A Submersible Global Positioning System Receiving Antenna. John D. Moore Sound Ocean Systems, Inc.

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1 A Submersible Global Positioning System Receiving Antenna John D. Moore Sound Ocean Systems, Inc. Abstract Long endurance missions using Autonomous Underwater Vehicles (AUVs) to collect ocean or geophysical data will require knowledge of the vehicle position. The present state of the art navigates the long range AUV using dead reckoning or inertial navigation. When the mission time is measured in days significant positioning errors can accumulate. Sound Ocean Systems, Inc. has developed a submersible Global Positioning System (GPS) receiving antenna and erection system. This antenna system will allow a surfaced AUV that is equipped with a GPS receiver to update its current position. Introduction Sound Ocean Systems, Inc. was approached several years ago by a party needing a submersible GPS antenna for use with a commercial inertial navigation system. We conducted a literature and manufacturers search and could not locate a source of a commercially manufactured one. Several manufacturers had antennas that could get wet but none that were water and pressure resistant at the same time. We undertook to find an existing manufacturer of GPS antennas that would be willing to manufacture them. When told of the small market for submersible antennas they politely declined interest in the project. We then undertook a back burner development project to find a solution for obtaining GPS data when an AUV was on or near the surface that could withstand being submerged while diving. The Challenge We identified several possibilities for getting GPS data into an AUV. Our first thought was to enclose a GPS antenna under a glass dome in a pressure housing. We also thought that we could pot an entire GPS antenna in epoxy or polyurethane with a molded on cable and connector set. Both of these approaches had one common problem. We could not find water and pressure resistant connectors that would work at the GPS L1 band frequency (1575 MHz). None of the manufacturers that we contacted could provide any test data or experience at that frequency for connectors they manufactured. We then thought that we might have to house or pot the entire GPS receiver in one housing. This would eliminate the need for water and pressure proof connectors and cabling that could operate at GPS frequencies. We looked at several small self-contained GPS receivers. They all had a common problem of only having a proprietary or NEMA 0183 interface. The physical size of these was also larger than could fit in the glass domes available to us. The problem of getting GPS L1 band signals from an antenna housing into a receiver housing still was our main stumbling block. While we had been researching GPS antennas looking for one that is water and pressure resistant we had noticed that they were all active (contained a low noise amplifier). The published gain figures for these antennas were in the order of +24 db. The manufacturers of GPS receivers had told us that typically you could expect good performance with up to 10 db loss in the connection between the antenna and receiver. We knew from past experience that Rochester CN-4 cable was a reasonably good water-resistant cable that approximated the performance of RG-58 C/U coaxial cable. We had also found through past experience that the D.G. O Brien (DGO) C10 series of connectors had good performance with this cable. Our problem still remained that DGO did not have any test data at the GPS frequency. We decided to carry out our own test to find if these connectors would work. We then setout to find if the C10 connectors and CN-4 cable could be combined to connect a GPS antenna and receiver. We did not have ready access to measuring equipment that operates at the frequencies and signal levels we needed for this test and did not wish to rent it.

2 We knew that we could borrow the cable/ connector sets and a GPS receiver. With these components we were able to setup a test for signal attenuation. The Test To model the effects of using the water and pressure resistant cable and connectors we built a representative vehicle set of cabling. This vehicle set of cabling was assembled as three subassemblies to represent the three sections that would normally be found in an AUV installation. Two of the sections were modeled as dry cable assemblies connected to a water and pressure resistant bulkhead connector. These 1.50 meter long RG-58 C/U cable assemblies were terminated on the dry end with a TNC male connector and on the bulkhead end with a C10C0001G01 connector. These two cable sections model the internal pressure vessel wiring that would be used in the antenna and receiver housings. The interconnecting cable section was modeled using a 4.57 meter long section of CN-4 coaxial cable with C10C1001G01 connectors on each end. These three sections where then connected in series to model a complete vehicle set of cabling. Our test setup was installed in a rural area with no tall buildings or obstructions nearby. The antenna was mounted atop a 3.65 meter tall mast and connected to the receiver via a 10 meter long RG-58-C/U cable. The GPS receiver and laptop computer used for controlling the receiver were installed in a wooden building. The GPS receiver was powered with a 12 volt deep cycle battery and the computer was powered from 120 volts ac building power. The computer was running the TSIP-Talker software from Trimble under Windows This software is able to display all the satellites in view and an arbitrary signal to noise ratio for each satellite. Results When we started the system running we saw that we had 10 to 12 satellites in view most of the time with 6 to 10 of them being used for the position solution. The S/N ratios for the satellites in use varied from 12 to 24 on a scale of 0 to 25. The first thing we did was a qualitative test to determine if the receiver would work at all with the underwater cable/connector set wired in series with the standard cable. We were pleased to find out that the GPS receiver continued to work with the underwater cable/connector set connected between the antenna and receiver. We were not able to detect any degradation in the performance of the receiver by simple observation. figure 2 Underwater cable/connector test arrangement The GPS receiver we borrowed, a Trimble Ag 122, is able to output a NEMA 0183 message type GSV. This message contains the identification, elevation angle, azimuth angle, and signal to noise ratio for each satellite in view. We logged the NEMA 0183 GSV message to log file on the computer for each test condition we needed to test the S/N ratio for. We then compared the signal to noise ratio Vs elevation angle in the normal Vs with the underwater cable/connector set. S/N Ratio Vs Elevation figure 1 Baseline Test Arrangement S/N Ratio Normal Cable With U/W Cable Set Elevation Degrees

3 We found that the average signal attenuation with the addition of the underwater cable/connector set was 3dB. This was well below the manufacturer s 10 db attenuation figure for acceptable receiver performance. We concluded that it was practical to use a commercially available underwater cable/ connector set to connect a low noise amplified GPS antenna to a GPS receiver. We still had the problem of protecting the antenna from water and pressure. The only material that we could identify that had the required mechanical properties and was still transparent to L band radio waves is glass. We proposed that any antenna pressure housing would need a glass end dome to admit the GPS signal. The question of the effect of the glass dome on the GPS antenna was asked. Using our existing test setup the effect on signal strength of covering the GPS antenna with a glass dome was measured. We used a 330mm diameter glass hemisphere with 10 mm wall thickness for our test. We think the hemisphere is made of BK-4 Borosilicate glass. Again we logged the NEMA 0183 GSV message to log file on the computer to test the S/N ratio. We then compared the signal to noise ratio Vs elevation angle with the underwater cable/connector set and with the antenna covered with the glass hemisphere. We found that the average signal attenuation with the addition of the glass hemisphere was 4 db. This held for all except elevation angles less than 15. Typically satellites with this low an elevation angle are not used for calculating the antennas position solution. This is still well below the manufacturer s 10 db attenuation figure for acceptable receiver performance. We concluded that it is practical to house a low noise amplified GPS antenna and receiver connected with a commercially available underwater cable/connector set in water and pressure resistant housings. S/N Ratio Vs Elevation 55 S/N Ratio Normal Cable With U/W Cable Set With Glass Dome and U/W Cable Elevation Degrees

4 Design A prototype design for a water and pressure resistant housing for a low noise amplified GPS antenna was made. This housing was designed using Commercial Off The Shelf (COTS) components to the fullest extent possible. The housing is rated to protect the GPS antenna to a depth of 6,000 meters. While this may appear to be a much greater depth than most commercial or research AUVs would need to operate we felt that the only way to justify the development cost was be able to use the design for all applications. Figure 3 illustrates the prototype housing we are constructing. figure 5 Antenna and Mast Deployed Conclusions figure 3 Prototype GPS Antenna Housing A typical AUV has very little if any freeboard while on the surface. This means that in any condition rougher than a swimming pool a hull mounted antenna is probably submerged or wet most of the time. The L1-band frequency used by GPS will not penetrate water. To solve this problem we propose to mount the GPS antenna on an extendable mast. Our proposed mast design uses water supplied by an integral pump as the working fluid to extend the mast. See figures 4 and 5 for a typical installation. For scale the AUV hull is shown at 21 inches in diameter. figure 4 Antenna and Mast Retracted Into Hull We concluded that it was practical to use a commercially available underwater cable/ connector set to connect a low noise amplified GPS antenna to a GPS receiver. We concluded that it is practical to house a low noise amplified GPS antenna and receiver connected with a commercially available underwater cable/connector set in water and pressure resistant housings. We concluded that it is practical to use COTS components to assemble a GPS antenna system that can survive hostile undersea environments. Future Work We have identified several areas that need further investigation to fully use the potential of the housed GPS antenna. The first area of investigation is to study the effect of refraction as the GPS signal passes through the glass dome on the bandwidth and center frequency tuning on the antenna. We believe that refraction is the cause of poor performance at low elevation angles in our test.

5 The second area of investigation is to work on reducing the size of dual band antennas to fit practical housing sizes. This has the potential for significant improvements in positioning accuracy through the use of both the L1 and L2 signals from the GPS satellites and the newly adopted Wide Area Augmentation System (WAAS) broadcast from geosynchronous satellites. The third area of investigation is to work on reducing the size and cost of the present system. By migrating the design to shallower depths and reduced size we hope increase the use and lower cost through economies of scale.