Trimble AgGPS 252 User Manual

F

Version 1.00

Revision A

Part Number 55510-00-ENG

February 2004

AgGPS® 252 Receiver

User Guide

Ag252_100A_UserGde_ENG.book Page i Wednesday, January 28, 2004 5:42 PM

ii AgGPS 252 Receiver User Guide

Contact Information

Trimble Navigation Limited
Agriculture Business Area
9290 Bond Street, Suite 102
Overland Park, KS 66214
USA

[phone] Phone

[e-mail]
www.trimble.com

Copyright and Trademarks

© 2004, Trimble Navigation Limited. All rights
reserved.

Trimble, the Globe & Triangle logo, and AgGPS are
trademarks of Trimble Navigation Limited, registered in
the United States Patent and Trademark Office and other
countries. EVEREST, MS750, and SiteNet are
trademarks of Trimble Navigation Limited.

Microsoft and ActiveSync are either registered
trademarks or trademarks of Microsoft Corporation in
the United States and/or other countries. All other
trademarks are the property of their respective owners.

Release Notice

This is the February 2004 release (Revision A) of the
AgGPS 252 Receiver User Guide, part number
55510-00-ENG. It applies to version 1.00 of the AgGPS
252 receiver.

The following limited warranties give you specific legal
rights. You may have others, which vary from
state/jurisdiction to state/jurisdiction.

Hardware Limited Warranty

Trimble Navigation Limited warrants that this hardware
product (the “Product”) will perform substantially in
accordance with published specifications and be
substantially free of defects in material and
workmanship for a period of one (1) year starting from
the date of delivery. The warranty set forth in this
paragraph shall not apply to software products.

Software License, Limited Warranty

This Trimble software product, whether provided as a
stand-alone computer software product, built into
hardware circuitry as firmware, embedded in flash
memory, or stored on magnetic or other media, (the
“Software”) is licensed and not sold, and its use is
governed by the terms of the relevant End User License
Agreement (“EULA”) included with the Software. In the
absence of a separate EULA included with the Software
providing different limited warranty terms, exclusions
and limitations, the following terms and conditions shall
apply. Trimble warrants that this Trimble Software
product will substantially conform to Trimble’s

applicable published specifications for the Software for
a period of ninety (90) days, starting from the date of
delivery.

Warranty Remedies

Trimble's sole liability and your exclusive remedy under
the warranties set forth above shall be, at Trimble’s
option, to repair or replace any Product or Software that
fails to conform to such warranty (“Nonconforming
Product”) or refund the purchase price paid by you for
any such Nonconforming Product, upon your return of
any Nonconforming Product to Trimble in accordance
with Trimble’s standard return material authorization
procedures.

Warranty Exclusions and Disclaimer

These warranties shall be applied only in the event and
to the extent that (i) the Products and Software are
properly and correctly installed, configured, interfaced,
maintained, stored, and operated in accordance with
Trimble's relevant operator's manual and specifications,
and; (ii) the Products and Software are not modified or
misused. The preceding warranties shall not apply to,
and Trimble shall not be responsible for defects or
performance problems resulting from (i) the
combination or utilization of the Product or Software
with hardware or software products, information, data,
systems, interfaces or devices not made, supplied or
specified by Trimble; (ii) the operation of the Product or
Software under any specification other than, or in
addition to, Trimble's standard specifications for its
products; (iii) the unauthorized, installation,
modification, or use of the Product or Software; (iv)
damage caused by accident, lightning or other electrical
discharge, fresh or salt water immersion or spray; or (v)
normal wear and tear on consumable parts (e.g.,
batteries). Trimble does not warrant or guarantee the
results obtained through the use of the Product.

THE WARRANTIES ABOVE STATE TRIMBLE'S ENTIRE
LIABILITY, AND YOUR EXCLUSIVE REMEDIES,
RELATING TO PERFORMANCE OF THE PRODUCTS
AND SOFTWARE. EXCEPT AS OTHERWISE
EXPRESSLY PROVIDED HEREIN, THE PRODUCTS,
SOFTWARE, AND ACCOMPANYING
DOCUMENTATION AND MATERIALS ARE PROVIDED

“

AS-IS” AND WITHOUT EXPRESS OR IMPLIED
WARRANTY OF ANY KIND BY EITHER TRIMBLE
NAVIGATION LIMITED OR ANYONE WHO HAS BEEN
INVOLVED IN ITS CREATION, PRODUCTION,
INSTALLATION, OR DISTRIBUTION INCLUDING, BUT
NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE, TITLE, AND
NONINFRINGEMENT. THE STATED EXPRESS
WARRANTIES ARE IN LIEU OF ALL OBLIGATIONS OR
LIABILITIES ON THE PART OF TRIMBLE ARISING
OUT OF, OR IN CONNECTION WITH, ANY PRODUCTS
OR SOFTWARE. SOME STATES AND JURISDICTIONS
DO NOT ALLOW LIMITATIONS ON DURATION OR
THE EXCLUSION OF AN IMPLIED WARRANTY, SO

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AgGPS 252 Receiver User Guide iii

THE ABOVE LIMITATION MAY NOT APPLY TO YOU.
TRIMBLE NAVIGATION LIMITED IS NOT
RESPONSIBLE FOR THE OPERATION OR FAILURE OF
OPERATION OF GPS SATELLITES OR THE
AVAILABILITY OF GPS SATELLITE SIGNALS.

Limitation of Liability

TRIMBLE’S ENTIRE LIABILITY UNDER ANY
PROVISION HEREIN SHALL BE LIMITED TO THE
AMOUNT PAID BY YOU FOR THE PRODUCT OR
SOFTWARE LICENSE. TO THE MAXIMUM EXTENT
PERMITTED BY APPLICABLE LAW, IN NO EVENT
SHALL TRIMBLE OR ITS SUPPLIERS BE LIABLE FOR
ANY INDIRECT, SPECIAL, INCIDENTAL OR
CONSEQUENTIAL DAMAGES WHATSOEVER UNDER
ANY CIRCUMSTANCE OR LEGAL THEORY RELATING
IN ANY WAY TO THE PRODUCTS, SOFTWARE AND
ACCOMPANYING DOCUMENTATION AND
MATERIALS, (INCLUDING, WITHOUT LIMITATION,
DAMAGES FOR LOSS OF BUSINESS PROFITS,
BUSINESS INTERRUPTION, LOSS OF BUSINESS
INFORMATION, OR ANY OTHER PECUNIARY LOSS),
REGARDLESS WHETHER TRIMBLE HAS BEEN
ADVISED OF THE POSSIBILITY OF ANY SUCH LOSS
AND REGARDLESS OF THE COURSE OF DEALING
WHICH DEVELOPS OR HAS DEVELOPED BETWEEN
YOU AND TRIMBLE. BECAUSE SOME STATES AND
JURISDICTIONS DO NOT ALLOW THE EXCLUSION OR
LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR
INCIDENTAL DAMAGES, THE ABOVE LIMITATION
MAY NOT APPLY TO YOU.

NOTE: THE ABOVE LIMITED WARRANTY
PROVISIONS MAY NOT APPLY TO PRODUCTS OR

SOFTWARE PURCHASED IN THE EUROPEAN UNION.

PLEASE CONTACT YOUR TRIMBLE DEALER FOR
APPLICABLE WARRANTY INFORMATION.

Notices
USA

NOTE – FCC Part 15 rules; paragraph 15.105

This equipment has been tested and found to comply
with the limits for a Class A digital device, pursuant to
Part 15 of the FCC Rules. These limits are designed to
provide reasonable protection against harmful
interference when the equipment is operated in a
commercial environment. This equipment generates,
uses, and can radiate radio frequency energy and, if not
installed and used in accordance with the instruction
manual, may cause harmful interference to radio
communications. Operation of this equipment in a
residential area is likely to cause harmful interference, in
which case, you, the user, will be required to correct the
interference at your own expense.

If this equipment does cause harmful interference to
radio or television reception, which can be determined
by turning the equipment off and on, the user is
encouraged to try to correct the interference by one or
more of the following measures:

– Reorient or relocate the receiving antenna.
– Increase the separation between the equipment and

the receiver.

– Connect the equipment into an outlet on a circuit

different from that to which the receiver is connected.

– Consult the dealer or an experienced radio/TV

technician for help.

Changes and modifications not expressly approved by
the manufacturer or registrant of this equipment can
void your authority to operate this equipment under
Federal Communications Commission rules.

Europe

This product has been tested and found to comply with
the requirements for the European Directive
75/322/EEC as amended by 2000/2/EC thereby
satisfying the requirements for e-mark compliance for
use in agricultural vehicles in the European Economic
Area (EEA).

This product has been tested and found to comply with
the requirements for a Class A device pursuant to
European Council Directive 89/336/EEC on EMC,
thereby satisfying the requirements for CE Marking and
sale within the European Economic Area (EEA).

C

Warning – This is a Class A product. In a domestic

environment this product may cause radio
interference in which case you may be required to
take adequate measures.

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iv AgGPS 252 Receiver User Guide

Declaration of Conformity

This product conforms to the following standards, and therefore complies with the requirements of the R&TTE
Directive 1999/5/EC, which specifies compliance with the essential requirements of EMC Directive 89/336/EEC and
Low Voltage Directive 73/23/EEC.

The technical file is maintained at Trimble Navigation Limited, 749 North Mary Avenue, PO Box 3642, Sunnyvale,
CA 94088-3642, USA.

EMC Emissions BSEN 55022:1998 (W/A1:00) Class A
EMC Immunity EN 55024:1998
Safety EN 60950:2000
Mark First Applied 03

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AgGPS 252 Receiver User Guide v

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Related Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Technical Assistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Your Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Standard Features of the AgGPS 252 Receiver . . . . . . . . . . . . . . . . 4

Receiver Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Receiver Input/Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

LED Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

GPS Positioning Methods. . . . . . . . . . . . . . . . . . . . . . . . . . 10

RTK GPS positioning . . . . . . . . . . . . . . . . . . . . . . . . 10

Differential GPS positioning (DGPS) . . . . . . . . . . . . . . . . 11

Autonomous GPS positioning . . . . . . . . . . . . . . . . . . . . 12

Sources of Error in GPS Positioning . . . . . . . . . . . . . . . . . . . . 13

Coordinate systems. . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Installing the Receiver. . . . . . . . . . . . . . . . . . . . 17

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

System Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Optional extra . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Mounting the Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Choosing a location . . . . . . . . . . . . . . . . . . . . . . . . . 19

Environmental conditions . . . . . . . . . . . . . . . . . . . . . . 20

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Contents

vi AgGPS 252 Receiver User Guide

Electrical interference . . . . . . . . . . . . . . . . . . . . . . . . 20

Connecting to an External Device . . . . . . . . . . . . . . . . . . . . . 21

Connectors and Pinouts . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Port A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Port B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4 Configuring the Receiver . . . . . . . . . . . . . . . . . . 27

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

AgRemote Home Screen . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Configuring Differential GPS. . . . . . . . . . . . . . . . . . . . . . . . 30

OmniSTAR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

WAAS/EGNOS . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Configuring the AgGPS 252 Receiver to Operate in RTK Mode . . . . . . 33

Configuring the Communication Ports . . . . . . . . . . . . . . . . . . . 33

Configuring input/output communication . . . . . . . . . . . . . . 34

5 Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . 39

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Problems and Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Troubleshooting Flowcharts . . . . . . . . . . . . . . . . . . . . . . . . 48

A Specifications . . . . . . . . . . . . . . . . . . . . . . . . 55

AgGPS 252 Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

GPS Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

L-Band Satellite Differential Correction Receiver . . . . . . . . . . . . . 58

Receiver Default Settings . . . . . . . . . . . . . . . . . . . . . . . . . . 58

B Third-Party Interface Requirements . . . . . . . . . . . . 59

Third-Party Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Third-Party Hardware. . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

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CHAPTER

1

AgGPS 252 Receiver User Guide 1

Introduction 1

Welcome to the AgGPS 252 Receiver User Guide. This manual:

• Describes how to install and configure the Trimble

®

AgGPS

®

252 receiver.

• Provides guidelines for connecting the receiver to an external
device.

• Provides guidelines for using the AgRemote utility to view and
configure the receiver correction sources and other operating
parameters.

Even if you have used other Global Positioning System (GPS)
products before, Trimble recommends that you spend some time
reading this manual to learn about the special features of this product.

If you are not familiar with GPS, go to the Trimble website at

www.trimble.com for an interactive look at Trimble and GPS.

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1 Introduction

2 AgGPS 252 Receiver User Guide

1.1 Warnings

Always follow the instructions that accompany a warning.

C

Warning – Indicates a potential hazard or unsafe practice that could result
in injury or property damage.

1.2 Related Information

Release notes describe new features, provide information that is not
included in the manuals, and identify changes to the manuals. You can
download release notes from the Trimble website.

1.3 Technical Assistance

If you have a problem and cannot find the information you need in the

product documentation, contact your local Trimble Reseller.

1.4 Your Comments

Your feedback about the supporting documentation helps us to
improve it with each revision. Email your comments to

[e-mail].

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CHAPTER

2

AgGPS 252 Receiver User Guide 3

Overview 2

In this chapter:

 Introduction
 Standard Features of the AgGPS 252 Receiver
 Receiver Connections
 Receiver Input/Output
 LED Indicator
 GPS Positioning Methods
 Sources of Error in GPS Positioning

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2 Overview

4 AgGPS 252 Receiver User Guide

2.1 Introduction

This chapter describes the AgGPS 252 receiver and gives an overview

of GPS, DGPS, and related information. When used with a Real-Time

Kinematic (RTK) base station, the AgGPS 252 receiver provides RTK

positioning for high-accuracy, centimeter-level applications. For
physical specifications, see Appendix A, Specifications.

2.2 Standard Features of the AgGPS 252 Receiver

A standard AgGPS 252 receiver provides the following features:

• 12 GPS (C/A-code) tracking channels, code carrier channels

• Horizontal RTK positioning accuracy 2.5 cm (0.98 in) + 2 ppm,
2 sigma; vertical RTK positioning accuracy 3.7 cm (1.46 in)
+ 2ppm, 2sigma

• Submeter differential accuracy (RMS), assuming at least five
satellites and a PDOP of less than four

• Combined GPS/DGPS receiver and antenna

• System level cable

• AgRemote utility with four-button keypad to configure and view

system properties (download from the Trimble website at

www.trimble.com)

• LED status indicator

• The receiver outputs a 1 PPS (pulse per second) strobe signal on
both ports. This signal enables an external instrument to
synchronize its internal time with a time derived from the very
accurate GPS system time.

• WAAS differential correction compatibility

• AgGPS 170 Field Computer compatibility

• EVEREST™ multipath rejection technology

• OmniSTAR VBS and HP positioning compatibility

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AgGPS 252 Receiver User Guide 5

Overview 2

• Two ports that support both CAN 2.0B and RS-232:

CAN

– J1939 and NMEA 2000 messages

Note – The AgGPS 252 is ISO 11783 compliant. It supports
some ISO 11783 messages.

RS-232

– NMEA-0183 output: GGA, GLL, GRS, GST, GSA, GSV,

MSS, RMC, VTG, ZDA, XTE (the default NMEA
messages are GGA, GSA, VTG, and RMC)

Note – PTNLDG, PTNLEV, PTNLGGK, PTNLID, and
PTNLSM are Trimble proprietary NMEA output messages.

– RTCM SC-104 output

– Trimble Standard Interface Protocol (TSIP) input and

output

2.3 Receiver Connections

Figure 2.1 shows the connector ports and the LED indicator on the

AgGPS 252 receiver.

Figure 2.1 AgGPS 252 receiver connector ports

LED indicator

Port A Port B

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2 Overview

6 AgGPS 252 Receiver User Guide

The two connectors (Port A and Port B) can perform the following
functions:

• accept power

• accept TSIP, RTCM, ASCII, and (if enabled) CMR inputs

• output RTCM, TSIP, and NMEA messages

• output 1 PPS signals

• provide support for the J1939 (CAN) serial bus

For more information about the inputs, outputs, and LED indicators,
see the information in the rest of this section.

2.4 Receiver Input/Output

The AgGPS 252 receiver data/power cable (P/N 50166) connects to a

receiver connector port to supply power. It also enables the following
data exchanges:

• TSIP, RTCM, and ASCII input from an external device

The receiver is able to receive ASCII data from an external
device, convert this data into an NMEA message, and export the
message to another device. TSIP command packets configure
and monitor GPS and DGPS parameters. The receiver is also
able to accept RTCM data from an external device, such as a
radio.

• CMR input from an external device

If the receiver is to be used in RTK mode, set the port that is

connected to the radio to the RtkLnk protocol. This protocol

enables the receiver to receive CMR messages.

• TSIP and NMEA output to an external device

When you are using an external radio, the receiver can also
receive DGPS corrections.

TSIP is input/output when communicating with AgRemote.

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AgGPS 252 Receiver User Guide 7

Overview 2

NMEA is output when the receiver is exporting GPS position
information to an external device, such as a yield monitor, or to
a mapping software program.

For more information on the National Marine Electronics
Association (NMEA) and Radio Technical Commission for
Maritime Services (RTCM) communication standard for GPS
receivers, go to the following websites:

– www.nmea.org

– www.rtcm.org

On the Trimble website ((www.trimble.com), refer to the

document called NMEA-0183 Messages Guide for AgGPS
Receivers.

• 1 PPS output

To synchronize timing between external instruments and the
internal clock in the receiver, the connection port outputs a
strobe signal at 1 PPS (pulse per second). To output this signal,
the receiver must be tracking satellites and computing GPS
positions.

• J1939 (CAN) bus

Both connection ports on the receiver support the J1939
Controller Area Network (CAN) bus protocol. This protocol
standardizes the way multiple microprocessor-based electronic
control units (ECUs) communicate with each other over the
same pair of wires. It is used in off-highway machines, such as
those used in agriculture, construction, and forestry.

For more information, go to the Society of Automotive
Engineers (SAE) International website at

www.sae.org/servlets/index.

• ISO 11783 messages

Both CAN ports support some ISO 11783 messages.

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2 Overview

8 AgGPS 252 Receiver User Guide

Position output format

The AgGPS receiver outputs positions in Degrees, Minutes, and

Decimal Minutes (DDD°MM.m'). This is the NMEA standard format
and is commonly used worldwide for data transfer between electronic
equipment.

2.5 LED Indicator

The AgGPS 252 receiver has an LED light that shows the status of the

receiver. The following tables describe the light sequences for each
positioning method.

Note – WAAS/EGNOS and OmniSTAR VBS use the Satellite
Differential GPS positioning method.

Table 2.1 LED sequences with Satellite Differential GPS or Autonomous positioning

LED color LED flash Status

Off Off No power
Green Solid Normal operation: computing DGPS positions
Green Slow No DGPS corrections: computing DGPS positions using old

corrections

Green Fast No DGPS corrections approaching DGPS age limit: computing

DGPS positions using old corrections

Yellow Solid DGPS corrections being received but DGPS positions not yet being

computed: computing autonomous GPS positions
Yellow Slow No DGPS corrections: computing autonomous GPS positions
Yellow F ast Not enough GPS signals: not tracking enough satellites to compute

position

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AgGPS 252 Receiver User Guide 9

Overview 2

Table 2.2 LED sequences with RTK positioning

LED color LED flash Status

Off Off No power
Green Solid Normal operation: computing fixed RTK positions
Green Slow Receiving CMR corrections but not initialized: computing float RTK

positions
Green Fast No CMR corrections: computing RTK position using old corrections
Yellow Solid Receiving CMR corrections but unable to calculate RTK position:

computing DGPS (if WAAS/EGNOS is unavailable) or autonomous

position
Yellow Slow No CMR corrections: computing DGPS or autonomous position
Yellow Fast Not receiving CMR corrections: not computing positions

Table 2.3 LED sequences with OmniSTAR HP positioning

LED color LED flash Status

Off Off No power
Green Solid Normal operation: computing converged OmniSTAR HP positions
Green Slow Receiving OmniSTAR HP corrections, but only able to compute

unconverged position
Green Fast Receiving OmniSTAR HP corrections, but an HP error occurred
Yellow Solid Receiving OmniSTAR HP corrections but unable to calculate a

position: computing DGPS or autonomous solution
Yellow Slow No OmniSTAR HP corrections: computing DGPS or autonomous

position
Yellow Fast Not tracking OmniSTAR HP corrections: no positions

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2 Overview

10 AgGPS 252 Receiver User Guide

2.6 GPS Positioning Methods

GPS positioning systems are used in different ways to provide
different levels of accuracy. Accuracy is measured in absolute terms
(you know exactly where you are in a fixed reference frame).

Table 2.4 summarizes the GPS positioning methods. Imperial units in
this table are rounded to two decimal places. The values shown are
2sigma.

Table 2.4 Absolute accuracy of GPS positioning methods

For more information about each positioning method, see below.

26.1 RTK GPS positioning

The AgGPS 252 receiver uses the RTK positioning method to achieve

centimeter-level accuracy. To use the RTK method, you must first set
up a base station. The base station uses a radio link to broadcast RTK

corrections to one or more rover receivers. The AgGPS 252 receiver is

a rover receiver, so another compatible receiver, such as a Trimble

MS750™ or AgGPS 214 GPS receiver, must be used as the base

station.

GPS positioning
method

Corrections used Approximate absolute accuracy

Real-Time Kinematic
(RTK) GPS

Trimble CMR
corrections broadcast
by a local base station

2.5 cm (0.98 in) + 2 ppm horizontal
accuracy,

3.7 cm (1.46 in) + 2 ppm vertical accuracy
Satellite Differential GPS OmniSTAR VBS 78 cm (30.71 in)
Satellite Differential GPS WAAS/EGNOS 95 cm (37.40 in)
OmniSTAR HP

Differential GPS

OmniSTAR HP 10 cm (3.94 in) after the signal has fully

converged

1

1

Convergence time can vary, depending on the environment. Time to the first fix (submeter accuracy) is typically

<30 seconds; time to the first high accuracy fix (<10 cm accuracy) is typically <30 minutes.

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AgGPS 252 Receiver User Guide 11

Overview 2

The rover receiver uses RTK corrections from the base station to
calculate its position to centimeter-level accuracy. As part of this
process, the rover receiver must calculate an initialization. This takes a
few seconds. While the receiver is initializing, an RTK Float solution
is generated. Once initialized, an RTK Fixed solution is generated. It is
the RTK Fixed solution that provides centimeter-level accuracy.

The parts per million (ppm) error is dependent on the distance
(baseline length) between the base and rover receiver. For example, if
the distance is 10 km, a 2 ppm error equals 20 mm.

For more information about RTK positioning, go to the Trimble
website at www.trimble.com/gps/

26.2 Differential GPS positioning (DGPS)

For differential positioning, the AgGPS 252 receiver uses corrections

from WAAS/EGNOS satellites or from OmniSTAR VBS or HP
satellites.

These differential systems use special algorithms to provide
differential corrections that allow the rover receiver to calculate its
position more accurately.

Free corrections

WAAS/EGNOS corrections are free in North America and Europe.
For more information about WAAS, go to the Federal Aviation
Administration website at

http://gps.faa.gov/Programs/WAAS/waas.htm.

For more information about EGNOS, go to the European Space
Agency website at

www.esa.int/export/esaSA/GGG63950NDC_navigation_0.html.

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2 Overview

12 AgGPS 252 Receiver User Guide

Subscription-based corrections

The AgGPS 252 receiver uses OmniSTAR HP or OmniSTAR VBS

differential corrections in the same way that it uses WAAS/EGNOS
corrections.

OmniSTAR corrections are provided on a subscription basis.

The corrections that are produced by OmniSTAR HP algorithms are
more accurate than the corrections that are produced by OmniSTAR
VBS algorithms. The accuracy of the positions reported using
OmniSTAR HP increases with the time that has elapsed since the

instrument was turned on. This process is called convergence.

Convergence to where the error is estimated to be below 30 cm
(approximate 12 inches) typically takes around 20 minutes. Factors
that influence the time to convergence include the environment, the
geographical location, and the distance to the closest OmniSTAR
corrections base station. OmniSTAR is continually improving the
service.

For more information about OmniSTAR, go to the OmniSTAR
website at www.omnistar.com. For information about activating an
OmniSTAR subscription, see OmniSTAR, page 31.

26.3 Autonomous GPS positioning

Autonomous GPS positioning uses no corrections. The rover receiver
calculates its position using only the GPS signals it receives. This
method does not have high absolute accuracy, but the relative
accuracy is comparable to the other methods.

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AgGPS 252 Receiver User Guide 13

Overview 2

2.7 Sources of Error in GPS Positioning

The GPS positioning method influences the accuracy of the GPS

position that is output by the AgGPS 252 receiver. The factors

described in Table 2.5 also affect GPS accuracy.

Table 2.5 Factors that influence the accuracy of GPS positions

Condition Optimum

value

Description

Atmospheric
effects

GPS signals are degraded as they travel through the
ionosphere. The error introduced is in the range of 10 meters.

The error is removed by using a differential or RTK positioning
method.

Number of
satellites used

> 5 To calculate a 3D position (latitude and longitude, altitude, and

time), four or more satellites must be visible. To calculate a 2D
position (latitude and longitude, and time), three or more
satellites must be visible. For R TK positioning, five satellites are
needed for initialization. Once initialized, four or more satellites
provide RTK positions. The number of visible satellites
constantly changes and is typically in the range 5 through 9.
The AgGPS receiver can track up to 12 satellites
simultaneously.

Note – To see when the maximum number of GPS satellites are
available, use the Trimble Planning software and a current
ephemeris (satellite history) file. Both files are available free from
the Trimble website at www.trimble.com.

Maximum PDOP < 4 Position Dilution of Precision (PDOP) is a unitless, computed

measurement of the geometry of satellites above the current
location of the receiver. A low PDOP means that the positioning
of satellites in the sky is good, and therefore good positional
accuracy is obtained.

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2 Overview

14 AgGPS 252 Receiver User Guide

Signal-to-noise
ratio

> 6 Signal-to-noise ratio (SNR) is a measure of the signal strength

against electrical background noise. A high SNR gives better
accuracy.

Normal values are:

•GPS 6

• WAAS 3+

• OmniSTAR HP/VBS 6+

Minimum
elevation

> 10 Satellites that are low on the horizon typically produce weak

and noisy signals and are more difficult for the receiver to track.
Satellites below the minimum elevation angle are not tracked.

Multipath
environment

Low Multipath errors are caused when GPS signals are reflected off

nearby objects and reach the receiver by two or more different
paths. The receiver incorporates the EVEREST multipath
rejection option.

RTCM­compatible
corrections

These corrections are broadcast from a Trimble AgGPS 214,
MS750, or equivalent reference station.

RTK Base station
coordinate
accuracy

For RTK positioning, it is important to know the base station
coordinates accurately. Any error in the position of the base
station affects the position of the rover; every 10 m of error in a
base station coordinate can introduce up to 1 ppm scale error
on every measured baseline. For example, an error of 10 m in
the base station position produces an error of 10 mm over a
10 km baseline to the rover.

For more information about how to make sure the position of
your base station is accurate, refer to the manual for your base
station receiver.

Multiple RTK
base stations

If you are using several base stations to provide RTK
corrections to a large site area, all base stations must be
coordinated relative to one another. If they are not, the absolute
positions at the rover will be in error. F or more inf ormation about
how to use several base stations to cover your site , contact your
local Trimble Reseller.

Table 2.5 Factors that influence the accuracy of GPS positions (continued)

Condition Optimum

value

Description

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AgGPS 252 Receiver User Guide 15

Overview 2

27.1 Coordinate systems

Geographic data obtained from different sources must be referenced to
the same datum, ellipsoid, and coordinate format. Different formats
provide different coordinate values for any geographic location. In
North America, the datums NAD-27 and NAD-83 are commonly used
in Agricultural mapping applications.

The AgGPS 252 receiver outputs position coordinates in several

datums and ellipsoids depending on the GPS positioning method
being used. See Table 2.6.

For more information, go to the National Geodetic Survey website at

www.ngs.noaa.gov/faq.shtml#WhatDatum

Table 2.6 DGPS coordinate systems

GPS positioning method Datum Ellipsoid

None – Autonomous mode WGS-84

1

WGS-84

OmniSTAR VBS North American Beams NAD-83

2

GRS-80

OmniSTAR VBS Rest of World Beams ITRF

3

GRS-80
OmniSTAR HP ITRF 2000 ITRF 2000
WAAS Beams WGS-84 WGS-84
RTK WGS-84 WGS-84

1

World Geodetic System (WGS) 1984. Datum and ellipsoid.

2

North American Datum (NAD) 1983. Equivalent to WGS-84 in North America.

3

International Terrestrial Reference Frame (ITRF). Contact the DGPS provider for details.

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2 Overview

16 AgGPS 252 Receiver User Guide

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CHAPTER

3

AgGPS 252 Receiver User Guide 17

Installing the Receiver 3

In this chapter:

 Introduction
 System Components
 Mounting the Receiver
 Connecting to an External Device
 Connectors and Pinouts

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