Optical Range Finding Technique
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Optical rangefinding applications using communications modulation technique William D. Caplan a , Christopher John Morcom b a NIRCM, Theresiastraat 279, 2593 AK, The Hague, Netherlands; b Instro Precision Limited, Broadstairs, Kent, CT10 2YD, England ABSTRACT A novel range detection technique combines optical pulse modulation patterns with signal cross-correlation to produce an accurate range estimate from low power signals. The cross-correlation peak is analyzed by a post-processing algorithm such that the phase delay is proportional to the range to target. This technique produces a stable range estimate from noisy signals. The advantage is higher accuracy obtained with relatively low optical power transmitted. The technique is useful for low cost, low power and low mass sensors suitable for tactical use. The signal coding technique allows applications including IFF and battlefield identification systems. Keywords: Laser rangefinder, low power, precision distance measurement, PDM, Optical Distance Measurement INTRODUCTION Laser rangefinders operate by measuring the delay of an optical signal reflected from the target of interest. This round trip propagation delay is proportional to range to the target. The optical signal decreases due to atmospheric attenuation and geometric spreading of the transmitted beam and reflected return signal. In order to compensate for these losses the rangefinder requires sufficient transmitted power, receiver aperture and electronic gain to produce an accurate range measurement. The size, weight, and power burdens of the sensor can be reduced by modulating the transmitted beam in order to take full advantage of advanced digital signal processing techniques that achieve the required range and accuracy with less transmitted power than a conventional laser rangefinder (LRF). As a consequence of the reduced burdens there are significant cost reductions realized as well. A novel range detection technique 1 is described here that combines optical pulse modulation patterns with signal cross- correlation to produce an accurate range estimate from low power signals. This technique, called precision distance measurement (PDM) is implemented in a family of low power LRFs currently in production. MODULATION TECHNIQUES There is a large body of knowledge in electronic signal processing that can be applied to select the best modulation technique; the pseudorandom binary sequence (PRBS) commonly used in communications systems is a useful modulation pattern for this application. The PRBS is reflected from the target and collected by the optical receiver. The receiver clock is synchronized with the transmitter clock which generates the modulation. The cross-correlation of the PRBS is calculated and range measurements are obtained by measuring the symbol phase delay of the coded sequence by means of the code sequence cross-correlation processor such that the phase delay, d, is proportional to the range to target. A schematic illustration of the process is shown in Figure 1. Unmanned/Unattended Sensors and Sensor Networks VII, edited by Edward M. Carapezza, Proc. of SPIE Vol. 7833, 78330B © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.864966 Proc. of SPIE Vol. 7833 78330B-1
Transcript of Optical Range Finding Technique
Optical rangefinding applications using communications modulation technique
William D. Caplana, Christopher John Morcomb
aNIRCM, Theresiastraat 279, 2593 AK, The Hague, Netherlands; bInstro Precision Limited, Broadstairs, Kent, CT10 2YD, England
ABSTRACT
A novel range detection technique combines optical pulse modulation patterns with signal cross-correlation to produce an accurate range estimate from low power signals. The cross-correlation peak is analyzed by a post-processing algorithm such that the phase delay is proportional to the range to target. This technique produces a stable range estimate from noisy signals. The advantage is higher accuracy obtained with relatively low optical power transmitted. The technique is useful for low cost, low power and low mass sensors suitable for tactical use. The signal coding technique allows applications including IFF and battlefield identification systems.
Keywords: Laser rangefinder, low power, precision distance measurement, PDM, Optical Distance Measurement
INTRODUCTION Laser rangefinders operate by measuring the delay of an optical signal reflected from the target of interest. This round trip propagation delay is proportional to range to the target. The optical signal decreases due to atmospheric attenuation and geometric spreading of the transmitted beam and reflected return signal. In order to compensate for these losses the rangefinder requires sufficient transmitted power, receiver aperture and electronic gain to produce an accurate range measurement. The size, weight, and power burdens of the sensor can be reduced by modulating the transmitted beam in order to take full advantage of advanced digital signal processing techniques that achieve the required range and accuracy with less transmitted power than a conventional laser rangefinder (LRF). As a consequence of the reduced burdens there are significant cost reductions realized as well.
A novel range detection technique1 is described here that combines optical pulse modulation patterns with signal cross-correlation to produce an accurate range estimate from low power signals. This technique, called precision distance measurement (PDM) is implemented in a family of low power LRFs currently in production.
MODULATION TECHNIQUES
There is a large body of knowledge in electronic signal processing that can be applied to select the best modulation technique; the pseudorandom binary sequence (PRBS) commonly used in communications systems is a useful modulation pattern for this application. The PRBS is reflected from the target and collected by the optical receiver. The receiver clock is synchronized with the transmitter clock which generates the modulation. The cross-correlation of the PRBS is calculated and range measurements are obtained by measuring the symbol phase delay of the coded sequence by means of the code sequence cross-correlation processor such that the phase delay, d, is proportional to the range to target. A schematic illustration of the process is shown in Figure 1.
Unmanned/Unattended Sensors and Sensor Networks VII, edited by Edward M. Carapezza, Proc. of SPIE Vol. 7833, 78330B
© 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.864966
Proc. of SPIE Vol. 7833 78330B-1
Transmitter
Receiver
Target
Range
Correlator timedelay
Transmitter
Receiver
Target
Range
Correlator timedelay
Figure 1 Laser rangefinder signal schematic
The measured range to the target is therefore calculated by R = d • (c/2) where c is the speed of light.
One implementation of the PRBS is the maximal length sequence (MLS). A portion of a typical MLS is shown in Figure 2. The main characteristic of this type of signal modulation is that it provides a very strong autocorrelation peak This type of signal coding is especially useful for recovering weak signals in high background noise by cross-correlation of the received signal with the transmitted modulation. A numerical simulation of this is shown in Figure 3 where the input signal to noise ratio is 1.0, and the result of the cross-correlation is a large peak.
Figure 2 partial MLS modulation pattern
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Figure 3 Cross-correlation peak
The theoretical result of a cross-correlation of an MLS with symbol time interval T with a delayed version of the original signal is a triangular peak of width 2T located at the sample corresponding to the delay d (Figure 4) Normally the range resolution of an LRF using this method would be limited to the length of the corresponding symbol pulse length T, because the correlator output interval is equal to the symbol time interval.
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T
timedelay
2T
MLS signal
Cross correlation
T
timedelay
2T
timedelay
timedelay
2T
MLS signal
Cross correlation
Figure 4 Cross-correlation interval
PRECISION DISTANCE MEASUREMENT
The precision distance measurement technique extracts higher resolution range measurements by increasing the time sample rate to be a shorter interval than the MLS symbol interval T. Typically the oversample factor would be 4, 8, 16, or 32 times faster. The same simulation data output as the previous figure is shown on a smaller scale in Figure 5, where the oversample rate of 8 produces a cross-correlation peak 16 samples wide.
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Figure 5 Cross-correlation peak in small scale
In order to extract higher accuracy distance measurement, the cross-correlation is run at an oversample rate such that the cross-correlation peak is spread by the number of oversamples. When this spread cross-correlation peak is analyzed by a post-processing algorithm, the precise location of the peak is calculated by a least squares estimator on the rising and falling sides of the peak. This technique produces a stable range estimate from a noisy input signal.
In the following Figure 6, the numerical simulation of the PDM is run with a higher fidelity simulation which includes the effects of finite electronic bandwidth. Here the shape of the cross-correlation peak is slightly distorted by the limited bandwidth.
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sample index Figure 6 High fidelity cross-correlation peak
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The core of the PDM technique is illustrated in the next Figure 7. Taking the cross-correlation output data on each side of the peak, a linear least squares fit is calculated and illustrated as a straight line. The two rising and falling LSE lines intersect, and the location on the x axis of that intersect (marked as a diamond shape) is a precise measure of the received signal delay. An interesting feature of the PDM technique is that range resolution is not limited to either the symbol time interval T or the sub-symbol interval determined by the oversample factor.
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sample index
target_delay
Figure 7 PDM technique
This feature allows the system design to be very flexible, where a trade-off between MLS sample rate, required range and range accuracy, and transmit power can be optimized for various applications.
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EXAMPLES
As an example for comparison, the Infrared Handbook2 shows a Nd:YAG rangefinder operating at 1.06 um, with the system parameters shown in Table 1. For the present work, the signal to noise ratio (SNR) as shown in Figure 8 differs slightly from the plot given in the handbook due to modifications to the reference target. The resulting SNR of the conventional LRF operating with peak power of 5 MW and an 8 ns pulse is the lower line in the plot.
Table 1 Conventional LRF parameters
Example Nd:YAG
laser pulsewidth 8 ns
laser peak power 5.0 E+6 W
detector type Si APD
unity-gain responsivity 0.59 A/W
receiver area 0.004 m2
receiver FOV 0.5 mrad
maximum range 8000 m
reference target 2.3 m X 2.3 m NATO standard
target reflectivity 0.10 lambertian
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0 2000 4000 6000 8000100
1000
1 104×
1 105×
1 106×
1 107×
1 108×
1 109×
1 1010×SNR conventional LRFSNR with PDM processing
SNR with standard target
meters
SNR
Figure 8 SNR of conventional and PDM laser rangefinders
The same sensor parameters are used for a model of an LRF with PDM processing; the resulting improvement in SNR is slightly over a factor of 1000, shown as the upper line. Conversely, in order to achieve the same SNR the LRF with PDM processing, transmit power can be reduced to slightly less than 5 kW peak power and the sensor achieves the same performance, as shown in Figure 9.
0 2 103× 4 103× 6 103× 8 103×100
1 103×
1 104×
1 105×
1 106×
1 107×
1 108×
1 109×
1 1010×SNR conventional LRFSNR with PDM processing
SNR with standard target
meters
SNR
Figure 9 SNR of conventional and PDM rangefinder with reduced power
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SUMMARY
The PDM coding technique can be used in applications to realize very small optical sensors wherever range sensing can be applied, such as security intrusion sensors and automotive driver assist. For light weight LRFs the PDM technique allows efficient operational devices with ranging abilities out to several kilometers to be produced with small mechanical size, as shown in Figure 10.
Figure 10 Compact PDM laser rangefinder
The PDM sensor can be adapted for cross-correlation of other modulation patterns. There are several techniques that allow data transmission to be embedded in the ranging signal, such that one sensor can provide distance measurements and a communications channel with roadside traffic information or automatic toll collection. The flexibility of the digital coding allows applications including identification friend or foe (IFF) and battlefield identification systems.
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ACKNOWLEDGEMENTS
The optical rangefinder technology described was developed through to series production by Instro Precision Limited (www.instro.com). The sensor system modeling described in this paper was carried out by NIRCM (www.nircm.com) under contract with Instro Precision Limited.
REFERENCES
[1] Patent, [Optical Distance Measurement], International Publication Number WO 01/55746 A1, Inventor MORCOM, Christopher John (GB), Instro Precision Ltd, Hornet Close, Pysons Road Industrial Estate, Broadstairs, Kent CT2 0HL (GB) www.instro.com
[2] Clifton S. Fox, Editor, [The Infrared and Electro-Optical Systems Handbook, Vol.6, Active Electro-Optical Systems], SPIE Press, 98-99, (1999), ISBN 0819410721
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