An apparatus has been designed and built for testing the effects, on moderate-sized objects, of cushioned decelerations having magnitudes ranging up to several hundred g [where g = normal Earth gravitational acceleration (approximately 9.8 m/s(exp 2))]

Feller, Jeffrey

Roach, Pat R.

NASA Technical Reports Server

NASA Tech Briefs, June 2007 5
Technology Focus: Communications
High-Accuracy, High-Dynamic-Range Phase-Measurement System
Phase differences can be measured to within a microcycle.
NASA’s Jet Propulsion Laboratory, Pasadena, California
A digital phase meter has been de-
signed to satisfy stringent requirements
for measuring differences between
phases of radio-frequency (RF) subcar-
rier signals modulated onto laser beams
involved in the operation of a planned
space-borne gravitational-wave-detecting
heterodyne laser interferometer. The ca-
pabilities of this system could also be
used in diverse terrestrial applications
that involve measurement of signal
phases, including metrology, navigation,
and communications. The capabilities
of this system include:
• Accuracy to within a millionth of a
cycle (greater than the accuracy of any
prior phase meter); 
• High dynamic range of phase (limited
only by numerical capabilities of a sup-
porting computer); 
• Wide input frequency range (≈10 kHz
to ≈20 MHz), 
• Maintenance of accuracy even when the
frequency of a signal slews by as much as
700 kHz/s within this range; and 
• The ability to distinguish and measure
multiple tones on the same carrier
with undiminished accuracy. 
In the operation of this system, two RF
modulated laser beams interfere on a
photodiode; their frequency difference
results in a RF heterodyne signal. The
RF signal output by the photodiode is
digitized at 40 Mega-samples/s, then fed
into a field-programmable gate array
(FPGA). The digitized heterodyne sig-
nal is split and multiplied by two ad-
justable local oscillators (LOs), which
separates the signal into in-phase (I) co-
sine and a quadrature phase (Q) sine
components. During stable operation at
a small phase difference between the RF
and LO signals, the Q multiplier output
includes a low-frequency component
proportional to the phase difference.
This phase-difference output is used to
update the LO frequency (and, thereby,
the LO phase) to keep the local oscilla-
tors locked to the incoming signal. The
tracking loop updates the LO frequency
every 0.1 ms.
While this makes for a stable digital
phase-locked loop, to achieve the de-
sired micro-cycle phase accuracy, the
residual phase-tracking error is com-
bined with the LO phase. The residual
phase difference is computed as the arc-
tangent of the ratio between the Q and I
signals, filtered appropriately, and
added to the phase used to update the
local oscillators.
For transmission to ground, the phase
measurements are decimated from 40
MHz to the frequency-and-phase-update
rate of 10 kHz, then again to a final out-
put rate of 100 Hz. To suppress aliasing
of noise in the signal band from 1 mHz
to 1 Hz in the decimation from 40 MHz
to 10 kHz, an anti-aliasing filter is ap-
plied prior to decimation. After the re-
construction of phase at 10 kHz and
prior to the final decimation to 100 Hz,
another finite-impulse-response anti-
aliasing decimation filter is applied. The
characteristics of the antialiasing filters
are tailored to the phase noise of the
input signal.
This work was done by Daniel Shaddock,
Brent Ware, Peter Halverson, and Robert
Spero of Caltech for NASA’s Jet Propulsion
Laboratory. Further information is contained
in a TSP (see page 1).
In accordance with Public Law 96-517,
the contractor has elected to retain title to this
invention. Inquiries concerning rights for its
commercial use should be addressed to:
Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
E-mail: iaoffice@jpl.nasa.gov
Refer to NPO-41927, volume and number
of this NASA Tech Briefs issue, and the
page number.
An apparatus has been designed and
built for testing the effects, on moder-
ate-sized objects, of cushioned deceler-
ations having magnitudes ranging up to
several hundred g [where g = normal
Earth gravitational acceleration (≈9.8
m/s2)]. The apparatus was originally in-
tended for use in assessing the ability of
scientific instruments in spacecraft to
withstand cushioned impacts of land-
ings on remote planets. Although such
landings can have impact velocities of
20 to 50 m/s, the decelerations must
not exceed a few hundred g.  This re-
quires the deceleration to occur over a
distance of as much as 50 cm in a time
of tens of milliseconds.  This combina-
tion of conditions is surprisingly diffi-
cult to simulate on the ground.  The ap-
paratus could also be used for general
impact testing.
The apparatus is simple to build. Rel-
ative to drop-tower apparatuses that
could produce equivalent impacts, this
apparatus is very compact. This appara-
tus is also relatively safe to operate be-
cause its design inherently prevents the
object under test or any debris from ac-
cidentally striking persons or equipment
in the vicinity.
The apparatus (see figure) includes a
steel pipe having an inside diameter of
20 cm and a length of 216 cm. A light-
weight polyethylene piston carries the
object under test. The piston is sealed to
the inner wall of the pipe by means of a
Simple, Compact, Safe Impact Tester 
Cushioned impact decelerations up to hundreds of normal Earth gravitation are easily produced. 
Ames Research Center, Moffett Field, California
https://ntrs.nasa.gov/search.jsp?R=20100002232 2019-08-30T08:35:08+00:00Z
6 NASA Tech Briefs, June 2007
Use of multiple-antenna radar sys-
tems aboard moving high-altitude plat-
forms has been proposed for measuring
rainfall.   The platforms contemplated
in the proposal would be primarily
spacecraft, but, in principle, the pro-
posal could also apply to aircraft. The
problem of measuring rainfall velocity
from a moving platform is especially
challenging because the velocity of the
platform (especially in the case of a
spacecraft) can be so large that it is dif-
ficult to distinguish between the rainfall
and platform contributions to Doppler
frequency shifts. Furthermore, nonuni-
form filling of radar beams can lead to
biases in Doppler estimates. Although it
might be possible to reduce these biases
through improved data processing, a
potential alternative is to use multiple
antennas positioned at suitable along-
track intervals.
The basic principle of the proposed
systems is a variant of that of along-
track interferometric synthetic-aper-
ture radar systems used previously to
measure ocean waves and currents. The
simplest system according to the pro-
posal would include two antennas that
would perform cross-track scans as in a
prior rainfall-measuring radar system.
The antennas would be located at dif-
ferent along-track positions. The along-
track distance between them would be
chosen, in conjunction with the along-
track velocity of the platform and the
radar pulse-repetition frequency
(PRF), such that this distance would
equal the distance traveled by the plat-
form between two successive pulses. (If
necessary, the PRF could be adjusted to
enforce this equality.) Thus, in effect,
two sets of measurements would be per-
formed at each platform position.
Under this condition, extraction of the
rainfall Doppler velocity without the
platform contribution is substantially
simplified and can be done with only
minor modification of processing tech-
niques traditionally used for ground-
based Doppler radars.
This work was done by Stephen Durden, Si-
mone Tanelli, and Paul Siqueira of Caltech
for NASA’s Jet Propulsion Laboratory. Fur-
ther information is contained in a TSP (see
page 1).
NPO-44018
Multi-Antenna Radar Systems for Doppler Rain Measurements
Use of multiple antennas would enable removal of platform Doppler contributions.
NASA’s Jet Propulsion Laboratory, Pasadena, California
Atmospheric Pressure Drives the Piston along the partially evacuated pipe toward the closed end. The piston is then decelerated strongly when it com-
presses the remaining air at the closed end of the pipe.
Latch
Object
Under Test
Air Compressed by
PistonPiston
Partial
Vacuum
To Vacuum
Pump
Pressure
GaugePiston at
Initial Position
Piston at
Maximum Excursion
(Maximum Compression)
gasket. One end of the pipe is open; the
opposite end of the pipe is closed and
connected to a vacuum pump.
At the beginning of a test, the piston is
held at the open end of the pipe by use
of a latch while the vacuum pump is acti-
vated to introduce a partial vacuum [typ-
ically, characterized by an absolute pres-
sure of 0.2 bar (20 kPa)] between the
piston and the closed end. Once the de-
sired partial vacuum has been reached,
the latch is released so that differential
pressure (atmospheric minus partial vac-
uum) accelerates the piston along the
pipe toward the closed end. The piston
reaches a maximum speed typically be-
tween 20 and 40 m/s, the exact value de-
pending on the mass of the object under
test and the starting pressure in the
partly evacuated volume. As the piston
approaches the closed end, it com-
presses the air ahead of it until, at a dis-
tance between 5 and 20 cm from the
closed end, the pressure becomes high
enough to stop the piston (and bounce
it back toward its starting position). An
accelerometer on the piston measures
the impact deceleration.
The impact deceleration is a strong
function of the amount of gas (and,
hence, of the pressure) initially in the
partially evacuated volume at the
closed end of the pipe. Therefore, it is
easy to adjust the peak deceleration by
adjusting the pressure of the partial
vacuum. The maximum speed is not a
strong function of the mass of the ob-
ject under test or of the starting partial-
vacuum pressure but can be tailored
over a wide range by using pipes of dif-
ferent length. 
This work was done by Pat R. Roach and Jef-
frey Feller of Ames Research Center. Further in-
formation is contained in a TSP (see page 1).
ARC-15085-1


Simple, Compact, Safe Impact Tester

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