Radio and optical alignment method based on GPS

Abstract

Point to point communication in free-space is severely dependent upon the alignment of the transmitter and receiver devices. The simplest low cost method for the alignment is achieved by utilising two geographical coordinates and an electronic compass. However, some regions of the Earth have a strong magnetic deviation that can introduce large errors to such systems. Other known methods, that can be utilised are a radio direction finder or stars sensor however these methods are too expensive. Here, we present a system which uses three GPS coordinates for the alignment of the transmitter and the receiver, of which two coordinates are measured on the transmitter side, while the receiver is previously known. The transmitter side positions can be relocated for convenience. The methods were tested using Google™ Maps for a long distance within the northern and southern hemisphere, while the experiment was performed for a short distance within the southern hemisphere. The system was developed based on the following considerations: •Algorithm Implementable into a Micro-Controller Unit (MCU) or a standard computer.•The local magnetic deviation does not have any influence on the method.•Can be use where the internet connection is not available, such as mountains and others remote areas.

Keywords: Coarse alignment for optical communication and fine alignment for radio communication; Optical transmission; Radio link; Tracking system.

Graphical abstract

Fig. 1

Magnetic deviation in Reunion Island. During an experiment, using a theodolite and a compass. The magnetic deviation in Piton Textor was greater than 20°.

Fig. 2

First alignment step: The initial direction of the laser $L_1$ is aligned with the $x$ -axis which is the same direction of the maximum gain of the antenna or the laser used for the transmission. The $z$ -axis is aligned with the direction of the position vector ${\overrightarrow{R}}_{\text{T}\text{X}}$ (position of the transmitter). In the first step the reference frame$x$, $y$ and $z$ is rotated around the y-axis until the $x$ -axis is directed towards the position of the reference station. The new reference frame becomes $x^{\text{'}}$, $y^{\text{'}}$ and $z^{\text{'}}$.

Fig. 3

Second step followed by the algorithm. The new reference frame $x^{\text{'}}$, $y^{\text{'}}$ and $z^{\text{'}}$ is rotated by an angle $\varphi$ in order to obtain a new reference frame $x^{″}$, $y^{″}$ and $z^{″}$, where the plane $y^{″}$ and $x^{″}$ lie on the plane $\pi$.

Fig. 4

The last step involves the alignment of the transmitter to the receiver: After the previous rotation, the laser will aim in the direction of the reference station. By the rotation around $z^{″}$ of an angle $\theta$, the transmitter is aligned with the receiver.

Fig. 5

The planes and the relative angle used to calculate the directions of rotations. The position vectors are taken with respect to the centre of the Earth and are calculated by the WGS84 standard. By the GPS it is possible to measure the position on the Earth surface, the altitude from the geoid and from the ellipsoid.

Fig. 6

Experimental verification of the system performed for a short distance using double precision.