Advanced HF Communications for Remote Sensors in Antarctica

Abstract

There is a strong research activity in Antarctica in the fields of geophysics, meteorology, wildlife, flora, oceanography, and environment, among others. This research activity often involves the installation of sensors in remote places under severe/extreme weather conditions. Most of those sensors are operated with a data logger that stores the data between campaigns until it is recovered after several months. If we need either to have access to the data throughout the year, or to install a sensor far away from the Antarctic station, we should install a radio transmission system. The standard VHF radios have a reach up to 50 km with line of sight. As the installation of a repeater station is not an option in Antarctica, it only remains to install either a satellite link or an HF link. The satellite services are expensive and are provided mostly by geostationary satellites. The geostationary orbits around the equator are not always easily visible from the poles, so the link is not fully reliable. The HF band (3–30 MHz) is well known from the beginning of the age of the radio. The ionization of the upper layers of the atmosphere changes the direction of the radio waves in that band, so the ionosphere behaves as a mirror for a certain set of frequencies. The reflection is strongly dependent on the solar activity, the solar radiation at any time, the terrestrial magnetic field, and the angle of incidence of the wave [1]. For oblique incidence, a range up to 3000 km for a single hop can be achieved, so we can establish a link all over the planet with a few hops. The antenna in those applications should have a maximum of radiation toward the horizon. For near vertical incidence skywave (NVIS), we can achieve a circular coverage area with a radius up to 250 km without the need of line of sight. The most suitable frequency for vertical incidence is lower than in oblique incidence, so a larger size of the antenna is needed. The antenna should have a maximum of radiation upward, so horizontal dipoles, inverted V, and loops are the best options [2]. As the distance is much lower, the transmission power required can be reduced to tens of Watt, so the application to remote sensors with power restrictions is straightforward. In the world where an increasing number of devices are going to be interconnected in an Internet of Things paradigm, a system able to communicate sensors located hundreds of kilometers away without any additional infrastructure is really welcomed. A first step when developing a new physical layer is the sounding and characterization of the channel. Apart from the classical model of Watterson [3] for narrowband communications and Mastrangelo [4] for wideband communications, a significant amount of research has been done in ionospheric channels for a single hop at high latitudes in the Arctic [5, 6, 7]. However, few works can be found about channel sounding for long-range links with multiple hops from the Antarctica. For the last 15 years, our research group has been working in the application of HF communications (3–30 MHz) with ionospheric reflection for data collection of remote sensors in Antarctica. In particular, we have developed a system to communicate the Spanish Antarctic Station (SAS) Juan Carlos I at Livingston Island (62.6°S, 60.4°W) and the Ebre Observatory in Roquetes (Tarragona, Spain) (40.8°N, 0.5°E). It is a 12,700 km link with multiple hops and without the use of any repeater. First, we started to sound the channel and estimate the main parameters of the channel [8, 9, 10, 11, 12]. Then, we have developed and tested a wide range of modulations, with its frame structure, the radio-modem, and several antennas for both the long range with oblique incidence and the near vertical incidence scenario [13, 14, 15, 16, 17]. We are also developing a self-organized network of NVIS nodes that can handle the delays and the unavailability of the ionospheric channel. The NVIS nodes may behave as a hub able to collect data from other neighboring sensors and transmit the joint data to the central node. This chapter is organized as follows. In Section 2, the evolution of the hardware of the radio-modem and the antennas is presented. The modem is prepared for both channel sounding and data transmission and has evolved to low-cost software radio platforms. In Section 3, the results of more than 15 years of experience in channel sounding are presented, for both the oblique and the vertical sounding. The variability of the channel as a function of time, season, and year is summarized. In Section 4, the physical layer of the communication system of the modem is introduced. The modulation, the frame structure, and the synchronization techniques for both the long-range modem and the NVIS modem are described. The performance in terms of bit rate and bit-error rate is presented. In Section 5, new routing strategies for NVIS networks are explained. The NVIS node has to collect the data from the sensors nearby and establish a delay tolerant network (DTN) with the rest of the nodes. Finally, Section 6 contains the conclusions and some other applications of HF communications.