GB2332798A - Radar range sensor - Google Patents

Abstract

A high speed three dimensional scanning radar system 20 provides range maps in operating environments plagued with dust, fog or rain. The system emits a sensor beam formed of high frequency, short duration, energy pulses. Its direction is controlled using a scanning mechanism incorporating a rapidly rotating reflector. The signal returned from the antenna 42 is conditioned and then passed through constant fraction discriminators (CFDs) 92, 94, 96, 98 to provide reliable detection of the time the returned signal is received. A high accuracy timer 24, which generates an analog voltage proportional to target distance, is started at a fixed time relative to signal emission and is stopped when the CFDs trigger detection. An apparatus for processing radar range signals comprising a digital processor for sorting range signals that are within a predetermined range of an intermediate range signal is also disclosed.

Description

SCANNED, PULSED RADAR RANGE SENSOR SYSTEM Technical Field This invention relates generally to radar range sensors and, more particularly, to an apparatus for obtaining radar range sensor data using pulsed time-of-flight techniques.

Background Art Sensing capabilities are a key technology for the success of robotic automation. Robots use data regarding the surrounding environment to navigate, detect obstacles, and perform task specific functions. Sensor systems acquiring such data need fields of view that encompass the desired regions of interest around the robot, high scan rates to provide fresh data, and the ability to function while mounted on moving equipment in harsh environments such as an automated machine operating in an earthmoving environment. Depending on the application, scanning range data up to 360 degrees and within a specified radius of the object is required. The sensor system's data resolution should be high enough to allow detection of the objects typically found in the subject environment including moving and stationary objects of various sizes, as well as to discriminate among a wide assortment of site materials, such as gravel and dirt.

Laser rangefinder based scanners have proven capable of creating very high resolution range maps of the environment surrounding an autonomous piece of equipment or machinery.

However, such optically based systems have found most utility on indoor robots. When subjected to dust, fog, and rain, performance of a traditional laser rangefinding sensor degrades and is of limited utility.

Radar sensor systems often prove to be more effective than laser-based sensor systems in outdoor environments. Radar scanning sensor systems emit energy beams having longer wavelengths that overcome the environmental challenges that affect laser sensors. Prior art radar sensor systems come with their own limitations, however, including large beamwidths, slow scan rates, concerns regarding significant antenna sidelobes, poor returns from specular targets, and poor range accuracy.

With radar sensor systems utilizing frequency modulated continuous wave (FMCW) radar techniques, it is difficult to resolve multiple targets in a cluttered operating environment.

Additionally, FMCW requires a dwell time equivalent to the duration of its frequency ramp in order to reliably resolve a target. This frequency ramp is proportional in bandwidth to the desired downrange resolution of the sensor, so the higher the desired accuracy, the harder it is to generate a linear frequency ramp. The combination of these challenges can make accurate, high speed scanning of FMCW radar sensors difficult.

A pulsed time-of-flight (TOF) technique provides the ability to easily discriminate the first target from the multipath and multi-target returns, but requires a high precision timing mechanism to determine the range to target.

Previous work has implemented a range gate based timing system in which the radar is pulsed once for each potential range bin, or range of interest.

Such a method greatly reduces the net data rate because the sensor must be aimed at the same target for a long period of time to complete the hundreds of samplings needed to fill the bins.

In most navigation and obstacle detection applications, a sensor system only needs to know the location of the nearest target, and can therefore measure time-of-flight for the first return. For applications involving robotic machines performing one or more tasks simultaneously in potentially harsh environments, a sensor system is required that provides a first target range reading for each pulse of the radar, allows high data gathering rates, yields high density range maps, and is not affected by the beam motion when scanning between sequential firings of the sensor. It is also desired to have means to accurately detect targets with poor return characteristics.

It is an object of the present invention to attempt to alleviate the aforementioned problems and/or to provide improvements generally.

According to the invention there is provided apparatus for determining range to a point as claimed in the accompanying claims.

Also according to the present invention there is provided a scanning radar sensor system as claimed in the accompanying claims.

Also according to the present invention there is provided a timing apparatus as claimed in the accompanying claims.

Also according to the present invention there is provided apparatus for processing range signals generated by a scanning radar sensor system as claimed in the accompanying claims.

In one embodiment of the present invention, a high speed, three dimensional scanning radar sensor system provides range maps in operating environments plagued with dust, fog, and rain. The sensor system emits a sensor beam formed of high frequency, short duration, energy pulses. The direction of the sensor beam is controlled through the use of a scanning mechanism having a high speed rotating reflector. The present invention includes a timing system to determine the elapsed time between the emitted and received signals. The time difference represents the distance to the target that reflected the energy. The signal returned from the antenna is conditioned and then passed through constant fraction discriminators (CFDs) to provide reliable detection of the time the returned signal is received. A high accuracy timer is started at a fixed tirne relative to the emission of the energy and is stopped when the CFDs trigger a detection.

The present timing system generates an analog voltage proportional to the downrange distance to the target.

Brief Description of Drawinqs Fig. 1 is a block diagram of components of a radar sensor system utilizing the present invention; Fig. 2 is a graph of a sample I signal as output by the front end of the radar system; Fig. 2a is a graph of a sample Q signal as output by the front end of the radar system; Fig. 3 is a flowchart of an algorithm for processing the range data; Fig. 4 is a side view of the components of the sensor system; Fig. 5 is a top view of an application the present invention at an excavating site; and Fig. 6 is a side view of an application of the present invention at an excavating site.

Referring now to the drawings, the block diagram in Fig. 1 shows the major components of the radar sensor system 20 incorporating the processing system according to the present invention, including the front end 22, timing system 24, a radar interface board 26, digital signal processor (DSP) 28, and a mechanical scanner and data acquisition system 30.

The sensor system 20 includes an antenna 42 that emits a scanning beam over a range of interest. The range of interest typically includes an area within 0 to 30 meters of the robot. A Gaussian optic lens antenna (GOLA) 42 is used to focus the beam in the ranges of interest. The lens diameter of the GOLA 42 may be of any size that achieves the desired results, such as a diameter of 8 to 25 centimeters.

The front end 22 of the sensor system 20 is based on a Gunn diode 34 coupled to an Impatt amplifier 36 and generates a radio frequency (RF) signal in the millimeter waveband having the power output desired for a particular application, such as 250 milliWatts, for example. A high speed switch 38 driven by a pulse generator 39 allows short pulses to pass through a circulator 40 to the antenna 42 for transmission into space at a desired pulse repetition frequency. The RF output pulses applied to the antenna 42 are radiated as an illuminating beam 46 which is directed to a desired surface area to develop a range map. Returned signals from the illuminated surface are received through the same antenna 42 and circulator 40, and are mixed with the transmitted RF signal in an I/Q mixer 48 as is well known in the art. The I/Q mixer 48 provides in-phase and quadrature phase detection and splits the transmitted and returned signals into an in-phase channel and a quadrature phase channel, the signals being then transferred as I (in-phase) 54 and Q (quadrature phase) 55 signals to the timing system 24. A low noise amplifier (LNA) 50, 52 on each channel amplifies the I and Q signals 54, 55 to usable levels for the timing system 24.

Sample I and Q outputs are shown in Figs.

2 and 2a. The leakage of the transmit pulse 56, 57 through the circulator 40 effectively blinds the receiver from looking for returns until transmission is complete. In practice however, a period must be allowed for the front end circuitry and amplifier to settle to a fully recovered state before the returns can be reliably sensed. Therefore, the high speed switch 38 minimizes the transmit pulse 56, 57 and provides controlled switching times having sharp rise and fall characteristics. The switch 38 in the preferred embodiment provides a 7 nanosecond full width half maximum pulse with 1.5 nanosecond switching time. Any type of switch 38 that delivers the desired performance characteristics may be used, however. This period of effective blindness, combined with a conservative amplifier settling time, results in data being unavailable for a short period of time during transmission of the pulse.

The time period of blindness results in a blind spot around the proximity of the sensor that is proportional to the time period. For example, a time period of 14 nanoseconds corresponds to a blind spot of 2.1 meters around the sensor.

The time-of-flight (TOF) timing system 24 is based on commercially available nuclear physics laboratory equipment and may be implemented using an application specific integrated circuit (ASIC). The leakage of the transmit pulse signal through the circulator 40 appears as large amplitude I and Q pulse signal outputs 56, 57, as shown in Fig. 2 and is used to start a time amplitude converter (TAC) 58. In the present invention, the Q signal 55 is split to provide input 60 to a level threshold detector based on a comparator 62, such as a model AD96685 comparator. The resultant emitter-coupled logic (ECL) signal 64 triggers a pulse generator or "one shot" circuit 66 which enables the timing system 24 for a desired length of time, for example, 400 nanoseconds, depending on the performance characteristics of the pulse generator 66. This ECL signal 64 is converted to negative logic at the start input 68 of the TAC 58.

A second set of amplifiers 72, 73 are used to amplify the I and Q signals 54, 55 to meet minimum signal input levels of the timing system 24.

High speed gallium arsenide (GaAs) switches 70, 71 are inserted between the primary 50, 52 and secondary 72, 73 amplifiers to prevent the transmit pulse 60 from reaching secondary amplification before it triggers the pulse generator 66 to start the timing system 24. An emitter coupled logic (ECL) to transistor-to-transistor logic (TTL) converter 74, such as a MC10125, is used to convert a signal 76 output by the pulse generator 66 to an appropriate voltage level and to delay the signal 76 the desired amount of time. The control signal 78 from the ECL-TTL converter 74 is used to trigger the switches 70, 71 so that the I and Q signals 54, 55 are input to secondary amplifiers 72, 73 after the transmission is completed. The voltage of the control signal 78 causes a slight bias on the output signals 80, 82 of the switches 70, 71 which is compensated for by an adjustable offset or bias voltage 85 fed into the opposite ports, or normally open contacts 84, 86, of the switches 70, 71.

The I and Q outputs 80, 82 of the secondary amplifiers 72, 73 are fed through splitters 88, 90 into a set of four constant fraction discriminators (CFDs) 92, 94, 96, 98. The CFDs 92, 94, 96, 98 are used to minimize the impact of return signal strength on the range reading by relying on the consistent rise time characteristics of the I/Q signals 54, 55. Since the CFDs 92, 94, 96, 98 are triggered only by a negative signal, the I and Q signals 80, 82 are each split and one half inverted such that the I signal 100, I inverted signal 102, Q signal 104, and Q inverted signal 106, are each input to CFDs 92, 94, 96, 98, respectively.

The four CFD outputs 108, 110, 112, 114 are input to an OR gate 116, and the resultant output signal 116 from the OR gate 118 is used to stop the timing sequence in the TAC 58. The CFDs 92, 94, 96, 98 are disabled from reporting transmit leakage and target ranges closer than a selected minimum range by a veto signal 120. The veto signal 120 is generated by delaying the output signal 76 of the pulse generator 66 for a desired time period, such as 18 nanoseconds, until the amplifiers 50, 52 and switch 38 have settled out the transmit pulse.

The TAC 58 outputs a range signal 122 proportional to the time to the target. An analog to digital (A/D) converter 124 may be used to digitize the range signal if it is provided in analog voltage form from the TAC 58. The combined time delays associated with the CFDs 92, 94, 96, 98, the logical OR gate 116, the TAC 58, and the A/D converter 124 in the timing system 24 introduce an error in the range reading. Typically, this error is approximately 2.5 centimeters or less.

When a pulse is transmitted by the radar system 20, the start signal 68 enables a timing sequence in the TAC 58. The TAC 58 outputs a "started" signal 126 when the TAC 58 timing sequence begins and a converted" signal 128 when the TAC 58 timing sequence stops. Switches 130, 132 send a trigger signal 134 to the A/D converter 124 to perform a range reading using the range signal 122 from the TAC 58. If the switch 130 connected to the "started" signal 126 is closed, the A/D converter 124 performs a range reading every time a pulse is transmitted. If the switch 132 connected to the "converted" signal 128 is closed, the A/D converter 124 performs a range reading every time that a pulse is received. If no return signal is received, the A/D converter 124 will report an invalid range value if the "started" switch 130 is used, whereas the system does not generate a value at all if the "converted" switch 132 is used. By using the "started" mode, the A/D converter 124 will output an indication each time a range reading is attempted.

The DSP 28 ignores the invalid readings and processes only the valid readings as described hereinbelow.

The A/D converter 124 outputs a conversion complete signal 136 which clocks a flip-flop device 138, such as a D-type flip-flop model 74HCT74, setting a discrete signal Q 140 to a logical "1".

The DSP 28 monitors the discrete signal Q 140 and reads data 144 from the A/D converter 124 upon detecting the rising edge of the discrete signal Q 140. After the DSP 28 successfully reads the data 144, it toggles the CLEAR (C) line 142 of the flip flop 138 to enable the system 20 for the next data acquisition.

Due to the varied reflectance and specularity of targets in the millimeter wave band, the range data is processed by the (DSP) 28 that is implemented in a data processing device such as a microprocessor connected to data storage and input/output devices. As shown in Fig. 3, a DSP algorithm 146 takes a number, n, of sequential range signals or values 148, some of which get no return due to specularity or weak reflectivity of the target, and determines a biased median of the data set. The algorithm 146 sorts the valid range readings, ignoring any invalid returns 150. Within the series of n valid data points, the range points may pertain to two or more targets at different ranges 152. This is known as "the mixed pixel problem", and is handled in the algorithm 146 by choosing a range signal at some intermediate position from the beginning of the data, such as 1/4 to 3/4 through the set of n data points 154. By selecting a range signal closer to the beginning of the data point series, range returns from closer targets are weighed more heavily. The algorithm 146 then averages all valid data points or range signals within a nominal distance 156, for example, 0.25 meters of the 1/3 point, to improve the accuracy of the reading. The DSP algorithm 146 thereby produces more accurate range data, and increases the likelihood of seeing irregularly specular and highly diffuse targets compared to a single range signal.

The number of data points, n, is a variable that is specified by the user, and is typically a value between 10 and 30 points, depending on the desired data frequency. For example, if the DSP algorithm 146 samples 20 data points taken at 250 kiloHertz, the result is a data rate of 12.5 kiloHertz.

Fig. 4 shows components of a scanning sensor system 159 mounted on two orthogonal axes 160, 162 in a manner that allows data to be acquired for the desired ranges and fields of view. A pan axis 160 of a rotatable shaft 167 of a first motor 166 is shown in Fig. 4 as being oriented vertically, and the GOLA antenna 164 is mounted on the shaft 167 so that the illuminating beam 168 is emitted in a substantially horizontal direction. Once the illuminating beam 168 is emitted, its direction is controlled by a reflector 170 which is mounted coaxially with a scan axis 162 on a rotatable shaft 171 of a second motor 172 adjacent to the GOLA antenna 164. Preferably, the reflector 170 is an elliptical disc having a highly reflective surface positioned at an angle relative to the direction of the emitted beam 168. As the shaft 171 of the second motor 172 rotates continuously through 360 degrees, the reflector 170 directs the beam 168 through a 180 degree scanning pattern. The second motor 172 is mounted at one end portion of a structural member 174 while the GOLA antenna 164 is mounted at the other end portion of the structural member 174.

With this configuration, the illuminating beam 168 is directed through 360 degrees horizontally as the first motor 166 rotates the shaft 167 continuously about the pan axis 160. The 180 degree scanning pattern about the scan axis 162 can be positioned such that the center of the scan looks downward from horizon to horizon or looks outward from straight down to straight up. The shafts 167, 171 of the motors 166, 172 rotate at rates that allow data to be acquired at the desired resolution, such as 60 degrees/sec about the pan axis 160 and 3600 RPM about the scan axis 162. With the scan axis 162 positioned to look downward, the antenna 164 only needs to rotate about the pan axis 160 +/-90 degrees to get an unobstructed view of the entire world below the horizon of sensor.

In Fig. 1, the front end 22 of the radar system 20 is triggered by a free running variable frequency oscillator 34. The preferred embodiment uses a 250 kiloHertz firing rate to trigger the front-end 22, however, the oscillator 34 may be selected to generate pulses at whatever frequency is desired. After a certain number of firings, for instance, 20 firings, the DSP 28 performs the filtering algorithm and triggers the data acquisition system 30 to latch the current angles of the pan and scan axes 160, 162. The combination of line-of-sight range to the target, pan (azimuth) angle, and scan (elevation) angle specifies a unique spherical coordinate in space.

The control system 32 of the data acquisition system 30 is based on a microprocessor 180 and contains interface cards for digital I/O 182, encoder quadrature 184, and communications to the host automation processor 186. Each servo axis 160, 162 is controlled by a motion controller/amplifier combination 188, 190 connected to the microprocessor 180. The encoder signal 192, 194 from each axis 160, 162 is input to the motion controllers for each individual axis 188, 190 and the encoder quadrature card 184 in the control system 32. The DSP 28 interrupts the control system 32 when a valid range reading is available, at which time the acquisition software captures the digital range on the digital input/output board 182 along with the current position 196, 198 of the pan and scan servo drives 188, 190. Depending on the number of firings included in each set of range data filtered in the DSP 28, the data is acquired at corresponding rates, such as 12.5 kiloHertz filtered or 250 kiloHertz unfiltered for a set of 20 data points, and is processed by other subsystems in the robotic automation system for navigation, obstacle detection, and trajectory generation.

The present invention is applicable in situations where autonomous or semi-autonomous control of mobile machinery is desired as in, for example, an earthmoving environment. Figs. 5 and 6 illustrate a typical excavation site with an excavator 200 positioned above a dig face 202 and a dump truck 204 located within reach of the excavator's bucket 206. In order for the excavator 200 to operate autonomously, the location of objects and obstacles within the excavator's area of movement, and the location of terrain to be excavated must be known. The sensor systems used must therefore be capable of providing current information regarding location of objects around the area of movement far enough in advance to provide the excavator 200 with adequate response time.

Figure 5 illustrates an implementation of the present invention with left and right sensors 208, 210 mounted at approximately symmetrical locations to the left and right of a boom 212 on the excavator 200. A dump truck 204 is positioned near the excavator 200 for receiving the excavated materials. During the digging and loading cycle, the control system (not shown) commands the left and right sensors 208, 210 to monitor the bucket 206 and adjacent areas. As the excavator 200 nears completion of the digging process, the control system commands the left sensor 208 to pan toward the dump truck 204 to check for obstacles in the path of movement of the excavator 200 and to determine the position and orientation of the dump truck 204. After completing the loading cycle the scan speed of the sensor 210 is coordinated with the pivotal rotation of the excavator 200 as it returns the boom 212 toward the dig face 202 to detect obstacles far enough in advance to allow adequate response time for the excavator 200.

The left and right sensors 208, 210 may be operated independently to improve efficiency. For example, as the excavator 200 swings toward the dump truck 204, the right sensor 210 retrogrades (i.e., pans in the opposite direction) to scan the excavated area to provide data for planning the next portion of the excavation. At the same time, the left sensor 208 scans the area around the dump truck 204. The scanning sensor 208 provides current information to the control system to allow it to determine an accurate location to unload the bucket 206, even if the dump truck 204 moved since the last loading cycle. While the bucket 206 is being unloaded, the right sensor 210 scans the area near and to the right of the bucket 206 to prepare for rotating toward the dig face 202. As the excavator 200 rotates to the right, the right sensor 210 pans ahead toward the dig face 202 to provide information for obstacle detection. When the excavator 200 begins to rotate toward the dig face 202 after unloading, the left sensor 208 retrogrades to view the distribution of soil in the bed of the dump truck 204 to determine the location in the bed to unload the next bucket of material. As the bucket 206 arrives near the dig face 202, the right sensor 210 scans the digging area. Once the left sensor 208 completes its scan of the dump truck 204, the control system commands the sensor 208 to also scan the digging area. The steps in the excavating process are repeated as outlined above until a dump truck bed is filled or the excavation is completed.

The control system uses information provided by the sensor systems to determine whether operations should be halted such as when the dump truck is filled, the excavation is complete, or an obstacle is detected. The information is also used to navigate movement of the equipment.

The application of the present invention to excavating and loading operations is illustrative of the utility of the sensor system. The sensor system may also be applied to other earthmoving machinery including wheel loaders, track-type tractors, compactors, motor graders, agricultural machinery, pavers, asphalt layers, and the like, which exhibit both (1) mobility over or through a work site, and (2) the capacity to alter the topography or geography of a work site with a tool or operative portion of the machine such as a bucket, shovel, blade, ripper, compacting wheel and the like. In an automated system, a sensing script can guide the scan pattern and scan rate for one or more sensor systems as a function of the machinery's progress in the work cycle.

Other aspects, objects and advantages of the present invention can be obtained from a study of the drawings, the disclosure and the appended claims.

Claims (17)

Claims

1. An apparatus for determining range to a point using a transmitted signal and a return signal provided by a pulsed millimeter waveband energy sensor system comprising: a plurality of constant fraction discriminators, the input to the constant fraction discriminators including the return signal from the sensor system, the constant fraction discriminators being operable to output a signal when the return signal from the sensor system is detected; an OR gate operably connected to receive the output signals from the constant fraction discriminators, and to output a stop signal when the return signal is detected; and a time amplitude converter operably connected to the start signal to begin a timing sequence and to the stop signal from the OR gate, wherein the time amplitude converter outputs a range signal proportional to the difference in time between detection of the start signal and detection of the stop signal.

2. The apparatus as set forth in claim 1 further comprising: a pulse generator to generate a start signal based on detection of the transmitted signal from the sensor system; signal processing means operable to output an invalid range signal if no return signal is detected during the timing sequence; and signal processing means operable to output a range data point based on a biased median value of a plurality of valid, sequential range signals.

3. The apparatus as set forth in claim 1 wherein the constant fraction discriminators are operably connected to a veto signal, the veto signal being generated by delaying the start signal from the pulse generator for a time period.

4. The apparatus as set forth in claim 2 wherein the signal processing means operable to output an invalid range signal if no return signal is detected during the timing sequence comprises: an analog to digital converter; a trigger switch operably connected between the time amplitude converter and the analog to digital converter to trigger operation of the analog to digital converter when the start signal is detected, the analog to digital converter operable to output an invalid range value if no range voltage is output by the time amplitude converter during the timing sequence.

5. The apparatus as set forth in claim 4 wherein the signal processing means operable to output a range data point based on a biased median value of a plurality of valid, sequential range signals comprises: computer software operable to sort a set of valid range values from the analog to digital converter, and to compute the biased median value for a subset of the sorted set of range values that are within a predetermined range of an intermediate range value in the set.

6. The apparatus as set forth in claim 5 wherein the intermediate range value is a range value that is between approximately 25 percent and 50 percent of the maximum range value and the subset of range values includes range values that are within approximately one meter of the intermediate range value.

7. A scanning radar sensor system comprising: a data acquisition assembly including an antenna having a first end operable to transmit an illuminating beam composed of a pulsed energy signal in the millimeter waveband generated by an oscillator, and to receive a return energy signal that is generated by a reflection of the transmitted signal off an object illuminated by the transmitted signal, and a rotatable reflector oriented at an angle adjacent the first end of the antenna operable to control the scanning direction of the transmitted signal and the return energy signal; a switch operable to provide a controlled signal representing the transmitted energy pulse; an I/Q mixer connected to the switch, the I/Q mixer being operable to process the controlled signal and the return energy signal to generate in-phase and quadrature-phase signals; and a timing system including constant fraction discriminators connected to the in-phase and quadrature-phase signals.

8. The scanning radar sensor system set forth in claim 7 further comprising: a first motor having a rotatable shaft, the antenna being mounted on the shaft so that the illuminating beam is emitted in a direction substantially perpendicular to the shaft; wherein the reflector is mounted on a rotatable shaft of a second motor, the second motor being mounted at one end portion of a structural member and the antenna mounted at the other end portion of the structural member.

9. A timing apparatus for a scanning radar sensor system comprising: a plurality of constant fraction discriminators operably connected to receive input signals comprised of a transmitted pulsed energy signal and a return pulsed energy signal, and to output a signal when the transmitted and return pulsed energy signals are detected, the pulsed energy signals being in the millimeter waveband; means to disable the constant fraction discriminators from outputting a signal when the transmitted pulsed energy signal is detected; and a time amplitude converter connected to receive input signals including the output signal from the constant fraction discriminators, the time amplitude converter being operable to output a signal proportional to the difference in time between detection of the transmitted signal and detection of the return signal.

10. The timing apparatus as set forth in claim 9 wherein the means to disable the constant fraction discriminators from outputting a signal while the transmitted pulsed energy signal is detected comprises: a veto signal connected as an input to the constant fraction discriminators, the veto signal being set to disable the constant fraction discriminators by delaying a signal from a pulse generator that is activated when the transmitted pulsed energy signal is detected.

11. The timing apparatus as set forth in claim 9 wherein the input signals comprised of a transmitted pulsed energy signal and a return pulsed energy signal to first and second constant fraction discriminators are time delayed, and the input signals comprised of a transmitted pulsed energy signal and a return pulsed energy signal to third and fourth constant fraction discriminators are negated.

12. An apparatus for processing range signals generated by a scanning radar sensor system comprising: a digital processor operable to sort a set of valid range signals, and to compute an average value for a subset of the sorted set of range signals, the subset comprising range signals that are within a predetermined range of an intermediate range signal in the set, the intermediate range signal being between approximately 25 percent and 75 percent of the value of the maximum range signal and the subset o range signals including range signals that are within approximately one meter of the intermediate range signal.

13. The apparatus as set forth in claim 12 further comprising: an analog to digital converter operatable to convert an analog voltage proportional to the range to an object to a digital signal, and to output a signal to a flip-flop device when the range data conversion is complete, the flip-flop device operable to output a discrete signal upon detection of the conversion complete signal; and wherein the digital signal processor is operable to monitor the discrete signal and to read data from the analog to digital converter upon detecting the discrete signal and to set a clear signal after the digital signal processor successfully reads the data to enable the flip-flop to detect the next conversion complete signal.

14. Apparatus for determining range to a point substantially as described herein with reference to the accompanying drawings.

15. A scanning radar sensor system substantially as described herein with reference to the accompanying drawings.

16. A timing apparatus substantially as described herein with reference to the accompanying drawings.

17. Apparatus for processing range signals substantially as described herein with reference to the accompanying drawings.