International Civil Aviation OrganizationACP WG-F/30IP 09

AERONAUTICAL COMMUNICATIONS PANEL (ACP)

30th MEETING OF WORKING GROUP F

ICAO Regional Offices, Bangkok, Thailand 13-19 March, 2014

Agenda Item 6: 5 GHz Band Planning

Initial C-Band Flight Test of Second Generation Prototype CNPC Radio

(Prepared by Kurt Shalkhauser, Robert Kerczewski, Joseph Ishac, and Steven Bretmersky)(Presented by Robert Kerczewski)

SUMMARY

This paper presents a description and initial results of flight tests of the second-generation, prototype, Control and Non-Payload Communications (CNPC) radio developed jointly under a cooperative agreement between the NASA Glenn Research Center and Rockwell Collins Inc. These tests are intended to support the validation of CNPC air-ground radio system requirements and the development of CNPC standards. The second generation prototype CNPC radio operates in the C-Band (5030-5091 MHz), whereas the first generation radio operated only in L-Band (960-977 MHz). Hence these tests are the first C-Band CNPC prototype radio flight tests.

1. INTRODUCTION

1.1 The US National Aeronautics and Space Administration (NASA) is executing the Unmanned Aircraft Systems Integration in the National Airspace System (UAS in the NAS) Project with the goal of reducing technical barriers to achieving routine access of unmanned aircraft (UA) to the airspace. The Communication Sub Project under the UAS in the NAS Project has among its objectives the development of technical data to validate requirements and enable the development of standards for the control and non-payload communications (CNPC) radio link between the UA and the ground control station. ACP-WG-F/25 WP 13 provided a description of the UAS in the NAS Project.

1.2 To develop the required technical data, NASA’s Glenn Research Center (GRC) is developing and testing prototype CNPC radios based upon the initial “seed” requirements from RTCA Inc. Special Committee 203. NASA GRC is utilizing a cost-sharing cooperative agreement with Rockwell Collins, Inc. to explore and perform the necessary development steps to realize the prototype UAS CNPC system. Previous contributions to WG-F ACP-WG-F /27 WP 20 and ACP-WG-F/27 WP

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21 have described technology assessment and waveform trade studies that provided the basis for the prototype CNPC system. ACP-WG-F/29 IP 4 provided design details for the prototype CNPC radio, noting that the functional designs for L-band and C-Band are identical.

1.3 The first generation CNPC radio, delivered to NASA Glenn on 28 February 2013, operated only in the L-Band, tunable from 960-977 MHz and produced approximately four watts of output power. Flight tests of this radio occurred during May and June, 2013. Flight test results were reported in ACP-WG-F/29 IP 6.

1.4 The second generation radios, which add the capability to operate over the C-Band frequency range of 5030-5091 MHz, were delivered to NASA on 27 September 2013. After laboratory testing and integration of the radios into the flight test system, the first flight test at C-Band occurred on October 25, 2013. The following sections of this paper provide a brief description of the C-Band test system, the parameters of the flight test, and initial flight test results. An overview of the upcoming C-Band test campaign is also presented, including the plans for in-flight hand-off of the CNPC link between two spatially-separated ground stations.

2. TEST SYSTEM DESCRIPTION

2.1 The CNPC radio flight test system consists of an airborne element and a ground element. The airborne element utilizes the NASA GRC Lockheed S-3B Viking aircraft (registration number N601NA), with the radio and support equipment mounted in the rear portion of the aircraft. The ground element consists of an 18 ft. trailer platform with equipment cabinet housing a similar radio and supporting equipment. Figure 1 shows the aircraft and trailer elements at Cleveland, Ohio.

Figure 1: Aircraft and Ground Station Trailer Elements

2.2 The ground station baseline configuration consisted of a Rockwell Collins CNPC radio, 28V power supply, spectrum analyzer, antenna controller, GPS time server, two computers, networking equipment and a C-Band antenna. A 60-foot pneumatic mast mounted on the trailer raised the antenna

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to a height of 65.5 ft. above the ground. A Global Positioning System (GPS) time server used the Network Timing Protocol (NTP) with the ground computers to allow for accurate time stamping of the data. Two computers were used during the tests, one to control the configuration of the radio and the other computer to populate the frames generated by the link stream to send user data. The C-band antenna was a directional antenna mounted on a mast of the trailer steered by the mast controller to the desired direction (all tests utilized a fixed azimuth and the aircraft was not tracked during flight).

2.3 The aircraft-based CNPC radio electronics system was very similar to that of the ground station, consisting of a Rockwell Collins radio, spectrum analyzer, GPS time server, two computers, and networking equipment. The electronics were rack mounted in two racks in the rear of NASA’s S-3B aircraft (Figure 2). A C-band omnidirectional antenna was installed on the bottom surface of aircraft, at the location shown in Figure 3.

Figure 2: Aircraft Equipment Racks

Figure 3: S-3B Aircraft showing C-band antenna location

3. FLIGHT TESTS

3.1 The flight test for the C-band radios occurred on October 25, 2013. The primary objective of the test was to operate the radio pair in an air-ground flight environment to determine communications range at various transmit power levels. For this test, the ground station was transported to a site at NASA’s Plum Brook Station near Sandusky, Ohio, USA where the ground radio and antenna could be placed in an open, rural environment (Figure 4). This site offered access to a low-traffic flight corridor that extended more than 150 nautical miles (nmi) across the state of Ohio, enabling extended periods of straight-line flight at constant airspeed and altitude. The Plum Brook Station site (elev. 653 ft MSL) was free from buildings, structures, and local man-made ground clutter. In this setting and with appropriate aircraft altitude planning, the CNPC signal could be line-of-sight (LOS) between the ground station and aircraft, without structural obstruction or significant terrain elevation changes. Sky conditions were generally clear with no precipitation.

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Figure 4: Ground Terminal at Plum Brook Station

3.2 The C-band ground antenna was elevated to 65 feet above ground level by the tower mast with the antenna boresight directed along the intended flight path on a magnetic compass heading of 213° (south-southwest). The antenna was a commercially-available dipole-and-reflector arrangement that offered nominal gain of 6 dBi with 3 dB beamwidths of 180° (azimuth) and 35° (elevation). The antenna had been used extensively in previous measurements and its performance was well understood. Low-insertion-loss coaxial cables were connected between the radio and antenna.

3.3 The flight test began with an overhead pass of the ground station antenna then proceeding directly into a long-range, straight-and-level, constant-velocity “outbound” flight leg on the 213° heading. This radial path provided the opportunity to collect received signal strength data for determining path loss versus distance, leading ultimately to radio operational range information. The outbound flight continued until the radio frequency signal received by the radio dropped below the radio sensitivity limit and the communications link was lost. The aircraft pilot then performed a course reversal, turning the aircraft 180 degrees onto the opposing, or “inbound” flight heading. The pilot continued on the inbound heading (33° magnetic) for a brief time until the CNPC signal was re-acquired by the radio, for confirmation of the radio slant range. The aircraft then performed two additional reversals to complete an “orbit” around the signal “drop-out” point. Once the orbit was completed, the aircraft continued inbound toward the ground station.

3.4 The flight track for the test (Figure 5) was constructed from global positioning system (GPS) data recorded by the S-3B aircraft avionics. The path shows the take-off from Cleveland, Ohio towards the ground station, then straight-line travel along the 213° heading from the ground station toward Cincinnati, Ohio. The aircraft orbit area is visible at the extreme southwest end of the flight path, occurring northeast of Cincinnati near Wilmington, Ohio. After the inbound flight and overflight of the ground station, the aircraft completed a second, shorter outbound/inbound run along the same flight vector (also visible in the figure). The airspeed for the testing was nominally 270 knots.

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Figure 5: Aircraft flight track, 25 October 2013

3.5 The airborne and ground radios were controlled by custom software that recorded radio performance data throughout the entire test flight. Data on the ground-to-aircraft signal performance was recorded on the aircraft, while aircraft-to-ground signal performance was recorded at the ground station. Post-processing of the data allowed the data sets to be joined and synchronized using the GPS time-stamps, then analysis could be performed of the uplink/downlink performances.

4. FLIGHT TEST RESULTS

4.1 The results of the 25 October 2013 flight test are plotted in Figure 6. A time scale is plotted along the horizontal (x) axis of the figure, which encompasses the entire test activity from aircraft pre-flight ground and taxi operations through flight maneuvers. Received signal strength is plotted along the vertical (y) axis in the uppermost section of the figure. Both the ground radio and aircraft radio data are presented on the same grid in separate traces.

4.2 The CNPC radio waveform supports several different modes for the uplink and downlink to accommodate various traffic characteristics and data loads. For this test, the uplink waveform operated in a single-slot (UL1) mode intended for a controlling (commanding) a single unmanned aircraft (UA). For the downlink, the waveform operated in the command & control (C2) mode, which supports the minimum set of downlink data necessary for operation of the UA. This includes C2 communications responses, navigational aids information, voice relay for Air Traffic Control (ATC) operations, and surveillance target data. The uplink UL1 data is presented as the dark blue trace, and the downlink C2 data is presented in the red trace.

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4.3 To aid in the analysis, a trace showing the expected (calculated) signal strength at the receiver was plotted in Figure 6. This trace (black) was computed using the theoretical free-space propagation loss for the ground-air path, transmit and receive antenna gain, transmit power level, and filter and cable losses. The areas where the theoretical trace is broken are areas where terrain blockage is predicted. Notably, the shape of the theoretical curve is close to the experimental data.

4.4 The Rockwell Collins CNPC radios have the ability to operate at reduced output power levels. This feature was used to create the other two traces shown in Figure 6 (light blue and salmon colors), now representing UL1 and C2 link data at a 4 watt radio output power level. The 6dB reduction in radio transmitter output power was intended to simulate the reduced gain of an omnidirectional antenna that would be used in subsequent tests. The data traces again show close tracking of the uplink and downlink performance.

4.5 In the space below the received signal strength trace plots, Figure 6 presents data on average percentage frame loss at the aircraft and at the ground station receivers. Individual traces for the UL1 and C2 data modes are shown. When the CNPC communications path is transferring all data without error, the data is presented as 0% loss and no colored trace is visible on the grid. When data packet errors occur in the radio link, the lost frame data creates a visible trace ranging from 1% up to 100% (total loss of radio link). The areas showing large blocks of color are periods when the CNPC link has been lost.

4.6 The bottom three plots in Figure 6 show the slant range between the aircraft and ground station, the aircraft altitude and the aircraft roll. Range and aircraft maneuvers will be shown to correlate with changes in signal strength.

4.7 Upon takeoff from Cleveland, Ohio, the NASA S-3B aircraft climbed to an altitude of 13,600 ft. MSL in preparation for the first segment of the test flight. All test operations during the trip from Cleveland to Plum Brook Station occurred in the back- and side-lobe regions of the ground antenna, and are not of interest in this test.

4.8 The first controlled test flight segment began with the overflight of the ground station (Figure 4) which is annotated near time 14:17 in Figure 6. Once the aircraft cleared the null directly above the ground antenna, the received signal strength maximized then began a progressive decrease as the aircraft traveled outbound, away from the ground station. The aircraft continued on the 213° outbound radial for nearly 24 minutes, until approximately 14:41:30, when the first UL1 and C2 data packet errors occurred (red and dark blue frame loss traces). This event occurred when the received signal strength reached approximately -118 dBm at both the aircraft and ground radios (red and dark blue upper traces). The aircraft slant range at the time of drop-out was approximately 120 nmi. The aircraft altitude throughout the flight segment was 13,600 ft MSL, approximately 13,000 feet above the ground antenna.

4.9 Once the aircraft reversed course and began traveling back toward the ground station, the radio re-acquired the CNPC signal at a signal strength near -118 dBm (at time 14:48:30), and data packet errors were significantly reduced. The slant range and signal strength values were confirmed a total of three times as the aircraft completed the orbit flight maneuvers near time 14:51:00. The S-3B aircraft was operated in flat and level attitude during the signal drop-out and re-acquisition events to reduce airframe shadowing influences.

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Figure 6: C-band Radio Performance Data and Associated Aircraft Parameters

4.10 The next major flight segment was the continuous inbound flight along the 33° heading to return to and overfly the ground station antenna. Immediately prior to this segment, the aircraft altitude was reduced to 9600 ft MSL to investigate the effects of predicted terrain obstruction. As expected, the radios reacquired the UL1 and C2 signals at a received signal strength of approximately -117 dBm (15:01:00), but did so at a closer range, 106 nmi. That is, the aircraft needed to be at closer range to the ground station at the lower altitude in order to achieve a reliable communications link. The reader will note that the UL1 and C2 curves exhibited an increased slope in the early minutes of the return flight due to diffraction caused by terrain obstructions. The predicted clear LOS to the ground station did not occur until 15:02:30. The inbound flight segment continued for approximately 24 minutes with the signal strengths increasing until the aircraft overflew the ground station antenna at 15:25:30.

4.11 The UL1 and C2 modes have very similar waveform properties, both utilizing a 75 kHz bandwidth for a 87.5 kbps data rate. As a result, the propagation characteristics recorded during the flight test are nearly indistinguishable, causing the red and dark blue signal strength traces to overlap throughout the entire flight. Both radios were operated at their full output power level of 18 watts, at a center frequency of 5080 MHz.

4.12 The second inbound/outbound test cycle on the 213° vector was initiated near time 15:28:30. For this test cycle, the low-power channels were reduced to a transmitted output power level of approximately 0.3 watts in order to observe channel loss due to low signal strength rather than terrain obstruction. Once the aircraft had separated from the ground station by approximately 40 nmi, the frame loss percentage began gradually increasing (near time 15:39) until it reached 100% packet loss near time 15:40. The aircraft performed an abrupt, banked turn at 15:41 which caused airframe shadowing that resulted in loss of the high-power, 18-watt channels as well (illustrated by the abrupt red and dark blue blocks in the high-power frame loss traces). The point to note from this flight cycle was that even at a transmit power of 0.3 watts, the UL1 and C2 CNPC channels were able to reach a range of 40 nmi before packet errors occurred.

5. SUMMARY OF TEST RESULTS

5.1 The test flight performed on 25 October 2013 demonstrated the general capabilities of the second generation, CNPC prototype radios operating in C-Band (5030-5091 MHz) and the performance of the test system. The received signal strength performance data from the radio followed the predicted signal strength curve well. The CNPC radio sensitivity also allowed measurement data to be collected when the aircraft was operating beyond LOS, thereby demonstrating diffraction effects caused by terrain obstructions. Depending on aircraft altitude and radio transmitter output power, LOS slant ranges of up to 120 nmi were achieved, which exceed the 69 nmi goal of the radio design.

6. FUTURE ACTIVITIES

6.1 A thorough test flight campaign using the Generation 2 radio sets is scheduled to begin in the Spring of 2014. Initial testing will again utilize the NASA S-3B aircraft and a single ground station, both equipped with a single C-band CNPC radio. Tests will first study air-ground communications in a single coverage “cell” area using omnidirectional antennas on both the aircraft and ground station. Once that performance has been recorded, a duplicate ground station will be installed at a location approximately 130 nmi away to facilitate a series of cell-to-cell hand-off demonstrations. To enable this demonstrate, a comprehensive ground-base network will be established which will utilize secure hand-off algorithms. Flight tests will first demonstrate and characterize range performance of the C-band radios, then progress into acquisition, hand-off, and release tests.

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7. ACTION BY THE MEETING

7.1 ACP WG-F is invited to consider the information provided in this paper describing the advances occurring in the development of the CNPC radio for operations in the 5030-5091 MHz bands.

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