Static Survey

APS-3 The following information is for use with an Altus Positioning Systems – APS-3 GNSS receiver and OPUS online post processing.

A typical Altus Surveying kit

OVERVIEW

A Static Survey is performed to produce a high accuracy post-processed coordinate for an unknown position on the surface of the earth.

Surveys by geodetic surveyors (new titles and relocation surveys) in the 21st century will required accurate Latitude/Longitude and Grid Coordinates to define survey points. The reliance on Static Survey and resulting reference pins will become more important. Traditionally, the installation of PM’s was a responsibility of the government survey agency (NAMRIA), but government resources are  primarily invested in developing a nation wide GNSS correction network.

CORS Monument

As continuously operating positioning systems (CORS) are developed across the country, the process of installing accurate reference markers become a necessity for the professional surveyor. By developing his own network of PM’s the coordinates should be used to reference and qualify the accuracy of surveys. As a consequence, a network of accurate static survey reference points will become a valuable investment.

DEFINITION

There are two methods for Static Positioning:

Static Positioning: Static positioning typically uses a network or multiple baseline approach for positioning. It may consist of multiple receivers, multiple baselines, multiple observational redundancies and multiple sessions. This method requires occupation times of 2 to 8+ hours of data collection. A least squares adjustment of the observations is made during processing. This method provides the highest accuracy achievable and requires the longest observation times. Static positioning is primarily used for ties to the National Spatial Reference System (NSRS) when observing Cadastral Project Control. This method may also be used for the Cadastral Measurement portion of a cadastral survey.

Fast-Static Positioning: This method requires shorter occupation times (i.e. 5 to 20+ minutes) than static positioning and may use a radial baseline technique, network technique, or a combination of the two. Fast static requires a least squares adjustment or use of processing software capable of producing a weighted mean average of the observations. Fast-static positioning may also be used for observing both the Cadastral Project Control and the Cadastral Measurements of a cadastral survey.

Processing can be done using proprietary software developed by GPS equipment manufacturers or using a FREE US Online service, known as OPUS. http://www.ngs.noaa.gov/OPUS/

THE SURVEY

Selecting a good site for a static survey is as important as the survey itself; the survey should be performed on common land at a location where the surveyor has permanent access without interruption for future access to utilize the point in survey control. When evaluating a site check for obstacles including buildings or tall trees. Holding the arm extended, raise it to an angle of 10 degrees to the horizon, then the rotate the body through 360 degrees sighting along the arm checking for obstacles. If there are any, evaluate their potential effect on preventing signal to reach the antenna or the potential for multipath. The elevation mask should be set to a default minimum of 10 degrees. Data from satellites lower than this are useless for surveying; the signal is too noisy going through the atmosphere. Anything over 15 degrees may be denying the processor access to critical data. As a “norm” use a cut-off elevation of 10 or 15 degrees.

Install a Monument or Star Post having a protrusion of 25-50mm above natural ground level in the ground (install at a minimum of 400mm deep). Complete the site with a concrete plinth formed around the marker (200mm square or round) to make it easier to find, more difficult to move and easy to identify. The plinth provides the surface to inscribe a identification number for future survey control.

RTK Base

Erect a tripod and tribrach over the survey point; level the tribrach as accurately as possible. Check the bubble and measurements several times over a 15 minute period to ensure stability and accuracy. Time and precision of set-up are relative to the accuracy of the survey. If adjustable height tripods are used, the height of the antenna (slope distance) above the mark should be measured accurately (millimeter). It is recommended that this measurement be made at a minimum of three locations around the antenna in two moments in time; at the beginning of the observing session and again at the end of the observing session.

Instrument Height

The height information is recorded in a field book or on log sheets for every occupation. The log sheets should contain local information about the site, however their main purpose is to record the height information of the setup and match the location for post processing. An Altus APS-3 antenna setup requires a diagonal (slope) distance to be measured from the bottom edge of the antenna (Grey/Orange intersection) to the monument/PM/pin.

Preparing Altus APS-3 GNSS receiver is very important; the batteries should be fully charged and capable of maintaining charge for the duration of the survey (2-8 hours). To protect the UHF radio, attach the UHF antenna (1/4 wave is suggested) for the survey. Screw the Altus APS-3 receiver on to the tribrach riser and press the ON button.

When the Altus APS-3 receiver’s SATELLITE LED has located sufficient satellites and is pulsing “normally”; press the POWER button twice in succession (within a second). The STATIC SURVEY RECORDING LED (Internal Data Logging) will begin flashing once per second.

LED's

All observers should be aware of the scheduled start and stop times for each session and  allow plenty of time to find the monument (which should be well marked and flagged before the day of the observation) and allow enough time to set up the antenna accurately so as to have time to record the data for the allotted time..

For fast static surveys (sufficient for local survey) record 2 to 8 hours of data. For Fast Static Survey Control Points a minimum of 2 hours is required; the longer the survey the greater the potential of sub centimeter results.

To end of the survey, press the POWER button twice in succession. The STATIC SURVEY RECORDING LED (Internal Data Logging) ceases operation.

The survey is complete and the data is recorded on the internal 2GB memory card ready for processing. Remove the memory card for processing in the office.

POST PROCESSING

If not already installed, install the Septentrio Tools from the Altus USB stick provided with the APS-3 receiver.

Select the APS-3 receiver used for the static survey, on the rear side, open the card compartment with either a coin or thumb nail. Remove the 2GB Memory Card.

Memory Card

Place the memory card in a card reader on a PC/Laptop and search the memory card for the folder containing the files. The files are numbered 001 ……, choose the file(s) with the same date as the static survey. Copy the file(s) to the C:/Program Files (x86)/Septentrio/RXTools folder. Rename the file to the Static Survey Point (number and date).SBF

If you choose make the static logging files increment rather than appending once SurvCE has been used?
Send the increment command via the Send File utility, which can be located via SurvCE –> Equip Tab –> GPS Utilities –>Send Command.

Then enter: setFileNaming, DSK1, Incremental, ALTUS

Select the Send button.

FILE CONVERSION

Open Septentrio RX Tools, SBF Converter.

Select the file (either single or multiple pending the number of static survey files (SBF) to be processed.

To convert an SBF file to RINEX format for post processing using vendor software or online services select the options for the output required. For OPUS processing select RINEX format.

635684120569372744

When settings are complete select “Convert”.

The linear graphs indicate progress.

On completion open the folder where the SBF file was stored and the conversion output is found here also.

OPUS PROCESSING

From a browser navigate to:  http://www.ngs.noaa.gov/OPUS/

635684123361892467

Upload the RINEX file and complete the settings (the default “none” works for Altus APS-3) from the Static Survey Log sheet enter the “height of antenna”.

For external antenna installations (APS-U or APS-3x) select the “SEPPOLANT_X_MF” antenna from the list.

Add your email address for return of solution from OPUS.

Process the data.

Pending the number of files waiting in line the processing at OPUS the data can take from some minutes to an hour to be processed.

Check your email. An email providing the output from the post processing/survey is provided, check the accuracy report to ensure the static survey met accuracy standards.

A similar service is available free also from AUSPOS – Online GPS Processing Service. This service is usually easier to access from the Philippines providing an equally exacting correction of a RINEX file to a Static Position.

AUSPOS

Copy the data from the OPUS or AUSPOS processing report to the Static Survey Log sheet.

The position and coordinates are ready to utilize in future surveys.

DOCUMENTATION  

A Final GPS Static Survey project file should made from the OPUS report and the Static Survey Log sheet including  the following information:

  • Project report
  • Project sketch or map (Google map capture) showing independent baselines used to create the network
  • Station descriptions
  • Station obstruction diagrams
  • Observation logs
  • Raw GPS observation (tracking) data files
  • Baseline processing results
  • Loop closures
  • Repeat baseline analysis
  • Least squares unconstrained adjustment results
  • Least squares constrained adjustment results
  • Final coordinate list

ACCURACY

How accurate is it?

Under normal conditions, positions are resolved to between sub- centimeter and few centimeters. Estimating the accuracy for a specific solution is difficult; however, as formal error propagation is notoriously optimistic for GPS reductions. Systematic errors, such as mis-identification of antenna or height, are not detected. Local multipath or adverse atmospheric conditions may also negatively impact your solution. If the processed position is outside the requirements for control, repeat the process over a series of days until an acceptable solution is resultant.

Static: Static processing provides peak-to-peak errors for each coordinate (X, Y, Z, Φ, λ, h, and H). These describe the error range, the disagreement between the 3 baseline solutions, as mentioned above.

One advantage of peak-to-peak errors is that they include any error from the CORS reference coordinates.

 Baselines

Rapid static: Absent of any warning messages, they are best estimates of coordinate accuracy using the standard deviations reported by single baseline analysis. Our experiments indicate that the actual error is less than these estimated inaccuracies more than 95 percent of the time. Results should be sub meter or better.