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ACP-WGF28IP03
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International Civil Aviation Organization / ACP WG-F/28
IP 03

AERONAUTICAL COMMUNICATIONS PANEL (ACP)

28th MEETING OF WORKING GROUP F

Lima, Peru11 – 22 March 2013

Agenda Item 8: / Any Other Business

Update on L-Band and C-Band Air-Ground Channel Measurement Campaign

(Prepared by Kurt Shalkhauser, David Matolak and Robert Kerczewski)

(Presented by Robert Kerczewski)

SUMMARY
This information paper is intended to inform ICAO WG-F of the progress of planned L-Band and C-Band channel propagation measurements that will support the development of Control and Non-Payload Communications (CNPC) for unmanned aircraft systems.

1.INTRODUCTION

1.1At the 26th Meeting of Working Group F, information paper ACP-WG-F /26 IP8 provided a description of plans by the National Aeronautics and Space Administration (NASA) to conduct a series of flight tests to measure the propagation characteristics of the air-ground (AG) channel at two frequency bands intended for Unmanned Aircraft Systems (UAS) Control and Non-Payload Communications (CNPC), as part of NASA’s UAS Integration in the National Airspace System Project. The data from these flight tests will support the development of detailed channel models for air-ground (AG) communications in the two bands.

1.2Since WG-F/26, work has progressed in the development of the flight test system and measurement hardware, and in the development of the analysis approach to be applied to the flight test results. Initial flight tests occurred in November and December, 2012 to enable flight and measurement system checkout, develop and validate flight test procedures, and obtain initial measurement data for the development of analysis techniques. This progress is described in the following sections.

2.Channel propagation measurement system

2.1 Propagation characteristics of the CNPC AG channels are being evaluated using a two-part measurement system consisting of a flight segment and a ground segment. Spread spectrum signals are transmitted from a self-contained, mobile ground platform equipped with radio frequency test electronics and a 60-foot tall extendable antenna mast. Test signals are received and recorded in-flight on-board a jet research aircraft equipped with specialized signal detection equipment. Propagation data is recorded while the aircraft executes a scripted set of flight maneuvers in airspace in the vicinity of the ground transmitter. Equipment in the air and ground segments is synchronized using Global Positioning System (GPS) timing to allow precise measurement of the desirable line-of-sight signal as well as the undesirable, multipath, interfering signals. The ground-mobile equipment is relocated to various terrestrial settings across the United States to quantify effects of local terrain and ground clutter on the AG channels.

2.2A dual-band channel sounder is the principal equipment used to receive and record the line-of-sight and reflected (multipath) radio signals in the propagation tests. The custom channel sounder system consists of one transmit unit, two receive units, and attendant control and signal processing equipment. The sounder transmitter (Tx) unit simultaneously produces signals in the L-Band (960-977 MHz) and C-band (5030-5091 MHz). Each of two identical sounder receiver (Rx) units receives simultaneously in both bands. This arrangement provides two physically-separate receivers for each band (two L-band and two C-band) that can be connected to four separate antennas, i.e. a single-input/multiple-output (SIMO) system for each band. The L-band signal bandwidth is approximately 5 MHz, and the C-band signal bandwidth approximately 50 MHz.

The channel sounder was procured from Berkeley Varitronics Systems Inc. and delivered in late June 2012. After laboratory testing and subsequent de-bugging and optimization, the channel sounder system was ready for the first flight test in November 2012. The channel sounder units are shown in Figure 1.

The channel sounder system precisely records amplitude and timing characteristics of the signals transmitted from the ground terminal and received on the aircraft. Processing of the data produces quantitative information on signals reflected by terrain and ground structures. The use of multiple aircraft antennae allows measurement of airframe shadowing effects of aircraft wings, engines, or other protuberances. The data analysis approach is discussed in section 3.

2.3
The ground terminal used in the propagation measurements is a self-contained assembly of electrical and mechanical equipment (Figure 2). Prime electrical power is supplied by a dedicated, on-board 7-kilowatt diesel generator, allowing the ground terminal to operate without utility connection at virtually any test location. The sounder transmitter, RF support components, power conditioning electronics, and computer control equipment is contained within weatherproof enclosures. A commercially-available, pneumatically-extended mast is used to elevate L-band and C-band antennas to heights of as much as 60 feet.

2.4The flight platform used in the propagation measurements is a Lockheed model S-3B Orion jet aircraft (Figure 3). The aircraft carries two pilots and two research crew members at a maximum speed of 0.79 mach (514 mph) to a maximum service ceiling of 40,900 feet. The aircraft was selected as a result of its cargo capacity, electrical power supply capability, in-flight stability and maneuverability, and low-speed/low-altitude flight capability. For the propagation testing, the baseline flight patterns are executed at a150 knot (173 mph) airspeed at 2000 feet altitude (above ground level). Two L-band and two C-band antennas are mounted on the underside surface of the aircraft. Antennas are spatially separated by at least ten wavelengths to avoid electromagnetic coupling. Low-loss coaxial cables connect each antenna to the individual sounder receivers.

Figure 3: Lockheed Orion S-3B research aircraft. Inset: antenna locations

2.5The channel sounding flight testing campaign will include multiple test locations across the United States, achieving data collection in a diverse set of terrestrial settings and signal-dispersion conditions. The test locations, shown graphically in Figure 4, enable data collection in mountainous, hilly, and flat terrain, over salt and fresh water, amidst urban and suburban structures, and at high and low elevation angles. Frequency authorizations for both test channels have been approved that allow flight operations within the shaded areas shown (190 kilometer radius).

Figure 4: Test locations with approved frequency authorizations.

Flight tests will be conducted with the ground station in several locations within the setting, for example with the ground station in both open and cluttered areas to characterize the effect of obstruction (or partial obstruction) of the line of sight between ground station and aircraft. The ground station antenna height may also be varied. Since the receivers have two separate antenna ports, if time and logistics permit, the transmission direction will be reversed to direct radiation from the aircraft toward the ground station, enabling assessment of local ground station antenna diversity gains.

The baseline flight pattern for sounder measurements is shown in Figure 5. Red traces are flight paths of the aircraft as plotted from actual GPS data recorded by the sounder receiver. Aircraft take-off, landing, holding, and alignment traces have been removed for clarity. Upon reaching altitude, the aircraft executes multiple inbound/outbound over-flights extending from directly over the transmitter to a downrange distance of approximately 18 km. Multiple “racetrack” loops are then executed at the downrange distance. Typical test flight duration is approximately 70 minutes from the time the aircraft arrives on station. Total sounder recording time is typically 40 minutes, which records approximately 2.5 Gbytes of data into the sounder.

Figure 5: Flight paths during sounder measurement periods

On-board computers allow monitoring of sounder data in near real time. A sample screen image recorded during flight testing is shown in Figure 6. Data from all four receivers is visible simultaneously. Figure 6 demonstrates clean reception of line-of-sight signals in both frequency bands as well as low-amplitude multi-path signals in the C-band.

Figure 6: Sounder real-time viewing screen - typical flight measurement

3.ANALYSis approach and channel model development

3.1Given that no wideband models exist for the air-ground (AG) channel over the range of environments expected for UAS, these measurements will collect data that will enable construction of such models. Channel models can generally take four forms: A. stored measured data that is “replayed” for analysis and simulations; B. deterministic models that use high-frequency approximations such as ray-tracing; C. stochastic models of the traditional tapped-delay line (TDL) form, based upon empirical data; and D. parametric stochastic models that employ random distributions (based on measurements) for TDL parameters or for parameters in a “randomized” geometry-based model. Models of type A are accurate, but limited to environments and conditions in which measurements were taken. Models of type B are site-specific, and highly complex when the environment geometry is not simple. For the AG channel, we envision a combination of type C and D, which can be well-approximated by a type-B model (a “2-ray” model) when elevation angle is large and the GS antenna is elevated with the GS in a clear (“clutter-free”) environment.

The well-known 2-ray model (~simplest type B) will generally form the starting point for analysis, and this will be augmented by multipath components (MPCs) whose characteristics are determined from measurements (~type C). Depending upon the complexity of the local GS environment (buildings, mountains, etc.), geometric analysis may be used to corroborate measurement findings and provide inputs to potential geometry-based model components (~type D). A diagram of the approximate modeling process appears in Figure 7. This diagram indicates primary model inputs, and also explicitly accounts for obstructions (“shadowing”), which can be due either to ground-based structures, or the airframe itself. The phenomenon of shadowing is not well-characterized in the literature, and since shadowing attenuations can exceed 25 dB (even for airframe shadowing), our measurements and subsequent models for this phenomenon will be vital for model accuracy.

Figure 7: AG channel modeling process.

Flight tests will provide data for the modeling of both shadowing and MPC variation (amplitude and phase), and will also provide information on MPC delays, relative powers, and Doppler spreads. This will be done as a function of GS setting, frequency band, flight paths and environment geometry, and time. Inter-antenna correlations will also be measured and modeled, as will correlations across the two bands, to enable assessment of antenna diversity and frequency diversity gains. Measurements will gather data that will be used to develop models for the AG channel impulse response:

which is the sum of a line-of-sight (LOS) component, a ground reflection, and L MPCs.

3.2The approximate signal bandwidths in the two bands are 5 MHz in the L band, and 50 MHz in the C band. This corresponds to delay resolutions of approximately 200 ns in L band, and 20 ns in C band, which in turn corresponds to distance resolutions between multipath components (MPCs) of 60 m and 6 m, respectively.

4.first flight tests

4.1Flight Test #1 took place 20 November, 2012. The ground terminal was located at the NASA Glenn Research Center aircraft hangar apron, directly adjacent to Cleveland Hopkins International Airport in Cleveland, Ohio. This test activity was the first flight of the test campaign, allowing system operators to successfully demonstrate the electrical power, computer control, and communications systems on both air and ground segments. Both L-band and C-band RF channels were confirmed and simple power level measurements were made. Additionally, the aircraft flight pattern execution was confirmed by mapping against the ground antenna alignments.

4.2Flight Test #2 took place 28 November, 2012 and was executed from the same ground terminal location at the NASA GRC facility. This test activity demonstrated time synchronization of the sounder transmitter (on the ground) and the sounder receivers (in the air). This test activity demonstrated the successful recording of propagation and GPS timing data into the sounder. Additionally, GPS position data was recorded into aircraft systems.

4.3Flight Test #3 took place 5 December, 2012 and was again executed from the location at the NASA GRC facility. This test activity demonstrated the process of collection, recording, and recovery of data from the sounder, as well as the data transfer, processing, and analysis cycle.

5.initial analytical results

Flight tests on 5 December 2012 enabled us to verify channel sounder operation, and establish flight and measurement procedures. From a segment of this flight, we were able to confirm GPS accuracy, link budget parameters, and Doppler shift of the LOS signal. Fig. 2 shows estimated received power and Doppler for a segment of flight at a link distance of approximately 21 km. Measured received power is computed using two methods (equivalent, with a known offset), and results were in perfect agreement with link budget computations. Computed Doppler was also in perfect agreement with flight velocity data.

Figure8: C-band received power (left) and LOS signal Doppler shift (right) for 5 December 2012 flight test.

6.Next steps

Work has been underway to optimize the resolution performance of the channel sounder, and planned modifications have been executed and results are being analyzed. The sounder equipment will be reinstalled into the ground terminal and aircraft in late February 2013. Propagation flight tests will resume as weather permits, at the planned test locations in Ohio, Pennsylvania, Colorado, and California. At the time of submission of this paper, the next flight tests were planned for the week of 11 March 2013 (Cleveland, Ohio) and 18 March 2013 (Latrobe, Pennsylvania).

7.action by the meeting

7.1ACP WG-F is invited to note the information provided.