Friday, December 10, 2010

Collecting Static Data with the Topcon GRS-1

Topcon GRS1

GRS-1 Overview

The GRS-1 is a fully integrated hand—held controller and GPS+GLONASS Ll1 receiver. Integrated into the unit is a 72-channel dual frequency GPS and Glonass receiver, and electronic compass, digital camera and optional cellular modem for internet connections.

Essential Components for Quality Surveying

Achieving quality position results requires the following elements:

image 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 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, ctc.) creates and transmit, along with DGPS corrections, data integrity information (for example, satellite health warnings).

— Current ephemerides and almanacs.

The GPS+ receiver component of the GRS-1 can receive and process GPS+GLONASS L1 and 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 and co-op tracking, provide under-canopy and low signal strength reception.

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 GLONASS signals. The number of channels available allows the receiver to track all visible GPS and Glonass satellites at any time and location.

An internal L1 GPS+Glonass 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.

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. With the collection of enough information to solve for position and time the receiver can solve an RTK position if provided with a differential correction from a base station or network, or the raw observation data may be stored in the optional SD card or to the internal memory of the unit for later post processing with a post—processing software package such as Topcon Tools.

Capabilities of the GRS-1 receiver include:

• Multipath reduction

• Satellite Based Augmentation Systems supported (WAAS and EGNOS)

• Dual-frequency static, kinematic, and Real time kinematic (RTK) survey modes (Use to external GNSS antenna MG-A8 or PG- A1)

Pre-Survey Preparation

Before beginning to survey with the GRS-1 receiver, the following steps and configurations need to be applied:
  • Charge the batteries
  • Install SD card
  • Gather the proper equipment needed for the survey.
  • Create a Post Processing configuration in TopSURV
Topcon GR-3 Geodetic Receiver, “Starting a Static Occupation”
  • Create a Static Configuration
  • Select: Configuration, Coordinate System, Geoid, Units, Display
  • Establish a Connection

Introduction to TopSURV

imageTopSURV is Topcon’s survey software available for hand-held controllers. TopSURV is used for surveying, common layout, and GIS purposes including:
• Field data collection with Topcon GPS receivers; Topcon, Sokkia, Nikon and Leica Total Stations; and Topcon Digital Levels
• Roads design to create cross section templates, horizontal, and vertical alignments
• Stakeout designed objects
• Data conversions to a variety of file formats
• COGO calculations
TopSURV installs on hand—held controllers that run Windows® Mobile operating system, such as Topcon’s GRS-1.
Starting TopSURVimage
To start TopSURV, select TopSURV from the applications menu.
TopSURV initially creates a Default job upon program installation.
By default, all jobs are stored in the Job folder of the TopSURV directory.
• Open - makes the Default job current and opens the main screen.
• New — press to create a new job. • Exit- quits the program.
• Browse — press to display browse directories to navigate to a desired job. Highlight the file and press OK to open the job.
Creating a new job in TopSURV
1. To create a new job, tap Job > New Job or tap the New button on the Open Job screen during initial startup. The New Job screen displays. Enter the Name of the job and corresponding information (that is, the name of the surveyor and any necessary comments). The date is stored automatically. Tap Next to move to the next screen. At any stage, select the Finish button to create a new job. See the following sections to create a new job for your mode of survey. A Job file contains all the pertinent data for the work being done; settings of the performed work and information on the Survey Configuration.
imageA Survey Configuration is a set of settings, such as instrument parameters or radio settings, which are independent of the job (one configuration can be used on several jobs). Configuration settings are applied to the equipment only after opening a screen that measures and stores data in the job file.
Survey configurations are stored in the Styles.tsstyles file in the TopSUimageRV directory.
2. By default all job files are stored in the Job folder of the TopSURV directory. To change the location of the job being created, tap Browse on the New Job screen.
3. On the Survey Style screen select the Survey Configuration, for GPS+ Next.
A Survey Configuration is a set of parameters that describe work conditions and depend upon the instrument used for the survey. The last Open configuration will initially display.
Creating a Configuration
A new configuration is performed with the help of a Wizard.
When creating a GPS+ configuration, use pre-defined configurations or create new ones. The pre-defined configurations are listed in drop-down menus in the corresponding fields. In the GPS+ Configuration field, choose one of the pre-defined configurations or tap the List button to create a new one or edit the parameters of an existing configuration. The Configurations screen displays. For this exercise create a configuration for GRS-1 Post Processing. Once the survey configuration has been created and selected, other job settings can be selected by tapping Next on the Survey Style screen. image
GPS+ Configuration
To configure a GPS+ survey, press the List button in the GPS+ Config field of the Survey Configuration screen.
The Configurations screen contains a list of Pre-defined configurations. Either edit an existing configuration or create a new configuration.
• Delete — deletes the highlighted configuration.
• Edit — opens the Survey screen to edit the configuration settings.
• Add — opens the Survey screen to create a new configuration.
• OK — returns to the Survey Configuration screen. .
• Name — the name of the configuration that is displayed in the Configurations screen.
• Type – Select the PP Static of configuration. (“PP" means Post-Processing.) Select Edit and change the name to “GRS-1 PP STATIC”. Select next.
•Simulation mode - check mark this box ONLY to run TopSURV in a GPS simulation mode!
imageWhen the mode is selected, then Simulation Setup icon appears in the Edit Job menu. For this configuration, leave Simulation mode unchecked.
•Static Receiver – For an L1/L2 solution, select the PGA-1 antenna.
• Raw Data Logging — imagethe set of logging parameters; log to the receiver, set the logging rate, and imageselect if the name of the receiver file is automatically set or user—defined. In the latter case, the corresponding dialog box displays at the logging start.
Complete the remaining screens to your preference and select Finish

Select a Configuration
Select the GRS-1 PP Static configuration from the pull-down list and select Next>>
imageCoordinate System
1. On the Coord System screen set the parameters of the coordinate system used: the projection, the Datum, and/or Geoid, then tap Next >>
Adding a Geoid File
Geoid is a physical reference surface. Its shape reflects the distribution of mass inside the earth. Geoid undulations are important for converting GPS-derived ellipsoidal height differences to orthometric height differences.
Install the geoid file on the controller prior to adding it to the list. Global geoid files can be opened and truncated in Topcon Link to fit the file to the job area.
Some geoid files can be installed on the controller during TopSURV installation. They are provided to the user with the TopSURV installation program as “.gif” files. image
To add a geoid file to the drop—down list in the Geoids field of the Coordinate System screen, tap the List button next to this field.
1. On the Geoid List screen, this is initially empty, press the Add button.
2. On the Add Geoid screen (Figure 5), select a Geoid tile from the controller directory to view the boundaries of the geoid application. Select Geoid 99/2003, Australian, Canadian 2000, Canadian 95, Geoid File Format, Mexico 97, Sweden, Denmark, Duteh2004 Files, or Norwegian Files.
3. Press OK to return to the Geoid List screen with the geoid tile added. This list corresponds to the Geoids drop-down list in the Coord System screen.
GEOID09 Data for Conterminous United States - PC binary format can be downloaded from:
Grid to/from Ground Transformation
If necessary, perform the Grid to Ground coordinate transformation.
A ground projection is a grid mapping projection rescaled to convert point coordinates to another reference surface (up to the average project elevation) to produce near ground distance values. The ground system can be rotated and shifted relative to the grid system. The ground coordinates can be converted back to the grid projection.
Set the distance and angle units of the job on the Units screen and tap Next (Figure 6). For the Total Station mode, also select the temperature and pressure units.
On the Display imagescreen, select the Display parameters: the type of Coordinates displayed, the plane coordinates order, the reference direction for Azimuth and representation type and the method for displaying position on the Centerline (Station or Chainage). If the Station is selected as representation type to display position on the CL, set the Full Station value. Then tap Next.
imageOn the Alarms screen, check and enable the Audible Alarm field to enable a sound for alarms in the Controller, Receiver, or Total Station, Place the check marks, where necessary.
Tap Finish to save for the newly created job.
Starting a Static Occupation
1. Turn on GRS-1
2. Create new job or open existing job in TopSURV
3. Select the PP Static configuration edited for the GRS-1.
Performing a Static Survey
1. To open the Static Occupation screen choose the GRS-1 PP Static configuration in the Survey Config screen (use the Configure icon) and tap Setup GPSimage > Static Occupationimage.
2. Enter the parameters of the occupation point: name, code and antenna height and height type.image
3. Tap Start Occ. The survey will start, and the Duration field displays the time passed, since the beginning of survey.
If User Defined was set in the configuration, TopSURV will prompt for a name for theimage static file (TPS file).
4. To stop the survey, tap the same button (when tapped, it changes (toggles) to Stop Occ).
The TPS files default to \Program Files\TPS\Raw Data on the GRS-1.

imageDifferential GPS
A GPS receiver calculates its position on earth based on the information it receives from a minimum of four located satellites. This method assumes the radio signals from the satellites will make their way through the atmosphere at a consistent speed (the speed of light). However, the Earth's atmosphere slows the electromagnetic energy down, particularly as it goes through the ionosphere and troposphere. The delay varies depending on where you are on Earth, which means it is difficult to accurately factor this into the distance calculations. Problems can also occur when radio signals bounce off large objects, such as buildings, giving a receiver the impression that a satellite is farther away than it actually is. On top of all that, satellites sometimes just send out bad almanac data, misreporting their own position.
Differential GPS (DGPS) helps correct these errors. The basic idea is to gauge GPS inaccuracy at a stationary receiver station with a known location. Since the DGPS hardware at the station already knows its own position, it can easily calculate its receiver's inaccuracy.
Differential GPS is what we use when we post process static data. By using a receiver that is collecting static data at the same time as the receiver at the unknown position we can have software, such as Topcon Tools, process the data and correct the unknown position.

Occupation Procedures

The base and “rover” receivers will follow the same procedure for each occupation.
The base receiver will be the first to be setup and the last to be shutdown.
The following steps can be used for each occupation:
1. Setup a tripod and tribrach, leveled and centered, over the occupation point.
2. Place the Antenna on the tribrach adapter and lock into the tribrach.
3. Mount or hang the GRS-1 on the tripod and connect to the antenna.
4. Turn on the receiver using the power button.
5. Create a New job in TopSURV.
6. Select the proper Configuration.
7. Measure and record the Slant Height Measure Mark (SHMM).
8. Start the Occupation and name the static file if necessary.
9. After confirming that the observation is recording allow the receiver to collect a minimum of 20 minutes of data.
10. Stop Occupation
11. Turn off the receiver.
12. Repeat as necessary for each control point.