1. 2 Observation Technique 2. Differential GPS (GPS) The selection of the observation technique ular requirements of the projec The desired accuracy especially I. Two(or more)receivers, where one(stationary)reference (base) receiver is located at a known point and the 1. Point positioning position of moving remote receiver is to be determined. For the SPs only the C/A-code is available and SA is incorporated into the service. This limits the (real-time accuracy The known position is used to calculate corrections to the 1. Horizontal accuracy: 100 m(95%), 300 m(99.99%) GPs derived position or to the observed pseudoranges 2. Vertical accuracy: 156 m(95%), and 500 m(99.99%) 4. These corrections are then transmitted via telemetry (i.e. controlled radio link)to the roving receiver The pps has access to both codes The higher accuracy is based on the fact that GPS error sources 1. Horizontal accuracy: 16 m(95%) re very similar over a distance of about 500 km and are 2. Vertical accuracy: 23 m(95%) therefore, virtually eliminated by the differential technique. 2. Two Correction Methods 3. Accuracy of DGPS I Position Correction. 1. Point positioning can not reach meter level because of The difference (differential") of the known and the SA but can be achieved by DGP calculated position yields position corrections. These 2. Using C/A-code ranges, accuracies at the 3-5 m level values are then applied to the roving receiver to obtain an can be routinely achieved improved position (conceptually simple, more comple 3. Phase smoothed code ranges or high performance C/A at. selection) code receivers can obtain the submeter level 2. Pseudorange Corrections 4. Carrier phases can obtain sub-decimeter level for up The difference between calculated ranges and observe to 20 km, to achieve this accuracy, the ambiguit (code or phase) pseudoranges at the re must be resolved on-the-fly and therefore(generally) site may be lual frequency receivers are required corrected by applying pseudorange corrections of the 1. The accuracy requirements of GPS users vary from reference station(more flexible, higher accuracy several hundred meters and centimeter level thera 2. Interested a real-time accuracy at the meter level. 4. Wide Area DGPS- WADGPS 5. Data Link WADGPS uSes a network of gPs reference stations with UHF (ultra high frequency)radio links for terrestrial coverage of a larger territo data links 2. RDS (radio data system) which is a standardized Main Advantages of wadgPs ethod for distributing digital data along with the 1. A more consistent accuracy throughout the regio onventional upported by the network(regular DGPS, the accuracy 3. LEO worldwide telecommunication satellite allows decreases at a rate of approximately 1 cm per 1 km) high frequencies in the GHz range and enables data 2. Inaccessible regions can be covered, e.g., large bodies rates up to 1200 bits per second over long distances. Correction update rates of 10 seconds or better are 3. The network will still maintain a relatively high level of adequate to remove SA effects at the 2 m accuracy level integrity and reliability compared to a collection of because SA is characterized by variations of the individual DGPS reference stations pseudorange error with an 100 m each about 10 minutes2 1.2 Observation Technique The selection of the observation technique • Particular requirements of the project • The desired accuracy especially 1. Point Positioning For the SPS only the C/A-code is available and SA is incorporated into the service. This limits the (real-time) accuracy 1. Horizontal accuracy: 100 m (95%), 300 m (99.99%) 2. Vertical accuracy: 156 m (95%), and 500 m (99.99%). The PPS has access to both codes. 1. Horizontal accuracy: 16 m (95%) 2. Vertical accuracy: 23 m (95%) 2. Differential GPS (DGPS) 1. Two (or more) receivers, where one (stationary) reference (base) receiver is located at a known point and the position of moving remote receiver is to be determined. 2. At least four common satellites must be tracked simultaneously. 3. The known position is used to calculate corrections to the GPS derived position or to the observed pseudoranges. 4. These corrections are then transmitted via telemetry (i.e., controlled radio link) to the roving receiver. 1. Basic Principle The higher accuracy is based on the fact that GPS error sources are very similar over a distance of about 500 km and are, therefore, virtually eliminated by the differential technique. 1. Position Correction: The difference ("differential") of the known and the calculated position yields position corrections. These values are then applied to the roving receiver to obtain an improved position (conceptually simple, more complex sat. selection). 2. Two Correction Methods 2. Pseudorange Corrections • The difference between calculated ranges and observed (code or phase) pseudoranges at the reference site. • The observed pseudoranges at the roving site may be corrected by applying pseudorange corrections of the reference station (more flexible, higher accuracy, general use). 1. Point positioning can not reach meter level because of SA but can be achieved by DGPS. 2. Using C/A-code ranges, accuracies at the 3-5 m level can be routinely achieved. 3. Phase smoothed code ranges or high performance C/A￾code receivers can obtain the submeter level. 4. Carrier phases can obtain sub-decimeter level for up to 20 km, to achieve this accuracy, the ambiguities must be resolved on-the-fly and therefore (generally) dual frequency receivers are required. 3. Accuracy of DGPS 1. The accuracy requirements of GPS users vary from several hundred meters and centimeter level. 2. Interested a real-time accuracy at the meter level. 4. Wide Area DGPS-WADGPS WADGPS uses a network of GPS reference stations with coverage of a larger territory. 1. A more consistent accuracy throughout the region supported by the network (regular DGPS, the accuracy decreases at a rate of approximately 1 cm per 1 km). 2. Inaccessible regions can be covered, e.g., large bodies of water, 3. The network will still maintain a relatively high level of integrity and reliability compared to a collection of individual DGPS reference stations. Main Advantages of WADGPS 5. Data Link 1. UHF (ultra high frequency) radio links for terrestrial data links. 2. RDS (radio data system) which is a standardized method for distributing digital data along with the conventional program. 3. LEO worldwide telecommunication satellite allows high frequencies in the GHz range and enables data rates up to 1200 bits per second over long distances. Correction update rates of 10 seconds or better are adequate to remove SA effects at the 2 m accuracy level because SA is characterized by variations of the pseudorange error with an 100 m each about 10 minutes