Functionalities of the EGNOS SDK

On tapping the bottom right-hand icon in the Skyplot screen you can access the Advanced Skyplot View that shows the live-video stream of the sky and displays the current position of the GPS/EGNOS satellites depending on where you’re pointing your mobile device.

An example of the Advanced Skyplot View

The Advanced Skyplot View also displays the horizon, represented as a line; the geographic directions, indicating the direction in which you’re pointing your device; an arrow pointing to the EGNOS satellites, indicating the direction to which the mobile device should be pointed to get one of the EGNOS satellites in view; the historical path of the EGNOS satellites, displaying the path of the EGNOS satellites from the past position to the current position.

Discover the Advanced Skyplot view through the fallowing Video

On tapping a GPS/EGNOS satellite a popup is displayed giving information about this satellite -  PRN, Name of the satellite, SNR, Elevation and Distance of that specific satellite is displayed.

On tapping the ‘About Advanced Skyplot View’ icon (in white) the Advanced Skyplot about page is displayed providing information on what each line, dotted line, arrow, satellite image or text represents.

An example of the Current location screen

On tapping Current location a map is displayed with your current position. On the map two points are shown:

• The GPS signal location point (in red)
• The EGNOS signal location point (in green – initially the EGNOS location point will be shown in Orange until EGNOS corrections are fully available)
• The EGNOS location point is provided with the Position Integrity circle based on Horizontal Protection Level (HPL)*

In case the ’R&D‘ feature is switched on, an additional purple location point will represent the R&D enhanced position. For this no integrity is available, since this position is not according to DO 229D.

The current location updates itself at regular intervals, based on the Minimum Time option specified in the Setting/Options menu.

*The horizontal protection levels (HPL) provided by this application are calculated according to SBAS RTCA MOPS DO229 standards, which are based on hypotheses applicable for aeronautical environments. They are provided as general integrity indicators but their values cannot be directly extrapolated to other environments as terrestrial or maritime

An example of Get direction screen

Get directions allows you to select the Start Point (current location or any location) and the End Point (any location). By tapping on ’Go‘ a route is drawn from the start to the end Point along with the current location point.

The Start tracking option allows you to track your current position, on regular intervals, based on the Minimum Time option specified in the Setting/Options menu. The screen displays the current and the past position:

• The GPS signal location point in red
• The EGNOS signal location point in green
• In case the ’R&D‘ feature is switched on, an additional location point in purple will represent the R&D
• The enhanced position, similar to ’Current location’

An example of the Start tracking screen

To explore the technical details behind the development of the EGNOS corrections implementation (GPS clock and orbit errors, ionosphere error, troposphere error, integrity) please have a look at the “EGNOS SDK Development” presentation. This document provides a detailed explanation of the development process and describes the key interactions between the SDK modules. The high-level modules (external receiver, ephemeris, satellite, positioning, EGNOS, SISNeT, ionosphere, troposphere, long corrections and fast corrections modules) are also described in depth.

By checking this option the satellite constellation used for the computation of the position is improved in terms of the number of satellites considered. The satellites previously eliminated – due to the restrictions imposed by the DO-229D specifications - are now introduced in the position computation.

The purpose of this functionality is to improve the resulting position accuracy by selecting the most suitable GPS satellites for the computation of the position, in particular when a high number of them are in view. To do that, some satellites are eliminated to obtain a better satellite geometry, which can reflect into a better position accuracy.

By checking this option the best satellite geometry is selected – through the values of the Dilution of Precision (DOP) which state how errors in the measurement of the position will affect the final estimation.

Dilution of Precision (DOP) is the ratio of the standard deviation of errors in the least-squares solution to the standard deviation of the measurement errors.

The idea of Geometric DOP is to state how errors in the measurement will affect the final state estimation. The DOP parameters are defined as geometry factors that relate parameters of the user position and time bias errors to those of the pseudorange errors.

For the EGNOS SDK the considered values are the GDOP – Geometry Dilution of Precision – and the HDOP – represent the satellite geometry effect on the horizontal component of the positioning accuracy.

By checking this option, a position is provided even if only 3 GPS satellites are in view (whereas 4 satellites are normally the mandatory minimum to compute a position). This is important in urban situations, where surrounding buildings can obstruct a clear view of the sky, making it difficult to track four satellites or more.

The rationale behind this calculation method is the following: the standard GPS Algorithm has four unknown elements: latitude, longitude, altitude and time. If you eliminate one of these unknown components (generally altitude) three satellites can be sufficient to obtain a position in 2D.

Moreover, a solution with a lower degree of uncertainty can be obtained, when more than three satellites are available.

By checking this option an “integrity check” is provided and you can achieve a better accuracy for your smartphone.

Receiver Autonomous Integrity Monitoring (RAIM) is a method of providing integrity with the capability of detecting when a satellite failure or a measurement error has occurred.

This option is based on an algorithm that detects faulty/unhealthy satellites or measurements possibly involved in the position computation. Thus, eliminating the faulty satellites enables to obtain a better positioning accuracy. This approach can be viewed as an integrity check on the information provided to the core of the receiver.

By checking this option the positioning can be improved by eliminating jumps that the Range Rate Correction (RRC) might cause in the fast corrections applied to the pseudorange (PR) measurements.

The RRC correction was introduced in the WAAS/EGNOS system to improve the performance with the GPS Selective Availability (SA) turned on. At the time it was a great improvement to the overall performance.

EGNOS messages MT2–5 provide pseudorange corrections, for satellite clock and ephemeris errors, that are simply added to the receiver’s raw pseudorange measurements for all visible satellites. The SBAS data does not include Range Rate Corrections. These are generated within the user equipment itself by differencing successive clock corrections.

By checking this option, the satellites involved in the position computation will be rated on the basis of the most beneficial weight matrix.

As the name suggests, the weight matrix gives a different importance to the satellites involved, satellites with a higher certainty should have a bigger importance in determining the position.

The weight matrix should contain indicators of the quality of the satellite signal, or the accuracy of the satellite data provided.

The most important sources of error in the GPS system are caused by the atmosphere, more precisely by the ionospheric and tropospheric layers. Due to this reason, a signal traveling a longer path through the atmosphere will be affected by a greater amount of error then a signal passing a shorter distance. This reflects in the fact that a satellite with low elevation will generally have a greater error then one at zenith.

The DO-229D standard specifies that the sigma values provided by EGNOS corresponding to each pseudorange have to be used as weight for the position computation algorithm. Several experiments showed using the Signal/Noise ratio (SNR) of each satellite signal as weight provides good results too.

An example of the SBAS ranging function visualization

The SBAS ranging function is implemented by enabling SBAS geostationary satellites to be used in the same way GPS satellites are used to compute a position.

The increased number of satellites will provide you with enhanced availability of the system and an increased quality of the satellite geometry.

The functionality can be enabled/disabled through the SBAS Ranging function options (Automatic mode, Forced mode and Off mode) within the R&D Position menu.

The EGNOS system broadcasts two different messages for ranging:

Message Type 9: GEO Navigation Message

The MT9 provides precise orbit information for the GEO satellites and is used in the position computation. The MT9 messages provide information on the position, velocity and acceleration of the geostationary satellite. It includes also an estimation of the accuracy of the message.

The Message Type 17: GEO Satellite Almanacs Message

The MT17 contains the rough location of the satellite and is used for speeding up the acquisition of the position. MT17 Almanacs for three satellites are broadcast periodically to alert you of their existence, location, health and status.

It should be noted that today EGNOS does not provide a reliable ranging function (contrary to the US WAAS), which is why, according to the standard, it is not used in Europe (and in Automatic mode in the EGNOS SDK). Using the forced mode in Europe might provide bad results in terms of accuracy (error of dozens of km).