Even with significant advancements in transceiver technology, there are limitations to transceivers. By understanding how the technology works, rescuers can better interpret the information given by the transceiver, and will be able to apply alternative search strategies to overcome problems.
The first transceivers in 1968, called SKADIs, were built using the 2.275 kHz transceiver frequency. This frequency was chosen because it is audible to the human ear, experiences little electrical interference, and is able to transmit well though obstacles. In 1997, this frequency was changed to 457 kHz to improve range. The standard for frequency tolerance was also improved in 1997, to +/- 100 Hz signal drift, and again in 2001, allowing only +/-80 Hz drift.
Transceivers are now built with three antennas. The type of antenna used is made of a cylindrical iron core with a copper coil wrapped around it. The electromagnetic field, which is induced by applying a voltage to the coil, is transmitted out of the poles of the transmitting antenna in a three-dimensional butterfly shape, commonly referred to as the flux-line-pattern.
The longest antenna is orientated in the long axis of the transceiver. This antenna is the transmitting antenna, and the effective long-range receiving antenna. Keep in mind that only one antenna transmits at a time. Some devices are able to detect a vertical orientation of the transmitting antenna and can then transmit the signal on the antenna that is most horizontal (Ortovox 3+).
The following two principles are fundamental to understanding the performance and limitations of modern transceivers.
A course search is the process of orientating the receiver to the best reception of the signal strength (parallel to the flux line) while moving closer to the buried transmitter. Signal strength increases according to the inverse square law, which states that the received signal intensity is inversely proportional to the square of the distance from the source. This means signal strength increases exponentially as the searcher approaches the source, rather than in a linear fashion. As a result, the accuracy of the search improves the closer one gets to the buried transmitter.
The signal is transmitted in a series of pulses. The timing characteristics of the pulses are critical for transceiver performance, as the on-off pattern can be used by a receiver to separate signals of multiple burials.
Longer pulse lengths were originally used in analog transceivers. These relied on human hearing to discern a change in volume, but were more likely to mask other signals in multiple burials. In digital transceivers, shorter pulse lengths make for a shorter overlap of multiple signals.
Pulse length and period length are randomized between units of the same manufacturer to that the signal lap is kept as short as possible. During signal overlap, a digital transceiver cannot separate multiple burials.
A single-antenna transceiver has only one antenna for sending or receiving a signal. To find the direction of the flux line, the searcher must pan the transceiver to align the receiving antenna with the flux lines (strongest signal). This process is repeated while moving closer to the transmitter. The searcher has to process the acoustic and/or LED signals to locate the buried transceiver.
Transceivers with two antennas have the antennas orientated to each other at 90 degrees (length and width of transceiver). Both antennas are always receiving any incoming signal; in order for the processor to receive consistent signal strength indications from each antenna, the transceiver must be held horizontally.
Due to the shorter length of the second antenna, it may not receive a useable signal until the coarse search is well underway. Initially in the coarse search phase, the searcher will be given distance and direction indication along the flux lines. At first, the direction indication can be 180 degrees in the opposite direction from the subject along the flux line. When the second antenna receives a usable signal, the processor in the device uses the strength of the signals from each antenna to calculate the resulting vector, which is then displayed as the direction indication.
During a fine search, a dual-antenna device may indicate false maxima, or “spikes” on the surface of the debris when the transmitting antenna orientation is in an unfavourable position. This situation can cause issues when pinpointing with a probe.
The third vertical antenna is typically short compared to the other two antennas, and therefore has a much shorter range. It comes into play during the fine search (between 3-10 meters from the buried transmitter), and is used to process the vertical dimension of the flux lines. This greatly helps to more accurately determine the location at which to start pinpointing, and gives an accurate indication of burial depth when held on the snow surface.
During the signal and coarse searches, the flux line field is roughly two dimensional. In most cases, the burial depth is small compared to the horizontal distance between the transmitter and the receiver. The processor only has to interpret 2D flux lines, making two horizontal antennas sufficient. However, as the distance to the transmitter decreases, the flux lines become more and more three dimensional, especially with deeper burials. This requires that the receiver processes 3D flux lines.
When a transceiver is buried vertically, the flux lines become perpendicular to both horizontal receiving antennas (as shown in the red curve). The third vertical antenna is aligned with the flux lines, and will receive a strong signal (as shown by the blue curve). This helps avoid false maxima.
All transceivers receive the same signal from the transmitter, via the receiving antenna in the form of a voltage output. The difference between analog and digital is how the received signal is presented to the user.
Modern Digital Transceivers:
When a transceiver works in analog mode, the user is also able to hear the raw analog beep sounds that are being processed by the device. Having the option of an analog mode can be advantageous in some scenarios, for example, solving a deep burial using the fine search in a circle technique, and aiding in alternate search strategies for multiple burials if the marking function fails.
The range of the receiving antenna plays an important role when searching for the first signal. The signal strength (range) of the transmitting field is limited by the length and quality of the transmitting antenna, the power of the transmitter (typically 1.5 watts), and the standard transmit frequency (457 kHz). The range can be divided into three axes based on the orientation of the receiving antenna:
Even if a device has a large maximum range, it does not necessarily mean that the signal may be effectively traced towards the transmitter at this distance. An early signal reception is usually at the expense of an accurate directional guidance. Some devices have problems in search mode at the very start of a coarse search, where distance and direction indication can be inaccurate. This is known as the “fuzzy range”.
When searching an avalanche for a transmitting beacon, the distance between your zigzagging paths is called the “search strip width.” A wide search strip allows you to cover a large debris field more quickly, but you risk reaching the bottom of the avalanche without receiving a signal. A narrow search strip ensures that you’ll receive a signal, but it will take longer to search the entire debris field. Searches should take an adaptive approach to the search-strip width, narrowing it in difficult or rough debris, and maximizing it in open debris. The following table shows the measured, calculated, and published ranges of most transceivers:
Digital triple-antenna transceivers with a marker function is regarded as the current standard. This helps increase the accuracy and speed of the fine search.
Some devices provide basic and advanced configurations. On these devices, the basic configuration offers only a digital search mode with distance and direction indication, and a marking function, whereas in an advanced configuration, the searcher can access analog sounds, scroll through a list of buried subjects, and manually control the receiver sensitivity.
Many modern devices also include a compass sensor to provide an independent reference grid for the processor to update direction indication between signal inputs, tilt sensors to warn users about holding devices horizontally, and 3D accelerometers to detect erratic rescuer movement (if they avalanche as well, it will automatically switch from receiver mode to transceiver mode).