Garmin GBR23 User Manual

owner’s

manual

and

reference

guide

GBR 23

beacon receiver

© 2000 GARMIN Corporation

GARMIN International, Inc.
1200 East 151st Street, Olathe, Kansas 66062, U.S.A.
Tel. [phone] or 800/800.1020
Fax [phone]

GARMIN (Europe) Ltd.
Unit 5, The Quadrangle, Abbey Park Industrial Estate, Romsey, SO51 9AQ, U.K.
Tel. [phone]
Fax [phone]

GARMIN (Asia) Corporation
No. 68, Jangshu 2nd Rd., Shijr, Taipei County, Taiwan
Tel. [phone]
Fax [phone]

All rights reserved. Except as expressly provided herein, no part of this manual may
be reproduced, copied, transmitted, disseminated, downloaded or stored in any
storage medium, for any purpose without prior written consent of GARMIN
Corporation. GARMIN Corporation hereby grants permission to download a single
copy of this manual onto a hard drive or other electronic storage medium to be
viewed for personal use, provided that such electronic or printed copy of this
manual contains the complete text of this copyright notice and provided further that
any unauthorized commercial distribution of this manual is strictly prohibited.

Information in this manual is subject to change without notice. GARMIN
Corporation reserves the right to change or improve its products and to make
changes in the content without obligation to notify any person or organization of
such changes. Visit the GARMIN website for current updates and supplemental
information concerning the use and operation of this and other GARMIN products.

Website address: www.garmin.com

GARMIN and GBR 23 are registered trademarks of GARMIN Corporation and may
not be used without the express permission of GARMIN Corporation.

May 2000 Part Number 190-00195-00 Rev. A Printed in Taiwan

GARMIN Corporation warrants this product to be free
from defects in materials and manufacture for one year
from the date of purchase. GARMIN will, at its sole option,

Limited Warranty

repair or replace any components that fail in normal use.
Such repairs or replacement will be made at no charge to
the customer for parts or labor. The customer is, however,
responsible for any transportation costs. This warranty does
not cover failures due to abuse, misuse, accident or
unauthorized alteration or repairs.

THE WARRANTIES AND REMEDIES CONTAINED
HEREIN ARE EXCLUSIVE AND IN LIEU OF ALL OTHER
WARRANTIES EXPRESS OR IMPLIED OR STATUTORY,
INCLUDING ANY LIABILITY ARISING UNDER ANY
WARRANTY OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE, STATUTORY OR OTHERWISE.
THIS WARRANTY GIVES YOU SPECIFIC LEGAL RIGHTS,
WHICH MAY VARY FROM STATE TO STATE.

IN NO EVENT SHALL GARMIN BE LIABLE FOR ANY
INCIDENTAL, SPECIAL, INDIRECT OR CONSEQUEN­TIAL DAMAGES, WHETHER RESULTING FROM THE
USE, MISUSE, OR INABILITY TO USE THIS PRODUCT
OR FROM DEFECTS IN THE PRODUCT. SOME STATES
DO NOT ALLOW THE EXCLUSION OF INCIDENTAL OR
CONSEQUENTIAL DAMAGES, SO THE ABOVE

The GARMIN GBR 23 has no
user-serviceable parts. Should
you ever encounter a problem
with your receiver, please contact
your GARMIN dealer or the
GARMIN Customer Service
department (913-397-8200 or
800-800-1020) for repairs.
Any attempt to open the case to
change or modify the unit in any
way will void your warranty and
may result in permanent damage
to the equipment.

LIMITATIONS MAY NOT APPLY TO YOU.

To obtain warranty service, contact your GARMIN
dealer or call the GARMIN Customer Service department
(913-397-8200) for a returned merchandise tracking
number. The unit should be securely packaged with the
tracking number clearly marked on the outside of the
package and sent freight prepaid and insured to a GARMIN
warranty service station. A copy of the original sales receipt
is required as the proof of purchase for warranty repairs.
GARMIN retains the exclusive right to repair or replace the
unit or software or offer a full refund of the purchase price
at its sole discretion. SUCH REMEDY SHALL BE YOUR
SOLE AND EXCLUSIVE REMEDY FOR ANY BREACH OF
WARRANTY.

Introduction

I

1

Introduction

Cautions

All differential beacon receivers decode correction data
determined at the beacon transmitter site via a GPS
receiver(s). The GPS system is operated by the govern­ment of the United States which is solely responsible for
their accuracy and maintenance. The DGPS beacon
transmitters are operated by the U.S. Coast Guard (or
similar government agency in other countries) which is
responsible for their accuracy and maintenance. The
Global Positioning System and the Differential Global
Positioning System are under development and are subject
to changes which could affect accuracy and performance
of all DGPS equipment. Although a DGPS system is a
precision electronic NAVigation AID (NAVAID), any
NAVAID can be misused or misinterpreted, and therefore
become unsafe. Use the DGPS system at your own risk. To
reduce this risk, carefully review and understand all
aspects of this Owner’s Manual and carefully compare
indications from your GPS receiver to all available
navigation sources including the information from other
NAVAIDs, visual sightings, charts, etc. For safety, always
resolve any discrepancies before continuing navigation.

NOTE: This device complies with Part 15 of the FCC
rules. Operation of this device is subject to the following
conditions: (1) This device may not cause harmful
interference, and (2) this device must accept any
interference received, including interference that may
cause undesired operation.

2

Introduction

Limited Warranty..............................................1

Cautions ..........................................................2

Table of Contents ............................................3

Product Registration ........................................4

Overview

Capabilities & Package Contents......................5

Installation

Mounting the GBR 23 .................................6-7

Connecting GBR 23 and GPS ....................8-10

Operation

Using the GBR 23 ....................................11-12

Introduction

Table of

Contents

Reference

DGPS: How It Works ...............................13-14

Sources of Error .......................................15-16

Troubleshooting Chart .............................17-18

GBR 23 Specifications ..................................19

Index .............................................................20

3

Introduction

Product

Registration

H

Why should you register your
GARMIN GPS unit:

• Notification of Product Updates

• Notification of New Products

• Lost or Stolen unit tracking

Customer Service Product Registration

Help us better support you by completing our on-line
registration today! Have the serial number of your GBR
23 handy and connect to our website (www.garmin.com).
Look for the Product Registration link on the Home page.

If you have previously registered a GARMIN product
purchase using a mail-in registration card, we invite you
to reregister using our NEW on-line system. Many
services provided by our new product registration system
are now being automated and reregistering your purchase
ensures you the best possible support from GARMIN.

Serial Number

Use this area to record the serial number (8-digit
number located on the bottom of the unit) in case it is
lost, stolen, or needs service. Be sure to keep your original
sales receipt in a safe place or attach a photocopy inside
the manual.

Serial Number:

*

*

The GARMIN GBR 23 has no user

I

serviceable parts. Should you ever encounter
a problem with your unit, please contact
your GARMIN dealer or GARMIN Customer
Service (913-397-8200 or 800-800-1020) for
repairs.

Repairs should only be made by an authorized
GARMIN service center. Unauthorized repairs or
modifications could void your warranty and authority to
operate this device under FCC Part 15 regulations. See
page 1 of this manual for further service details.

4

Capabilities

The GBR 23 offers a host of powerful capabilities to
enhance the performance and accuracy of your GPS
receiver:

• Performance— The single channel receiver has

high-sensitivity automatic and manual tuning
throughout the beacon broadcast band.

• Ease of Use— Once installed, operation is

controlled using your GARMIN GPS unit. The unit
may also be allowed to operate autonomously.

• Convenience— May be remotely mounted in an

out-of-the-way location. Receiver status information
is displayed directly on the GARMIN GPS unit.

• Low Power Consumption— Draws approximately

95 milliamps during normal operation.

• Accuracy— 1-5 (2DRMS) meters when used with a

GARMIN 12 channel GPS.

Package Contents

Your GARMIN GBR 23 package includes:

• GBR 23 unit

• 30’ Power/Data Cable

• Owner’s Manual & Reference Guide

• Beacon Reference Card

If any parts are missing or broken, please
contact your GARMIN dealer immediately.

Overview

Capabilities &

Package Contents

The GBR 23 may be used with a
variety of GARMIN GPS
receivers. The list below
identifies most, but not all,
compatible models. If your GPS
receiver is not listed, you may
check with your GARMIN
dealer or contact GARMIN
product support at 1-800-800­1020 to verify compatibility.

•GPS II/II+/III/III+/III Pilot

•GPS eMap/eTrex/NavTalk

•GPS StreetPilot/ColorMap

•GPS 12/12XL/12CX/12Map

•GPS 38/40

•GPS 45/45XL/48

•GPS 50/75

•GPS 65

•GPS 89/90/92/95

•GPS 120/125

•GPSCOM 170/190

•GPSMAP 130/135/180/185

•GPSMAP 162/168

•GPSMAP 175/195/295

•GPSMAP 210/220/215/225

•GPSMAP 230/235

5

Installation

Mounting the

GBR 23

I

Note: As a general rule,
mount the receiver at least
three feet from all other
antennas and the vessel’s
electrical system components
(alternator/ignition system).

6

Mounting the Receiver

Please read through these instructions thoroughly before
attempting installation. Make sure you completely understand
these instructions before you begin. When in doubt, seek
professional assistance.

The following additional items are needed or optional
to complete the installation of your GBR 23:

• Antenna mount— Since antenna mounting
locations and methods vary, you will need to obtain
an antenna mount to install the GBR 23. The
receiver base fits a standard 1-inch, 14 threads per
inch mount. Check with your GARMIN dealer or a
marine supply company for this item.

• On/Off switch (optional)— Power to the
receiver may be controlled by an on/off switch, such
as an accessory switch on the control console.
Check with your GARMIN dealer or a marine/
electric supply company for this item.

• Fuse or cucuit breaker (optional)— It is
recommended to install a 2-6 Amp slow response fuse
or circuit breaker to protect the curcuit that the
GBR 23 will be attached to.

• GARMIN 60’ Power/Data Cable— For cable
installation lengths greater than 30’.(010-10284-00)

It is recommended that the system be temporarily
hooked up with the wiring and unit placement
approximating at the desired final installation. Then, check
operation with potential local interfering equipment turned
on and off. For example, other electronic equipment, fan
motors, engine ignition, alternators, generators, radars and
VHF radio transmissions can be sources of interference.

Position the receiver so that the clearest possible view
of the horizon in all directions is obtained. Avoid mounting
the antenna next to large areas of conductive material
(metal, aluminum, etc.) as this may cause poor signal
reception. The GBR 23 is supplied with a 30 feet power/
data cable. When routing the wiring to the GBR 23, avoid
routing the cable near the vessel’s alternator or ignition
system components or parallel to other power lines.

Three common sources of interference for DGPS

units are:

ABOVE- OK

Radar

BELOW- OK

Installation

Mounting the

Receiver

3'

Radar

EMI

EMI (Electromagnetic Interference) from engine components

If a problem is found, try altering the location of the

VHF Radio

When routing the power/data
cable, try to avoid:

• Sharp edges which may cut the
cable

• Routing the cable parallel to
other power lines

• Excessively twisting, straining or
bending the cable

unit or wiring. Often moving the antenna a few feet away
from the source of interference will solve the problem.
When a suitable configuration is found, a permanent
installation should be made. Select the mounting location
according to your preferences. Keep in mind that from
this mounting location cables will be routed to the
antenna and to the GPS unit.

1. Once a suitable mounting location has been
determined and tested, secure the antenna mount
at the desired mounting location.

2. Make sure the GBR 23 is aligned properly to avoid
thread damage. Thread the GBR 23 onto the
antenna mount and hand tighten clockwise until
snug (Fig. 1). Do not overtighten.

3. Align the notches on the power/data cable and GBR
23 connector and push it connector in until it seats.
Turn the locking ring clockwise until the power/data
cable is firmly locked into position.

4. Route the power/data cable from the GBR 23 to the
GPS unit. Excess cable may be shortened or coiled
together and secured in an inconspicuous location.
If additional cable is needed you may use GARMIN’s
optional 60’ cable (010-10284-00) or similar 24
AWG shielded wiring. The entire length of the
power/data cable should not exceed 200’ (60.9m).

I

Antenna
Mount

Figure 1: Mounting

Power/Data
Cable &
Connectors

the GBR 23

7

Installation

Connecting the

GBR 23

F

You may also output corrected
NMEA data from a GARMIN GPS/
DGPS combination to a PC or
another NMEA device (autopilot,
radar, chartplotter, etc.). GARMIN
GPS units can support NMEA data
output for up to three devices (the
GBR 23 counts as one). Most
NMEA device configurations only
require that the GROUND (-) and
the DATA OUT (+) wires from the
GPS (which are also connected to
the GROUND (-) and DATA IN (+)
wires of the GBR 23) to be
connected. Please refer to the
instructions or manufacturer of the
NMEA device for correct wiring
procedures.

PC Connections

1

2345

67

8

9

Connect the harness leads
from the GPS to a 9-pin serial
connector:
Data Out From GPS - Pin 2
Data In From GPS - Pin 3
Ground From GPS - Pin 5

* When wiring a GARMIN GPS/
DGPS combination, connecting the
DATA OUT from the PC (pin 3) to
the DATA IN of the GPS will
disable the DGPS (since the GPS
only allows for one input). If you
need to upload/download data
between the GPS and PC, it is
advisable to install a switch which
toggles the input to the GPS
between the DGPS or PC.

8

*

Connecting the GBR 23 To Your GPS Unit

The final step in installing the GBR 23 is to connect
the receiver’s DATA IN, DATA OUT and GROUND lines
to your GPS unit. The GBR 23 is designed to transmit/
receive data at 4800 baud or BPS (bits per second), which
is suitable for use with all differential-ready GARMIN GPS
receivers and select products made by other manufactur­ers.

For reliable communication, it is essential that the
GBR 23 and the GPS unit share the same ground. This
ground connection acts as the (current) Return line. It is
recommended to wire the unit to it’s own circuit to avoid
interference from other electronics.

Some non-GARMIN GPS units may have a separate
data line labeled “RETURN”, “DATA GROUND” or “DATA

-”. If one of these lines exist, connect the BLACK wire
from the power/data cable to it.

1. You may reference Figure 3 for the following wiring:
Connect the BLUE (Data Out) wire from the GBR
23’s power/data cable to the DATA INPUT line of
the GPS unit.

2. Connect the BROWN (Data In) wire to the DATA
OUTPUT line of the GPS unit.

3. Connect the BLACK (-) wire to the GROUND wire of
the GPS unit. If the BLACK wire is already connected
to the same ground terminal as the GPS unit, no
additional connection is required (unless a separate
RETURN line is provided by the GPS unit).

4. Connect the RED (+) wire from the power/data
cable to a 8-35 vDC power source. It is advisable to
use a slow response fuse or circuit breaker in the
range of 2A to 6A to protect the curcuit.

5. If a remote power switch is being installed, see
Figure 2. This will allow the GBR 23 to remain
connected to a power source, but manually
powered on and off. If the receiver is being wired to
a circuit which is already switched, (with the GPS
for example) connect the WHITE wire to ground or
the same location as the BLACK wire. When the
BLACK and WHITE wires are combined, the GBR 23
will turn on/off when power is applied/removed to
the RED (+) and BLACK (-) wires.

Auto-Tuning

Without external commands, the GBR 23 will
automatically scan frequencies between 283.5-325.0 kHz
@ 0.5kHz steps for 100 and 200 bps stations only. To use
25 & 50 bps, you will need to manually tune the beacon
via the GPS receiver or other source. For non-GARMIN
receivers which do not supply a valid tuning interface (pg.
11-12) to the GBR 23, it is not necessary to connect to the
DATA IN (BROWN) wire of the GBR 23. The GBR 23
should tune into a DGPS station within about 10 minutes.
The unit will store the last used frequency in non-volatile
memory and default to that frequency on next use. If you
lose DGPS signal, the unit will attempt to reconnect to the
last frequency and if necessary, tune to another station.

7

6

5

Closed- ON

Open- OFF

(-) (+)

8-35 VDC

Switch

Fuse

GBR 23

2-6A

Power/Data Cable

Remote Power Switch

(1) RED:

8-35 VDC

(2) BLACK: GROUND

(5) WHITE: REMOTE ON/OFF*

Installation

Connecting

GBR and GPS

1

2

3

4

Figure 2: Wiring a Remote Power Switch For the GBR 23

UNITS WITH
18 PIN PLUG

Power / Data Cable

2

1

3

6

11
15

7
12

18

16

UNITS WITH 4 PIN,
RECTANGLE PLUG

Data Cable

(Not used)

4

1

23

Figure 3: Sample Wiring to GARMIN GPS Units

(-) (+)

Fuse
2-6A

8-35 VDC

(15) RED: POWER INPUT
(18) BLACK: GROUND
(17) BLUE: DATA OUT
(16) BROWN: DATA IN
(11) YELLOW: ALARM
(1-10,12-14) NOT USED

(4) BLACK: GROUND
(3) WHITE: DATA OUT
(2) GREEN: DATA IN

* When the black and white wires are combined, the GBR 23 will turn on/off

when power is applied/removed to pins 1 & 2. To remotely control power to
the GBR 23, see Figure 2.

(-) (+)

8-35 VDC

(1) RED:

8-35 VDC

(2) BLACK: GROUND
(5) WHITE: REMOTE ON/OFF*
(4) BROWN: DATA IN
(3) BLUE: DATA OUT
(6) GREEN: NOT USED
(7) YELLOW

:

NOT USED

Fuse
2-6A

8-35 VDC

(1) RED:
(2) BLACK: GROUND
(5) WHITE: REMOTE ON/OFF*
(4) BROWN: DATA IN
(3) BLUE: DATA OUT
(6) GREEN: NOT USED
(7) YELLOW

:

NOT USED

GBR 23

Power / Data Cable

GBR 23

Power / Data Cable

9

Installation

Connecting

GBR and GPS

UNITS WITH
7 PIN PLUG

4

3

5

2

6

1

7

UNITS WITH
13 PIN PLUG

Power / Data Cable

1

2

13

UNITS WITH
6 PIN PLUG

Power / Data Cable

(1) RED: POWER INPUT
(2) BLACK: GROUND
(3) BLUE: ALARM
(4) BROWN: DATA OUT
(5) WHITE: DATA IN
(6) RED/BLK: NOT USED

(1) RED: POWER INPUT
(2) BLACK: GROUND
(3) BLUE: DATA OUT
(4) BROWN: DATA IN
(5) WHITE: NOT USED
(6) GREEN: NOT USED

(7) YELLOW: ALARM

(2) RED: POWER INPUT
(1) BLACK: GROUND
(4) BLUE: DATA OUT
(5) BROWN: DATA IN
(6) YELLOW: ALARM
(3, 7-13) NOT USED

(-) (+)

8-35 VDC

(-) (+)

8-35 VDC

(-) (+)

8-35 VDC

Fuse
2-6A

(1) RED:
(2) BLACK: GROUND
(5) WHITE: REMOTE ON/OFF *
(4) BROWN: DATA IN
(3) BLUE: DATA OUT
(6) GREEN: NOT USED

(7) YELLOW

Fuse
2-6A

(1) RED:

8-35 VDC

(2) BLACK: GROUND

(5) WHITE: REMOTE ON/OFF *
(4) BROWN: DATA IN
(3) BLUE: DATA OUT

(6) GREEN: NOT USED

(7) YELLOW

:

NOT USED

Fuse
2-6A

(1) RED:

8-35 VDC

(2) BLACK: GROUND
(5) WHITE: REMOTE ON/OFF *
(4) BROWN: DATA IN
(3) BLUE: DATA OUT
(6) GREEN: NOT USED
(7) YELLOW

:

NOT USED

8-35 VDC

:

NOT USED

GBR 23

Power / Data Cable

GBR 23

Power / Data Cable

GBR 23

Power / Data Cable

UNITS WITH
4 PIN ROUND PLUG

Power / Data Cable

1

4

3

2

* When the black and white wires are combined, the GBR 23 will turn on/off

when power is applied/removed to pins 1 & 2. To remotely switch power to
the GBR 23, see Figure 2.

10

Figure 3 (cont.): Sample Wiring to GARMIN GPS Units

(1) RED: POWER INPUT
(2) BLACK: GROUND
(3) BROWN: DATA OUT
(4) WHITE: DATA IN

(-) (+)

8-35 VDC

Fuse
2-6A

(1) RED:

8-35 VDC

(2) BLACK: GROUND
(5) WHITE: REMOTE ON/OFF *
(4) BROWN: DATA IN
(3) BLUE: DATA OUT
(6) GREEN: NOT USED
(7) YELLOW

:

NOT USED

GBR 23

Power / Data Cable

Using the GBR 23

When using the GBR 23, tuning may be either done
automatically or controlled by the GPS unit. Your
GARMIN GPS unit provides a Beacon Receiver Setup Page
and, in some cases, Beacon Log Page for this purpose.

The operating instructions provided below are
generalized to cover several different GARMIN
models and may not exactly match your display. For

additional information on operating your GPS unit with
the GBR 23, refer to the owner’s manual for your GPS
unit. Screen examples from various model GARMIN units
are provided on the following pages.

1. Apply power on to the GBR 23.

2. Turn the GPS unit on.

3. From the Interface Setup Page, select an RTCM
input. If a baud rate selection is also provided,
specify 4800. On GARMIN GPS units, RTCM
input/NMEA 0183 output should be selected. The
baud rate will be set to 4800. If additional
devices, such as an autopilot or plotter, are
connected to the GPS unit, they must be set to
work with NMEA 0183 as well.

On some newer model GARMIN GPS receivers,
you may set the interface to “Garmin Bcn Rcvr”.
This is a scan option, seperate from the GBR 23
scan, which will automatically scan for a DGPS
frequency and bit rate. Please refer to your GPS
owner’s manual for using this function. If this
setting is used, you may skip step 4.

4. From the Interface Setup Page, enter the
“Tuned To:” frequency and the “Bit Rate” for the
nearest DGPS beacon transmitter. (Refer to the
Beacon Reference Card for this information.)

5. Once the frequency and bit rate have been
entered, “Tuning” will be displayed at the bottom
of the page.

6. If the beacon signal is received by the GBR 23,
the “SNR” field will display a signal-to-noise ratio
and “Receiving” will appear on the page. The
SNR scale ranges from zero (no reception) to 31
(best reception).

Operation

Using the

GBR 23

From the Interface Page,
select ‘RTCM/NMEA’ and
‘NMEA 0183 2.0’ to properly
communicate with the
GBR 23.

Use the Beacon Receiver
Setup Page to enter the
beacon operating frequency
and bit rate for the nearest
DGPS beacon site. This
information is automatically
transferred to the GBR 23.

11

Operation

Using the

GBR 23

The Main Menu Page on the
GPSMAP 130/135/175/
180/185/230/235. Select
‘Input/Output’ to make the
desired interface settings.

From the Input/Output
Settings Page, select ‘RTCM
In/NMEA Out’ to properly
interface with the GBR 23.

12

7. If the beacon signal is received, but no DGPS
correction data is included, the message “No
Data” will appear at the bottom of the page.
If no beacon signal is received, the “SNR” field
will remain blank, “No Status” will appear at the
bottom of the page.

8. To select another beacon transmitter, enter a new
frequency and bit rate.

9. Some Garmin units have a list of the last five
beacon frequencies used on the Beacon Log
Page. As an alternative to reentering frequency
and bit rate information each time you use the
GBR 23, you may select a beacon from this list.
(NOTE: The Beacon Receiver Setup Page will
automatically default to the last beacon
transmitter used each time the GPS unit is turned
on.)

10.Select the Satellite Status Page. A “D” will
appear at the bottom of the signal strength
bar for each satellite with a corresponding
differential correction or “3D Differential” will be
displayed at the top of the satellite page. This
page is useful for determining the quality of
differential coverage available. (Remember, the
more satellites with corresponding differential
corrections, the more accurate your position will
be.) Some models will show a distance from the
DGPS tranmsitter only if this information is
contained in the DGPS signal.

Additional manual tuning for the GBR 23

If your GPS does not support a tuning interface for
the GBR 23 and you wish to manually tune it, transmit
the following tuning sentence, based on NMEA 0183
ASCII (pg. 19), through a PC terminal program or other
compatible source to the DATA IN of the GBR 23:

$PSLIB,<1>,<2><CR><LF>

<1> Beacon tune frequency 0.0 (Auto), 283.5-

325kHz in 0.5 kHz steps.
<2> Beacon bit rate 0 (Auto), 25, 50, 100, 200
<CR><LF> Carriage Return, Line Feed
Example: $PSLIB,305.0,200<CR><LF>

DGPS: How It Works

Differential GPS (DGPS) is a technique used to
improve the accuracy of the Global Positioning System.
DGPS reduces the effects of atmospheric variations and
can improve position accuracy to better than 5 meters.

Reference

DGPS:

How It Works

GPS SIGNAL

SHIPBOARD GPS AND
DGPS RECEIVERS

GPS RECEIVER
STATION AND
DGPS TRANSMITTER

GPS SIGNAL

CORRECTION

DATA

DGPS (CORRECTION) SIGNAL

Figure 4: The DGPS System

A DGPS system consists of the following:

• DGPS Beacon Transmitter and GPS Receiver at
a known location

• Shipboard DGPS Beacon Receiver

• Shipboard GPS Receiver (DGPS capable)

• GPS Satellites

The DGPS Beacon Transmitter is placed at a known
location (i.e., the exact position of the site has been
previously determined). At the beacon transmitter site,
the GPS satellites are monitored using a GPS receiver.
This receiver is equipped to calculate corrections for each
satellite received. The correction is the difference between
the distance to the satellite (from the beacon site) as
measured by the GPS receiver, and the actual distance to
the satellite based on the known location of the beacon
site. These corrections are communicated to the users
GPS set through the DGPS Beacon Station and the
GBR 23. The users GPS set then uses the corrections to
remove errors from its measurements.

After entering the beacon’s
operating frequency, set the
bit rate as indicated on the
Beacon Reference Card.
Beacon information is also
available from the other
sources listed on the card.

Select GARMIN DGPS
when using the GBR 23 with
GARMIN GPS units.

13

Reference

DGPS:

How It Works

The Main Menu Page on the
GPS III/III+/12Map/
Navtalk. Select ‘Setup’ to
make the desired interface
settings.

Satellites received by the shipboard GPS receiver, but
not by the GPS receiver at the beacon transmitter site, will
not have corresponding corrections. When four or more
satellites received by the shipboard GPS receiver have
corresponding corrections, the result is a highly accurate
position reading. The more satellites with corrections, the
more accurate the position.

The GBR 23 receives RTCM-SC-104 format signals
from ground based DGPS Beacon Stations operating in
the 283.5 kHz to 325.0 kHz frequency band, with MSK
modulation and data rates of 25,50,100, or 200 bits per
second. These stations are typically operated by
government agencies such as the U.S. Coast Guard.

Generally, the closest DGPS station will provide the
most accurate correction data. Individual stations vary
and the user may wish to verify the suitability of the
signal for the intended application with the station
operator. The station power is normally set to provide a
usable range somewhere near 300 km. Other factors such
as local interference, lightning, time of day and season, if
the path to the station is over ground or water, and
antenna selection or installation affect the usable signal
range.

From the Interface Page,
select ‘RTCM/NMEA’ to
properly interface with the
GBR 23.

14

DGPS BEACON

SIGNALIN

GBR 23 H-FIELD
BEACON RECEIVER

CORRECTION

(RTCMSC-104)

Figure 5: Shipboard DGPS System

DATA

GPS SIGNAL IN

RECEIVER

GA-29 GPS
ANTENNA

GPS

Sources of Error

Using a DGPS Beacon Receiver with your existing
GPS Receiver can provide substantial improvements in
accuracy; however, there may be occasions when the best
possible accuracy will not occur. Several factors can
contribute to a degraded DGPS accuracy.

Loss of DGPS Beacon Signal— Obviously, the lack
of DGPS correction data will result in reduced accuracy.
Accuracy will be the same as if no beacon receiver was
being used. Several conditions can cause a loss of the
beacon signal:

• Poor data between DGPS receiver and the GPS
receiver can result in intermittent or nonexistent
correction data.

• The range of a DGPS beacon transmitter (see the
accompanying Beacon Reference Card) is typically
a few hundred miles, or less. Beyond this range,
the beacon signal cannot be reliably received.

• Interference to the beacon signal can be experi­enced during periods of thunderstorm activity.
Other sources of static interference, such as
alternator motors, ignition systems and VHF, can
also affect signal reception. Alternator/ignition
interference can be minimized through proper
shielding of the ship’s wiring, by using an EMI/RFI
filter, and by mounting the beacon receiver’s
antenna away from these sources of interference.

Multipath— Multipath error occurs when the GPS

signal is reflected before it reaches the GPS receiver. The
reflected signal takes slightly longer to reach the GPS
receiver than a non-reflected signal. This added time delay
results in position error. (The distance to each satellite is
calculated based upon the time it takes the GPS signal to
reach the GPS receiver.) Multipath error can be minimized
by mounting the GPS antenna at a location which
minimizes the potential for reflected signals. Generally,
the GPS antenna should be mounted on a large, flat
horizontal surface and away from any vertical structure
(cabin walls, large mast, etc.) which could reflect the GPS
signal.

Reference

Sources of

Error

After entering the beacon’s
operating frequency, set the
bit rate as indicated on the
Beacon Reference Card.
Beacon information is also
available from the other
sources listed on the card.

The Beacon Log Page lists the
last five beacon frequencies
used. As an alternative to
retyping the frequency and bit
rate each time, you may select
a beacon from this list.

15

Reference

Sources of

Error

Number of Satellites Visible— As previously

stated, the number of satellites available can affect
position accuracy. To apply the corrections provided for
the satellites received at the beacon transmitter station, the
same satellites (at least in part) must be received by your
GPS receiver. And, certainly, if there aren’t enough
satellites to determine a GPS position, there aren’t enough
satellites to calculate a DGPS position.

Atmospheric Conditions— Differences in the

ionosphere and/or troposphere between the DGPS
broadcast station and the DGPS receiver can result in
decreased position accuracy. Although this does not cause
significant error, the amount of error can increase as you
get further away from the DGPS broadcast station.

Satellite Geometry— A minimum of 4 satellites are

required to determine a 3D position. At times, additional
satellites are required due to their placement with respect
to each other. This relative placement is referred to as
“satellite geometry”. Ideal satellite geometry exists when
the satellites are located at wide angles with respect to
each other. When satellites are located in a line, satellite
geometry is considered poor.

This same requirement applies to DGPS. If
corrections are available for four different satellites, but
they are all located in the same general area or in a line,
the DGPS corrections will be minimal. However, if the
same four satellites are placed farther apart, in several very
different directions from our position, the corrections will
have a much greater effect and the position accuracy will
be greatly improved.

16

Reference

Problem

1) “Check Wiring”
displayed on GPS unit

Power switch/wiring faulty.

Fuse/breaker blown (if used).

GBR 23 not wired properly with

GPS unit. Check wiring.

If the BROWN (Data In) wire is

not used (Auto-tuning), this

message may be displayed. If the

receiver status screen displays

the GPS is receiving differential

corrects, no action is needed.

2) “No DGPS Position”
displayed on GPS unit

Not enough data is available to

compute a DGPS position.

Please refer to Troubleshooting

problems 5 and 6.

3) “No RTCM Input”
displayed on GPS unit

Beacon receiver is improperly

connected or baud rates do not

match.

Please refer to Troubleshooting

problems 5 and 6.

4) “RTCM Input
Failed” displayed on
GPS unit

Intermittent connection between

GBR 23 and GPS unit. Check

wiring.

Interference from ship’s electrical

system, other electronic items,

thunderstorm activity, or

another source is preventing

signal lock-on.

Also refer to Troubleshooting

problems 5 and 6.

Possible Cause

Troubleshooting

Chart

The receiver status indication
at the top left of the Satellite
Status Page will indicate ‘3D
Diff’ or ‘2D Diff’ when
differential corrections are
available. A ‘D’ appears at
the bottom of the signal
strength bar for each satellite
with a corresponding
differential correction.

5) GPS interface
status screen displays
“No data” or “No
Signal”

Wrong frequency or bit rate

selected on GPS unit.

Beacon transmitter not

transmitting correction data.

17

Reference

Troubleshooting

Chart

If the ‘No Differential GPS
Position’ message is displayed
on your GPS receiver,
correction data may not be
available. This may be due to a
missing or weak beacon signal,
not enough GPS satellites or
bad wiring connection between
the GBR 23 and GPS.

Problem

6) GPS interface
status screen displays
“None” or “No Status”

7) Beacon signal weak
or not received.

8) Accuracy not as
expected

Possible Cause

GBR 23 not wired properly with
GPS unit. Check wiring.

No power to GBR 23. See
Troubleshooting problem 1.

Possible damage to the with
DGPS antenna.

Baud rates do not match.
Beacon transmitter out of range.

Try a different station.

Interference from ship’s
electrical system, thunderstorm
activity, or another source is
inhibiting signal lock on.

Also refer to Troubleshooting
problems 5 and 6.

Poor satellite geometry
coverage exists.

Interference to GPS antenna
and/or beacon antenna exists.

Portable antenna being used
on GPS unit. Use remote
antenna.

Multipath signals being
received by GPS unit.

Too far from DGPS transmitter
or not tuned to closest station.

If either ‘No Status’, ‘None’, or
‘Check Wiring’ messages
appear (depending upon model
of GPS) see Troubleshooting
problem number 7.

18

9) Distance from
DGPS station is blank
or zero on GPS unit

The distance reading to the
beacon is dependent upon
reception of the RTCM
message transmitted from the
DGPS broadcast site. This
message is infrequently
broadcast and may vary from
station to station.

Physical

Size: 4.6”W x 5.3”H

(117mm x 135mm)

Weight: 23.9 ounces (0.678 kg)
Operating Range: -68°F to +158°F

(-20°C to +70°C)

Waterproof: -1 meter submersion for 30 minutes

IPX7 rating for IEC529

Mount Thread Size: 1” diameter at 14 threads/inch

Power

Voltage: 8 - 35 vDC using supplied

power/data cable. Surge and reverse
polarity protected for up to 80 vDC.

Current Drain: 95 mA @ 12 vDc

Signal Processing

Frequency Range: 283.5 - 325.0 kHz @ 0.5kHz
Data Rates: 200/100/50/25 BPS
Auto Tuning: All 200 & 100 BPS Channels
Minimum Signal: 10 µV

Data Processing

Demodulation: MSK (Minimum Shift Keying)
MSK Bit Rates: 25, 50, 100, 200 bps

Interfaces

Input: RS-232 or NMEA 0183, 4800 baud
Input Sentences: Binary

$PSLIB
$--MSK

Output: RS-232 4800 baud
Output Sentence: RTCM SC-104

(6 of 8 bit format)

Reference

GBR 23

Specifications

H

Complete information concerning
NMEA & RTCM formats and
sentences is available for
purchase at:

National Marine Electronics

Association (NMEA)

PO Box 3435

New Bern, NC 28564-3435

USA

252-638-2626

252-638-4885 FAX.

www.nmea.org

Radio Technical Commission For

Maritime Services (RTCM)

1800 Diagonal Road, Suite 600

Alexandria, VA 22314-2480,

USA

703-684-4481 (Info Only)

703-836-4229 FAX

www.rtcm.org

19

Reference

Index

A

Antenna mount..................................6, 7

Auto-tuning................................9, 11-12

B

Baud rate....................................8, 11-12

Beacon Log Page..................................12

Beacon Range.......................................14

Beacon Reference Card......................5,11

Beacon Receiver Setup Page............11-12

Beacon System................................13-16

M

Mounting the Receiver........................6-7

Manual tuning..................................11-12

N

NMEA..........................................8, 12, 19

O

On/Off Switch...................................6, 8-9

Operation........................................9, 11-12

P

Package Contents.....................................5

PC Connection.........................................8

Power/Data Cable...............................5-10

C

Capabilities............................................5

Cautions................................................2

Compatibility.........................................5

Connections, Wiring.........................8-10

Contents, Package..................................5

D

DGPS System..................................13-14

E

Error, Sources of.............................15-16

F

Frequencies..............................11-12, 14

I

Installation.......................................6-10

Interface Setup Page.......................11-12

Interference.............................6-7, 15-16

20

R

Registration..............................................4

RTCM-SC-104 format............................14

S

Satellite Status Page................................13

Serial Number..........................................4

Sources of Error................................15-16

Sources of Interference..........................6-7

Specifications.........................................19

T

Table of Contents.....................................3

Troubleshooting...............................17-18

Tuning..........................................9, 11-12

W

Warranty Information.............................1

Wiring Connections............................8-10

© 2000 GARMIN Corporation

GARMIN International, Inc.

1200 East 151st Street, Olathe, Kansas 66062, U.S.A.

GARMIN (Europe) Ltd.

Unit 5, The Quadrangle, Abbey Park Industrial Estate, Romsey, SO51 9AQ, U.K.

GARMIN (Asia) Corporation

No. 68, Jangshu 2nd Rd., Shijr, Taipei County, Taiwan

www.garmin.com

Part Number 190-00195-00 Rev. A