Last Pulse Measuring Technique

The RIEGL Last Pulse distance meters enable laser range measurements even under conditions of bad visibility. Generally spoken, the distance meter provides the range of the last target, even if the measuring beam partially hits or penetrates other targets before. Thus, the technique is addressed as Last Pulse detection.

Fig.1

The diagram shows a typical situation (1) where the emitted laser pulse (2) first travels through a glass pane, then penetrates a cloud of dust or fog, afterwards e.g. a wire fence, and finally hits the last, solid target.

Every target provides an echo resulting in several received optical pulses (3), exceeding the trigger level and, consequently, yielding corresponding trigger pulses (4).

• A CONVENTIONAL pulsed time-of-flight rangefinder would simply provide the range of the first target, in our example the glass pane itself nearby the rangefinder. Short-range suppression techniques allow to ignore nearby target echoes, so that the range of the next target, i.e. the cloud of dust or fog would be displayed.
  A range gate (5) limits the maximum measuring range; target echoes outside this range gate cannot be detected by the trigger and are, therefore, ignored. Clock pulses (6) generated by a quartz-stabilized oscillator enable an electronic counter to measure the time delay of the echo signals and, consequently, to calculate the range of the 1^st target (7).
• The RIEGL LAST PULSE measuring technique, on the other hand, ignores all undesired target echoes with the exception of the desired echo from the last target (8).
  Targets can be distinguished if the range differs by at least 2 meters. The Last Pulse measuring technique also provides the range difference between the last target and the target next to the last (NTL)¹⁾. If there is only one target present, the range of that target is given, and the range difference to the NTL target is set to zero.

 Serial Data Output

 Figure 2 shows experimental data obtained with an LD90-3100EHS-LP aimed at a stationary target at about 20 m distance. At about 11 m distance, dense fog is generated temporarily resulting in NTL-readings as indicated in the diagram. As to be seen, the last target readings remain absolutely undisturbed!

Fig.2

Figure 3 shows a similar situation with simulated dense snowfall at approximately 15 meters: Again, no disturbance of the last target display!

Fig.3

Figure 4 shows data of another experiment. Dense snowfall is simulated at two different positions as indicated (distances approximately 16 m and 19 m). Measurements to the target at 34 m are again undisturbed.

Fig.4

Information contained herein is believed to be accurate and reliable. However, no responsibility is assumed by RIEGL for its use. Technical data are subject to change without notice.

RIEGL General Information GI009 (6/97)

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Last updated 05/23/02