From the Desert to Space

In my last post, guest blogger Bashar Rizk described the three cameras of the OSIRIS-REx Camera Suite (OCAMS). These imagers will provide unprecedented documentation of Bennu’s size, shape, geology, and environment. Have you ever wondered how those images, taken on board the OSIRIS-REx spacecraft, get back down to Earth? Read on to learn about the critical role of the NASA Deep Space Network in communicating with OSIRIS-REx and a myriad of other interplanetary spacecraft that are exploring our Solar System.

The 70-m DSN antenna placed inside the Rose Bowl for scale – talk about making some noise!

The 70-m DSN antenna placed inside the Rose Bowl for scale – talk about making some noise!

Step 1 – Take an Image

The long-distance photography of Bennu involves three steps: taking the picture, transmitting the data back to Earth, and receiving and assembling the data into a recognizable image. This complicated process depends on our ability, here on Earth, to communicate with the spacecraft, even when it is on the other side of the Solar System.

Taking the picture is the job that requires coordination between OCAMS and the spacecraft computer system. When OCAMS looks at Bennu, light from the asteroid surface passes through the lenses and then through a filter before falling on an electronic chip called a charge-coupled device, or CCD. The surfaces of the OCAMS CCDs are divided into 1,024 parallel lines, each of which is further divided into 1,024 light-sensitive pieces, for a total of 1,048,576 picture elements, or pixels. The OCAMS imagers are therefore 1-megapixel cameras. Each pixel records brightness based on the amount of energy striking each capacitor on the array (the photoactive region). Basically, each capacitor accumulates an electric charge proportional to the light intensity at that location. The camera electronics convert these values into digital code, made up of 0s and 1s called bits, and transfers the code to the spacecraft’s central computer, which queues it up for transmission back to Earth at the next opportunity.

Step 2 – Transmit the Image Back to Earth

The spacecraft telecommunications system then relays the bitstream of data to Earth. The data are first sent through one of our Small Deep Space Transponders. The transponder then sends it through one of our 100 Watt Travelling Wave Tube Amplifiers, which boosts the signal. The TWTA delivers the boosted signal out to one of our three different types of antennas.

The speed of transmission depends on which antenna we use to talk to the Earth. Our highest data rates are achieved when we can point our big 2.0-m (6-foot) wide High-Gain Antenna at the Earth. In addition, we can maintain communication with the Earth using either our Circular Horn Medium-Gain Antenna, or one of the Choked Horn Low-Gain Antennas. As we switch across these different antennas we trade bandwidth for field-of-view. The High-Gain Antenna provides our highest data rate but has a very narrow field of view – requiring us to point the antenna directly at the Earth to get our data back. The Low-Gain Antenna, on the other hand, is always pointed at the Earth (we have two of them), but cannot be used to downlink a lot of data – we use this antenna to maintain communication with the spacecraft for safety monitoring. The Medium-Gain antenna provides a happy medium – providing a moderate data rate for an expanded field of view.

Step 3 – Receive the Image on Earth

Listening back on Earth is one of the antennas from the NASA Deep Space Network. I had the opportunity to tour the Goldstone Deep Space Network tracking station north of Barstow, California last week to see these assets in action and meet the team members responsible for their success. I was accompanied by members of the OSIRIS-REx Flight Dynamics Team, the group responsible for flying the spacecraft to Bennu and back.

The OSIRIS-REx flight dynamics team got a tour of the Goldstone DSN station. From left to right: Eric Carranza, Felicia Sanders, Ed Beshore, Pete Antresian, Brent Barbee, Dale Stanbridge, Dante Lauretta, Kenny Getzandanner, Kevin Berry, Bobby Williams, Joel Fischetti, and Coralie Jackman.

The OSIRIS-REx flight dynamics team got a tour of the Goldstone DSN station. From left to right: Eric Carranza, Felicia Sanders, Ed Beshore, Pete Antresian, Brent Barbee, Dale Stanbridge, Dante Lauretta, Kenny Getzandanner, Kevin Berry, Bobby Williams, Joel Fischetti, and Coralie Jackman.

The NASA Deep Space Network – or DSN – is an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe. The network also supports selected Earth-orbiting missions. The DSN consists of three deep-space communications facilities placed approximately 120 degrees apart around the world:

This strategic placement permits constant observation of spacecraft as the Earth rotates, and helps to make the DSN the largest and most sensitive scientific telecommunications system in the world.

Field of view of the Deep Space Network antennas, looking down from the North Pole. Missions >30,000 Km from Earth are always in view of at least two stations. Image source: http://en.wikipedia.org/wiki/File:DSNantenna.svg

Field of view of the Deep Space Network antennas, looking down from the North Pole. Missions >30,000 Km from Earth are always in view of at least two stations. Image source: http://en.wikipedia.org/wiki/File:DSNantenna.svg

These three stations require huge antennas, ultra-sensitive receivers, and powerful transmitters in order to transmit and receive over the vast distances across the Solar System. All DSN antennas are steerable, high-gain, parabolic reflector antennas. At each station there is: one 34-meter (112 ft) diameter High Efficiency antenna; one or more 34-meter (112 ft) Beam waveguide antennas; one 26-meter (85 ft) antenna; and one 70-meter (230 ft) antenna.

OSIRIS-REx team members standing in front of Deep Space Station 22 – one the 34-m Beam Waveguide Antennas at the Goldstone DSN complex.

OSIRIS-REx team members standing in front of Deep Space Station 22 – one the 34-m Beam Waveguide Antennas at the Goldstone DSN complex.

The DSN provides the vital two-way communications link that will guide and control OSIRIS-REx during his entire journey. The antennas of the DSN will also bring back the images and the new scientific information he collects.  The antennas and data delivery systems make it possible to acquire data from spacecraft, transmit commands to spacecraft, track spacecraft position and velocity, and gather our science data. We are very grateful to our hard-working team members of the DSN – without their tireless efforts, we would not be able to talk to OSIRIS-REx!

Al Hewitt and I standing in front of the MARS 70-m DSN antenna at Goldstone. Al is the Network Operations Engineer (NOPE) for OSIRIS-REx.

Al Hewitt and I standing in front of the MARS 70-m DSN antenna at Goldstone. Al is the Network Operations Engineer (NOPE) for OSIRIS-REx.

So how fast can we downlink our data using this great communication system? Our top data-transmission speed will be a whopping 914 kilobits per second – only slightly faster than your average DSN/cable connection. Fortunately, we have many weeks and months at Bennu – plenty of time to take those great images and beam them back to Earth.

OSIRIS-REx data will be transmitted back to Earth at less than 1 Mbps.

OSIRIS-REx data will be transmitted back to Earth at less than 1 Mbps.

3 comments

  1. Will Cutlip · · Reply

    D Really enjoying these posts! Found my self saying “pronounced Twee-Ta” and “like the Three Bears’ porridge, the middle antenna is ‘just right’”

    Great stuff! Keep up the good work!

    Cheers Bill

    🚀 Sent from Bill’s mobile office 🚀

    >

  2. […] The aft deck supports the batteries, electronics, another LGA, a medium gain antenna, the reaction wheels, and solar array gimbals. Four gusset panels connect the forward and aft decks and provide additional support. Loads from the forward deck are reacted by the central cylinder and gusset panels on the diagonals. The gusset panels support telecom components, propulsion isolation and control components, and the helium pressure tank. The forward and aft decks and two of the gusset panels combine to support the 2-meter high gain antenna. […]

  3. […] pole of the asteroid at a distance of seven kilometers while tracking the spacecraft using the Deep Space Network. The asteroid mass will tug on the spacecraft and alter its orbit. This effect shows up as a […]

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