Technical Details

The ApRES radar uses the frequency-modulated continuous wave (FMCW) technique, in which a transmitted tone sweeps from 200 to 400MHz over a period of 1 s to form a chirp. The chirp received from a reflector arrives a short time later, and so is always at a frequency slightly lower than the tone presently being transmitted, the difference in frequency being linearly dependent on the range of the target. Mixing the received signal with the transmitted signal generates sum and difference frequencies, and the difference frequency (anything from 0 to a few kHz) can be selected by an appropriate filter and recorded. In the case of ApRES, the phase of the transmitted tone is carefully defined, and by tracking the phase through the processing it is possible to process the data using phase-sensitive techniques. The design and technical details for the instrument’s radar board are described by Brennan and others (2013).

The instrument has been specifically designed for long-term autonomous polar deployment. The instrument runs on a 9-20V source, with a maximum current at 12V of ~0.5A, which reduces to ~200 μA during sleep between measurements. A typical measurement will last for 1–2 min, coherently stacking 20–100 consecutive chirps, so that 100 Ah of battery capacity would power 10–20 measurements each day for a period of a year. The components used in the instrument are specified to a minimum temperature of –40°C. An Iridium data link is built into the controller, and also a GPS receiver, primarily to help with the precise timing of observations. The GPS fix is included in the transmission via the Iridium link, helping in the recovery of an instrument deployed on a moving ice sheet. The Iridium link is two-way, allowing remote reconfiguration of most aspects of the radar’s operation.

Instrument Installation

An ApRES installation consists of a main electronics box (housed in a waterproof plastic case with dimensions 410mm x 320mm x 170 mm, and weight 4.2 kg). Data are stored on SD memory cards, which should be SDHC SLC or MLC type and rated to low temperature. Attached to the radar are GPS and Iridium antennas, and a receive (Rx) and transmit (Tx) antenna pair, located 5-10 m apart. For surveys over multiple sites where antennas must be robust for transport, a demountable skeleton slot antenna is available (antenna pair 0.72m x 0.72m x 0.12m, 16 kg when packed). For autonomous deployments at a single site, it is important that the properties of the antennas do not change significantly during the deployment. An open-structure antenna such as the skeleton slot devices would need to be protected from filling with snow. A cavity-backed bow-tie antenna was developed to provide a cost-effective solution suitable for long-term deployment; it is housed in a corrugated plastic box, yielding a lightweight, flat-pack solution, at less than one-tenth the cost of the skeleton slot antenna, and which can be buried just below the snow surface.

A Quick-Start guide to ApRES set-up as well as more detailed instructions for both short-term and long-term deployments are found below. These instructions are designed for deployments using the demountable skeleton slot antenna. If a bow-tie antenna set-up is preferred, these can be constructed by the user. Assembly guidelines for the bow tie antenna are also provided below.