A laser ranging instrument is being developed to measure the spatial variations in the Earth's gravity field. It will range in space to a cube corner on a passive co-orbiting sub-satellite with a velocity accuracy of 20 to 50 microns/sec by using AlGaAs lasers intensity modulated at 2 GHz

Abshire, James B.

Millar, Pamela S.

Sun, Xiaoli

English

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(NASA-CR-199776) SEMICONDUCTOR N96-15752
LASER-BASED RANGING INSTRUMENT FOR
EARTH GRAVITY MEASUREMENTS (NASA.
Goddard Space Flight Center) A p Unclas
G3/36 0083578
https://ntrs.nasa.gov/search.jsp?R=19960008586 2020-06-16T06:01:52+00:00Z
76 /TuB5- l
• - - - - , ., -
Semiconductor Laser-Based Ranging Instrument for Earth Gravi ty Measurements
NASA-CR-199776 James B. Abshire
Pamela S. Millar
Xiaoli Sun*
NASA - Goddard Space Flight Center
Experimental Instrumentation Branch, Code 924
GreenbekMD 20771, USA
Phone: (301) 286-261 1 Fax: (301) 286-1761
Internet: jba@eibl .gsfc.nasa.gov
*-Johns Hopkins University
Electrical and Computer Engineering Department
Baltimore MD 21218
Abstract: A laser ranging instrument is being developed to measure the spatial variations in the Earth's gravity
field. It will range in space to a cube comer on a passive co-orbiting sub-satellite with a velocity accuracy of 20 to
50 urn/sec by using AlGaAs lasers intensity modulated at 2 GHz.
Need for Gravity Measurements: Spatial variations
in a planet's gravity field is an important indicator of
its internal structure, which is often otherwise
inaccessible. Knowledge of the spatial structure in
the Earth's gravity field is also essential for the
precise determination of satellite orbit heights. These
are critical for space-based ocean altimetry missions,
where radar measurements to the ocean surface are
being used to investigate the flow of ocean currents.
Spatial variations in the gravity field cause both the
local sea surface and satellite orbit heights to vary
with location. Lack of knowledge of the gravity
field's higher order terms now limits in the spatial
resolution of the Topex ocean height measurements.
Mission Overview: The Gravity and Magnetics
Earth Surveyor (Games) mission has been
developed by NASA-GSFC to measure the Earth's
gravity and magnetic fields. The mission is planned
as a small Earth-probe for a fall 1998 launch. A
small rocket will be used to launch a SMEX-type
spacecraft into a 325 km sun-synchronous circular
polar orbit The spacecraft carries a vector and scalar
magnetometer on several meter long boom. A GPS
receiver is used to determine the low degree and
order terms of the gravity field.
The high degree and order gravity field terms
are determined from the measurements of a satellite-
to-satellite laser ranging instrument (LRI). Once in
orbit, the spacecraft ejects a small passive
subsatellite, which carries a laser retro-reflecior. The
main spacecraft then maneuvers ahead of the
subsatellite and co-orbits with it, while leading it by
~ 150 km. The LRI is pointed toward the
subsatellite, and it tracks and continuously measures
the intersatellite range.
As the spacecraft pair pass over the spatial
variations in the gravity field, they experience along-
track accelerations which change their relative
velocity. These time displaced velocity changes are
sensed by the LRI with a resolution of 20-50 um/sec.
When drag causes the intersatellite range to deviate
outside the desired 100-200 km interval, the main
spacecraft performs an orbit adjustment to re-
establish the desired co-orbiting conditions.
Instrument Description: The LRI is laser diode-
based ranging system, which is fix-mounted to the
spacecraft The present LRI configuration is shown
in Figure 1 and its block diagram is shown in Figure
2. The LRI measures the range with 3 different
intensity modulated signals, which determine the
absolute range to the subsatellite. The fine and
medium range signals are 2 GHz and 31.25 MHz
sinusoids which are intensity modulated onto the 815
nm ranging laser diode. The relative ranges are
measured by resolving the phase angles of the
reflected ranging tones. The 2 GHz tone is used to
measure the most accurate relative range to the
subsatellite. The 31.25 MHz medium ranging tone is
measured to better than the 75 mm ranging period of
the fine tone once per second. The coarse ranging
signal is a 31.25 MBit/sec pseudo-noise (PN) code,
which is intensity modulated onto the tracking laser
diode at 845 nm. It is used to resolve the absolute
range to the subsatellite to within a bit-time. The
combined ranging signals are used for the gravity
determination as well as to guide the orbit correction
TuB5-2 / 77
maneuvers and maintain the 100 to 200 km distance
lothesubsaiellite.
The subsatellite is passive and carries a 9 cm
diameter hollow cube comer. After deployment, the
spacecraft body points the LRI to the subsatellite.
The spacecraft keeps the ranging beam centered on
the subsatellite by using the LRI measured angular
position to the control the spacecraft pointing.
Instrument Design: The LRI consists of the
transceiver, door, electronics and the subsatellite
comer reflector. The details of the LRI design are
summarized in Table 1. The transceiver is attached
to the spacecraft via an interface plate, which
provides the common optical bench for the Laser
Diode (LD) transmitters and receiver telescope. The
transmitter consists of 3 ranging and 5 tracking LDs.
The ranging LDs are used for both medium and fine
ranging and the tracking LDs are used for both
tracking and coarse ranging. The beams from all
lasers are co-aligned and fixed with respect to the
receiver's optical axis.
The LRI collects the reflected signal from the
subsatellite with a 20 cm diameter receiver telescope.
The aft optics split the received optical signal to the
ranging and tracking detectors. A protective door is
open during normal ranging operation, but is closed
for launch and thruster firings. The door is used to
prevent contamination from the spacecraft thrusters.
The door also carries passive optical reflectors, which
serve as a test target when the door is closed. The
reflectors return attenuated LD beams to the receiver,
which permits verification tests of the LRI without
the subsatellite.
Acquisition: The tracking LDs transmit a 10 mrad
wide beam at 845 nm along the spacecraft's negative
velocity vector. Three tracking laser diodes are used
at one time, leaving two as spares. To acquire the
subsatellite, the spacecraft systematically scans its
attitude in pitch and yaw. This scans the fixed-
mounted LRI beam. When the subsatellite is
illuminated, the reflected power is collected by the
telescope, and the tracking signal is split in the aft-
pptics assembly. Approximately 50% of the power
is focused onto the CCD array while the remainder is
focused onto the coarse ranging PMT. The CCD
readout is processed by the on-board computer,
which generates pitch and yaw error signals
proportional to the offset of the tracking spot from
the array's center. These angular errors are input to
the spacecraft control system, which corrects the
spacecraft pointing to center the signal on the CCD
array.
Tracking: Once acquisition occurs, the tracking
^gorithm continuously update the angular error
signals to the spacecraft. The spacecraft keeps the
LRI pointed at the subsatellite to better than 100 urad
by adjusting its pitch and yaw angles in response to
the error signals.
The coarse ranging signal from the PMT is
converted to a photon counting bit stream and is
integrated by the receiver histogram electronics for -
1 sec. The histogram is correlated with a stored
replica of the transmitted code to obtain the most
likely range bin (the pseudo range). The multiple
code length range ambiguity is resolved by
alternating the PN code length between 127 and 511
bits and applying the Chinese Remainder Theorem
on the pseudoranges.
Ranging: Ranging occurs simultaneously with
tracking. The ranging LDs transmit an intensity
modulated beam at 815 nm with a 1 mrad
beamwidth. A single ranging LD operates at any
time, leaving two as spares. Approximately 80% of
the LD's intensity is modulated by the 2 GHz tone,
10% is used by the 31.25 MHz tone and the residual
10% is not modulated.
The LRI laser signals reflected by the
subsatellite are collected by the telescope and
directed to the ranging PMT. The PMTs electrical
output is split into the fine and medium receiver
electronics. Both ranging signals are independently
down converted to the 7.6 KHz intermediate
frequency (IF) by multiplying them with phase-
locked local oscillator (LO) signals. Each LO is
offset from its ranging tone by 7.6 KHz. The fine
and medium IF signals are independently digitized at
30.4 kHz rate. The computer computes the in-phase
and quadrature (I&Q) components from the samples
at ~ 260 Hz.
The instrument computer collects the received
signal components to transmit to the ground. The
ground station computes the magnitude and phase
angles for both ranging tones from the I&Q samples.
The fine ranging phase angle is used to determine the
subsatellite velocity. The medium ranging phase
angle is used to measure the cycle number of the fine
ranging tone to within a bit of the coarse ranging
signal. The coarse ranging measurements are used to
reference the medium ranging tone to absolute range.
The gravity measurement errors, which are
referenced between the main spacecraft and
subsatellite CG's, are summarized in Table 2. These
are slightly larger than the instrument errors, since
they also include the geometrical effects of the
positions of the LRI and cube comer.
Details on the mission and instrument design, the
fine ranging breadboard measurements, and expected
instrument performance will be discussed in the talk.
78 /TuB5-3
Table 1 - Summary of LRI Transceiver Desien
Pointing:
Laser Transmitters:
Ranging:
Tracking:
Telescope:
Ranging Detectors:
Tracking Detector:
Fine Range Receiver:
IF frequency:
Sampling rate
Phase components:
Fine Ranging Measurement
200km:
100km:
Drift rates:
Body pointed by spacecraft to - 100 urad
Single mode AIGaAs Laser Diodes:
60 mW average power, single mode, 1 mrad beam width
815 nm, 2 GHz mod., 3 each -1 on at a time
90 mW average power, single mode, 10 mrad beam width
845 nm, 31.25 Mbit/sec PN code, 5 each - 3 on at a lime
20 cm diameter Be
PMT with GaAs photocathode, 20% QE (810- 845 nm)
256 x 256 CCD. - 10 Hz frame rate
2 GHz & 31.25 MHz ranging tones
7.6KHz
30.4 KHz
Compute I&Q, 260 times/sec
Precision:
« 45 um/sec (1.5 x signal shot noise)
= 10 um/sec (1.5x signal shot noise).
< 2 um/sec ,
Table 2 - GAMES-LRI Error Summary in Gravity Measurements
Error Source
LRI random ranging error (pm/sec)
LRI measurement drift rate (urn/sec)
LRI-spacecraft CG offset (residual)
Subsatellite (um/sec)
Cube vertex-CG offset: (2 um/sec)
Drag induced accel . : (10 |im/sec)
Margin - other ranging errors
RSS Gravity Ranging Errors (ujn/sec)
Subsatellite Range (km)
200
45
2
2
12
10
48
100
10
2
2
12
10
18
Comments
From link analysis
LRI requirement
Dom. error at 100 km
Mounting errors, etc.
RSS: Root Sum Square
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Semiconductor laser-based ranging instrument for earth gravity measurements

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