NECKLACE primarily uses Autonomous phase-sensitive Radio-Echo Sounders (ApRES) to measure ocean-driven melt on Antarctic ice shelves. The ApRES instrument is a robust, lightweight radar designed by researchers at BAS and UCL to allow long-term, unattended monitoring of ice-shelf and ice-sheet thinning. By using phase-sensitive processing techniques, the instruments can measure the depth of internal layers and the glacier base to millimetre precision

Ice shelf melt rates are calculated by tracking the change in ice shelf thickness, and removing the effects of internal thickness change by tracking internal layers in the ice shelf. Instruments are capable of monitoring melt rates at seasonal, weekly, and tidal timescales (Vaňková and others, 2020).

ApRES can also be used for a wide range of other glaciological applications, including mapping internal reflectors using an ApRES array in multiple-input multiple-output imaging mode (Young and others, 2018), inference of ice fabric characteristics from polarimetric measurements (Brisbourne and others, 2019; Jordan and others, 2020), and changing englacial meltwater content (Kendrick and others, 2018; Vaňková and others, 2018). ApRES are being used to obtain data in increasingly diverse environments including Antarctica, Greenland, and Arctic Canada.

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.


Manual VersionDateEquipment versionSummary of changes
Version 100.006-10-2014RMB2bNew version for RMB2 + VAB Issue C
Version 100.113-10-2014RMB2bUpdating figures. Minor text edits. Mention File Manager
Version 100.220-10-2014RMB2bDiscussion of power supply requirements; change to GPSon; contents list
Version 101.011-06-2015Description of uptell functionality; change to AFGain settings as a result of move to RMB2 Issue C.
Version 101.112-11-2015Corrections to screenshots and VAB photo. Active GPS antenna jumper
Version 101.230-01-2016Addition of Appendix with checklist for long-term deployment
Version 101.309-11-2016RMB2cMinor mods
Version 101.427-07-2017RMB2cDescription of additional functionality (TOD sync, battery check), FTP server functionality
Version 101.525-05-2018RMB2cDescription of additional config commands (ER_ICE, GPS_TIMEOUT, NO_IR_RETRIES, MessageTimeout), and averaging option for trial subburst.
Version 101.609-10-2018RMB2cDescription of additional commands (MonoTx, MonoRx, Alternate, TxAnt, RxAnt, Antenna_Select). Change to functionality of CheckEthernet SAF modes.
Version 101.707-11-2018RMB2cDescription of additional command (NSAFData). Revised guidance for updating firmware.
Version 102.022-08-2019Description of additional CheckEthernet (FTP) mode.
Version 102.110-03-2020Correction to Housekeeping data order.