For almost thirty years now, the Cospas-Sarsat worldwide Search and Rescue satellite system has been in operation, rescuing more than 26 000 distress victims. The scope in Cospas-Sarsat Antennas –project is to develop a body-worn antenna, suitable to be integrated into a life vest platform together with a commercial Cospas-Sarsat distress transmitter. As a result, a functional demonstrator is achieved.
The project work contains complete antenna design flow, from substrate material selection and characterization to actual design implementation, integration and verification. The resulting demonstrator system is capable to operate as a distress transmitter system in close proximity of human body as part of life vests.
The primary objective of this activity is to study, design and analyze the body-worn antennas, suitable to be integrated into a life vest platform. Moreover, the antennas are manufactured and measured to ensure the performance of the antenna element.
The secondary objective is to establish an interface between antenna elements and a commercial transmitter. This requires the development of a special interfacing circuitry network, especially if the final solution includes several antenna elements.
The following picture shows an example user case: A shipwreck victim has fallen into sea and activated the distress transmitter. Depending on the health status, the victim may try to save energy as much as possible by floating in a still position (left picture) or try to reach dry land / dry floating platform by swimming (right picture) Depending on the user position, one of the antenna elements is under water. The connection with the Cospas-Sarsat satellite is lost when antenna is under water. Therefore, the project objective is also to consider methods (number of elements / antenna placement / antenna control) to ensure that the satellite connection can be maintained despite the predicted user movement.
Finally, the ultimate goal is to demonstrate full compliance in a simulated distress situation.
Project output in hardware point of view is a functional system demonstrator, which is tested in the abovementioned real life simulated distress situation.
There are four key issues in the project.
The first issue is related to the antenna performance near human body. The close proximity of human body degrades the antenna efficiency and disrupts the antenna radiation pattern. This has to be taken into account in the antenna design. Design target is to create an antenna structure which is insensitive against the human body proximity, especially the effect caused by variation of the body distance from the element under user nominal movement.
The second issue concerns the size of the antenna elements. Operating frequencies are at VHF/UHF band, which means that free space wavelengths are around 0.7- 2.5 meters. Therefore, the antenna needs to be miniaturized employing suitable design methodology in order to fit it in the life vest platform. The key issue is to maintain reasonable antenna bandwidth, gain and efficiency.
The third issue is the material selection for the antennas. Antenna needs to be body-worn, which means that it cannot be completely rigid. Furthermore, the operating environment (maritime environment) exposes the antenna element to direct contact with (salt) water. The effect of water and moisture to the antenna performance has to be controlled via suitable material selection.
Finally, the last issue deals with the manufacturing and integration. The body-worn antenna has to be robust against manufacturing tolerances. Moreover, life vest as a body-worn antenna integration platform is considered to be demanding.
The conventional integration scheme of Cospas-Sarsat transmitters into life vests uses whip antennas. This is a quite bulky solution and it limits the user movement. The Cospas-Sarsat Antennas project aims to integrate the antenna elements in a seamless way into the life vest.
- Integrated body-worn antennas do not restrict the user movement,
- Seamless integration makes the system more reliable and robust, because whip antennas can stick to surrounding obstacles,
- Several antenna elements can be used to ensure that system is operable whether the user is in floating or swimming position,
- Integrated solution is more user-friendly.
The operating frequency bands for commercial Cospas-Sarsat user equipments are 121.5 MHz and 406.0 - 406.1 MHz. The military frequency band (243 MHz) is not supported in this project work. Antenna(s) can employ linear polarisation on both supported frequency bands. The system features:
- Body-worn antenna(s) for dedicated VHF/UHF bands.
- Seamless integration into a life vest platform.
- Interfacing circuitry between transmitter and antenna elements.
- Integration of the developed modules together with a selected commercial transmitter.
Antenna materials are selected to be compatible with the life vest platforms. Examples of material selection criteria:
- Robust against water exposure and moist conditions.
- Durable (resistant against wear and tear) Light weight.
- Suitable for body-worn antenna elements (electrical parameters).
The integration platform (inflatable life vest) is shown in the figure below. Antenna element(s) are fitted on areas that remain above the water surface when life vests are being used. The body-worn antenna can be fitted inside the cover fabric when the life vest is deflated. Small size Cospas-Sarsat transmitter can also be fitted inside the cover pouch.
The life vests are provided by VIKING Life-Saving Equipment A/S. Body-worn antenna element fits inside the cover of the deflated vests as shown in the right hand side of the picture below.
Antenna simulations and preliminary measurements have shown that the antenna will meet the official Cospas-Sarsat specifications for a beacon antenna. The 406 MHz body-worn antenna 3-D radiation pattern is shown in the figure below. The maximum realized gain is about 3 dBi.
The body-worn antennas were manufactured and measured. Measurement data showed similar performance for the body-worn antenna modules as the simulations predicted. Therefore the complete body-worn unit was assembled and tested in a field test as shown in the figure below.
The field test was a success. The satellites detected the activated transmitter within minutes and the location of the test site was resolved by the satellite system. Body-worn antennas operated as expected. This success story is also reported on ESA’s web page: http://telecom.esa.int/telecom/www/object/index.cfm?fobjectid=31339.
The project started on the 1st of January 2010. The total project length is 18 months. The work to be done during this time is divided into nine separate Work Packages (WP):
WP1: Management & Project control
WP2: Establishing the requirements
WP3: Study and selection of body-worn materials
WP4: Characterization of body-worn materials
WP5: Preliminary body-worn antenna design
WP6: Detailed body-worn antenna design
WP7: Manufacturing of the body-worn antenna
WP8: Measurement of the body worn antenna/transmitter unit
WP9: Conclusions and development plan
WP10: Development of compact body-worn unit
The policy to monitor project status is to arrange progress meetings in 2 month intervals. Moreover, design review meetings will be organized according to the technical milestone schedule. The project will end on 30th of September 2011. An additional work package (WP10) is to be added to the project via CCN procedure.
The project started at the beginning of January 2010. The project status currently is:
- The requirements for the antennas and subsystem were established in Work Package 2.
- Materials for the antenna have been selected and the characterization methods have been defined in Work Package 3.
- Conductive and non-conductive material characterization was performed in Work Package 4. Materials are now characterized to support antenna design work.
- Preliminary body-worn antenna design was completed in Work Package 5. This work package included also an interface module design, which is needed to establish a connection between the COTS transmitter and the antenna elements.
- Final design iterations were performed during the Detailed Design phase (Work Package 6). As a result, extremely robust body-worn antenna design was achieved: Antenna performance will not degrade due to nominal material or fabrication tolerances. Furthermore, the interface module design was demonstrated to be functional in a test with a real transmitter.
- Manufacturing of the body-worn antenna modules and interface modules is completed.
- Module level measurements are completed. Measurement results are compliant with simulations and antenna performance meets the specifications.
- The body-worn unit was tested in a field test using the actual Cospas-Sarsat satellite system to locate the transmitter. The test was a success, and the body-worn unit worked as planned.
- Project was finished 30.9.2011 (as scheduled).