Space-based composite Ads-b and multilaTeration systEm validation thRough scalable simulAtions

Introduction

Automatic Dependent Surveillance–Broadcast (ADS-B) is a surveillance technology in which airborne equipment automatically broadcasts aircraft location to ground stations.

ADS-B is one of the pillars of state-of-the-art Air Traffic Control (ATC) systems and today it is considered as the future air traffic surveillance systems.

Unfortunately, it has some drawbacks related to the use of GNSS data and the use of open (and not secured) protocol, and usually, a secondary independent surveillance system is needed.

For this purpose, SATERA aims to formulate and validate the concept of a space-based ADS-B signals multilateration system leveraging a constellation of LEO satellites.

SATERA will use hybrid localization techniques combining time-of-arrival (ToA), angle-of-arrival (AoA) and frequency-of-arrival (FoA) measurements, leading to the concept of enhanced multilateration (E-MLAT) from the Space.

Concept

SATERA proposes the design, development, and proof of a space-based composite ADS-B + E-MLAT system concept. This idea arises mainly from the need to verify and validate the GNSS data received on the ground from space-based ADS-B systems by means of an independent (i.e., non GNSS-based) system.

The concept proposed by SATERA will consider that the receiving stations are on board of small satellites in a constellation deployed in the low Earth orbit (LEO) to provide the space-based ADSB service. The ADS-B information broadcasted by the aircraft will be received on-board each satellite of the constellation by specific equipment that will measure one or more characteristics of the electromagnetic waves carrying ADS-B messages (e.g., ToA, AoA, and FoA).

This information will then be relayed between the different satellites and sent through a downlink connection to the central processing station (CPS) on the ground. As the space channel is harsh and quite band-limited, appropriate data encoding (including compression) and routing algorithms will be used to prevent the measured data from being affected by the channel noise and by network congestion, thus ensuring that the required information reaches the CPS on time to be properly processed.

Once data reach the CPS, they will be processed using multilateration techniques, and the position of the aircraft will be estimated. The estimation will be compared with the reported position provided in the ADS-B messages and an integrity indicator will be added to the ADS-B Target Report. Lastly, both reports (ADS-B and E-MLAT) will be injected into the ATM surveillance chain for ATC purposes.

The satellite constellation. In a space-based multilateration system, the receiving stations are on board of the constellation satellites. Hence, the quality of the aircraft computed position will be affected by the dilution of precision (DOP) caused by the relative position and speed of the aircraft and the satellites, as well as by the number of satellites within the range of the aircraft ADS-B emitter. Thus, the configuration of the constellation shall ensure that the satellite’s orbits and revisit times (that will determine the coverage area for each satellite at any time) will allow the system to meet the required performance expressed in terms of an error bound estimator (e.g., the Cramér-Rao lower bound) while optimally using the space resources (i.e., minimising the number of satellites) and guaranteeing the required network connectivity. To ensure the independence of the MLAT-generated position data from GNSS-derived ones, satellite positions shall not solely rely on GNSS data. Therefore, the SATERA satellite constellation design will include mechanisms to accurately determine the satellites’ positions along their orbits without using GNSS positioning or, at least, to enable an integrity assessment of GNSS data before they are used by the localization algorithms in the central processing station.

The receiving stations. Receiving stations will be in charge of listening to the ADS-B messages broadcasted from aircrafts and measuring the physical characteristics of the electromagnetic waves carrying them. The receiving stations will consist of a radio front-end in charge of receiving the electromagnetic signal at 1,090 MHz and down-converting it to a frequency at which the signal may be digitised and processed to measure the time of arrival and the Doppler shift. The radio front-end will also allow to measure the angle of arrival of the received signal. Measuring other aircraft position related parameters (not only the ToA) can be very useful in satellite based MLAT because they can contribute to improve the DOP of the solution (or, in other words the condition number and the ill-conditioning of the inverse problem to be solved to compute the aircraft position).To measure the ToA and to timestamp the digitised messages, the receiving stations shall have a local onboard clock synchronised with those onboard the rest of the cluster of satellites used to multilaterate the aircraft position. For similar reasons, the local oscillators used to measure the Doppler shift shall have a controlled drift. SATERA will design and validate the receiving stations’ architecture considering the components currently used in receiving stations on the ground (i.e., not space grade). Since these components (generically commercially off-the-shelf -COTS- components) are sensitive to space radiation and may have relatively high rates of hardware transient faults, the proposed architecture will use software implemented fault tolerant (FT) techniques (or hybrid FT techniques) to ensure that the receiving stations and associated processing can tolerate the transient faults induced by space radiation and provide a similar level of reliability as receiving stations on the ground.

The communications network. This component will be tasked with getting the relevant data from each receiving station to the CPS, where they will be correlated (i.e., associated to a single emitter) and the position of the aircraft calculated. The communication will be performed through two subnetworks: inter satellite link (ISL) and downlink (DL). The ISL will connect nearby satellites between them so that data measured on board a satellite can be seamlessly routed through the constellation until reaching a satellite from which a downlink connection with a suitable ground station is feasible. Being ATC a real time process, it is crucial that aircraft positions are timely computed, and the ADS-B data validated. Moreover, data measured from the same emitted signal must be received within a rather narrow time window. The communication latency will be mainly introduced by the ISL subnetwork. Therefore, to prevent ADS-B validated data from exceeding the data age required by EUROCAE standards ED-142A and ED-129C, the ISL subnetwork will use: (1) a routing algorithm that will consider the state of the constellation to find the optimal path between the satellite where the information is generated and that from which it will be downloaded to the ground; and (2) robust error detection and correction encoding, as well as efficient compression algorithms to optimally address the bandwidth and noise issues posed by space communication channels. On the other hand, the DL will be a direct radio link between one satellite of the constellation and a ground receiving station (i.e., the ground segment of the constellation). In any case the performance of the communication network shall be taken into consideration when assessing the constellation’s DOP for design purposes.

The central processing station. This component will be responsible for receiving and processing the data sent from the satellites through the communication network and generating Target Reports (TR) suitable for ATC purposes in ASTERIX Cat. 20 and 21/ 533 Data received at the CPS will encompass ADS-B reports and signal’s parameters measured on board the satellites by the receiving stations. The CPS will first correlate the measurements (i.e., associate them to a particular emitter) and will check their integrity (e.g., by detecting garbled or apparently erroneous data). Afterwards, the CPS will compute the time difference of arrival between a reference station and the rest of the cluster (TDOA), collect the other available measurements (such as AoA or frequency difference of arrival – FDOA–), and set up the corresponding system of non-linear equations using the best suited combination of measurements. The CPS will carry out an independent estimation of the aircraft position by solving this set of equations using advanced mathematical algorithms. To ensure the independence of the computed MLAT position, the E-MLAT system will be designed so that is has no common failure mode with the ADS-B system (i.e., the knowledge of the satellites’ position will not rely solely on GNSS data, and neither will the synchronisation scheme).
Once MLAT positions are obtained, they will be processed by an advanced tracking algorithm to further improve them (by using the receiving station error model to remove any bias and to smooth the measurement noise) and to estimate additional state parameters (e.g., velocity vector, acceleration, etc.). The position estimated by the multilateration system, along with other measurements coming from the satellites, will be finally used to compute an integrity estimator for the ADS-B reported position. The estimator will consist of a scale of discrete values, each of them representing a range of differences between the reported GNSS position and the one provided by the multilateration system. Finally, the received data will be used to assemble target reports in ASTERIX CAT020 (MLAT data) and CAT021/053 (ADS-B data).

SATERA Expected Outcomes

1. Functional requirements of a space-based composite ADS-B + E-MLAT
2. Evaluation tool of space-based composite ADS-B + E-MLAT surveillance system  ​
3. Simulator of MLAT receiving stations
4. Communication network simulator ​
5. Simulator of the central processing station​
6. Space-based E-MLAT systems’ performance prediction tool ​