Fixing

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Rovers using high precision services provided by AXIO-NET utilise the so-called RTK method (Real Time Kinematic) in order to determine precise coordinates in the centimetre range. In this process the GNSS Rover uses the GNSS satellite signals (e.g. GPS and GLONASS), which it received itself, together with the relevant GNSS correction data supplied by AXIO-NET. The correction data are transferred in the standardised RTCM format. The measurement method is based on carrier phase measurements and supposes a “fixation” of phase ambiguities which are still unknown at the start of measurements. After solving the ambiguities, what is also called “fixing”, positions with centimetre accuracies in real-time are available.

 

The pace and reliability of setting ambiguities depend on deviations that have an impact on the rover. In dependence on local measurement environments these deviations can vary according to the site of operation. Due to its observations and correction data the rover calculates ambiguities and the position. These are constantly verified of plausibility with appropriate statistical methods in the rover and are released after a successful test (fixed solution). The higher the amount of all residual deviations is, the longer it will take to fix ambiguities.

 

Owing to the regular contact to and communication with our customers as well as to test results established on our own, 80 percent of all longer fixing times can be ascribed to four main influential factors:

  • Multipath and shadowing effects
  • Interruption of the flow of correction data (GPRS or GSM disconnections)
  • Erroneous adjustment and/or interpretation of the GLONASS level
  • Ionospheric residual errors

Multipath and Shadowing Effects

If satellite signals are not received in situ by the rover due to objects (e.g. buildings), this is called shadowing effects. Due to a small number of received satellites, the so-called satellite geometry worsens so that the search for ambiguities can be made more difficult. Five GPS satellites are at least necessary. If satellite signals are reflected by objects and the rover has a multiple reception of signals, Multipath Effects occur. The rover has to distinguish between several possible signal pathways. The stronger the surface reflects (e.g. metal surfaces or glazed office buildings), the more critical is this effect. Similarly, measurements carried out at or under trees have to be considered as critical. Therefore, it is always advisable to accomplish control measurements in such environments.

 

Interruption of the Flow of Correction Data

In the ideal case, the rover receives all available correction data within seconds. Depending on the brand and the rover adjustment, interruptions of the flow of correction data can be bridged for up to several seconds without interfering the fixing performance. A bad mobile phone connection, however, can cause an incomplete transfer of correction data. This, in turn, leads to the problem that not all received satellite signals can be corrected. Please pay attention during your measurements to the quality of the data connection by verifying the specific signals of your rover.

 

 

GLONASS Level

During GPS measurements the satellite signal is tracked via the CDMA method (Code Division Multiple Access). Using this method the measured values provided by the various brands can be regarded as almost identical. This means that the rover of one brand can process carrier phase corrections of each other brand without any additional information.

 

GLONASS is currently based on the FDMA method (Frequency Division Multiple Access). The conversion of GLONASS signals into observation data takes place dependently on the hardware used. The code observations and carrier phase observations vary slightly according to the brands. This difference is referred to as “Bias”. In order to process the GLONASS information fully, the rover has to take this bias into consideration accordingly during the RTK process. Erroneous adjustments often lead to increased deviations with longer fixing periods.

 

Using AXIO-NET services, all GLONASS observations are calibrated in the networking on a so-called standardized level. For historical reasons this level corresponds to the level of Topcon GNSS receivers, in simple terms: Topcon level. Using AXIO-NET services, you are able to dial in into our service via two different communication media: via GSM and, Internet based, via the NTRIP protocol (GPRS).  

 

When dialling in per GSM the GLONASS corrections are issued on Topcon level (Legacy-E-Class).

 

When dialling in via NTRIP the receiver type is automatically identified and the GLONASS level is adapted to the brand type/receiver type.

 

Furthermore, the information of the used GLONASS level is transmitted in the RTCM 3 Message via Type 1033.

 

If the rover cannot process or interpret the record of Message 1033, we recommend that you define manually the GLONASS level by choosing the reference receiver.

 

The currently 22 active GLONASS satellites complement perfectly the GPS constellation and can, when used properly, enhance and stabilize the RTK performance.

 

Please note: It is highly important that your rover is adjusted correctly.

 

Ionospheric Residual Errors

When using an own base the classical RTK procedure is limited because of distance-related residual errors in the field of application. In particular, the influence of the ionosphere can limit the possible employable radius to a few kilometres around the base.

 

Given a mutual adjustment of a number of reference stations, one of the major tasks of the reference-based networking principle is to model the distance-related errors.

 

Estimating the influence of the ionosphere, residual errors occur according to the intensity of ionospheric activities. These errors have influence on RTK positioning and correlate with three cycles of the ionospheric activities: the 11-year-cycle of the sun, the maximum being expected in 2013, a seasonal cycle with a maximum in winter months and a day cycle with a maximum around noon.

 

In reference networks these residual errors are included in the estimation for each satellite and are transferred to the rover in form of the RTCM 3 message types 1030 and 1031. An automatic weighting of the respective satellites at the rover can lead to an improvement in performances. The prerequisite for this is that the rover can process and interpret these messages.

 

Summary

Longer fixing times, when using services of AXIO-NET, are mainly the result of the influences as mentioned above. The repercussions on the rover, however, have different effects depending on local conditions and the equipment used.

 

In order to use our service to optimum effect and reliability, we kindly ask you to verify and, if necessary, to change the corresponding adjustments at your rover (as for GSM = Topcon, as for NTRIP = your type of receiver). Please do not hesitate to contact us, if you have questions or if you need our support. We are always at your disposal on +49 (0) 511 123718-87.

Please keep in mind independent measurement controls in difficult measurement surroundings (Multipath, power lines etc.).

 

 

 

 

 

 

 

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