Issue 1, March 2005
As a result of experience with the V8SC line, users have been pressing us to produce even smaller transmitters. Our objective is to provide the smallest transmitters in the world - acoustic or radio - while maintaining reliable Vemco performance. This article outlines our progress to date as well as upcoming introductions.
Tag Size and Model Designation
With the exception of the V8 family, the family name designates the tag diameter. For example, V16 tags are 16 mm in diameter and V13 Tags are 13 mm in diameter. However, for historic reasons, this doesn't hold for the V8 family. The original V8 was indeed 8 mm in diameter but as a result of two generations of improvements the current V8-SC is in fact 9 mm in diameter. We are in the process of regularizing this by gradually changing the designation to V9. The recently announced pressure-sensing version of the V8-SC is called a V9P.
In 2004, we introduced the V7 tag family (7 mm diameter) and later this year we will be introducing the smaller V6 tag family (6 mm diameter).
||Length of Smallest Model
||Weight of Smallest Model
||2 g in Water
||0.7 g in Water
||0.6 g in Water
With few exceptions, Vemco miniature transmitters have had an operating frequency of 69 KHz which has served our clients well since this low frequency gives good range in both salt water and fresh water. For instance, using V8s with arrays of easily deployed VR2 Monitoring Receivers, researchers have been able to study large scale, long term movements of salmon starting right from smolt stage. For examples, see the Atlantic Salmon Foundation Project and The Pacific Ocean Salmon Tracking Project.The use of a low operating frequency does, however, involve compromises and, as the size gets smaller, one reaches a point of diminishing returns due to the very low efficiency one can achieve when driving a small transducer far below its resonant frequency. In fact for the V6, a low frequency version becomes impractical. Therefore, the V6 family will have a standard transmission frequency of 200 kHz. Other frequencies will be offered if demand warrants it.We will also introduce high frequency versions of selected Vemco tags from other families to address fresh water applications where two advantages accrue:
- Certain locations - around hydroelectric dams, for example - exhibit noise conditions which make the use of lower frequency transmitters virtually impossible.
- Higher electric to acoustic efficiency due to operating the transducer at resonance translates to increased range in fresh water - but not in the ocean where absorption rises significantly with frequency.
The small V6 transmitters will have some application in salt water studies where the disadvantages of shorter range and limited operational life are offset by the ability to tag smaller fish than previously possible.
Range Implications and Caveats
Claims about range have to be taken with a grain of salt and, for this reason, Vemco has been reluctant to make any. Rather, we have specified our transmitters in terms of their acoustic output which is the only objective measure. However, since both frequency and output level affect range in the ocean, we need to provide some guidance and will do so by introducing the concept of "range benchmarks". We will stipulate two of these:
- Tracking: The range at which the signal to noise ratio in the ocean under typical conditions (Sea State 3) falls to the minimum level that can be detected automatically with a conventional receiver such as the VR60 (i.e. 12 dB in a 1 kHz bandwidth).
- Monitoring: The range at which a simple monitoring receiver like the VR2 will reliably detect codes under typical conditions. Again Sea State 3 conditions are assumed.
The table presents range benchmarks information for our two smallest 69 KHz products along with the soon to be available 200 kHz V7. Information for the first V6 products will be available later this year.
||Output Level (dB re 1 uPa @ 1 m)
||Range Benchmarks (Metres)
1. Current product literature shows an output level of 139 dB for the V8SC-6L but recent
improvements have raised this to 142 dB.
2. A monitoring range benchmark will be determined once characteristics of the planned
200 kHz Monitoring Receiver are finalized.
Again, we emphasize that the "range benchmarks" expressed above are merely provided to help understand the influence of output level and frequency. Actual range experienced could be quite different depending on local conditions (spreading, temperature profiles, etc.), the characteristics of the receiver and hydrophone used, hydrophone movement, noise generated by the boat in tracking applications, etc. Because of the large number of variables, there is no substitute for on-site range testing, particularly when positioning arrays of VR2 monitoring receivers.Further discussion of the impact of power, frequency and receiver characteristics on range and code detectability will be provided in a white paper currently under development.
What about Receivers?
Traditionally, Vemco receiving technology has focused on frequencies under 100 kHz. With the introduction of high frequency transmitter options, we obviously need complementary receiving capability and the recent introduction of the VR100 Receiver is the first step. This platform is capable of acquiring signals up to 500 kHz and is configured to a particular requirement with DSP and Controller Firmware. Currently, we offer a general purpose 100 kHz tracking algorithm capable of simultaneously detecting transmitters on eight different frequencies. Other algorithms will follow - the first being a 200 kHz tracking algorithm to be used with our new high frequency transmitters. This will be followed by the introduction of Monitoring Receiver products.