FieldGenius Technical Notes GPS Differential Corrections

Transcription

1 FieldGenius Technical tes GPS Differential Corrections Introduction The accuracy requirement of survey grade or mapping grade GPS applications for real time positioning requires the use of differential corrections. The corrections can be supplied as code differential messages (DGPS) for meter level positioning accuracy or carrier phase differential messages (RTK) for centimeter level positioning accuracy. Correction messages are produced by a GPS receiver operating as a reference station supplied by the user, a public service or commercial service. The roving GPS receiver must be able to receive the generated messages from the reference station and apply the necessary corrections. The medium for communication between the reference station and rover station can be a direct cable connection, a radio modem or a GSM device. Radio modems provide the greatest flexibility and allows for one reference station to service many rover stations. This document will focus on the aspects of using radio modems for transferring correction messages. Correction Messages Several protocols have been developed for transferring differential corrections and each has its advantages and disadvantages. Each message protocol will consist of multiple records to include reference station coordinates, reference station information, pseudorange observations and carrier phase observations. Table 1 lists many of the available message protocols and their intended observation type. Protocol Description Observation RTCM 1 Code differential corrections DGPS RTCM 9 Partial code differential corrections DGPS RTCM 18&19 Uncorrected carrier phases and pseudoranges RTK RTCM 20&21 Carrier phase and pseudorange corrections RTK CMR Trimble s compact measurement record RTK CMR+ Trimble s compact measurement record plus RTK Leica Leica proprietary RTK Javad Javad/Topcon proprietary RTK Ashtech Ashtech proprietary (DBEN) RTK Table 1. Correction Protocol Types. RTCM (Radio Technical Commission for Maritime Services) is an industry standard to allow different manufacturer s GPS receivers to be used as reference station and rover station pairs. The list also has several protocols which are proprietary to manufacturer and these proprietary protocols are usually more efficient than RTCM for message transmission. The reference station GPS receiver and rover station GPS receiver

2 must each be configured with identical protocols to ensure successful reception of the correction messages. Data Connections The diagram shown in Figure 1 illustrates the connectivity of devices in order to establish real time differentially corrected positions. Within the data stream there are important communication parameters which must be matched between devices and are discussed later in this document. REFERENCE STATION ROVER STATION DATA COLLECTOR GPS RECEIVER RADIO MODEM RADIO MODEM GPS RECEIVER DATA COLLECTOR RADIO LINK Figure 1. Real Time Positioning Equipment Connectivity. Each connection between devices (Example: data collector - GPS receiver or GPS receiver - radio modem) must have the communication parameters (baud rate, parity, data bits, stop bits) matched in order for the two devices to communicate. But the communication parameters do not need to be match between different pairs of devices. Radio link settings have to be identical for both the reference station s radio modem and the rover station s radio modem and can be modified by using a utility supplied by the radio manufacturer. These settings can include data rate, modulation type, scrambling and frequency/channel. Some key notes to remember if not already mentioned by the radio modem manufacturer to ensure maximum productivity from the equipment: 1. Transmitting data on a reference station radio without being properly terminated by an antenna may cause internal damage to the radio. Therefore always connect the antenna first to the radio in the course of assembling and remove the antenna last from the radio during disassembly. 2. Keep all antenna and cable connectors free from dirt and moisture. Failure to maintain clean conductors will cause a gradual decrease in performance. Unless using a broadband antenna, ensure that your antenna is tuned for the designated frequency to ensure maximum power output. 3. Keep spare cables readily available. 4. Mount the antennas as high as possible to achieve the best coverage range. For omni-directional antennas, keep the antenna vertically leveled to maintain an even gain pattern in all directions. 5. Be aware of overhead power lines, electrical storms and high winds for antenna mast installations.

3 6. Prior to equipment configuration always use a radio scanner to select the frequency/channel with the least amount of activity. 7. Confirm that the reference station is fully operational prior to departing to start surveying with the rover station. FieldGenius Link Parameters GPS configuration within FieldGenius has a page called Link to permit setup of receiver s communication to the modem device, configuration of the modem device and selection of the message protocol. Figure 2. GPS Configure - Communication Link. 1. Mode Possible options for mode include ne, Radio and Wireless Network depending on the capabilities of receiver. The selection of ne for mode will disable the GPS receiver for any type of differential corrections and force the use of autonomous positions (excluding WAAS). A setting of Radio will use an RF wireless device for differential communication. Detailed communication device settings can be configured by pressing. For a radio device the available parameters are Radio Model and Channel or Frequency depending on the selected radio model. 2. Enable WAAS Wide Area Augmentation System (WAAS) is a set of geostationary satellites which provide differential corrections to achieve meter level positioning accuracies. The reception of the correction messages are handled internally by receiver and therefore the availability of this option is dependant upon the selected GPS model.

4 Enabling WAAS will provide an improvement over the autonomously generated positions and not interfere with RTK or DGPS corrections when available. 3. Link Communication This section is for configuring receiver to match the communication parameters of the link device which can be a radio modem or wireless network modem (see Figure 1). The GPS Port setting refers to the port on receiver to which the communication link device is connected. Knowledge of the link device s current communication settings (baud rate, parity, data bits, stop bits and flow control) are necessary in order to configure receiver s communication parameters. 4. Message Type The options available for this setting are shown in Table 1 and are dependent on the selected GPS model. As discussed previously the message type determines the protocol for transmitting corrections from the reference station GPS receiver to the rover station GPS receiver. If an RTCM protocol is selected, a corresponding version number might be selectable depending on model. The reference station and rover station must each be configured with an identical message type in order to successfully obtain differential position solutions. It is generally recommended to use the proprietary message type which has been designed for the selected GPS manufacturer. 5. The reference station ID must be matched at both the reference station and rover stations. A possible option for setting the reference station ID mask on the rover station is Any which will accept differential corrections from any reference station ID. Reference Station Configuration Figure 3. Reference Station Identification Setup. Rover Station Configuration

5 Trouble Shooting Guide The GPS toolbar as shown below supplies a wealth of information regarding the status of receiver. The left most button on the toolbar which is called the Solution Button will provide the first indication of a communication problem by denoting Link ( ). If receiver has established a differentially derived solution from the correction messages, the Solution Button will either indicate WAAS( ), DGPS( ), RTK Float( ) or RTK Fixed( ). Figure 4. GPS Toolbar. Prior to starting the process of trouble shooting real time positioning problems, ensure that receivers to be used have been updated with the latest firmware available from the manufacturer and that the necessary options/upgrades have been purchased to enable real time positioning. The flowchart shown in Figure 4 is designed to easily and efficiently remedy any problems which can occur with real time positioning techniques. tes: 1. The receive light indicator on the radio modem should be blinking at a repeating one second interval. Irregular or random flashing typically denotes interference by other users on a similar frequency and can be confirmed with the use of a handheld radio scanner. Random flashing of the light can also indicate that the reference station radio modem is not transmitting. 2. There are several factors which can influence the amount of time for a rover GPS receiver to generate a differential solution. The reference station as part of its correction messages sends a coordinate record approximately every 15 seconds. The rover station GPS receiver must have this coordinate information in order to produce corrected positions and if the quality of the RF communication is poor, this record can easily be neglected and require more time to obtain. The quality of observations has a direct impact on the time to generate a solution. Therefore always ensure sufficient satellites are being tracked, the PDOP should be within acceptable levels and avoid areas prone to multipath. Finally receiver needs to have a valid satellite almanac to determine positions. An invalid almanac can occur if receiver has been reset or has not been in use for several weeks. New almanacs can be obtained by operating as a stand alone for at least 30 minutes with an unobstructed view to the sky.

6 Start Link Is the radio modem s receive (RX) light flashing at the rover station? (te 1) Is the radio modem s transmit (TX) light flashing at the reference station? Is the rover GPS receiver tracking sufficient SVs for a position? Wait for GPS receiver to acquire at minimum 6 SVs. Is the rover station within RF range of the reference station? Move the rover station closer to reference station until an RF link is established. Is the rover station s reference ID mask rejecting the correction messages? Set the Reference ID parameter on the Rover page of Configuration to Any or match the ID set at the reference station. Is there RF interference from other users on a similar frequency? Change frequencies or channels to minimize interference. Are the message types (CMR, RTCM) matched between the reference station and rover station? Confirm message types on the Link page of Configuration. Is the frequency or channel matched between the reference s radio modem and the rover s radio modem? Use the radio modem manufacturer s configuration utility to confirm settings. Are the communication parameters (port, baud rate, parity, data bits) matched between the rover s GPS receiver and radio modem? Confirm parameters on the Link page of Configuration. Is the power to the reference s radio modem turned on? Connect and turn on power to the radio modem. Are the radio link parameters (link rate, modulation, scrambling) matched between the rover s radio modem and the reference s radio modem? Use the radio modem manufacturer s configuration utility to confirm settings. Is the reference GPS receiver tracking sufficient SVs for a position? Are the communication parameters (port, baud rate, parity, data bits) matched between the reference s GPS receiver and radio modem? Wait for receiver to acquire at minimum 6 SVs. Confirm parameters on the Link page of Configuration. Does the rover s GPS receiver and radio modem have enough battery power? Has the rover GPS receiver been given enough time to compute a solution? Replace batteries. Wait for at least 10 to 15 minutes for a solution to be determined. (te 2) Are the configured reference station coordinates within tolerance to the true WGS84 position? Confirm coordinates on the Reference page of Configuration and select Determine Autonomously if necessary. Figure 5. Trouble shooting Flowchart