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There is a collision between requirements to maintain radar's coherence and provide radar's jamming immunity. To resolve this collision, radar shall be made operating coherently and at the same time in the frequency agile mode. This paper offers a method to enable radar's coherent and frequency-agile operation.

 

I. INTRODUCTION

Shipboard radars are now widely used for navigation, environment monitoring and detecting targets at sea. Most of them operate in X and S frequency bands. However, the progress in radar engineering is closely followed by improvements in radar countermeasures. Therefore, there is a need to keep enhancing the jamming immunity of radars. One of ways to achieve this goal is the use of outgoing signal carrier frequency pulse-to-pulse agility. The signal carrier frequency agility (i) prevents from spurious pulses, which could otherwise come into adjacent probing period due to super refraction, and (ii) complicates radar countermeasures thanks to extension of the frequency range the radar is to be searched in, therefore, time of the search considerably increases and efficiency of repeater jamming reduces. Besides outgoing signal carrier frequency agility, coherent integration of radar signals is required for subsequent Doppler filtering and discriminating the moving targets.

 

II. RADAR STRUCTURE AND OPERATIONAL PECULIARITIES

The main functional assembly that may enable radar's coherent operation with frequency agility is a digital signal synthesizer (hereinafter referred to as DSS). To determine requirements to DSS, we will review the functional diagram of the synthesizer as part of a radar as shown in Figure 1.


Fig. 1


The DSS has two channels which shape signal UOUT at the carrier frequency ƒOUT and signal ULO1 at the local oscillator frequency ƒLO1, respectively; these signals are spaced by the intermediate frequency ƒLO2 value. Signal ULO1 is shaped at the output of direct digital synthesizer AD9914. Once multiplied by 4 and subsequently filtered, signal ULO1 comes to the inputs of the quadrature modulator and the reception path mixer. Modulating signal UIQ is shaped by direct digital synthesizer AD9910, subsequently filtered and amplified, and finally fed to the quadrature modulator as a modulating signal. The quadrature modulator uses signal UIQ to modulate the carrier frequency. The main advantage of this approach to signal shaping is that an initial phase of signal UOUT can be set independently.

Once amplified, outgoing pulse UOUT is emitted by the antenna assembly. The incoming echo signal is amplified in the reception path and mixed with local oscillator signal ULO1 in the mixer resulting in signal UIF at intermediate frequency ƒIF, which differs from signal ULO2 by phase only. Phase difference ∆φ between signals UIF and ULO2 is found out in the phase detector of the digital signal processor.

When the radar operates at a constant carrier frequency, its coherence is provided by continuity of signals UIQ and ULO1. Here, change in phase difference ∆φ will be caused by a Doppler frequency present in the incoming signal and by signal modulation.

When outgoing pulse frequency is changed, frequency ƒLO1 of signal ULO1 is changed. Signal UIQ from synthesizer AD9910 acts as a modulating signal; its frequency does not change and integrity (continuity) is maintained. Whenever frequency is changed, value ∆φ will change according to the radar's phase-frequency response. The phase-frequency response may be arbitrary depending on the radar peculiarities. A possible phase-frequency response is shown in Figure 2.


Fig. 2

The shaped signals UIQ and ULO coming to the quadrature modulator's input can be generally represented as follows:

where φRPLO1 is a random initial phase of signal ULO1 resulting from change in frequency ƒLO1; παt2 - is a signal UIQ frequency modulation function; φIQ is an initial phase of signal UIQ as set in synthesizer AD9910; A(t) is a signal UIQ amplitude modulation function.

Signal UOUT shaped at the quadrature modulator's output may be represented as production of signals ULO and UIQ:

After signal UOUT has traveled through the space to a reflecting target and back, the phase of incoming signal UIN is shifted by values φD and φSRP, which depend on the range and on the phase-frequency response of the radar's transmission and reception paths, respectively. Once amplified in the reception path, signal UIN comes to the down mixer's input, and signal ULO1 is fed to the down mixer's another input.

The mixing of signals UIN and ULO1 results in signal UIF shaped in the reception channel's mixer; once amplified and filtered, this signal comes to the input of the digital signal processor (which includes a phase detector), while signal ULO2 comes to another input of the digital signal processor. The signal processor finds out phase difference Δφ. Signals UIN, UIF and Δφ are defined by the following expressions:

where φRP is signal UIF's generalized random phase equal to the sum of random phases (φRPSRPLO1RPLO2). Terms φRPLO1 and φRPLO2 are random phases of signal ULO at the input of the quadrature modulator and of the down mixer, respectively. For the sake of convenience, we will further consider generalized random phase φRP of signal UIF which depends on the radar's phase-frequency response.

When using the proposed functional diagram of the DSS, random phases φRPLO1 and φRPLO2 are compensated.

If signal UIF features random initial phase φRP in each sensing period, it makes coherent integration and further processing of such a signal impossible, when the radar operates in the frequency agile mode.

 

III. RADAR PHASING

To enable coherent operation of radar in the frequency agile mode, it is proposed to phase the radar in order to compensate for random initial phase of signal UIF whenever the frequency is changed. The primal problem of signal line and local oscillator line phasing is to determine random phase φRP of the signal for various frequencies and subsequently compensate for this phase in signal UIQ. Phasing shall be carried out after the radar has been installed on the platform and adjusted.

To phase the radar, it is necessary to have a well reflecting target present within the radar's direct visibility range. The phasing shall be carried out with the radar platform and the reflective target as immovable as possible. If the above conditions could not be satisfied, the radar may be phased before it is installed on the platform using a certified reflecting delay line to be connected to the radar in place of the antenna assembly.

The phasing shall be carried out with no frequency modulation in signal UIQ. The radar phasing process includes sequential irradiation of the reference reflecting target at all the frequency points. Based on the response received from the reference target, Δφ value is estimated for radar's each frequency point.

With no frequency modulation and no relative motion of the radar and reflecting target, signal Δφ (6) shaped by the phase detector will be defined as:

Adjusting initial phase φIQ of signal UIQ changes Δφ, and φIQ corresponding to the minimum Δφ is stored in FPGA for each frequency point. Thus, an array of UIQ's initial phases φIQi for each frequency point is populated.

Then in the course of normal operation of the radar in the frequency agile mode, the signal shaper generated signal UIQ is admixed with respective compensatory phase value from array of φIQi. This value faithfully follows radar's function φRPS shown in Figure 2. As a result of phasing, expression (5) for signal UIF will be as follows:

where Δφi is a phasing error at i-th frequency.

Phasing error Δφ normally should not exceed an initial phase φIQ adjustment increment, which, in turn, depends on capabilities of FPGA and synthesizer AD9910 and does not exceed several degrees. Such phasing accuracy enables coherent integration of signal UIF in the frequency agility mode.

It shall be noted that initial phase compensation φIQi shall be admixed to signal UIQ between the probing periods, i.e. before local oscillator signal frequency ƒLO1 is changed. Then, based on the probing period starting strobe, frequency modulation and amplification of the signal take place. This condition is required to have transient processes completed before the outgoing pulse shaping starts. Time necessary for completing a transient process is determined individually for each radar.

IV. SUMMARY

The described method of phasing radar's signal channel and local oscillator channel enables coherent operation of the radar in the frequency agile mode. It makes it possible to significantly increase the radar immunity and to suppress target signals that may be caused by super refraction thanks to unambiguous frequency correspondence between outgoing and incoming pulses.

Now the AstroSoft company is developing a hardware-and-software simulation system for testing radars to refine and verify various radar design/upgrade solutions.

REFERENCES

[1] M. Skolnik “Radar handbook”, 2nd ed, 1990

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