Passive imaging system equipped with a range finder

• US 20100214554A1
• Filed: 12/17/2009
• Published: 08/26/2010
• Est. Priority Date: 12/23/2008
• Status: Active Grant

• Current Assignee: Thales SA
• Original Assignee: Thales SA
• Inventors: Audier, Marcel-Francis, Belhaire, Eric
• Granted Patent: US 8,107,057 B2
• Time in Patent Office: Days
• US Class Current: 356/5.10
• CPC Class Codes: G01S 17/10 using transmission of interrupted, pulse-modulated waves determination of distance by phase measurements G01S17/32
• CPC Class Codes: G01S 17/894 3D imaging with simultaneous measurement of time-of-flight at a 2D array of receiver pixels, e.g. time-of-flight cameras or flash lidar
• CPC Class Codes: G01S 7/487 Extracting wanted echo signals , e.g. pulse detection

Abstract

The invention relates to a range finding method for a target by means of a pulsed laser (4) and a device for detecting pulses backscattered by the target, which comprises a step for emitting pulses with a repetition frequency f. The detection device comprises an array of photodiodes (21) associated with integrators (22′). The method comprises steps for roughly then finely locating the target, the rough locating step comprising the following substeps:

• • integration of a backscattered pulse, over a time window of duration F and offset by an offset Off between the instant of emission of the pulse and that of the start of said integration,

with Off<1/f and F<1/f,

• • as long as the signal is less than a first threshold, the integration steps are reiterated with a new offset Off incremented by approximately F relative to the preceding offset Off,
  • on completion of these iterations, the target being roughly located within a distance range corresponding to the time window of duration F and offset by the offset Off designated Off_g, the fine locating step is implemented, which comprises the following sub steps:
  • integration of a backscattered pulse over a time window of duration F′ and offset by an offset Off_f equal to Off_g,
  • the integration steps are reiterated with a new offset Off_f of the time window incremented by an offset d relative to the preceding offset Off_f, with d<F′ and Off_g Off_f≦Off_g+F according to the detected signal.

Claims

1. Range finding method for a target by means of a pulsed laser (4) and a device for detecting pulses backscattered by the target, which comprises a step for emitting laser pulses at determined instants and with a repetition frequency f, characterized in that the detection device including an array detector (2) which comprises an array of photodiodes (21) associated with integrators (22′

), the method comprises the following steps of roughly locating then finely locating the target, the rough locating step comprising the following substeps;

integration by the integrators (22′

) of a backscattered pulse, received by the photodiodes (21) over a time window of predetermined duration F and offset by an offset Off between the instant of emission of the pulse and that of the start of said integration,

with Off<

1/f and F<

1/f,

comparison of said integrated signal with a first predefined threshold,

as long as the signal is less than this threshold, the integration and comparison steps are reiterated with a new offset Off of the time window incremented by approximately F relative to the preceding offset Off,

on completion of these iterations, the target being roughly located within a distance range corresponding to the time window of duration F and offset by the offset Off designated Off_g, the fine locating step is implemented which comprises the following substeps;

integration by the integrators (22′

) of a backscattered pulse received by the photodiodes (21) over a time window of predetermined duration F′ and

offset by an offset Off_f equal to Off_g,

comparison of said integrated signal with a second predefined threshold,

the integration and comparison steps are reiterated with a new offset Off_f of the time window incremented by an offset d relative to the preceding offset Off_f, with d<

F′ and

Off_g≦

Off_f Off_g+F, until the signal exceeds this second threshold if the signal is less than this threshold on the first iteration, or until the signal passes below this threshold if the signal is greater than this threshold on the first iteration, the target being located on completion of these iterations.

2. Range finding method for a target according to the preceding claim, characterized in that F and Off are determined according to a distance range within which the target is likely to be located.

3. Range finding method for a target according to one of the preceding claims, characterized in that d is determined according to the required location accuracy.

4. Range finding method for a target according to one of the preceding claims, characterized in that F′

≦

F.

5. Range finding method for a target according to one of the preceding claims, characterized in that it comprises a substep for post-integration of the pulses received before the comparison substep.

6. Range finding method for a target according to one of the preceding claims, characterized in that the thresholds are determined according to a target estimated minimum signal and the number of any post-integrations.

7. Imaging optronic system which comprises an array detector (2) and a digital processing unit (8′

) for the images obtained from the detector, characterized in that it comprises a device for emitting laser pulses (4) and in that the processing unit (8′

) comprises means (10, 9, 3′

) for implementing the method according to one of the preceding claims.

8. Imaging optronic system according to the preceding claim, characterized in that the array detector (2) comprises an array of photodiodes (21) associated with integrators (22′

) and is equipped with array row (25) and column (26) addressing circuits.