Permanent object monitoring is the solution to the problem of monitoring the geometry of an object continuously and largely automatically. The aim is to detect the occurrence of critical changes through their relationship with the continuously recorded measurements, and thus to set off an appropriate alarm as promptly as possible. Depending on the dynamic behaviour of the object, the monitoring interval may vary from once per hour to ten or more times per second.
A GNSS such as GPS or GLONASS provides the basis for this solution, if the usual requirements such as GPS visibility are met.
For a statistically valid deformation analysis, reliable coordinates supported by full covariance matrix data are essential. These can only be provided by means of a rigorous multi-station solution. In order to guarantee a rapid reaction time, real time processing of the GPS data is necessary.
To meet the highest accuracy requirements (better than 1mm over short distances), residual systematic errors must be dealt with, in particular including antenna phase centre variations and multi-path effects.
The practical solution
With GNPOM, Geo++® provides a complete system of permanent object monitoring to meet all such needs. Being based on the Geo++® GNRT and GNNET-RTK software packages, all the performance features of these systems are also available within GNPOM.
GNPOM can process observations from single- or dual- frequency GPS and/or GLONASS receivers. Over very small networks, dual-frequency receivers do not necessarily yield great improvements in accuracy, so significantly less expensive single-frequency receivers can also achieve high quality results.
The transmission of the observations from the GNSS receivers to a central computer can be achieved flexibly over a variety of communications links. In the simplest case each receiver is connected over a serial line (RS232 interface). Over greater distances an alternative protocol (RS422) may be used. If a cable connection cannot be installed, radio connection is possible; the addition of the GNMUX module allows data transmission over a single frequency. If the necessary infrastructure is available, modem connections using fixed or mobile (GSM) networks, or computer networks (LAN, TCP/IP, NETBIOS), may also be used for data transmission.
GNNET multi-station processing
GNPOM works with the rigorous GNNET multi-station GPS processing software, so that several object points to be monitored, several base points (assumed to be stable), and several reference stations can all be processed simultaneously. If necessary, track errors and ionospheric and tropospheric effects can be included and modelled through multi-station observations using an appropriate distribution of reference stations. In addition to the three-dimensional coordinate vectors, the results may include a full variance-covariance matrix, an important requirement for any subsequent rigorous deformation analysis.
Processing of results
The values of the parameters determined (e.g. coordinates) can be further processed using a variety of simultaneously-operating dynamic filters in order to suppress the remaining residual errors.
For monitoring purposes, either the coordinates themselves or derived quantities such as orientation angles or distances may be used.
The visualization of coordinate differences and positions is provided via html service.
By means of the Geo++® GNALARM alarm system, alarm plans can be set up and managed, so that the occurrence of specified events will set off pre-determined actions, including for example screen messages, printouts of reports, notifications by fax, or the start-up of processes, even on remote computers.
The important error sources in close range applications of GPS (up to 1km) are variations in antenna phase centres and multi-path effects in the neighbourhood of the antenna. To meet the highest accuracy demands, these errors can be largely eliminated by a special measurement arrangement. The elimination of antenna phase centre variations (PCV) is effected by means of elevation- and azimuth-dependent antenna calibrations. The successful elimination of multi-path effects is due to the analysis of sidereal day differences with GNDIF, which
exploits the fact that the constellation of satellites, and hence the indirect path geometry of the satellite signals, is repeated after almost exactly one sidereal day. When using this procedure, movements of object points must not exceed a few centimetres per day.
Relative differential GPS (RDGPS)
GNPOM also allows a highly accurate carrier-phase solution to be computed between several moving GNSS antennas, without the need for a fixed reference station. This is known as relative differential GPS (RDGPS). With this process, GNPOM also lends itself to the monitoring of moving platforms, where it is not so much the absolute position of an object but rather the monitoring of the internal geometry of the object with regard to dynamic distortions which is required.
The GNPOM system may be applied to many fields in which the position of objects must be determined continuously and with high accuracy. These may include:
- Monitoring of civil engineering works (dam walls, bridges, towers, buildings) with an accuracy of 1mm
- Monitoring of geomorphological change (e.g. landslides), accuracy 1cm
- Monitoring of vessel geometry (e.g. container or other ships or any other floating installations), accuracy 1cm
|GNSS reciever:||GPS and/or GLONASS
Single- or dual-frequency
Geo++® GNSAT integrated GPS sensor with antenna,
receiver and radio
|Sampling rates:||Depending on the hardware, up to 10 Hz or more|
|Accuracies:||Daily changes for object points :|