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A set of definitions that has become pervasive in the UAV community stems from an attempt to define a hierarchy of UAV requirements in each of the US services. The levels in these hierarchies were called “tiers” and terms such as “tier II” are often used to classify a particular UAV or to describe a whole class of UAVs.

The tiers are different in each US service, which can lead to some confusion. These tiers are tabulated with brief descriptions in Tables 2.2 through 2.4 for US Air Force, Marine Corps, and Army respectively.

Table 2.2 US Air Force tiers

No.	Tier	Mission/Group	Example
1	N/A	Small/micro‐UAV
2	I	Low altitude, long endurance
3	II	Medium altitude, long endurance (MALE)	MQ‐1 Predator
4	II+	High altitude, long endurance (HALE) conventional UAV. Altitude: 60,000–65,000 ft (19,800 miles), less than 300 knots (560 km/h) airspeed, 3,000 nautical‐miles (6,000 km) radius, 24 h time‐on‐station capability. Tier II is complementary to the Tier III aircraft.	RQ‐4 Global Hawk
5	III−	HALE low‐observable (LO) UAV. Same as the Tier II+ aircraft with the addition of LO.	RQ‐3 DarkStar

Table 2.3 Marine Corps tiers

No.	Tier	Mission/Group	Example
1	N/A	Micro‐UAV	Wasp
2	Tier I	Mini‐UAV	Dragon Eye
3	Tier II	Small	RQ‐2 Pioneer
4	Tier III	Medium	Shadow

Table 2.4 Army tiers

No.	Tier	Mission/Group	Example
1	Tier I	Small UAV	RQ‐11A/B Raven
2	Tier II	Short‐Range Tactical UAV	Role filled by the RQ‐7A/B Shadow 200
3	Tier III	Medium‐Range Tactical UAV

The most recent classification of systems in use in the United States is related to missions although the old Tier system is still in existence. Eighteen missions relate to four general classes of UAVs – small, tactical, theater, and combat. It is quite specific to US military requirements and is not presented in this book.