Loading...
Please wait, while we are loading the content...
Aireon Independent Validation of Aircraft Position via Space-based Ads-b
| Content Provider | Semantic Scholar |
|---|---|
| Author | Dolan, John |
| Copyright Year | 2018 |
| Abstract | The Aireon Hosted Payload (AHP) on the Iridium NEXT satellite constellation provides global ADS-B (Automatic Dependent Surveillance – Broadcast) coverage that is achieved by each payload covering a portion of the Earth’s surface. The nature of the satellites’ polar orbits and the size of the payload’s footprints allows for coverage overlap between adjacent payloads. This overlap creates regions where ADS-B transmissions are detected by more than one AHP and allows for two or more measurements of the same information. These measurements can be used to perform Time Difference of Arrival (TDOA) calculations that Aireon has incorporated into a position validation algorithm allowing for verification of an aircraft’s reported position independent of GPS. This independent validation algorithm augments Aireon’s surveillance system to be resistant to spoofers (devices that are intentionally transmitting incorrect positions), faulty avionics, and GPS outages. This validated data allows Aireon to greatly improve the safety and robustness of any system using its ADSB data. The validation algorithm utilizes two primary techniques: first, using two or more satellites to perform TDOA calculations and verify the aircraft reported position is within a configurable distance from truth; second, use of the aircraft’s kinematics to persist and verify their validation state in regions where there is no satellite overlap. Given the size of the AHP’s coverage footprint the regions of single satellite coverage only exist near the equator, in fact above 43° and below -43° latitude all aircraft are always covered by at least two satellites. But even at the equator where coverage overlap is reduced, an aircraft still has an 80% chance of being covered by more than one satellite. The regions where TDOA calculations cannot be performed change rapidly due to the procession of the satellite constellation around the Earth. In the worst case if an aircraft is only covered by a single satellite at the equator it will reenter redundant coverage in less than four minutes. During those times the kinematic portion of the validation algorithm will take over to ensure the reported data is correct. Aireon has been receiving operational ADS-B data from the available satellites in orbit for over a year. Using recent data while 47 satellites were in operational orbit showed that there were over one million TDOA opportunities in sixteen million total reports collected in a single hour. In April of 2018 Aireon began receiving Precision Timing and Position (PTP) messages from Iridium which provides the necessary accuracy in both timing and satellite position to perform the TDOA calculations. With these PTP messages Aireon has begun testing the validation algorithm on operational data with great success. Figure 1 shows the results of successfully validating the position of transmitters in the Azores that are part of the ERA surveillance system to within 0.5NM via TDOA opportunities. Furthermore, Aireon has identified multiple cases of faulty avionics that are reporting incorrect positions in their ADS-B data and have successfully invalidated them using this algorithm. This paper will outline the validation algorithm at a high level and show measured results of the algorithm from data recorded by the Aireon system. Figure 1: TDOA Results on ERA Ground Transmitters in the Azores [1] I. Overlapping Coverage & TDOA The primary component to the validation process is the ability to perform TDOA calculations which requires coverage by more than one satellite. In the context of the Aireon system coverage is defined by the elevation angle of the aircraft, Figure 2 shows the angular relationship between the satellite, target, and Earth. In this context the target is the aircraft of interest with ε representing the elevation angle [2]. Figure 2: Definition of Angular Relationships [2] The original design target of the Aireon hosted payload’s coverage was 8.2 degrees of elevation [3] from the aircraft perspective, shown in Figure 3. It was immediately clear upon receiving data from the first Aireon payload that the actual performance far exceeded that objective. The measured elevations have been recorded as low as -4.6 degrees for some aircraft and it has been concluded that a more realistic coverage elevation is 0 degrees [4]. Figure 3: Satellite Overlapping Coverage at Elevation Angle of 8.2° This new information opened the possibility for a heavier reliance on TDOA in the validation process which would be far superior to earlier design ideas such as pure range checking, beam-based, and probability-based validations. The new footprint size changes the predominant coverage type from single satellite to triple (or greater) satellite coverage. Figure 4 shows the updated overlapping coverage using the 0-degree footprint which provides for: 1. Persistent overlapping coverage at ±43° 2. Global overlapping coverage roughly 94% of the time 3. 80% probability of overlapping coverage at the equator (worst case) Figure 4: Satellite Overlapping Coverage at Elevation Angle of 0° Figure 5 shows the difference between the peak time of single satellite coverage using the original 8.2-degree design and the 0-degree footprint observations for given latitude values calculated via a three-hour simulation. These values are averaged across multiple single satellite coverage events which can vary in time but gives a representative of how much time an aircraft can expect to experience inside a single satellite coverage event. Figure 5: Peak Single Satellite Coverage Durations by Latitude This improved duration of TDOA opportunities from the 0° satellite footprint leads to a high probability and average number of satellites in view of any given location. Figure 6 shows the probability of being under two or more satellites at any given time at any given location. This value bottoms out at about 80% meaning there is a very high probability, even at the equator, of having a TDOA opportunity. Figure 7 presents this information differently and shows the average number of satellites covering any given point, with the lowest value being two. Figure 6: TDOA Probability Figure 7: Average Satellite Overlap The impact of this redundant coverage has been seen in the operational data with over one million TDOA opportunities observed in sixteen million reports collected in a single hour shown geographically in Figure 8. In this test we are checking for any messages that are seen on two satellites and contain the same information and are spaced by less than 20 milliseconds. Figure 8: Example of Global TDOA Measurements in 1 Hour The importance of the TDOA is that it provides an independent measurement that can be compared against the position data provided by the target aircraft, which in a noiseless environment would be calculated as: TDOA = (TOMR1 − TOMR2) = (TOMR1 − TTX) − (TOMR2 − TTX) ∆robs = c ∙ TDOA = |rsat1 − racft| − |rsat2 − racft| Where TOMR1 and TOMR2 are the Time of Message Reception at each satellite and TTX is the time of transmission, which is unknown. The r values indicate the position of the two satellites and the aircraft as vectors. II. Validation Methods Validation as described in this document is the indication that a position can be trusted to within a certain distance. It is the intent of Aireon to validate all ADS-B data delivered regardless of the type of satellite coverage. The previous section described the use of the overlapping satellite coverage to get TDOA calculations, but data will still need to be validated during the single satellite coverage periods. There are three possible validation states, each of which can be broken down into further levels of granularity: Valid, Invalid, and Unknown. After TDOA validation has been performed the validity of an aircraft’s position information will be known: either that it’s invalid beyond a configurable distance threshold or valid within some quantized distance (e.g. 1.9 NM). Once the aircraft exits overlapping satellite coverage the validation state can be updated using the reported velocity. ADS-B velocity does not use GPS position but instead utilizes doppler shift calculations using the relative motion of the GPS satellite with respect to the aircraft. This method of calculation makes the GPS derived velocity, in a way, independent from the ADS-B position, therefore will not need to be independently validated. Given the duration of single satellite coverage and requiring a TDOA validation to initiate any validation state using the velocity introduces a very low risk to report false validation information. Provided with two possible scenarios: a malicious spoofer or an unintentional piece of faulty avionics. In either case the goal is to report when the reported position does not correspond to the actual. In the first case the spoofer can be eliminated using several techniques, first and foremost is a simple range check; if the aircraft is outside the maximum possible range of the satellite it is clearly a bad position. When the spoofer is detected by more than one satellite the TDOA calculation will prove that it is invalid. In the second case, unintentional bad data, the procedure is the same unless the issue is sporadic. In that case an aircraft can start as valid and then begin reporting bad data at any time. In this case if observed by two or more satellites the TDOA will invalidate the aircraft position and if under a single satellite the “bad” position will not line up with any previously validated positions and the velocity. Finally, the validation metrics must be reported. In the CAT021 ASTERIX report there is a field that indicates if an Independent Position Check has been performed and failed [5]. Aireon intends to use this field to indicate if a CAT021 report is suspect (validation distance is beyond a configurable value). To augment the single bit reporting, there is currently work ongoing in ED-142A with proposals o |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.icao.int/NACC/Documents/Meetings/2018/ADSB/App4-Aireon%20Independent%20Position%20Validation%20Paper.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
