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600px-Aliens Sentry

The UA 571-C is a man-portable automatic perimeter defense system currently deployed by the US Army and the Colonial Marine Corps. A remote sentry unit weighs in at 19.6 kg and can be assembled in under 150 seconds. The major components consist of a snap-open tripod mounting, battery pack, breech and barrel assembly, sensor unit and 500 round ammunition drum. A microwave datalink connects the sentry to a remote command console. Once in place, the system may be set to sweep wide arcs up to 360 degrees (subject to terrain and emplacement restrictions), though doctrine is set to several sentries to cover smaller, interlocking fields of fire on a narrow frontage. The UA 571-C mounts a pulse action machine gun, using the M250 10 mm x 28 HEAP round. The weapon has a cyclic rate of 1100 rpm and is air cooled, with an automatic cut-out preventing the loading of any more rounds into the breech should overheating cause a chance of ammunition 'cook-off'.

The sentry unit's sensor array is mounted above the barrel, aligned to cover a 60 degree cone in front of the weapon. This sensing suite consists of a cooled infrared detector in the 3-5 um and 8-13 um band, ambient light optics, an ultrasonic motion tracker and LIDAR (Laser Imaging Detection And Ranging). If a target’s visual or thermal profile is known, the system may be set just to monitor those specific targets using infrared or optical. However, the system is usually set to multi-spectral mode, where the sentry's software cross-correlates received data from the different sensors to obtain a full target profile.

If the sentry is set to 'auto-remote', it will interrogate all targets in its sensor cone, using an Identification Friend Foe (IFF) transponder. All Colonial Marine personnel and vehicles carry an IFF transponder in their communication gear which sends back a coded radio signal when interrogated. If the IFF return is positive, the weapon will let the soldier or vehicles pass; otherwise it will open fire. If the system is set to 'manual override' or 'semi-automatic', this information is flashed back to the command console, where the system operator can decide whether or not to fire.

When the weapon fires, automatic servos in the tripod boresight the weapon on the target. The number of rounds fired in a burst is dependent on the target profile, so that the most economical use of ammunition is made.


The UA 571-C is one of a family of remote sentry systems including the -D variant, which mounts a 20 mW HF laser, and the -F version, which employs a 40 mm automatic grenade launcher.