Aircraft Tracking and Diagnostics in C-Band– Added diagnostic metrics • Signal dropouts and data...

NAVAIR Public Release 2019-328. Distribution Statement A – “Approved for public release; distribution is unlimited”

Aircraft Tracking and Diagnostics in C-Band

Joe Martin, NAWC-AD/AVMI, [email protected] Day, PAE/ATR, [email protected]

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mailto:[email protected]

mailto:[email protected]


Introduction

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This presentation will describe the efforts of a team at NAS Patuxent River which undertook an investigation of the various factors that can affect C-Band telemetry performance when compared with traditional telemetry bands. During testing of a new C-Band telemetry datalink, differences in performance of the system under test were observed between flights at different service’s test ranges. Several C-Band specific contributing factors were examined, including relatively tighter beam widths, differing multipath effects, and the limitations of existing telemetry infrastructure. This presentation will address how these challenges were identified and analyzed, including how data was gathered from ground tracking antennas and test articles, and what quantitative analysis was performed. After this analysis, it was determined that antenna auto-tracking performance could be improved using target slaving, and this approach was put into use on the Atlantic Test Range, and has shown promising results so far.


Challenges: Band Comparison

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L/S – Band Characteristics • f=1-4 GHz/15-7.5 cm wavelength

• Wider RF Beam width (𝐵𝐵𝐵𝐵° = 70𝜆𝜆𝑑𝑑

)

• Lower loss through cables• Longer wavelength -> less variable

peaks/nulls due to multipath• Lower attenuation over equivalent distance

C – Band Characteristics• f=4-8 GHz/7.5-3.75 cm wavelength

• Tighter RF Beam width (𝐵𝐵𝐵𝐵° = 70𝜆𝜆𝑑𝑑

)

• Higher loss through cables• Shorter wavelength -> more densely

packed peaks/nulls due to multipath for a given area

• Higher attenuation over equivalent distance

Key Takeaway: C-Band receiver systems need to be more “responsive” than legacy systems did


Challenges: Side Lobes

• Side lobe gain is sufficient to provide antenna auto-tracking lock, but provides degraded signal reception

• Size, shape, and quantity of side lobes depend on a number of factors, including antenna design and signal frequency

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Reception Diagnostic Plan

• Primary Objective:– TM in multiple bands and compare– Tracking accuracy

• Slant range• Expected power• Received power• Pointing error angle (Δ)• Slaving vs. auto-tracking

• Secondary Objectives:– Radiation pattern– Equipment diagnostics

Δ

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Data Sources: Aircraft

• Parameters– Latitude– Longitude– Altitude– Heading (geographic)– Attitude (pitch and roll)– Recorder Time

• Acquisition– Onboard instrumentation– Advanced Range Data

System (ARDS) pod w/ GPS downlink

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Altitude


Altitude

Data Sources: Antenna Control Unit (ACU)

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• Parameters– Azimuth angle– Elevation angle– Position (altitude, latitude,

longitude)– Tracking mode (slaving,

auto-tracking, manual)– 4 channel polarized

Received Signal Strength Indication (RSSI) acquisition

– Built-in logging functionality (10 samples/sec)


Calculations: Expected RSSI

• Gives notional value for Received Signal Strength Indication (RSSI) at ground antenna

• Ignores some factors (antenna polarization, antenna pattern nulls, ground cable losses, etc.)

• Used to detect relative changes in signal strength, due to transient events (blockage, track loss, etc.)

• Calculated for S and C-Band signals• Validated calculation using ground tests

𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝐸𝐸𝐸𝐸 = 𝑃𝑃𝑇𝑇𝑇𝑇 − 𝐿𝐿𝑇𝑇𝑇𝑇 + 𝐺𝐺𝑇𝑇𝑇𝑇 − 𝐹𝐹𝑅𝑅𝑃𝑃𝐿𝐿 + 𝐺𝐺𝐺𝐺𝐺𝐺𝑑𝑑

𝑃𝑃𝑇𝑇𝑇𝑇 = Transmitter Power𝐿𝐿𝑇𝑇𝑇𝑇 = Test Article Cable Loss (incl. filters, splitters, etc.)𝐺𝐺𝑇𝑇𝑇𝑇 = Test Article Antenna Gain𝐺𝐺𝐺𝐺𝐺𝐺𝑑𝑑 = Ground Antenna Gain

FSPL = Free Space Path Loss =20 ∗ log104𝜋𝜋𝑑𝑑𝜋𝜋𝑐𝑐

d = Slant Rangef = TM Frequencyc = Speed of Light

All parameters in dBm, Meters, Hz, Seconds

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Calculations: Pointing Error

• Calculate correct Azimuth/Elevation angles from aircraft and antenna positions

• Find errors in antenna Az/El as separate values

• Take errors as new direction vector, and convert to Phi/Theta form

• Pointer error emerges as Theta: the angular difference between the x-axis (zero error, towards target) and the new direction vector formed from the Az/El Errors

• Implemented using MATLAB’s azel2phitheta() function

Az Error

El Error

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Calculations: Slant & Ground Ranges

• Geometrc Relationship – tan α = 𝑅𝑅𝑒𝑒+ℎ𝑎𝑎 cos θ −(𝑅𝑅𝑒𝑒)

𝑅𝑅𝑒𝑒+ℎ𝑎𝑎 sin(θ)

∗(Assuming α > 0)

– 𝑠𝑠 ≈ 𝑅𝑅𝑒𝑒θ– Takeaway: s < projected surface

distance

• Implemented using MATLAB’s geodetic2aer() function

*See Earth Referenced Aircraft… for derivation. 10

θ 𝑅𝑅𝑒𝑒

D

ℎ𝑎𝑎

𝑅𝑅𝑒𝑒

α

ψ =π2− α − θ

ψ

s =θ𝑅𝑅𝑒𝑒


Software Workflow

Data Handling•Trimming•Sample & hold interpolation

•Time interpretation and alignment

Calculations•Slant Range•geodetic2aer()

•Phi-Theta Transfer•azel2phitheta()

•Expected RSSI

Evaluations•Mode identification•Tracking accuracy & mode intersection

Mapping•Mapping Toolbox•.Geotiff files

•Mapping•geoshow()

•Heading vectors•quiverm()

Inputs: Aircraft & Antenna

Data

Outputs: Plots & Maps

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Initial Flight: Degraded C-Band Datalink

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• A customer was testing a new C-Band telemetry datalink on the Atlantic Test Range

• This datalink had previously been tested on other ranges and in a lab setting with promising results

• The performance of this datalink at ATR was different from what was expected

• The team analyzed the “over-the-air” portion of the link


Initial Flight: RSSI Plot

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Lobe trackingManeuver

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Trending

Diverging


Initial Flights: RSSI Plots

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Azimuth tracking

Elevation tracking


Initial Flights: Mapping

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• Legend– Red: Flight path– Blue: Automatic tracking

mode, within C-Band pointing angle

– Green: Manual tracking mode, within C-Band pointing angle

– Purple: Heading arrows, taken at given intervals



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• Map Analysis– Manual tracking was

largely out of beam



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• Map Analysis– Tracking quality not

affected by geography

– Tracking quality not clearly affected by maneuver


Follow-On: Strong C-Band Datalink

• After studying the tracking issues of the initial flight, the team decided to test “slaving” the tracking antenna to the location of the test article

• As a control, the team used conventional automatic tracking for the last portion of the flight

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Follow-On Flight: RSSI Plot

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Trending

Within C-Band pointing margin



Trending

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Auto-tracking undulation


Follow-On Flight: RSSI Plots

Azimuth trackingsomewhat worse

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Azimuth tracking

Elevation tracking


Results: Mapping

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• Legend– Red: Flight path– Cyan: Slaved tracking mode,

within C-Band pointing angle– Blue: Automatic tracking mode,

within C-Band pointing angle– Purple: Heading arrows, taken

at given intervals

• Map Analysis – Excellent tracking accuracy in

auto-tracking and “slaving”– No apparent dropout when

transitioning between tracking modes


Analysis

• Test flight data supports the theory that automatic tracking locking on side lobes may be contributing to poor C-Band performance

• Antenna slaving largely eliminated tracking “wobble”• Attempts to recreate side-lobe tracking for comparison

with slaving were inconclusive• Other factors may be contributing as well (Multipath?)

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Survey Summary

• Expected RSSI model tracked well with measured RSSI• Saw side-lobe tracking in some situations• Tracking in azimuth found to be worse than in elevation• Tracking with geodetic slaving does not yield significantly higher

signal levels than auto-tracking under “normal” conditions• However, because of the ancillary benefits of slaving, NAS Pax

River has adopted this as a routine tracking mode, largely as a result of this survey

• Further flights are planned later this year

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Moving Forward

• Planned Diagnostics Improvements– Added diagnostic metrics

• Signal dropouts and data quality• Incorporation of full aircraft attitude

– 3D aircraft tracking model• Upper/Lower aircraft transmitter detection

– Improvements to RSSI model• Emission pattern simulation

– Analysis • Frequency analysis for sidelobe detection

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Questions

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Sources of Error

• Time Cohesion– Multi source time alignment– Slave clock updating

• GPS– Accuracy & precision

• Data Analysis– Slant range– RSSI polarization synthesis

• Geodetic Model– Mercator projection– Datum plane

• Data Rates– ACU @ 10 samples/sec, – Aircraft @ ~ 20 samples/sec

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Sampling

Propagation


References

1. https://www.phys.hawaii.edu/~anita/new/papers/militaryHandbook/antennas.pdf

2. https://en.wikipedia.org/wiki/Side_lobe3. Earth-Referenced Aircraft Navigation and Surveillance Analysis (Volpe

National Transportation System Center)4. https://en.m.wikipedia.org/wiki/File:F16_drawing.svg5. https://openclipart.org/detail/191682/gps-gdop-2-satellites-good

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https://www.phys.hawaii.edu/%7Eanita/new/papers/militaryHandbook/antennas.pdf

https://en.wikipedia.org/wiki/Side_lobe

https://en.m.wikipedia.org/wiki/File:F16_drawing.svg

https://openclipart.org/detail/191682/gps-gdop-2-satellites-good

Aircraft Tracking and Diagnostics in C-Band– Added diagnostic metrics • Signal dropouts and data...

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