Garmin GPS 25 LP Manual

GPS 25 LP SERIES
GPS SENSOR BOARDS
GPS25-LVC, GPS25-LVS, GPS25-HVS
TECHNICAL SPECIFICATION
_______________________________________________________________________
GARMIN· 1200 E. 151st St· Olathe, Kansas 66062· [phone]· [phone] FAX

©2000 GARMIN Corporation
1200 East 151st Street, Olathe, KS 66062
All rights reserved. No part of this manual may be reproduced or transmitted in any
form or by any means, electronic or mechanical, including photocopying and recording,
for any purpose without the express written permission of GARMIN.
Information in this document is subject to change without notice. GARMIN reserves the
right to change or improve their products and to make changes in the content without
obligation to notify any person or organization of such changes or improvements.
190-00125-00 Rev. G

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Table of Contents
Table of Contents.............................................................................................................i
Section 1 .........................................................................................................................1
Introduction...................................................................................................................1
1.1 Overview...........................................................................................................1
1.2 Naming Conventions ........................................................................................1
1.3 Features ...........................................................................................................2
1.4 Technical Specifications...................................................................................3
1.4.1 Physical Characteristics .............................................................................3
1.4.2 Environmental Characteristics....................................................................3
1.4.3 Electrical Characteristics............................................................................3
1.4.4 Performance...............................................................................................4
1.4.5 Interfaces....................................................................................................4
1.5 Application........................................................................................................5
1.5.1 Application Considerations.........................................................................5
Section 2 .........................................................................................................................7
Operational Characteristics..........................................................................................7
2.1 Self Test ...........................................................................................................7
2.2 Initialization.......................................................................................................7
2.3 Navigation.........................................................................................................8
2.4 Satellite Data Collection...................................................................................8
Section 3 .......................................................................................................................10
Hardware Interface.....................................................................................................10
3.1 Mechanical Dimensions..................................................................................10
3.2 Connector Specifications................................................................................10
3.3 Connector Pin-Out..........................................................................................11
3.4 Antenna Connection.......................................................................................14
Section 4 .......................................................................................................................16
Software Interface ......................................................................................................16
4.1 NMEA Received sentences............................................................................16
4.1.1 Almanac Information (ALM)......................................................................16
4.1.2 Sensor Initialization Information (PGRMI)................................................17
4.1.3 Sensor Configuration Information (PGRMC)............................................17
4.1.4 Additional Sensor Configuration Information (PGRMC1).........................18
4.1.5 Output Sentence Enable/Disable (PGRMO).............................................19
4.1.6 Tune DGPS Beacon Receiver (PSLIB) ....................................................20
4.2 NMEA Transmitted Sentences........................................................................20
4.2.1 Sentence Transmission Rate....................................................................20
4.2.2 Transmitted Time......................................................................................21
4.2.3 Global Positioning System Almanac Data (ALM) .....................................22
4.2.4 Global Positioning System Fix Data (GGA)..............................................22
4.2.5 GPS DOP and Active Satellites (GSA).....................................................23
4.2.6 GPS Satellites in View (GSV)...................................................................23

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4.2.7 Recommended Minimum Specific GPS/TRANSIT Data (RMC)................24
4.2.8 Track Made Good and Ground Speed with GPS Talker ID (VTG)...........24
4.2.9 Geographic Position with LORAN Talker ID (LCGLL)..............................24
4.2.10 Track Made Good and Ground Speed with LORAN Talker ID (LCVTG)..25
4.2.11 Estimated Error Information (PGRME) .....................................................25
4.2.12 GPS Fix Data Sentence (PGRMF) ...........................................................25
4.2.13 Sensor Status Information (PGRMT)........................................................26
4.2.14 3D velocity Information (PGRMV).............................................................26
4.2.15 DGPS Beacon Information (PGRMB).......................................................27
4.3 Baud Rate Selection.......................................................................................27
4.4 One-Pulse-Per-Second Output.......................................................................27
4.5 RTCM Received Data.....................................................................................28
Appendix A....................................................................................................................29
Earth Datums..............................................................................................................29
Appendix B....................................................................................................................32
GPS 25LP Connectors...............................................................................................32
Appendix C....................................................................................................................33
GPS 25LP Evaluation Kits..........................................................................................33
Appendix D....................................................................................................................35
Phase Output Data Binary Format .............................................................................35
GARMIN Phase Monitor Program - gps25pm.exe......................................................38

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Section 1
Introduction
1.1 Overview
The GARMIN GPS 25LP Series are GPS sensor boards designed for a broad spectrum
of OEM (Original Equipment Manufacturer) system applications. The GPS 25LPs will
simultaneously track up to twelve satellites providing fast time-to-first-fix, one second
navigation updates and low power consumption. Their far-reaching capability meets
the sensitivity requirements of land navigation as well as the dynamics requirements of
high performance aircraft.
The GPS 25LPdesign utilizes the latest surface mount technology as well as high-level
circuit integration to achieve superior performance while minimizing space and power
requirements. All critical components of the system including the RF/IF receiver
hardware and the digital baseband are designed and manufactured by GARMIN to
ensure the quality and capability of the GPS 25LP sensor board. This hardware
capability combined with software intelligence makes the board set easy to integrate
and use.
The GPS 25LP is designed to withstand rugged operating conditions, however it should
be mounted in an enclosure as part of a larger system designed by an OEM or system
integrator. A minimum system must provide the sensor board with conditioned input
power and L1 GPS RF signal. The system may communicate with the board set via a
choice of two CMOS/TTL or two RS-232 compatible bi-directional communications
channels. A highly accurate one-pulse-per-second (PPS) output can be utilized in
applications requiring precise timing measurements. An on-board memory
rechargeable backup battery allows the sensor board to retain critical data such as
satellite orbital parameters, last position, date and time. Non-volatile memory is also
used to retain board configuration settings even if backup battery power fails. End user
interfaces such as keyboards and displays are added by the application designer.
1.2 Naming Conventions
The GPS 25LP sensors are delineated with a three letter extension to designate the
operating voltage range and the serial data voltage specification.
High Voltage - GPS25-HVx designation indicates that the unit will accept a high
input voltage. The internal switching regulator will operate from a 6VDC to 40VDC
unregulated supply.

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Low Voltage - GPS25-LVx designation indicates that the unit is designed to
operate from a low voltage 3.6VDC to 6.0VDC supply. Operation at about 4VDC
is the most power efficient mode of operation for the GPS25LP sensor. The unit is
protected if a high voltage is inadvertently applied to the input.
RS-232 Serial Data - GPS25-xVS designation means that the two bi-directional
serial data ports are true RS-232 ports conforming to the RS-232E standard.
CMOS Serial Data - GPS25-xVC designation means that the two bi-directional
serial data ports use CMOS output buffers. The inputs buffers will accept either
CMOS(TTL) voltage levels or RS-232 voltage levels. This configuration is
adequate for communicating directly with serial devices over short cable lengths
(less than 20 meters).
1.3 Features
The GPS 25LP sensor boards provide a host of features that make it easy to integrate
and use.
1) Full navigation accuracy provided by Standard Positioning Service (SPS)
2) Compact design ideal for applications with minimal space
3) High performance receiver tracks up to 12 satellites while providing fast first fix
and low power consumption
4) Differential capability utilizes real-time RTCM corrections producing less than 5
meter position accuracy
5) On-board clock and memory are sustained by a rechargeable memory backup
battery which recharges during normal operation or by optional external standby
power
6) User initialization is not required.
7) Two communication channels and user selectable baud rates allow maximum
interface capability and flexibility. The standard channels are CMOS/TTL levels
for the -xVC version or RS-232 for the xVS versions.
8) Highly accurate one-pulse-per-second output for precise timing measurements.
The default pulse width is 100 msec, however it is configurable in 20 msec
increments from 20 msec to 980 msec.
9) Binary Format Phase Data Output on TXD2

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10) Flexible input voltage levels of 3.6Vdc to 6.0Vdc with overvoltage protection in the
-LVx, and 6.0Vdc to 40Vdc in the -HVx versions.
11) Fully shielded construction for maximum EMI and RFI protection
12) FLASH based program memory. New software revisions upgradeable through
serial interface
1.4 Technical Specifications
Specifications are subject to change without notice.
1.4.1 Physical Characteristics
1) Single board integrated with complete component shielding
2) Weight: 1.3 ounce [38g]
3) Size: 1.83" (w) x 2.75" (l) x 0.45" (h)
1.4.2 Environmental Characteristics
1) Operating temperature: -30°C to +85°C (board temperature)
2) Storage temperature: -40°C to +90°C
1.4.3 Electrical Characteristics
1) Input voltage: +3.6VDC to 6.0VDC regulated, 150 mVp-p ripple -LVx versions.
+6.0VDC to 40VDC unregulated -HVx version.
2) Input current: 120 mA typical 140 mA max -LVx versions, 20 mA while in power
down.
870mW typical 1000mW max -HVx version, 300uA while in power
down.
3) Backup power: 3V Rechargeable Lithium cell battery, up to 6 month charge
4) Auxiliary battery recharge voltage: 4Vdc to 35Vdc at 4mA typical.
5) Receiver sensitivity: -165dBW minimum

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1.4.4 Performance
1) Tracks up to 12 satellites (up to 11 with PPS active)
2) Update rate: 1 second
3) Acquisition time
-15 seconds warm (all data known)
-45 seconds cold (initial position, time and almanac known, ephemeris unknown)
-1.5 minutes AutoLocateTM (almanac known, initial position and time unknown)
-5 minutes search the sky (no data known)
4) Position accuracy:
Differential GPS (DGPS): Less than 5 meters RMS
Non-differential GPS: 15 meters RMS (100 meters with Selective Availability on)
5) Velocity accuracy: 0.2 m/s RMS steady state (subject to Selective Availability)
6) Dynamics: 999 knots velocity, 6g dynamics
7) One-pulse-per-second accuracy: ±1 microsecond at rising edge of PPS pulse
(subject to Selective Availability)
1.4.5 Interfaces
1) Dual channel CMOS/TTL level (-xVC versions) or RS-232 compatible level (-xVS
versions), with user selectable baud rate (300, 600,1200, 2400, 4800, 9600,
19200)
2) NMEA 0183 Version 2.0 ASCII output (GPALM, GPGGA, GPGSA, GPGSV,
GPRMC, GPVTG, PGRME, PGRMT, PGRMV, PGRMF, LCGLL, LCVTG)
Inputs
-Initial position, date and time (not required)
-Earth datum and differential mode configuration command, PPS Enable,
almanac
Outputs
-Position, velocity and time
-Receiver and satellite status
-Differential Reference Station ID and RTCM Data age
-Geometry and error estimates
3) Real-time Differential Correction input (RTCM SC-104 message types 1,2,3 and 9)
4) One-pulse-per-second timing output
5) Optional binary TTL output data format
6) Binary Format Phase Data

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1.5 Application
GPS 25LP Series Block Diagram
TYPICAL APPLICATION ARCHITECTURE
1.5.1 Application Considerations
1) The GPS 25LP sensor boards contain a sensitive receiver. Additional
electromagnetic shielding may be required to prevent undesirable interference
from other nearby circuits.
2) The GPS 25LP sensor boards use approximately 0.5 W to .85 W, depending on
supply voltage, and require minimal cooling. Forced air cooling is not

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recommended since it may cause rapid temperature changes which may
temporarily affect the frequency stability of the internal oscillator.
3) Interruptions in the RF signal can increase acquisition time. Antenna location with
clear line-of-sight to all directions in the sky will yield the best performance.
4) The GPGSV sentence contains signal strength information for the visible
satellites. Typical values will be between 30 db and 50 db. A majority of values
near the lower limit may indicate a marginal RF signal.

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Section 2
Operational Characteristics
This section describes the basic operational characteristics of the GPS 25LPsensor
boards. Additional information regarding input and output specifications are contained
in Section 4.
2.1 Self Test
After input power has been applied to the GPS 25LP sensor boards and periodically
thereafter, the units will perform critical self test functions and report the results over
the output channel(s). The following tests will be performed:
1) RAM check
2) FLASH test
3) Receiver test
4) Real-time clock test
5) Oscillator check
In addition to the results of the above tests, the board set will report software version
information.
2.2 Initialization
After the initial self test is complete, the GPS 25LP will begin the process of satellite
acquisition and tracking. The acquisition process is fully automatic and, under normal
circumstances, will take approximately 45 seconds to achieve a position fix (15
seconds if ephemeris data is known). After a position fix has been calculated, valid
position, velocity and time information will be transmitted over the output channel(s).
Like all GPS receivers, the GPS 25LP utilizes initial data such as last stored position,
date and time as well as satellite orbital data to achieve maximum acquisition
performance. If significant inaccuracy exists in the initial data, or if the orbital data is
obsolete, it may take 1.5 minutes to achieve a navigation solution. The GPS 25LP
AutoLocateTM feature is capable of automatically determining a navigation solution
without intervention from the host system. However, acquisition performance can be
improved if the host system initializes the board set following the occurrence of one or
more of the following events:

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1) Transportation over distances further than 1500 kilometers
2) Failure of the on-board memory battery
3) Stored date/time off by more than 30 minutes
See Section 4 for more information on initializing the GPS 25LP.
2.3 Navigation
After the acquisition process is complete, the GPS 25LP will begin sending valid
navigation information over its output channels. These data include:
1) Latitude/longitude/altitude
2) Velocity
3) Date/time
4) Error estimates
5) Satellite and receiver status
The GPS 25LP sensor boards will select the optimal navigation mode (2D or 3D) based
on available satellites and geometry considerations. When navigating in the 2D mode
the board set utilizes the last computed altitude or the last altitude supplied by the host
system, whichever is newer. The host system must ensure that the altitude used for 2D
navigation is accurate since the resulting position error may be as large as the altitude
error. See Section 4 for information altitude initialization.
The GPS 25LP will default to automatic differential mode -- “looking” for real-time
differential corrections in RTCM SC-104 standard format using message types 1,2,3 or
9 and attempting to apply them to the satellite data in order to produce a differential
(DGPS) solution. The host system, at its option, may also command the board set to
choose differential only mode. When navigating in the differential only mode, the board
set will output a position only when a differential solution is available.
2.4 Satellite Data Collection
The GPS 25LP sensor boards will automatically update satellite orbital data as they
operate. The intelligence of the board set combined with its hardware capability allows
these data to be collected and stored without intervention from the host system. A few
key points should be considered regarding this process:
1) If the sensor board is not operated for a period of six (6) months or more, the unit
will “search the sky” in order to collect satellite orbital information. This process is
fully automatic and, under normal circumstances, will take 3-4 minutes to achieve

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a navigation solution. However, the host system should allow the board set to
remain on for at least 12.5 minutes after the first satellite is acquired (see Section
4 for more information on status indications).
2) If the memory backup battery fails, the sensor board will search the sky as
described above. The system designer should be aware of the availability of
standby power input to the board set to prevent this situation.
3) If the initial data is significantly inaccurate, the board set will perform an operation
known as AutoLocateTM. This procedure is fully automatic and, under normal
circumstances, will require 1.5 minutes to calculate a navigation solution.
AutoLocateTM, unlike search the sky, does not require that the sensor board
continue to operate after a fix has been obtained.

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Section 3
Hardware Interface
3.1 Mechanical Dimensions
The GPS 25LP sensor board mounts via four #4 or M-3 size screws (see drawing).
3.2 Connector Specifications
The GPS 25LP sensor boards features a single row, right angle, 12-pin male
connector. The mating connector is available from JST Corporation (see appendix B).

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3.3 Connector Pin-Out
PIN-OUT DIAGRAM
The following is a functional description of each pin of the interface connector. The
pins are numbered from left to right as viewed from the connector end of the board with
the connector on top.
Pin 1: TXD2 - Second Serial Asynchronous Output. Electrically identical to
TXD1. This output provides phase data (Ver 2.03) see Appendix D
Pin 2: RXD2 - Second Serial Asynchronous Input. Electrically identical to
RXD1. This input may be used to receive serial differential GPS data
formatted per “RTCM Recommended Standards For Differential Navstar
GPS Service, Version 2.1” (see Section 4 for more details).
Pin 3: PPS - One-Pulse-Per-Second Output. Typical voltage rise and fall times
are 300 nSec. Impedance is 250 ohms. Open circuit output voltage is 0V
and Vin. The default format is a 100 millisecond high pulse at a 1Hz rate,
the pulse width is programmable from a configuration command in
20msec increments. Rising edge is synchronized to the start of each
GPS second. This output will provide a nominal 700 mVp-p signal into a
50 Ohm load. The pulse time measured at the 50% voltage point will be
about 50 nSec earlier with a 50 Ohm load than with no load.
Pin 4: TXD1 - First Serial Asynchronous Output. CMOS/TTL output levels vary
between 0V and Vin in the -LVC version. In the -LVS and -HVS
versions a RS-232 compatible output driver is available. This output
normally provides serial data which is formatted per “NMEA 0183, Version
2.0”. Switchable to 300, 600, 1200, 2400, 4800, 9600 or 19200 BAUD.
The default BAUD is 4800. This output data functions in parallel with pin
12.

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Pin 5: RXD1 - First Serial Asynchronous Input. RS-232 compatible with
maximum input voltage range -25 < V < 25. This input may be
directly connected to standard 3 to 5Vdc CMOS logic. The
minimum low signal voltage requirement is 0.8V, and the maximum
high signal voltage requirement is 2.4V. Maximum load impedance
is 4.7K ohms. This input may be used to receive serial
initialization/configuration data, as specified in Section 4.1.
Pin 6: POWER DOWN - External Power Down Input. Inactive if not
connected or less than 0.5V. Active if greater than 2.7V. Typical
switch point is 2.0V @ 0.34 mA. Input impedance is 15K Ohms.
Activation of this input powers the internal regulators off and drops
the supply current below 20mA in the -LVx version and below 1mA in the
-HVx version. The computer will be reset when power is restored.
Pin 7: VAUX - Auxiliary External Backup Battery Charge circuit. If used,
a 4VDC to 35VDC @ 4mA power source is required to supply a trickle
charge to the battery. During normal operation a trickle charge is
supplied to the battery from an internal regulator. On-board
rechargeable battery capacity is 7 mA hour.
Pin 8: GND - Power and Signal Ground
Pin 9: Vin - Connected to Pin 10 (VER 11 and above).
Pin 10: Vin - Regulated +3.6V to +6V, 200 mA (maximum) in the -LVx versions.
Typical operating current is 120 mA plus antenna power. Transients and
overvoltages are protected by an internal 6.8V transient zener diode and
a positive temperature coefficient thermistor. With voltages greater than
6.8Vdc the thermistor will power the unit off until proper supply voltages
are returned. Antenna supply is derived from Vin after passing through a
50mA current limiter. The CMOS/TTL output buffers are powered by Vin,
therefore a 3.6Vdc supply will create 3.6V logic output levels.
In the -HVS version, Vin can be an unregulated 6.0Vdc to 40Vdc. This
voltage drives a switching regulator with a nominal 4.4Vdc output, which
powers the antenna connector current limiter, the internal linear
regulators, and the CMOS output buffers.
Pin 11: NC - This pin is floating on GPS 25LP but reserved for future.
Pin 12: NMEA - NMEA 0183, Version 1.5 electrical specification compatible serial
output. This output is CMOS compatible with a no load voltage swing of
0.2Vdc to 0.9xVCC. This output normally provides ASCII sentences
formatted per “NMEA 0183, Version 2.0”. User selectable baud rates of
300, 600, 1200, 2400, 4800, 9600 and 19200 are available, with 4800 as
the default. The data output on this pin is identical to the data output on
pin 4.

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Typical Embedded Application Interconnect
For embedded applications where the sensor board will be connected directly to the
host system microprocessor through standard CMOS logic, the following
connections are suggested.
Typical Personal Computer Application Interconnect
The following connections are suggestions for applications where the sensor board will
be connected to a personal computer via a standard RS-232 interface.

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3.4 Antenna Connection
The GPS 25LP sensor boards make their antenna connections via a 50 ohm MCX style
connector attached directly to the sensor board (+Vin-0.4V @ 15 mA power is supplied
on the center conductor for the antenna).
Coaxial cable supplies +Vin-0.4V @15 mA to antenna/preamp. 1.57542 GHz signal
returns to the sensor board.
The recommended GARMIN remote antenna for the GPS 25LP series is the GA27. It
has eight feet of RG-174A/U type cable with a MCX connector installed for attachment
to the sensor board. An extension cable with up to -5dB additional attenuation at 1.5
GHz may be used with the GA27 (either 50 Ohm or 75 Ohm impedance cable is
permitted).
A passive antenna may be used with the GPS 25LP series sensor boards, provided no
more than -2dB or -3dB cable loss is inserted between the antenna and the sensor.
Higher gain competitors’ antennas may be used provided there is adequate cable
attenuation to prevent overloading the sensitive GPS 25LP receiver. 15dB excessive
antenna system gain (LNA Gain - Cable Loss) should be considered a maximum.
Check to insure that the antenna in question will operate properly with the antenna bias
voltage that is supplied by the GPS 25LP, when powered by the desired Vin voltage.
The GPS 25LP will current limit for loads above 50mA to prevent damage, should the
antenna cable become accidentally shorted.
The following table may be used to estimate the typical attenuation of various cable
types at 1.5 GHz. For example, a 15 foot length of RG-188 may have 4.5 dB
attenuation. The attenuation of different styles of the same cable type may vary. Refer
to the specifications of the actual cable that will be used.

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Type Typical Attenuation dB/100’ @ 1 GHz
RG-211A/228A 4.5
RG-217/224/293/14 5.8
RG-8/215/216 9
RG-6 11
RG-59 11.5
RG-58 20
RG-188 30
RG-174 31
RG-178 45

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Section 4
Software Interface
The GPS 25LP interface protocol design on TXD1/RXD1 is based on the National
Marine Electronics Association’s NMEA 0183 ASCII interface specification, which is
fully defined in “NMEA 0183, Version 2.0” (copies may be obtained from NMEA, P.O.
Box 50040, Mobile, AL, 36605, U.S.A.) and the Radio Technical Commission for
Maritime Services’ “RTCM Recommended Standards For Differential Navstar GPS
Service, Version 2.1, RTCM Special Committee No. 104” (copies may be obtained from
RTCM, P.O. Box 19087, Washington, D.C., 20036, U.S.A.). The GPS 25LP interface
protocol, in addition to transmitting navigation information as defined by NMEA 0183,
transmits additional information using the convention of GARMIN proprietary
sentences.
Binary phase data information is output on TXD2, see Appendix D for details.
The following sections describe the NMEA data format of each sentence transmitted
and received by the GPS 25LP sensor board. The baud rate selection, one-pulse-per-
second output interfaces and RTCM differential GPS input are also described.
4.1 NMEA Received sentences
The subsequent paragraphs define the sentences which can be received on RXD1 by
the GPS 25LP sensor boards. Null fields in the configuration sentence indicate no
change in the particular configuration parameter.
All sentences received by the GPS 25LP must be terminated with <CR><LF>, but do
not require the checksum *hh. The checksum is used for parity checking data and it is
recommended that the checksum be used in environments containing high
electromagnetic noise. It is generally not required in normal PC environments.
Sentences may be truncated by <CR><LF> after any data field and valid fields up to
that point will be acted on by the GPS 25LP.
4.1.1 Almanac Information (ALM)
$GPALM,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,<10>,<11>,<12>,<13>,<14>,<15>
*hh<CR><LF>
The $GPALM sentence can be used to initialize the sensor board's stored almanac
information if battery back-up has failed.