OXTS GPS-Base User manual

Confidently. Accurately.
GPS-Base
GPS
Base
Station
GPS-Base
User Manual

2 Oxford Technical Solutions
Legal Notice
Information furnished is believed to be accurate and reliable. However, Oxford
Technical Solutions Limited assumes no responsibility for the consequences of use of
such information nor for any infringement of patents or other rights of third parties
which may result from its use. No license is granted by implication or otherwise under
any patent or patent rights of Oxford Technical Solutions Limited. Specifications
mentioned in this publication are subject to change without notice and do not represent
a commitment on the part of Oxford Technical Solutions Limited. This publication
supersedes and replaces all information previously supplied. Oxford Technical
Solutions Limited products are not authorised for use as critical components in life
support devices or systems without express written approval of Oxford Technical
Solutions Limited.
All brand names are trademarks of their respective holders.
Copyright Notice
© Copyright 2013, Oxford Technical Solutions.
Revision
Document Revision: 131025 (See Revision History for detailed information).
Contact Details
Oxford Technical Solutions Limited
77 Heyford Park
Upper Heyford
Oxfordshire
OX25 5HD
England
Tel: [phone]
Fax: [phone]
http://www.oxts.com
mailto:[email protected]

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Table of contents
Introduction 5
Overview 5
Correction types 6
File logging 7
Features 7
Scope of delivery 9
Specification 11
Warranty 12
Conformance notices 13
Regulatory testing standards 13
Operation and configuration 14
Choosing a suitable location 14
Connections 14
GPS-Base software 16
Choosing the connection 16
Choose port 17
Scan ports 17
Remember this port selection 18
Configuring the GPS-Base 18
Average position 19
Restore position from file 19
Enter antenna position 20
Leave unchanged 20
Advanced 20
Status page 21
Status 22

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Communication 23
Latitude, longitude, altitude 23
Base Station ID 23
Logging Novatel binary 23
Logging RTCM V3 24
Save position to file 24
Start logging 25
LED status 26
SATEL radio status 26
Freewave radio status 26
Discussion on repeatability 28
Differential correction format details 30
Revision history 32

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Introduction
The GPS-Base is a GPS Base Station suitable for transmitting differential corrections to
the OxTS RT3000, RT4000, RT2002, RT2004, Survey+ and OEM products that use
GPS. The position accuracy of differential and RTK GPS receivers is improved when
using the GPS-Base.
Four models of the GPS-Base exist, as listed in Table 1. All models are identical in
their operation but are able to track different satellite signals.
Table 1. GPS-Base Models
Model Measurement
GPS-Base-20 L1 GPS corrections suitable for 20cm positioning
GPS-Base-20G L1 GPS and L1 GLONASS corrections suitable for 20cm positioning. Some RT
products can achieve 2cm accuracy using this model.
GPS-Base-2 L1/L2 GPS corrections suitable for 2cm positioning
GPS-Base-2G L1/L2 GPS and L1/L2 GLONASS corrections suitable for 2cm positioning
The GPS-Base is available with several different radio options. Different radios are
required for license free operation in different countries.
Overview
Figure 1 gives an overview of how differential GPS works. The information from each
satellite is measured by both the GPS-Base and by the GPS in the car. The GPS-Base
works out the error in the satellite’s information and transmits it to the car using a radio
link. The GPS in the car then applies the correction to each satellite’s measurement
before it computes position and time.

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Figure 1. Differential GPS overview
For RTK (Real-Time Kinematic) carrier-phase measurements the principle is the same,
but the GPS in the car also has to figure out the difference in the number of carrier-
phase cycles between the GPS-Base and the car. The GPS-Base measures the carrier-
phase of the signals from each satellite and transmits it to the car.
Differential GPS works in real-time because the corrections from each satellite vary
slowly and predictably. The GPS in the car uses a model to predict the error from each
satellite. It can update its model when the radio link transmits new data. It is not
necessary for the GPS in the car to wait until the radio has transmitted the correction
before it outputs its latest value.
Correction types
The GPS-Base can transmit differential corrections in different formats. The
recommended format for OxTS products is RTCA (GPS-only) or RTCA2 (GPS and
GLONASS). The RTCM V3 format can also be used. Table 2 lists the different formats
supported by the GPS-Base and gives their suitability to other OxTS products.

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Table 2. Differential correction formats supported by the GPS-Base
Format Purpose
RTCA The RTCA format is suitable for GPS differential corrections for the RT3000,
RT4000 and RT2002 products. The RTCA format is compatible with older RT3000
products (serial numbers below about 150). The RTCA format is not suitable for
GLONASS.
RTCA2 The RTCA2 format is suitable for GPS and GLONASS differential corrections for
the RT3000, RT4000, RT2002 and RT2004 products. It is not compatible with older
RT3000 products (serial numbers below about 150).
RTCM V3 The RTCM V3 format is suitable for GPS and GLONASS differential corrections for
the RT3000, RT4000 and RT2002 products. It is not suitable for RT2004 products.
More detailed information on the different formats is given at the end of the manual in
the section called “Differential correction format details”.
File logging
The GPS-Base software can log data on to the hard disk of the PC. Files can be logged
in Novatel binary format and in RTCM V3 format.
Features
The GPS-Base is a self-contained unit that includes:
The Base-Station GPS receiver.
External Radio Modem.
GPS Antenna, 15m Cable and Tripod.
Radio Modem Aerial, Cable and Magnetic Mount.
All that is needed to operate the GPS-Base is an external power source (normally a
battery) and a PC or Laptop.
The GPS-Base also includes a radio modem and antenna for use on the vehicle.
The radio modem in your GPS-Base will be factory configured for use in a particular
country or territory. Typically the radio can transmit between 2 km and 5 km line-of-
sight. Trees, buildings, hills and other obstructions limit the range that can be used.
Table 3 lists the different radio options that are commonly used with the GPS-Base.

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Table 3. Overview of different radios
Radio Specification
SATEL 380 - 480 MHz band, up to 1 W, typically 5 km. License free bands
available for many European countries. Radio will typically cover 8 bands
with 25 kHz channel spacing.
SATEL 869 MHz band, up to 500 mW, typically 2 km. License free across most of
European Union.
Freewave 900 MHz band, up to 1 W, typically >10 km. License free in USA, Brazil,
Canada.
Futaba 2.4GHz band, 10mW, typically more than 1km. License free in Japan.

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Scope of delivery
Table 4, and Table 5, list all the items that are delivered with a GPS-Base and the
respective radio modem.
The customer must check that the radio can be used without a license or obtain a
suitable license before using the GPS-Base. Oxford Technical Solutions cannot be held
responsible for using this equipment illegally without the correct radio license.
Table 4. Summary of the GPS-Base components with SATEL radio
Qty Description
1 GPS-Base Unit
1 GPS-C006 15m GPS Antenna Cable
1 GPS-702-GG GPS Antenna
2 SATEL Satelline-3ASd Radio Modem
2 Radio Modem Aerial/Antenna with 3m cable and Magnetic Mount
1 Tripod
1 Power cable
1 Radio modem cable
1 PC-USB cable
1 GPS-Base User Manual
1 GPS-Base Quick Guide

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Table 5. Summary of the GPS-Base components with Freewave radio
Qty Description
1 GPS-Base unit
1 GPS-C006 15m GPS antenna cable
1 GPS-702-GG GPS antenna
2 Freewave FGR-115RC 900 MHz radio
2 14C0044B Freewave radio cable
1 Car antenna (short) with 20ft cable
1 Base-station antenna (long) with 20ft cable
1 Lightweight tripod
1 Power cable
1 Radio modem cable
1 PC-USB cable
1 GPS-Base user manual
1 GPS-Base quick guide
Figure 2. GPS-Base components with SATEL radios

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Specification
The technical specification of the GPS-Base unit is shown in Table 6, below.
Table 6. Technical specification (except radio modem)
Parameter Specification
Power 6-18 V DC, 2 W
Operating temperature -40 °C to +75 °C
Environment Dry environments
Relative humidity 95%, non-condensing
Corrections RTCA (Differential, L1, L2), RTCA2, RTCM V3
Frequency 1 Hz
Format RS232

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Warranty
Oxford Technical Solutions Limited warrants the GPS-Base products to be free of
defects in materials and workmanship, subject to the conditions set forth below, for a
period of one year from the Date of Sale.
‘Date of Sale’ shall mean the date of the Oxford Technical Solutions Limited invoice
issued on delivery of the product. The responsibility of Oxford Technical Solutions
Limited in respect of this warranty is limited solely to product replacement or product
repair at an authorised location only. Determination of replacement or repair will be
made by Oxford Technical Solutions Limited personnel or by personnel expressly
authorised by Oxford Technical Solutions Limited for this purpose.
In no event will Oxford Technical Solutions Limited be liable for any indirect,
incidental, special or consequential damages whether through tort, contract or
otherwise. This warranty is expressly in lieu of all other warranties, expressed or
implied, including without limitation the implied warranties of merchantability or
fitness for a particular purpose. The foregoing states the entire liability of Oxford
Technical Solutions Limited with respect to the products herein.

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Conformance notices
The GPS-Base complies with the radiated and conducted emission limits for CISPR 25
Level 2 and Class B of Part 15 of the FCC rules, and with the radiated emission and
immunity limits for Class B of EN 61326. These limits are designed to provide
reasonable protection against harmful interference in a residential installation. This
equipment generates, uses and can radiate radio frequency energy and, if not installed
and used in accordance with the instructions, may cause harmful interference to radio
communications. However, there is no guarantee that interference will not occur in a
particular installation. 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:
Re-orient or relocate the receiving antenna
Increase the separation between the equipment and the receiver
The GPS-Base conforms to the requirements for CE.
Regulatory testing standards
EN55025
CISPR 25 Level 2
EN 61000-4-2
EN 61000-4-3
EN 55001 (EN 61326) Class B

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Operation and configuration
Follow these steps to operate your GPS-Base unit correctly.
Choosing a suitable location
For correct operation of the GPS-Base it is essential to locate the GPS antenna in a
location where it has a full view of the sky, down to an elevation of 10 degrees in all
directions. It must also be away from reflective objects, like buildings and trees.
The GPS-Base unit should not be left in the rain or other wet conditions.
Figure 3. GPS-Base location away from buildings, trees and reflective objects
Connections
Connect the GPS antenna to the tripod or to a secure pole. The mounting should ensure
that the antenna does not move, including in wind or gusts (such as when a car drives
past). Connect the GPS antenna cable to the GPS antenna and to the GPS antenna input
on the GPS-Base unit.
Note: Never extend or shorten the GPS antenna cable. The loss in the cable is
carefully matched to the GPS-Base and lengthening or shortening the cable will reduce
the performance of the GPS-Base system.

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Note: Never connect the GPS antenna to the radio aerial connector. The use of two
TNC connectors is required since they have much better ground properties compared to
BNC connectors. The radio aerial output has a high-power signal that may damage the
GPS antenna.
Locate the radio modem aerial at least 2m away from the GPS antenna. Clip the radio
modem aerial on a metal object, such as the roof of a car. Connect the radio modem
aerial to the radio aerial connector on the GPS-Base unit. OxTS has alternative aerials
that are suitable for attaching to buildings and do not require a ground-plane under the
aerial.
Figure 4. GPS-Base connections
Figure 4, above, shows the connections for the GPS antenna and the radio aerial.

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GPS-Base software
The GPS-Base software is used to configure the GPS-Base and monitor its operation.
With the software you can:
a) Average base-station location
b) Enter base-station location, if known
c) Restore a saved base-station location
d) Program a location in to the GPS-Base so it will use it after power is removed
and restored
e) Set the base-station identifier in to the GPS-Base
f) Change the format of the corrections
g) Monitor the status of the GPS-Base
h) Save differential correction data to disk
The GPS-Base software is organised as a wizard that will guide you through the
operation of the GPS-Base.
Choosing the connection
Figure 5 shows the software when it is first run. The left hand side shows which step of
the wizard you are currently on. Pages can be jumped by clicking on the steps on the
left hand side.

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Figure 5. GPS-Base software
Choose port
To communicate with the GPS-Base the software needs to know which “port” the GPS-
Base is connected to. The GPS-Base connects using USB but the USB drivers makes
the GPS-Base appear as a serial COM port. Normally the driver will install three COM
ports for the GPS-Base: COM5, COM6 and COM7. Any of these can be used. If these
ports are taken by other software drivers then other ports will be used and it will be
necessary to “search” until the correct port is found.
The GPS-Base automatically scans all the available ports on start up and only lists the
ports where a valid GPS-Base has been detected.
Scan ports
If the GPS-Base was not connected when the software was run then the list will be
blank. Press the Scan Ports button to rescan the ports and find the GPS-Base.

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Remember this port selection
If this option is checked then the software will skip the port scan, which makes it
quicker when the software is run. Normally the GPS-Base will remain on the same port
so it is not necessary to visit this page each time.
If the GPS-Base cannot be found on the saved port then it will scan all the ports, which
can take time. It is best to have the GPS-Base on and connected before starting the
software.
Configuring the GPS-Base
Before the GPS-Base will output corrections it needs to know the position of the GPS
antenna. This can be restored from a file (i.e. using a position that has been saved), it
can be restored from the GPS-Base, it can be entered by the user or the GPS-Base can
average its position for a period of time to find an approximate location for the antenna.
There is some discussion at the end of the manual describing the benefits of each
technique. In general it is sufficient to average for 3 minutes. For repeatable work save
the averaged location and then restore this location from a file or use the “Save Position
To GPS Base” feature. To be accurate to a map it will be necessary to average for a
long period of time (and hope that the person who made the map did this too, which
they may not have).
When this page is first entered the software will default to “Average Position”. If a
GPS-Base with a valid position is detected then the software will quickly change to the
“Leave Unchanged” option.

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Figure 6. GPS-Base configuration page
Average position
Use this option to let the GPS-Base find its own position using GPS measurements.
This is the most common option to use. For temporary installations a 3 minute period is
sufficient. Longer periods can be used to find the location of the antenna more
precisely.
Note: if you average for 3 minutes then all the data for this average will be accurate to
2cm compared to other data obtained during this average. If you re-average you will
end up in a different location and the data from the new location will not overlay the
old location. Use the Save/Restore features so that future data will overlay the current
data.
Restore position from file
If you have saved the location of the GPS antenna to a file then you can restore that file
using this option. This guarantees repeatable data if you can locate the GPS antenna in
exactly the same location each time. The GPS-Base can also store an antenna location
internally, this will be explained in the “Save Position To GPS Base” section.

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Enter antenna position
If you know the position of the GPS antenna then you can enter it using this option.
The position of the antenna might be known by writing down a previous location
(rather than saving it to disk) or if a professional surveyor has measured the position of
the GPS antenna.
Note that the altitude must be entered in EGM96, not WGS-84.
Leave unchanged
Use this option if the GPS-Base already knows its location and you do not want to
change it. If the GPS-Base has a location stored internally, or a location has already
been acquired during the session, this option will be selected automatically.
Advanced
Using the Advanced Settings the correction type and the Base Station ID can be set.
This is an identifier that is transmitted with the corrections so that the mobile GPS
receiver knows which base-station is sending corrections. Figure 7 shows the Advanced
Settings page.