A 550 GHZ NEAR-FIELD ANTENNA MEASUREMENT SYSTEM FOR THE NASA SUBMILLIMETER WAVE ASTRONOMY SATELLITE
Antenna Measurement Techniques Association Conference
October 3 - October 7, 1994
Nearfield Systems Incorporated
1330 E. 223rd Street #524
Carson, CA 90745 USA
This paper describes a 550 GHz planar near-field
measurement system developed for flight qualification of the radio
telescope carried onboard the NASA submillimeter wave astronomy
satellite (SWAS). The very high operating frequency required a
new look at many near-field measurement issues. For example the
short wavelength mandated a very high precision scanner mechanism
with an accuracy of a few microns. A new thermal compensation
technique was developed to minimize errors caused by thermally
induced motion between the scanner and spacecraft antenna.
Keywords: Near-Field, Antenna, Submillimeter Wave, SWAS, Thermal Compensation
This paper describes a submillimeter planar near-field
antenna measurement system developed for flight qualification
of the radio telescope carried onboard the NASA submillimeter
wave astronomy satellite (SWAS). This telescope will produce high
resolution radio maps of molecular cloud chemistry in the Milky
Way galaxy. The SWAS satellite is part of the NASA small explorer
(SMEX) satellite program, a series of small and inexpensive spacecraft.
The SWAS will be placed into orbit by a Pegasus launch vehicle.
The SWAS radio telescope consists of an off-axis
elliptical Cassegrain antenna coupled to a dual channel submillimeter
wave receiver that operates at 490 and 548 or 553 GHz. The receiver
outputs are simultaneously processed in an acousto-optic spectrometer
and transmitted back to earth. The radiotelescope antenna is electrically
quite large, providing a beamwidth less than 5 arcminutes wide.
(See Figure 1.)
The SWAS telescope operates in a wavelength region
from 0.54 to 0.61 millimeters. Even though the near-field scanner
is physically small, it is electrically very large, as the scanner
aperture width is greater than 1400 wavelengths. The SWAS telescope
was designed to a 25 micron phase-front accuracy.
2.0 DESIGN REQUIREMENTS
The SWAS near-field measurement system required a
15 micron or better phase-front measurement accuracy. This paper
will concentrate on the technology needed to provide a high phase
measurement accuracy in this application. Three elements will
be discussed in detail: the scanner design, phase reference system
design and thermal compensation.
SWAS near-field antenna measurement system design
- The scan area was to be 0.8 by 0.8 meters.
- The system would require an rms Z planarity of 15 microns or better. The error budget was allocated as follows:
a. Scanner Z planarity ..........5 microns rms
b. Phase reference arm ..........12 microns rms
c. Other errors .................6 microns rms
- The system would be shipped to several facilities. The scanner Z planarity needed to be easily checked.
- The system would need to be operationally and thermally compatible with the spacecraft assembly area.
- A vertical scan plane orientation was required for compatibility with the flight article handling fixture.
3.0 SCANNER DESIGN
Four basic scanner design elements were used to provide
the extreme precision needed in this application. These elements
- Granite structure - The scanner was built from
granite. Granite provides several advantages in this application,
including excellent long term mechanical stability and compatibility
with precision surface lapping methods. Lapped surfaces were necessary
to produce the required planarity.
- Air bearing carriages - Air bearings were used
to provide a better planarity than can be provided by conventional
ball or roller bearings. Air bearings have a large surface area
which minimizes the effect of local surface errors.
- Sideways H scanner geometry - A sideways H scanner
geometry using a vertical surface plate reference was selected
as the method to provide the best planarity.
- Reference surface plate - The scanner would include
an integral surface plate to be used for checking scanner Z planarity.
A vertically oriented surface plate at the back of
the scanner served both as the foundation and as a Z planarity
reference when probed with a precision dial indicator. Two horizontal
granite rails separated vertically by several feet were attached
to the vertical surface plate. An air bearing carriage rides on
each of the horizontal granite rails. The two air bearing carriages
are interconnected with a vertical granite rail. A third air bearing
carriage rides on the vertical rail. Both the horizontal and vertical
axes are positioned by precision stepper/leadscrew drives. The
scanner as delivered had a Z planarity of 1.2 microns rms (2 wavelengths
of red light). See Figure 2 for a photo of the SWAS scanner.
4.0 RF SUBSYSTEM
The RF subsystem design was primarily driven by the
need to use the internal receiver in the SWAS payload. For this
reason, the near-field scanner probe was configured as a transmitter.
A frequency multiplier on the probe carriage converted a 5.11
GHz phase reference signal to the submillimeter wave test frequency.
All submillimeter hardware for the system was developed by Millitech.
The 5.11 GHz phase reference path to the moving probe
carriage was required to be extremely phase stable, since any
phase error at 5.11 GHz would be multiplied approximately 100
times by the frequency multiplier. A two arm mechanism interconnected
with three specially selected rotary joints was used to meet the
performance requirement of a 12 micron rms path length stability.
The arm design used a thermally shielded exoskeleton that enclosed
a low temperature coefficient SiO2 cable. The phase
accuracy of the rotary joint arm system was verified by an S11
measurement of the phase delay to a short mounted at one end of
the phase reference cable.
The SWAS telescope receiver IF output was at S-band.
A standard Hewlett-Packard 8753C network analyzer was used to
convert the S-band IF signal into digital form for sending to
the NSI near-field measurement software. The SWAS telescope receiver
also provided the phase reference signal for the network analyzer.
5.0 THERMAL DESIGN
Thermally induced phasefront errors become an important
issue when the near-field scanner is electrically large, that
is when the scanner dimensions are large when measured in wavelengths.
Because of the high operating frequency of the SWAS telescope,
the near-field measurement system has a 15 micron rms phase measurement
accuracy requirement. This specification requires that the rms
phase error from all sources including mechanical scanner errors,
phase reference cable systematic errors, receiver errors and thermal
drift will result in less than a 15 micron rms change in RF path
length. Temperature changes in a near-field test facility will
cause the following thermal effects:
- Thermally and time dependent changes in the electrical
length of the probe antenna phase reference cable, test antenna
cable and receiver phase reference cable will cause phase changes
that appear to be along the antenna boresight direction. A one
meter long RF cable with an 8 ppm//C
temperature coefficient and 1/C
temperature change will have an electrical path length change
of 8 microns.
- The SWAS payload is mounted onto an aluminum handling
fixture which can differentially expand. In general, any Z axis
motion components of the SWAS telescope hves the same effect and
magnitude as any Z errors in the NFR scanner.
- Thermal changes in the test antenna mount can cause
the antenna location (X, Y, Z) and orientation (yaw, pitch, roll)
to drift unpredictably. The SWAS near-field region is similarly
quite sensitive to unmodeled azimuth and elevation changes as
these include a differential Z motion of reflector. A Z component
of motion is present for relative Z, azimuth and elevation motion.
A .1 arcsecond pointing change of the antenna mount would move
one side of the SWAS aperture 15 microns closer to the scanner
than the other side of the antenna. This would use up the entire
- Temperature changes can warp both the scanner
and antenna under test.
A number of techniques can be used to reduce the
effects of thermally induced phase errors. These techniques include:
- Thermally stabilizing the environment minimizes
all error terms. Electrically large systems are often thermally
stabilized to a "0.5
/C range or better.
- Low temperature coefficient materials can be used.
Granite has a low temperature coefficient (5 ppm//C)
and a high thermal inertia. The SWAS RF components are interconnected
with specially selected RF cables that have a very low thermal
- Thermally induced cable phase errors can be reduced
by minimizing the length of the RF cables and other mechanical
- As the RF subsystem of the near-field range is
a form of a two arm interferometer, a differential thermal drift
cancellation technique can be used. In this technique, the receiver
phase reference cable is made equal in length to the sum of the
probe and AUT cable lengths. The assumption here is that all cables
have the same temperature environment, coefficient and time constants.
This would minimize the first error term only. Because the receiver
phase reference cable was internal to the SWAS payload, this method
could not be used.
- The tie scan technique can be used to measure
cable drift and the component of relative motion aligned with
the beam boresight direction.
- A new thermal compensation technique called motion
tracking interferometry (MTI) was developed for the SWAS near-field
measurement system. This method measures several thermal drift
components including multiaxis solid body motion and time varying
low order distortions of the AUT and scanner.
6.0 MOTION TRACKING INTERFEROMETRY
The motion tracking interferometer (MTI) system is
an extension of the tie scan concept sometimes used for thermal
drift compensation of near-field measurements. Unlike the tie
scan, MTI provides a multidegree of freedom measure of the relative
rigid body motion between the scanner and test antenna during
the test. Additionally, the MTI system provides an estimate of
the measurement uncertainty.
The MTI processor measures the relative azimuth,
elevation and Z motion between the scanner and SWAS antenna. Measurements
of other degrees of freedom (X, Y, roll) are not needed because
the significant SWAS antenna energy is aligned with the scanner
boresight axis. The MTI data is acquired by periodically interrupting
the normal data acquisition process and then scanning four spatially
separated points. The MTI scan is performed at a single frequency
with an unsteered beam, even in the case of multifrequency and
multipolarization measurements. The MTI measurements are phase
unwrapped and distance normalized to remove frequency dependence.
A series of least squares solutions to the plane orientation provides
a time history of the relative Z, azimuth and elevation motion
between the scanner and SWAS antenna. Even though the MTI measurements
were made at a single frequency, polarization and beam steering,
the results apply to all polarizations and frequencies.
Because the solution is overdetermined, the measurement
uncertainty can be readily estimated. The measurement uncertainty
is a function of the RF signal to noise ratio, scanner repeatability
and unmodeled nonsolid body motion. For example, if the scanner
or antenna became thermally warped, the MTI measurement uncertainty
would increase. Thermal drift in the phase reference cable appears
as a solid body motion aligned with the MTI reference antenna
beam direction, in this case, aligned with the Z axis.
The MTI measurements can be used to correct for the
unwanted solid body relative motion in two ways. First, the MTI
measurements can be nulled by periodically rotating and translating
the scan plane. Second, the MTI Figure 3measurements can be interpolated
over the duration of the scan to estimate the relative solid body
motion history. A postprocessor is used to perform a time varying
derotation and translation of the phasefront that effectively
nulls the drift. The second technique was used in the SWAS system
as the scanner did not include a motorized Z axis.
MTI, as used in the SWAS near-field measurement system
does not measure X, Y or roll motion nor does it explicitly separate
out the cable thermal term. SWAS near-field measurements are relatively
insensitive to these motions because the antenna beam is directed
along the scanner Z axis.
See results illustrated in Figures 3 through 5.
The operation of an electrically large submillimeter
wave near-field antenna measurements system required several extensions
to normal near-field test methods. These extensions included a
very high precision granite scanner mechanism, a highly stable
phase reference system, and the development of an RF based motion
tracking (MTI) system. The new phase reference and MTI systems
are being applied to other electrically large near-field measurement
systems operating at lower frequencies. The MTI system provides
a simple and highly accurate alternative to autocollimator, tiltmeter,
and other mechanial referencing methods. The need for the customer
to provide a thermally stable test article mount is reduced. The
MTI system is patent pending.
Figure 1. SWAS Satellite Payload Drawing
Figure 2. SWAS Scanner
Figure 3. Z Drift as a Function of Time
Figure 4. Measurement Uncertainty as a Function of Time
Figure 5. Azimuth Drift as a Function of Time
© Copyright 1996, Nearfield Systems Inc., All Rights