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The Spaceflight Now article Photos: Sentinel 3B satellite prepared for launch from Plesetsk Cosmodrome has of course lots of photos. One of them shows a dish antenna covered with shiny, metallized film of some kind.

I believe I see an small, compact array of seven corner cube retroreflectors to the left of the dish antenna. Why were these included on Sentinel 3B? Is this actually common for LEO Earth measuring satellites, or is this something new or unusual?

I've included a cropped, magnified, and heavily sharpened view of the device below as well.

enter image description here

enter image description here

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That is definitely the Laser Retro Reflector (LRR). They are used to provide very precise ranging information. The Sentinel 3 LRR is a hemisphere of 7 corner cube reflectors on the Earth-facing side of the satellite to allow ground stations to get ranging data accurate down to millimeters, by shining lasers at the satellite and measuring the round trip time of the reflection. enter image description here

LRRs are used in satellites as either a scientific payload in their own manner (such as LAGEOS, to give data about the Earth's internal structure by measuring gravity's effects on the orbit), or as a calibration tool to enable precise location knowledge - such as in the GPS system, where precision location data is critical to network functionality.

In the case of Sentinel it is likely used to calibrate the radar altimetry data collected by the SRAL instrument.

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