Corona Calibration Targets

Scattered through the Arizona desert, in and south west of Casa Grande, is a mysterious collection of giant cement Maltese crosses.  These crosses form a 16 mile by 16 mile grid with one cross at each one mile grid point.  Research tells me they are an artifact of the Cold War and were used to calibrate the then top secret Corona project spy satelite cameras. 

AN52 Target

What Was The Corona Project

Project Corona was a series of spy satellites (144 in all) launched between 1959 and 1972 that used panoramic, and later stereoscopic panoramic, cameras to photograph the Soviet Union, Peoples Republic of China and other areas of intrest.  When the satellite was finished taking pictues it would jettison the film canister, which would fall back to earth and be recovered midair by aircraft (not always succesfully).

Corona launch and recovery sequence

Recovery aircraft

Who Is In Charge Of Secrets

The project was headed by the CIA with major help from the Air Force. It’s 13 year run had more hardware and technology updates than I’m willing to go into here.  Besides, this is supposed to be about the test targets.

Satillite Painting

Start of the Calibration Targets

The project of laying out and pouring the crosses was given to the US Army Map Service .  The Arzona Realestate Office was hired to lease office space for the surveyors and obtain leases for needed sites.  Most leased sites were 100’x100’ with access rights.

Target From Google Earth

The Cover Story

Tom Brock, a member of the area Arizona Realestate Office at the time, recalls, "In the mid ‘60s we received a visit from surveyors from the Army Map Service.  They briefed us on a project involving the placement of concrete aerial camera markers at approximately one-mile intervals throughout a 16x16-mile area in and mostly south of Casa Grande, Calif." (sic)  Brock went on to say, "It is my understanding that the survey was both vertical and horizontal with extreme accuracy not off by more than the width of a pencil lead at a mile,"

X47 Survey Marker 1

X47 Survey Marker 2

Target Disposition

The test range served as a calibration source for the Corona spy satillites from around 1966 until the end of the project in 1972. A few years after that, in the mid to late 70s, the Army Map Service said they no longer needed the sites because a large portion had become inaccurate due to subsidence caused by ground water pumping for the many cotton farms in the area.  Well, of course they didn’t need the targets anymore, the Corona program had ended a few years earlier. 

Mr. Brock recalls, "We wrote letters to the lessors in the portion of the range to be abandoned asking if they wanted the markers removed from their property.  Most did. The concrete rubble was disposed of along the banks of a wash over toward Eloy, Calf." (sic) 

Target Dimensions

Total width is 60 ft,

Each arm is 25 ft long
      16 ft wide at the wide end
      2 ft 8 in wide at the narrow end


There is a round cement pour at the exact center with a survey marker. The survey markers have writing in 4 concentric rings, with the year of install and survey mark at the center.  
     The next ring in is made up of two parts each, upper and lower:
          Upper portion is the station designation in a curved box.  This is the row and column at which the marker is located, referencing the 16 mile x 16 mile grid.  For example the marker AC-48 (pictured below) falls in the north/south column AC and the east/west row 48.
          Lower portion is the agency responsible for the survey marker.  For all the markers it's the "Army Map Service."
     And again, the center of the survey marker has the survey point and the year the survey marker was placed. To the best of my knowledge all the markers were place in 1966 and 1967.
     I've seen survey markes with other numbers in the space above the center survey mark, but I'm not sure what they mean.

AC48 Survey Marker

Some test targets have two survey markers.  The second survey was used when the test target was not oriented due north/south.  These second markers can have a couple different configurations, and I'll try to get some posted here.

The west arm has a manhole, with cement cover and rebar handles.  Around the cover protrudes 6 equadistant pieces of rebar. Gary Morgan, member of the Cold War Museum in Warrenton VA., emailed me to say he had seen somewhere that the pits may have been intended to hold laser lighting to give a more accurate fix on each target. Thanks for the info, Gary.


How Many Targets

Looking at the actual target locations you will notice the grid size is not exactly 16x16. Some rows and columns had 17 targets. For example the "51" row had targets from "W" to "AN," which is 17. While the next row south ("52") only went from "X" to "AN," which is 16 targets.

Although the vast majority of the northern most row ("43") is missing, it seems that row had a target at each mile. For example, there is still a target visible at W43 then again one mile to the east at X43. Moreover, the eastern most column ("AN") only has two visible targets left, but those two targets are at rows "51" and "52," which would indicate that column was not skipping.

Conversely, if you plot the target placements on a map for the west ("W") column, and south ("59") row, you find they only have a target every other mile, giving the grid a serrated look on those sides.

In essence what you end up with is a 16x16 grid comprised of rows 43 through 58 (16 rows), and columns X through AN (16 columns).  However, there is one additional row ("59"), and one additional column ("X"), each with a target at every other mile mark.  As a consequence every other row and column has 17 targets. Per info supplied by my brother, Ron, who works in government intel, the most likely reason for the difference is to differentiate north, south, east, and west on the photos.

Taking the above into account there should have been a total of 273 targets.  That would be  (16x16)+17, the extra 17 coming from the extra column and row. However, there appears to be at least one location that may never have had a target, and that's "AA56." The grid location of "AA56" puts it on an incline in the Silver Reef Mountains with no level ground in the area. The cost and trouble to level the area for this target may not have been worth the benefits of having one more target.  What we are left with is 272 target locations, assuming all other target locations were developed.

Target Locations

Quite a few of the crosses remain to the south west of Casa Grande and most can be visited without too much problem. 

 Target Grid

Green Circle: Target survey marker designation confirmed or confirmed missing.
Yellow Circle: Target visible using Google Earth but survey marker designation not confirmed.
Red Circle: Target missing.
Blue Circle: Target status questionable.

Corona Target Location

This is the Google Earth .kmz file showing all the Corona spy satellite test target locations around Casa Grande AZ.

You will need to have Google Earth loaded on you computer to view this file.

File size
23.64 KB


Corona Target Rows and Columns

This is the Google Earth .kmz file showing the Corona spy satellite test target row and column designations.

You will need to have Google Earth loaded on you computer to view this file.

File size
2.97 KB