GB2133256A - A PPI display apparatus - Google Patents

Abstract

A radar or sonar image for example, or bearing marker is displayed on a circular display, e.g. on a circular CRT screen through sweeping on the basis of a plan position indicator system. Generation of a radar or sonar antenna angle signal, generation of a bearing angle signal representing the angle of a bearing cursor and generation of sine and cosine signals corresponding to the antenna angle or bearing angle are all carried out through digital processing. The digital bearing angle signal representing the bearing cursor angle is converted into a display code and numerically displayed on a display. <IMAGE>

Description

SPECIFICATION PPI display apparatus The present invention relates to a plan position indicator (PPI) display apparatus for displaying e.g.

radar or sonar information on a circular display for example on a circular screen of a cathode-ray tube (CRT) through electron beam sweep based on a polar coordinate deflection system (a PPI system).

A conventional PPI display apparatus which can display radar or sonar information etc., is shown in FIG. 1, for example.

Referring to FIG. 1, an antenna angle generator 10 ordinarily uses a resolver or the like to generate a sine signal (sin ot) and a cosine signal (cos 0,) proportional to the antenna angle 8,. An electronic bearing marker angle generator 12 generates a bearing marker angle signal representing the angle of a bearing marker which is displayed on the CRT screen. It ordinarily uses a resolver of sine/cosine function generating potentiometer to generate signals sin 62 and cos 62 proportional to a bearing angle 62 which is set by a manual operation signal.

The outputs from the antenna angle generator 10 and the electronic bearing marker angle generator 12 are inputted to a signal selection switch 14. The signal selection switch 14 is switched under the control of an externally supplied switching control signal to selectively supply the sine and cosine signals from either the antenna angle generator 10 or the electronic bearing generator 12 to a sweep signal generator 1 6. The sweep signal generator 1 6 can integrate the sine and cosine signals supplied as DC voltage signals from the signal selection switch 14 to produce sweep signals in the form of saw-toothed signals. The sweep signals are inputted to a sweep current amplifier 18.The sweep current amplifier 1 8 supplies amplified deflection currents to perpendicular deflection coils 20 and 22 of a CRT, whereby the image of radar, sonar, information etc. or bearing marker is displayed on the PPI display through polar coordinate deflection of an electron beam.

The sweep signal generator 1 6 receives externally supplied X- and Y-axis (i.e. horizontal and vertical) off-center control signals, and has an off-center function capable of shifting the sweep start point of an antenna angle sweep signal or that of a bearing marker angle sweep signal independently to a given point on the CRT screen. To be more specific, off-center control signals for the antenna angle deflection signal and bearing marker angle deflection signal are supplied on a time division basis in a synchronous relation to a switching control signal externally supplied to the signal selection switch 14.

Thus, the sweep start points for the antenna information and the bearing marker angle information can be set independently.

In the prior art PPI display system as described above, however, the sine and cosine signals representing the antenna angle ot and the sine and cosine signals representing the bearing marker angle 62 are generated as analog signals from a resolver or the like. In this case, the precision of the sine and cosine signals depends on the mechanical and electrical precisions of the resolver or like transmitter. These precisions can only be increased with an increase in cost. In addition, limitations are imposed on the precisions, so that it is difficult to increase the resolution of a target detected by the radar, sonar, etc. beyond a certain limit.

In addition, all the signals involved are processed as analog signals, and the processing is susceptible to the influence of temperature. Electric temperature compensation is of course provided.

However, since the system is used under comparatively severe conditions, such as in ships, the influence of temperature cannot be perfectly compensated for, and the stability of the system is inferior.

In a further aspect, it is desired that the angle 62 of the bearing marker displayed on the prior art PPI display according to the output signal of the electronic bearing marker angle generator 1 2 be readily read out. A scale 26 having bearing angle graduations is provided around the CRT screen 24, as shown in FIG. 2A. The bearing marker angle 62 can thus be readily read out from the scale 28.

However, when the bearing marker is displayed by off-center display according to the off-center control signals as shown in FIG. 2B, the reading of the leading end B of the off-center bearing marker 28 drawn from the off-center point P no longer represents the correct bearing angle because the scale 26 is graduated with its origin 0 at the center of the CRT screen.

Accordingly, with the prior art PPI display a pseudo-bearing marker 30, as shown by a broken line, is drawn from the origin 0 so that it is parallel to the off-center bearing marker 28 drawn from the offcenter point P, and the graduation corresponding to the leading end A of the pseudo-bearing marker 30 is read out. This procedure of reading the bearing marker angle in the off-center display is very cumbersome and inconvenient. Besides, the recognition of the CRT display image is obstructed by the pseudo marker.

An embodiment of the present invention can provide a PPI display apparatus, in which the processing of signals for the PPI display is carried out digitally, thus permitting the improvement of the precision of the display and eliminating the influence of temperature, and hence ensuring high stability.

An embodiment of the present invention can provide a PPI display apparatus, in which an antenna angle signal, a bearing marker angle signal representing a preset bearing marker angle and sine and cosine signals corresponding to these angle signals are all generated as digital signal.

An embodiment of the present invention can provide a PPI display apparatus, in which a sweep signal for the sweep of an electron beam is generated through conversion of the digital sine and cosine signals into corresponding analog signals.

An embodiment of the present invention can provide a PPI display apparatus, in which the digital sine and cosine signals are integrated to obtain digital sweep signals which are converted to analog signals for sweeping the electron beam.

An embodiment of the present invention can provide a PPI display apparatus, in which the digital sine and cosine data are stored in a memory in individual addresses thereof to be made access to by respective digital angle signals so that given digital sine and cosine data can be readily obtained when the corresponding digital angle signal is given.

An embodiment of the present invention can provide a PPI display apparatus, in which a digital bearing signal representing a bearing marker angle set by an operator is converted into a display code and numerically displayed on a display provided near the CRT screen or on a vacant space thereof so that the angle of the bearing marker which is displayed even in an off-center display with respect to the display center of the CRT screen can be readily read out.

Reference is made, by way of example, to the accompanying drawings, in which: FIG. 1 is a block diagram showing a prior art PPI display apparatus.

FIG. 2A is an explanatory view showing a bearing marker display by on-center display on a CRT screen of the prior art apparatus.

FIG. 2B is an explanatory view showing a bearing marker display by off-center display on a CRT screen of the prior art apparatus.

FIG. 3 is a block diagram showing one embodiment of the PPI display apparatus according to the present invention.

FIG. 4 is a block diagram showing another embodiment of the present invention.

FIG. 5 is a block diagram showing a further embodiment of the present invention which can also numerically display the bearing marker angle.

FIG. 6A is an explanatory view showing a bearing marker display by on-center display on a CRT screen of the embodiment of FIG. 5 along with a corresponding numerical display of the bearing marker angle.

Fig. 6B is an explanatory view showing a bearing marker display by off-center display on the CRT screen of the embodiment of FIG. 5 along with a corresponding numerical display of the bearing marker angle.

Fig. 3 is a block diagram showing one embodiment of the present invention.

As for an antenna angle generator 32, there may be used a synchro transmitter or shaft encoder (angle encoder). It generates an antenna angle signal 01 as a digital signal according to the antenna angle. A digital converter 34 converts the signal 01 into a binary digital antenna angle signal. It is a synchro digital converter in case where the antenna angle generator 32 is a synchro transmitter, while it is a pulse counter in case where the antenna angle generator 32 is an incrementary shaft encoder.

Where the binary digital antenna angle signal produced from the digital converter 34 is a 12-bit signal, for example, the bit place of the individual binary bits of the angle signal and the corresponding binary-coded decimal (BCD) weight of angle are related as shown in Table 1 below.

TABLE 1

Bit place BCD weight of angle Most significant bit a1 180.0 a2 90.0 a3 45.0 a4 22.5 a8 11.25 a6 5.625 a7 2.8125 as 1.40625 as 0.703125 a10 0.3515625 all 0.17578125 Least significant bit a2 0.087890625 As is clear from Table 1 above, the most significant bit a1 of the binary digital antenna angle signal represents an angle of 1800.The second most significant bit a2 represents one half the angle, i.e. 900.

The least significant bit a12 represents an angle weight of 0.0878906250.

The binary digital antenna angle signal representing the antenna angle 1 as shown in Table 1, is fed as one input to a digital signal selection switch 38, which may be a digital multiplexer.

An electronic digital bearing marker angle signal generator 36 generates a signal representing the angle 62 of a bearing marker displayed on a CRT screen. The bearing marker angle 62 may be freely set by an operator. The generator 36 may be a binary up/down counter, which can up- or down-count clock pulses in response to an up- or down-counting command signal coupled by an external operation and produce a binary digital signal having the same number of bits as the binary digital antenna angle signal shown in Table 1 as its count that represents the given marker angle 62. The binary digital bearing marker angle signal 62 from the electronic binary digital bearing marker angle signal generator 36 is fed as the other input to the digital signal selection switch 38.The selection switch 38 selectively feeds either the digital antenna angle signal 6i from the binary digital converter 34 or the digital bearing marker angle signal 62 from the digital bearing marker angle signal generator 36 under the control of an externally supplied switching control signal.

The digital signal slection switch 38, which is a digital multiplexer operated under the control of the switching control signal, can selectively provide the 12-bit binary digital data from the digital converter 34 and from the digital bearing marker angle signal generator 36 as parallel binary data.

The digital antenna angle signal 6i or digital bearing marker angle signal 62 that is selectively fed through the digital signal selection switch 38 is inputted to a sine/cosine generator 40. The sine/cosine generator 40 is, for instance a P-ROM (programmable read-only memory), in which digital data representing sine and cosine values of angles up to a sweep angle of 3600 are stored, these angles being spaced apart by a predetermined unit angle, for each of addresses, which can be made access to by the 12-bit binary digital signal of Table 1 as an address signal. Sine and cosine value data can be read out by the digital antenna signal fl1 or digital bearing marker angle signal 62 supplied as the address signal from the digital signal selection switch 38.The sine and cosine value data read out from the sine/cosine generator 40 are fed to digital-to-analog converters 42 and 44, respectively, for conversion into corresponding analog voltages.

The digital-to-analog converters 42 and 44 may each include constant voltage sources which can provide output voltages corresponding to the angle weight values for the individual binary bit positions shown in Table 1. These output voltages are selectively added together according to the sine and cosine digital data, whereby analog voltages having a precision determined by the weight of the last or least significant bit of binary digital data can be obtained.

The sine and cosine signal voltages from the digital-to-analog converters 42 and 44 are fed to a sweep signal generator 46. The sweep signal generator 46 integrates the sine and cosine signal voltages to produce saw-toothed sweep signals, which are current-amplified by a sweep-current amplifier 48, the output of which is in turn supplied as sweep currents to perpendicular deflection coils 50 and 52 of a CRT.

X-axis (i.e. transversal) and W-axis (i.e. vertical) off-center control signals are externally supplied to the sweep signal generator 46, whereby the sweep start point of antenna angle and bearing marker angle sweep signals can be independently moved to a desired point on the CRT screen.

in operation, the antenna angle generator 32 generates the antenna angle signal 01 according to the rotation of the radar or sonar antenna. The digital converter 34 converts the input antenna angle signal into, for instance a 12-bit binary digital antenna angle signal 01 as shown in Table 1. This signal is fed as one input to the digital signal selection switch 38.

Meanwhile, when the angle 62 of the bearing marker to be displayed on the CRT screen is set by an operator, the electronic binary digital bearing marker angle signal generator 36 generates the digital bearing marker angle signal 62 which is fed as the other input to the digital signal selection switch 38.

The digital bearing marker angle signal 62 is generated according to an addition signal or subtraction signal that is inputted by the operator to the electronic digital bearing marker angle generator 36 including the up-down counter. To be more specific, the up-down counter up- or down-counts clock pulses according to the addition or subtraction signal, and the digital bearing marker angle signal 62 generated represents the count of the up-down counter corresponding to the given bearing marker angle 62. Of the digital antenna and bearing marker angle signals 0, and 62 fed to the digital signal selection switch 38, either one is selectively inputted to the sine/cosine generator 40 according to the externally supplied switching control signal.The selectively inputted signal 0, or 62 serves as an address signal for address specification of the P-ROM in the sine/cosine generator 40, whereby the digital sine and cosine data stored in the specified address are read out. These read-out data are converted by the digital-to-analog converters 42 and 44 into corresponding analog voltage signals which are fed to the sweep signal generator 46. The sweep signal generator 46 integrates the input analog sine and cosine voltages to generate sine and cosine sweep signals as saw-toothed signals. The sweep current amplifier 48 proportionately current-amplifies the sweep signals to supply amplified sweep currents to the perpendicular deflection coils 50 and 52 of the CRT.According to these sweep currents, the PPI electron beam is deflected to effect sweeping, whereby the data of the radar or sonar is displayed.

When an off-center operation is effected by an operator for off-center PPI display, X-axis and Yaxis off-center control signals corresponding to the preset off-center position are supplied to the sweep signal generator 46. As a result, the start point of sweeping of the sine and cosine sweep signals from the sweep signal generator 46 is shifted to a given off-center position, so that off-center sweep of the electron beam is obtained.

FIG. 4 is a block diagram showing another embodiment of the present invention. This embodiment is the same as the preceding embodiment of FIG. 3 except for that a digital sweep signal generator 54 is provided on the input side of digital-to-analog converters 56 and 58. The digital sweep signal generator 54 has a function of digitally integrating digital sine and cosine data corresponding to the digital antenna angle or bearing angle data 0, or 62 outputted from the sine/cosine generator 40.

The digital sweep signal generator 54 accumulates the input digital sine and cosine data and outputs the accumulated data for every constant period of time. As a specific example, when digital sine data X, is inputted at a timing T,,the generator 54 stores this value X, and outputs the same. When next data X2 (provided that X2 = X,) is inputted at the next timing T2, the generator 54 adds this data X2 to the stored data X1, stores the sum X2 + X, = 2Xg and outputs the same. Likewise, data X3 (provided that X3 = X2 = X,) inputted at the subsequent timing T3 is added to the stored data, and the sum X3 + X2 + X, = 3X, is stored and outputted. This accumulating operation is progressively effected.

Digital cosine data Y, is accumulated in the same manner as well.

X-axis and Y-axis off-center control signals may be externally supplied to the digital sweep signal generator 54. For the off-center display, the digital data corresponding to the X- and Y-axis coordinates of the off-center position are algebraically added to the accumulated values of the digital sine and cosine data, and the sum data are outputted.

Digital-to-analog converters 56 and 58 convert the accumulation of the digital sine and cosine data as respective outputs of the digital sweep signal generator 54, which outputs may also include the off-center data, into analog voltages, respectively. These analog voltages are saw-toothed voltages inclusive of the off-center value.

The output from the digital-to-analog converters 56 and 58 are fed two a sweep current amplifier 48.

The rest of the construction is the same as in the embodiment of Fig. 3.

In operation, unlike the embodiment of Fig. 3 in which the output signals of the sine/cosine generator 40 are converted into the analog signals which are integrated in an analog manner to obtain the saw-toothed signals, in the embodiment of FIG. 4 the output signals from the sine/cosine generator 40 are first integrated digitally before conversion to the analog signals.

The embodiment of FIG. 4 is advantageous in that it is possible to eliminate such causes of deterioration or precision non-linear distortion and temperature drift that usually occur in an analog integrator as in the embodiment of FIG. 3.

More specifically, in the digital integration or accumulation, non-linear distortion, temperature drift, etc. may be theoretically eliminated by providing digital data bits in number necessary for the required precision of calculation.

The embodiment of FIG. 4 thus can provide for higher precision and stability (with respect to temperature) than the embodiment of FIG. 3 while the function is the same.

In the preceding embodiments of FIGS. 3 and 4, the digital sine and cosine data corresponding to the antenna angle or preset bearing marker angle are generated by using a P-ROM. Alternatively, the digital sine and cosine data can be calculated directly from the digital signal representing the angle through a programmed arithmetic operation in a microprocessor or using digital calculators for performing digital sine and cosine operations. Particularly, since a microprocessor can perform programmed operations not only for the generation of the digital sine and cosine data but also programmed operations of other digital processings, it is advantageous from a standpoint of cost reduction.In the cases of FIGS. 3 and 4 using the P-ROM, however, highly precise digital sine and cosine data can be generated by preliminarily storing sine and cosine data in the P-ROM and using an input digital angle signal as an address signal for addressing the P-ROM. In this case, the necessary processing time may be only the time required for reading data out of the P-ROM. This is desirable from a standpoint of materializing a high processing speed.

As is clear from the embodiments shown in FIGS. 3 and 4, according to the present invention, the generation of the sweep signal as well as the generation of the antenna angle signal, preset bearing marker angle signal and the sine and cosine signals based on either of the angle signals for generating the sweep signal, is obtained entirely through digital processing. Therefore, the precision of the digital processing can be considerably enhanced by increasing the number of bits in the binary digital signal. In addition, it is possible to ignore errors due to analog signal fluctuations in an angle range which is determined by the weight value of the last or least significant bit of the binary digital data. Further, since the construction is less sensitive to the influence of temperature, stable performance can be ensured.

Further, commercially available digital circuit elements can be used for the individual digital processing sections, so that the cost can be reduced as compared with the prior art construction based on the analog system.

Fig. 5 shows a further embodiment of the present invention. In this embodiment, a circuit for numerically displaying the bearing angle which is displayed as a corresponding bearing marker on the CRT screen, is added to the structure of the embodiment of Fig. 3.

Referring to Fig. 5, the binary digital code output of an electronic digital bearing marker angle generator 36, representing the preset angle of the bearing marker displayed on the CRT screen, is fed both to a binary/BCD converter 60 and to a digital signal selection switch 38.

The binary/BCD converter 60 may consist of a P-ROM. In this instance, the digital data output of the electronic digital bearing marker angle generator 36 representing the preset bearing marker angle 62 is used as an address signal to read out a BCD code, which corresponds to the digital data representing the bearing marker angle 62. The BCD code read out from the binary/BCD converter 60 representing the preset bearing marker angle 62 is latched in a latch circuit 62 for every constant period of time. The latched data representing the preset bearing marker angle 62 is numerically displayed on a LED angle display 64 by driving LED display segments.The angle may be displayed in a unit of 0.1 , i.e. from 000.00 to 359 9 With this arrangement, the preset bearing marker angle 62 of the bearing marker is numerically displayed as real time display on the LED angle display 64 according to the binary digital data from the electronic digital bearing marker angle generator 36. An operator thus can set a given bearing marker angle 62 while observing the display on the LED angle display 64.

The rest of the circuit construction and operation are the same as in the previous embodiment of FIG. 3.

The bearing marker angle display operation of the embodiment of FIG. 5 will now be described in detail.

When a given angle of the bearing marker is set by an operator, the count of the up-down counter of the electronic digital bearing marker angle generator 36 is changed according to the addition or subtraction signal supplied to the generator 36 in accordance with the preset bearing marker angle. The binary digital data from the electronic digital bearing marker angle generator 36 representing the preset bearing marker angle 62 is supplied as address data to the binary/BCD converter 60. The BCD code corresponding to the address data thus supplied is read out and latched in the latch circuit 62 for every constant period of time to be numerically displayed on the LED angle display 64. A desired bearing marker angle may be set while observing the displayed numerical figure on the LED angle display 64.

The preset bearing marker angle data 62 from the electronic digital bearing marker angle generator 36 is also fed as binary digital data to the digital signal selection switch 38. Either the digital antenna angle signal from the digital converter 34 or the digital bearing marker angle signal from the electronic digital bearing marker angle generator 36 is supplied under the control of the externally supplied switching control signal to the sine/cosine generator 40. That is, the digital data from the digital signal selection switch 38 is supplied as an address signal, whereby the corresponding sine and cosine data representing the antenna angle 0, or bearing marker angle 62 is read out from the P-ROM in the sine/cosine generator 40.The read-out data are converted in the digital-to-analog converters 42 to 44 into corresponding analog voltage signals which are fed to the sweep signal generator 46. If a start point of sweeping of X- and Y-axis off-center control signals externally supplied to the sweep signal generator 46 corresponds to the origin, i.e. the center of the CRT screen, the angle data is displayed in on-center display on the CRT screen 24, as shown in FIG. 6A. More specifically, the sweep signal generator 46 provides saw-toothed sweep signals by integrating the sine and cosine voltage signals.

These sweep signals are current-amplified in a sweep current amplifier 48 to obtain deflecting currents, which are passed through the perpendicular deflection coils 50 and 52, whereby the display of FIG. 6A is obtained. If the bearing marker angle preset in the electronic digital bearing marker angle generator 36 shows that 62 = 299.90, a bearing marker 28 directed from the origin 0 at the preset angle 62 is displayed. At the same time, this bearing marker angle is numerically displayed as 62 = 299.90 on the LED angle display 64 which is provided near the CRT screen 24.

If the start point of sweeping of X- and Y-axis off-center control signals externally supplied to the sweep signal generator 46 corresponds to an off-center point P, as shown in FIG. 6B, the displayed bearing marker 28 is directed from the off-center point P at the preset angle of 62 = 299.90, while this angle 62 is numerically displayed on the LED angle display 64.

In the case of the off-center display as shown in Fig. 6B, it may sometimes be impossible to read the accurate bearing marker angle 62 depending on the scale 26 provided around the CRT screen 24.

With this embodiment, the bearing marker angle of 62 = 299.90 is displayed on the LED angle display 64 provided near the CRT screen 24. Thus, the angle of bearing marker 28 displayed in the off-center display can be readily read out from the LED angle display 64.

In the above embodiment shown in FIG. 5, the circuit for numerically displaying a bearing marker angle has been added to the construction of the embodiment of FIG. 3. It is also possible to add a similar circuit to the construction of the embodiment of FIG. 4, thereby enabling the bearing marker angle to be numerically displayed.

Further, instead of the embodiment of FIG. 5 which numerically displays the bearing marker angle on the angle display provided near the CRT screen, it is possible to convert the display code signal obtained from the binary/BCD converter 60 into a video signal representing the numerical value of the bearing marker angle and display this numerical value on a space of the CRT screen which will not interfere the display of the bearing marker.

Although the present invention has been described above in the context of the display of radar or sonar information, for example, derived from a radar or sonar antenna, on a circular screen CRT, it will be appreciated that embodiments of the invention can employ circular displays other than a circular screen CRT, and can display orientation related information other than radar or sonar information along corresponding orientations in the display (i.e. for each orientation with respect to a central point, information pertaining to that orientation can be displayed along a correspondingly oriented line in the display).

Claims (6)

1. PPI display apparatus for displaying orientation related information along a corresponding orientation in a circular display, comprising digital bearing marker angle generating means for generating digital bearing marker angle data representing an angle of a bearing marker displayed on the circular display, said bearing marker angle being preset by a bearing marker angle setting operation; orientation angle generation means for generating an orientation angle signal, digital angle converting means for converting said orientation angle signal from said orientation angle generating means into a corresponding digital angle signal; signal selection switch means for selecting the output of said digital bearing marker angle generating means and the output of said digital angle converting means in accordance with a present timing; sine/cosine generating means for generating digital data representing the sine and cosine of the digital angle signal selected by said signal selection switch means, display driving signal generating means for generating analog orientation angle or bearing marker angle display driving signals corresponding to the digital sine and cosine data outputs of the sine/cosine generating means, for driving the display.

2. A PPI display apparatus for displaying data through sweeping of an electron beam on a circular CRT screen in accordance with a PPI system, which comprises digital bearing marker angle generating means for generating digital bearing marker angle data representing an angle of a bearing marker displayed on said circular CRT screen, said bearing marker angle being preset by a bearing marker angle setting operation; antenna angle generating means for generating an antenna angle signal; digital angle converting means for converting said antenna signal from said antenna angle generating means into a corresponding digital angle signal; signal selection switch means for selecting the output of said digital bearing marker angle generating means and the output of said digital angle converting means in accordance with a preset timing; sine/cosine generating means for generating digital data representing the sine and cosine of the digital angle signal selected by said signal selection switch means; sweep signal generating means for generating analog antenna angle or bearing marker angle sweep signals corresponding to the digital sine and cosine data outputs of said sine/cosine generating means, the sweep start point of said sweep signals being variable according to externally supplied signals; and current amplifying means for proportionately current-amplifying the output of said sweep signal generating means to provide corresponding deflection currents to perpendicular deflection coils of said CRT; said antenna angle signal, said bearing marker angle data representing the bearing marker angle and the sine and cosine signals corresponding to the antenna angle and bearing marker angle being all generated through digital processing.

3. A PPI display apparatus according to claim 2, wherein said sweep signal generating means comprises digital-to-analog converters for converting the digital sine and cosine data outputs of said sine/cosine generating means into corresponding analog voltages, and a sweep signal generator for generating antenna angle or bearing marker angle sweep signals in proportion to the outputs of said digital-to-analog converters, the sweep start point of said sweep signals being variable according to externally supplied X- and Y-axis off-center control signals.

4. A PPI display apparatus according to claim 2, wherein said sweep signal generating means comprises a digital sweep signal generator for generating digital antenna angle or bearing marker angle sweep signals through digital integration of the digital sine and cosine data outputs of said sine/cosine generating means, the sweep start point of said sweep signals being variable according to initial value data of the digital integration operations corresponding to externally supplied X- and Y-axis off-center control signals, and digital-to-analog converters for converting the output data of said digital sweep signal generator into corresponding analog voltages.

5. A PPI display apparatus according to claim 2, wherein said sine/cosine generating means comprises a programmable read-only memory, digital sine and cosine data corresponding to angle data being stored therein in respective addresses, said addresses being made access to by the digital angle signal.

6. A PPI display apparatus according to claim 2, wherein said digital bearing marker angle generating means comprises a binary code signal generating circuit for generating a binary digital code signal corresponding to a bearing marker angle set by an operator, a convertion circuit for converting the binary digital code signal from said binary code signal generating circuit into a display code signal, and a numerical display unit for numerically displaying the preset bearing marker angle according to the display code signal from said convertion circuit.