Saturday, 1 October 2011

How do Aylal and Naders’ satellite tags work?

It has been mentioned in previous posts that the tags on Aylal and Nader have different levels of accuracy and this is due to the fact that two different systems are being used: Argos and GPS. Both tags use the Argos system but one tag also has a GPS transmitter.

The Argos system is named after the mythical giant with 100 eyes, who was the perfect, all-seeing guardian. This system is based on a series of six polar-orbiting satellites, i.e. which pass over both poles, at an altitude of 850 km. These satellites belong to the National Oceanic and Atmospheric Administration (NOAA) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and other agencies will join in the future.

Each platform, i.e. the animal or object carrying the transmitter, can be reached, in theory, about 14 times per day. The position of the platform is calculated using the Doppler Effect, i.e. the variation in the wave frequency (produced by the transmitter) between two objects moving relative to each other (in our case the transmitter and the satellite).

Overlap of the areas covered by two successive passes of the Argos satellite system (http://www.argos-system.org)

With the Argos system there is a certain limit to the number of locations that can be received from each transmitter but it is ideal for following long-distance movements of birds. It is also small and light - the transmitter on Nader weighs just 9 grams (1/4 ounce).

The transmitter on Aylal also carries a GPS unit. The GPS system relies on a denser system of satellites, with 24 satellites (four in each of six different orbits), at an altitude between 10,100 and 20,200 km. Each transmission from the platform is picked up by several satellites and the position can be calculated by measuring the distance from each one. This provides an accurate, three-dimensional location (latitude, longitude and altitude). Not surprisingly, this accurate technique was originally developed for military use.





This type of transmitter was deployed on Aylal to provide us with detailed data on small-scale movement patterns of the species within its normal range. This will help us to identify any potential threats and will complement the day to day work done by the guards on the ground. The transmitter on Nader will provide us with more broad-scale data on the large-scale dispersal patterns of juveniles of this species. This is the same system which has been used to tag ibises in Syria and Turkey.


The fundraising campaign for the conservation of the bald-headedibis aims to raise money to buy more transmitters such as these, to tag moreindividuals. The data provided from these tags greatly increases our knowledgeof species movements and habitat use and allows us to identify potentialthreats.  

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