Assessment of ARAIM Availability in Brazilian Territory: Operational Evaluation for Civil Aviation
Glaucia Balvedi, Osamu Saotome, ITA; Joao Francisco Galera Monico, UNESP
GNSS integrity methods were developed to improve aircraft navigation through integrity monitoring, crucial in safety critical applications scenarios. One of the first integrity monitoring implementation based only on airborne sensors, namely Aircraft Based Augmentation Systems (ABAS), is the classical Receiver Autonomous Integrity Monitoring (RAIM) algorithm, widely used in current aviation since mid-1990´s to support supplemental navigation in the en-route and terminal phases of flight, and lateral guidance during the approach phases of flight. Based on GPS L1 signals, the RAIM algorithm are not used for vertical navigation, as it cannot support the vertical errors bounds required for vertical guidance.
A second generation of integrity algorithms using multiple constellations and dual-frequency was developed in order to ensure integrity in both lateral and vertical navigation in some circumstances: the Advanced RAIM (ARAIM). Two different ARAIM modes are envisioned: horizontal (H-ARAIM), to support near-term, proposed multi-constellation applications, and vertical (V-ARAIM), to be implemented subsequently once sufficient data is collected and safe operations is demonstrated. One of the main differences between ARAIM and RAIM is the capacity of ARAIM to adjust its operation to different requirements. Integrity parameters, fault probabilities, and even the probability of missed detection are hard-coded in RAIM, whereas in ARAIM these parameters vary according to the requirements.
It is expected that H-ARAIM is more robust than RAIM for two levels of service, RNP 0.1 and RNP 0.3. The ARAIM Working Group also focuses on ARAIM architectures to support LPV-200 or LPV-250 services globally.
In this paper, an assessment of ARAIM availability in Brazilian territory is presented, together with an operational evaluation considering civil aviation requirements. Data from Global Navigation Satellite Systems (GNSS) receivers belonging to the Brazilian GNSS-NavAer network were used in order to calculate protection levels derived from ARAIM algorithm, horizontal and vertical modes.
Six different locations in Brazilian territory were considered for this assessment, chosen based on their distance to the magnetic equator. In this way, results from different geomagnetic latitudes could be compared. For each station, two months of data were analyzed: one month corresponding to the period of high ionospheric scintillation activity (September to March), one month corresponding to the period of low ionospheric scintillation activity (April to August), both during the year of 2020.
The following ARAIM modes were simulated: H-ARAIM GPS+GAL IFREE, and V-ARAIM GPS+GAL IFREE. The RAIM GPS L1 mode were also simulated in order to compare the availability results for RNP(x) with those provided by the H-ARAIM GPS+GAL IFREE. The availability for each considered station were calculated for the following scenarios: different months, different periods of the day (day and night), using all satellites in view, and using satellites presenting scintillation index S4 below certain thresholds in order to simulate loss of continuity.
Simulation results indicated that the H-ARAIM GPS+GAL IFREE mode is very robust for RNP 0.1 in Brazilian territory. Protection levels well below 220m were achieved in the considered scenarios, different from what was achieved using RAIM GPS L1.
V-ARAIM GPS+GAL IFREE also presented interesting results: LPV-250 is fully operational in northeast of Brazil, and partially operational depending on the period of the day in other locations. Considering a time window of 24 hours, horizontal protection levels less than 40m is achieved 96% or more of the time, and vertical protection levels less than 50m is achieved 86% or more of the time.
These results demonstrate the positive impact of the deployment of dual-frequency and multi-constellation systems and ARAIM integrity algorithm for RNP and LPV implementation in Brazilian territory.
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