Section 4.3. Interference to GPS Receivers

4.3. Interference to GPS Receivers

In this section, we discuss the interference effect of multiple UWB transmitters on GPS receivers. GPS devices are satellite-based radio navigation services that provide worldwide access to accurate position and time information. These systems are used in numerous critical applications in military, aviation, law enforcement, public safety, emergency response, personnel, and vehicle monitoring and tracking. Since GPS is a space-based system and the signals sent from its satellites to Earth are as weak as 130 dBm, GPS receivers are very vulnerable to interference. Therefore, GPS providers and users around the world have expressed a great deal of concern about UWB interference with weak GPS signals.

The FAA [2], TEM Innovations [3], the U.S. GPS Industry Council [4, 5], American Airlines [5], the General Aviation Manufacturers Association [5], Stanford University [5], and United Airlines [5] have expressed concerns that UWB operations will produce harmful interference in the GPS bands. The FCC has tested the impact of UWB emissions on a GPS receiver from a Time Domain Corporation Part 15-qualifiable radar. The emissions from the radar prevented the GPS receiver from tracking a satellite when it was 1 foot away from the radar. The GPS receiver was prevented from acquisition when the radar was 10 feet away.

Time Domain Corporation [6] performed a similar test in an open field using a handheld GPS receiver, and it has also carried out some theoretical analyses on the compatibility of a GPS system and UWB emissions. The required isolation between a GPS receiver and a UWB device was calculated to determine the range at which GPS no longer works. The required isolation was calculated assuming that the UWB device was limited to Part 15 Class B unintentional radiator emission levels. The range was determined by assuming free-space propagation between a UWB device and a Navstar GPS receiver. The theoretical ranges were found to be 19.8 meters for acquisition and 7.1 meters for tracking and demodulation. These values are greater than the results of the FCC tests, and also greater than measurement tests performed by Time Domain Corporation, which found that reliable positioning information was lost for a separation of 4 to 6 feet. The handheld GPS receiver was kept relatively level at a normal operating height. The discrepancy between measured and theoretical results can be accounted for by considering gains in the GPS system not accounted for in the theoretical analysis. This means the theoretical analysis gives a conservative estimate of the range.

In this section, the performance degradation of GPS devices is analytically presented based on UWB transmitter density similar to the analysis performed for WLAN devices in Section 4.2. The analysis is based on the derivation of path loss curves when a victim GPS receiver is surrounded by uniformly distributed UWB transmitters operating within Part 15 emission limits (12 nW/MHz) and 1 GHz UWB bandwidth.

4.3.1. Interference Model

As in the earlier WLAN analysis, we model the impact of the interference from multiple UWB transmitters to a victim GPS receiver using the arrangement shown in Figure 4-23.

Figure 4-23. The interference model. Top: Multiple UWB transmitters, denoted with crosses, are distributed about a 2D plane, and a cellular receiver is at the position marked with a black dot. Bottom: An enlargement of the center of the top diagram showing some dimensions required to derive the interference power from the UWB transmitters to the receiver.

In Figure 4-23, the victim receiver is shown as a dot and the UWB transmitters are shown as crosses. The interference needs to be estimated to determine the maximum UWB power spectral density that would inflict negligible interference on a GPS receiver. We assume that the UWB transmitters are distributed with a distance r throughout a 2D plane. The PSDs of the UWB transmitters, G_p, are assumed to be equal. Consider a victim GPS receiver, labeled RX in Figure 4-23, located in a 2D plane, having a receiver bandwidth B_RX.

The total interference power at the receiver from the UWB transmitters, I_UWB, can be calculated by evaluating the following expression:

Equation 4-12

where G_p denotes the ultra-wideband power spectral density, B_RX represents the victim receiver's bandwidth, and is F(l, r, d₀, n) described by:

Equation 4-13

In this equation, l is the wavelength, r denotes the UWB transmitter density, d₀ is the free-space reference distance, and n is the path loss coefficient. The detailed derivation of this equation is presented in Appendix A.

4.3.2. Isolation Between UWB Sources and a GPS Receiver

In this section, we calculate the maximum UWB density that would degrade the performance of the GPS receiver used in Time Domain Corporation's theoretical analysis [6]. The required isolation was determined by Time Domain Corporation to be 62.4 dB and 53.4 dB for acquisition and tracking, respectively. The derivation of the isolation figures have accounted for the ratio of UWB-to-GPS bandwidth. The GPS operating frequency is 1575 MHz, which is equivalent to a wavelength of 0.19 meter in our calculations. The predicted isolation can be derived from Equations 4-10 and 4-11 from the previous section.

Figure 4-24 shows the predicted path loss plotted against the UWB density in an urban area, for example, with d₀ = 100 meters. The maximum UWB densities to cause the GPS acquisition and tracking to fail are approximately 300 and 1500 sources per square kilometer. This is more than the 100 sources per square kilometer previously considered as an appropriate density of UWB devices in an urban area.

Figure 4-24. Isolation between UWB sources located in an urban area and a GPS receiver plotted against the density of UWB sources

A similar graph appears in Figure 4-25: predicted path loss for a rural area, for example, with d₀ = 1000 meters. The maximum UWB densities to cause the GPS acquisition and tracking to fail are approximately 100 and 750 sources per square kilometer. This is more than the 1 source per square kilometer previously considered as an appropriate density of UWB devices in a rural area.

Figure 4-25. Isolation between UWB sources located in a rural area and a GPS receiver plotted against the density of UWB sources

The Global Positioning System was designed to receive signals as low powered as 130 dBm at its terminals. Time Domain Corporation assumed this value for the received power at a GPS receiver in its derivation of the required isolation between a UWB source and the receiver. The isolation for acquisition and tracking was determined so that the SNR was equal to the minimum required SNR for acquisition and tracking, respectively. This method of quantifying the performance degradation of a GPS receiver is fair, rather than determining the isolation that degrades the noise floor by 1 dB. Therefore, we can conclude that the interference caused by a proliferation of UWB devices to GPS receivers is of less concern than the interference caused to cellular systems. Of course, if you put a UWB device in the near vicinity of a GPS receiver, it will cause a problem, but this is going to be the case for practically any electronic device. There are methods of isolating a GPS receiver from other electronic devices; for example, Multispectral Solutions [7] has UWB devices operating in the GPS band without interference with an integral GPS receiver built in.