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ACP-WGF29 / IP06/
International Civil Aviation Organization / ACP WG-F/29
IP 06
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
29th MEETING OF WORKING GROUP F
Nairobi, Kenya05-12September 2013
Agenda Item 6: / 5 GHz Band PlanningFlight Tests of First Generation Prototype CNPC Radio
(Prepared by Kurt Shalkhauser, James Griner and Robert Kerczewski)
(Presented by Robert Kerczewski)
SUMMARYThis paper presents a description and example results of flight tests of the first generation prototype 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.
1.INTRODUCTION
1.1The 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.
1.2To 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. NASAGRC 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. These activities include investigation of signal waveforms and access techniques, development of representative CNPC radio hardware, and execution of relevant testing and validation activities. These NASA/Rockwell activities do not intend to manufacture the CNPC end product, rather, the goals are to study, demonstrate, and validate a typical CNPC system that allows safe and efficient communications within the L-band and C-band spectrum allocations and develop the necessary data to inform the requirements and standards development processes.
1.3Previous WGF papers have described the results of the communications technology assessment (ACP-WG-F/27 WP20) and communications waveform trade study (ACP-WG-F/27 WP21) which provided the basis for the first generation prototype CNPC radio design. Rockwell Collins implemented the selected waveform using an existing hardware platform to build the first generation CNPC prototype radio, delivered to NASA Glenn on 28 February 2013. The first generation radio operates only in the L-Band, is tunable from 960-977 MHz and produces approximately four watts of output power. The second generation radio now in development will also operate over the C-Band (5030-5091 MHz). Thus flight testing described in this paper occurred using the 960-977 MHz band, however a preliminary version of the C-Band radio was tested during the final flight test and a brief description of that test is also provided.
1.4The first generation radios were tested extensively in NASA Glenn’s UAS laboratory. Software code to control, monitor, and flow data through the radio system developed by NASA Glenn and was tested with the radios prior to installation of the radios into the flight test system.
1.5The following sections of this paper provides a description of the test system, the parameters of the flight tests, several examples of flight test results, a brief description of the preliminary C-Band radio tests, and plans for future activities.
2.TEST SYSTEM DESCRIPTION
2.1 The test system is comprised of an airborne element and a ground station element. The airborne element consists of the NASA GRC Lockheed S-3B Viking aircraft (registration number NA601A) with the radio and support equipment mounted in the rear of the aircraft. The ground element consists of an 18 ft. trailer platform with equipment cabinet housing the radio and supporting equipment. Figure 1 shows the aircraft and trailer elements.
Figure 1 – Aircraft and Trailer Elements
2.2The 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 an L-band antenna. A 60 foot pneumatic mast mounted on the trailer raised the antenna 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 computers located on the ground to allow for accurate time stamping of the data. A similar set-up was on the aircraft. Two computers were used during the tests. One was used to control the configuration of the radio and the other computer was used to populate the frames generated by the link stream to send user data. The L-band antenna is a directional antenna mounted on a mast of the trailer steered by the antenna controller to the desired direction (all test utilized a fixed azimuth and the aircraft was not tracked during flight).
2.3The aircraft baseline configuration is very similar to the ground station configuration consisting of a Rockwell Collins radio, spectrum analyzer, GPS time server, two computers, networking equipment and an L-band antenna mounted in NASA’s S-3B aircraft. The components are rack mounted in two racks in the rear of the aircraft and the L-band antenna is an omni-directional antenna mounted on bottom of the aircraft, at the location shown in Figure 2.
Figure 2 – Aircraft Equipment Racks and Aircraft Antenna Placement
3.Flight Tests
3.1The flight test campaign for the radios consisted of seven separate flights occurring on seven days over the period of May 22, 2013 through June 18, 2013. Each flight test consisted of multiple, pre-planned aircraft maneuvers and flight path segments. The objective of the flight test campaign was to operate the radios in an air-ground flight environment to determine possible limitations to communications range and data throughput performance. Flight and ground data was sent to Rockwell Collins for a more detailed examination, in order to enable changes before delivery of the Generation 2 radios. Flight plans were individually tailored to address the type of test being performed. Weather and air traffic issues had only minor impact on the flights.
Table 1 – Summary of Generation 1 CNPC Prototype Radio Flight Tests
Flight Test # / Date / Ground Station Location / Test Setting / Objectives / Comments1 / 22 May 2013 / Cleveland, Ohio
Ant.: 312° / Flat terrain; suburban structures in antenna field of view; over areas of fresh water / Verify test systems; Verify radio operation / First flight tests of the radios, performance verified. Operated in congested air traffic environment.
2 / 23 May 2013 / Sandusky, Ohio
Ant.: 212° / Open, rural terrain. / Test radio operational range / 90-100 nmi slant range achieved at 7500 ft. AGL
3 / 24 May 2013 / Sandusky, Ohio
Ant.: 212° / Open, rural terrain with maximum hills 490 feet above ground station antenna. / Assess measurement system repeatability, altitude influence, and impact of LOS obstructions / Confirmed presence of terrain obstructions. Radio data shows excellent stability and test-to-test repeatability.
4 / 28 May 2013 / Sandusky, Ohio
Ant.: 178° / Open, rural terrain with 800ft max. hills; aircraft in guided approach to MSL / Assess radio performance during guided airport approach / Demonstrated continuity of CNPC radio communications down to less than 500 feet altitude.
5 / 29 May 2013 / Sandusky, Ohio
Ant.: 178° / Open, rural terrain with 800ft max. hills; aircraft performs touch-and-gos at MSL runway 23. / Assess radio repeatability; examine signal interruption during runway touchdown. / Observed line-of-sight signal interruptions and re-acquisitions. All signals lost at 200 feet above runway surface. Radio performance similar at both high and low ends of CNPC L-band.
6 / 31 May 2013 / Sandusky, Ohio
Ant.: 212° / Open, rural terrain with maximum hills 490 feet above ground station antenna. / Assess data flow and networking performance using real-time aircraft data / Confirmed no measurable impact using actual aircraft data. Confirmed that actual in-flight parameters can be successfully transferred in CNPC channel.
7 / 18 June 2013 / Cedar Rapids, Iowa
Ant.: Omni / Open, flat, rural terrain. No hills. Aircraft at 10,000 ft. MSL (~9,000 ft.AGL) / Assess LOS signal range with ground antenna installed on 300-ft. radio tower / Slant range of 130 nmi achieved, limited only by Earth curvature.
3.2Each flight test in the campaign included an overhead pass of the ground station antenna and a long-range, straight-and-level, constant-velocity “outbound” leg on a predetermined flight vector/heading. This radial path provided the opportunity to collect data on line-of-sight (LOS) path loss versus distance, leading ultimately to radio operational range information. The outbound path typically continued until the power 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 180 degrees for an “inbound” run on the opposing flight heading. The radios, one at the ground station and one in the aircraft, communicated throughout the inbound and outbound runs. Multiple inbound and outbound segments were sometimes flown on a given test day. Aircraft altitude and airspeed were typically constant during a given segment, but were sometimes altered between runs to collect data to investigate terrain obstructions. Airport approach and touch-and-go maneuvers were flown to investigate the effects of terrain and airport-area structural obstructions, and to demonstrate radio signal recovery upon take-off and ascent. Table 1 provides a summary of the radio test flight campaign.
3.3The radios were controlled by custom software that permitted configuration of the radios and recorded radio performance data throughout the test flight. Each data set also includes radio performance during set-up, course reversal, altitude change, and other miscellaneous maneuvers.
3.4Seven flight tests were performed during the campaign. The initial flight test occurred with the ground station located at NASA GRC and aircraft operations from the adjacent Cleveland Hopkins International Airport (CLE). This flight test provided the first air-ground radio communications and allowed for test system validation and adjustment. Average airspeed for Flight Test 1 was 168 knots.
3.5For the second through sixth flight tests, the ground station was relocated approximately 50 miles west of CLE to NASA’s Plum Brook Station (PBS) facility near Sandusky, Ohio. This placed the ground station in a slightly quieter electromagnetic environment, away from the sources of possible interfering signals. More importantly, the PBS site allowed the aircraft to operate in a less-congested airspace where long-range flight paths and maneuvers could be executed with fewer deviations from the flight plan. An air corridor running approximately 100 nautical miles to the southwest of PBS would also be free of tall structures and major terrain elevation changes. Air traffic control rules in the Plum Brook region allowed greater flexibility in requested aircraft altitudes. Average airspeed for flight tests 2 through 6 was 250 knots.
3.6The seventh flight test occurred in Cedar Rapids, Iowa at the Rockwell Collins facility. For this test, a 300-ft. tall tower provided a significantly higher elevation of the ground station antenna. This coupled with the relatively flat and open terrain of eastern Iowa offered the opportunity to measure the maximum range of the radio. Average airspeed for flight test 7 was 297 knots.
4.Flight test RESULTS
4.1Test results for three of the seven flight tests are summarized below: the first flight test demonstrating the initial operations of the Generation 1 CNPC radios; the second flight test demonstrating the maximum range of radio operation for the 65.5 ft. ground station height; and the seventh flight test demonstrating the maximum range of radio operation for the 300 ft. ground station height.
4.2Flight Test #1 – 22 May 2013 – Cleveland, Ohio, USA
4.3The first flight test of the radios occurred in airspace northwest of CLE. This area is of generally flat terrain with a limited number of tall structures entering the ground station antenna field of view. The northern portion of the flight area is over Lake Erie, offering the opportunity to observe the impact of possible signal reflections from the freshwater surface.
4.4The objective of test 1 was to verify that the bi-directional communications link could be established between the two radios, and that the link could be maintained during aircraft maneuvering, as well as verifying operations of the attendant ground electronics and aircraft electronics, including computers, timing equipment, direct current power systems, radio frequency components, and supporting air-ground communications.
4.5The flight track for test flight #1 is shown in figure 3. Upon takeoff, the NASA S-3B aircraft climbed to 3000 ft. MSL and maintained that altitude for the duration of the test flight.
4.6The results of the flight test are plotted in Figure 4. A time scale is plotted along the horizontal (x) axis of the figure, which encompasses the entire test activity from pre-flight ground and taxi operations through flight maneuvers. Signal strength is plotted along the vertical (y) axis in the top plot. Both the ground radio and aircraft radio data are presented on the same grid in red and blue traces, respectively. The calculated expected theoretical free space is also show as the black line in this plot.
Figure 3 – Flight Test #1 Flight Track, 22 May 2013 – Cleveland, Ohio, USA
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ACP-WGF29 / IP06Figure 4 – Flight Test 1 Radio Performance Data and Associated Aircraft Parameters
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ACP-WGF29 / IP064.7The first controlled test flight segment begins at map point B (Figure 3) and is annotated near time 15:18 in Figure 4. The received signal strength shows a steady increase as the aircraft approaches the ground station inbound. As the aircraft overflies the ground station antenna, the received signal strength drops rapidly (time 15:22) as the aircraft moves into a null in the antenna radiation pattern, then into the background of the antenna. Once the aircraft has reversed course and again overflies the ground station antenna, the received power peaks (time 15:25) and begins a steady decrease that continues throughout the 25 nmi outbound flight.
4.8Test flight 1 included two sets of “racetrack” orbits, each performed approximately 11 nmi downrange from the ground station. Signal strength data for the first set begins near time mark 15:40 and data for the second set begins near time mark 16:20. Throughout the racetrack maneuvers the signal strength dwells near -80 dBm with slightly higher received power during travel on the near-side leg and slightly lower received power during travel on the far-side leg. The 180° turns at the ends of the racetrack required aircraft banking, which caused shadowing of the aircraft antennas and breaks in the line-of-sight signal path. This shadowing effect caused received signal power level to drop abruptly and significantly at each turn. The shadowing periods are highlighted in Figure 4 (racetrack periods only) for aircraft roll angles greater than 10 degrees.
4.9The aircraft remained in relatively close range to the ground station throughout Flight Test #1, so the LOS between the 65-foot tower and the aircraft was never interrupted by terrain obstructions. The periods of packet loss in the figure occurred during radio configuration change, wing shadowing (aircraft roll), or when the aircraft was out of the antenna beam.
4.10 In graphs below the received signal strength trace, Figure 4 presents data on average percentage frame loss at the aircraft and at the ground station receivers. Individual traces for the UL1 (uplink, one subframe transmitted per frame), UL20 (uplink, all 20 subframes per frame), C2 (downlink data channel), and video (downlink video channel) 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 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 first portion of the flight test operated in configuration 1, with only UL1 and C2 data modes active. The in-flight changeover to configuration 2 (UL20 and video modes) was made near time marker 15:53. This is made obvious by the brief periods of 100% frame loss. Data traces for these modes show 0% frame loss throughout the inbound and outbound passes. Some frame loss was recorded during the aircraft course reversals, as expected. The radio was changed to configuration 3 near time mark 16:32, which returned to UL1 and C2 data modes and flowed a different form of digital data during the final racetrack orbit. The alternate data had no detectable effect on the received error percentage.
4.11The bottom three plots in Figure 4 show the range between the aircraft and ground station, the aircraft altitude and the aircraft roll. Range and aircraft maneuvers can be seen to correlate with changes in signal strength.
4.12Flight Test #2 – 23 May 2013 – Sandusky, Ohio, USA
4.13The objective of the second CNPC flight test was to begin examination of the operating range of the radios. This test would establish the communications link between the ground station and aircraft radios then fly along a fixed radial vector until the communications link was lost. Data from the test would determine if the CNPC link was lost due to either radio sensitivity limits (from increased propagation losses) or because of LOS signal blockage by ground obstructions.