NASA's Deep Space Network (DSN)

The Deep Space Network is the link between all of the past and present space missions. It consists of receiving and transmitting sites located at sites in Spain, Australia and California. These sites are located 120 degrees apart in longitude, which allows for overlapping coverage. This provides continuous coverage and easy transfer of the radio link from the spacecraft from one site to the next as the earth rotates. This is illustrated in Figure 1.

These sites all are equipped with at least 4 deep space stations using ultra-sensitive receiving systems and large microwave parabolic antennas. The sizes of these antennas are as follows:

Two 34-meter (111-foot) diameter antennas.

One 26-meter (85-foot) antenna.

One 70-meter (230-foot) antenna.

26-meter Antenna

The 26-meter antennas are the ones used for tracking the earth orbiter satellite spacecraft that usually are in orbits between 160 and 1,000 kilometers high (100-630 miles). By using the X-Y mount, the antenna can point low on the horizon and pick up the fast moving orbiters as soon as they clear the horizon and can be seen by the antenna. Maximum tracking speed of these antennas is 3 degrees per second. These antennas were originally built to support the Apollo moon missions that took place between 1967 and 1975. If this antenna is used in the S-band range, 5-6 Gc. the gain will be nearly 62 dBd and the beam width will be less than .15 degree.

70-meter Antenna

This antenna with its diameter of 230 feet is the largest and the most sensitive of all the antennas and is capable of tracking a spacecraft that is flying more than 16 billion kilometers from earth. In order for this antenna to maintain its high gain and directivity, the surface of the parabolic reflector must be accurate within a fraction of a wavelength across the entire surface of the parabola. This means that across the entire 3,850 square meter surface (41,400 square ft.) the surface must be maintained within 1 centimeter (.4 in.). The gain of this antenna in the S-band range will be on the order of 68+ dBd and the beam width will be less than .1 degree. Total weight of the antenna, azimuth-elevation mount and supporting pedestal is 2.7 million kilograms which is 8,000 U.S. tons

There are new antennas currently under construction, and the DSN Antenna project will add five new 34-meter (111-feet) antennas to the system to handle the increased demands on the DSN. These antennas will have gains in the S-band of 63 dBd and beam widths between .12 to .10 degrees.

By having the ability to array several antennas, the improved sensitivity using this technique-improved reception of data that comes in from the Galileo spacecraft. This array electronically links the 70-meter antenna at Goldstone, California with the identical antenna located at the Canberra site in Australia. These two sites are the ones concurrently receiving the communications with Galileo. Each site is located in semi-mountainous, bowl-shaped terrain which shields the antennas from extraneous radio interference. Figure 2 shows this.

All of the stations are remote controlled from a central Signal Processing Center at each complex. The Centers contain all of the controlling subsystems that direct the antennas to the appropriate spacecraft, receive and process all telemetry data and transmit the required commands to the spacecraft for navigation and other functions that the spacecraft was required to carry out.

After the initial data processing at each complex, the data is then transmitted to JPL for further processing and distribution to other science teams for analysis. This is done by a modern high-speed ground communications network.

DSN Ground Communications

The Ground Communications Systems are part of the NASA global DSN network, linking the Central Communications Terminal at the main JPL site in Pasadena, California to the other DSN sites located in Canberra, Australia and Madrid, Spain. These circuits carry all the data for the missions and supporting traffic for all missions and also to support radio and radar astronomy observations pertaining to exploration of the Solar System and the universe. All data from earth-orbiter, lunar and shuttle missions is also handled by this network.

Ground Communications Traffic

Traffic on the DSN Ground Communications Network consists of telemetry, command, tracking, monitor and control data. Also included is radio science and voice traffic, to include facsimile traffic

NASA to antenneX

antenneX has been in communications with NASA regarding its methods of communicating with Deep Space Missions such as the present Mars Mission. When asked about how signals are processed so efficiently with the equipment recently landed on Mars, lander and rover, here is what NASA had to say:

We do not communicate directly with the rover from earth. The rover only communicates with the lander. The lander relays back-and-forth all communications between the earth and the rover. The lander/rover link is a simple UHF FSK design that operates just above 400 MHz. Communications between the lander and earth are done through a unified X-band design that provides Doppler, range, command and telemetry links. The uplink to the lander operates at about 7175 MHz and the downlink is at about 8429 MHz. The uplink and downlink are coherent such that the downlink frequency and phase are exactly 880/749 of the uplink frequency and phase. Relative to the downlink frequency (8429 MHz) the space loss right now is about 277 dB. That probably sounds like a lot compared to UHF cellular links! So how do we maintain communications?

The lander has an X-brand transmitter that generates about 10 watts of RF output power. The lander also has a high gain antenna (HGA) that is articulated to keep it pointed at earth. The HGA has a peak gain of about +24.9 dBic. So the EIRP for the lander is roughly +63.4 dBm which is a little more than 2 kilowatts. Here on earth we receive the signal with the large parabolic antennas of the Deep Space Network which are located at three complexes around the world. We’ve been using the 70-meter diameter antennas which have an X-band gain of about +73.6 dBic. The front end amplifier is a ruby maser which is cooled with liquid helium that boils at about 3K. The overall system noise temperature of the receiver is about 33K at 20 degrees elevation angle and improves at higher elevations due to reduction of water vapor in the ray path through earth’s atmosphere.

The channel bandwidths are narrow where we can use narrow bandwidths. The carrier is tracked with a phase-locked loop with a single-sided bandwidth of 3 Hz. The subcarrier and symbol tracking loops are 0.5 Hz each. But the data channel must be wide enough to transport the data spectrum. So the data channel bandwidth is several tens of kilohertz depending on the data rate being used.

Finally, if you are going to juggle the numbers, you need to know that we use convolutional coding on the downlink telemetry. This gives us about an eight-fold increase in performance relative to an uncoded digital channel. Pathfinder uses a constraint length 15, rate 1/6 code so we send six binary symbols for each data bit. We then add Reed-Solomon parity words to correct any bit errors that occur in detection. Overall, the telemetry link requires an ‘energy-per-bit-to-noise-spectral-density-ratio’ (Eb/No) of about 0.4 dB to operate above our defined threshold bit error rate.”

Our thanks to those at NASA and JPL for being so helpful and responsive to our questions. We expect to have more on this series about deep space communications.

Originally posted on the AntennaX Online Magazine by Information as Supplied by NASA and JPL

Last Updated : 8th March 2024

NASA’s Deep Space Network (DSN)