Flight Tests of First Generation Prototype CNPC Radio



|[pic] | |ACP WG-F/29 |

| |International Civil Aviation Organization | |

| | |IP 06 |

| | | |

AERONAUTICAL COMMUNICATIONS PANEL (ACP)

29th MEETING OF WORKING GROUP F

Nairobi, Kenya 05-12 September 2013

|Agenda Item 6: |5 GHz Band Planning |

Flight Tests of First Generation Prototype CNPC Radio

(Prepared by Kurt Shalkhauser, James Griner and Robert Kerczewski)

(Presented by Robert Kerczewski)

|SUMMARY |

|This 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. 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.

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. 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.

3. Previous 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.

4. The 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.

5. The 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

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.

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Figure 1 – Aircraft and Trailer Elements

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 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).

3. The 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.

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Figure 2 – Aircraft Equipment Racks and Aircraft Antenna Placement

3. Flight Tests

1. The 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 |Test Setting |Objectives |Comments |

|# | |Location | | | |

|2 |23 May 2013 |Sandusky, Ohio |Open, rural terrain. |Test radio operational |90-100 nmi slant range achieved at |

| | |Ant.: 212° | |range |7500 ft. AGL |

|3 |24 May 2013 |Sandusky, Ohio |Open, rural terrain with |Assess measurement system|Confirmed presence of terrain |

| | | |maximum hills 490 feet |repeatability, altitude |obstructions. Radio data shows |

| | |Ant.: 212° |above ground station |influence, and impact of |excellent stability and test-to-test |

| | | |antenna. |LOS obstructions |repeatability. |

|4 |28 May 2013 |Sandusky, Ohio |Open, rural terrain with |Assess radio performance |Demonstrated continuity of CNPC radio |

| | | |800ft max. hills; |during guided airport |communications down to less than 500 |

| | |Ant.: 178° |aircraft in guided |approach |feet altitude. |

| | | |approach to MSL | | |

|5 |29 May 2013 |Sandusky, Ohio |Open, rural terrain with |Assess radio |Observed line-of-sight signal |

| | | |800ft max. hills; |repeatability; examine |interruptions and re-acquisitions. All|

| | |Ant.: 178° |aircraft performs |signal interruption |signals lost at 200 feet above runway |

| | | |touch-and-gos at MSL |during runway touchdown. |surface. Radio performance similar at|

| | | |runway 23. | |both high and low ends of CNPC L-band.|

|6 |31 May 2013 |Sandusky, Ohio |Open, rural terrain with |Assess data flow and |Confirmed no measurable impact using |

| | | |maximum hills 490 feet |networking performance |actual aircraft data. Confirmed that |

| | |Ant.: 212° |above ground station |using real-time aircraft |actual in-flight parameters can be |

| | | |antenna. |data |successfully transferred in CNPC |

| | | | | |channel. |

|7 |18 June 2013 |Cedar Rapids, Iowa|Open, flat, rural |Assess LOS signal range |Slant range of 130 nmi achieved, |

| | | |terrain. No hills. |with ground antenna |limited only by Earth curvature. |

| | |Ant.: Omni |Aircraft at 10,000 ft. |installed on 300-ft. | |

| | | |MSL (~9,000 ft. AGL) |radio tower | |

2. Each 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. The 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.

4. Seven 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.

5. For 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.

6. The 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

1. Test 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.

2. Flight Test #1 – 22 May 2013 – Cleveland, Ohio, USA

3. The 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. The 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.

5. The 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.

6. The 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.

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Figure 3 – Flight Test #1 Flight Track, 22 May 2013 – Cleveland, Ohio, USA

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Figure 4 – Flight Test 1 Radio Performance Data and Associated Aircraft Parameters

7. The 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.

8. Test 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.

9. The 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.

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.

11. The 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.

12. Flight Test #2 – 23 May 2013 – Sandusky, Ohio, USA

13. The 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.

14. For the second flight test, the ground station was relocated approximately 50 miles west of Cleveland, Ohio to NASA’s Plum Brook Station (PBS) facility. 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-

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Figure 5 – Flight Test #2 Flight Track, 22 May 2013 - Sandusky, Ohio, USA

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 is also 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. The flight track for Test 2 is presented in Figure 5.

15. Plots of the received radio frequency signal strengths for test flight 2 are presented in Figure 6. Data for the first outbound pass began at an approximate time of 15:33, when the aircraft passed overhead of the ground station antenna. As expected, the received signal strength curves show a steady decrease as the aircraft traveled outbound. Approximately 19 minutes later, near the 15:52 time mark, the radios begin to experience interruption (drop-out) of the downlink video signal and uplink UL20 signal, as evidenced by the abrupt increase in percentage of received packet losses. While these interruptions appear to occur simultaneously in both the aircraft and ground radios, the signal actually is lost first by the ground radio, then seconds later by the aircraft radio. Approximately one minute later the C2 and UL1 signals are lost on the respective radios. The received signal strength was approximately -107 dBm when the video signal was lost and approximately -119 dBm when the C2 signal was lost. The corresponding slant range distance between the ground station and the aircraft was 65 nmi at video drop-out and 75 nmi at C2 drop-out. The aircraft was operating at an altitude of approximately 4200 ft. MSL for this test. To verify the locations at which the communications were lost, the aircraft reversed course to head inbound, remained at the 3500 AGL altitude, and entered into the elongated orbit pattern shown near location area A in Figure 13. During the first orbit, near time mark 16:00, the received signal strength had increased sufficiently that the C2 signal returned to 100% operation (0% losses), followed shortly thereafter by return of the video signal. A second orbit then confirmed the signal strengths for signal interruption and re-acquisition. These repeated interruptions/re-acquisitions help to demonstrate the repeatability of the radio receivers in this flight situation.

16. At approximately 16:12 the aircraft began a full inbound pass on the 212° radial. As expected the received signal strengths continued to increase until the aircraft made the overhead pass of the ground station antenna. As the overflight occurred the signal strength dropped abruptly in the overhead null of the ground antenna resulting in data frame errors. The radios continued to operate, at reduced performance, in the back lobe of the ground antenna.

17. At approximately 16:32 the aircraft began the second outbound pass. Since pre-flight calculations predicted that natural obstruction due to earth curvature was likely, the aircraft altitude was raised to 8200 ft. MSL. This allowed the radios to achieve a LOS distance of over 90 nmi before signal dropouts. The region of signal loss is identified as area B on the flight track (Figure 5).

18. For test flight 2, the maximum slant range was approximately 90 nmi for the video/UL20 signal and approximately 100 nmi for the C2/UL1 signal. Analysis shows, however, that terrain obstructions due to Earth curvature impose these limits and the radio is capable of greater range. The 90 nmi range reached during this test indicates that the radio capability exceeds the 69 nmi requirement for ground station cell size.

19. The graphs in Figure 6 are of the same parameters as in Figure 4 and the explanations of sections 4.10 and 4.11 generally apply. However, there is an additional trace plotted with the signal strength in Figure 14. This second trace labeled “Terrain Based”, utilizes the Terrain Integrated Rough Earth Model (TIREM) calculation which adds reflection and diffraction effects to forecast radio frequency propagation loss over actual terrain. There is good correlation between the radio received signal power and the RF signal attenuation due to terrain.

20. Flight Test #7 – 18 June 2013 – Cedar Rapids, Iowa, USA

21. The seventh and final flight test of the radio test campaign examined the radio signal range from a 300-ft. fixed radio tower. The increased elevation of the ground station antenna coupled with the relatively flat and open terrain of eastern Iowa offered the opportunity to increase the unobstructed LOS slant range to well over 100 nmi. The first goal of flight test 7 was to verify that the CNPC communications links could be sustained to greater distances using the standard four-watt RF output power level from the radio. The second test goal was to determine if communications channel drop-outs were caused by LOS obstruction or by free-space (distance-induced) propagation loss.

22. For this test, another copy of the radio was temporarily installed in the radio room at the base of the 300-ft. tall tower at the Rockwell Collins facility in Cedar Rapids, Iowa. A low-loss coaxial cable was installed up the tower to an omnidirectional antenna at the top of the tower. The aircraft radio and antenna system remained unchanged from previous tests. The flight track for the Cedar Rapids test is presented in Figure 7.

23. The radio performance data for test 7 is presented in Figure 8. Ground-to-aircraft data is displayed for two different operating center frequencies, 963 MHz and 974 MHz. The test period began with the aircraft passing outbound over the transmitting antenna, which occurred at approximately 13:57 in the figure. With the radio operating at the nominal four-watt output power level, the aircraft was able to fly for approximately 26 minutes, until time mark 14:23, before continual signal interruptions occurred. Close examination indicates that the 974 MHz signal actually performed slightly better than the 963 MHz in the outbound run. This was expected as the tower installation had an additional 1 dB of attenuation at 963 MHz. Even in the worst case, the UL20 dropout occurred at a slant range of approximately 130 nmi, well beyond the 69 nmi target range. Consistent with the previous test flights, the communications signal dropouts occurred near -107 dBm for UL20 signals and -119 dBm for UL1

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Figure 6 – Flight Test 2 Radio Performance Data and Associated Aircraft Parameters

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Figure 7 – Flight Test #7 Flight Track, 18 June 2013 – Cedar Rapids, Iowa, USA

signals. The flight testing also included the uplink UL4 and downlink Weather modes (not shown). These modes performed as expected and had similar range performance to the UL1 and C2 modes.

24. One interesting anomaly is the brief loss of communications in the UL20 signals near time mark 14:19. The multi-minute dropout and return occurred at both frequencies, but the dropouts were not identical. It is suspected that the cause of the dropout was destructive multipath interference. The variations in the signal strength plot give support to that hypothesis. Terrain mapping predicts that the Earth curvature should begin to obstruct the LOS path at these distances, even considering the 300-ft antenna elevation. The UL1 signals operated continuously throughout the interference period, which is reasonable considering that it has 13 dB more link margin.

25. While the aircraft executed a course reversal, the output power level of the transmitting radio at the tower was adjusted downward from 4 watts to 0.2 watts. By using this lower power level for the inbound run, test engineers could demonstrate that it was terrain obstruction that caused the drop in received signal strength, not the free-space attenuation of the LOS signal. The UL1 963 MHz trace, for example, clearly shows that signal was lost at approximately 14:23 at a slant range of approximately 130 nmi during the outbound pass. The same signal returned to operation at 14:29 on the inbound pass at the same slant range distance of 130 nmi. Even though the radio output power level had changed from 4 watts to 0.2 watts, the point of signal dropout stayed the same, indicating that terrain obstruction was causing the dropout. Had the LOS been clear of obstructions a slant range of greater than 130 nmi should have been possible with a radio output level of 4 watts.

26. Summary of test results

27. Seven flight tests performed during May and June of 2013 verified the performance of the first generation CNPC prototype radios operating in L-Band (960-977 MHz) and the performance of the test system. Depending on aircraft altitude and ground terminal height, slant ranges of 100 nmi and 130 nmi were achieved, essentially to the edge of radio line of sight horizon. This exceeds the 69 nmi goal of the radio design. Communications between the aircraft and the ground station was demonstrated down to an altitude of 200 ft. Operation of the radio link in congested air traffic environment was demonstrated as was successful communication of real-time aircraft data from the aircraft to the ground. The data gathered over the seven test flights was highly repeatable and similar performance was measured at the high and low ends of the 960-977 MHz band.

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Figure 8 – Flight Test 7 Radio Performance Data and Associated Aircraft Parameters

5. preliminary c-band tests

1. Immediately following L-band flight test #7 at Cedar Rapids, Iowa, Rockwell Collins was given the opportunity to operate a very preliminary version of the C-band CNPC radio. The radio was still under development at the time, so the test flight was intended as a debugging activity of the prototype radio. The radio was tuned for the UAS C-band operating frequency of 5080 MHz. The goal of this test was simply to observe radio-to-radio connectivity. The same system configuration was used as in test #7, with a ground terminal radio feeding the antenna on the 300-ft tower, and the second radio in the NASA GRC aircraft. A flight path extending west from the tower on the 270° radial direction was used.

2. Although the radio was only at a pre-production, breadboard level of development, the operational results looked quite promising. The radios demonstrated links with UL1, C2, UL4, and Weather data channels to a slant range of over 110 nmi, with the aircraft operating at 10,000 ft. altitude. (Video and UL 20 modes were not operational during testing.) The first versions of the formal Generation 2 C-band radios are scheduled for delivery to NASA in the Fall of 2013, with multiple sequential upgrades occurring prior integrated flight testing in Spring of 2014.

6. future activities

1. Test flights using the Generation 2 radio sets are scheduled to begin in the Spring of 2014. The Generation 2 radio system will include both L-Band and C-Band capabilities, and will also be able to demonstrate cell-to-cell hand-off of a communications channel using a single aircraft and multiple ground stations. This can only be accomplished through a comprehensive “ground” network and 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.

7. action by the meeting

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 960-977 MHz and 5030-5091 MHz bands.

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