Sokkia GRX1 User Manual

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Introduction

The GRX1 receiver is a multi-frequency, GPS+ receiver built to be the most advanced and
compact receiver for the surveying market. The 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 GRX1 can receive and processes multiple signal types ( including the latest GPS L1, L2, C/A,
L2C GLONASS L1, L2, C/A signals ) improving the accuracy and reliability of the survey points
and positions, especially under difficult jobsite conditions. The multifrequency and 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. The receiver provides the functionality, accuracy, availability, and integrity
needed for fast and easy data collection.

Figure 1-1. GRX1 Receiver

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 so that basic
operating principles can be applied.

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GNSS Overview

Currently, the following two global navigation satellite systems ( GNSS ) offer line-of-site radio
navigation and positioning, velocity, and time services on a global, all-weather 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

Coordinational Scientific In-formation Center website ( http://www.glonass-ianc.rsa.ru/ ).
Despite numerous technical differences in the implementation of these systems, satellite
positioning systems have three essential components:
 Space - GPS and GLONASS satellites orbit approximately 12,000 nautical miles above Earth

and are equipped with a clock and radio. These satellites broadcast ranging signals and

various digital information ( ephemerides, almanacs, time and frequency corrections, and

so forth ).
 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.
 User - The community and military that use GNSS receivers 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 ), the receiver must lock onto f ive or
more satellites to account for the different time scales used in these systems and to obtain an
absolute position.

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Calculating Differential Positions

DGPS, or Differential GPS, is a relative positioning technique where the measurements from two
or more remote receivers are combined and processed using sophisticated algorithms to
calculate the receivers' relative coordinates with high accuracy. DGPS accommodates various
implementation techniques that can be classified according to the following criteria:
 The type of GNSS measurements used, either code-phase differential measurements or

carrier-phase differential measurements
 If real-time or post-mission results required. Real-time applications can be further divided

according to the source of differential data and communication link used.
With DGPS in its most traditional approach, one receiver is placed at a known, surveyed location
and is re ferred to as the referen ce 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 code-phase and carrier-phase measurements from each GNSS satellite in view.
 For real-time applications, these measurements and the reference station coordinates are

then built up to the industry standard RTCM - or various proprietary standards established

for transmitting differential data - and broadcast to the remote receiver ( s ) using a data

communication link. The remote receiver applies the transmitted measurement

information to its observed measurements of the same satellites.
 For post-mission applications, the simultaneous measurements from reference and rover

stations are normally re-corded to the receiver's internal memory ( not sent over

communication link ). Later, the data are downloaded to computer, combined, and

processed. 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.
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 satellite based augmentation systems ( WAAS, EGNOS, MSAS ).
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 1.0 cm horizontal and 2.0 cm vertical.

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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.

 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 GNSS 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 scenario

 Satellite Based Augmentation Systems ( WAAS, EGNOS, and so on ) 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 Sokkia Topcon website.

Receiver Overview

When power is turned on and the receiver self-test completes, the receiver's 72 channels
initialize and begin tracking visible satellites. Each of the receiver's channels can be used to
track any one of the GPS or GALILEO signals. The number of channels available allows the
receiver to track all visible global positioning satellites at any time and location.

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An internal GPS+ antenna equipped with a low noise amplifier ( LNA ) and the receiver's radio
frequency ( RF ) device are connected with a co-axial 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, its absolute position in WGS-84 and the
time offset between the receiver clock and GPS time are computed. This information and the
measurement data can be stored in the optional SD card and downloaded later onto a computer,
then processed using a post-processing software package. When the receiver operates in RTK
mode, raw data measurements can also be recorded into the receiver's internal memory. This
allows the operator to double check real-time results obtained in the field.
Depending on your options, capabilities of the receiver include:

 Satellite based augmentation systems ( WAAS, EGNOS, and so forth ).
 Adjustable phase locked loop ( PLL ) and delay lock loop ( DLL ) parameters
 Dual- or multi-frequency modes, including static, kinematic, real-time kinematic ( RTK ),

and differential GPS ( DGPS ) survey modes ( DGPS modes include static, kinematic, and

RTK )

 Auto data logging
 Setting different mask angles
 Setting different survey parameters
 Static or dynamic modes

Getting Acquainted

The GRX1 is a 72-channel GPS receiver, which includes the following:

 External, detachable batteries
 One data ports
 Interface for controlling and viewing data logging
 External memory card slot
 Internal radio modem
 Bluetooth wireless technology module
 Optional GSM/GPRS module
 Optional CDMA module ( only with the Digital UHF radio modem )

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