Impact of ARAIM Nominal Bias Bounding Techniques on Final ARAIM User Performance

Receiver Autonomous Integrity Monitoring (RAIM) has been certified to provide lateral guidance in flight operations ranging from En-route to Non-Precision Approach (NPA). Recent developments in the RAIM algorithm science, namely Advanced RAIM (ARAIM), have suggested a future role in vertically guided operations down to LPV with a decision height of 200ft. However, more stringent requirements as a result of the vertical guidance application question the external risk or trust that is placed on the constellation service provision and may require the partial reduction of this risk through the use of a ground segment, identifying and removing threats and providing data through an ISM (Integrity Support Message). This ground segment should ideally be light and low-cost so not to replicate that implemented for SBAS. In addition the ISM latency should ideally be allowed as long as possible to obviate the challenging and expensive communications requirements as imposed, for example, on SBAS (6 sec Time to Alert). Furthermore, the ISM should be as simple as possible to ensure the data broadcast requirements can be met with a number of solutions from ATC, from local ground communications to GEO relay. Finally, the network should be light, in the sense of a sparse and global distribution of stations. In order to meet the defined role of the ground segment and its monitoring capability; three possible methodologies were identified: No ground monitoring, Offline Monitoring, Real-Time Monitoring. The parameters of interest to this monitoring are the input parameters defined for the ARAIM baseline airborne algorithm as given below: • URA/SISA: Standard deviation of ranging measurement for integrity • URE/SISE: Standard deviation of ranging measurement for nominal accuracy/continuity • Bnom: Maximum nominal bias on ranging measurement • Psat: Prior probability of fault in satellite per approach • Pconst : Prior probability of fault affecting more than one satellite in constellation per approach This list of parameters contains the maximum nominal bias Bnom. A nominal bias is a fault-free bias, both to account for near-constant uncorrected errors (signal deformation and antenna bias) and non-Gaussian behaviour. However, some small bias may be included in this Bnom parameter. Indeed, after application of all possible corrections, iono-free smoothed code ranges are affected by residual ephemeris plus satellite clock and payload group delay error w.r.t constellation reference frame and clock. The residual ephemeris plus clock error (URE), residual tropospheric error, and multipath plus noise error, are all assumed to be random errors overbounded by zero mean gaussian errors with known modeled variance. However, note that the residual ephemeris plus satellite clock error may include a permanent bias, labeled here nominal bias. These ionofree smoothed code ranges are also affected by the receiver clock offset, defined as the common propagation delays from antenna to signal processing stages, also defined as the error identical to all measurements of the same constellation, which varies across constellations (time reference, signal) and the receiver design. Note that the receiver clock offset may include residual payload plus ephemeris delays identical to all satellites used in the position computation, so may vary depending on the set of satellites used in this computation. The iono-free nominal bias may then be defined as the permanent bias in excess of the residual error identical to all measurements of the same constellation, and from this definition may therefore depend on the receiver clock offset. Proper overbounding of this nominal bias by a ground segment to be sent via an ISM may therefore be difficult depending on the size of unmodeled values. On the ARAIM user side, the nominal bias affects the measurements but is not used in the MHSS detection threshold determination, although it is partially reflected with the URE. So the problem is that bounds on the nominal bias are to be known by the ARAIM user receiver, and impacts ARAIM user integrity and availability. Therefore, Bnom bounds on biases from ISM shall cover with high probability all nominal biases for all users. On the ARAIM ground segment side, it is anticipated that Bnom is determined from integrity analyses and accounts for nominal signal deformation and antenna biases as well as other potentially non-zero mean error contributions. But it is felt that clarification on the definition of these nominal biases, analysis on potential techniques to be used on the ARAIM ground segment, determination of their performance, and final impact on ARAIM user is needed. The aim of this paper is therefore to determine the size of the nominal biases affecting the user measurements, the capacity of the ground monitoring network to provide a pertinent Bnom, and the impact on the ARAIM user performance. This methodology could allow determining possible restrictions on ARAIM user receiver characteristics.