# The Impact of Space-Based AIS Antenna Orientation on In-Orbit AIS Detection Performance
## Materials and Methods  
### AIS Satellite Antenna  
The AIS satellite is typically small; a nano-sized or micro-sized satellite is suggested. The size limitation of the AIS satellite may cause a variety of problems that need to be addressed, and one of the main problems is the antenna placement. The small size of the satellite will limit the space and number of onboard antennas, which will further cause problems for communication. Smaller antennas lead to smaller power-to-noise ratios, and therefore reduced signal reception. For a nano-sized satellite, the reflector antennas are likely to require too much mass and volume and are therefore mostly ruled out as an option even though there is a nanosatellite using deployable Yagi-Uda type antenna [16]. Still there exists a variety of thin linear antennas suitable for this type of satellite. A short linear conductor is often referred to as a short dipole. The length of the conductor must be in the order of magnitude of the wavelength of the carrier for the antenna to function properly, and different conductor lengths result in different field patterns. Even though LAPAN-A2 is not a nanosatellite, it is a small satellite carrying multiple payloads; hence there is a space limitation to place the directional antenna or dual polarization antenna. Figure 3 illustrates the field pattern for a vertically polarized single \(\lambda\)/4 dipole antenna. This \(\lambda\)/4 dipole antenna is used when there is not enough space in satellite body for a \(\lambda\)/2 dipole antenna. A conductor of \(\lambda\)/4 can act as the antenna, and the frame of the satellite can be impedance-matched to fit as the other half. The disadvantage of this technique is the reduction of efficiency by a factor of two, compared to the full-sized \(\lambda\)/2 antenna [17]. The polarization of the electromagnetic field, which is radiated by a dipole-transmitting antenna corresponds to the orientation of the antenna. A dipole antenna can be oriented horizontally, vertically, or at a slant. The receiver antenna is most sensitive to electric fields whose polarization is parallel to the orientation of the antenna. This means that a vertically polarized antenna will not be able to receive a horizontally polarized carrier. To compensate for this, a cross dipole might be an adequate solution. However, this type of antenna for a small satellite is typically difficult to accommodate. The signal loss due to polarization mismatch can be found by:  
\[\text{LpoldB}=-20\cdot\log\cos\left[(\Delta\partial)\right] \tag{1}\]  
where \(\Delta\partial\) is the polarization mismatch angle. Figure 3: The typical field pattern of a monopole \(\lambda\)/4 antenna. Besides the polarization mismatch angle, AIS Satellite reception also suffers from atmospheric influence since the typical linear polarization AIS signal from the ships will pass the atmosphere to the satellite. The typical atmospheric influences, are Faraday rotation, Rain attenuation, and also Multipath distortion. The Faraday rotation is the changing or rotation of linear polarization due to ionospheric influence and is expressed by:  
\[\Delta\partial_{\mathrm{Faraday}}=\left(\frac{e^{3}}{2\pi m^{2}c^{4}}\int_{0}^ {d}n_{e}B\,ds\right)\lambda^{2} \tag{2}\]  
where \(e\) is a charge of an electron, \(m\) is the mass of an electron, \(c\) is the speed of light, and \(B\) is the magnetic flux density in the direction of propagation. The density of the electron is denoted as \(n_{e}\) and may vary along with path \(d\). The choice of antenna technology can positively influence the probability of detecting an AIS message in a signal band with interference. This is accomplished by exploiting the following phenomena, which are related to the choice of antenna technology:  
* Polarization diversity
* Frequency diversity (i.e., Doppler shift)
* Power diversity
* Geometric diversity (radiation pattern)  
for the choice of antenna, various technology options are available.