How Does an Avalanche Beacon Work?

What is DVA?

• A standardized frequency

A DVA is a single-frequency transceiver that allows devices of different makes and models to receive each other. A European standard sets the frequency used by DVA (457 kHz) (EN 300718), applied worldwide. All devices sold since 1992 respect it.

On the other hand, all the models manufactured before 1992 are to be prescribed, whether they devise mono-low frequency (2,275 kHz), incompatible with the current devices or models bi-frequency, which have insufficient performances.

• Analog or digital?

Until 1998, manufacturers used analog technology. In this case, the signal of the transmitting DVA is converted by the receiver into sound waves. When a receiving DVA picks up a transmitting DVA, it emits a “beep” all the louder as the two devices are close to each other or correctly oriented relative to each other. Other. A potentiometer makes it possible to vary the sound intensity of the signal, by varying the sensitivity of the receiver. All DVA manufactured before 1998 are analog.

In 1998, the second generation of DVA appeared using digital technology exclusively or associated with analog technology. This is called digital DVA. The signal from the transmitter is analyzed by a microprocessor which converts it into visual indications of two types:

– a progress indication (numerical value appearing on a screen); 
– a direction indication (LED or arrow that illuminates when the receiving DVA is correctly oriented relative to the transmitting device).

The audible signal that has been stored in several digital DVAs is not sufficient for searching, unlike analog DVAs. 
The interpretation of visual signals is often easier than that of a sound signal. But from a practical point of view, both technologies make it possible to locate a DVA precisely and quickly (provided one has trained). The use of any one of these technologies is not in itself a decisive criterion of quality for a DVA.

• Main Features

Shape, size, and weight are not critical criteria.

Moreover, their appreciation is quite subjective. However, a DVA should not hinder or hurt. The absence of sharp edges and angles, reduced weight and volume will be sought. But the small footprint of a DVA must not interfere with functionality criteria: handling, use and manipulation of switches, buttons, or potentiometer (especially with gloves). 
The straps must allow an adjustment closer to the body, but without hindering, as well as a quick and easy removal.

• Ease of use

It is a criterion of fundamental choice. It covers several aspects that are not devoid of subjectivity. 
The operating mode of the DVA must be clearly and unambiguously indicated. 
Without notice, one must know quickly and without possible error whether it is off or on, and in the latter case, transmission or reception. 
A DVA should be easy to handle, even with gloves, especially when searching. Switching the device on and off, switching on reception and return to transmission must be simple and fast. However, to avoid any mishandling, they must not be able to be done unintentionally or unconsciously.
The search is performed by interpreting sound and visual indications. Regardless of other parameters, the secondary search will be more natural as the interpretation of the received signals will be simple.

• DVA and cell phone, radio, GPS, etc.

A receiving DVA emits a signal when it receives an electromagnetic wave of frequency 457 kHz. 
All electromagnetic devices (mobile phone, radio, GPS, etc.) and even high-voltage power lines emit electromagnetic waves at a given preferential frequency. It is possible that there is interference between electronic devices and DVA reception, which may disturb a search. Thus, a DVA placed in reception within 40 to 50 cm of a mobile phone may give the impression that it receives the signal from another DVA. 
However, the indications received will generally not be as regular and consistent as those given by a DVA in the issue. This may help to realize that the signal received is not that of

Each will have to check with the electromagnetic devices that it carries in the mountain and its DVA, and to draw the consequences, in particular on the distance to which the apparatus will have to be placed to avoid any interference.

When in doubt, it will be prudent to turn off your laptop, radio, or GPS while searching. If their use remains necessary, entrust them to someone who does not participate in the research.

Some reminders about litters

• The interpretation of the values given by the manufacturers is not always easy. It is even sometimes misleading. A reminder of some definitions to recognize it …
• The range: a DVA R reception receives the signal of a DVA E emission if the distance between them is less than a certain value. This value is called the range of the DVA R. It is not fixed but depends on the sensitivity of the receiver R and the power of the emitter E, which vary according to the brand, the model (or even the device itself), the state of the batteries and the temperature. The range also depends on the respective position in the space of the two apparatuses R and E.
• The maximum range is obtained when the receiving antenna is oriented in the axis of the transmitting antenna. It can reach 100 m for analog DVA. This value is of little interest since it corresponds to a positioning of the two exceptional DVA in practice.
• The minimum range is obtained when the receiving antenna is oriented perpendicular to the axis of the transmitting antenna. In theory, it is zero. In practice, it is very variable (the slightest variation in the angle formed by the two antennas causes a significant variation in its value).
• The useful range: between these two extreme cases lies the useful range. This is the distance below which a receiving DVA picks up the signal from any transmitting DVA, regardless of the other parameters (battery status, temperature, type and brand, respective positions of the devices in the space).
• The knowledge of this parameter is fundamental. It defines the search strategy, based on the number of rescuers and the size of the avalanche. In all cases, the spacing between two rescuers must be less than or at most equal to twice the effective range.
• The useful range of value can also influence the duration of the search. The first phase (primary search) aims to obtain a signal from the transmitter DVA: the rescuer moves on the avalanche until a signal is received. If his DVA has a large useful range, he will pick up this signal sooner than if it is weak. Indeed, the displacements before the first signal will be reduced. The more useful a DVA is, the more the primary search will be abbreviated. 
  However, this value should not be considered as an exclusive criterion. 
  It will have to be even bigger than:

– the number of potential rescuers will be small (case of a small group)

– the area to prospect will be important;

– the area of the deposit of the Avalanche will make the progression difficult (powdery snow, in which one sinks, a deposit of large blocks of hard snow).

Be careful; the useful ranges are well below the ranges announced by some manufacturers (they often give “their” maximum range) or those that can be read in some magazines.

How does an avalanche beacon work

• The ARVA principle is relatively simple. The antenna consists of a ferrite bar surrounded by an electric coil. By supplying this coil with an electrical signal, an electromagnetic field is created. These lines of electromagnetic fields are elliptical and form closed loops. With another antenna, they can be tracked one way or the other to find the transmitting antenna.
• The device runs on batteries, must be stick to the body of the person wearing it and has two modes: transmission & reception.
• In a normal situation, the DVA is placed in a “transmission” mode. The power of the transmitted signal depends on the power provided by the batteries, so it is essential to have batteries well charged! The range of the signal generally varies between 30 m and 60 m according to the models, the batteries, and the weather conditions of the moment (cold).
• To save the batteries, the signal is not sent continuously but by short pulses according to the ETS standard. Also, to reduce the sensitivity to external disturbances, the signal is further modulated in amplitude at a frequency of 457 kHz +/- 80 Hz.
• A standard is set up by the ETS (European Telecommunication Standard), number 300 718, published in March 1997 to standardize all the DVAs marketed, and thus make them inter-compatible. 
• When read, we see that the frequency 2275 kHz was gradually abandoned. Moreover, one hardly finds any more on the market of apparatus emitting at this frequency.
• When a victim is caught under an avalanche, the DVA must be placed in the “receive” mode to receive the signal emitted by the DVA under the snow. The received signal is maximum when the two antennas are approaching and when the receiving antenna is parallel to the field line. Not knowing in advance in which position the DVA is in the snow, it is not easy to follow exactly the line of field and especially to refine the search close to the victim.
• During a search, it is necessary to have in mind this principle of lines of the field and not to follow blindly the indications given by the apparatus. It must also be borne in mind that the calculation made by the DVA takes time and therefore avoid going too fast.