urn:nasa:pds:context:instrument:nlr.near 1.0 1.7.0.0 Product_Context 2016-10-01 1.0 extracted metadata from PDS3 catalog and modified to comply with PDS4 Information Model urn:nasa:pds:context:instrument_host:spacecraft.near::1.0 instrument_to_instrument_host Cheng, A.F., A. Santo, K. Heeres, J. Landshof, R. Farquhar, et al., Near-Earth Asteroid Rendezvous: Mission Overview, Journal of Geophysical Research, Vol. 102, pp. 23695-23708, 1997. reference.CHENGETAL1997 Cole, T., M. Boies, A. El-Dinary, A.F. Cheng, M. Zuber, and D.E. Smith, The Near Earth Asteroid Rendezvous Laser Altimeter, Space Science Reviews, Vol. 82, pp. 217-253, 1997. reference.COLEETAL1997 Gardner, C.S., Ranging performance of satellite laser altimeters, IEEE Trans Geoscience and Remote Sensing, Vol. 30, pp. 1061-1072, 1992. reference.GARDNER1992 Smith, D.E., M.T. Zuber, S.C. Solomon, R.J. Phillips, J.W. Head, J.B. Garvin, W.B. Banerdt, D.O. Muhleman, G.H. Pettengill, G.A. Neumann, F.G. Lemoine, J.B. Abshire, O. Aharonson, C.D. Brown, S.A. Hauck, A.B. Ivanov, P.J. Mcgovern, H.J. Zwally, and T.C. Duxbury, The Global Topography of Mars and Implications for Surface Evolution, Science, Vol. 284, pp. 1495-1503, 1999. reference.SMITHETAL1999B Zuber, M.T., D.E. Smith, S.C. Solomon, D.O. Muhleman, J.W. Head, J.B. Garvin, J.B. Abshire, and J.L. Bufton, The Mars Observer Laser Altimeter Investigation, J. Geophys. Res., 97, 7781-7797, 1992. reference.ZUBERETAL1992 Zuber, M., D.E. Smith, F.G. Lemoine, and G. Neumann, The Shape and Internal Structure of the Moon from the Clementine Mission, Science, Vol. 266, pp. 1839-1843, 1994. reference.ZUBERETAL1994 Zuber, M., D.E. Smith, A.F. Cheng, and T.D. Cole, The NEAR Laser Ranging Investigation, Journal of Geophysical Research, Vol. 102, pp. 23761-23773, 1997. reference.ZUBERETAL1997 NEAR LASER RANGEFINDER Altimeter not applicable not applicable NEAR Laser Rangefinder Instrument Overview ========================================== NLR is a direct-detection, time-of-flight laser altimeter that determines the range from the NEAR spacecraft to the surface of Eros by measuring the round-trip travel time of laser pulses with 0.312 m range resolution (single count). It employs direct, incoherent detection using an all solid-state, diode-pumped laser transmitter, a Cassegrain receiver telescope, and a receiver electronics package that incorporates an IR-enhanced, silicon Avalanche Photo Diode (APD) with thresholding and timing electronics. Figure 1 shows the NLR flight hardware and its principal subsystems. NLR is the first laser altimeter to employ continuous, in-flight calibration using a fiber-optic delay assembly (FODA), which is a spooled, 109.5-m fused silica fiber-optic cable. Part of each optical pulse formed within the laser resonator is injected into the FODA, which is connected to the baffle system in front of the receiver telescope. Using a 45 deg elliptical mirror, the delayed optical signal is directly introduced into the receiver. This allows end-to-end calibration for each emitted pulse even when no target surface is available. The laser transmitter (Fig. 1) is a diode-pumped, solid-state (DPSS) Cr:Nd:YAG using an active, lithium niobate (LiNbO3) Q-switch to control the formation of the laser pulse. Such Q-switched Nd:YAG DPSS lasers provide high reliability and were used in the Clementine [ZUBERETAL1994] and Mars Global Surveyor [ZUBERETAL1992]; [SMITHETAL1999B] laser altimeters. The laser transmitter thermal design minimizes thermal excursions of the optical resonator assembly, using thermostatically controlled heaters and thermal isolation from the spacecraft. The laser pulse repetition rate (PRF) can be selected from among 1/8 Hz, 1 Hz, 2 Hz, and 8 Hz, but firing at 1 Hz PRF will be the nominal mode of operation at Eros. NLR is a bistatic system, with the transmitter separate from the receiver. The transmission of the laser optical pulse is accomplished using a 62 mm Galilean refractive telescope (Fig. 1). The large output aperture significantly reduces the transmitter beam divergence. The receiver (Fig. 1) uses an 8.89 cm diameter, gold- coated aluminum Dall-Kirkham telescope to focus back-scattered laser energy onto the APD detector. The APD is implemented in a trans- impedance circuit that automatically compensates for thermal variations. Detected signals from the APD are amplified, passed through a 30 MHz Bessel filter and sent to a commandable comparator to generate the digital stop pulse that is used to determine the time-of-flight (TOF). The TOF system measures the time from laser firing, as indicated by the laser transmitter start pulse, to the first receiver stop pulse (from the FODA calibration signal) and to the second stop pulse produced from the backscattered laser light returning from the target. The Bessel filter integrates the analog pulses to maximize the probability of detection for returned pulses that have been stretched in time ('dilated') by scattering from the rough, tilted target surface. The filter also attenuates high frequency electronic noise. The comparator tests whether the filtered analog signal exceeds a commandable threshold voltage; when this occurs, the digital stop pulse is generated. The threshold level used by the comparator is set by ground command or autonomously through an auto- acquisition sequence. After the two TOF counters are stopped, a digital processing unit (DPU) reads the count values and formats them into science data packets together with instrument status and housekeeping information. Table. 1 NLR Characteristics PARAMETER MEASUREMENT max. range (altitude) 327 km range accuracy < 6 m range resolution 0.312 m pulse energy 15.3 mJ @ 1.064 um energy jitter < 1 % rms knowledge pulse-width 15 ns pulse-width jitter 0.82 ns rms knowledge wavelength spread +- 1 nm pulse frequency 1/8, 1, 2, 8 Hz T-0 mask 0 to 511.5 usec, delta = 500 ns range gate 81ns to 42.7us, delta = 41.7 ns TEM00 (% Gaussian fit) 91 % (TEM00 mode-like) divergence (1/e2 -points) 235 urad calibration power jitter +- 5 % calibration timing jitter < 31.2 cm thermal control < +- 2 degC shots (lifetime) ~10^9 effective RX aperture, f/# 7.62 cm, f/3.4 spectral receiver bandwidth < 7 nm temporal receiver bandwidth 30 MHz APD dark noise voltage 150 uVrms APD hybrid responsivity 770 kV/W optical receiver FOV 2900 urad threshold levels 16mV*2^n, for n = 0,1,.,7 data rates 51 and 6.4 bps boresight shift, TX-to-RX 345 urad Operation ========= The measured laser pulse characteristics are summarized in Table 1 (data from [COLEETAL1997]). The temporal, spatial (near- and far- field), and pulse energy characteristics were measured during NLR flight system development. Pulse energy measurements of the transmitter beam and FODA port output were measured using NIST- traceable equipment accurate to +5%. Measured near-field spatial beam distributions included beam diameter (1/e2), modal structure, and energy distribution characteristics. Far-field measurements included beam divergence, jitter, and wander. The receiver sees two optical pulses per transmitted pulse. The first arrives ~558 ns after laser firing and is the calibration pulse routed through the FODA. Detection of this pulse stops the calibration counter, and the calibration counts value ('Calibration' in Table A1, see Appendix) is reported for each shot when operating at 1/8 Hz, 1 Hz or 8 Hz, but not when operating at 2 Hz. The next detected return is the surface backscattered pulse, whose detection stops the range counter. The range count value ('Range' in Table A1) is always reported for each laser shot. NLR's receiver is a leading-edge detector, meaning that a stop pulse is generated as soon as the optically produced signal (the filtered, analog voltage from the calibration pulse or the target return) exceeds a threshold level ('Threshold Voltage' in Table A1). The time at which this signal crosses the threshold determines the measured TOF, which therefore depends on the threshold value (threshold-driven 'range walk'). This threshold can be commanded to any of eight values (Table 1), although no calibration signal is detected at the highest value. Figure 2 shows the sequence of events that occur with each laser shot. After the fire command, the laser pulse is generated after a time delay that varies from 170 ?s to 190 ?s. The firing time delay varies slowly on ten-minute time scales. The emitted optical pulse generates the start pulse for the two TOF counters. The calibration pulse is received about 0.558 ?s after firing, and the laser return from the target is received much later, at a time determined by the range (the range counter overflows after 2.18 ms). The NLR receiver is `blanked', or prevented from detecting a pulse, for a time interval T0 after the fire command. This T-0 mask (see Table 1) is set by ground command with a default value of 180.498 ?s. In-flight tests have shown that this default value, as well as values half as large, are sufficient to suppress any electronic noise from firing. The value of T0 is adjusted by two parameters returned in the NLR data; the first is called 'T-0 COUNT' and is represented here by the symbol ?, while the second is called 'Range gate' and is represented by the symbol rg. This 'Range gate' parameter does not define a range gate in the usual sense of a brief time interval, during which the receiver is active, encompassing the time of the expected range return. Rather, the NLR receiver becomes active once the time T0 has elapsed, and it remains active until two stop pulses are generated, one from the calibration return and one from the target. The relation between T0 and the commandable parameters ? and rg is where T0 is in units of ?s, and where ? and rg are both integers in the range [0, 1023]. The NLR timing shown in Fig 2 applies to normal operation at any of the four PRF values. NLR also includes two special failure modes of operation, that can be used in the event of failures involving either the start pulse or the calibration pulse. In the former case, the FAILSAFE mode starts the TOF counters at the final value of the 'T-0 COUNT' countdown. In the latter case, the ONESTOP mode causes NLR to use the first received stop pulse as the range pulse from the target. As of September, 1999, the start and calibration pulses are functioning nominally, and there is no plan to use either FAILSAFE or ONESTOP. NLR has two additional special operating modes designed to enable autonomous operation without excessive ground commanding. The first is called 'AUTO ACQUIRE', in which NLR uses the calibration pulse to find a threshold voltage above the noise level in the receiver. NLR fires 16-shot bursts at each of the eight possible values for the voltage threshold (listed in Table 1), which are labeled TH = 0 through TH = 7. The results of all pre-launch and in-flight tests through September, 1999 have shown that AUTO ACQUIRE sets the threshold at TH = 3. The second special operating mode is called 'CALIBRATE', in which NLR autonomously sets the value of ?. During 'CALIBRATE', NLR increments ? until it finds as large a value as possible that still assures detection of a valid calibration pulse. Fig 2 shows that if ? becomes too large, the time T0 that the receiver is blanked will include the arrival time of the calibration pulse, which cannot then be detected. As of September, 1999 there is no plan to use CALIBRATE during asteroid operations because the default values of ? and rg are completely satisfactory. The timing of NLR events relative to other NEAR spacecraft events is shown in Figure 3. All spacecraft events are coordinated by synchronization pulses sent over the Mil-Std-1553 data bus at each one second interval of mission elapsed time (MET) under control of the command and telemetry processor (CTP). These one second intervals are referred to as 'major frames'. Every major frame is divided into 8 minor frames (Fig 3) numbered 0 through 7, each lasting 125 ms. The NLR DPU controls instrument timing which is locked to the receipt of the MET synchronization pulses once per second from the CTP. NLR fires the laser in specific minor frames, depending on the PRF mode as shown in Table 2. In all cases, laser firing occurs within ~3 ms of the start of the minor frame(s) shown in Table 2. Table 2. NLR firing times PRF Mode MinorFrame(s) 1/8Hz 0 1 Hz 0 2 Hz 0,4 8 Hz all NLR can sustain continuous firing at 1/8 Hz, 1 Hz, or 2 Hz PRF for indefinite periods. However, because of thermal limitation NLR cannot fire indefinitely at 8 Hz. In the 8 Hz mode, NLR fires 16 shots in two seconds, after which NLR is quiescent for 14 s. Hence, in this mode NLR fires an average of one pulse per second. An important NLR science objective is to correlate the laser altimetry data with imaging data from the NEAR Multispectral Imager (MSI), as discussed by [ZUBERETAL1997]. To accomplish this objective, it is necessary to determine the boresight offset between the NLR and the MSI. Both instruments have been designed to enable a direct measurement of the boresight offset, by using MSI to image the laser spot over the darkside of Eros. To obtain sufficient signal in the image, NLR is operated at 8 Hz to produce 8 shots during the maximum imager exposure time of 999 ms. The relative timing of NLR operation and MSI image exposures is shown in Fig 3. The MET synchronization pulses at NLR and at MSI are time-aligned within a few ms. During the maximum MSI exposure, the first NLR shot occurs ~80 ms after the start of the exposure, and the last occurs ~44 ms before the end. NLR Data collection The current plan is to leave NLR powered on throughout the prime mission phase in Eros orbit, firing laser pulses whenever the instruments are pointed at Eros for data acquisition (all NEAR instruments can observe Eros simultaneously). NEAR will point its instruments at Eros for approximately 16 hours per day, and it will point its high gain antenna at Earth for approximately 8 hours per day to downlink data. During downlink periods, NLR will not be pointed at the asteroid but will remain powered on. If NLR is operated at its 1 Hz nominal PRF mode, it will obtain a total of ~2x107 range measurements during the year-long prime mission and generate up to ~1.7 Gbit of data. The NLR investigation will begin ranging to Eros once the spacecraft descends to within 300 km range from the asteroid in early March, 2000. The NLR investigation will control spacecraft pointing for the first two weeks in May, 2000, while the spacecraft is in an approximately circular, 50 km polar orbit. Most of the NLR data will be acquired while nadir pointing, i.e., with NLR pointed to the Eros center of mass. Figure 4 shows the ground coverage for 1 week of nadir-pointed observations in this orbit. The footprints of successive laser shots at 1 Hz PRF will be significantly overlapping while NLR is nadir- pointed during most of the 50 km orbit, except when NEAR is directly over an elongated end of Eros ([ZUBERETAL1997]; [COLEETAL1997]). The footprint diameter is 7 m at an altitude of 30 km and normal incidence. The sub-spacecraft point on Eros moves at 3-5 m/s. Even one week of nadir-pointed observations will yield a data set well suited for determining global shape, size, and rotation state [ZUBERETAL1997]. Although each NLR track is densely sampled in the 50 km orbit, Figure 4 shows that the successive tracks are spaced more than a kilometer apart at the equator of Eros. The NLR investigation plans to allocate a portion of the 50 km orbit time to cross-track scanning of NLR, using spacecraft maneuvers, to obtain a more uniform areal coverage of Eros. Since the laser footprints are overlapping or nearly so at 1 Hz PRF while nadir pointed in the 50 km orbit, NLR operations through the first half of the rendezvous year will use the 1 Hz mode. Due to a spacecraft interface error, the spacecraft overcurrent protection for NLR must be switched off when NLR fires at 2 Hz. Nevertheless, NLR may be operated in the 2 Hz mode late in the rendezvous year, from the 35 km circular orbit. Note All NLR data after 6/22/00 (2000 204) will appear in the version 7 NLR format, which is the same as the version 6 normal format (ID = 4) EXCEPT for the field labeled 'CALIBRATION_WORD_1' which has been changed. 'CALIBRATION_WORD_1' will take the following values: in AutoAcq, 'first cal' = 30 in 1 Hz, 'first cal' = 584 (first shot in minor frame 4, at packet MET plus 0.5 second) in 2 Hz, 'first cal' = 364 (usually) but 804 is possible. If 364, the first shot was in minor frame 2 (at packet MET plus 0.25 second). Otherwise, the first shot was in minor frame 6.