:

\~ Harbor and Harbor Approach navigation (HHA)

\~ Vessel Traffic Service (VTS) surveillance

\~ Aids To Navigation (ATON) positioning

\~ Exclusive Economic Zone (EEZ) surveying

HHA is the only listed mission that requires a navigation capability for
both government and public users. The other three are government
missions requiring a positioning service. HHA, being considered the
highest risk mission, was therefore chosen for concentrated analysis.
The goal was to accurately model the HHA mission; then utilize the model
to derive measurable performance values for the DGPS service.

**2.2 OPERATIONAL PERFORMANCE DEFINITION**

Traditionally, radionavigation system critical performance is defined in
terms of accuracy, integrity, availability, reliability, and coverage to
be provided. Table 2-1 gives definitions for these five performance
measures from the 1992 Federal Radionavigation Plan. Boundary values are
set for four of these performance terms; the exception is integrity. The
U.S. Coast Guard had extensive experience in specifying navigation
service alarm thresholds to ensure safe oceanic (OMEGA) and coastal
(LORAN) radionavigation. However, in the transition from coastal to
harbor navigation the probability of vessel collision or grounding
clearly increases. In such constricted and congested waterways,
navigation service unreliability can also contribute to mishaps with the
most severe human, economic, and environmental penalties. The DGPS
service is intended to improve HHA navigation safety under all weather
conditions. Therefore, integrity and reliability parameters had to be
established to levels that would significantly reduce the risk of any
navigation casualty while traversing harbor areas.

+-----------------------------------------------------------------------+
| -   **Accuracy** -- the degree of conformance between the estimated   |
|     or measured position of a platform at a given time and its true   |
|     position.                                                         |
|                                                                       |
| -   **Availability** -- the percentage of time that the services of   |
|     the system are usable.                                            |
|                                                                       |
| -   **Coverage** -- the surface area or space volume in which signals |
|     are adequate to permit the user to determine position to a        |
|     specified level of accuracy.                                      |
|                                                                       |
| -   **Reliability** -- the probability of performing a specified      |
|     function without failure under given conditions for a specified   |
|     period of time.                                                   |
|                                                                       |
| -   **Integrity** -- the ability of a system to provide timely        |
|     warning to users when it should not be used for navigation.       |
+-----------------------------------------------------------------------+

**Table 2-1 Navigation System Parameter Definitions from the\
1992 Federal Radionavigation Plan**

**3.0 HARBOR NAVIGATION RISK ANALYSIS**

The U.S. Coast Guard DGPS Project Team chose to study marine operations
in the Saint Mary's River to refine integrity and reliability
requirements for the Harbor and Harbor Approach (HHA) mission. The river
provides the waterway connection between Lake Superior and Lake Huron on
the border of Canada and the United States. The Saint Mary's River is
the navigation choke point for bulk cargo vessels, 600 to 1000 feet in
length, that connect western Lake Superior product ports with the Lake
Huron and Lake Michigan industrial centers. Economics of the steel
industry, coupled with winter ice closure of the Saint Mary's River,
have driven the construction of iron ore bulk vessels to the maximum
length, beam, and draft physically capable of navigation through the
narrow locks and rock-cut channels that characterize the waterway.

**3.1 NAVIGATION RISK MODEL**

A rate of 4.0x10-5 navigation casualties per ship-hour was calculated
from U.S. Coast Guard operational and incident data covering ten years
(1981-1990) on the Saint Mary's River. This value is accepted as the
navigation-related casualty risk for the harbor and harbor approach
(HHA) scenario while using visual and radar navigation methods combined.
The Project Team set as its goal for the addition of the DGPS service to
result in a two-fold improvement in marine safety and efficiency. A goal
was therefore established that DGPS service implementation will achieve
a factor of two reduction in HHA navigation casualties. This equates to
deriving DGPS service reliability and integrity specifications to meet a
HHA navigation risk factor of 2.0x10-5 casualties per ship-hour. A
navigation risk model was derived by the project team and based on the
average number of waypoint maneuvers, maneuver duration, and vessel
speed used to transit the Saint Mary's River. A full discussion of the
model used is provided in reference \[9\]. By setting the model equal to
the design risk factor of 2.0x10-5, a set of performance specifications
are derived to achieve satisfactory levels of service integrity and
reliability. These specifications were summarized in Table 3-1 along
with the other service performance requirements taken from the Tentative
Operational Requirements (TOR) and 1992 Federal Radionavigation Plan
(FRP).

+-------------------------------------+--------------------------------+
| Accuracy                            | 8 -- 20m, 2drms                |
+-------------------------------------+--------------------------------+
| Availability                        | 99.7 percent                   |
+-------------------------------------+--------------------------------+
| Reliability, by Maneuver Category\* | 1: 2000\                       |
|                                     | 2: 1000 outages per\           |
|                                     | 3: 500 million hours           |
+-------------------------------------+--------------------------------+
| Protection Limit                    | 13m (at 8m, 2drms accuracy)    |
|                                     |                                |
|                                     | 32m (at 20m, 2drms accuracy)   |
+-------------------------------------+--------------------------------+
| Time to Alarm                       | 10 seconds                     |
+-------------------------------------+--------------------------------+
| Probability of Missing Detection    | 1x10^**-6**^ per hour          |
+-------------------------------------+--------------------------------+
| Total Alarm Rate,\                  | 1: 2000\                       |
| by Maneuver Category\*              | 2: 1000 outages per\           |
|                                     | 3: 500 million hours           |
+-------------------------------------+--------------------------------+
| \* Categories are defined by        |                                |
| maneuver duration                   |                                |
|                                     |                                |
| -   1: \<140 seconds                |                                |
|                                     |                                |
| -   2: 140 to 280 seconds           |                                |
|                                     |                                |
| -   3: 280 to 500 seconds           |                                |
+-------------------------------------+--------------------------------+

**Table 3-1 U.S. Coast Guard DGPS Service Requirements\
**

**3.2 INTEGRITY PARAMETER DEFINITIONS**

Four parameters are introduced in Table 3-1 to fully define the DGPS
service integrity. The parameters and definitions are adopted from the
Radio Technical Commission for Aviation Services (RTCA), Special
Committee No. 159's work on GPS integrity requirements during aircraft
nonprecision approach. The parameters and their working definitions are
as follows:

\~ **Protection Limit** - the user position error which shall not be
exceeded without display of an alarm to the user

\~ **Time to Alarm** - the maximum allowable time between appearance of
an error outside the protection limit at the user location and display
of an alarm to the user

\~ **Probability of missed detection** - the probability that an error
larger than the protection limit appears at the user location for longer
than the time to alarm without display of an alarm to the user

\~ **Total alarm rate** - the frequency with which the alarm is raised
at any user location (either through false alarm or detection)

While these parameters were developed by the RTCA for aircraft GPS
integrity, it follows that there is direct application to the marine HHA
DGPS integrity. The risk and penalties associated with a modern
passenger airliner approaching Kennedy Airport and a large hazardous
product tank vessel transiting New York harbor are of similar magnitude.
While there are differences in the precision of platform maneuverability
and navigation accuracy required, the two scenarios are analogous in
terms of navigation service integrity.

3.  **OPERATIONAL RESTRICTIONS**

Two vessel operational restrictions must be applied to ensure the
validity of the specified DGPS service requirements. This is due to risk
model variables that were held constant during the analysis. The first
restriction is that for vessel operating speeds of ten knots the free
half channel width must be at least 50 meters. Free half channel is
defined as half the available channel width minus half the vessel's
beam. For situations where the free half channel is less than 50 meters,
minimum DGPS service performance is predicted to be achievable only if
vessel speeds are restricted below ten knots as required by the risk
model. The second restriction is that vessels utilizing the DGPS service
must have minimal maneuvering capabilities defined by the track-keeping
performance bounds used in the risk model. As an example, with a free
half channel of 50 meters and navigation fix accuracy of eight meters
(2drms) the vessel's required track error standard deviation is between
seven and eight meters. This second restriction implies that vessel
track-keeping performance certification may be a future user requirement
for the DGPS service in low-visibility navigation of some identified
restricted channels.

**4.0 SYSTEM ARCHITECTURE ANALYSIS**

For U.S. Coast Guard system engineers to prepare individual equipment
functional requirements, a DGPS service architecture is required in
addition to the operational performance requirements. Three candidate
architectures were considered during the last three months of 1992. The
set of architectures reflected the most promising configurations of the
DGPS service functional elements based on the experience with the U.S.
Coast Guard prototype broadcasts and the results of international DGPS
work. Each architecture had to be individually evaluated without bias.
Meeting DGPS service minimum functions and rating performance of each
function are the primary measures for architecture evaluation.

**4.1 ARCHITECTURES CONSIDERED**

The three candidate architectures evaluated for the DGPS service are
presented by Figure 4-1 in block diagram format. The commonality shared
by each is:

\~ The DGPS data link is implemented by MSK modulation of a marine
Radiobeacon signal.

\~ The existing marine Radiobeacon monitor and control equipment would
be used and renamed Differential Broadcast Site Monitor (DBSM).

\~ The broadcast messages would adhere to the RTCM SC-104 standard.

\~ 47 specific geographic sites of existing U.S. Coast Guard
radiobeacons are used to provide the DGPS service coverage.

\~ Two Control Stations, one each on the East and West U.S. Coasts with
responsibility for nominally equal halves of field site control; each
would normally store operational data on all remote sites and be capable
of controlling the entire DGPS service for redundancy.

As would be expected, each architecture also has unique characteristics.
Architecture 1 locates a Reference Station at each of the 47 Broadcast
Sites with a site-associated monitor for integrity and broadcast
coverage placed near the limit of each site's coverage. Architecture 2
locates the Integrity Monitor and Reference Station at each of the 47
Broadcast Sites with additional monitors located in the far field of ten
selected radiobeacons. Architecture 3 uses half the number of Reference
Stations (24), each providing corrections to two Broadcast Sites with 47
Integrity Monitors each located at a Broadcast Site. Architecture 3 also
restricts far field monitors to ten, but these are located at selected
Reference Stations. The three architectures offer a variety of Reference
Station and Integrity Monitor equipment placement schemes. This variety
is expected to sift out the optimal configuration in terms of the
overall performance, reliability, and cost during the evaluation
process.

2.  **ARCHITECTURE EVALUATION**

Evaluation criteria are grouped into four broad categories: Functional,
Performance, Cost, and Intangibles. As much as possible, quantitative
measures were developed to support bias-free comparison of each
architecture alternative. A category score between zero and 100 was
assigned based on the architecture's total measure for the category. An
architecture's ability to meet all functional and performance
requirements was considered more important than cost and intangible
benefits. Therefore, to determine the overall architecture score, each
category was multiplied by a weighting factor determined by unanimous
decision of the project team. The following formula presents the
selected category weights:

Architecture Score = 0.30 X Function Criteria Score

0.35 X Performance Criteria Score

0.25 X Cost Criteria Score

0.10 X Intangible Criteria Score

+-----------------------------------------------------------------------+
| **ARCHITECTURE 1**                                                    |
|                                                                       |
| PSS DBSM & PSS                                                        |
|                                                                       |
| PSS                                                                   |
|                                                                       |
| PSS: Packet Switched Service of X.25 Protocol\                        |
| DBSM: Differential Broadcast Site Monitor                             |
|                                                                       |
| ARCHITECTURE 2                                                        |
|                                                                       |
| PSS DBSM & PSS                                                        |
|                                                                       |
| Through PAD                                                           |
|                                                                       |
| PSS: Packet Switched Service of X.25 Protocol\                        |
| DBSM: Differential Broadcast Site Monitor\                            |
| PAD: Packet Assembler / Dissasembler                                  |
|                                                                       |
| ARCHITECTURE 3                                                        |
|                                                                       |
| DBSM DBSM                                                             |
|                                                                       |
| PSS                                                                   |
|                                                                       |
| PSS: Packet Switched Service of X.25 Protocol\                        |
| DBSM: Differential Broadcast Site Monitor                             |
+-----------------------------------------------------------------------+

**Figure 4-1**

***\
The following paragraphs provide a brief discussion of each evaluation
category.\
*4.2.1 CRITERIA FOR FUNCTION**

Each architecture was scored by a checklist that reflects the minimum
functions required to implement a DGPS service that achieves the defined
operational requirements. The list, which entails 581 individual
function checks, is divided into sub-lists. There is one sub-list for
each of the common DGPS service block functions of broadcast, Reference
Station, monitor, control, and user. The sub-lists were developed to be
independent of architecture details. A candidate architecture was scored
on the percentage of checklist functions met without any identified
risk. In all, 581 distinct functions were checked for in each
architecture. Any redundant provisions for a single function was noted
and used as a factor during the reliability portion of the
architecture's performance scoring. All three of the candidate
architectures supported all functions with varying degrees of functional
redundancy. Therefore, all three received equal functional scores of 100
percent.

**4.2.2 CRITERIA FOR PERFORMANCE**

An architecture's performance score was based on the percentages of
performance requirements, presented in Table 3-1, that are fully met
without identified risk. As would be expected, these criteria consumed
the largest portion of the architecture evaluation effort. Evaluation
criteria are consolidated into three categories. The first category
considered accuracy, availability, and coverage together as
interdependent performance elements. Reliability and integrity were
examined separately in the other two categories.

1.  **ACCURACY, AVAILABILITY, AND COVERAGE**

The first category focus was on determining if the specified accuracy is
available to the level required throughout the desired coverage area.
Each architecture's duration of service outage is determined by the
combination of two distinct components; 1)outages due to failures of the
DGPS service equipment, and 2)outages due to the signal propagation
environment. Ground system available time for one year was calculated
using Reference Station and broadcast equipment interconnection models
for each architecture. The model was identical for Architecture 1 and
Architecture 2. In Architecture 3, the significant difference was the
communications link equipment between the geographically separated
Reference Station and Broadcast Sites. Failure and repair interval data
for each equipment type, including primary and backup power, was
compiled and inserted in each model to calculate ground system
availability as percentages of one year. A software tool named COAST was
utilized to compute the outage time due to environmental signal effects.

The COAST acronym stands for Coverage Analysis Support Tool and is a
custom software package developed under contract for the U.S. Coast
Guard DGPS Project Team. The software uses Millington's Method to model
Radiobeacon signal propagation over variable ground conductivity's and
includes signal-to-noise degradation due to noise seasonal variations
\[10\]. The software also predicts bit error rates in the DGPS
corrections due to environmental impulse noise (lightning). Finally a
GPS satellite geometry and availability model is used with selectable
DGPS error budget values to predict accuracy. An interactive menu allows
creation of variable power radiobeacons at any U.S. location. Results
are displayed graphically by contour plots of signal strength,
signal-to-noise ratio, bit error rate, and accuracy based on minimum
availability values selected. The COAST evaluation of each
architecture's accuracy, availability, and coverage was performed over a
selected set of six Radiobeacon sites to reduce computation time for
each architecture. The sites were selected to give a cross section
representation of the U.S. marine radiowave propagation environment. The
DGPS error budget values and key assumptions used in the COAST work are
given in Table 4-1 and 4-2 respectively.

  ------------------------ ---------------- ---------------------------------
  **ERROR COMPONENT**      **RMS VALUE      **RATIONAL**
                           (m)**            

  DGPS correction error    1                Assumed dominated by multipath at
                                            reference antenna.

  Local user error         0.5              Assumed dominated by multipath at
                                            user antenna.

  Spatial decorrelation    3 ppm of         Selected to be consistent with
                           baseline         RTCM\
                                            SC-104 reported range of 1 to 11
                                            ppm (reference 6)

  Reference-to-broadcast   3 ppm of         Assumes additional error due to
  decorrelation\           reference\       separation of reference and
  (architecture 3 only)    to broadcast     broadcast is common to service
                           baseline         area
  ------------------------ ---------------- ---------------------------------

**Table 4-1 DGPS Error Budget Values**

+-----------------------------------------------------------------------+
| -   No beacon coding                                                  |
|                                                                       |
| -   All correction messages are Type 1                                |
|                                                                       |
| -   100 bits / second MSK transmission rate                           |
|                                                                       |
| -   100 microvolts / meter beacon signal strength at limit of         |
|     coverage area                                                     |
+-----------------------------------------------------------------------+

**Table 4-2 Key Assumptions in Accuracy, Availability, and Coverage
Analysis**

**4.2.2.2 RELIABILITY**

The reliability performance analysis was based on the probability of an
unforecast DGPS service outage occurring during a crucial user maneuver.
Failure probabilities of two distinct systems were separately
considered; DGPS service equipment and failure of GPS satellite signals.
However, GPS satellite reliability is not dependent on DGPS service
architecture and is therefore a "constant" for all architectures
evaluated.

Each system's unreliability can be defined by the probability of
critical equipment failure occurring during a crucial user maneuver,
given that all critical equipment was operational at the start of the
maneuver. The equipment interconnection models developed for each
architecture (discussed in Section 4.2.2.1) were used to establish
failure relationships. Critical subsystem reliability was then
calculated with a Markov Chain methodology. This methodology factors in
the effect of equipment failure and repair for subsystems with redundant
components. Equipment mean-time-between-failure and mean-time-to-restore
data is required as input. Assuming that equipment failures are equally
likely to occur at any given time, the calculated probabilities of
failure over specific crucial time interval can be converted to a value
in terms of failure rates. Once converted, each architecture's service
reliability was directly compared to the reliability performance
requirement expressed in outages per million hours (Table 3-1).

**4.2.2.3 INTEGRITY**

This portion of the performance evaluation centers on quantifying each
architecture's effect on maintaining integrity service at the required
level of availability within the coverage area. The DGPS service will
provide integrity service through monitor and alarm capabilities.
Integrity service is considered available when total alarm rate and
probability of missed detection specifications are met for the required
protection limit and time-to-alarm values (Table 3-1). Integrity
availability within the coverage area was assessed by evaluating the
following three factors separately:

\~ Availability of equipment associated with monitor and alarm functions

\~ Availability of differential GPS corrections at the monitor

\~ Normal variability of DGPS errors

Availability of integrity ground equipment was computed from
interconnection models of each architecture's critical Reference
Station, broadcast, and monitor equipment. Each equipment's mean time
between failures and mean time to repair data was then applied to
determine an architecture's availability of integrity ground equipment.

Differential corrections must be received regularly by the monitor for
integrity service to be provided. For this analysis, a broadcast was
defined to be unmonitored if more than half of the broadcast correction
messages are not received by the assigned monitor. In other words, a
broadcast is unmonitored when the probability that a correction message
will be missed by the monitor is 0.5 or greater. Assuming that
corrections are broadcast by Type 1 message for seven satellites, each
correction message is 420 bits in length by definition of the RTCM
SC-104 standard. Assuming that the bit-to-bit error is uncorrelated, the
probability of receiving a Type 1 message can be expressed as:

P(420 good bits) = (1-BER)420

Setting the above equation equal to 0.5 and solving for the bit error
rate (BER) yields 0.0016. The previously defined condition for an
unmonitored broadcast can now be expressed in terms of a bit error rate
value of 0.0016. This portion of the evaluation is only applicable to
Architecture 1 as the other two architectures locate the monitor at the
broadcast source. Architecture 1 places the Integrity Monitor at the
limit of each broadcast's coverage area. COAST was used to calculate the
availability of a 0.0016 bit error rate at the limit of specified
coverage areas. The COAST calculations were completed for the same six
representative radiobeacon sites used for the accuracy, availability,
and coverage evaluation discussed in Section 4.2.2.1. The results
predicted that Architecture 1 Integrity Monitors will be available 93.0
to 99.7 percent of a year based on correction loss alone. At best, this
predicts that Architecture 1's DGPS service availability will not meet
the Table 3-1 performance requirements due to unmonitored time alone.

The final factor in the integrity evaluation was to determine the
unavailability caused by variability of DGPS error between the monitor
location and any DGPS service user location. If this error variability
is too large, user errors will occur beyond the protection limit while
monitor error is below alarm threshold. The reverse will occur when a
user's error is less than the service area protection limit during a
broadcast alarm due to error at the monitor beyond the threshold. Either
scenario results in unavailability of integrity service due to missing
detection of a user error, or a false user alarm. The tolerable number
of missed detection of user error and numbers of false user alarms are
set by the DGPS service requirements for probability of missed detection
and total alarm rate given in Table 3-1. The unavailability of the
integrity service due to DGPS error variance was calculated in three
steps. First, the probability of a missed user error detection was
computed as a function of horizontal dilution of precision (HDOP) given
that the total alarm rate requirement is met. Second, the HDOP value was
found for the condition where the required probability of missed user
error detection is met. The final step was to define integrity service
unavailability as the unavailability of the HDOP value solved for.

The unavailability of integrity service was calculated for two user
locations for each architecture. The first location considered was at
the limit of the specified coverage area. The second location was at
half the limit of the specified signal coverage area. Architectures 1
and 2 calculations result in identical unavailability levels of
integrity service in both user locations and Architecture 3 has somewhat
higher unavailability. The poorer integrity availability of Architecture
3 was no surprise with the addition of higher spatial error over the
service area. The identical results for Architectures 1 and 2 indicate
that integrity availability is not affected by placement of a single
point monitor when the evaluation is done over the entire service area.

**4.2.2.4 FINAL PERFORMANCE SCORE**

Architecture performance score is a percentage from 0 to 100. Accuracy,
availability, coverage, reliability, and integrity were given equal
weight in determining a final architecture score. An architecture is
given twenty percent if it supports a performance category. The
accuracy, availability, and coverage performance of all three
architectures was considered nearly equal. Architectures 1 and 2
received credit for reliability while Architecture 3 did not due to it's
marginal rating. Architecture 2 showed a significant advantage over the
other two architectures in integrity and is therefore the only
architecture to receive credit for integrity.

**4.2.3 CRITERIA FOR COST**

A life-cycle cost methodology was used for this evaluation with a cost
score assigned relative to the lowest total cost of the three
architectures. The cost score was calculated by the formula:

Cost Score = 100 (Lowest Total Cost/Total Architecture Cost)

The evaluation is divided into three broad cost categories of
acquisition, maintenance, and operations. Where possible, actual market
quotes from late 1992 were used for equipment acquisition costs.
Developmental costs for equipment not commercially available and all
maintenance costs were estimated, along with installation and
construction costs. Included in each architecture maintenance estimate
is technician labor, spare components, site upkeep, and service
contracts. All estimates were based on past U.S. Coast Guard experience
with other radionavigation services.

Operational cost estimates for each architecture considered the required
staff labor, property rent, utilities, and communications network.
Estimating was again accomplished using historical data from other U.S.
Coast Guard radionavigation services with the exception of
communications. A commercial packet switched service (PSS) in the X.25
protocol was used for network communications. Use of PSS technology
promises greater network flexibility to future expansions and lower
long-term costs compared to U.S. Coast Guard leased dedicated data grade
circuits for established radionavigation services. Quotes were obtained
for X.25 protocol PSS available through a U.S. Government service
contract. Considerable effort was then expended to develop a message
structure for each architecture. Each structure is a complete set of the
individual messages required to support all functions identified by the
functional checklist discussed in Section 4.2.1. The message structures
provided the means to calculate, with reasonable confidence, the packet
traffic and associated PSS costs for each architecture.

The total architecture life-cycle cost is calculated over the 25 year
baseline taken from Table 1-2. Two assumptions were made and applied to
all architectures. First, all acquisition costs occur in year one.
Second, all recurring maintenance and operations costs are constant from
year to year. The final overall cost is adjusted for the future value of
money using a return on investment of ten percent \[11\]. Table 4-3
presents the total life-cycle cost calculated for each architecture.
Other system costs which are common to all architectures are not shown.

+-----------------+-----------------+-----------------+-----------------+
| **COST          | **COST (in      |                 |                 |
| ELEMENT**       | thousands of    |                 |                 |
|                 | U.S. dollars)** |                 |                 |
+-----------------+-----------------+-----------------+-----------------+
|                 | **ARCHITECTURE  | **ARCHITECTURE  | **ARCHITECTURE  |
|                 | 1**             | 2**             | 3**             |
+-----------------+-----------------+-----------------+-----------------+
| Acquisition     | 3,037           | 3,063           | 2,349           |
|                 |                 |                 |                 |
| Operations      | 1,138           | 1,004           | 1,087           |
|                 |                 |                 |                 |
| Maintenance     | 356             | 357             | 339             |
+-----------------+-----------------+-----------------+-----------------+
| Life -- Cycle   | 16,598          | 15,417          | 15,292          |
| Cost            |                 |                 |                 |
+-----------------+-----------------+-----------------+-----------------+

**Table 4-3 Life-Cycle Cost Comparison**

**4.2.4 CRITERIA FOR INTANGIBLES**

These criteria were developed to provide for a systematic evaluation of
each architecture's benefits and drawbacks that are not accounted for
under the first three criteria categories. The following are considered
as intangibles:

\~ Expandability - can the architecture readily accept addition of
radiobeacons, Reference Stations, and monitors should an increase in
DGPS service coverage area be desired

\~ Flexibility - can the architecture easily support additional
requirements to meet future needs of other agencies, or readily take
advantage of evolving DGPS technology such as wide-area corrections

\~ Vendor Independence - an architecture should not require equipment or
services of a sole supplier thereby eliminating the benefits of
competition

\~ User Operations - an architecture should not add operational burdens
or restrictions on DGPS service users

\~ Risk to Schedule - an architecture must not require more than 48
months to implement

A relative score of 0 to 100 percent was assigned to each architecture
based on the importance of intangibles the architecture does, or does
not, support.

**4.3 SELECTED ARCHITECTURE**

The weighted overall scores for each candidate architecture are
summarized in Table 4-4. Architecture 2 achieved the highest total. From
Table 4-4 the performance criteria is what separates the three
architectures. Architecture 2's higher performance score is attributed
to an advantage in integrity over both of the other architectures and in
reliability over Architecture 3.

+-----------+----------+---------------+--------------+---------------+
| *         | *        |               |              |               |
| *CRITERIA | *WEIGHED |               |              |               |
| C         | SCORE**  |               |              |               |
| ATEGORY** |          |               |              |               |
+-----------+----------+---------------+--------------+---------------+
|           | **TOTAL  | *             | **           | *             |
|           | POINTS** | *ARCHITECTURE | ARCHITECTURE | *ARCHITECTURE |
|           |          | 1**           | 2**          | 3**           |
+-----------+----------+---------------+--------------+---------------+
| F         | 30       | 30.0          | 30.0         | 30.0          |
| unctional |          |               |              |               |
|           | 35       | 7.0           | 14.0         | 0.0           |
| Pe        |          |               |              |               |
| rformance | 25       | 22.9          | 24.8         | 25.0          |
|           |          |               |              |               |
| Cost      | 10       | 7.1           | 10.0         | 7.1           |
|           |          |               |              |               |
| In        |          |               |              |               |
| tangibles |          |               |              |               |
+-----------+----------+---------------+--------------+---------------+
| Life --   | 100      | 67.0          | 78.8         | 62.1          |
| Cycle     |          |               |              |               |
| Cost      |          |               |              |               |
+-----------+----------+---------------+--------------+---------------+

**Table 4-4 Architecture Weighted Scores**

![](media/image1.png){width="6.493055555555555in"
height="4.366666666666666in"}Architecture 2 was selected for the
implementation of the U.S. Coast Guard DGPS service; however the
evaluation process identified several probable areas of marginal
performance. Most concerning is the predicted shortfall of accuracy,
availability, and coverage requirements. Techniques devised to improve
the differential correction data link immunity to noise, particularly
impulses, promised the highest improvement in overall DGPS service
performance. The U.S. Coast Guard DGPS Navigation Service implemented
with the chosen architecture is presented in Figure A-1.

**5.0 OPERATIONAL DOCTRINE**

With the architecture decision finalized in December 1992, the U.S.
Coast Guard DGPS project effort turned to formally documenting the DGPS
service signal specifications, architecture, and performance parameters.
A signal or broadcast specification is the root document for the
operation of all public radionavigation services. This document lists
DGPS service performance parameters and provides the broadcast
implementation details required by the user equipment designers and
manufacturers. As importantly, the broadcast standard document provides
the foundation for validation of radionavigation service performance.
The architecture evaluation and selection process predicted that the
U.S. Coast Guard would fall short of accuracy, availability, and
coverage requirements for harbor and harbor approach navigation (Table
3-1). This gloomy forecast was supported by the current experience from
the Northeast Testbed prototype broadcasts and the COAST predicted
Radiobeacon signal strength coverage contours. Several key changes to
operational doctrine were adopted in the BROADCAST STANDARD FOR THE USCG
DGPS NAVIGATION SERVICE approved for publication in April 1993. These
operational changes are discussed in the following paragraphs and all
are associated with improving on the predicted shortfalls in DGPS
service performance.

**5.1 BROADCAST SITE SELECTION**

COAST-generated signal strength contours showed several critical
coverage gaps with the initial selection of 47 existing Broadcast Sites.
Figure 5-1, provided as one example, shows the signal strength coverage
of the four Northeast U.S. Testbed radiobeacons. The Radiobeacon range
circles all have radii of 250 nautical miles and are displayed for scale
reference. The lighter gray regions indicate overlapping, or redundant,
coverage. Note the New York City harbor coverage "hole" predicted due to
the attenuation of the Montauk Point broadcast by the Long Island
landmass. Spot field measurements verify that the predicted "hole" does
exist. A refined list of 50 Radiobeacon sites resulted after running
numerous COAST signal strength coverage scenarios. A Radiobeacon site at
Sandy Hook, New Jersey has been proposed to fill the New York City
example cited. Sites are also adjusted to maximize coverage overlap in
harbors with Vessel Traffic Systems (VTS) to meet a higher VTS signal
availability requirement of 99.9 percent \[12\]. There is an
implementation cost associated with optimizing U.S. marine coverage and
availability performance through site selection. One-third of sites on
the refined list of 50 Broadcast Sites are not existing marine
radiobeacons. The cost tradeoff for installation of new radiobeacons is
easily justified by the large reduction in the risk of failing to meet
the DGPS service availability and coverage performance requirements.

**5.2 RTCM TYPE 9 CORRECTIONS**

The COAST performance computations for the architecture analysis
discussed in Section 4.2.2.1 assumed the RTCM Type 1 message is used for
broadcasting differential corrections. Marginal DGPS service accuracy,
availability, and coverage performance is predicted primarily due to the
correction data link susceptibility to propagation path noise. Reducing
the probability of bit error to any given message frame suggests a large
return in improved performance.

The RTCM standard, Reference \[6\], defines the Type 9 message as a
second format for transmission of satellite corrections. The Type 9
message format is variable in length to contain corrections for any
chosen number of satellites, from one to all. Grouping satellite
corrections into sequential Type 9 messages reduces the opportunity for
any single message loss due to noise induced bit errors. The associated
cost is increased overhead, as each RTCM message has a universal header
contained in the first two message words. The universal header is
required for synchronization and time coordination of the DGPS service
user's equipment, in addition to identifying the message type, message
length, originating station, and alarm conditions. The optimal balance
between overhead and correction latency, without bit errors, is
theoretically found with Type 9 messages that contain corrections for
three satellites \[13\].

The U.S. Coast Guard DGPS service Broadcast Standard specifies these
corrections will be broadcast exclusively in the Type 9 format. Each
Type 9 message frame will contain corrections for three satellites.
Sequential Type 9 messages will provide a complete set of differential
corrections for all satellites in-view of the Reference Station. The
final Type 9 in a sequence will contain corrections for one, two, or
three satellites, depending on the in-view satellite count. For example,
for eight satellites in view a complete sequence will consist of three
Type 9 messages; the first two messages containing corrections for three
satellites each and the last with corrections for two satellites. This
scheme of implementing the Type 9 format ensures that an equal number of
corrections are broadcast for each in-view satellite.

![](media/image2.png){width="6.496527777777778in"
height="5.504861111111111in"}**Figure 5-1**

In addition to improved noise immunity over the Type 1 broadcast, there
are two other significant benefits gained with the Type 9-3 format; the
"-3" notation is for three satellites as described in the preceding
paragraph. First, the average correction latency is reduced over the
Type 1 format. This results in increased accuracy performance,
particularly in the presence of high pseudorange accelerations for one
or more satellites. Second, the RTCM universal message header occurs
more often over a given time period. The result is increased opportunity
for user equipment synchronization and reduced intervals between alarm
transmission windows.

Reference \[14\] reports on the first U.S. Radiobeacon field tests of a
Type 9 broadcast. These tests began in early June of 1993 at a new U.S.
Coast Guard Radiobeacon, installed as an experimental site near New
Orleans, Louisiana. The preliminary results reported strongly
corroborate the theoretical Type 9 advantages. Briefly, Type 9 messages
from the New Orleans Radiobeacon were received at a known location in
Houston, Texas (over 250 Nautical miles away) with position accuracy
better than 5 meters (2drms). Type 1 format messages were simultaneously
broadcast from the New Orleans beacon with a second carrier frequency 1
kilohertz below the Type 9 modulated carrier. The Type 1 messages were
not received at the Houston location. The Type 9 broadcast from New
Orleans will be resumed later this year. Far-field data will be logged
for performance comparison to the Northeast U.S. Testbed Type 1
broadcasts.

**5.3 TRANSMISSION EFFICIENCY**

The Radiobeacon has evolved over the past seventy years from the primary
U.S. marine radionavigation aid to its present status of a tertiary aid.
Throughout this evolution, the Radiobeacon priority for labor and fiscal
support has become less competitive within the overall U.S. Coast Guard
mix of navigation aids. Steps have been taken to minimize the required
Radiobeacon maintenance, particularly over the last ten years. Major
savings resulted by installing fiberglass whip antennas to replace aging
guyed towers. The loss in antenna radiation efficiency was justifiable
at the time. The new Radiobeacon role within the DGPS service
architecture indicates that reconsideration of higher efficiency
antennas is in order.

A guyed, top-loaded, 90-foot tower is installed at the experimental New
Orleans Radiobeacon. Matched to a U.S. Coast Guard inventory one
kilowatt transmitter, the antenna is achieving a radiation efficiency
that approaches fifteen percent \[13\]. This is a considerable
improvement over the best fiberglass whip efficiency of three percent.
While impressive from the electrical engineer's view, the New Orleans
antenna has raised numerous structural engineering concerns over
increased maintenance. Also in question is the antenna's ability to
survive the ice, wind and salt spray conditions over the spectrum of
U.S. coastal environments. The impact to DGPS service reliability of a
more maintenance intensive and less environmentally robust antenna needs
to be assessed. This will be possible from the experience gained at the
New Orleans site.

A second modification is under experimentation at the New Orleans
Radiobeacon. The beacon has been operating with a single carrier since
mid-June of 1993 when the simultaneous Type 1 and Type 9 broadcast test
was completed. A secondary carrier is required at operational U.S.
radiobeacons. This carrier is modulated to provide each individual
beacon with a unique Morse Code identification tone. This secondary
carrier is not used at the New Orleans site. Obviously, this provides
full amplification capacity to the MSK modulated primary carrier with
significant improvement to coverage and availability performance.
Current Radiobeacon policy and user requirements must be consulted prior
to implementing single carrier operation at radiobeacons integral to
international navigation plans. It is believed that the individual
broadcast identifier available within the universal header of each RTCM
message will satisfy all Radiobeacon user identification requirements.

**6.0 SUMMATION**

This paper provides a background sketch of U.S. Coast Guard differential
efforts prior to 1991. Discussion continues to briefly report on each
major element of the design progression over the first half of the
five-year DGPS service implementation project. The design has logically
progressed through the following steps:

\~ Evaluation of current DGPS technology through prototype broadcasts

\~ Requirements definition through analysis of the most demanding DGPS
service mission

\~ Systematic evaluation of candidate DGPS service architectures

\~ Exploration of existing operational doctrine to achieve higher DGPS
service performance

The procurement process is now being carried out. Specifications are
being developed based on the experience gained and decisions made to
date. Delivery of the first field equipment is expected by August 1994.
The Broadcast Site list and site installation schedule is being
finalized for public release early in calendar year 1994. Installations
will be accomplished as the Reference Station and Integrity Monitor sets
are received. The Control Station computer hardware has been procured;
Control Station software design began in earnest in May 1993. The first
version of the Control Station software is scheduled to be installed at
the eastern U.S. Control Station in late 1993 or early 1994. The Control
Station software beta test will be conducted with the Reference Station
and Integrity Monitor capabilities available from the Northeast U.S.
Testbed sites.

One of the most formidable remaining tasks is the drafting of a DGPS
service Concept of Operations document. This U.S. Coast Guard
organizational document includes internal support responsibility
assignments. This document, coupled with electronic equipment support
plans, will begin the recurring budget processes to provide future
moneys for the DGPS service life-cycle maintenance costs. This document
will raise the level of effort devoted to marine radiobeacons. DGPS
radiobeacons will be a top priority on the national level as the
critical segment in the newest public navigation service. Current
Radiobeacon policy, from equipment failure response to preventive
maintenance, must be revised as the majority of beacons will be much
more than individual navigation aids. Despite the grandest of
engineering design endeavors, the DGPS service availability and
reliability requirements are NOT achievable without long-term
infrastructure support commitment.

The U.S. Coast Guard will continue to fully cooperate on international
fronts with the International Association of Lighthouse Authorities
(IALA) and the International Maritime Organization (IMO) to achieve
global DGPS commonality. Nationally, the U.S. Coast Guard is consulting
with other agencies to adapt the DGPS service to meet their needs. U.S.
Coast Guard representatives are participating within a RTCM Special
Committee 104 working group. This group is developing commercial
standards for Reference Station, Integrity Monitor, and user equipment
specifications. Through significant enhancement of maritime safety and
the inherent water transport efficiency increase, the DGPS service will
be a high value navigational asset.

**ACKNOWLEDGEMENTS**

The author would like to extend thanks to LTJG David R. Cragg of the
ONSCEN DGPS Operations Staff for many late hours of editing and
formatting this paper.

**REFERENCES**

1\. O'Brien, Arthur E., History and Status of U.S. Marine Radiobeacon
System, U.S. Department of Transportation, DOT-CG-N-1-82, National
Technical Information Service, Springfield, VA, February 1983.

2\. 1992 Federal Radionavigation Plan, U.S. Department of Defense,
DOD-4650.4 and U.S. Department of Transportation, DOT-VNTSC-RSPA-92-2,
National Technical Information Service, Springfield, VA, January 1993.

3\. Pietraszewski, D., Spalding, J., Viehweg, C., Luft, L., U.S. Coast
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6\. RTCM Recommended Standards for Differential NAVSTAR GPS Service,
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134-89/CS 104-68, January 1990.

7\. Alsip, D., Butler, J., Radice, J., The U.S. Coast Guard's
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June 1992.

8\. Recommendations for Government Provided DGPS Service, International
Association of Lighthouse Authorities, October 1991.

9\. Creamer, P., Alsip, D., Radziszewski, J., Performance Requirements
for the Coast Guard's Differential GPS Service, ION National Technical
Meeting, January 1993.

10\. Pisano, J., Creamer, P., Desrochers, G., Radziszewski, J., Marine
DGPS Requirements and Expected Coverage, RTCM General Assembly, June
1993.

11\. Wilson, R., Decision-Making Guide for the Proposed Coast Guard
Differential Global Positioning System, Thesis, Naval Postgraduate
School, Monterey, CA, June 1991.

12\. Tentative Operational Requirements for a Differential Global
Positioning System, U.S. Coast Guard, November 1990.

13\. Broadcast Standard for the USCG DGPS Navigation Service, U.S. Coast
Guard COMDTINST M16577.1, April 1993.

14\. Enge, P., Young, D., Sheynblatt, L., Westfall, B., DGPS/Radiobeacon
Field Trials Which Compare Type 1 and Type 9 Messaging, ION National
Technical Meeting, June 1993.

15\. Braasch, M., van Graas, Mitigation of Multipath in DGPS Ground
Reference Stations, Proc. ION National Technical Meeting, San Diego,
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