Assessing Joint Strike Fighter Defence Penetration Capabilities

US DoD Band Allocation Chart

Spectral Range - Electronic Threat Environment 2008

Since 1991 most new Russian radar designs have been in the L-band and VHF-band, with the Cold War era UHF band designs re-engineered and digitised. Significantly the L-band designs are capable of tracking -20 dBSM RCS targets in the 50 - 70 NMI range window, and -30 dBSM targets in the 40 - 50 NMI range window. The "shoot and scoot" LEMZ 96L6E Cheese Board is used in the SA-21 system and is an option for the SA-20 system. The 67N6E and 59N6E are available as options for the SA-21 system. Performance curves based on Russian datasheets.

Russian S-band acquisition radars used with SAM systems. While these are mostly legacy Cold War designs, some remain in production at this time. The 36D6/ST-68U/UM Tin Shield family of radars remains widely used and variants are deployed with SA-10 and SA-20 SAM batteries for long range acquisition. Estimated performance for the SA-20B 64N6E2 Big Bird is based on increased 48N6E2 missile range compared to the 48N6 used with the 64N6/64N6E, a shorter detection range would not be compatible with the weapon. Performance curves based on Russian datasheets.

Most Russian SAM engagement radars operate in the X-band. There is a clear disparity between power aperture requirements for the medium range SAM systems (SA-3, SA-10A, SA-12) and the newer long range SAM systems (SA-20, SA-21). Estimated performance for the SA-21 92N2E Grave Stone and SA-20 30N6E1 Tomb Stone is based on increased 48N6E1/2/3 missile range compared to the 5V55K used with the 30N6, a shorter detection range would not be compatible with the weapon. Performance curves based on Russian datasheets.

VHF band radars present a major risk for fighter sized VLO and LO aircraft, as virtually all airframe shaping becomes ineffective or at best partly effective in this band, in addition to problems arising with absorbent material skin depth at ~100 MHz. Numerous Russian source cite an RCS in the VHF band for the F-117A Nighthawk at 0.5 m², a figure which would be similar for the Joint Strike Fighter. Recent designs such as the Nebo SVU include solid state AESA technology and provide comparable angle and range accuracy to S-band acquisition radars. Performance curves based on Russian datasheets.

Provisional data - Russian sources.

Provisional data - Russian sources

S-125 / SA-3 and S-125-2T Pechora 2T block upgrade firing trial results. The Pechora 2T is a characteristic of contemporary digital block upgrades to widely used Soviet era SAMs. The improved autopilot algorithm significantly extends the engagement envelope of the weapon system. The best range achieved was 16 NMI. Provisional data - Tetraedr JSC.

The Radar Cross Section (RCS) performance of the lower fuselage section and nozzle was assessed at three important frequencies of interest 1 GHz (L-band), 4 GHz (S-band) and 10 GHz (X-band), as these present the most likely bands in which a range of legacy and contemporary acquisition and engagement radars operate. Refer Annex A.

Modelling was performed using the POFacets 3.1 simulator which produces optimistic results as the simulator does not include edge diffraction effects and surface traveling wave effects. The actual and measurable Radar Cross Section would be higher due to impact of shaping features excluded in the model, and the contributions not predicted by the simulator. A calibration check was performed to validate simulator results against published test data, using Fig 6-3 at 5.8 GHz, and Fig 6-17 at 6 GHz from Knott, Schaeffer and Tuley, 2nd and 1st Editions respectively. The simulator yielded close agreement with measurement. Other shapes specifically applicable to this problem were also modelled, with consistent agreement against published measurements.

Because the simulator uses the physical optics model for computing the target cross section, shapes must be subdivided into triangular facets. The shape models designed and implemented for both the nozzle and lower fuselage were constructed with fidelity in mind, the aim being to avoid unwanted quantisation errors.

The strategy adopted in modelling was to look at a number of operationally and tactically critical azimuth angles from which a threat might illuminate the aircraft in the bands of interest, and within each of these azimuth angles the simulation covers the full range of elevations of interest. The 'traditional' and popular model in which a polar plot of RCS is produced within the horizontal plane of the aircraft might present nicely and be easy to interpret, but it is not very useful if we need to consider real world threats which may appear at a wide range of elevation angles relative to the aircraft in flight.

The diagram below shows the cardinal depression angles for an aircraft at the tropopause, accounting for the curvature of the earth and atmospheric refractive effects which 'bend' the ray path between the aircraft and threat radar. The specific angles in this diagram were determined using Russian specifications for missile kinematic range, the SBF refractive model for short ranges, and an exponential CRPL refractive model for ranges in excess of 100 nautical miles. It is important to observe that in straight and level flight at medium and high altitudes all surface based threats are firmly in the lower hemisphere, putting a premium on low Radar Cross Section in the angular range between 3.7° and 36.5° , as area defence missile systems and associated acquisition radars will illuminate the aircraft within this angular range.

Threat depression angles as a function of type and missile kinematic range.

Equally so, careful consideration was given to the choice of azimuth angles for modelling.

Lower Centre Fuselage Model

The Joint Strike Fighter lower centre fuselage area represents an area of major concern in the aircraft’s overall radar cross section, as a result of a series of design changes introduced during the SDD program.

These result primarily from the extension, downward, of the two outboard main weapon bays, and the use of additional symmetrical blended fairings on the lower fuselage, parallel and toward the aft and centre of the weapon bays. Both of these design features destroy the essentially flat lower fuselage geometry demonstrated in the X-35 prototypes.

Modelling was performed to determine the order of magnitude Radar Cross Section contribution of this part of the airframe shape, with a specific focus on vulnerabilities arising to threats in the beam sectors of the Joint Strike Fighter aircraft.The solid shape model for this portion of the design is optimistic in its Radar Cross section prediction. This is because it excludes the doubly curved surfaces at the front and aft ends of the weapon bays and the fuselage fairings. The absolute and relative dimensional accuracy of the fuselage section employed is better than 3%, and original photographic imagery was employed to generate this section.

Wireframe rendering of the solid model for JSF lower fuselage geometry employed for RCS modelling. This model accurately represents the complex singly curved section of the lower centre fuselage, but does not represent the longitudinal taper or the problematic doubly curved shapes at the weapon bay and ventral blister transitions. The model was produced by digitising a section from a photograph and after scaling, extending the section into a solid using a custom C language program (Author).

Lower Centre Fuselage Analysis

The results of modelling the RCS performance of the lower centre fuselage are entirely consistent with 'rule of thumb' RCS estimation techniques which predict very poor performance in bands where the shaping features no longer produce good effect. This is evident in the results for the L-band and S-band, with performance generally improving in the X-band, and best performance achieved in the Ku-band.

Angular dependencies are interesting. In terms of depression angles the best performance was achieved generally for distant threats, as the distance is reduced RCS grows strongly due to the steeper depression angles and increasing exposure of the contoured lower fuselage to impinging illumination. The fuselage slab sides and chines, copied from the F-22 design, are the principal reason for good upper band performance at shallow depression angles.

In terms of azimuths, best performance was generally achieved for threats in the forward quarter of the aircraft, with the RCS contribution of the lower centre fuselage consistent with public claims of -30 dBSM RCS performance in the X-band. That the simulation yields such consistency with publicly disclosed RCS performance data demonstrates that it performed accurately.

The most problematic aspects for the lower centre fuselage are those where the contoured lower surface is exposed, especially if the azimuth of illumination is close to the broadside beam aspect of the aircraft. This is especially prominent for angles of ±10° to ±20° off the beam, where average RCS values of up to 0 dBSM are observed in the lower bands.

In conclusion, of the 48 angular sectors assessed in four bands, only three yielded safe RCS performance, with 58 percent of these angular/frequency extents yielding poor or very poor results.

At 1 GHz where newer Russian acquisition radars such as the 96L6 Cheese Board (SA-21 and optional for SA-20) the JSF is in difficulty even at its best performing beam sector lower hemisphere aspects. Peak RCS exceeds a square metre and the skin depth will present problems for absorbent coatings and laminates.

The JSF was optimised to perform best in the X-band (above) and Ku-band, the intent being to frustrate engagement radars associated with mobile battlefield point defence weapons. While its lower hemisphere signature is better in these bands relative to the L-band and S-band, medium and long range SAM engagement radars waiting in ambush and engaging from shorter ranges will be able to detect dense peaks produced by the contoured lower fuselage. Turning away from the threat will significantly increase the observable RCS - the bank angle adds to the depression angle relative to the threat (below).

Axisymmetric Nozzle Model

The Joint Strike Fighter axisymmetric exhaust nozzle represents an area of major concern in the aircraft’s overall radar cross section, and arises due to the use of this specific class of nozzle geometry in preference to the slit geometry models employed in all other US stealth designs.

The nozzle and aft fuselage interface designs employ three distinct shaping features to reduce external shaping Radar Cross Section. These are a serrated trailing edge, a serrated fuselage fairing, and very low curvature nozzle petals.

The model employed assumes the internal tailpipe cavity is a perfect absorber and makes no significant contribution to the performance of the nozzle, ie the model represents specular returns from the outer surface of the petals and nozzle aperture rim.

Wireframe rendering of the solid model for JSF nozzle geometry employed for RCS modelling. This model accurately represents the complex nozzle shape, but does not represent the problematic singly curved surface of the petal exterior. The model was produced using a custom C language program with cardinal dimensions scaled from photographs and public US DoD line drawings (Author).

JSF Axisymmetric Nozzle RCS Model L-band @ 1.0 GHz - Peak/Average RCS [dBSM] in Angular Range V-pol
Depression Angle / Angle Off Beam	45°	60°	80°	90° (Tail)
Long Range SAMs [-3.6° to -20°]	-13.0/-17.0	-8.0/-13.0	-8.0/-15.0	-4.0/-15.0
Medium Range SAMs [-20° to -36°]	-4.0/-7.0	-8.0/-13.0	-11.0/-16.0	-12.0/-17.0
Short Range SAMs [-36° to -60°]	+8.0/-2.0	+5.0/-8.0	-2.0/-12.0	-5.0/-12.0

JSF Axisymmetric Nozzle RCS Model S-band @ 3.0 GHz - Peak/Average RCS [dBSM] in Angular Range V-pol
Depression Angle / Angle Off Beam	45°	60°	80°	90° (Tail)
Long Range SAMs [-3.6° to -20°]	-10.0/-14.0	-13.0/-17.0	-12.0/-17.0	+1.0/-17.0
Medium Range SAMs [-20° to -36°]	-7.0/-14.0	-12.0/-17.0	-13.0/-17.0	-17.0/-22.0
Short Range SAMs [-36° to -60°]	+13.0/-3.0	0.0/-12.0	-8.0/-18.0	-6.0/-18.0

JSF Axisymmetric Nozzle RCS Model X-band @ 8.0 GHz - Peak/Average RCS [dBSM] in Angular Range V-pol
Depression Angle / Angle Off Beam	45°	60°	80°	90° (Tail)
Long Range SAMs [-3.6° to -20°]	-12.0/-17.0	-12.0/-22.0	-15.0/-22.0	+1.0/-22.0
Medium Range SAMs [-20° to -36°]	-7.0/-17.0	-13.0/-22.0	-12.0/-22.0	-20.0/-25.0
Short Range SAMs [-36° to -60°]	+12.0/-6.0	0.0/-12.0	-12.0/-18.0	-15.0/-18.0

JSF Axisymmetric Nozzle RCS Model Ku-band @ 16.0 GHz - Peak/Average RCS [dBSM] in Angular Range V-pol
Depression Angle / Angle Off Beam	45°	60°	80°	90° (Tail)
Long Range SAMs [-3.6° to -20°]	-12.0/-17.0	-17.0/-25.0	-19.0/-25.0	+1.0/-30.0
Medium Range SAMs [-20° to -36°]	-9.0/-17.0	-12.0/-22.0	-13.0/-23.0	-17.0/-25.0
Short Range SAMs [-36° to -60°]	+14.0/-10.0	-5.0/-15.0	-6.0/-18.0	-10.0/-25.0

Colour Coding (Average RCS Only)	Safe (<-30 dBSM)	Borderline (>-30 dBSM)	Vulnerable (>-20 dBSM)
Notes:	The table shows simulated RCS contributions by the axisymmetric nozzle for four representative bands. The actual RCS at these aspect angles will be higher due to the contributions of the double curved surfaces not included in the model, backscatter from the tailpipe cavity, plus other airframe shaping features. Colour coding is based on average RCS as this magnitude guarantees a reliable track by a radar, and is very conservative. In some instances the peaks are dense enough to permit tracking. The -30 dBSM threshold for "safe" is based on the assumption that application of an absorbent coating or laminate will reduce RCS by a further 10 dB or more thus driving the RCS contribution of the lower fuselage into the VLO or "genuine stealth" category.

Axisymmetric Nozzle Analysis

The results of modelling the RCS performance of the nozzle are entirely consistent with 'rule of thumb' RCS estimation techniques which predict very poor performance in bands where the serrated trailing edge is no longer effective. This is evident in the results for the L-band and S-band, with performance generally improving in the X-band, and best performance achieved in the Ku-band.

Angular dependencies are also of much interest. In terms of depression angles the best performance was achieved generally for distant threats, as the distance is reduced RCS grows strongly due to the steeper depression angles. In terms of azimuths best performance was generally achieved for threats directly aft of the aircraft, but with the caveat that a narrow cone centred on the axis of the nozzle exhibits an RCS of around 1 square metre for the upper three bands, which is also consistent with 'rule of thumb' estimators for nozzles.

The most problematic aspects for the nozzle are those in close proximity to the normal to the nozzle taper angle, which result in large specular returns over an angular extent of up to 10°, observed in all bands and with all azimuths.

In conclusion, of the 48 angular sectors assessed in four bands, only one yielded safe RCS performance, with 73 percent of these angular/frequency extents yielding poor or very poor results.

Best case performance was observed in the Ku-band for threats directly behind the aircraft, with the caveat that a single lobe with a 1 square metre RCS is seen about the axis of the nozzle.

Worst case performance was observed in the L-band and S-band, as theory predicts. While the ratio of wavelength to nozzle dimensions was borderline for the L-band simulation, the results were sufficiently similar to the S-band to be considered representative. Peak RCS in the critical -30° depression angle sector is of the order of -10 dBSM.

Imagery Sources: Author; www.jsf.mil, US DoD.

Air Power Australia Analyses ISSN 1832-2433

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Annex A Threat Operating Bands

Annex B Engagement Envelopes - Representative Threats

Annex C Joint Strike Fighter Lower Fuselage and Nozzle RCS Modelling

Lower Centre Fuselage Model

Lower Centre Fuselage Analysis

Axisymmetric Nozzle Model

Axisymmetric Nozzle Analysis