Aircraft Carrier Multipath Modeling for Sea-Based JPALS - AWE ...

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Figure 12: Comparison of real and simulated code multipath, PRN 23. Overall multipath statistics also assess how well the simulated multipath matches the live data. The overall multipath 1-σ statistics per 10 degree elevation by are shown in Figure 13, where the left and right set of bars show the real and simulated multipath statistics, respectively. This comparison shows that the model captures multipath quite well in the statistical sense. Overall, the model not only correctly predicts the trends across the elevation bins but also predicts the 1-σ magnitudes to within 90% across all bins. Figure 13: Real and simulated aircraft carrier code multipath 1-σ statistics. Carrier Phase Multipath Results The model also computes carrier phase error due to multipath directly for a given site. This is advantageous because carrier phase multipath cannot be isolated in live measurements. Double-difference or inter-frequency carrier phase multipath observables always contain a combination of multipath from two sites and two different satellites or frequencies [2]. The simulated carrier phase multipath 1-σ statistics corresponding to the validated code multipath are shown in Figure 14. It is promising that 1-σ magnitudes are well below 1 cm. The model predicts that maximum carrier phase errors for this satellite geometry are no larger than 1.5 cm, as shown in Figure 15. Figure 14: Carrier phase multipath 1-σ. Figure 15: Carrier phase multipath maximum. Computational Costs The computational cost of a given simulation depends upon a number of factors driven by the ray-tracing, antenna models, and tracking loops. For ray-tracing, these factors include the environment complexity and number of surfaces, the number of interactions to consider, the number of transmitters (i.e., number of epochs per satellite), and the number of prediction points (receiving antennas). After ray-tracing is completed, the antenna pattern is applied and the ray-tracing outputs are converted to multipath parameters. This step is fastest with an omni-directional antenna, but slows down when using a FRPA or CRPA due to the required gain/phase lookups and/or weighting matrix calculations. The code and carrier tracking loop models are mostly affected by the precision to which the multipath delays are computed. Presented at the Institute of Navigation GNSS 2005, 13-16 September, Long Beach, CA 6
This affects the simulation speed because the code correlation functions must be sampled to this precision, so an increase in precision from 1 cm to 1 mm will result in correlation function vectors which are ten times as long. We have found that 1 cm precision is a good tradeoff between computational speed and the ability to capture multipath errors with sufficient accuracy. The simulation tasks and associated computational time for the validation simulation are summarized in Table 3. The breakdown is given for both a single and double interaction simulation corresponding to the 13 hour live data set. Both simulations were run on the same Pentium 4 / 2.8 GHz PC. The double interaction results were of course used because they represent the live data much more accurately. The increased accuracy comes at a much higher computational expense, though, as the total simulation time increased by about a factor of 70. While the total simulation time is still acceptable in this case, we are currently exploring the use of additional processors as well as the distribution of tasks to multiple PCs via a local area network. Table 3: Computational tasks for aircraft carrier validation simulation. Computational Task Aircraft Carrier @ One Interaction Aircraft Carrier @ Two Interactions MATLAB file I/O 10 min 150 min Ray Tracing 30 min 7700 min Apply Pattern Antenna 15 min 40 min Code/Carrier Tracking Loops 45 min 250 min Total: ~2 hours ~137 hours COMPARATIVE MULTIPATH PERFORMANCE STUDIES One important application of this multipath model is the evaluation of expected multipath error for different antenna types and locations in a given environment. Such studies can be very helpful in identifying candidate antenna equipment and sites which reduce overall multipath error. In the context of the aircraft carrier, the expected performance of beam-steering CRPA equipment and the level of multipath on the middle yardarm are of interest. Sample results for a beam-steering antenna are shown next. This simulation uses a 7-element GPS Antenna System 1 (GAS 1) CRPA model provided by Rockwell Collins [1,5]. This antenna model utilizes spatial adaptive processing (SAP) only and the geometry of the array is shown in Figure 16. A weighted sum of the outputs of each of the antenna elements is computed to create a composite antenna pattern that can be controlled to enhance direct and attenuate multipath signals. In our implementation, the RCP and LCP antenna patterns for each element are only loaded once while the weighting algorithms and gain/phase lookups are performed at each epoch. To improve computational performance, the composite gain/phase pattern are computed only for angles at which signal rays are present (rather than for the complete pattern). Figure 16: GAS-1 CRPA diagram. Courtesy of Rockwell Collins. The CRPA performance is evaluated for two sample satellites by comparing CRPA multipath to a non-choke ring FRPA at the top-level yardarm location. Since this is a comparison of simulated data only, receiver noise is not included in the multipath observables. Sample time histories are shown below. As one can see, the beamsteering CRPA is able to reduce multipath because the antenna array applies positive gain towards the direct signal while attenuating multipath signals arriving from other directions. It is notable that at certain times, such as between hours 5-6 in Figure 17 and hours 10.5-11 in Figure 18, the CRPA errors are comparable to those of the FRPA. This is not unexpected since the composite CRPA pattern sometimes exhibits “parasitic beams” where arriving signals are not significantly attenuated [5,9]. An example of a composite CRPA pattern with parasitic beams is given in Figure 19. Here the pattern is steering towards an azimuth of 90 deg but one can see that signals arriving from azimuths around 220 deg and 320 deg would not be significantly reduced by the antenna. Presented at the Institute of Navigation GNSS 2005, 13-16 September, Long Beach, CA 7
Page 1 and 2: BIOGRAPHY Aircraft Carrier Multipat
Page 3 and 4: Figure 1: CVN 69 CAD model. Figure
Page 5: MODEL VALIDATION To validate the mo
Page 9 and 10: Figure 22: Sample multipath impulse

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