Modem warfare often involves enemy troops located close to civilian population and to friendly troops. While it is desirable to engage the enemy troops and enemy sites, care must be used to minimize or eliminate unintentional engagement of friendly troops and/or collateral damage.
In modern warfare the targeting of enemy sites is typically focused on the increasing probability of munitions hitting the desired target, typically with means to improve overall weapon accuracy. Certain countries or groups of people place air defense systems and other military significant systems near buildings such as hospitals, schools or places of religious worship (e.g. churches, temples or mosques) in hope that an attempted targeting of the military significant systems will be tempered by the desire not to hurt civilians in the hospitals, schools or places of religious worship or to harm the buildings themselves.
Present day munitions used in warfare are increasingly Precision Guided Munitions (PGMs). A “PGM” is a munition with sensors that allow it to know where it is and actuators that allow the munition to guide itself towards an intended target. The PGM's guidance system provides a generally accurate target area for the munitions to strike. These munitions target an aim point. The aim point has an area around it referred to as the Circular Error Probable (CEP). The CEP defines an area about an aim point for a munition wherein approximately fifty percent of the munitions aimed at the aim point of the target will strike. While fifty percent of the munitions will strike within the CEP area, the remaining fifty percent will strike outside the CEP area, in some cases potentially very far away. It is munitions that strike away from the intended target that result in unintentional engagement of friendly troops or friendly sites or provide collateral damage to civilians and civilian structures.
One system used to provide guidance of a PGM is known as a Laser Guidance System (LGS) used with Laser Guided Bombs (LGBs). In use, a LGB maintains a flight path established by the delivery aircraft. The LGB attempts to align itself with a target that is illuminated by a laser. The laser may be located on the delivery aircraft, on another aircraft or on the ground. When alignment occurs between the LGB and the laser, the reflected laser energy is received by a detector of the LGB and is used to center the LGB flight path on the target.
Another type of PGM is known as an Inertial Guided Munition (IGM). The IGM utilizes an inertial guidance system (IGS) to guide the munition to the intended target. This IGS uses a gyroscope and accelerometer to maintain the predetermined course to the target.
Still another type of PGM is referred to as Seeker Guided Munitions (SGMs). The SGMs attempt to determine a target with either a television or an imaging infrared seeker and a data link. The seeker subsystem of the SGM provides the launch aircraft with a visual presentation of the target as seen from the munition. During munition flight, this presentation is transmitted by the data-link system to the aircraft cockpit monitor. The SGM can be either locked onto the target before or after launch for automatic munition guidance. As the target comes into view, the SGM locks onto the target.
Another navigation system used for PGMs is known as a Global Positioning System (GPS). GPS is well known to those in the aviation field for guiding aircraft. GPS is a satellite navigation system that provides coded satellite signals that are processed by a GPS receiver and enable the receiver to determine position, velocity and time. Generally four satellite signals are used to compute position in three dimensions and a time offset in the receiver clock. A GPS satellite navigation system has three segments: a space segment, a control segment and a user segment.
The GPS space segment is comprised of a group of GPS satellites, known as the GPS Operations Constellation. A total of 24 satellites (plus spares) comprise the constellation, with the orbit altitude of each satellite selected such that the satellites repeat the same ground track and configuration over any point each 24 hours. There are six orbital planes with four satellites in each plane. The planes are equally spaced apart (60 degrees between each plane). The constellation provides between five and eight satellites visible from any point on the earth, at any one time.
The GPS control segment comprises a system of tracking stations located around the world. These stations measure signals from the GPS satellites and incorporate these signals into orbital models for each satellite. The models compute precise orbital data (ephemeris) and clock corrections for each satellite. A master control station uploads the ephemeris data and clock data to the satellites. The satellites then send subsets of the orbital ephemeris data to GPS receivers via radio signals.
The GPS user segment comprises the GPS receivers. GPS receivers convert the satellite signals into position, velocity and time estimates. Four satellites are required to compute the X, Y, Z positions and the time. Position in the X, Y and Z dimensions are converted within the receiver to geodetic latitude, longitude and height. Velocity is computed from change in position over time and the satellite Doppler frequencies. Time is computed in satellite time and GPS time. Satellite time is maintained by each satellite. Each satellite contains four atomic clocks that are monitored by the ground control stations and maintained to within one millisecond of GPS time.
Each satellite transmits two microwave carrier signals. The first carrier signal carries the navigation message and code signals. The second carrier signal is used to measure the ionospheric delay by Precise Positioning Service (PPS) equipped receivers. The GPS navigation message comprises a 50 Hz signal that includes data bits that describe the GPS satellite orbits, clock corrections and other system parameters. Additional carriers, codes and signals are expected to be added to provide increased accuracy and integrity.
A system used to provide even greater accuracy for GPS systems used in navigation applications is known as a Space Based Augmentation System. One type of SBAS is known as a Wide Area Augmentation System (WAAS). WAAS is a system of satellites and ground stations that provide GPS signal correction to provide greater position accuracy. WAAS is comprised of approximately 25 ground reference stations that monitor GPS satellite data. Two master stations collect data from the reference stations and produce a GPS correction message. The correction message corrects for GPS satellite orbit and clock drift and for signal delays caused by the atmosphere and ionosphere. The corrected message is broadcast through one of the WAAS geostationary satellites and can be read by a WAAS-enabled GPS receiver.
Some PGMs combine multiple types of guidance. For example, the Joint Direct Attack Munition (JDAM) uses GPS, but includes inertial guidance, which it uses to continue an engagement if the GPS signal becomes jammed.
A drawback associated with all these types of PGMs is the unintentional engagement of friendly or neutral targets. While LGBs have proven effective, a variety of factors such as sensor alignment, control system malfunction, smoke, dust, debris, and weather conditions can result in the LGB not hitting the desired target. SGMs may be confused by decoys. The image obtained by the SGM may be distorted by weather or battle conditions such as smoke and debris and result in the SGM not being able to lock onto the target. There are several areas where GPS errors can occur. Noise in the signals can cause GPS errors. Satellite clock errors, which are not corrected by the control station, can result in GPS errors. Ephemeris data errors can also occur. Tropospheric delays (due to changes in temperature, pressure and humidity associated with weather changes) can cause GPS errors. Ionospheric delays can cause errors. Multipath errors, caused by reflected signals from surfaces near the receiver that either interfere with or are mistaken for the signal, can also lead to GPS errors.
Despite the accuracy provided by LGBs, IGMs, SGMs, and GPR-based munitions the PGMs still occasionally inadvertently engage at or near friendly troops, sites, civilians or important collateral targets. This may be due to other factors as well, such as target position uncertainties, sensor errors, map registration errors and the like. This problem is increasingly important, both because domestic and world opinion is becoming increasingly sensitive to friendly fire and collateral damage, and because adversaries are more frequently deliberately placing legitimate military targets near potential targets of substantial collateral damage.