(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 76 /TuB5-l • - - - - , ., - Semiconductor Laser-Based Ranging Instrument for Earth Gravity Measurements James B. Abshire NASA-CR-199776 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 subsatellite, and it tracks and continuously measures in a planet's gravity field is an important indicator of the intersatellite range. its internal structure, which is often otherwise As the spacecraft pair pass over the spatial inaccessible. Knowledge of the spatial structure in variations in the gravity field, they experience along- the Earth's gravity field is also essential for the track accelerations which change their relative precise determination of satellite orbit heights. These velocity. These time displaced velocity changes are are critical for space-based ocean altimetry missions, sensed by the LRI with a resolution of 20-50 um/sec. where radar measurements to the ocean surface are When drag causes the intersatellite range to deviate being used to investigate the flow of ocean currents. outside the desired 100-200 km interval, the main Spatial variations in the gravity field cause both the spacecraft performs an orbit adjustment to re- local sea surface and satellite orbit heights to vary establish the desired co-orbiting conditions. with location. Lack of knowledge of the gravity field's higher order terms now limits in the spatial Instrument Description: The LRI is laser diode- resolution of the Topex ocean height measurements. based ranging system, which is fix-mounted to the spacecraft The present LRI configuration is shown Mission Overview: The Gravity and Magnetics in Figure 1 and its block diagram is shown in Figure Earth Surveyor (Games) mission has been 2. The LRI measures the range with 3 different developed by NASA-GSFC to measure the Earth's intensity modulated signals, which determine the gravity and magnetic fields. The mission is planned absolute range to the subsatellite. The fine and as a small Earth-probe for a fall 1998 launch. A medium range signals are 2 GHz and 31.25 MHz small rocket will be used to launch a SMEX-type sinusoids which are intensity modulated onto the 815 spacecraft into a 325 km sun-synchronous circular nm ranging laser diode. The relative ranges are polar orbit The spacecraft carries a vector and scalar measured by resolving the phase angles of the magnetometer on several meter long boom. A GPS reflected ranging tones. The 2 GHz tone is used to receiver is used to determine the low degree and measure the most accurate relative range to the order terms of the gravity field. subsatellite. The 31.25 MHz medium ranging tone is The high degree and order gravity field terms measured to better than the 75 mm ranging period of are determined from the measurements of a satellite- the fine tone once per second. The coarse ranging to-satellite laser ranging instrument (LRI). Once in signal is a 31.25 MBit/sec pseudo-noise (PN) code, orbit, the spacecraft ejects a small passive which is intensity modulated onto the tracking laser subsatellite, which carries a laser retro-reflecior. The diode at 845 nm. It is used to resolve the absolute main spacecraft then maneuvers ahead of the range to the subsatellite to within a bit-time. The subsatellite and co-orbits with it, while leading it by combined ranging signals are used for the gravity ~ 150 km. The LRI is pointed toward the determination as well as to guide the orbit correction TuB5-2 / 77 maneuvers and maintain the 100 to 200 km distance signals to the spacecraft. The spacecraft keeps the lothesubsaiellite. LRI pointed at the subsatellite to better than 100 urad The subsatellite is passive and carries a 9 cm by adjusting its pitch and yaw angles in response to diameter hollow cube comer. After deployment, the the error signals. spacecraft body points the LRI to the subsatellite. The coarse ranging signal from the PMT is The spacecraft keeps the ranging beam centered on converted to a photon counting bit stream and is the subsatellite by using the LRI measured angular integrated by the receiver histogram electronics for - position to the control the spacecraft pointing. 1 sec. The histogram is correlated with a stored replica of the transmitted code to obtain the most Instrument Design: The LRI consists of the likely range bin (the pseudo range). The multiple transceiver, door, electronics and the subsatellite code length range ambiguity is resolved by comer reflector. The details of the LRI design are alternating the PN code length between 127 and 511 summarized in Table 1. The transceiver is attached bits and applying the Chinese Remainder Theorem to the spacecraft via an interface plate, which on the pseudoranges. provides the common optical bench for the Laser Ranging: Ranging occurs simultaneously with Diode (LD) transmitters and receiver telescope. The tracking. The ranging LDs transmit an intensity transmitter consists of 3 ranging and 5 tracking LDs. modulated beam at 815 nm with a 1 mrad The ranging LDs are used for both medium and fine beamwidth. A single ranging LD operates at any ranging and the tracking LDs are used for both time, leaving two as spares. Approximately 80% of tracking and coarse ranging. The beams from all the LD's intensity is modulated by the 2 GHz tone, lasers are co-aligned and fixed with respect to the 10% is used by the 31.25 MHz tone and the residual receiver's optical axis. 10% is not modulated. The LRI collects the reflected signal from the The LRI laser signals reflected by the subsatellite with a 20 cm diameter receiver telescope. subsatellite are collected by the telescope and The aft optics split the received optical signal to the directed to the ranging PMT. The PMTs electrical ranging and tracking detectors. A protective door is output is split into the fine and medium receiver open during normal ranging operation, but is closed electronics. Both ranging signals are independently for launch and thruster firings. The door is used to down converted to the 7.6 KHz intermediate prevent contamination from the spacecraft thrusters. frequency (IF) by multiplying them with phase- The door also carries passive optical reflectors, which locked local oscillator (LO) signals. Each LO is serve as a test target when the door is closed. The offset from its ranging tone by 7.6 KHz. The fine reflectors return attenuated LD beams to the receiver, and medium IF signals are independently digitized at which permits verification tests of the LRI without 30.4 kHz rate. The computer computes the in-phase the subsatellite. and quadrature (I&Q) components from the samples at ~ 260 Hz. Acquisition: The tracking LDs transmit a 10 mrad The instrument computer collects the received wide beam at 845 nm along the spacecraft's negative signal components to transmit to the ground. The velocity vector. Three tracking laser diodes are used ground station computes the magnitude and phase at one time, leaving two as spares. To acquire the angles for both ranging tones from the I&Q samples. subsatellite, the spacecraft systematically scans its The fine ranging phase angle is used to determine the attitude in pitch and yaw. This scans the fixed- subsatellite velocity. The medium ranging phase mounted LRI beam. When the subsatellite is angle is used to measure the cycle number of the fine illuminated, the reflected power is collected by the ranging tone to within a bit of the coarse ranging telescope, and the tracking signal is split in the aft- signal. The coarse ranging measurements are used to pptics assembly. Approximately 50% of the power reference the medium ranging tone to absolute range. is focused onto the CCD array while the remainder is The gravity measurement errors, which are focused onto the coarse ranging PMT. The CCD referenced between the main spacecraft and readout is processed by the on-board computer, subsatellite CG's, are summarized in Table 2. These which generates pitch and yaw error signals are slightly larger than the instrument errors, since proportional to the offset of the tracking spot from they also include the geometrical effects of the the array's center. These angular errors are input to positions of the LRI and cube comer. the spacecraft control system, which corrects the Details on the mission and instrument design, the spacecraft pointing to center the signal on the CCD fine ranging breadboard measurements, and expected array. instrument performance will be discussed in the talk. Tracking: Once acquisition occurs, the tracking ^gorithm continuously update the angular error 78 /TuB5-3 Table 1 - Summary of LRI Transceiver Desien Pointing: Body pointed by spacecraft to - 100 urad Laser Transmitters: Single mode AIGaAs Laser Diodes: Ranging: 60 mW average power, single mode, 1 mrad beam width 815 nm, 2 GHz mod., 3 each -1 on at a time Tracking: 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 Telescope: 20 cm diameter Be Ranging Detectors: PMT with GaAs photocathode, 20% QE (810- 845 nm) Tracking Detector: 256 x 256 CCD. - 10 Hz frame rate Fine Range Receiver: 2 GHz & 31.25 MHz ranging tones IF frequency: 7.6KHz Sampling rate 30.4 KHz Phase components: Compute I&Q, 260 times/sec Fine Ranging Measurement Precision: 200km: « 45 um/sec (1.5 x signal shot noise) 100km: = 10 um/sec (1.5x signal shot noise). Drift rates: < 2 um/sec , Table 2 - GAMES-LRI Error Summary in Gravity Measurements Subsatellite Range (km) Error Source 200 100 Comments LRI random ranging error (pm/sec) 45 10 From link analysis LRI measurement drift rate (urn/sec) 2 2 LRI requirement LRI-spacecraft CG offset (residual) 2 2 Subsatellite (um/sec) 12 12 Dom. error at 100 km Cube vertex-CG offset: (2 um/sec) Drag induced accel . : (10 |im/sec) Margin - other ranging errors 10 10 Mounting errors, etc. RSS Gravity Ranging Errors (ujn/sec) 48 18 RSS: Root Sum Square GAMES LRI 2 GAMES Laser Ranging Instrument 2 System Block Diagram V.2 MulwOae Fn« Rang t 2.0 LDDtM GHz EtaOionia fM FRLO UGHLr • 7.6 kHz) J 61 Sampling Clock Co*n«U> X . M5 nm. Up 10 3S MHz -60 mW Co*r»»an4 fintHanBitv aP,M*. *»"•". Sub-MUBR* Sub-ut DiS«nc» lOOkm-JMlun FRLO —' 61 kHi S*mpl« _? 2-22-M KOC