Field Strength Measurements of DGPS and FAA Beacons
in the 285 to 325 kHz Band

PURPOSE

The purpose of these measurements is to determine absolute DGPS signal strengths at various distances from eight different Coast Guard beacons transmitting DGPS signals and to measure signal strength at various distances from an FAA beacon located in the same frequency band. Four of the measured DGPS beacons are located along the Gulf Coast and four are on the West Coast. The measured FAA beacon is located in Bennett, Colorado east of Denver. Each of the transmitters are described in Table 1. Results of the measurements are used to provide model inputs and to compare results to actual model predictions.

Figure 1. Transmitter at Aransas Pass.

Measurements in the southern region were conducted by driving along the Gulf between Corpus Christi, Texas and Mobile Alabama. On the first day, the measurement van was driven to the Coast Guard station at Aransas Pass (Figure 1). Starting approximately 17 km from the transmitter, signal strength measurements (304 kHz band only) were performed as the measurement vehicle was driven along the southwest edge of the bay, through Corpus Christi, circling around the bay to the north. On the second day, measurements were conducted between Corpus Christi, Texas and Lafayette, Louisiana while traveling along highways 77, 59, and Interstate 10. All four Coast Guard transmitters were monitored continuously (except for a short break as the vehicle came in close proximity to the Galveston site). The vehicle was driven to the town of Galveston via Highway 6 and then transported by ferry across the bay. Measurements resumed on the opposite shore.

Figure 2. Transmitter at Mobile Point.

Data were acquired to within five km of the Galveston transmitter. Shortly after reaching the opposite side of the bay (approximately 6:00 p.m.), the sun set. For the remainder of the distance between Galveston and Lafayette, data were acquired at nighttime. On the third day, measurements were conducted continuously on all four Gulf State transmitters as the vehicle traveled between Lafayette, Louisiana and Mobile, Alabama along Interstate 10. En route, the measurement vehicle came within 11 km of the English Turn transmitter. On the fourth day, measurements were conducted near the Mobile Point transmitter (300 kHz band only) as the vehicle was driven to the transmitter site from the west side of the bay. Data were acquired to within 10 km of the transmitter (Figure 2).

On the West Coast, signal strength measurements were conducted on each of the four DGPS transmitters by driving the following routes: Data were acquired on the Cape Mendocino signal while driving north from Santa Rosa, California along Highway 101 within 12 km of the transmitter and continuing north to Crescent City, California. From there, the measurement vehicle was driven along Highway 199 to Grants Pass, Oregon. Cape Mendocino is located approximately halfway between Santa Rosa and Grants Pass along Highway 1. Data were acquired on the Fort Stevens signal starting at a distance of 10 km from the transmitter on Highway 30, traveling along the Columbia River east to The Dalles. Both nighttime and daytime data were acquired on the Pigeon Point and Point Blunt transmitters. Pigeon Point is located approximately 45 km north of Santa Cruz along Highway 1, and Point Blunt is located on Angel Island in the San Francisco Bay. Nighttime data were acquired by driving south from Willits, California along Highway 101 to the Golden Gate Bridge and continuing along Highway 1 to Santa Cruz. Daytime data were acquired by starting 6 km south of the Pigeon Point transmitter traveling south to Santa Cruz, across the coastal mountains on Highway 17, north on the eastern border of the bay along Interstate 680, east along Interstate 80, over the Sierra Nevada mountains, and past Reno, Nevada.

Data on the FAA beacon at Bennett, Colorado were acquired on two different routes. The first route started at the transmitter traveling west along Interstate 70 over the Rocky Mountains to Grand Junction, Colorado. The second route started at the transmitter traveling west along Interstate 70 to Interstate 25 and then north to approximately 50 km north of Cheyenne, Wyoming. The primary reason for measuring the signal strength on the FAA beacon is to examine the propagation characteristics through the Rocky Mountains west of Denver.

Figure 3. Block diagram of measurement system.

The measurement system (seen in Figure 3) consists of a receiving antenna, low pass filter, amplifier, spectrum analyzer, GPS/DR receiver, and a computer. The antenna was calibrated for antenna correction factors at 2 kHz intervals between 285 kHz and 325 kHz (see Appendix D). The low pass filter serves to filter out all frequencies above 400 kHz, particularly the AM broadcast bands. The computer is used to control the spectrum analyzer, download raw data, gather GPS information, and perform various computations. The overall gain of the system (including cables) is approximately 24 dB. The noise figure of the system is 9.6 dB, giving a sensitivity of 7.85 dB µV / m for a bandwidth of 300 Hz (see Appendix C). The GPS receiver has a dead reckoning system such that, if satellite lock is lost, the proper coordinate information is maintained.

The computer controls the acquisition of data using GPIB and RS232 commands sent to the spectrum analyzer and GPS receiver, respectively. Data are also downloaded via GPIB and RS232. Two types of data are acquired: noise and DGPS signal strength.

Prior to acquiring data on the DGPS signal, the power of the noise is determined by making measurements 500 Hz offset from the DGPS carrier frequency. The purpose of measuring noise is simply to determine if the signal, as a whole, can be detected against the noise background. The purpose is not to determine environmental noise, even though, in many cases, this may be the primary source of noise. Three types of measurement procedures were used: type 1 in the Gulf States, type 2 for Cape Mendocino, and type 3 for all the remaining beacons along the West Coast and Colorado. Type 1 measurements use a 500 Hz span and sweep across the frequencies in that span. To avoid measuring power from the adjacent DGPS signal itself, the resolution bandwidth is set to 30 Hz. A single sweep is performed in 360 ms, after which the mean, the standard deviation, and the peak noise power are determined from 601 data points evenly spaced across the 500 Hz bandwidth. Type 2 measurements use a zero span with a 300 Hz bandwidth and sweep in time. A single sweep is performed in 700 ms, after which the mean, the standard deviation, and the peak noise power are determined from 601 data points evenly spaced in time. In addition, 50 samples evenly spaced across the 601 data points are stored in the record header. Because the filters on the HP8563 spectrum analyzer have a gentle roll-off and the measured signals are only 500 Hz offset from the location of the noise measurements, this technique results in an upward bias due to the contribution of power by the adjacent signal. Therefore, the measurement technique was changed to a type 3 noise measurement shortly after completing data acquisition on the Cape Mendocino beacon. The type 3 measurement uses a 200 Hz span with a 10 Hz bandwidth and sweeps across the frequency band. A single sweep is performed in 350 ms, after which the mean, the standard deviation, and the peak noise power are determined from 601 data points evenly spaced in time. In addition, 50 samples evenly spaced across the 601 data points are stored in the record header. For all three techniques, there is an upward bias when the noise measurements are conducted in close physical proximity to the transmitter because the signal can significantly contribute to the power received within the bandwidth. This, again, is because the filters in the HP8563 spectrum analyzer have a gentle roll-off and the measured signals are only 500 Hz offset from the location of the noise measurements.

The DGPS signal strength is measured by performing 50 separate sweeps of a 500 Hz span spaced between one to five seconds apart. After each sweep, the signal power is determined and recorded in the data file. For a type 1 acquisition, the peak power over the entire span is determined and recorded. For a type 2 and 3 acquisition, the power is determined at the DGPS center frequency. The latter is believed to be better since the former may cause a slight upward bias from spurious noise spikes. Based on the antenna correction factor and the known frequency, the signal field strength is calculated as described in Appendices A and B. With each sweep, a data string from the GPS / DR system is downloaded and placed in the file to mark the location of the acquisition. Each of the four parameters (frequency, power, field strength, and GPS coordinate string) are recorded for each of the 50 sweeps. The DGPS transmission rates are either 100 or 200 bps with minimum shift keyed modulation (MSK). For MSK, 99% of the power is contained within a bandwidth equal to 1.17 times the bit rate. Therefore, the resolution bandwidth of the spectrum analyzer is set at 300 Hz when acquiring DGPS signal strength data. When more than one band is monitored, the system acquires noise data and DGPS signal strength data (on the 50 sweeps for a particular frequency band) and then performs the same operations on each of the successive bands. After completing the data acquisition on each of the bands, the entire process is repeated continuously. The structure of the data files is described in greater detail in Appendices A and B.

Two calibration procedures are performed before acquiring data. The first procedure is used to determine the total gain between the output of the antenna and the measured power determined by the spectrum analyzer. This gain is subtracted from the measured power so as to determine the power at the output of the antenna. The procedure amounts to putting a signal of known power into the cable normally connected to the antenna and measuring the power on the spectrum analyzer. The difference between the known power and the measured power is the gain of the system, which in this case is approximately 24 dB.

The second calibration procedure is used to determine any differences in antenna characteristics between those measured in a laboratory (during which the antenna correction factor is determined) and the characteristics of the antenna when placed on the measurement van. First, the antenna is placed at the center of a 1.3 meter round backplane which, in turn, is mounted on a tripod approximately one meter above ground. This is used to simulate the measurement conditions in the laboratory. The power of a known transmitted signal (such as a DGPS signal) is measured. Then the antenna is mounted on the measurement van and the power of the same signal is measured at eight different azimuth orientations by the van (approximately 45^o apart). Results show essentially no difference between the two different antenna mounts and among the eight different azimuth orientations.