Home Articles Placement of WAAS Reference Station (WRS) for Indian airspace

Placement of WAAS Reference Station (WRS) for Indian airspace

Arjun Singh


Arjun Singh & K. Ramalingam
Airports Authority of India

D. C. Reddy, P. Laxminarayna
Osmania University
Hyderabad
[email protected]

Abstract
The standalone basic Global Positioning System (GPS) service fails to meet the required accuracy, availability, and integrity, which are critical to safety of Civil Aviation for precision approach. In the recent year, there has been wide spread growth in the development of satellite based augmentation system which is part of the Global Navigation Satellite System (GNSS), for enroute, precision approach and landing of the aircraft. This paper is presenting, briefly Wide Area Augmentation System (WAAS) principles, bowing effect for coverage analysis and identification of the WRS for Indian Air space based on FAA’s WAAS for GPS signal range correction error. The visibility of the satellite at each WRS with their PRN are observed at all the reference station and presented in the paper. In this paper, we developed some guidelines for optimal placement of the WRS for Indian WAAS (INWAAS). The method of placement of WRS has been discussed in length. The monitored area and bowing effects is calculated which demonstrate that if WRS number increased, the bowing effects will decrease and monitored area will increase.

Introduction
The GPS satellite signals are available worldwide; GPS represents a unique opportunity for the international aviation community to start converging toward the goal of a single, integrated and universalized GNSS for air navigation. This will eventually allow aviation users to reduce the number of different types of receivers required for navigation services in all phases of flight. The GPS will contribute to increased safety and efficiency of international Civil Aviation by supporting real-time surveillance of aircraft, thereby reducing separation requirements to increase the number of flights possible on the busy, most favorable transoceanic routes by overlying Geostationary Earth Orbit Satellite (GEOS).

The basic requirements for designing of WAAS reference station are discussed in shape of rectangular and Hexagonal configurations. It is verified that increase in number of WRS will increase the monitored area and reduction in bowing effect. The bowing effect is noticed only when distance between WRS stations are very high. When consider for regional coverage, the distance between WRS is decreased and monitored area is increased. Now in present discussion, it is very difficult to choose the site, which can provide the regular geometrical shape for easy way to calculate the coverage area. When Indian geographical condition is considered, the tedious task is to decide the place and getting coordinate of the place.

World wide WAAS
The major Airlines in the United States and around the world are flying to more international destinations; co-operation and joint alliances are being developed among them. Therefore the Airlines will implement WAAS only if it will lead to a seamless, worldwide system and allow them to reach their objective of flying and landing anywhere in the world with the single system. ICAO has already started the process of defining and implementing of worldwide GNSS Standard and Recommended Practices (SARPs). They have developed the Future Air Navigation Systems (FANS) concept and are in the process of defining the Satellite Based Augmentation Systems (SBAS) and its infrastructure, i.e. GNSS , will be part of FANS. The GNSS panel was convened in Montreal in October 1994 to start defining the process. Civil Aviation Authorities can follow to make the transition from the current, ground base navigation system to SBAS. The transitions involve the use of GPS and WAAS, as parts of an initial component of GNSS, to provide a smooth evolution to a worldwide, integrated GNSS with international participation and control.

WAAS is the first step toward an international GNSS, since countries or groups of countries can implement it, will have a voice over the control and management of GNSS and its use over their sovereign airspace. Since WAAS is more effective when it is an integrated system over a large area, its implementation will require regional coordination and cooperation. This requirement will encourage Civil Aviation Authorities to work together, and to initiate the process needed for an eventual worldwide and internationally based GNSS. Cooperation and coordination are essential. Since GNSS or any satellite-based system is too expensive for implementation by any one country, and by its nature is an inherently worldwide system, it requires standardization. WAAS implementation will require countries covering either large or small airspace, to work together towards a jointly operated and controlled navigation system. WAAS also represents a de-facto test bed to determine whether Civil Aviation Authorities around the world are ready to work together in a jointly operated system. The implementation of a seamless WAAS would be a first step toward a worldwide, internationally controlled GNSS.

WAAS Description
To provide enroute, non-precision approach and precision approach navigation service throughout Indian airspace, WAAS in its final form will use 10-12 stations placed throughout India to cover the entire Indian controlled Airspace. The GPS master station will collect the data and calculate the differential correction for GNSS core constellation. Ground Earth Station (GES) will transmit the corrections to Geo stationary Earth Orbit Satellites (GEOS), which will retransmit them to users. The GES transmission will employ a GPS wave form, including the coarse acquisition (C/A) code, which GEO will preserve, when it retransmit at the GPS L1 frequency (1575.42 Mhz); GEOS will thus appear to the user receiver to be additional GPS satellite that can be used for ranging as well as source of differential corrections to be applied to all visible satellites.


Fig: 1 Typical Wide Area Augmentation System (WAAS) Network
Corrections to be provided on a satellite-by-satellite basis are of two types. Long-term ephemeris and clock corrections are to be transmitted once a minute or longer. Fast corrections, transmitted every six seconds accounts for Selective Availability (SA) but also partially compensate for interim errors in the long-term corrections. The WAAS will also collect measurements of signal delay through the ionosphere at each station using dual frequency techniques taking advantages of GPS signals being broadcast on two widely separated frequencies, L1(1575.42 Mhz) and L2 (1227.60 Mhz) . This data will then be compiled at the master station to compute values for an imaginary grid over the service area, at 5 degrees spacing of the vertical Ionospheric delay (in meters) to be subtracted from the users range calculation to each satellite. The “grid” is broadcast to user via GEOS. To make use of this data, the user receiver must interpolate between the grid values to calculate the delay at the point at which its line-of-sight to each satellite in view pierces the Ionospheric shell. This vertical Ionospheric delay must then be rotated to the satellite line of sight angle. The WAAS station will therefore provide a continuous set of Ionospheric delay estimates that replace the generic, less accurate model of Ionospheric delay that is broadcasted by GPS satellite. Fig (1) shows the typical WAAS architecture.

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.

To reduce the unserviceable area as compared to serviceable area a hexagonal model is proposed and analyzed as follows. Let us consider an area to be covered is having a hexagonal shape is shown in fig (3). We place six WRS station at six corners, only the shade region is service area. Other area is not under service area due to bowing effect. The service area covered by each satellite on average for hexagonal shape with six satellites is 21.63* 10 6 Km 2. The service area covered by each satellite on the average for square shape with four satellites is 13.414942*106 Km2. This is very clear that increases of WRS in number will increase the service area drastically and also indicates the reduction of the bowing effects. By placing the reference station at all the corners of Hexagon, calculation for service area is as follows.
Side of the hexagon (rs) = 7900Km
Total service area = Area of the Hexagon
From fig (4), Area of the equilateral triangle ABC = Ö (3)/4 * side2 = 27.02 *106 Km2
Area of the sector = (q/ 360) *p* rs2
Area of the sector = 32.667106*106 Km2
Deepest point (segment depth) d = 7900-6814.6 = 1058.4 Km
Uncovered area (unshaded) each station BDC = sector area – triangle area
Total uncovered area =6* 5.657 *106 Km 2 = 33.942*106 Km2
Total area covered by hexagon= 162.12*106 Km2
Useful coverage area = Hexagonal area – total uncovered region= 128.178*10 6 Km2
Service area of each station =21.630 *106 Km2 Adding two WRS, the service area of hexagon model is increased by 37.33 *106 Km2
Table 1

  Rectangle (4) Hexagon (6)
Area of the WRS 124.81*106 Km2 162.12 *106 Km2
Uncovered due to bowing effect 71.153816 *10 6 Km2 33.942*106 Km2
Useful service Area 53.65*106 Km2 128.178 106 Km2

The above methods suggest that increase of WRS station will reduce the bowing effects as well as increase in the monitoring area in turn service area will get increased. In the case of regional coverage, total area get reduced depending upon the size of the country as well as controlled airspace. Hence the distances between WRS get reduced to counter the bowing effect. The concept of the optimization to place the WRS is discussed below.

WRS placement for satellite Integrity monitoring over Indian Airspace:
Geographical condition of Indian air space is versatile in nature. The positioning of the WRS stations for providing monitored air space and reliable correction signal to user in all the phases of flight is challenging task. The coordinate of the place, accessibility to the point for installation of the equipment and also should fulfill the siting criteria defined in International Civil Aviation Organisation (ICAO) document for navigation equipment. For sake of simplicity, the coordinates of Doppler Very High Frequency Omni-Range (DVOR) and Distance Measuring Equipment (DME) Stations, Non Directional Beacon (NDB) and Airport Reference Point (ARP) have been taken into consideration . The coordinates of the navigational aids are published in Air Radio of Airports Authority of India (AAI) and used by pilot for navigation of the aircraft in all the phases of flight. The coordinates of the proposed WRS placement over Indian Airspace are placed in the Table –II as plan I &II.
Table – II (Proposed Plan – I)

Place Latitude (N)Deg Min Sec. Longitude (E)Deg Min Sec Site / facilities
Srinagar (1) 34       00      12 74      45      23 DVOR
Delhi (12) 28       34      00 77      05      46 DVOR
Jamnagar (2) 22       29      35 70      03      30 NDB
Bhopal (13) 23       16      56 77       20       13.5 DVOR
Mumbai (3) 19       07      01 72      50      04 NDB
Hyderabad (14) 17       26      59 78      21      13 DVOR
Kavatati (4) 10       30      00 72      36      00 NDB
Nagarcoil (5) 20       42      45 70      55      20 NDB
Port Blair (6) 21       38      26 69      39      46 NDB
Chennai (7) 12      59       30.50 80      10      09 DVOR
Bhuneshwer (8) 20       14      38 85      49      02 DVOR
Aziwal (9) 23       44      32 92      48      22 NDB
Itanagar (10) 27       06      01 93      44      37 Proposed Airport
Gagtok (11) 27       21      03 88      56      41      Proposed Airport

(Proposed Plan – II)

Place Latitude (N)Deg Min Sec Longitude (E)Deg Min Sec Site / facilities
Srinagar (1) 34  00  12 74  45  23 DVOR
Jamnagar (2) 22  29  35 70  03  30 NDB
Hyderabad (9) 17  26  59 78  21  13 DVOR
Kavatati (3) 10  30  00 72  36  00 NDB
Nagarcoil (4) 20  42  45 70  55  20 NDB
Port Blair (5) 21  38  26 69  39  46 NDB
Jodhpur (8) 26  14  00 73  03  00 DVOR
Itanagar (6) 27  06  01 93  44  37 Proposed Airport
Gagtok (7) 27  21  03 88  56  41 Proposed Airport

Source: Air Radio AAI SEcond Edition

The Plan -I has 14 number of WRS where as Plan- II has nine number of WRS stations. The positioning of WRS station to cover the Indian Airspace have nonlinear pattern of triangles or squares. It is very difficult to calculate the total / Actual monitored area for users. The methodology adopted for calculating the monitored area is as follows.

Calculate the circumference of earth where radius of earth is 6378Km (taken)

Calculate the distance between 1 degree, 1 minute and 1 second in KM

Calculation based on:
Radius of the earth = 6378 Km, Circumference of the earth = 2ð * 6378 = 40,054 Km
One degree = 40,054/360 = 111.3 Km
One Minute = 111.3/60 = 1.855 Km
Calculate the distance between the WRS
We have mentioned a solution given by Gauss. Its limitations is that the distance d between the WRS station should be smaller than 50 Km . For longer distance Bessel’s Mid latitude formulas is suggested.

One Second = 1.855/60 = 0.04 Km


Fig :4 – Plan I
Used Hero formula to calculate the area of isosceles triangle which are the part of the WRS monitored area.
S= (a + b + c) / 2
Where a, b, c are the sides of the triangle
Area of triangle = [s (s-a) (s-b) (s-c)]1/2
When station is connected to each other, the final shape of the monitored area is irregular shape. To ease out the calculation of WRS monitored area, the isosceles triangle pattern has been considered for calculating the monitored area of the WRS station. The area calculation of Plan I &II and the station coverage diagram as Fig (4) & (5)
The total Area covered by plan-I = 8185051.789 Km2
The total Area covered by plan-II = 5007448.85 Km2

It is evident that Plan –I monitored area is much higher than Plan II monitored area due to increased in number of WRS stations. To prepare the coverage diagram for both plans, for sake of simplicity thumb of rules has been followed. The coverage of the each WRS is calculated based on averaging of sides of triangle and also taken least value of the average as same station sharing or the part of the other triangle. The coverage diagram of Plan-I and Plan –II are placed as Fig (4) and Fig (5). In plan-I, Entire Indian Airspace is covered and monitored area is calculated including bowing effect coverage. The bowing effect will be more prominent between station 1&2, 2&3, 3&4, 4 & 5, 5 & 6, 6&9 , 9 &10 , 10&1, 11&9. Some of the station coverage is overlapping due to more number of stations in less area. The WRS coverage will over lap to each other WRS coverage. The station (12) &(14) will not play major role as station will fall within the monitoring station of the other station. Therefore, station (12) and (14) can be utilized for WMS. The coverage diagram for plan-II as shown Fig (5) does not cover entire Indian air space. This plan needs to more WRS to cover entire airspace. The additional WRS required for plan-II are either at Lucknow or Patana . There is less overlapping of correction signal compare to plan-I .


Fig :5 – Plan II
Visibility of the GPS Satellite at WRS:
The GPS data was taken on 22-06-2001 and it was utilized to verify the visibility of the satellites at each WRS station. The GPS satellites visibility and also monitored the GDOP, PDOP and HDOP of the each WRS station place with GPS constellation (in view) which is presented in the Table-III (a),(b),(c).
PLACE VISIBLE GPS SATELLITES ID GDOP PDOP HDOP
SRINAGAR 6,10,15,17,18,23,26,30 2.3 2.0 1.1
DELHI 6,10,15,17,18,23,26,30 2.3 2.0 1.1
JAMNAGAR 5,6,10,15,17,18,21,23,26,30 1.8 1.6 1.0
BHOPAL 5,6,10,15,17,18,21,23,26,30 1.9 1.7 1.0
MUMBAI 5,6,10,15,17,18,21,23,26,30 1.8 1.6 1.0
HYDERABAD 5,6,10,15,17,18,23,26,30 1.9 1.7 1.0
KAVATATI 5,6,9,15,17,18,21,23,26,30 2.1 1.9 1.1
NAGARCOIL 5,6,10,15,17,18,21,23,26,30 1.8 1.6 1.0
PORT BLAIR 5,6,10,15,17,18,21,23,26,30 1.8 1.6 1.0
CHENNAI 5,6,9,10,15,17,18,21,23,26,30 1.7 1.5 1.0
BHUBNESHWAR 5,6,10,15,17,18,23,24,26,30 1.8 1.6 .09
AZIWAL 6,10,15,17,18,23,24,26,30 1.9 1.7 1.0
ITANAGAR 6,10,15,17,18,23,24,26,30 1.9 1.7 1.0
GANGTOK 6,10,15,17,18,23,24,26,30 1.9 1.7 1.0
JODHPUR 6,10,15,17,18,23,26,30 2.2 2.0 1.1

(a) TIME : 0600 to 12:30       DATED : 22-06-2001

PLACE VISIBLE GPS SATELLITES ID GDOP PDOP HDOP
SRINAGAR 11,14,20,21,25,29,30 2.7 2.3 1.4
DELHI 11,14,20,21,22,25,29,30 1.9 1.7 1.0
JAMNAGAR 1,11,14,20, 22,21,25,29,30 1.8 1.7 0.9
BHOPAL 11,14,21, 22,25,29,30 2.4 2.1 1.1
MUMBAI 11,14, 18,21,22,25,29,30 2.4 2.1 1.0
HYDERABAD 11,14,18,21,22,25,29,30 2.2 1.9 1.0
KAVATATI 3,11,14,21,22,25,29,31 2.4 2.2 1.1
NAGARCOIL 1,11,14,20,21, 22,25,29,30 1.8 1.7 0.9
PORT BLAIR 1,11,14,20,21,22,25,29,30 1.8 1.9 0.9
CHENNAI 11,14,18,21,22,25,29,30 2.2 1.9 1.0
BHUBNESHWAR 11,14,18,21,22,25,29,30 2.2 1.9 1.0
AZIWAL 11,14,18,21,22,25,29,30 2.3 2.0 1.0
ITANAGAR 5,11,14,18,21,22,25,29,30 1.8 1.6 1.0
GANGTOK 11,14,18,21,22,25,29,30 2.3 2.0 1.0
JODHPUR 11,14,20,21,22,29,30 1.9 1.7 1.0

(b) TIME : 12.30 to 1800         DATED :22.06.2001

PLACE VISIBLE GPS SATELLITES ID GDOP PDOP HDOP
SRINAGAR 1,3,8,13,27,28,31 3.3 2.8 1.6
DELHI 1,3,8,13,27,28,31 3.4 2.8 1.7
JAMNAGAR 1,2,3,8,13,20,27,28,31 2.0 1.8 1.1
BHOPAL 1,3,8,13,27,28,31 3.4 2.9 1.8
MUMBAI 1,2,3,8,13,20,27,28,31 2.0 1.8 1.1
HYDERABAD 1,2,3,8,11,13,20,27,28,31 1.9 1.7 1.1
KAVATATI 1,2,3,8,11,13,20,27,28,31 1.9 1.7 1.0
NAGARCOIL 1,2,3,8,13,20,27,28,31 2.0 1.8 1.1
PORT BLAIR 1,2,3,8,13,20,27,28,31 2.0 1.8 1.1
CHENNAI 1,2,3,8,11,13,20,27,28,31 1.9 1.7 1.1
BHUBNESHWAR 1,3,8,11,13,20,27,28,31 2.0 1.8 1.2
AZIWAL 1,3,13,22,27,28,31 4.6 3.7 2.1
ITANAGAR 1,3,13,22,27,28,31 4.5 3.7 1.9
GANGTOK 1,3,8,13,22,27,28,31 3.0 2.6 1.9
JODHPUR 1,2,3,8,13,27,28,31 2.2 2.0 1.2

c) TIME : 1800 to 00:00         DATED : 22-06-2001
Table –III (a) (b),( c) shows that 95% of the GPS satellites are visible to all WRS over the entire Indian Air space. Therefore monitored area will be doubled due to same set of satellites visible to all WRS.

Future Works

  • Generate the data base of visibility of GPS satellite and PDOP for the selected WRS
  • Identify the blind and dense spot
  • Based on the results modify the number and locations of WRS

Conclusion
The basic requirements for designing of WASS reference station is discussed along with two WRS placement configurations. The results are very much encouraging. Moreover, geographical condition of India is such that any particular suggested model may not suit the requirement but with some trade-off model can be suggested for Indian air space. It also suggests that if 10 numbers of WRS are installed, seamless monitored service area will be provided to the users in all the modes of flight.

In non-linear model of WRS position, it shows that Plan-I has more coverage than Plan-II. It indicates the bowing effect is less in plan-I as compare to Plan-II. In this paper , it has been investigated that same set of satellite is visible over the entire Indian airspace. The total monitored area will increase and bowing effects will reduce in the inner side of the monitored area and more prominence at the side of the WRS station coverage. The GPS satellites constellation provides good PDOP to all the WRS stations. Hence ten numbers of WRS is sufficient to cover the entire Indian Airspace.

References

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  • ROBERT P. DENARO, RUDOLPH M KALAFUS “differential operation of NAVSTAR GPS enhance accuracy”.
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  • P. LAXMINARAYANA, K.V.S SAIPRASAD, Dr. D.C. Reddy “Wide Area Reference Station Placement for an Indian WAAS” ION NTM-2001, 22-24 January 2001, Long Beach,CA