Trimble CenterPoint RTX Post-Processing Services FAQs

Transcription

1 Trimble CenterPoint RTX Post-Processing Services FAQs What is Trimble RTX technology? 30 September 2013 Trimble RTX TM (Real Time extended) is a high-accuracy, global GNSS correction technology that combines real-time data with innovative positioning and compression algorithms to provide centimeter level positioning accuracy How does Trimble RTX technology work? Trimble RTX technology utilizes real-time data from a global reference station infrastructure to compute and relay satellite orbit, satellite clock and other system adjustments to the receiver, resulting in centimeter level positions that deliver repeatable high accuracy positions worldwide. These adjustments are transmitted to the receiver via satellite, Internet Protocol (IP or Cellular), and through post-processing delivery. What solutions are enabled by Trimble RTX Technology? Trimble RTX Technology now enables several different solutions: Trimble CenterPoint TM RTX TM post-processing service Trimble CenterPoint TM RTX TM correction services Trimble RangePoint RTX correction service Trimble xfill TM technology Trimble Pivot RTX App Trimble Pivot RTX-PP App What is the Trimble CenterPoint RTX post-processing service? CenterPoint RTX post-processing uses the Trimble RTX technology with the GPS, GLONASS and QZSS satellite systems to post-process user submitted observation files. Dual frequency data files can be submitted through the web service, at When processing is complete, a report will be sent via . The CenterPoint RTX post-processing PREVIEW service uses the Trimble RTX technology with the GPS, GLONASS, QZSS AND Galileo and BeiDou satellite systems to post-process user submitted observation files. Because the Galileo and BeiDou satellite systems have not been certified for commercial use, the PREVIEW service is for testing purposes only.

2 Where are the CenterPoint RTX post-processing services avaliable? The CenterPoint RTX post-processing services are accessible at Click on the Now Supporting Galileo and BeiDou image to access the PREVIEW service. Who can use the CenterPoint RTX post-processing services? The CenterPoint RTX post-processing service and the PREVIEW service are provided as free services to all users of supported GNSS hardware. For more information, or to check if your hardware is supported, visit What are the requirements of my observation file? Observation files must meet the following requirements: Data formats accepted include RINEX 2 and RINEX 3 data format and for Trimble proprietary data formats (e.g. DAT, T01, T02 files) Data must be collected for a minimum of 1 hour and cannot exceed 24 hours in length Data files must be static only Data files must contain dual frequency observations (L1/L2) Receivers must be on the supported receiver list at The service does not work with data files from unsupported receivers. Data must have been collected after 14 May 2011 The CenterPoint RTX post-processing PREVIEW service has similar requirements as noted above, however: 0nly Trimble receivers are supported Data must have been collected after 12 September 2013 What receivers can I use with the CenterPoint RTX postprocessing services? Receivers must be on the supported receiver list, which can be found at The CenterPoint RTX post-processing PREVIEW service only supports Trimble hardware at this time.

3 What is the recommended length of a data session? The achievable accuracy is very closely correlated to the length of the observation file. It is recommended to use data sets that are a minimum one hour in length, and this will deliver 2cm horizontal accuracy. Longer data files will yield even greater accuracy and can approach 1cm. The post-processing services do not accept observation files that are longer than twenty-four hours in length. What is the recommended maximum observation update rate? The maximum observation update rate is one second. How does the obervation rate affect the accuracy? The observation update rate should not impact accuracy but higher update rates allow for easier detection and fixing of carrier phase cycle slips. The minimum recommended observation update rate is therefore ten seconds. What coordinate systems are supported? CenterPoint RTX corrected positions are originally derived in ITRF2008 (current epoch), however it has been identified that users often need their data in a different reference frame. The CenterPoint RTX post-processing services therefore offer many different reference frames by which data can be transformed. The reference frames currently supported include: ITRF 1988 to ITRF 2008 NAD83, NAD83-CSRS, NAD83-CORS96, NAD , NAD83-MA11, NAD83-PA11 ETRS89, ETRF2000-R05 GDA94 SIRGAS2000, SIRGAS95, SIRGAS-CON What accuracy level can I achieve using the CenterPoint RTX postprocessing services? The achievable accuracy level of the CenterPoint RTX post-processing services is two centimeters or better horizontal accuracy and approximately 4 centimeters in vertical. This is based on a minimum one hour data session. Accuracy can approach one centimeter horizontal accuracy and approximately two centimeters in vertical, as the data session approaches but does not exceed 24 hours in length.

4 When should I use the tectonic plates option? If individual station velocities are unknown and the user desires to determine a position on a specific tectonic plate the post-processing service allows transforming the ITRF2008 position to another frame with a different reference epoch and on a selected tectonic plate. The service uses plate rotations provided by Altamini (2007) and Bird (2003). The following tectonic plates are supported: Africa, Amurian, Antarctica, Arabia, Australia, Caribbean, Cocos, Eurasia, India, Juan de Fuca, Nazca, North America, Nubia, Okhotsk, Pacific, Philippine, Rivera, Scotia, South America, South Bismarck, Somalia, Yangtze. Is the accuracy achieved in any coordinate system or only in ITRF2008? CenterPoint RTX post-processing is done in the IRTF2008 reference frame at the observation epoch. Transforming the positions computed at the observation epoch to a fixed epoch, requires knowledge of the station velocity due to the tectonic plate motion. If the velocity is not available, it must be determined from a tectonic plate model such as NNR-MORVEL56 or NNR-NUVEL-1A. The accuracy of the velocity estimate will degrade the accuracy of the position computed for a fixed epoch in ITRF2008. If transforming to another reference frame such as NAD83, the accuracy of the transformation parameters will further degrade the accuracy of the computed position. What is ITRF and ITRF 2008? The Earth is constantly changing shape. To be understood in context, when the motion of the Earth's crust is observed, it must be referenced. A Terrestrial Reference frame provides a set of coordinates of some points located on the Earth's surface. It can be used to measure plate tectonics, regional subsidence or loading [1] and/or used to represent the Earth when measuring its rotation in space. This rotation is measured with respect to a frame tied to stellar objects, called a celestial reference frame. The International Earth Rotation and Reference Systems Service (IERS) was created in 1988 to establish and maintain a Celestial Reference Frame, the ICRF, a Terrestrial Reference Frame, the ITRF. The Earth Orientation Parameters (EOPs) connect these two frames together. These frames provide a common reference to compare observations and results from different locations [1]. Nowadays, four main geodetic techniques are used to compute accurate coordinates: the GPS, VLBI, SLR, and DORIS. Since the tracking network equipped with the instruments of those techniques is evolving and the period of data available increases with time, the ITRF is constantly being updated. 11 realizations of the ITRS were set up from The latest is the ITRF2005. All these realizations include station positions and velocities. They model secular Earth s crust

5 changes that s why they can be used to compare observations from different epochs. All the higher frequencies of the station displacements can be accessed with the IERS conventions, chapter 7 [2]. Continuity between the realizations has been ensured as much as possible when adopting conventions for ITRF definitions. The relationship linking all these solutions is of utmost importance. They are supplied here by the transformation parameters. The International Terrestrial Reference System (ITRS) is a world spatial reference system corotating with the Earth in its diurnal motion in space. The IERS, in charge of providing global references to the astronomical, geodetic and geophysical communities, supervises the realization of the ITRS. Realizations of the ITRS are produced by the IERS ITRS Product Center (ITRS-PC) under the name International Terrestrial Reference Frames (ITRF). ITRF coordinates were obtained by combination of individual TRF solutions computed by IERS analysis centers using the observations of Space Geodesy techniques : GPS, VLBI, SLR, LLR and DORIS. They all use networks of stations located on sites covering the whole Earth. (Source: International Terrestrial Reference Frame, What is the accuracy of the coordinate transformation? This will depend on the process used to compute the transformation parameters and will likely be different for each reference frame. The accuracy will also be time dependent. What is the difference between ITRF and WGS84? ITRF is a global datum used primarily by the scientific community and is realised by a large network of fiducial (i.e. fundamental trust) sites around the globe. ITRF sites are typically continuously operating GPS stations (including the Australian Regional Geodetic Network (ARGN) managed by Geoscience Australia), Very Long Baseline Interferometry (VLBI) and Satellite Laser ranging (SLR) stations. The ITRF is defined by the coordinates and velocities of the stations at a specified reference epoch. ITRF sites are located on different tectonic plates which move at up 10 cm per year with respect to each other (Figure 2). As a consequence, the velocity for each ITRF site with respect to a stable earth enables ITRF coordinates to be computed for any specified epoch. Because ITRF coordinates are constantly changing, ITRF is referred to as a dynamic datum. The latest realisation of ITRF is ITRF2005. (Source: International Terrestrial Reference Frame, WGS84 is a global datum used by the United States Global Positioning System. The datum is currently defined by the coordinates and velocities of 18 GPS tracking stations maintained by the US Air Force (USAF) and US National Geospatial Intelligence Agency (NGA). The latest realisation of WGS84 is WGS84(G1150) where 1150 refers to the GPS week of realisation. WGS84 is now coincident with the latest realisation of ITRF at the 10 cm level (NGA, 2003). WGS84 is kept in alignment with ITRF to ensure that the GPS broadcast ephemeris is not degraded by holding coordinates of the GPS tracking stations fixed when they are subject to relative deformation of up to 10 cm a year. (Source: Stanaway, Richard, Quickclose, Can I use any GPS antenna type in my observation? No, antennas need to be listed in the supported antenna list found at

6 What happens if my antenna type is not supported or unknown? The CenterPoint RTX post-processing services will not process unsupported antenna types. Does the CenterPoint RTX post-processing service use the geoid model or the ellipsoid model to establish elevation/height? The CenterPoint RTX post-processing service uses the ellipsoid model. How does the ionosphere affect CenterPoint RTX positioning? GNSS signals travel through a part of the earth s atmosphere called the ionosphere. When the signal is travelling through the ionosphere, refraction, or bending of the wave, occurs. The level of ionospheric activity is dependent on: Solar activity; it is highest at solar maxima during an eleven year solar cycle; the next solar maximum is expected to be in2013 Time of day (highest at local noon) Season (highest at equinoxes March/September) The ionospheric effect is frequency dependent, i.e. under normal conditions, dual-frequency (L1 and L2) code/carrier phase observations can be used to essentially remove ionospheric errors. This is what is done in RTX positioning, i.e. the position estimate is independent of the level of ionospheric effects as long as no ionospheric scintillation is occurring. Under extreme conditions, the ionosphere can become highly stratified (irregular distribution of charged particles) leading to GNSS signal scintillation. Ionospheric scintillation involves fluctuation in the phase and amplitude of GNSS signals. In extreme cases, scintillation can cause loss of signal tracking (i.e. cycle slips). It is important to note that the effects of scintillation are not removed by dual-frequency observations. Trimble has setup a global ionospheric scintillation sounding network, which detects scintillation effects and is able to give up-to-date warning information on scintillation effects in different parts of the world. Typically scintillation occurs in equatorial regions after sunset for several hours. In polar regions, scintillation can occur at any time. Mid-latitude regions are sometimes affected by Travelling Ionospheric Disturbances (TIDs).