|Argos Newsletter N° 52 - July 1997|
Paul W. Howey, Ph.D.
President Microwave Telemetry
Since 1991 Microwave Telemetry has been supplying miniature Platform Transmitter Terminals (PTTs) designed especially for tracking migratory birds with Argos. Back then, the lightest PTT that we produced weighed 95 grams. This version is still being used by many researchers to globally track large birds weighing in the 3 kg and greater range, e.g. large eagles. (The weight of the PTT limits the species that can be tracked as it is generally considered that the package should not weigh more than 3-5% of the weight of the bird.). It has always been our goal to continually reduce the weight of our ptts using the latest miniature components and power sources. In 1994, we converted the electronic module to a hybrid circuit that weighs only 3.5 grams and, along with new batteries, this allowed us to produce PTTs that weigh less than 30 grams. Recently by incorporating new power supply circuitry we have been able to produce an even smaller ptt, the PICO-PTT100 that weighs a little under 20 grams (Figure 1).
These tiny PTTs have opened up the ability to research the movements of many species which, until now were quite mysterious. Dr. Bill Seegar and his colleagues attached the first two PICO PTTs to male Peregrine falcons on Padre Island on the Gulf coast of Texas in April 1996. Figure 2 shows the movements of these two birds. PTT id 11978 was programmed to last 3 months while id 5696 was set to less intense duty cycles to follow the bird for a longer period. The map is self explanatory and shows what can be achieved with the present system.
The advent of the enhanced Argos-2 equipment aboard NOAA-K, that is due to be launched in 1998, may allow us to further reduce the PTT weight and so make it possible to track even smaller species of birds.
Improved Receiver Sensitivity and Its Implications
We have concluded over the years that with the present generation satellites
Advantage of the increased sensitivity could be taken to possibly produce even lighter PTTs, reducing the transmitted output power by 3 dB to match the improvement of the receiver sensitivity (i.e., reducing the minimum transmitted output from 100 mW to 50 mW). This would probably not allow for a significant reduction in the weight of the ptt electronics, which already weigh only 3.5 grams, but it should allow the use of lighter power sources. This could potentially reduce the minimum ptt weight by up to 6 grams. PTTs with such a reduction in power level however may not be suitable for use in all parts of the world. Observations indicate that in some areas, especially Western Europe, receiver sensitivity is effectively limited by the background RF noise level emitting from the radio sources on the ground below. The weak signals from these tiny PTTs might not be heard above this background noise. However in radio quiet areas, very low power (50 mW) ptts should allow the tracking of many new species.
Increased Receiver Bandwidth
It is proposed that the Argos-2 receiver's 80 kHz bandwidth be split into 2 sub-bands, one of which will be reserved for low power transmitters. Along with the increased receiver sensitivity this should be an advantage for tracking low power ptts. At the present time the signals from the low power PTTs compete for DRU (Data Recovery Unit) assignment on board the satellite. A higher signal strength transmission emitting from a high power ptt may pre-empt assignment to a dru of a weaker signal from a low power ptt at the same received frequency. Hence by assigning this separate sub-band for low power ptts the competition for drus will be purely between low power ptts and hence, the assignment will become unbiased.
The addition of 4 extra DRUs will also obviously help in recovering more uplinks especially in more PTT populated areas. During the time while NOAA-K is in operation alongside the first generation satellites, we will have the opportunity to explore the possibilities of this "low power" sub-band; however transmitters set up to operate in this band will not be able to be received by the current satellites. At first, this may seem a disadvantage; however it may also be a unique opportunity to track some very low power PTTs while the sub-band is very quiet.
We are looking forward to trying these advanced features of Argos-2 and hopefully using this unique asset to solve the mysteries of the movements of some of the world's smaller birds.