Known as "LAPAS" (Luftgestütztes, abstandsfähiges Primär-Aufklärungssystem, engl: airborne, long distance primary reconnaissance system) in Germany and "Senior Guardian" in the US, the program initially attempted to acquire the Lockheed TR-1 (U-2) for this role, but when this did not succeed, a new aircraft was sought.

[1] During the design phase, the programme was viewed by E-Systems as a major opportunity to increase its prominence in the international markets and grow the business, which offset the difficulties associated with the export of technologies that were considered to be highly sensitive at that time.

[1] According, during September 1988, the D-450 established three separate world records related to altitude and time to climb; specifically: During 1989 and 1990, a pair of pre-production aircraft, referred to as the D-500 EGRETT II, were completed.

[1][6] In 1992, the German Air Force placed an order for production of nine EGRETT IIs; these were intended to be supplemented by at least one G 520T twin seat trainer and one of the demonstrator aircraft.

[1][6] However, during February 1993, before much of the manufacturing work had taken place, the procurement process came under intense scrutiny when accusations of corruption (Amigo Affair) surrounding the former Bavarian Minister President Max Streibl became public.

The G 520T was grounded in 2007 due to a lack of funds and was repurchased by Grob Aircraft during 2013,[1] after which it was upgraded, returned to flightworthy condition, and flown back to Germany.

[1] These include the adoption of a Pratt & Whitney PT-6, a total-authority autopilot, and other fittings to turn it into an optionally piloted vehicle that can be flown remotely from a ground station.

The prospective G 520NG has been promoted as being a more affordable alternative to unmanned surveillance and reconnaissance aerial vehicles such as the Northrop Grumman RQ-4 Global Hawk as well as the aircraft not falling under International Traffic in Arms Regulations (ITAR) restrictions.

[citation needed] Power is typically provided by a single turboprop Honeywell TPE 331-14F that drives a four-blade Hartzell propeller.

When functioning as a maritime patrol aircraft, it can be furnished with a suitable radar, or be used as a satellite relay system to bolster ground communications via satcom apparatus.

[1] An optional operator console can be installed in the rear cockpit of twin seat aircraft; these furnishing include a full keyboard, hand controller units, and a camera grip.