2.2.8. VERSION NUMBER
The Version Number is required as per DO-260A and identifies the format of ADS-B messages. Version Number 0 (or when no version number is transmitted) refers to DO-260 and Version Number 1 to DO-260A. Version Number 2 is expected for DO-260B.
2.2.9. RECEIVER AUTONOMOUS INTEGRITY MONITORING (RAIM) / FAULT DETECTION AND EXCLUSION (FDE)
The RAIM/FDE function performs a monitoring of the GPS position integrity. It measures the confidence in the correctness of the parameters provided by the GPS constallation. It enables the detection and, when possible, the exclusion of a faulty satellite. A GPS receiver is able to perform the RAIM/Fault Detection (FD) function when 5 satellites are visible (4 satellites enables the calculation of a 3D position; the 5th satellite enables the fault detection).
The FDE function is an enhanced version of RAIM. In addition to the fault detection, FDE is able to exclude the faulty satellite. As a consequence, the navigation can rely on GPS satellites without interruption. To that end, the FDE function requires 6 satellites (4 satellites for the 3D position, 1 satellite for the fault detection and 1 for the exclusion).
It has to be noted that the FDE function is able to detect and exclude one and only one faulty satellite. If a second satellite fails, it may be detected but it may not be excluded.
2.2.10. GPS HORIZONTAL FIGURE OF MERIT (HFOM)
HFOM defines the estimated accuracy of the GPS position assuming there is no satellite failure. HFOM is the radius of a circle centered on the current position, such that the probability of the actual position lying inside the circle is 95% or more (or outside the circle with a probability of 5% or less). The higher the HFOM, the lower the estimated accuracy in the GPS position.
HFOM is also known as Estimated Position Uncertainty (EPU)6 .
2.2.11. GPS HORIZONTAL PROTECTION LIMIT (HPL)
HPL is the radius of a circle centered on the current position, which defines an area that is assured to contain the indicated horizontal position with a given probability.
HPL is also known as Horizontal Integrity Limit (HIL).
HFOM reflects the estimated accuracy of a satellite geometry whereas HPL indicates its estimated integrity. Figure 2-5 illustrates a satellite geometry with:
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A good accuracy (HFOM) thanks to satellite A
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A poor integrity (HPL) due to satellite A.
Figure 2-5: Accuracy (HFOM) vs. integrity (HPL) Indeed, the satellite A position relative to satellites B, C, D reduces the satellite range errors. The satellite A upgrades the accuracy (HFOM). The high correlation between satellites B, C and D allows an easy detection of failure from any of these three satellites. Conversely, the position of satellite A cannot be correlated with other satellites. Consequently, a failure of satellite A may not be detected. The satellite A downgrades the integrity (HPL).
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