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.
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 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:
- the Goldstone Deep Space Communications Complex (35°25′36″N 116°53′24″W) outside of Barstow, California.
- The Madrid Deep Space Communication Complex (40°25′53″N 4°14′53″W), 60 kilometres (37 mi) west of Madrid, Spain
- The Canberra Deep Space Communication Complex (CDSCC) in the Australian Capital Territory (35°24′05″S 148°58′54″E), 40 kilometres (25 mi) southwest of Canberra, Australia.
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.
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.
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!
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.