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Placement of WAAS Reference Station (WRS) for Indian airspace
WAAS Reference Station (WRS) Differential GPS is a practical solution in many situations providing few-meter level accuracy under dynamic conditions. It has been noted that the majority of the error sources are related to the satellites and propagation environment. The subject of differential GPS is a simple concept, but implementation details can be more complex and handling of all cases is taken special care to achieve continuously reliable result. DGPS is a concept that eliminates some of the common bias errors experienced by GPS. Differential GPS derives its potential from the fact that the measurement errors are highly correlated between differential users. By employing second GPS receiver with comparison to true, slow varying correlated errors can be isolated and eliminated. In addition, depending on relative rates, selective availability (SA), Intentional degradation of the C/A signal, may be eliminated by DGPS as well. In DGPS, a receiver reference station is located in the local area where greater accuracy is desired. The correlated errors that a receiver experiences (such as satellite ephemeris errors) should be common to all users, and are a relatively closer geographical area. If the reference station can obtain the reliable estimate of its actual error and transmit it, to dynamic users, the dynamic users may be able to compensate for a large portion of their error. WRS Placement for Satellite Integrity Monitoring: GPS or GLONASS standalone or in combination, is unable to provide the Required Navigation Performance (RNP) in the all phases of flight. To achieve the RNP the WAAS in DGPS mode is the answer for aviation application. A Wide Area Differential Global Positioning System (WADGPS) augmentation system provides differential correction to GPS / GLONASS user at large within the bounded service area. The Wide Area Augmentation System Reference Station (WRS) are the system component that does the ranging measurements from the GNSS core satellites. The measurements are then used to calculate the correction. Wide Area differential corrections are i) satellite ephemeris / clocks corrections, and ii) Ionospheric-delay correction. The underlying difference between the two types of corrections and measurement used to calculate them impose differential requirements on the placement density of the reference station. There are unique reference station placement requirements near the boundaries of the service area. This work presents guidelines for reference station placement to provide the RNP for aviation enroute (non-precision) & precision approach, in all the phases of the flight.  Fig: 2 Four stations grid showing bowing effect Now consider the case of world coverage. For illustrative purpose, place four station as apart as they can be seamless coverage in a square with diagonal 2*rs =15,800 Km and side of 15800 *Cos (45°) = 11,172 Km as shown in fig (2). The temptation would be to claim that the users could be provided satellite monitoring service of the entire area of the square. However, the actual service area turns out to be only the shaded area, with a significant segment exclude from every side of square. Generally accepted assumption that users mask angle is not lower than reference station (in case of 5 degrees), then their visibility range is ru equal to rs. Consider a user located right at one of the station, every satellite that the user “sees”, the station will see too, within the circle of radius ru=rs. But now consider a user between two adjacent stations say at “d” that labels that segment depth. A satellite that happened to be south of the user at the lowest 5 degrees elevation angle would be in the notch between the two stations coverage. i.e. not visible to either . Therefore a user at “d” would not be in the service area. Integrating over the entire area within the square, where a satellite may be visible to a user, but not to either station, yields the segment between two stations defined by the arc and chord of the circle centered at notch’s intersection point and with radius ru . The depth d of the segment at its deepest point is 2314 Km as shown in fig (2). The calculation is as follows ru=rs = 7900 Km Side of the square (a) =15800 cos (45°) = 11,172 Km A/2 = h =5586 Km Depth d = 7900-5586 = 2314 Km (due to bowing effect) Area of sector ABC = (90/360)* p * (7900)2= 48.991850*106 Km 2 Area of triangle ABC =(1/2 ) 5586*11,172 = 31.203396*106 Km 2 Area of the shaded area region BDC = 48991850 – 31203396 = 17.788454*106 Km2 Uncovered area = 4* 17788454 Km2 =71.153816*106 Km2 Area of the square = 124.813584 *106 Km2 Total service area = 53.659768*106 Km2 If the station mask angle is greater than the user’s mask angle, the service area becomes even smaller by the difference in their visibility ranges i.e. ru-rs. With a user’s angle of 5 degrees and station 10 degrees, an additional 7900-7362 = 538Km would get excluded from every side of the service area.  Fig: 3 Six Stations grid showing bowing effect The bowing effect would have to be overcome by reducing the distance between WRS or placing additional station(s) outside of service area. The bowing effect is analyzed in “The closer the stations, the smaller the bowing effect”, as measured by corresponding segment depth d. |