Digital weather radar including target severity analysis capability

A digital weather radar system having a mode that clears the display of all clutter and lighter precipitation to reveal or positively present only the contoured areas, i.e., the severe "cores" or "hard spots." Data representative of the searched space is stored at least temporarily in a digital memory. In the preferred embodiment, a controllable decoder operates on this data according to the particular decoder state selected, the decoder output is D/A converted, and the resultant analog signal is used to intensity modulate a cathode ray tube (CRT).

This invention relates to a digital weather radar and more particularly to 
weather severity discernment. 
Airborne weather radars have traditionally incorporated a type of visible 
presentation wherein the target echos from a given searched area are 
presented on a viewing screen as a picture whose intensity increases in 
proportion to the severity or degree of precipitation encountered by the 
search beam. Due to the limited dynamic range of the high retentivity, 
storage type CRT's commonly used for radar display, this type of 
presentation has proved of minimal value to the operator who must analyze 
and interpret the picture to determine the weather severity. More 
particularly, storage tube display devices provide little contrast between 
targets of different magnitude and permit little discernment of the areas 
of most intense weather. 
The isoecho-contour display evolved as a means for overcoming this lack of 
contrast problem. In the isoecho-contour display only the target 
magnitudes producing echo strengths above mds (minimum discernible signal) 
and below a predetermined threshold are positively painted on the viewing 
screen. That is, only those return levels between mds and a predetermined 
threshold are presented as light areas. Target magnitudes above the 
predetermined threshold and target magnitudes below mds appear as black or 
dark areas on the viewing screen. Also, from the width of the light areas, 
the viewer can estimate the rainfall gradient and thus the degree of 
turbulence. A variation of this display technique involves positively 
painting on the screen only the lowest magnitude and the next to highest 
magnitude, while the highest magnitude and the next to lowest magnitude 
both appear as dark areas on the screen. For additional details on the 
contour type display see (i) Merrill I. Skolnik's book "Introduction to 
Radar Systems" Pages 582, 583, McGraw Hill 1962 (ii) U.S. Reissue Pat. 
No. 24,084, and (iii) U.S. Pat. No. 2,996,678. 
In the isoecho-contour type display an ambiguity in presentation exists. 
That is, the safe areas and the dangerous areas appear the same; i.e., as 
dark areas on the screen. In an attempt to resolve this ambiguity several 
radars provide alternate presentations of contoured pictures and 
non-contoured pictures. However depending on several variables such as 
range and azimuth resolution capability of the radar, storm geometry, and 
storm intensity gradient, the total presentation can still be misleading 
or at least can require more time to correctly interpret than the pilot 
can or should spend during periods of heavy workload. 
A further technique occasionally used by some pilots but considered as poor 
practice by several of those skilled in the art involves reducing the RF 
gain. Reducing the RF gain eliminates the lower intensity targets from the 
presentation but also reduces the intensity and the contrast with which 
the remaining targets are displayed and can thus mislead the viewer as to 
their absolute strength. Moreover, due to pilot workload the gain can be 
reduced and forgotten thus causing the pilot to later misinterpret the 
data presented. Thus this technique is far from foolproof and can also 
require more time than should be devoted during periods of heavy workload.

In the system illustrated in FIG. 1, antenna 13 is presently caused to scan 
90.degree. in azimuth substantially in 90 regularly occurring 1.degree. 
increments, the rate of incrementing being approximately 24Hz. For each 
1.degree. antenna increment or azimuth position there are four regularly 
occurring pulse periods. During any one pulse period, transmitter 15 is 
caused to emit a radar pulse, and following the transmission the radar 
return therefrom is processed at RF and IF in 17 and the amplitude 
detected in 19. The resultant video is then processed in the digital video 
processor 21 as follows. 
Generally, in item 23, each of the four video returns is digitized, the 
four are then accumulated or summed together, and this accumulation 
further summed with digital data representative of the "line of data" 
which corresponds to the same antenna position but which was derived 
during the next preceding antenna scan. Herein, the terms line, line of 
display, line of information, or line of data, will be used to indicate 
the information displayed, or the data used in displaying, during one 
outward sweep with range of the display CRT electron beam. A new line of 
digital data representative of the total sum is then entered in memory 25 
at the address corresponding to this azimuth position. That is, the total 
sum becomes a new line of data and replaces the old line of data for this 
azimuth position. 
More particularly each of the four returns from a particular azimuth 
position are in turn digitized at a predetermined bit rate into 128 
serially occurring two-parallel-bit digital words. The time interval per 
bit corresponds of course to one range resolution interval, and 128 bit 
time intervals correspond to the maximum radar range. These four digitized 
returns are summed such that the resultant word for any particular bit 
time comprises the sum of the four digital words for that particular bit 
time. The resultant is of course 128 serially occurring digital words. To 
this sum is also added 128 words representative of the line of data stored 
in memory 25 which corresponds to the same antenna position but which was 
derived during the next preceding antenna scan. At a time compatible with 
similar processing for the next antenna position, a line of data 
representative of this total sum and comprising 128 two-parallel-bit 
digital words is entered into memory 25 as the new line of data for this 
particular azimuth position, replacing the old line of data therein. 
At any particular point in time, memory 25, which comprises two 11,520 bit 
shift registers operating in parallel, contains 90 different lines of data 
which correspond respectively to the 90 different antenna azimuth 
positions, and any particular line comprises 128 two-parallel-bit digital 
words which correspond respectively to the 128 different range intervals. 
For instance, the 23rd line of data represents the space searched or 
interrogated by the radar at the 23rd antenna azimuth position, and the 
41st digital word of the 23rd line of data represents the target magnitude 
at the 41st range interval along antenna azimuth position 23. 
The presently preferred method of presentation generation is to generate or 
paint the information on the CRT as it becomes available at the memory 
output. Since the entire 90 lines of data are recirculated in memory 25 
once each pulse period, this results in the generating, per pulse period, 
of one complete picture of all 90 lines of memory data. As shown in FIG. 1 
timing coordination for transmitting, antenna incrementing, digital 
processing, and CRT deflection, is provided by master timing generator 27. 
The digital video processor 21 is the subject of U.S. Pat. application Ser. 
No. 720,165 entitled Radar Signal Processor and assigned to the assignee 
of the present invention. For additional detail on the processor 21, the 
system portions preceding same, the timing coordination and other such 
background of the present invention, said application is hereby 
incorporated by reference into the present disclosure. 
In accordance with the present invention and the preferred embodiment 
thereof, the data available at the memory 25 output is visually presented 
on the face of CRT 29 in accordance with the state of target analysis 
circuit 31. More particularly each two-parallel-bit digital word appearing 
at memory 25 output is one of the four words 00, 01, 10, or 11. As 
presently assigned, the memory word 11 represents the largest target 
magnitudes and more particularly represents targets whose magnitude is 
equal to or greater than contour threshold. (Contour threshold or 
isoecho-contour threshold is an industry standard term used to define an 
absolute level of storm severity or intensity; e.g., a particular rainfall 
rate. See ARINC CHARACTERISTIC 564-1 by Aeronautical Radio, Inc., issued 
Nov. 1, 1967, Section 3.7.) Also as presently assigned the memory word 00 
represents target magnitudes below system mds, the memory word 01 
represents target magnitudes from mds to a predetermined intermediate 
level, and the memory word 10 represents target magnitudes from said 
intermediate level to the contour level or threshold. Typically the 
rainfall rate represented by the intermediate level is approximately 25% 
to 50% of the rainfall rate represented by the contour level. The four 
different memory output words are converted into new digital words by the 
controllable decoder 33. This conversion depends on whether 
operator-controllable switch 35 is open or closed and is defined in the 
table below. 
TABLE 
______________________________________ 
Decoder 33 Decoder 33 
Memory 25 Digital Word Digital Word 
Digital Out (Switch Out (Switch 
Word Out 35 Open) 35 Closed) 
Gate 43 Gate 45 Gate 43 Gate 45 
Ch. 1 Ch. 2 Out Out Out Out 
______________________________________ 
0 0 1 1 1 1 
0 1 1 0 1 1 
1 0 0 1 1 1 
1 1 0 0 0 0 
______________________________________ 
The decoder 33 digital words out are converted by D/A converter 37 into an 
analog signal S.sub.out suitable for controlling or modulating the 
electron beam intensity (i.e., the Z axis) of the CRT 29. As shown in FIG. 
1 controllable decoder 33 presently comprises three NAND gates 41, 43, and 
45, and one EXCLUSIVE OR gate 47. Gates 43 and 45 are of the type whose 
logic 0 output corresponds substantially to ground potential and whose 
logic 1 output corresponds to an open circuit. Item 49 is an inverter 
which outputs a logic 0 when switch 35 is open and outputs a logic 1 when 
switch 35 is closed. The presently employed D/A converter 37 is shown in 
FIG. 2. Item 51 is a differential input operational amplifier. 
The waveforms of FIG. 3 represent a typical conversion by target analysis 
circuit 31 of input digital words from memory 25 into the CRT 
intensity-controlling analog signal output S.sub.out. Therefrom it should 
be noted that, for the switch 35 open condition, the memory words which 
correspond to the largest target magnitudes (i.e., all memory words 11) 
are each converted to the highest S.sub.out level and thus result in the 
highest intensity areas on the CRT. The memory words corresponding to the 
target magnitudes below mds (i.e., all memory words 00) are each converted 
to the lowest S.sub.out level L.sub.o and thus result in the dark areas on 
the CRT. The memory words corresponding to the second and third largest 
target magnitudes (i.e., 10 and 01) are converted respectively to second 
and third highest S.sub.out levels and thus result in the second and third 
brightest intensity levels respectively on the CRT. In other words all 
target magnitudes above system mds are represented as light areas whose 
intensity increases as a monotonic function of target magnitude. 
It should also be noted from FIG. 3 that for the switch 35 closed condition 
all memory words which correspond to target magnitudes less than the 
contour level are converted to the lowest S.sub.out level and thus result 
in dark areas on the CRT. Significantly, the memory words which correspond 
to the largest target magnitudes (i.e., contour level or above) are still 
converted to the highest S.sub.out level and thus result in areas on the 
CRT of the same intensity as for the switch 35 open condition. More 
particularly, in the switch 35 closed condition, memory words 00, 01, and 
10 are prevented from contributing to the light pattern on the CRT, and 
memory words 11 still appear on the CRT unaffected in image size or 
brightness. 
The difference between switch 35 open and closed conditions is also 
represented in FIGS. 4 and 5. FIG. 4 represents a CRT screen displaying a 
particular searched space and targets therein, and assumes switch 35 is 
open. FIG. 5 assumes the same conditions as FIG. 4 with the exception that 
switch 35 is closed. In FIG. 4 the lightest area 61 represents the target 
magnitude above contour level; the 2nd and 3rd lightest areas 63 and 65 
respectively represent the 2nd and 3rd target magnitude levels; and the 
darkest area 67 represents the target magnitudes below mds. In FIG. 5 the 
lighest area 61 represents the target magnitudes above contour level; and 
the darkest area 67 represents the target magnitudes below contour level. 
As seen from FIGS. 4 and 5 the severe "core" of the target appears 
identical in both figures. The second and third levels which appear in 
FIG. 4 are however eliminated from view in FIG. 5. As earlier mentioned, a 
complete picture is painted every pulse period or more particularly at a 
96Hz rate, and thus the time of transition from FIG. 1 to FIG. 2 
(approximately 10 milliseconds) is essentially instantaneous to the 
viewer. 
Thus, with all returns except those at contour level and above eliminated 
from view, and with the contour and higher level returns still displayed 
at the same intensity, an unambiguous representation of high contrast is 
instantly available to permit the pilot to quickly locate the severe storm 
cores and plan a judicious route around them. 
It should be apparent that the target analysis circuit 31 may be variously 
embodied. One alternative embodiment is shown in FIG. 6. Therein an 
appropriate D/A converter 71 converts the four different memory output 
words into four different and corresponding analog levels. Comparator 73 
closes switch 75 only when the converter 71 analog output exceeds an 
appropriate reference. For the FIG. 3 inputs, the waveform at pole 77 of 
switch 78 would appear essentially identical to the FIG. 3 S.sub.out 
waveform for switch 35 open conditions, and the waveform at pole 79 of 
switch 78 would appear essentially identical to the FIG. 3 S.sub.out 
waveform for switch 35 closed conditions. Selection between the two poles 
is of course afforded by switch 78 and thus the actual form of S.sub.out, 
and the actual presentation, would depend on the state of switch 78. 
Thus, while particular embodiments of the present invention have been shown 
and described, it is apparent that changes and modifications may be made 
therein without departing from the invention in its broader aspects. The 
aim of the appended claims, therefore, is to cover all such changes and 
modifications as fall within the true spirit and scope of the invention.