Eight Reasons Why Saab Has Chosen Line Of Sight (LOS) Guidance For The RBS 70 NG System

On the short range GBAD missile system market today there are two major guidance principles used. One of them is Proportional Navigation (PN), used for homing systems with Infrared (IR) or radar (RR) seekers. The other one is Line Of Sight guidance (LOS) used for Beam Riding and Command to Line Of Sight (CLOS) systems. What guidance system has Saab chosen for its RBS 70 NG system and why?

For short range GBAD (up to 20 km), Saab has chosen the LOS principle. But why? Below, Dr Bo Almqvist, Director Strategic Business at Saabs Business Area Dynamics, compares the different guidance principles and explains why LOS is the best solution for a short range GBAD system. Systems with a range of less than 10 km are often Man-Portable and will below be called MANPADS (Man Portable Air Defence System), while systems with a range up to 20 km will be called SR-GBADS.

1. Reaction time

Lock on is a procedure which takes time and is necessary for IR homing MANPADS. LOS guidance will have shorter reaction times since lock on, or rather the start of the auto tracker, does not need to take place before launch, or at all. As soon as a target is detected by the operator the missile can be fired. Furthermore the ground system is more sophisticated than a missile seeker and can detect a target faster and at longer range.

A SR-GBADS with PN guidance can be designed to have a lock on after launch and in those cases there is no difference between LOS systems and Homing systems. If the SR-GBADS are vertically launched it might take time to get the missile on a trajectory towards the target and the reaction time will be longer compared to slant launch. However, this is not directly connected to the guidance principle.

2. Target manoeuvre

As soon as the target starts to manoeuver, the PN guidance needs more endurance. The reason for this is the guidance principle itself, where the seeker is locked on to the target and the missile is travelling towards a predicted intercept point. When the target makes a manoeuvre, the missile must change course and make a manoeuvre that is three to four times as large as that of the target, to be able to guide the missile to a new predicted intercept point. This means that if the target makes a manoeuvre of typically 8-9g to evade the approaching threat, the missile has to make a manoeuvre of 24-30g. This requires a constant high manoeuvrability by the missile throughout the entire engagement, since the target will continue to make evasive manoeuvres until the threat is eliminated. Every pilot knows that they stand a good chance of outmanoeuvring a PN missile - i.e. as soon as a warning is given that the aircraft is engaged by a missile, the pilot can start to manoeuvre.

3. Fire and Forget and multiple target capability

A PN MANPAD is used for achieving Fire and Forget capability. This means that the operator can leave the position immediately after firing or fire the next missile while the operator of a LOS system needs to stay until the missile hits the target. Due to very short time it takes to intercept for MANPADS there is no practical difference in this sense and reloading might be the time consuming part anyway.

For SR-GBADS there is no practical difference between the two guidance principles when it comes to re-deployment, but there is an advantage for PN systems in having several missiles in the air. Due to the short time it takes to intercept SR-GBADS, this advantage will only come to full effect for GBADS with ranges over 20 km. For some CLOS systems a multi firing is a possibility.

4. Aiming Point Correction

There is always a possibility with LOS systems to adjust the aiming point within the target (e.g. cockpit or tail) or change target after launch. This can be done either automatically or by support from the operator. Similar changes are not always possible with a homing system, which in such cases has to be done automatically without operator support.

5. Low level targets and clutter resistance

For a MANPADS any target close to or on the battlefield, and that can be seen by the ground station, can be engaged. There is a limited possibility for clutter to prevent a missile launch for LOS systems, while homing systems might have problems with discriminating clutter on the battlefield or in the background, both from heat sources and radar reflections 

6. Active jamming

Active jamming of GBAD missiles is mainly to prevent homing missiles to reach the target. This can be all types of jamming, Directional Infrared Counter Measures (DIRCM) or flares and chaff. For a LOS system the communication sensor is in the rear end of the missile and it is more or less impossible for the target to affect the sensor in any way, while for a homing system the sensor is pointing at the target and it is wide open for jamming while approaching the target.

7. Self-Destruction Command

The LOS systems can destroy the missile during the engagement phase if needed, while a homing system without up-link will continue to the target after lock on. This is why LOS is always chosen for event protection.

8. Missile Price

With a costly seeker in the missile it will become more expensive than a system where the costly parts belongs to the ground based equipment and can be re-used.