Technical comparison: OLS-35 on Russian fighters vs. Sniper, LITENING, ATFLIR, and IRST21 pods. Performance, constraints, effects on missions, and consequences.
Summary
The OLS-35 is the integrated IRST sensor on the Su-35, combining infrared (3–5 µm), TV and laser rangefinder/designator. It passively detects and tracks, with ranges varying depending on the aspect (up to several tens of kilometers), and laser rangefinding of around 20 km (10.8 nm). In contrast, the American Sniper ATP, LITENING, and ATFLIR pods offer large optical apertures, HD generation detectors (up to 1K×1K), multi-axis gyro-stabilized stabilization, and accurate firing coordinates for air-to-ground targets. For long-range passive air-to-air, the US Air Force deploys the IRST21 (Legion Pod), optimized in the long band (8–12 µm) and capable of cooperative triangulation. Key limitations of the OLS-35: smaller aperture, restricted field of view, limited aerial telemetry, and a processing/link chain less geared towards fine geolocation. As a result, in mission, the Su-35 relies more on aircraft sensor fusion and tactics, while Western pods maximize identification and designation at long range.
An overview of the optoelectronic technologies available
Optoelectronic systems for fighter aircraft fall into two categories: integrated IRSTs (such as the OLS-35) and suspended designation pods (Sniper, LITENING, ATFLIR), to which dedicated IRST pods (IRST21/Legion Pod) can be added.
An IRST passively searches for and tracks aerial targets using infrared. Its advantages include discretion (no emissions), immunity to electromagnetic jamming, and sensitivity to low EPR targets. Its physical limitations include dependence on weather (humidity, clouds), thermal gradients, and background/target contrast. A designation pod prioritizes air-to-ground: high-definition FLIR, day camera, rangefinder/designator, automatic tracking, generation of precise geographic coordinates, and video link. The latest American pods also provide useful air-to-air capabilities (detection and visual/IR tracking, identification).
In terms of spectrum, the OLS-35 operates in MWIR (3–5 µm), which is favorable for hot signatures from nozzles and high-contrast targets. The IRST21 favors the LWIR band (8–12 µm), which is more robust against certain atmospheric conditions and more sensitive to “cold” targets against the sky background.
Technical description of the OLS-35 integrated into the Su-35
The OLS-35 is an “optical station” integrated into the front right of the Su-35. It combines:
- an IR channel (MWIR, 3–5 µm) for scanning and imaging;
- a daytime TV channel;
- a laser rangefinder/designator (typical ranges: approx. 20 km on air targets, up to ~30 km on ground targets, depending on size and atmosphere);
- a multi-target tracking computer and an interface with the avionics.
The usual field of view is approximately ±90° in azimuth and +60/-15° in elevation, covering the forward hemisphere. In air-to-air detection, ranges depend heavily on aspect: the rear (afterburner, exposed nozzles) offers a marked gain, while the front reduces thermal contrast. In practice, detections of several tens of kilometers face-to-face are commonly reported, and greater distances from behind when the signature is high. The system can track several targets simultaneously and provide the fine angle for sensor fusion (N035 Irbis-E radar, RWR, links).
Strengths: native integration (no additional drag), optimized mass/energy, permanent availability without occupying a hardpoint. Weaknesses: aperture limited by fairing, field of view fixed by installation, aerial laser telemetry capped (≈ 20 km), hardware upgrade less trivial than a modular pod.
State of the art in American pods: Sniper, LITENING, ATFLIR
The Sniper ATP (Lockheed Martin), LITENING (Northrop Grumman/Rafael), and ATFLIR (Raytheon) pods are 2.2–2.5 m (7.2–8.2 ft) long with large apertures, 2- to 4-axis gimbals, gyrostabilized stabilization, and HD generation sensors. As a rough guide:
- Sniper: length ≈ 2.52 m (98.2 in), diameter ≈ 0.30 m (11.9 in), weight ≈ 202 kg (446 lb).
- LITENING: length ≈ 2.20 m (7.2 ft), diameter 0.406 m (16 in), mass ≈ 208 kg (459 lb).
- ATFLIR: length ≈ 1.83 m (72 in), weight ≈ 191 kg (420 lb), slant range air-to-ground greater than 48–64 km (26–35 nm) depending on altitude.
Recent generations feature 1K×1K MWIR FLIR matrices, super-resolution algorithms, multi-target auto-tracking, an HD day camera, laser designation, a laser spot tracker, and video/data links. They deliver geolocated target coordinates with reduced circular errors, crucial for the use of laser- or GPS-guided munitions, while enabling visual identification at long range. In air-to-air, these pods provide detection/tracking and visual identification (appearance, payloads), although telemetry for BVR firing often relies on radar or multi-platform cooperation.
A US IRST dedicated to air-to-air: IRST21 / Legion Pod
For long-range passive detection of aircraft, the USAF/USN deploys the IRST21 (Legion Pod) in LWIR. The pod (diameter 406 mm, length 2.50 m, < 250 kg) serves as the main passive air-to-air sensor. Its added value: cooperative triangulation thanks to an integrated datalink. Two equipped aircraft share their IR lines of sight to estimate the distance of a target without radar. This is a major advantage: a single IRST knows the angle very precisely, but not the long-range distance (the laser only covers a few tens of kilometers); the fusion of two IRSTs removes this ambiguity, opens up BVR firing, and preserves discretion.
A comparison of capabilities: what the OLS-35 does, what the pods do
Field of view and aperture
An internal sensor such as the OLS-35 mainly sees the front hemisphere. A pod can pivot widely downwards and sideways, cover ground targets in a wide cone, and maintain tracking despite maneuvers. The optical aperture of a pod (large-diameter lenses) captures more photons at low F/, improving effective range and low-contrast identification.
Resolution and image processing
Recent American pods have “1K” FLIR matrices and highly mature video/processing chains (stabilization, noise reduction, multiple trackers, picture-in-picture). The earlier OLS-35 is credited with more modest resolutions. However, in practice, visual identification (ID) of an aircraft at > 30–40 km depends as much on resolution and zoom as on simple thermal detection.
Telemetry and geolocation
The OLS-35 has a rangefinder/designator; on aerial targets, the laser range is around 20 km (10.8 nm). Beyond that, the IRST provides angles, but the range remains ambiguous without assistance (radar, cooperative). American pods generate precise air-to-ground coordinates (INS correction, boresight, georeferencing). For passive air-to-air beyond 40–60 km, IRST21 takes the advantage via multi-platform triangulation.
Integration and modularity
A pod can be replaced, upgraded, and maintained independently of the aircraft. The LITENING G4/SE and Sniper “Advanced” versions show a trajectory of continuous improvement (HD sensors, links, detection AI). An integrated sensor such as the OLS-35 benefits from low drag and permanent availability, but evolves more slowly in terms of hardware: any upgrade requires work on the aircraft.
Concrete limitations of the OLS-35 in mission
Earlier or later identification?
During interception, the Su-35 can thermally “lock on” to a target at useful ranges, especially from behind. But positive visual identification (to avoid friendly fire and confirm the type) requires detail. American HD pods obtain this ID earlier, thanks to their optics and fine stabilization at high zoom factors. On a high-altitude alert departure, the difference translates into a few dozen seconds gained, which are precious for ROE decisions.
Passive BVR firing, yes, but under certain conditions
Beyond 40–60 km, if radar silence is to be maintained, an IRST must estimate the distance. The OLS-35 struggles to do this on its own outside laser range. The Su-35 can certainly correlate with its radar measurements in brief “probe shots,” but loses some of the benefit of discretion. IRST21, on the other hand, performs a collaborative passive measurement. As a result, an equipped F-15C/EX can engage more confidently without continuous radar emissions.
Air-to-ground penetration: designate and survive
The OLS-35 can designate ground targets with a laser. However, for detailed reconnaissance, damage assessment, or dynamic attack (CAS) at long range, US pods remain more effective (HD TV/IR, high zoom, two-way video links, precision coordinates). The operational consequence is simple: a Su-35 can deliver guided weapons, but an F-16/F-15E equipped with Sniper/LITENING can identify, assign, and strike multiple targets further away from the threat.
Physical and industrial factors that should not be overlooked
IR performance depends on weather conditions (water vapor, aerosols), background, aperture size, detector quality (HgCdTe, T2SL), cryogenic coolers, and real-time processing. American manufacturers have pushed the limits of cooled 640×512 and then 1024×1024 matrices and vibration/acceleration resistance. Access constraints to cutting-edge components also play a role: the availability of high-performance MWIR/LWIR detectors, reliable cryogenic coolers, and computers can determine the pace of development of an integrated system.
A “mission” analysis: what this means for employment
On the Su-35, the OLS-35 is a useful sensor: passive surveillance, multi-target tracking, opportunistic laser designation, support for sensor fusion. But its limitations are as follows:
- in air policing, visual ID comes later, resulting in shorter firing windows under strict rules of engagement;
- in passive BVR, the “confident firing” range without radar is reduced, except with external support;
- in air-to-ground, very long-range imaging/coordination is less rich than with a modern Western pod.
For Western fighters, pods provide:
- long-range reconnaissance/identification with stable metrics (coordinates, video quality);
- cross-platform collaboration (links, video, IRST21 tri-points) improving passive long range;
- a “multi-mission” capability (CAS, ISR, opportunistic air-to-air) without changing sensors.
A word on survivability and maintenance
An integrated sensor adds no drag or payload signature, which is an advantage in terms of fuel consumption and speed. A pod, on the other hand, occupies one point, weighs ≈ 190–210 kg, and generates drag; but it can be quickly removed, replaced, and upgraded. Operationally, this modularity allows the latest version of sensors/software to be deployed without immobilizing the fleet. The OLS-35 is progressing via aircraft standards (e.g., Su-35 avionics upgrades), at a slower pace.
A likely trajectory of evolution
On the Russian side, the next generation (OLS-50M of the Su-57) suggests the arrival of more powerful detectors, distributed antennas, and better network integration. On the Western side, pods are becoming “sensor platforms”: insertion of IRST, compact SAR, dedicated data links, AI-assisted onboard processing for detection/identification. The race will focus as much on software and sensor fusion as on the sensors themselves.
Compared to American pods, the OLS-35 provides real services but has structural limitations: reduced aperture and field of view, later ID, limited aerial telemetry beyond 20 km, and less modularity. The American forces compensate for the cost/drag of the pods with superior imaging and geolocation, and with a collaborative LWIR IRST for discreet air-to-air operations. Tactically, this translates into more options: earlier identification, more confident passive firing, and richer inter-platform coordination. Technology isn’t everything, but here it clearly shifts the balance in favor of podded and cooperative optronic chains.
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