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Automatic Dependent Surveillance-Broadcast (ADS-B) is an air traffic surveillance technology in which aircraft broadcast position, identification and status an average of 6.2 times per second on 1090 MHz. The Royal Military College of Canada has developed an ADS-B receiver that is scheduled to fly as a technology demonstrator on the Canadian Advanced Nanospace eXperiment-7 (CanX-7) nanosatellite. A signal propagation model was developed to determine the power level and number of signals that will be detected by CanX-7. Since the ADS-B messages are alternately transmitted from upper and lower antennas, both the direct and reflected signals were considered. A simulation using the model was run over the North Atlantic with aircraft data supplied by air traffic services and a satellite altitude of 800 km. Power at the receiver for reflected ADS-B signals ranged from -109.5 to -118 dBm depending on aircraft-satellite geometry and was approximately 18 dBm less than the direct path signal strength. With a sensitivity of -103 dBm, the CanX-7 ADS-B receiver should detect virtually all of the direct path signals while reflected signals are below the detection threshold. Although the reflected signals should not be a factor for the CanX-7 mission, they could be a consideration for a large operational satellite with a more sensitive receiver. The reception of both direct and reflected ADS-B signals from multiple aircraft could lead to signal collisions and subsequent loss of aircraft tracking information, particularly in coastal regions where there are additional sources of the 1090 MHz signal.

Automatic Dependent Surveillance-Broadcast (ADS-B) is an air traffic surveillance technology in which aircraft routinely transmit aircraft identification, position, velocity and status during flight. Commercial air carriers use 1090 MHz to broadcast the 120-bit message at random periods between 0.4 and 0.6 seconds to prevent aircraft from having synchronized transmissions. The ADS-B signal alternates between top- and bottom-mounted quarter-wave monopole antennas, with transmitter power between 75 W and 500 W depending on the aircraft category [

ADS-B coverage is currently limited by the placement of ground stations that cannot be installed in mid- ocean and are difficult to maintain in Polar Regions. An orbital ADS-B system has the potential to provide precise aircraft surveillance in remote and oceanic airspace, which would transform existing traffic procedures that use standardized routes and large inter-aircraft spacing to provide separation. An operational spaceborne ADS-B system would reduce aircraft spacing requirements in oceanic regions leading to more efficient routes and subsequent reductions in fuel consumption. These efforts are topical with the announcement that ADS-B receivers are planned as secondary payloads on the Iridium Next constellation [

The Royal Military College of Canada (RMCC) has been involved in space based ADS-B research since 2009, conducting successful high altitude balloon missions and publishing research in the field [

This paper describes the method and results for ADS-B signals reflected from the ocean surface. Section 2 describes the simulation setup; Section 3 focuses on sea surface scattering; Section 4 discusses the simulation results and Section 5 contains conclusions.

The signal propagation model used for the simulation is written in the Python programming language. Aircraft and satellite positions, environmental data, and antenna radiation patterns are input to determine the power of the reflected ADS-B signals reaching a satellite receiver. The model updates once per second, interpolating both aircraft and satellite position and determining relative geometry between transmitter and receiver. The following is a description of aircraft-satellite geometry, reflected signal power and atmospheric effects. Specific parameters used for the simulation are described in Section 4.

The geometry for ADS-B signals transmitted by aircraft to satellite receiver is shown in

in which G_{1} is the distance from the sub-aircraft point to the specular point, Point B. Once G_{1} is determined, the remaining unknowns are readily calculated. This method uses an effective Earth radius, a_{e}, described by Equation (2)

where k is the radius coefficient and R_{e} is the Earth radius (6371 km). This factor corrects for atmospheric refraction and permits the assumption of straight ray paths. The value of k varies between 1 for paths perpendicular to the surface to 4/3 for very low grazing angles [

Rather than develop a full ray tracing algorithm that would account for the gradient of the refractive index with increasing altitude, a relationship to vary k as a function of elevation angle, θ_{d}, was developed in which

For each aircraft-satellite geometry Equation (1) is initially solved for a value of k = 1.0, then a new k value is calculated from the resultant θ_{d} using Equation (3). This iterative process is repeated until the solution converges with a tolerance of ∆k < 1 × 10^{−7}. This solution permits k ≈ 4/3 at low grazing angles and reduces to k ≈ 1.0 at high grazing angles. This approximation was deemed acceptable in light of the greatly reduced computation requirement for the model.

Referring to

where

and

Noting that,

for i = (1, 2). The reflected signal path lengths (R_{1}and R_{2}) are determined using the law of cosines to triangles ABC and BCD as shown in Equation (8)

The depression angle, θ_{r}, of the reflected path, is found by using the law of cosines on triangle ACD

The grazing angle, ψ, can be found using triangle ABC

The bi-static radar equation used to calculate the signal strength observed at the satellite for the sea surface reflection is Equation (11)

where P_{t} is the transmitter power, G_{t} and G_{r} are the gains of the transmitting and receiving antennas respectively, λ is the wavelength of the electromagnetic wave (0.275 m for ADS-B), L_{a} represents atmospheric losses, D is divergence and σ is the radar cross section (RCS).

Divergence is the weakening of the field strength caused by more rapid spreading caused by reflection from a spherical surface compared to the spreading rate before reflection. In accordance with Blake [

In the case of purely specular reflection, D is significant only when the grazing angle is small and can be simplified to Equation (13),

The determination of σ, which defines the amount of incoming energy reflected from the sea surface boundary, will be discussed in Section 3.

Potential atmospheric effects on the 1090 MHz signal include Faraday rotation, time delay dispersion, phase dispersion, time delay, ionospheric scintillation and absorption, tropospheric scintillation and absorption and rainfall.

Effect | Value |
---|---|

Faraday rotation | 12.5º |

Time delay | 26 ns |

Time delay dispersion | 83 ps |

Phase dispersion | 16.3˚ |

Ionospheric scintillation | Up to 20 dB at the geomagnetic equator at Equinox |

Ionospheric absorption | Negligible |

Tropospheric absorption | 0.114 dB in rain for multipath case |

Tropospheric scintillation | Negligible for elevation angles > 1˚ |

Rainfall | 0.017 dB in 100 mm/hr rain |

signal power. For reflected signals, neutral atmosphere effects are considered for the downward path of the signal and the subsequent reflected path toward the satellite receiver.

The scattering of the 1090 MHz signal from the sea surface is a key factor in the determination of power received at the satellite. The following subsections describe the calculation of the RCS used in the simulation in terms of scattering theory, reflection coefficients, mean square slope and glistening surface.

Efforts to quantify the magnitude of reflected energy from the ocean surface draw on scattering physics, surface wave hydrodynamics and the structure of the marine boundary layer [

The two most commonly used electromagnetic scattering models are the Kirchhoff Approximation (KA) [

The KA method may be compared with an exact numerical computation of Maxwell’s equations using a Multi-Grid Iterative Approach (MGIA) [

The radar equation, Equation (11), used to calculate the magnitude of the ADS-B signal relies on determining the NRCS (σ˚), for a given surface roughness and then multiplying it by the area that contributes to the reflection to arrive at the RCS (σ). Kodis [

where η and ξ are the scattered and incident polarization states respectively, mss is the mean square of the total slope at a point on a two-dimensionally rough surface, ι is the local angle of incidence at the specular point, β is the angle between the mean normal to the surface and the local surface normal at the specular point, and R is the refection coefficient.

ADS-B antennas on aircraft are mandated to operate within 3 dB of an ideal quarter-wave monopole in the same plane of the aircraft [

where

R_{vh} is the reflection coefficient for a vertically polarized incident wave and a horizontally polarized scattered wave, and R_{vv} is the reflection coefficient for a vertically polarized incident wave and a vertically polarized

scattered wave. The quantities R_{v}(ι) and R_{h}(ι), shown in Equation (19) and Equation (20) are the Fresnel reflection coefficients for vertical and horizontally polarized waves [

where ε is the complex dielectric constant of seawater. For the simulation, ε was calculated as a function of sea surface temperature and salinity using a double Debye relaxation law [

With the geometry described in _{s} is zero or near zero, which means that R_{vh} = 0 from Equation (15). With φ_{s} = 0, the reflection coefficient for the simulation is a simplification of Equation (16) and is expressed in Equation (21)

Elfouhaily et al. [

Two main parameters used in oceanographic research to calculate wave spectra are the wind at 10 m reference height above the sea surface (U10) and the Significant Wave Height (SWH). SWH is defined as the mean wave height from trough to crest of the highest third (H_{1/3}) of the waves as this is the mathematical value closest to that estimated by trained observers. SWH values are available from oceanographic prediction tools such as the NOAA Wave Watch III model. These parameters alone are not sufficient to characterize wave spectra since distance the wind acts on the surface, or fetch, also has an effect. Wave height increases asymptotically with fetch to a limit where gravitational effects balance wind energy. The Elfouhaily model uses a parameter, known as inverse wave age (Ω_{c}), to describe the effects of fetch. For example, a fully developed sea has a value of Ω_{c} close to 0.84. The omnidirectional elevation spectrum, S(k), is the sum of the low and high wave spectra [

where k is the wavenumber, B is the curvature spectrum, and the subscripts l and h indicate the low and high frequencies respectively. ^{−3} to 10^{2} for U_{10} wind speeds between 3 and 21 m/s. Of note is the relative insensitivity of S(k) to changes in wind speed for wavenumbers between 2 and 10 rad/m.

For isotropic Gaussian distributed slopes, mss can be derived from the sea surface elevation spectrum by integrating over wavenumbers k as shown in Equation (23)

Elfouhaily’s model sets the limits of the integral to 0 and ∞. The upper limit is problematic as the wave spectra at high wavenumbers does not approach an asymptote as the spectra does at the low wavenumbers. To counter this problem while fitting experimental observations, Zavorotny and Voronovich [

As the calculations to determine the NRCS rely on the contribution of a unit area to the aggregate, it is necessary to calculate the area of the glistening surface (AGS). The term glistening surface defines the portion of the Earth’s surface that can contribute to the reflection for a given transmitter/receiver geometry. The model used by Beckmann and Spizzichino [^{2}β_{o} represents the mean square slope of the irregularities. This relationship allows the calculation of the maximum surface slope from S(k) as shown in Equation (25)

Normalizing by the NRCS at the specular point (β = 0) yields a step function [

This function represents the multiplication of the NRCS by the area bounded by β = β_{o}. An easy way to visualize this is to think of a cone with a vertex angle of β_{o} coming from the aircraft and centered on the path from the aircraft to the specular point (path AB in

As the maximum signal is received when the scattered angle, θ_{s}, is the same as the incident angle θ_{i}, β goes to zero and i = θ_{i}. This simplifies Equation (14) for the VV polarization case to Equation (28)

Determining AGS by the step function described in Equation (26) and Equation (28) results in Equation (29)

This value of σ can be directly substituted into the bi-static radar equation shown in Equation (11).

The simulation presented in this paper is based on a satellite in LEO passing over the north Atlantic Gander and Shanwick OCAs. The scenario was run for altitudes between 400 and 800 km. The receiving antenna is a circularly polarized patch antenna with a half-beam width of 55˚, similar to that planned for the CanX-7 mission. This gives a receiver footprint of approximately 2800 km at 800 km altitude. Aircraft position and altitude data was obtained from NAV CANADA for a 24-hour period on 29 April 2012 as shown in

Aircraft are fitted with antennas on the upper and lower surfaces to achieve spherical coverage. An average of 6.2 ADS-B messages are transmitted per second, alternating between the upper and lower antenna. The aircraft transmitting antenna for the scenario was the S65-5366 made by Sensor Systems, which is used by Boeing and Airbus commercial fleets.

from 3 to 21 m/s had no effect on the reflected signal strength since the increase in the AGS for rougher seas compensates for the reduced forward scattering per unit area. There is a null directly beneath the satellite and a marked decrease in strength at about 17˚ to 19˚. The null that occurs when the aircraft is at the satellite nadir is typical of a quarter wave monopole (

With a sensitivity of −103 dBm, the CanX-7 ADS-B receiver should detect virtually all of the direct path signals while reflected signals are below the detection threshold. For an operational constellation such as Iridium NEXT, which is likely to feature ADS-B receivers with greater sensitivity than a nanosatellite, the reflected signals could potentially be detected. The increase in signal density could lead to signal collisions and subsequent loss of aircraft tracking information, particularly near coastal areas where there are other sources of 1090 MHz including Modes A, C, S transponders and Traffic Collision Avoidance System (TCAS).

The CanX-7 nanosatellite, due for launch in 2016, will host an RMCC payload that will monitor aircraft-gener- ated 1090 MHz ADS-B transmissions over the North Atlantic. In an effort to determine the number and strength of signals reaching the receiver, a signal propagation model was created, taking into account neutral atmosphere and ionospheric effects, aircraft-satellite geometry and antenna radiation patterns. A signal propagation model was created to determine the strength of direct path ADS-B signals arriving at the CanX-7 receiver [

A simulation using the model was run over the North Atlantic with aircraft data supplied by NAV CANADA at satellite altitudes of 400 - 800 km. The simulation examined the power received at the satellite of reflected ADS-B signals for wind speeds of 3 to 21 m/s. The key findings of the simulation are as follows.

1) At an altitude of 800 km the received power of the reflected signal ranges between −109.5 to −118 dBm between 2˚ and 60˚ from satellite nadir and is approximately 18 dBm lower than the direct path signal.

2) There is no relationship between the wave height, which is related to near surface wind speed, and the received signal.

3) There is a null when the aircraft is at satellite nadir as a result of the quarter-wave monopole antenna radiation pattern.

4) For every 100 km reduction in satellite altitude below 800 km there is approximately a 1.5 dBm increase in signal strength for both the direct and reflected ADS-B signal.

The simulation indicates that the CanX-7 ADS-B patch antenna, with a sensitivity of −103 dBm, will not detect the reflected signals at the anticipated altitude of 800 km. However, the scattered signals could be a consideration for a larger satellite with a more sensitive receiver. In coastal regions in which there are increased sources of 1090 MHz, the cumulative effect of direct and reflected ADS-B signals could lead to signal collisions and loss of aircraft tracking information.

RichardVan Der Pryt,RonVincent, (2016) A Simulation of Reflected ADS-B Signals over the North Atlantic for a Spaceborne Receiver. Positioning,07,51-62. doi: 10.4236/pos.2016.71005