Non-linear radar range scale display arrangement

A non-linear range scale for a radar system is implemented whereby the full range capability of the system is shown with emphasis on close-in targets. The arrangement is adaptable to a variety of non-linear functions having the advantage of selection by the user to provide good close-in visibility and visibility of distant conditions simultaneously.

BACKGROUND OF THE INVENTION 
Users of radar systems experience a problem when, for example, using the 
radar system for guiding an aircraft through thunderstorms and other 
adverse weather conditions. Such users are aware of adverse weather 
conditions in close proximity to the craft. In order to be aware of 
adverse weather conditions beyond a presently selected range scale of the 
radar system, the user needs to constantly switch from close-in to distant 
range scales. Failure to do this for one reason or another can result in 
potentially hazardous flying situations. 
Prior art radar systems utilize linear range scales. That is to say, the 
displayed distance is directly proportional to the actual distance. The 
present invention, on the other hand, features a non-linear range scale 
which emphasizes the close-in range relative to a more distant range. With 
this arrangement, close-in targets such as weather disturbances are 
displayed larger and with substantial detail while more distant 
disturbances are displayed in a manner so as to alert the user to same. 
SUMMARY OF THE INVENTION 
This invention contemplates a non-linear radar range scale display 
arrangement wherein close-in targets are displayed with substantial 
detail, while more distant targets are simultaneously discernable. For 
example, when using a radar system for detecting weather disturbances, a 
displayed weather pattern might be that of a long frontal weather 
disturbance. With prior art radar systems, only the first ten miles or so 
in front of the system antenna/receiver would be seen, perhaps showing 
only a single target or weather cell. To alleviate this situation, the 
scale displayed in accordance with the present invention is proportional 
to the logarithm (log) to the base ten of the range (R). The displayed 
scale can be commensurate with a variety of non-linear functions in 
addition to log R as aforenoted, such as the square root of R, or R.sup.a, 
where "a" is a positive value less than one. The selection of a specific 
non-linear function is dependent upon how much emphasis is required on 
close-in versus distant ranges.

DETAILED DESCRIPTION OF THE INVENTION 
In implementing the invention, values for a plurality of range rings are 
determined for a non-linear scale. With reference to FIG. 1, four such 
rings are shown and are designated by the numerals 2, 4, 6 and 8. Range 
ring 8 is at full distance, range ring 6 is at three-quarter distance, 
range ring 4 is at one-half distance and range ring 2 is at one-quarter 
distance. Part of a target such as a weather cell 5 is within ring 2; the 
rest of target 5 and most of a target 7 are within ring 4; the rest of 
target 7 and targets 9, 11, 13 and part of a target 15 are within ring 6; 
and the rest of target 15 and targets 17 and 19 are within ring 8. The 
weather pattern depicted might be typical of a long frontal weather 
pattern. With a non-linear range scale the close targets are shown in 
detail, but it can be discerned that the front extends for a long distance 
in front of an observer. With the above in mind, consider the following: 
EQU D=(log R)/K, (1) 
where D equals displayed distance (1.0 is full scale); R equals radar range 
in nautical miles (nm) and K is a constant to normalize the radar range to 
the displayed range. 
For outer ring 8, D equals 1. Thus substituting appropriate values in 
equation (1), the following is obtained: 
EQU 1=(log 320)/K, (2) 
where K equals 2.505. 
For ring 6, log R equals 0.75.times.2.505, or R equals 75.7 nm, nominally 
80 nm, as shown in the Figure. Similarly, for ring 4, log R equals 
0.5.times.2.505 or R equals 17.9, nominally 20 nm, and for ring 2 log R 
equals 0.25.times.2.505, or R equals 4.2, nominally 5 nm, as also shown in 
FIG. 1. 
In order to judge as to which non-linear function would be most desireable 
for use with a particular radar system, several non-linear functions are 
plotted, as illustrated in FIG. 2. Thus, FIG. 2 shows curves obtained by 
plotting displayed distance against radar range for the following three 
non-linear functions: log R; R .sup.0.5 ; and R.sup.0.1. For purposes of 
illustration, the selected function for implementation is the log R 
function, since this function appears to provide the best compromise of 
resolution and display area utilization, as is desireable. 
To best understand the aforenoted implementation, reference is made to the 
weather radar system block diagram illustrated in FIG. 3. Thus, a 
transmitter is designated by the numeral 10. Transmitter 10 may be a 
magnetron which is connected to an antenna/receiver 12 via a circulator 
14. 
Pulses are directed from antenna 12 to a target such as a weather cell as 
shown in FIG. 1. The pulses are reflected from the target to 
antenna/receiver 12 and are directed to a preamplifier 16 via circulator 
14 so as to provide an amplified signal. The amplified signal is mixed by 
a mixer 18 with a signal from a local oscillator 20. Mixer 18 mixes the 
frequencies of the amplified signal from pre-amplifier 16 and the signal 
from local oscillator 20 to provide an intermediate frequency (IF) signal. 
Local oscillator 20 is driven by an automatic frequency control (AFC) 
signal provided by a microprocessor 21. Microprocessor 21 also provides a 
START signal and a RESET signal for purposes to be hereinafter described. 
Signal AFC is a digital signal which is converted to an analog signal by a 
digital to analog (D/A) converter 23 and the analog signal drives the 
local oscillator. The mixed signal from mixer 18 is amplified and filtered 
by an amplifier/filter 22. The amplified and filtered signal is detected 
by a detector/discriminator 24. 
Detector/discriminator 24 provides an analog signal which is applied to a 
sample and hold circuit 25. Sample and hold circuit 25 is responsive to 
the START signal from microprocessor 21 for applying a sampled and held 
signal to the microprocessor. 
The analog signal from detector/discriminator 24 is converted to a digital 
signal by an analog to digital (A/D) converter 26 which is driven by a log 
clock generator 30. The digital signal is stored in a memory device 28 to 
be later applied to a display device 29 in an appropriate format. In this 
regard, it is noted that display device 28 is an external device and is 
connected to memory device 28 by a self-clocking high speed bus 31. 
Log clock generator 30 provides a proper clock frequency to quantize a 
selected non-linear radar range scale to the actual radar range scale. In 
this regard, it will be understood that the round trip time for the pulses 
from and to antenna/receiver 12 is approximately 12.35 .mu.s per nm. To 
create a log R range scale, a clock generator must be provided that varies 
its frequency logarithmically with time. Log clock generator 30 which is 
responsive to the START and RESET signals from microprocessor 21 and 
which, in turn, drives memory device 28 through A/D converter 26 serves 
this purpose. 
The approach taken to configure log clock generator 30 is to use a 
"piece-wise" linear approximation to the actual clock signal desired and, 
in this regard, reference is made to FIG. 4 which is a plot of the 
"piece-wise" linear approximation of range bins versus range. With eight 
linear range segments, the desired log R function is obtained with 
reasonable accuracy. It is to be noted that this approach is adaptable to 
any particular non-linear function, with the log R function being 
described for illustration purposes. 
FIG. 5 is a block diagram of log clock generator 30 shown generally in FIG. 
3. Thus, log clock generator 30 includes four basic components: a cycles 
counter 32; encoding logic 34; a divide ratio register 36; and a 
programmable divider 38. The arrangement is such that log clock generator 
30 provides a pre-set clock frequency for a predetermined number of clock 
cycles and then switches to a frequency at one-half that rate. The new 
clock rate is active for a predetermined number of clock cycles before 
switching occurs to the next frequency, now one-quarter of the original. 
This repeats for a total of eight different frequencies. 
The function of cycles counter 32 is to count the number of range bins 
produced. The radar system contemplated uses a total number of two hundred 
fifty-six range bins for a complete range scale. The following table 
outlines the switching points for the overall function of log clock 
generator 30. 
______________________________________ 
Segment Divide Ratio Range Bins 
Preload 
______________________________________ 
1 2 51 255 
2 4 39 254 
3 8 38 252 
4 16 29 248 
5 32 29 240 
6 64 39 224 
7 128 25 192 
8 256 6 128 
256 total 
______________________________________ 
Encoding logic 34 which is connected to cycles counter 32 via eight cycles 
counter outputs (LC0-LC7) monitors the output of cycles counter 32, 
looking for a match for the aforementioned switching points and provides 
outputs DO-D6. When a switching point is observed, an appropriate preload 
value is sent to a seven bit divide ratio register 36. Divide ratio 
register 36 holds the preload values listed in the table above. Divide 
ratio register 36 drives programmable divider 38 through inverters 
39A-39G. 
A "D" type flip-flop (F/F) 40 is connected at an input D to an overflow 
output (OVF) of cycles counter 32; a "D" type flip-flop (F/F) 42 is 
connected at a clear (CLR) input to the output of an OR gate 41; and a 
"J-K" type flip-flop (F/F) 44 is connected at its J and K inputs to an 
overflow output (OVF) of programmable divider 38. OR gate 41 receives an 
output (Q) from flip-flop 40 and the RESET signal from computer 21. 
A clock output from a main systems clock 43 which may be a crystal 
oscillator is applied to a clock input (CLK) of programmable divider 38 
and is applied through an inverter 45 to a clock input (CLK) of flip-flop 
44. All of the 39A-39G inverter outputs are set to a logic "1" by a logic 
start signal (LSTRT) applied to a reset input of divide ratio register 36 
from flip-flop 42 and are applied to programmable divider 38. Signal LSTRT 
is applied to a clear (CLR) input of flip-flop 44. The RESET signal from 
computer 21 clears cycles counter 32 and flip-flop 40 and 42, which when 
applied to a clear (CLR) inputs thereof resets the system. A preload value 
of all logic "1's" causes an overflow condition (OVF) to always be present 
at the output of programmable divider 38 so that the output of J - K 
flip-flop 44 toggles with every clock pulse, creating a divide by two 
output, i.e. LGCK and LGCK. Output LGCK is applied to the clock input 
(CLK) of cycles counter 32 and output LGCK is applied to the clock input 
(CLK) of divide ratio register 36 and to the clock (CLK) input of 
flip-flop 40. As the bits in divide ratio register 36 are cleared, the 
proper preload value is obtained. - With reference to the chart above, 
after two hundred fifty-six range bins have been produced, cycles counter 
32 overflows setting flip-flop 40 which clears flip-flop 42 through OR 
gate 41, thus resetting the system awaiting the next sequence of pulses 
from microprocessor 21 (FIG. 3) to start another log clock sequence via 
the START signal from microprocessor 21. 
Programmable divider 38 is a binary counter which can be programmed to 
produce different output frequencies. The counter starts at a preloaded 
value and then counts up until it overflows. The overflow condition allows 
the final stage, flip-flop 44, to toggle, as aforenoted. The overflow 
condition also causes programmable divider 38 to load the starting value. 
When this value is left constant, the result is a fifty percent duty cycle 
clock signal at a rate of one-half the overflow rate of the counter. Since 
the value used in the preload is programmable, the output frequency is 
correspondingly programmable. It should be noted that the arrangement can 
be used on any range scale by simply changing the main clock input. 
With the above description of the invention in mind, reference is made to 
the claims appended hereto for a definition of the scope of the invention.