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Section 8: GPS RTK Surveying

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Overview

Real-time kinematic (RTK) positioning is similar to a total station radial survey. RTK does not require post processing of the data to obtain a position solution. This allows for real-time surveying in the field and allows the surveyor to check the quality of measurements without having to process the data.

RTK positioning may be used for Level 3 and 4 surveys as mentioned in Section 5 of this chapter. Level 3 surveys require that a second base station be set up for the purpose of creating a second baseline. Trimble units (and most others) will allow the averaging or adjustment of the two or more baselines while still standing at the point. Level 4 surveys will accept the single radial baseline solution. The surveyor must also follow the manufacturers prescribed methods.

Real-time surveying technology may utilize single or dual-frequency (L1/L2) techniques for initialization, but the subsequent RTK survey is accomplished using only the L1 carrier phase frequency. Therefore, all RTK surveys are currently subject to the limitations of the L1 frequency which is 10 kilometers from the base station. In order to maintain a 2 cm level of accuracy, distances are usually considerably less than this but there may be circumstances where this maximum range may be extended.

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Planning

As with static GPS surveying, mission planning is an important step in performing a RTK survey. There are times of the day when the numbers of satellites available will vary. The positions of the satellites at various times of the day are also a factor. Planning your work around these times greatly increases productivity and the quality of your results. Most, if not all, software packages include a utility allowing suers to predict satellite coverage.

The Trimble planning utility supported by Technology Services Division (TSD) is simply called “Planning” and offers charts, graphs, and sky plots to aid in determining the best times for GPS reception and data quality. Number of satellites and PDOP are the most important indicators. A recent almanac (approximately 12 minutes of broadcast ephemeris data collected within the last couple of days) is needed. The surveyor can collect this or download it from several sites on the internet. The Trimble site, www.trimble.com, uses a file extension of .ssf on their daily almanac files.

The selection of the base station sites will also affect the success of the RTK observations. Users who select a poor base stations site will likely have problems throughout the entire survey. Select a site with good sky visibility down to 10 or 15 degrees from the horizon. Be aware of high power transmitters such as microwave, TV stations, military installations, high voltage transmission power lines, etc.

Multi-path may be caused by radio wave reflective objects such as trees, buildings, large signboards, chain link fences, etc. Because of the orbits of the satellites, obstacles to the north of the antenna setup are not as detrimental to reception.

It is worth the effort to get the base stations in optimum locations. A problem at the base is a problem at all rovers. A problem at one rover is only a problem at that one rover.

If possible, users should take part in the selection of any project control points in the beginning stages of a project. This is to insure that the points can be surveyed with GPS and well spaced for project coverage of real-time kinematic (RTK), if GPS is likely to be used. Of course, the primary project control points selected should always be GPS friendly.

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Preparing the Data Collector

It is important to have the Feature Code Library or a feature table loaded into the data collector before going to the field. A list of available existing control points that may be available should also be included to prevent having to type in long coordinate values. On the Trimble TSC and ACU data collectors the control file can simply be an ASCII file with the format: point name, northing, easting, elevation, feature and feature code. The extension can be .txt or it can be a .csv file.

The “job” can be created in the office and exported to the data collector or the user can enter the job parameters in the field. Some of the settings involved are datum, projection, scale, and scale factor and measurement units. The Trimble data collection software is called, “Survey Controller.”

A survey “style” is used as a template to repeat settings for a particular type of real-time kinetic (RTK) survey. The style contains dozens of settings to include all the base and rover radio settings, receiver settings, data and accuracy parameters, etc. The software comes with several styles, but the user will eventually want to create their own to accommodate their particular brand of radios, or cell phones. The user’s styles can be created in the “configuration” icon and will be saved under the Trimble Data folder on the hard drive of the data collector.

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Setting Up the Base Station

Set the base station at one second collection rate and at 13 degrees elevation mask. It is advisable to use a fixed height tripod (usually 2-meters). It is possible to do the entire survey without ever having to make an H.I. measurement – a considerable advantage over the conventional survey.

Obviously, it will be of little value to have data based on a coordinate system or datum that is unusable. However, control local in NAD 83 datum may sometimes be unavailable within range of the RTK system. Perhaps control points have been destroyed or maybe no control was extended beyond the several project control points on a large project. In these instances, a proper GPS methods can be used to bring control points to areas suitable for RTK base stations.

There is also the option of starting an RTK survey with an autonomous position for the base. While collecting the RTK data at the rover, have the base station log raw observables for post-processed GPS. After post-processing the data to establish correct coordinates, users can apply the corrected coordinates to edit the base station positions and shift the collected RTK data to its proper positions. Trimble data collectors have a selection for this purpose, which is called a “here” option.

A similar option is to start your base station with an autonomous (here) position but then observe control points and calibrate or localize (manufactures use different terms) to shift the data to the existing control values.

There are drawbacks to using these two approaches. The problems arise from the inaccuracy of the position of the base stations. For each ten meters of error in the base position, introduce an additional 1ppm (1mm per kilometer) error in our baselines. (This rule holds true with all GPS surveying techniques users might choose.)

Even after the base station data is post-processed and the coordinates are determined and shifted, error will remain in the baselines if the 10 meter fault is present. The finding of an autonomous position of more than 10 meters error, especially if baselines of more than a few kilometers were used, may mean a redo of the survey. For this reason, it is strongly recommended that base stations be known control points.

If it is not possible to use a control point, there are different approaches that can be applied to assure that inaccurate base station coordinates are held to a minimum.

  1. Use the Wide Area Augmentation System (WAAS) corrected signal (still only accurate to about 7 meters).
  2. Use equipment that allows for averaging of code derived autonomous positions.
  3. Predetermine position with code equipment using a correction service.

Using an unknown position for the base station in the methods described above is a poor substitute for the practice of occupying a known position.

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The Rover

Set the rover receiver at one (1) second collection rate and 15 degrees elevation mask. The rover rod should always be a fixed height. Unlike conventional surveying from a total station, line of sight is not needed – there is no need to raise or lower the rod height. Usually a 2-meter rod is used. Not having to make this measurement eliminates one more chance of error.

Before getting too far away from the base station, check the radio (or cell phone) link to the rover.

The first thing that must be done upon “starting the survey” on the data collector is to initialize the system (resolve the integer ambiguity). Several methods that are acceptable when preformed properly.

  • Known point initialization – this is the fastest and safest way to initialize. Where possible and practical this should be the method of choice. The firmware uses the three-dimensional deltas of the relative WGS84 positions of the base stations and the known point occupied by the rover as an aid in solving the integer ambiguities.
  • New point initialization – this is a technique that is usually used on equipment that does not have the ability to solve the integer ambiguity on the fly (OTF).
  • On-The-Fly (OTF) initialization – this is the most common technique used by most equipment today. There must be care taken when using this method. The possibility of an incorrect initialization may be remote but remains a possibility.

To avoid the possibility of an undetected incorrect initialization use one of the following methods to check the system.

After the OTF initialization, observe a point, this can be a temporary mark or a point in the survey. Discard the first OTF initialization and OTF re-initialize by moving more than forty (40) feet away from the point to be used as check. After the new OTF initialization has been accomplished return to the point being used as a check and re-shoot. Compare the first and second shots to within an acceptable tolerance. If the points check, proceed with data collection with the confidence of surveying with a correct initialization.

If the error between the two points is beyond the expected error one or both of the OTF initializations used for a check are incorrect. The user must change the location by a difference of more than two (2) feet of H.I. or more likely move more than forty (40) feet away in a different direction. This will usually provide enough information to identify the OTF initialization that is incorrect. Once the problem is solved, users can begin the survey. This procedure must be repeated with any loss of initialization.

As with any surveying techniques the user would want to check a known point in the survey before beginning work, apply the same logic to your RTK survey.

The user is now ready to observe points. The amount of time of occupation will vary depending on conditions such as obstruction, multi-path, noise, etc. The user may have to resort to:

  • increasing occupation time to a couple of minutes at one second epochs
  • a more stable setup (use of a tripod or bipod)
  • use of a ground plane when in a multi-path environment

The use of the most recent list of TxDOT feature codes is mandatory. As of November 2003, the “txdot2k” is the most recent. This TxDOT list is available in Trimble format as “txdot2k.fcl” and in CAiCE ™ format as “txdot2k.ftb.” Topographic data should be collected in a manner similar to a conventional topographic survey in that the rover operator(s) must be aware of the fact that they are collecting chains of connected points to create break lines that will not be crossed in the creation of the TIN file.

The selection of feature codes for “as-builts” and various features will also determine what points will or will not be included in a DTM. TxDOT’s Technology Services Division (TSD) provides a class on Survey Data Management System® (SDMS) for data collection that would be helpful in understanding the procedures for collecting data with RTK for topo work.

There are two screens associated with each measurement on the Trimble data collector: the “measure” screen and the “attribute” screen. The initial (default) screen is the “measure” screen, which will allow the user to key in the feature code. Before pressing “measure” however, open the “attribute” to answer the prompts such as FG: or GM: or whatever else appears, then make the measurement.

If the user has not checked the “prompt for attributes” box this will not appear. Users can continue shooting points without going back to the attributes screen, until the situation changes when a new figure number, geometry setting, or whatever else the user might want to change is needed.

A good rule of thumb is to reoccupy about 25% of all points requiring the accuracy of a Level 3 survey after a new initialization or about 10% of the points in a topo survey.

Upon successful completion of the user’s observations, the user will now have a radial survey. Users must move the base station to a second control point and repeat the process for surveys that will not allow single baseline solutions (Level 3).

If radial lines are permitted, such as on a topographical survey or wing panel locations, it is still a good idea to occupy a second base station if another control point is nearby to randomly check a few of the points already established.

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Post Processing

An alternative method of performing a kinematic survey is to collect the data and process it at a later time. This does not require the use of a communications link (i.e. radio or cell phone) and can be combined with RTK to perform infill when the link is temporarily down. Post processed kinematic survey methods provide the surveyor with a technique for high production measurements and can be used in areas with minimal obstructions of the satellites.

PPK uses significantly reduced observation times (i.e. 0.5 to 3 minutes, usually 10-30 seconds per point) compared to static or fast-static/rapid-static observations. This method requires a least squares adjustment or other multiple baseline statistical analysis capable of producing a weighted mean average of the observations. Post processing will allow kinematic surveying to be used for some Level 3 surveys.

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Integrating Conventional Measurements

To be added in a later edition.

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Using Networked RTK (VRS)

Networked RTK is a new variation of RTK data collection. Rather than setting up a base station on the project, a number of permanent and continuously operating base stations are set up at about a 30 - 40 mile spacing, providing that augmentation to the basic position is determined at the rover.

These stations send GPS data into a central computer that streams the correction data to the Internet. The data can then be accessed by way of a cell phone/cell modem at the rover receiver. The data collector then uses this information to provide real time solutions with the same speed and accuracy as Base Station RTK but without the complication of setting up a receiver and radio on a local control point.

This system yields the same accuracies as the normally accepted three (3) miles of a standard radio-linked base station and rover. TxDOT has installed a number of these RTK networks. Coverage is growing across the state but presently covers the major metropolitan areas and some of the rural districts. Refer to Figure 3-5 in this chapter.

The name virtual reference station (VRS) was coined for this method because a “virtual” base station point is determined by the computer from the network of base stations. The virtual base station is never more than 3 miles from the rover and is automatically redefined when the rover goes beyond that preset distance.

The TxDOT RTK Network uses the VRS technology. A virtual base point near the project is computed by the central computer. The user operating a rover unit dials in to the TxDOT IP address for connection to the system. The data-ready cell phone/modem must then be physically carried by the user to maintain constant communication between the rover and the Internet. For information about this and other features that vary from place to place and time to time, contact your district survey coordinator, who in turn, may contact the administrators of the system at TSD.

Specific cell phone services and connection information should be obtained from the local cell phone service provider.The same procedures and precautions as outlined for Base Station RTK should be followed using the TxDOT RTK networks. The difference is simply that users are not working from a base station set up by the user for a particular project. Users will not need to occupy the known station with a GPS receiver transmitting correction data to the rover(s). The work will be accomplished from a network of GPS base receivers.In the case of a Level 3 point, where users would normally occupy a point more than once and from two or more base stations; three to six RRP’s are already being used in the coordinate calculation using the networks. The point should still be occupied twice at different times of the day.

There is an option in Survey Controller to do an “Observed Point,” which will automatically collect for a specified amount of time; usually three minutes. This gives the mark a special status in the priority of stations in the TGO program.

The TxDOT RTK Network is based on the National Spatial Reference System, which means that all coordinates are in the NAD83 datum and accuracy and compatibility should not be a problem. This however, can work against users when all previous work was done on local coordinates or the area of previous control may carry local biases.

To overcome the clash of coordinate values, the process of “calibrating” to the existing control is used. This was not used as extensively via the base station method where the control point coordinates were the start of subsequent GPS work.

Most RTK network surveys should be done after a calibration to existing control. Even if the horizontal component doesn’t require a calibration, consider performing the vertical calibration. GPS solutions require the aid of a geoid model for elevations. In several areas around the state, the geoid model has a difference of more than a tenth of a foot from known elevations. If any known bench marks exist in the area; calibrate to them.

In TxDOT districts with RTK networks, users should apply for a password through their district survey coordinator. This password will allow users to access the system through the Internet.

Consultants with active contracts are allowed to apply for a password through the district survey coordinator but are limited to just TxDOT work with the TxDOT system. The use of a private real time network for TxDOT work by consultants is up to the discretion of the district survey coordinator.

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The End Product

When topo data is collected with real-time kinematic (RTK), the output format most often will need to be in Survey Data Management System® (SDMS) format. The TxDOT supported TGO software exports directly to the SDMS® format. There must be point connectivity (break lines) and the standard TxDOT feature codes will insure this. The file is available at district offices in Trimble format for Trimble TSC-1, TSC-e and ACU data collectors. As mentioned above, the file name is “txdot2k.fcl”. Conventional data collected on the Trimble data collector using Survey Controller can be included in the same .dc (job file).

The .dc (job file) can be downloaded to the PC with the Trimble Data Transfer program or using Microsoft ActiveSync with a USB connection. By importing into Trimble TGO, the user can see the work graphically and do some editing if necessary before exporting the final product as an ASCII or SDMS.cal file.

The use of LandXML format is being investigated as an alternative standard of transfer.

If an ASCII file of final coordinates is needed, the most often requested format is: name, northing, easting, elevation, feature code.

NOTE: that all data passing hands should include notes on datum, projection, geoid model, and date of survey. Coordinates should be designated as state plane coordinates or surface adjusted coordinates with an accompanying CAF or SAF factor.

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