Topcon America 80501WL, Q2438F-M Users Manual

DRAFAFT Chapter 1

Introduction

The GRS receiver is a single-frequency, GPS+GLONASS L1 L2receiver and hand-held controller built to be the
most advanced, compact, and portable receiver for the GIS surveying market. An integrated electronic compass and
digital camera make the GRS an all-purpose, GIS field mapping unit.

The GRS receiver is a multi-function, multi-purpose receiver intended for precision markets. Precision markets
means markets for equipment, subsystems, components and software for surveying, construction, commercial
mapping, civil engineering, precision agriculture and land-based construction and agriculture machine control,
photogrammetry mapping, hydrographic and any use reasonably related to the foregoing.

The GRS provides the functionality, accuracy, availability, and integrity needed for fast and easy data collection.

Principles of Operation

Surveying with the right GPS receiver can provide users accurate and precise positioning, a requirement for any
surveying project.

This section gives an overview of existing and proposed Global Navigation Satellite Systems (GNSS) and receiver
functions to help you understand and apply basic operating principles, allowing you to get the most out of your
receiver.

GNSS Overview

Currently, the following three global navigation satellite systems (GNSS) offer line-of-site radio navigation and
positioning, velocity, and time services on a global, all-weather, 24-hour scale to any user equipped with a GNSS
tracking receiver on or near the Earth’s surface:

• GPS – the Global Positioning System maintained and operated by the United States Department of Defense. For

information on the status of this system, visit the US Naval Observatory website (http://tycho.usno.navy.mil/) or
the US Coast Guard website (http://www.navcen.uscg.gov/).

• GLONASS – the Global Navigation Satellite System maintained and operated by the Russian Federation

Ministry of Defense. For information on the status of this system, visit the Ministry of Defense website
(http://www.glonass-center.ru/frame_e.html).

• GALILEO – an upcoming global positioning system maintained and operated by Galileo Industries, a joint

venture of several European space agencies working closely with the European Space Agency. Unlike GPS and
GLONASS, this is a civil endeavor and is currently in the development and validation stage. For information on
the status of this system, visit the Galileo Industries website (http://www.galileo-industries.net).

Despite numerous technical differences in the implementation of these systems, satellite positioning systems have
three essential components:

• Space – GPS, GLONASS, and GALILEO satellites orbit approximately 12,000 nautical miles above Earth and

are equipped with a clock and radio. These satellites broadcast digital information (ephemerides, almanacs,
time&frequency corrections, etc.).

• Control – Ground stations located around the Earth that monitor the satellites and upload data, including clock

corrections and new ephemerides (satellite positions as a function of time), to ensure the satellites transmit data
properly.

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Introduction

• User – The community and military that use GNSS receivers and the corresponding satellites to calculate

positions.

Calculating Absolute Positions

When calculating an absolute position, a stationary or moving receiver determines its three-dimensional position
with respect to the origin of an Earth-Center Earth-Fixed coordinate system. To calculate this position, the receiver
measures the distance (called pseudo-ranges) between it and at least four satellites. The measured pseudo-ranges are
corrected for clock differences (receiver and satellites) and signal propagation delays due to atmospheric effects.
The positions of the satellites are computed from the ephemeris data transmitted to the receiver in navigation
messages. When using a single satellite system, the minimum number of satellites needed to compute a position is
four. In a mixed satellite scenario (GPS, GLONASS, GALILEO), the receiver must lock onto at least five satellites
to obtain an absolute position.

To provide fault tolerance using only GPS or only GLONASS, the receiver must lock onto a fifth satellite. Six
satellites will provide fault tolerance in mixed scenarios.

Calculating Differential Positions

DGPS, or Differential GPS, typically uses the measurements from two or more remote receivers to calculate the
difference (corrections) between measurements, thus providing more accurate position solutions.

With DGPS, one receiver is placed at a known, surveyed location and is referred to as the reference receiver or base
station. Another receiver is placed at an unknown, location and is referred to as the remote receiver or rover. The
reference station collects the range measurements from each GPS satellite in view and forms the differences
(corrections) between the calculated distance to the satellites and the measured pseudo-ranges to the satellites.

These corrections are then built up to the industry standard (RTCM or various proprietary standards) established for
transmitting differential corrections and broadcast to the remote receiver(s) using a data communication link. The
remote receiver applies the transmitted DGPS corrections to its range measurements of the same satellites.

Using this technique, the spatially correlated errors—such as satellite orbital errors, ionospheric errors, and
tropospheric errors—can be significantly reduced, thus improving the position solution accuracy of the GPS.

A number of differential positioning implementations exist, including post-processing surveying, real-time
kinematic surveying, maritime radio beacons, geostationary satellites (as with the OmniSTAR service), and the
wide area augmentation system (WAAS) service.

The real-time kinematic (RTK) method is the most precise method of real-time surveying. RTK requires at least two
receivers collecting navigation data and communication data link between the receivers. One of the receivers is
usually at a known location (Base) and the other is at an unknown location (Rover). The Base receiver collects
carrier phase measurements, generates RTK corrections, and sends this data to the Rover receiver. The Rover
processes this transmitted data with its own carrier phase observations to compute its relative position with high
accuracy, achieving an RTK accuracy of up to 1 cm horizontal and 1.5 cm vertical.

Essential Components for Quality Surveying

Achieving quality position results requires the following elements:

• Accuracy – The accuracy of a position primarily depends upon the satellite geometry (Geometric Dilution of

Precision, or GDOP) and the measurement (ranging) errors.

– Differential positioning (DGPS and RTK) strongly mitigates atmospheric and orbital errors, and

counteracts Selective Availability (SA) signals the US Department of Defense transmits with GPS signals.

– The more satellites in view, the stronger the signal, the lower the DOP number, the higher positioning

accuracy.

GRS Operator’s Manual

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• Availability – The availability of satellites affects the calculation of valid positions. The more visible satellites

available, the more valid and accurate the position. Natural and man-made objects can block, interrupt, and
distort signals, lowering the number of available satellites and adversely affecting signal reception.

• Integrity – Fault tolerance allows a position to have greater integrity, increasing accuracy. Several factors

combine to provide fault tolerance, including:

– Receiver Autonomous Integrity Monitoring (RAIM) detects faulty GPS and GLONASS satellites and

removes them from the position calculation.

– Five or more visible satellites for only GPS or only GLONASS; six or more satellites for mixed scenarios.
– Wide Area Augmentation Systems (WAAS, EGNOS, etc.) creates and transmit, along with DGPS

corrections, data integrity information (for example, satellite health warnings).

– Current ephemerides and almanacs.

Conclusion

This overview simply outlines the basics of satellite positioning. For more detailed information, visit the TPS
website.

GRS Overview

The GRS is a fully integrated hand-held controller and GPS+ receiver. Included in the system is an electronic
compass and digital camera.

The hand-held controller component of the GRS

includes the Windows® Mobile operating system and color LCD touch screen. Integrated Bluetooth® /Cell phone
modem (option)wireless technology allows this system to be a cable-free controller/receiver for maximum
portability. The rugged casing is durable and built for rugged use.

As a field controller, the GRS can run a full suite of field software for working with total stations and RTK GPS
systems.

The GPS+ receiver component of the GRS

can receive and process GPS+GLONASS L1/L2 signals improving the accuracy of your survey points and positions.
The GPS+ features of the receiver combine to provide a positioning system accurate for any survey. Several other
features, including multipath mitigation, provide under-canopy and low signal strength reception.

When power is turned on and the receiver self-test completes, the receiver’s 50 channels initialize and begin
tracking visible satellites. Each of the receiver’s channels can be used to track any one of the GPS or GLONASS
signals. The number of channels available allows the receiver to track all visible GPS satellites at any time and
location.

An internal GPS antenna equipped with a low noise amplifier (LNA) and the receiver’s radio frequency (RF) device
are connected with a coaxial cable. The wide-band signal received is down-converted, filtered, digitized, and
assigned to different channels. The receiver processor controls the process of signal tracking.

Once the signal is locked in the channel, it is demodulated and necessary signal parameters (carrier and code phases)
are measured. Also, broadcast navigation data are retrieved from the navigation frame.

After the receiver locks on to four or more satellites, it is possible to solve the so-called “absolute positioning
problem” and compute the receiver’s coordinates (in WGS-84) and the time offset between the receiver clock and
GPS time. All this information can be stored in the the optional SD card and internal flash memory, then processed
using a post-processing software package.

Capabilities of the GRS receiver include:

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