www.nasa.gov

National Aeronautics and 

Space Administration

Currently, NASA uses radio frequency (RF) 

transmitters, but it is also developing 

optical communications technologies that 

use infrared lasers to send more data than 

traditional RF. Optical terminals are also 

smaller and lighter than comparable RF 

dishes, which can leave more room on the 

spacecraft for science instruments.

NASA’s Laser Communications Relay Demonstration (LCRD) will 

demonstrate the advantages of optical communications, which uses 

infrared lasers to provide missions with higher data rates. While this 

infrared light is invisible to the human eye, it is similar to light from other 

sources like the Sun.

How does LCRD’s laser light compare to the Sun’s light as we 

experience it on Earth’s surface?

Just like a light bulb, electromagnetic energy—light—disperses in all 

directions as it moves away from its source. The further away from the 

source, the weaker the light will be. Consider two planets, Mercury and 

Earth: Mercury’s orbit averages 36 million miles from the Sun, while 

Earth’s orbit is around 93 million miles.

To compare the light received by the planets, let’s discuss units of 

irradiance. Irradiance is a specific term for the amount of radiant 

energy flowing through a two-dimensional area. It is usually expressed 

in units of power (Watts) over area in square meters (m2

). 

The closer to the source, the more energy per square meter is 

received and the higher the irradiance. The irradiance on Mercury 

is a scorching 9,082 W/m2. Combined with its lack of atmosphere 

and magnetosphere, this contributes to its incredibly volatile 

temperature, which swings from 800 °F to –290 °F. On Earth, we 

only receive 1,368 W/m2 at the top of our atmosphere, which results 

in our more temperate climate.

The concept of irradiance doesn’t just apply to the Sun; we can also 

measure the irradiance of other energy sources, like the ones we 

use for space communications.

NASA Space Communications 

and Navigation

ACTIVITY SHEET

Irradiance GRADES 10–12

(continued)

Optical 

Telescope

6-inch diameter

3510mW

PAGE 1

Sun

Earth

93 million miles

1,368 W/m2

Mercury

36 million miles

9,082 W/m2

PROBLEMS

We use about 3,510 milliwatts (mW) of power to send information from an 

LCRD ground station to the LCRD spacecraft in orbit. Let’s compare the 

irradiance of the LCRD ground terminal to the irradiance of the Sun.

1. The LCRD ground terminal uses an optical telescope to focus the 

infrared light it generates. This telescope has a circular aperture 

(opening) 6 inches in diameter. What is the area of this aperture in 

square inches (in2)? Recall that Areacircle = πr 2, where r is the radius.
PAGE 2

Irradiance

PROBLEMS (continued)

2. Convert the LCRD telescope aperture area above into square centimeters (cm2) 

and then to square meters (m2). Recall that 2.54 cm = 1 in and 100 cm = 1 m. 

3. Given that we are putting 3,510 mW of power into the telescope and we now 

know the area of the aperture in m2, what is the irradiance of the LCRD ground 

station at the aperture in W/m2? Recall that 1,000 mW equals 1 W.

4. A few important factors alter how much energy from space reaches the surface of 

Earth. Our atmosphere plays a critical role in protecting us from too much energy! 

Let’s apply two of those factors to get a more realistic sense of the energy felt on 

the surface compared to the energy felt at the top of the atmosphere:

Approximately 56% of the Sun’s energy (in all wavelengths) reaches the surface. 

26% is reflected by the upper atmosphere, and ozone absorbs another 18%.

Of the light that does reach the surface, approximately 50% of that is infrared 

energy—the kind emitted by the LCRD ground station.

Apply these atmospheric factors to Earth’s irradiance value provided above. 

What is the total solar irradiance at Earth’s surface? What is the infrared solar 

irradiance at Earth’s surface?

5. How does the infrared solar irradiance at Earth’s surface compare to the infrared 

LCRD ground station irradiance at the telescope aperture? Which one is larger? 

By how much?

Solar irradiance 

at the top of the 

atmosphere is 

1,368 W/m2

(full spectrum)

Approximately 

56% of the 

Sun’s energy 

reaches the 

surface

Of the energy 

that does reach 

the surface, 50% 

is infrared energy

NASA’s Laser 

Communications Relay 

Demonstration (LCRD)

is the next step in optical 

communications. 

Hosted on a U.S. Space Force spacecraft, LCRD 

will demonstrate optical technology in geosynchronous orbit 22,300 miles above Earth’s surface. 

LCRD is built and managed by NASA’s Goddard 

Space Flight Center in Greenbelt, Maryland.

LCRD is a technology demonstration that will pave 

the way for future optical communications missions. 

When NASA has developed a new way to solve 

a problem and wants to show how it works, they 

create a technology demonstration. LCRD’s first 

orbiting experimental user will be the International Space 

Station's Integrated LCRD Low-Earth Orbit User Modem 

and Amplifier Terminal (ILLUMA-T). The terminal will 

receive science data from experiments and instruments 

aboard the space station and then transfer the data to 

LCRD, which will then transmit them to a ground station. 

After the data arrive on Earth, they will be delivered to 

operation centers and scientists to help make decisions 

about the mission.

LEARN MORE

Learn more about LCRD’s predecessor, the Lunar 

Laser Communications Demonstration (LLCD), at 

http://go.nasa.gov/LLCD_history.

Learn more about LCRD at 

http://go.nasa.gov/LCRD_overview.