Large Reflector Uplink Arraying - nebula.wsimg.com

American Institute of Aeronautics and Astronautics

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Large Reflector Uplink Arraying

G. Patrick Martin1 and Kathy Minear

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Harris Corporation, Melbourne, Florida, 32902-0037

Barry J. Geldzahler3

NASA HQ, Washington DC, Mail Suite 7K80, 300 E Street SW, Washington, DC, 20546-001

and

Jason Soloff4

NASA Lyndon B. Johnson Space Center, 2101 NASA Parkway, Mail Stop ZF, Houston, TX 77058

Abstract -- The promise of array technology in support of space operations has long been

appreciated, and receiving array technology is now an important operational asset. Notable

examples include the NRAO (National Radio Astronomy Observatory) array of twenty seven

25m reflectors and the DSN (Deep Space Network) ad hoc arraying of various assets,

particularly 34m antennas , to realize significant G/T increases providing needed bandwidth

and range extensions. However, uplink (transmit) arraying has not kept pace due to the

difficulty of ensuring „open loop‟ beam formation under the conditions of wide spacing, due

to 1) transmission line and circuit variability, 2) precise antenna reference point

determination, and 3) tropospheric effects especially at higher frequencies. Notable success

in solving uplink array calibration issues has been achieved by two different groups at JPL,

one arraying five 1.2m reflectors and the other three-34m reflectors. However, there are

operational issues with each of these approaches. We present an approach for mitigating

these difficulties, offering the potential for continual readiness operationally and

extensibility to Ka band. Additionally the approach is suitable for use on a wide range of

antenna sizes, including both 34m and 12m reflectors. Currently a transmit uplink

experiment is underway at Harris Corporation using three-12m reflectors operating at

DSCS X band. This array architecture provides continuous internal self-calibration using

the transmit signal itself, a method to dynamically solve for the antenna reference points,

and mitigation of propagation effects by using received signals from known sources.

Nomenclature

ARP = Antenna Reference Point, usually the intersection of azimuth and elevation axes

A2D = Analog to Digital Conversion

COTS = Consumer Off the Shelf

DISA = Defense Information Systems Agency

DSCS = Defense Satellite Communications System

DSN = Deep Space Network

EIRP = Effective Isotropic Radiated Power

FO = Fiber Optic

FPGA = Field Programmable Gate Array

GRASP = General Reflector Antenna Software Package, from TICRA Corporation

G/T = Gain divided by Temperature, usually expressed in dB

1 Senior Scientist, R&D Technology, Harris Corp., P.O. Box 37, Melbourne, FL 32902 M/S 19-21T, Member.

2 Senior Mathematician, R&D Technology, Harris Corp., P.O. Box 37, Melbourne, FL 32902 M/S 19-21T R&D

Technology, Harris Corp., P.O. Box 37, Melbourne, FL 32902 M/S 19-11B, Member. 3 Chief Scientist, Space Communications and Navigation Division, NASA HQ, Washington DC, Mail Suite 7K80,

300 E. Street SW, Washington, DC, 20546-001, Member. 4 Avionics & Communications Office Lead, Constellation Program, NASA Lyndon B. Johnson Space Center, 2101

NASA Parkway, Mail Stop ZF, Houston, TX 77058, Member (exp.)

SpaceOps 2010 Conference Delivering on the Dream Hosted by NASA Mars25 - 30 April 2010, Huntsville, Alabama

AIAA 2010-2175

Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.


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GUI = Graphical User Interface

HW = Hardware

JPL = Jet Propulsion Laboratory, Pasadena, California

LED = Light Emitting Diode

Rx = Receive

SAR = Synthetic Aperture Radar

STK = Satellite Toolkit, a precision orbit prediction application from Analytical Graphics Inc.

TLE = Two Line Element set, a standard method for orbit description

Tx = Transmit

I. Introduction

ASA is moving forward in space communication capabilities on three fronts: upgrades to ground systems to

incorporate arraying technology, operations at Ka band, and optical communication. The International

Telecommunications Union allocation at X-band for both near Earth and deep space missions is a total of 50 MHz.

Hence, each deep space mission is fortunate to get some 5-7 MHz in spectrum allocation and generally 10 MHz for

near Earth missions although 15 MHz is granted on rare occasion. This in turn limits maximum data rates.

Alternatively, Ka band offers higher data rates via greater spectrum availability and substantial link advantages.

Additionally, substantially higher accuracy spacecraft navigation and tracking can be realized via Ka band Doppler

precision increase and medium scattering error reduction.

Downlink arraying technology is employed today, and for the foreseeable future, on the Deep Space Network

(DSN) in a limited way: relatively few antennas are phased together to form a larger synthesized aperture. One of

the questions NASA has wrestled with is uplink capabilities in the array era. There are two main possibilities: use of

single dishes with higher power transmitters or coherent uplink arraying. The aggregate EIRP of an N -element

array of identical antennas is proportional to 2N wherein a few antennas with moderate power transmitters can

achieve a substantially stronger EIRP than a single dish can alone. JPL has demonstrated this potential both in an X

band uplink using three 34m dishes to the EPOXI spacecraft in 2008 and at Ku band with five 1.2m lower power

reflector antennas to a geosynchronous communications satellite. Although impressive accomplishments, each of

those arraying methods require calibration procedures that are not conducive to operational application and they do

not extend readily to Ka band.

This paper is concerned with operationally feasible uplink arraying and extensibility to Ka band. We present an

alternative approach having the potential for immediate readiness operationally, potential for extension to Ka band,

and suitability for use on a wide range of reflector sizes, including both 34m and 12m.

On the basis of our modeling and simulation results which were first presented at the AIAA 2006, an

experimental demonstration of our method is now in progress using three 12m reflectors operating at X band. This

paper begins with a summary of the three principal challenges, describes the experimental set-up and algorithms,

discusses methods of mitigating the challenges, and discusses future work including extensibility of these methods

to a larger array. Our method offers[1]

:

Continuous calibration using the transmitted signal itself

Instant availability without the need for calibration repointing

Potential for direct extension to Ka band

Mitigation of ARP errors and tropospheric propagation variation

Tolerance for COTS hardware and relaxed circuit stability requirements

Applicability to a wide range of antenna and array sizes Important experimentally measured results are presented in Section 4A, where differential closed-loop transmit

circuit phase was maintained within 8 degrees over 83 hours despite circuit phase variation of hundreds of degrees

due to environmental conditions. These results, obtained with error detection assemblies ready for installation on

the antennas, are encouraging since they indicate that similar performance will be obtained during imminent uplink

testing.

Control algorithms discussed include open loop continuous model-based transmit, continuous Blind Signal

Sorting optimum aperture combining for receive, a novel closed-loop continuous transmit algorithm to mitigate

propagation effects (demonstrated on a small prototype array in 2008), and automatic continuous self calibration of

transmit and receive circuit paths using forward and reverse traveling wave properties.

N


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II. Uplink Arraying Considerations

A. Three Principal Error Contributors

Principal error contributors in arrays of large reflector antennas are:

1. Circuitry variation between the point where a signal is phased for transmission and the location from which

that signal is radiated

2. Imprecise knowledge of the ARP (Antenna Reference Point) and relationship of a particular antenna‟s phase

center to the ARP so that proper phasing can be calculated and generated

3. Uncompensated differential propagation phase variation (tropospheric effects)

Circuitry variation over time and temperature (including transmission lines, frequency converters, filters, and

power amplifiers) is the dominant error in beam formation, and distributing precisely phased signals to the antenna

elements becomes increasingly difficult as separation and frequency increase. For example, suppose a Ka Band

frequency is to be transmitted and that array elements are located within a diameter of hundreds of meters, then the

distribution network alone will span tens of thousands of wavelengths. Effective arraying requires that circuit phase

error contribution to the total error budget be held to no more than about 10° peak, implying tolerable variation less

than a few parts per million. Typical circuitry includes not just the fiber optic transmission lines, but up-converters,

filters, FO transmitters and receivers, and power amplifiers. All of these components are temperature sensitive and

time variable.

While it may be extremely difficult to accurately determine and continually update the positions of widely

spaced antenna elements in general, in the case of large reflector antennas in fixed installations such determination

can usually be achieved with very high precision; consider for instance the NRAO VLA or multiple 34m BWG

reflectors at Goldstone. Basically, the mechanical accuracy and stability required for achieving high gain and

precise pointing in turn ensures a very stable ARP. Consequently, once determined using a variety of means, such

as laser surveying, interferometry of known stellar objects or spacecraft, the ARP tends to be constant barring

significant geological disturbance such as an earthquake.

The last major error, Uncompensated Tropospheric Propagation, is almost negligible at X-Band except at low

elevation where it can be moderate, but, this error can be very large for Ka band at any elevation. Algorithms for

mitigating this error using known emitters are discussed in Section IV.

B. Alternate Uplink Arraying Methods

Uplink arraying methods have been demonstrated by two different groups at JPL, one using two and three 34m

antennas at Goldstone5 and another using five 1.2m antennas

6. Both of these methods were successful in a

succession of experiments.

Vilnrotter, et al5, recently arrayed three 34m BWG antennas at X-Band. These antennas have circuitry that is

compensated locally or environmentally controlled, and great effort has been applied to make these antennas the best

worldwide. Despite this attention, residual amplitude and phase errors remain which must be sensed and calibrated.

Due to far field considerations and lack of a suitable in-orbit transponder, a lunar bounce was used to derive the final

corrections. As a tribute to the experiment concept, quality and equipment stability, the calibration solution obtained

was applied to transmit to a distant spacecraft for hours, even though the reflectors had to track the target and

continuous background Doppler corrections were necessary. This experiment applied intensive SAR (synthetic

aperture radar) processing to focus on a specific small size lunar target in the calibration process to prevent

averaging of range-dependent coefficients seen differently by each antenna.

Principal disadvantages of this approach include the need to point away toward the moon to obtain calibration,

dependence on circuit stability between recalibration events, lack of availability when calibrating, and the 12 hour

lunar visibility. Additionally, this method does not extend readily to Ka Band due to tropospheric propagation

considerations.

5 V. Vilnrotter, D. Lee, R. Mukai, T. Cornish, and P. Tsao, “Three-Antenna Doppler-Delay Imaging of the Crater

Tycho for Uplink Array Calibration Applications,” The Interplanetary Network Progress Report, vol. 42-169, Jet

Propulsion Laboratory, Pasadena, California, pp. 1–17, May 15, 2007.

http://ipnpr.jpl.nasa.gov/progress_report/42-169/169D.pdf 6 Larry D'Addario, Robert Proctor, Joseph Trinh, Elliott Sigman, and Clifford Yamamoto, “Uplink Array

Demonstration With Ground-Based Calibration”, Interplanetary Network Progress Report, vol 42-176, February

15, 2009: http://ipnpr.jpl.nasa.gov.


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D'Addario, et al.6, explored the use of five relatively small 1.2m apertures, using low cost commercial

transponder components and an advanced time-transfer method to obtain circuitry synchronization. This method

also must address an unknown phase coefficient that includes filters, antenna feed and internal reflector variations.

Calibration was achieved by providing multiple external sensing points, mostly towers, which enabled simultaneous

solution for the unknowns. D'Addario has noted that with good knowledge of the array elements and sensor position,

only one tower is sufficient. Given small elements, a feasibly tall tower in the center of the array is adequate to

satisfy relaxed far-field constraints. This experiment transmitted to commercial Ku Band satellites, and was entirely

successful, maintaining good beams continuously for many days. Such impressive performance is a tribute to the

concept, architecture and circuit design.

Disadvantages of this approach include the need for a high, stable calibration tower, the need to point away, at

least occasionally, to the calibration tower, and potential internal reflector variations between calibrations due to

temperature differentials, especially with larger reflector sizes. Required calibration tower height increases as the

square of element diameter (to satisfy near field constraints), with a 6m diameter element probably close to the

upper practical limit. Array cost increases quickly for aperture diameter less than about 10m, since the number of

elements required to achieve a particular G/T becomes very large (overcoming potentially low cost per element).

Our models predict that cost of an array of 6m reflectors would be about 58% greater than for an array of 12m

reflectors with the same G/T, and an array of 4m reflectors would be almost 300% more expensive. This cost

increase is almost entirely due to low noise amplification and electronics.

The possibility of employing a geosynchronous satellite for calibration has been noted, eliminating the need for a

stable high tower and removing the small reflector size limit. In the case of the DSN, at least three of these

calibration satellites would be required worldwide. Another problem arises for Ka Band frequencies; due to spatially

dependent tropospheric propagation, as such calibration will include two way propagation variations for a particular

spatial direction that is likely not the direction required. Since propagation phase can change rapidly7, even if spatial

directions were similar, two-way latency would likely invalidate the calibration. Ka band tropospheric compensation

is also an issue with lunar calibration methods for the same reasons.

C. Harris‟ Uplink Arraying Approach

Previously discussed alternate methods are all characterized by a residual unknown phase at each element in the

array which must be determined using a known external calibration target or tower. In contrast, Harris‟ approach

applies closed loop circuit phase control to all transmit pathway circuitry, including transmission lines, frequency

converters, filters, power amplifier, feed with polarizer and diplexer, and the antenna itself. By closing a loop on the

entire pathway, there is no unknown phase coefficient. Consequently, the array is always ready for transmit at the

carrier frequency, does not require calibration periods or pointing away to a calibration target.

Time delay adjustment can then be realized at baseband (zero frequency) with no impact on carrier phase shift.

In a customary system, time delay accuracy must satisfy both carrier phase precision and information content

alignment. With this approach, only information content alignment is required. Since carrier phase is unaffected,

coarse control to nanoseconds or hundreds of picoseconds conveniently realized through digital processing is

sufficient versus analog femtosecond precision required for traditional methods at Ka Band.

All Receive pathways are also placed under closed-loop control in order to support precision interferometry,

precise range and range-rate measurements, precision ARP refinement and tropospheric propagation error mitigation

using known location angularly nearby sources (Discussed in Section IV).

7 Nessel, James, A.; Acosta, Roberto, J.; Morabito, David, D.,” Phase Fluctuations at Goldstone Derived From One-

Year Site Testing Interferometer Data”, National Aeronautics and Space Administration, John H. Glenn Research

Center at Lewis Field, Cleveland, Ohio 44135-3191,

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090020407_2009019738.pdf


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Figure 1 conceptually illustrates this method.

Both receive and transmit pathways are

controlled, but we will focus on the right hand

side transmit pathway (shown in red). Assume

that a means is available for generating a precise

phase reference at widely separated physical

locations. This can be accomplished customarily

with two-way time transfer or by Harris‟ novel

phase transfer method.

At a point including as much circuitry as

feasible, a sample of the signal being transmitted

is compared with the remote phase reference.

Any phase deviation can be attributed to circuit

variability. A weighting device in the transmit

pathway applies the measured correction, forcing

the entire pathway to a net zero phase shift

between generation point and transmission.

Figure 2 illustrates how the feedback point can

be located on the reflector surface, thus including

all temperature sensitive feed components and

internal reflector variation. This is the last

accessible physical point before the wave leaves

the reflector. As the reflector and its components

expand and contracts due to temperature changes,

the circuit path length change is sensed and

corrected.

In this way, all unknown circuit phase errors

can be sensed and controlled, making the antenna

ready for instantaneous, blindpointing transmit

operation.

In addition to negligible latency, immediate

availability and no point-away requirement, this

method is potentially low in cost since ordinary

COTS hardware can be substituted for precision,

temperature stabilized components. Transmission

lines need not be environmentally controlled.

Since time delay compensation is accomplished at

baseband, only approximate delays are needed to

satisfy time*bandwidth constraints (RF transmit

phase is unaffected). This approach applies to arrays of any size and is particularly well suited for array expansion.

Since each array element is independently ready for service at any time, partitioning the array into subapertures

(such as 34m equivalents) is straightforward.

III. Experimental Setup

Based on modeling and simulation results for the Harris approach in 2006, an experimental effort was

undertaken to validate and demonstrate the concept. The following paragraphs describe the experimental setup.

A. Antennas

Three 12m Cassegrain reflectors manufactured by Patriot Corporation8 were installed on a deliberately scalene

lattice, each side having more than 60m length. Aside from electrical power, all other interconnection is via optical

fiber. (This includes RF transmit, RF receive, transmit circuit control, receive circuit control, Antenna Control Unit

interface, and hardware status.) No effort was made to protect the fiber from temperature or mechanical variation.

A photograph of the array appears in Figure 3.

8 Patriot was acquired by Cobham. http://www.cobham.com/about-cobham/avionics-and-surveillance/about-

us/satcom/albion/products/large-aperture-antennas/120m-earth-station-antenna-system.aspx

Figure 1. Closed Loop Circuit Compensation

F074N-010ppt (0) 051209F074N-010ppt (0) 051209

Figure 2. Circuit Feedback Sensor on Reflector Surface


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Figure 3. Array of Three 12m Reflector Antennas on a Scalene Lattice

B. Wave Phase Sensors

Figure 4 is a photograph showing a

wave phase sensing probe installed on a

reflector. Transmit signals are sampled

and compared with a precision

reference, enabling closed loop control

of the entire transmission path. This

process ensures that the transmitted

signal has exactly the desired

beamsteering phase applied remotely

by the weighting hardware at the Ops

Center. Error measurement assemblies

fed by the sensor are located

immediately behind the reflector

surface. These assemblies also receive

a precision reference signal via optical

fiber.

C. Signal Processing Facility

All signals and controls are routed

to a nearby Ops Center, containing

signal generators, FPGA based digital weighting and combining at baseband, up and down converters, a DSCS

modem, and the computers required to execute the arraying and control processes as well as reflector pointing and

tracking. All of the algorithms and control reside in Harris‟s ArrayLab, a MATLAB language modeling and

simulation tool with real-time interface and control capability, described in greater detail in Section IV. Figure 5

shows some of the Ops Center equipment.

Field SensorField Sensor

Figure 4. Phase Error Sensor Installed on the Reflector


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Figure 5. Op Center Panoramic View; hardware racks, control, status, and algorithm monitoring displays.

D. Satellite Targets

Arrangements were made through DISA to transmit and receive through geosynchronous X-Band DSCS

satellites. Several satellites are visible from the array, including one at about 10° elevation. Instant availability of

the arraying method can be tested by rapidly moving among the several satellites. The low elevation satellite

provides an opportunity to experience modest propagation variation, allowing a test of the propagation

compensation algorithms.

Figure 6 is a STK (Satellite Toolkit) depiction of the experiment. Using TLE‟s from DISA, STK generates real-

time pointing directions, range and range rate, for each aperture during an experiment whenever requested by

ArrayLab. (ArrayLab is described shortly in Section II-F.)

Figure 6. Satellite Toolkit Produces Pointing, Range and Range Rate Using DISA TLE‟s

Array geometry must be known to support beamforming calculations. Each reflector comprising the array is

described by a table of parameters beginning with its ARP latitude, longitude and elevation. These coordinates are

determined initially with surveying methods using a laser rangefinder and theodolite establishing location relative to

a nominal surveyed site reference. Relative coordinates are determined nominally to within a fraction of a

millimeter. Using calibrated receive capability, observation of known emitters allows adaptation of the geometry

model through solution for each ARP, resulting in refinement of the initial location. Interferometry (Angle of

Arrival) is also useful in this context as is the “Instant Return” process for mitigation of tropospheric variation,

which also affects ARP refinement.

Reflector geometry table parameters also include values for typical errors, including gravity distortions, azimuth

axis tilt, and RF axis offset from the mechanical elevation axis. It is expected that these array geometry parameters

are highly stable, but they are to be continually refined and tracked.

Array geometry, reflector orientation, imperfections, etc., are parametric values in an overall model of the array.

Collectively, these parameters in association with the model establish a basis for determining all beamforming

weight settings, thus a model based form of calibration [2]

. By sensing and refining these parameters continually, the

model is adapted to present conditions.


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E. Algorithm-Centric vs. Hardware-Centric System

The transmit adaptive combining experiment (Tx ACE) prototype system is algorithm-centric in contrast to

state-machines where the HW runs the system. Advanced algorithms provide the modeling, signal processing, and

calibration needed to run each phase of the experiments. The algorithms also control the HW accelerators which are

used for quick multiplies and parameter settings. There are 20+ algorithms for this experiment that use combinations

of the four signal paths: Rx, Tx, Receive circuit calibration, and Transmit circuit calibration.

Figure 7 is a flow diagram showing relationship of the algorithms (tan rectangular boxes) with the hardware.

Note that the error detection assemblies (blue boxes) are physically on the antenna mounted near the back side of the

surface. The hardware accelerator cards are in the upper left yellow box. The two square boxes under the

assemblies are physically located in the pedestals of the antenna and are for signal throughput.

Figure 7. Functional Set-up for Model-Based Adaptive Combining for Tx

F. Harris‟ ArrayLab: Comprehensive Array “Simulation to Prototype” Laboratory

ArrayLab (Array Laboratory) is Harris‟ in-house comprehensive array “simulation to prototype” software suite.

It combines specific mission scenarios with custom communication systems and with advanced proprietary

algorithms providing realistic system-level analysis. It was used on a 2-year IR&D to design and model the

performance of phased array systems consisting of widely spaced dishes; and in particular to model and simulate

solutions to specific operational and mission problems. Analysis from these studies was used to quantify system and

component-level capabilities and limitations .in order to create error budgets for system subcomponents. In

hardware prototype mode, ArrayLab‟s advanced signal processing algorithms and hardware control modules now

run the large reflector transmit uplink array. In both simulation and prototype modes, each element in an array is

modeled with 6 degrees of freedom. The element patterns are either inputs from COTS electromagnetic field

analysis software or analytic formulations. For model-based uplink arraying, the directivity pattern is (particularly in

the main beam) adequately estimated using a Bessel function model with input parameter: frequency, diameter, feed

offset, efficiency, edge taper, and angle off boresight. For spillover analysis the directivity was numerically

calculated in GRASP and the output file was ingested by ArrayLab. It provides 6 degrees of freedom element

configuration and element pattern modeling accepting both analytic patterns as well as electromagnetic software

outputs. Live simulations of the Transmit uplink arraying methods presented in this paper using up to 400, 12m

antennas were first presented for NASA and JPL guests at the AIAA Conference in 2006.

Though not intended for operational usage, the 40K+ lines of code in this baseline are archived and versioned-

controlled using Clearcase (Commercial software providing complete development software configuration

management through version control). It is comprised of a modular baseline containing 8 main modules: array

modeling, signal generating, blind signal sorting, advanced proprietary signal processing, error analysis, link budget,


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custom analysis, and utilities. The modules were designed for ease of transition from simulations to control of

hardware prototype systems.

The top path in Figure 8 depicts ArrayLab used in simulation or design mode while the bottom path depicts

hardware prototype mode. Both paths are driven by the ArrayLab Baseline. Some proprietary algorithms contained

in this baseline and used in this experiment include widely-spaced continuous optimum model-based adaptive

aperture combining on Tx, widely-spaced continuous optimum blind signal sorting-based aperture combining on Rx,

instant return, and advanced calibration methods such as antenna reference point refinement. The algorithms are

written for both a bit-by-bit signal-level as well as a quick-look or expected value analysis. Custom interfaces

provide the user with control over algorithm and array parameters. In prototype mode additional interfaces provide

hardware parameter control and status. During the simulation phase statistics of various errors sources were varied

using ArrayLab‟s Error Tool. These included compensated and uncompensated circuit error, phase reference point

errors and atmospheric effects.

Figure 8. ArrayLab: Simulation to SW Prototype Algorithm & Control Suite.

It can be noted in Figure 8 that for both simulation and hardware prototype modes, ArrayLab controls AGI‟s

COTs software, Satellite Toolkit (STK). In prototype mode, the target line-of-sight, range, range rate are provided

in real-time. STK provides interactive 4D visualization of the planetary and communications platform motion while

ArrayLab provides instantaneous communications performance analysis through plots, graphs, and raw data.

The bottom path in Figure 8 depicts ArrayLab used in real-time hardware prototype mode. The hardware control

for this experiment includes the racks of equipment in both the operations center and the pedestals, the error

detection assemblies located behind each probe sensor, and the three 12m Patriot antennas. Real signals are

substituted for simulated ones via signal generators and antenna elements. While the antennas are substituted for the

simulated array, the array modeling component remains and is used to calculate an initial estimate of the relative

beam steer weights. Statistical error modeling is replaced with circuit phase errors due to the up/down converters,

phase discriminators, line receivers, fiber optic lines, and other hardware as well as tropospheric effects.

Before beginning the prototype study, custom analyses for this experiment were conducted. Object obscuration

analysis (including terrain, buildings, and trees) was performed on 4 potential antenna sites. Element shadowing was

also analyzed. Various other simulations were also performed including: spillover effects on array G/T, G/T

degradation due to planetary thermal emissions on pattern sidelobes, the ability of the Patriot antennas to track

various targets (slew rate analysis) for various array locations on the Earth, near-field radiation effects on over-head

low-flying aircraft, and determination of how the arrayed beam forms in the array near field region.


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Other signal processing factors considered include Doppler effects due to planetary rotation motion and wide

reflector element separation, especially at Ka-band, and speed-of-light adjustment as a function of atmospheric

pressure and temperature.

G. Graphical User Interfaces & Controls

ArrayLab has 3 main interfaces for this program as shown in Figures 9 and 10 allowing the experimenters to

control the algorithms to be run, input parameters, which targets to communicate with, test, monitor, and control all

the hardware in the system. Real-time output data is captured and displayed during the tests as well as STK target

information. The first keeps track of the algorithms being run and the real-time STK updates from each of the

targets. New targets can be added and orbit parameters can be updated as new TLE‟s are available. Currently this

occurs once a week. The second interface is dedicated to circuit calibration. Calibration of the circuit error

assemblies themselves are also controlled and monitored here.

Figure 9. ArrayLab Graphical User Interfaces for the Tx ACE Experiment. Top Left: Main Interface

controls algorithms and STK real-time connection. Top Right: Circuit calibration and monitoring.

The third GUI (See Figure 10) is for all the HW

interfaces including the three antenna control units, up and

down converters, modem, electro-optical modules,

attenuator, reference signal generator, Transmit signal

generator, remote digitizers residing in the pedestals,

temperature control plates at the error assemblies and in the

pedestal, as well as measurement devices such as spectrum

and network analyzers. Red, green, and yellow simulated

LEDs provide status for each component.

These interfaces are designed for scientific investigation

and neither for operational nor mission control. The

ArrayLab advanced algorithms, array modeling, and STK

connectivity could be used within mission control software

such as Harris‟ OSCOMET.

H. Patriot Antenna Pointing Calibration

The antennas required pointing calibration after

installation. An algorithm was developed to scan in a grid-

like pattern while tracking a target. A low and high

resolution scan is performed. (See Figure 11) The antenna

pointing biases are the difference between the line-of-sight calculated using the orbit parameters and the centroid of

the 3dB down contour on the 3D antenna pattern formed during the scan.

Azimuth and elevation line-of-sight pointing biases are saved for many targets. These are used to create the

nine-parameter (five azimuth and four elevation) corrections incorporated by the antenna control unit. This

procedure is repeated for each dish. All targets are searched for sequentially and autonomously.

Figure 10. ArrayLab Graphical User Interfaces

for the Tx ACE Experiment. Hardware control

and status


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Figure 11. Results from Antenna Pointing Calibration Algorithm.

IV. Mitigations of Arraying Challenges

A. Continuous Self-Calibration of Tx and Rx Circuitry

Circuit phase control is considered to be the enabling technology in this experiment [3],[4]

. This control in turn

depends upon a precision phase reference which is derived from forward and reverse traveling waves in a reference

distribution network. Theoretically, this reference is insensitive to variation in physical properties of the distribution

network. To verify the control process, a closed-loop circuit control experiment was conducted using the set-up in

Figure 7. The signal goes to a network analyzer instead of the two antennas shown in the figure. In this way, the

network analyzer can measure phase difference that would otherwise be inaccessible. All the hardware depicted

across the top of the figure as well as variable fiber length is a source of phase error and is compensated for

continuously and automatically. The goal was 3 degrees RMS phase error and 10 degrees peak. After running at

many frequencies and power levels, an extended run was conducted with the results given in Figure 12. The RMS

was 1.4 degrees, the peak was 7.9 degrees and there was a bias of -0.42 degrees.

Figure 12. Results for Closed Loop Circuit Control Experiment

Run Time: 83+ Hours Requirement: 3° RMS 10° Peak Achieved: 1.4° RMS 8.0° Peak.

During this 83+ hour run, Figure 13 shows the phase changes in the Transmit circuit for assembly 1 (yellow) and

assembly 2 (cyan). The large changes are primarily due to temperature changes: the air conditioner coming on and

off (faster cycling) and overnight periods when room temperature dropped due to lack of heat (at about times 1 and

2 times 10^5 seconds). Even these gross changes in circuit phase were controlled successfully as shown in Figure 12

where time is on the x-axis and phase differential between the assemblies is on the y-axis. About two degrees peak-

to-peak short term phase variation is attributable to up converter phase noise.


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Figure 13. Wide range of phase errors in the transmit circuit paths were corrected during Closed Loop

Circuit Control Experiment.

Because of these results, exceeding the design objective, we have increased confidence that this performance

will be realized on the antennas, enabling control of transmit circuit phase errors during transmit uplink arraying.

B. Antenna Reference Point Calibration

Refinement of the phase center reference point location for each of the dishes is the second major source of

differential phase error. It is assumed that the dishes are stable and the initial calibration of the reference points

resulted in less than one wavelength of error. The phase center refers to the point from which the electromagnetic

radiation generated by the antenna element spreads spherically outward, with the phase of the signal being equal at

any point on the sphere. If the phase center location information is inaccurate, incorrect interference patterns will be

generated during beamforming, resulting in reduced signal strength during reception or misalignment during

transmission. These difficulties are further exacerbated as the size of the array is increased and the distance to the

object of interest is increased. A system of equations is formed where each row or observation represents signals

from various sources and angles of arrival. The phase center locations can be calculated based on differential

distances for the antenna elements [5]

. The term “differential distance” refers to the additional distance a wavefront

needs to travel to reach a phase center of the second antenna element after the wavefront has reached a phase center

of the first antenna element.

An algorithm was developed to minimize the error in the spatial positions of any number of antenna elements

using receive signals. This algorithm is run after the initial locations of the elements are measured using the total

station. It can theoretically correct up to one wavelength of error or about 36mm at X-band and 9mm at Ka-band.

Simulations for various antenna phase center errors have been conducted. In the Figure 14 example, 20mm (mean

spherical random) error or about 0.48 wavelengths at X-band was added to each antenna reference point. Also

compensated receive circuit errors of 5 degrees RMS were added. Initial antenna element estimates were modeled

with 6 degrees of freedom plus analytic antenna patterns. Four satellites were used as targets for the signal

downlink. The satellite and planetary motion were modeled using STK. After 20 observations (5 from each target),

the RMS mean error was

reduced to 1.8 mm from

20.0mm. The true

Cartesian positions of the

antennas are shown in

green and the corrected

positions are shown in

black. We will conduct

this calibration before the

adaptive combining expe-

riments using several

DSCS satellites and other

appropriate targets.

Figure 14. Results from Calibration of Antenna Reference Point Simulation.


13

C. Tropospheric Phase Error Correction

The last of the three primary considerations for Tx uplink arraying is uncompensated differential propagation

phase variation due to tropospheric effects. Part of the Transmit Adaptive Combining Experiment includes

evaluating the feasibility of uplink at higher frequencies such as Ka-band. The tropospheric contribution to phase

errors is significant and unpredictable The tropospheric contribution to phase errors is significant and unpredictable

as evidenced by Nessel, et al.9 of Glenn Research Center, “There does not appear to be any apparent relationship

between surface meteorological data and phase fluctuations. This implies surface measurements are not accurate

indicators of what is occurring higher in the atmosphere and developing a model to predict phase stability at a

particular site would be extremely difficult.” Harris‟ Instant Return‟ algorithm[6]

will be used to mitigate these

effects during our final experiment for this study.

During the 2008 Tx Uplink IR&D,

ArrayLab was used to test the Instant Return

algorithm on a hardware prototype

communications system. A mock-up of the

Orion exploration vehicle was constructed in a

compact range. It was fitted with 2-4 element

phased arrays. (See Figure 15). In this study,

tropospheric effects were not the cause of the

unknown phase errors but rather an anomalous

condition such as mechanical or thermal

variation of a circuit (phase detection

assemblies were not an option due to SWAP

constraint). To simulate these unknown phase

errors, random differential beamsteering errors

were added to the steering weights. The system

then switched to receive mode and used the

downlink signal and „instant return‟ to calculate the correct transmit weights. The Instant Return Method is thought

to be a viable solution for Transmit Uplink Arraying at higher frequencies. We will test this on a target with a low

elevation angle (~10 degrees) at X band. The method can use any signal arriving within an elements field of view.

ArrayLab was also used to model and

simulate tropospheric effects and the Instant

Return algorithm for the Norway Facility site

(See Figure 16a). The simulation includes 30

degrees RMS tropospheric phase errors, half

wavelength antenna phase reference error, and 50

degrees phase circuit errors. Sample results for 3

moments in time are shown in Figure 16b. The

elevation angles at the 3 time samples are about

7.1, 19.4, and 27.1, respectively. In this

simulation the target is the AQUA polar orbiting

satellite. Figure 17 shows a similar simulation

using an array of 25 dishes. For each time

sample a 3x3 plot of pattern with uncorrected

random phase errors, the pattern after using a

Receive signal to determine those errors and

adjusting and applying them for correction to the

Transmit signal. The bottom row depicts the

beam pattern in an ideal system for the same time

sample.

9 Nessel, James, A.; Acosta, Roberto, J.; Morabito, David, D.,” Phase Fluctuations at Goldstone Derived From One-

Year Site Testing Interferometer Data”, National Aeronautics and Space Administration, John H. Glenn Research

Center at Lewis Field, Cleveland, Ohio 44135-3191,

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090020407_2009019738.pdf

Figure 15. Instant Return HW Orion Mock-up

Prototype Comm System (2008).

Figure 16a. Norway Facility Instant Return

Modeling & Simulation


14

Figure 16b. Norway Facility Simulation Results Mitigation of Tropospheric Effects Using Instant Return.

Each column of the graphics in Figure 16b represents transmission at a particular time sample. There are 3 different

line-of-sight elevation angles. The red line denotes the cross-section of the antenna pattern before using a receive

signal to determine the beam steer weights. The black curve is the beam pattern after the beam steer correction. The

black vertical line is where the peak of the beam should be.

Figure 17. Instant Return Simulation Results Applied to an Array of 25 dishes. Each of the 25 elements is

affected by varying phase errors due to the atmosphere. The top 3 plots show the 3D, 2D, and 1D slice of the

antenna pattern with simulated tropospheric errors. The peak is about 20 dB down. The middle 3 plots show the

corrected patterns after using Receive signals to determine the correct beam steer weights for each element. The

bottom 3 plots show the adaptively combined beam form the 25 elements without errors.


15

V. What‟s Next

A. Harris Experiment Plan

Three experiments remain for the array of 12m reflectors:

Calibrated Uplink arraying

Calibrated Receive arraying

Instant Return Uplink with correction of modest propagation variation After these experiments complete, assuming success with propagation correction, a reasonable next step would

extend the X band experiments to Ka band. The three 12m reflectors offer low impact means for extended testing.

B. Future of Array Technology

The DSN is currently upgrading the transmitter on one 34m beam wave guide (BWG) antenna per Tracking

Complex to 80 kW versus the 20 kW available today. This will provide a backup to the 70m DSN antenna‟s uplink

capability should the 70m experience an anomaly. However, on a near term basis, uplink arraying two 34m

antennas each with the currently deployed 20 kW transmitters could alternatively provide the same EIRP but for

only half the transmitter power budget and without the expense of new 80KW amplifiers. Uplink arraying three of

these antennas provides up to a 9.5dB increase.

As NASA spacecraft push farther out into the reaches of the solar system, larger receive apertures at radio

frequencies will be required to capture the data from increasingly weak transmission signals and higher uplink

radiated power will be required to support both routine and emergency communications at great distances from

earth. The need for higher EIRP and the role of uplink arrays in providing this has been succinctly noted by

D‟Addario10

, showing that an array of smaller antennas with modest power amplifiers could readily produce more

than 1.0 TW EIRP and that this would be needed for emergency spacecraft communications to Neptune and beyond.

An important benefit of arrays of smaller reflectors is the field of view they provide for the array. This greatly

increases the usefulness of the array in signal acquisition and navigation and tracking since more known position

quasars or other spacecraft can be seen simultaneously. Such enhanced visibility would also be beneficial for Ka

band propagation error detection and correction.

Another advantage of arrays of smaller reflectors is that lattices can be constructed that greatly minimize grating

lobes. For future array expandability considerations, optimization of antenna element spatial placement with respect

to minimizing sidelobes was also simulated. Figure 18 compares far field array patterns for three different lattices

populated with 12m reflectors. Random and aperiodic arrangements are seen to offer substantial suppression in

comparison with a regular hexagonal lattice. The pattern for a random lattice with 57 12m antennas (center

illustration) compares favorably with that of a single large dish having about the same overall diameter, while arrays

of a few large antennas with the same gain display pronounced grating similar to that of the hexagonal lattice or the

three element array patterns seen in Figure 16b.

Figure 18. Array Configuration Optimization With Respect to Sidelobe Minimization

10

D‟Addario, Larry R., “Large Transmitting Arrays for Deep Space Uplinks, Solar System Radar, and Related

Applications,” Jet Propulsion Laboratory, California Institute of Technology, M/S 11-116, 4800 Oak Grove Drive,

Pasadena, CA 91109, USA. Email: [email protected]


16

VI. Conclusions

Uplink Arraying architecture providing continuous internal self-calibration using the transmit signal itself, a

method to dynamically solve for the antenna reference points, and mitigation of propagation effects by using

received signals from known sources was presented, offering the potential for continual readiness operationally and

extensibility to Ka band. The approach is suitable for use on a wide range of antenna sizes, including both 34m and

12m reflectors. Recently completed experimental testing has confirmed the critical component of the approach,

closed-loop circuit phase control, and transmit experimentation is imminent using three-12m reflectors operating at

DSCS X band.

In conclusion, uplink arraying is a cost effective means of providing very high EIRP that is ready to be deployed

operationally, at least at X band.

Acknowledgments

The authors would like to thank Irene Bibyk for contributions in the early day of this work, the NASA teams at

the Johnson Space Center and the Glenn Research Center, John Rogers of DISA, the Harris teams at Palm Bay, and

Harris for the use of their facilities. In particular, the authors would like to thank Kathryn Morrison, Harris Program

Manager for handling the many details of this experiment and Wade Minear, Electrical Engineer, who has been a

critical part of the modeling and simulation team since 2005, and whose expertise was key in the transition to a

hardware prototype system. A special thanks to Nancy Gazzola Harris R&D Technology support, whose help

getting this document prepared, was crucial to a timely submission.

References

Patents

[1] Martin, G. P, Minear, K. M., Roach, J.(III), Dianic, A., Adams(Jr.), William C., Ralston, Lynda M Harris Corporation.,

Melbourne, FL, U.S. Patent Application for a “Compensation of beamforming errors in a communications system having widely

spaced antenna Elements,” U.S. Patent Application S/N: 12/273,760 filed 29 Nov. 2008.

[2] Martin, G. P, Minear, K. M., Adams (Jr.), William C., Harris Corporation., Melbourne, FL, U.S. Patent Application for a

“Model-based system calibration for control systems,” U.S. Patent Application S/N: 12/273,001 filed 19 Nov. 2008.

[3] Martin, G. P, Roach, J. Adams (Jr.),William C., Minear, K. M., Hash, R. J., Ralston, L. M., Harris Corporation.,

Melbourne, FL, U.S. Patent Application for a “Closed loop phase control between distant points,” U.S. Patent Application S/N:

12/273,839 filed 19 Nov. 2008.

[4] Martin, G. P, Harris Corporation., Melbourne, FL, U.S. Patent Application for a “Systems for determining a reference

signal at any location along a transmission media,” U.S. Patent Application S/N: 12/273,797 filed 19 Nov. 2008.

[5] Minear, K. M., Martin, G. P, Harris Corporation., Melbourne, FL, U.S. Patent Application for a “Systems and methods for

providing corrected antenna element phase center locations in a communications system,” U.S. Patent Application S/N:

12/273,981 filed 19 Nov. 2008.

[6] Martin, G. P, Minear, K. M., Harris Corporation, Melbourne, FL, U.S. Patent Application for a “Systems and methods for

compensating for transmission phasing errors in a communications system using a receive signal” U.S. Patent Application S/N:

12/273,935 filed 19 Nov. 2008.

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