System and modifications are presented which allow existing artillery and rtar projectile proximity fuzes to have a near-surface burst (NSB) option enabling low height of bursts ranging between one and three meters. The additional circuitry needed to implement this NSB into an existing fuze is a single operational amplifier. The velocity of the fuze is calculated by the micro-controller counting the number of Doppler cycles over a pre-determined sample period of time. Thereafter, using the fuze velocity, the delay time needed for a NSB detonation is computed.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates to techniques for providing a near surface burst 
function for artillery proximity fuzes by adding a feature to the existing 
multi-option fuze for artillery (MOFA) hardware. 
2. Background of the Invention 
The current multi-option fuze for the artillery does not have a near 
surface function. Rather, the current multi-option fuze for artillery 
(MOFA) is designed to operate in four functional modes: Proximity, Time, 
Impact, and Delay. In the PD, or Point Detonate Mode, the detonation 
occurs upon impact with the target. The PD mode acts as a back-up mode in 
the case of any failures in the other modes. In the Delay mode, detonation 
occurs with a slight delay after impact. This mode is beneficial for the 
penetration of certain structures. The third mode, Time mode, detonates 
after a set time predetermined by the setter of the fuze. This mode is 
useful with smoke or illumination rounds. For the multi-option fuze for 
artillery in the Proximity mode, detonation occurs at a nominal height of 
nine meters above the ground. There exists a need to provide a near 
surface burst function to be added to the existing proximity sensor of the 
fuze which could provide low height-of-bursts ranging between one and 
three meters above the ground. 
SUMMARY OF INVENTION 
It is an object of this invention to provide an added feature to the 
existing PROX Sensor of the Multi-Option Fuze for Artillery for near 
surface bursts 1-3 meters above the ground instead of nine (9) meters. 
It is still a further object of this invention to provide a fuze option to 
the existing fuze which achieves low height of bursts ranging between one 
and three meters. 
It is a further object of this invention to describe the additional 
circuitry required to implement this near-surface burst capability into 
the existing multi-option fuze for the Artillery. 
It is finally an object of this invention to provide a near surface burst 
feature at low cost to any fuze program that incorporates a 
micro-processor board proximity function which extracts the Doppler 
signature for processing by adapting their sensor design. 
The low cost of this invention is due to the low part count and ease in 
adapting the proximity sensor designs to have a near-surface burst option 
capability.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a block diagram which depicts the proximity fuze's theory of 
functioning. In particular, when the MOFA sensor turns on several seconds 
before a preset target time, the fuze begins to transmit a frequency 
modulated RF signal. The receiver 11 in the fuze picks up Doppler-shifted 
signal reflected from the target. The frequency difference between the 
transmitted signal and the received signal is extracted for further 
processing and is known as the "IF" signal 12. 
The IF signal 12 is then processed through the signal processor 13 which 
generates a Doppler range response 14 as shown at FIG. 2. The horizontal 
scale represents the distance of the fuze from the target in meters. As 
the fuze approaches the ground, there is little response above nine meters 
because the Doppler signal is not within the range cell window. The 
Doppler range cell window for this particular simulated fuze starts at ten 
meters and ends at five meters. This signal is rectified, integrated, 15 
and then fed into a voltage comparator which comprises the fire-decision 
logic circuitry 16 to determine the proper instant to detonate the fuze. 
When the signal 17 exceeds the voltage threshold (to fire the fuze), the 
output comparator signal goes High (transition from a LOW to a HIGH 
state), which then triggers detonation of the fuze. The system is designed 
so at nine meters the Doppler signal will exceed the voltage threshold and 
thus produce a nine meter height of burst (HOB). 
The NSB functional mode now takes the basic design described above and 
operates the fuze in its normal Proximity mode, with the range cell peak 
at seven meters. Once the fuze senses a normal proximity function, instead 
of detonating, the fuze introduces a processed time delay. Thus, the 
near-surface burst function will occur some time after the fuze reaches 
nine meters. 
The following equations describe the theory in producing a Near-Surface 
Burst (NSB) by using a time delay after nine meters has been reached: Once 
the fuze reaches nine meters, the Doppler frequency is derived from: 
EQU Fd=N/dt (1) 
which counts the number of Doppler cycles (N) within a sample time (dt). 
Knowing the Doppler Frequency, Fd, the approach fuze velocity of the 
projectile is calculated by: 
EQU V=(Fd.times.c)/2.times.Fo (2) 
wherein the approach velocity is the Doppler frequency, Fd, multiplied by 
the speed of light, c, and divided by two times the RF frequency, Fo. Now 
knowing the velocity of the projectile, the appropriate time delay 
required for the fuze to reach the desired NSB height can be determined 
by: 
EQU Td=(2*Fo*D)/(Fd*c) (3) 
Where, Fo is the transmitted FM frequency in hertz; Fd is the Doppler 
frequency; c is the speed of light; D is the desired distance from nine 
meters to the near-surface burst function height; N is the number of 
Doppler cycles counted within a pre-determined sample time; and dt is the 
pre-determined sample time length. 
FIG. 3 describes the block diagram of the near surface burst functional 
mode. The voltage comparator 22 output transitions high when the rectified 
Doppler signal output 23 from the rectifier integrator 15 reaches the set 
voltage threshold. 
This transition is recognized by a micro-controller 28 as a nine meter 
proximity function. If this were in the Proximity mode, the fuze would 
detonate. But in the near surface burst mode, the output 23 of the 
comparator 22 enables a micro-controller 28 NSB routine. The existing MOFA 
micro-controller 28 performs many functions. It accepts inputs, makes 
decisions based on these inputs, and executes appropriate output signals. 
With the added NSB feature, the micro-controller 28 will have a few extra 
responsibilities. Concurrently, the Doppler signal is passed to an op-amp 
which functions as a zero-crossing detector 24 to square up the signal. 
This signal is then fed into the micro-controller 28 to calculate the 
Doppler frequency and to calculate the proper delay 26 needed for a 
near-surface burst. 
The voltage comparator 22 and the micro-controller 28 are existing parts of 
the MOFA hardware. With only the addition of the zero-crossing detector 
required, the design becomes valuable because it is very inexpensive and 
does not require any hardware changes to the existing MOFA fuze. 
FIG. 4 depicts the circuitry of the zero-crossing detector 24. This 
zero-crossing detector 24, has two specific functions. One is to condition 
the Doppler signal. The Doppler signal rides on a five volt D.C. level and 
its peak-to-peak voltage can reach up to ten volts. Since the 
micro-controller 28 cannot accept inputs above five volts, the 
zero-crossing detector 24 limits the signal to five volts zero-to-peak and 
converts the Doppler signal to a square wave to make it manageable for the 
micro-controller 28 to process. 
This hard-limited waveform, as the original Doppler signal is altered in 
magnitude, but contains the same frequency content. To calculate the fuze 
velocity, only the Doppler Frequency is needed. Hence, alteration of the 
signal magnitude has no detrimental consequence. 
The second function of the zero-crossing detector 24 is to reject any 
noise. This is accomplished by providing a one volt hysteresis. 
Any Doppler signal that does not exceed this one volt hysteresis level will 
be considered noise and not recognized by the micro-controller 28 as 
Doppler. 
FIG. 5 depicts the NSB subroutine of the micro-controller 28. When the fuze 
senses a nine meter HOB, the output of the voltage comparator 22 
transitions HIGH. The transition is recognized by the micro-controller 28 
to branch to the NSB subroutine described at FIG. 5. 
First, the micro-computer 28 sets a timer 31 to a pre-determined sample 
time to count Doppler cycles 32. The Doppler count is accomplished by 
incrementing a counter 32 for every high transition of the Doppler signal. 
In the interim, the counter 32 continuously checks 33 to see if the timer 
31 has elapsed. If not, it continues to count Doppler cycles 32. 
Once completed, the micro-controller 28 calculates the Doppler frequency 
(samples/second) 32 by dividing the number of cycles counted, N, by the 
sample timer (dt) 31 using formula (1). The next step is to calculate the 
time delay 34 needed for, e.g,, a two meter HOB. This is computed by the 
time delay formula (3). Finally, with the timer delay calculated 34, the 
subroutine performs the calculated timer delay 35 to reach the desired two 
meter height and executes a DETFIRE signal 36 for detonation of the fuze. 
The micro-controller is the MC68HC705C8 or equivalent. 
FIG. 6 shows two oscilloscope traces taken during laboratory testing using 
an RF simulation chamber. FIG. 6(a) represents a proximity function and 
FIG. 6(b) an NSB function. The top channel of both traces represents the 
Doppler range response. The second channel (where the signal transitions 
from a LOW to a HIGH state) represents the DETFIRE signal which initiates 
the detonator. In the proximity mode, when the voltage threshold of the 
comparator is reached, the DETFIRE signal goes HIGH, and initiates the 
round at nine meters. In the NSB mode, notice the time delay between the 
nine meter PROX DETFIRE signal (FIG. 6(a)) and the two meter NSB DETFIRE 
signal (FIG. 6(b)). This is the time delay calculated by the 
microprocessor 28. 
Field tests were conducted on four mortar rounds set to the NSB functional 
mode. Two rounds functioned properly and produced a near-surface burst of 
one meter. The other two rounds functioned point detonation (PD) backup 
due to drop outs in the doppler range response. 
FIG. 7 depicts the range response retrieved from an artillery projectile 
whose fuze failed to activate the Near-Surface Burst (NSB) routine. The 
horizontal line represents the one volt hysteresis level, Vh, provided by 
the zero-crossing detector 24 op-amp. The Doppler signal that does not 
exceed this one-volt hysteresis level will be considered to be noise and 
will not be recognized by the micro-controller 28 as Doppler. 
The drop outs in the range law response, known as the coke-bottling 
effects, are due to multi-path return signals reflected back to the 
receiver 13 of the fuze. Some of these returned signals act destructively 
with one another and cancel each other out. Therefore, when the 
micro-controller tried to determine the Doppler frequency by counting 
Doppler cycles, it missed a few cycles due to the coke-bottling effect and 
predicted that the fuze was traveling at a slower rate then it actually 
was. This in turn generated a larger time delay than required for the fuze 
to read the Near Surface Burst height. This caused the fuze to function PD 
backup before the micro-controller 28 finished its delay loop. 
To overcome this problem, it was determined that instead of determining the 
time delay required for the fuze to reach the desired NSB function height 
using a single sample time window 37 to count Doppler cycles, one could 
break up the window 37 into three smaller frames. Each frame would be of 
length equal to one third of the original sample time window 37. 
Therefore, the micro-computer 28 can calculate the Doppler frequency 
within each frame. 
Thus, an accurate frequency can be determined by discarding any erroneous 
data given by a particular frame, and averaging the valid ones, which in 
turn, will produce an accurate time delay. 
For example, one approach would be to make sure there are at least two 
accurate Doppler frequency measurements taken. The time delay can then be 
computed by using the average of these two Doppler frequency measurements. 
If by chance there are not two valid frames to compute the Doppler 
frequency, the fuze can revert to detonate instantly. This detonation 
would occur at a function height somewhere between the nine meter PROX 
height and the near-surface burst (NSB) height which is a favorable 
alternative, rather than function PD back-up. 
The NSB subroutine described herein is called only when the fuze is 
detected to be at the proper function height of nine meters. Therefore, 
there must be Doppler present at the output of the Doppler filters. Using 
the fact that there is some prior knowledge of the Doppler signal because 
one knows what the Doppler frequency should be within certain thresholds, 
it can be determined by the micro-controller 28, that the calculated 
Doppler frequency, for a particular frame, is not Within the theoretical 
Doppler thresholds, and hence to consider the calculated Doppler frequency 
erroneous. FIG. 8 illustrates the subroutine's flow chart which is 
self-explanatory for this alternate NSB algorithm. 
Thus, it is apparent that in accordance with the present invention, a 
functional design that fully resolves an important military munitions 
problem is set forth above. While the invention has been described in 
conjunction with a specific embodiment and one related alternative, it is 
evident that many alterations, modifications, and variations will be 
apparent to those skilled in the art in light of the foregoing 
descriptions. Accordingly, it is intended that the present invention 
embrace all such alternatives, modifications, and variations as fall 
within the spirit and broad scope of the appended claims.