The Corona Satellite Calibration Targets are two hundred and seventy two concrete markers in and around Casa Grande, Arizona  that helped to calibrate satellites of the Corona spy satellite program.

FOVto focal length

Each of the targets has a manhole or hatch on the west arm of the cross, The manhole has a cement cover and steel reinforcement bars ("rebar"). According to a member of the Cold War Museum in Warrenton, Virginia, the 6 pieces of rebar, which protrude at an equal distance from each other, may have been used to hold laser lighting to give a more accurate fix on each target.

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While caching I had seen a few of these targets but had no idea of their use.  Mystery now solved for me.  Thank you team RStarrT for enlighting me.

Following the launch of Corona satellites in the 1960s, the National Reconnaissance Program determined that there was a need for calibration under the Controlled Range Network. Working with the Arizona Real Estate Office, the Army Map Service was directed to lease land for office space in Casa Grande, Arizona. Land was leased in one hundred by one hundred foot parcels, with access to a road. Large concrete Maltese crosses in the ground, sixty feet in width, and were in place by 1967. The crosses were arranged in a sixteen by sixteen grid, each roughly a mile apart from the next one.

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Cache is located on Arizona State Trust Land so you will need a ASTL recreational permit. These are available online at: https://land.az.gov/natural-resources/recreational-permits and can be printed at home.

The majority of the targets were abandoned when the program ended in 1972. According to Army Map Service, the government considered the targets to be obsolete because the land on which they were situated had subsided because of groundwater extraction.  Land lessees were then given the option of having the targets removed and dumped near Eloy, Arizona. The remaining 84 targets (counted July 2016) have been left in place, unless they have been removed because the location has been redeveloped.