Evolution of Integrity Concept – From Galileo to Multisystem - Global Navigation Satellite Systems: Signal,Theory and Applications - page 82

The objective of HPCA is to compute the predicted HMI probability in any integrity
exposure time interval (150 s) for a given GMS integrity by monitoring state and user
geometry.

This algorithm includes the following modules:

• UERE Computation Module

• Position Solution Matrix Computation Module

• Fault Free Position Error Computation Module

• Faulty Position Error Computation Module

• HMI Probability Computation Module

Figure 2 shows the block diagram of the HPCA Algorithm.

Obtain Geometry
and Integrity Data

UERE Computation

Position Solution
Matrix Computation

Faulty
Position Error
Computation

FaultFree Position
Error Computation

HMI Probability Computation

Set
HPCA_index

Fig. 2. HPCA algorithm

The “UERE Computation module” computes the predicted standard deviation of total
pseudorange error on each signal from visible satellites. The principal formula is as follows:

[] [] []

2 2

σ i = σ i + σ i (3)

uRX , SISA uL ,

It takes into account the signal in space as well as the local errors.

SISA σ is the SISA value for the i th satellite used at user level; it is equal to the SISA value
broadcasted in the navigation message increased by a factor of 1.1.

[]

( ) 2

[] []

2

σ i =⋅ 1.1 SISA i (4)

SISA