Deep Space Network
The Deep Space Network (DSN) is an international network of antennas owned and operated by NASA located on Earth and dedicated to the communication with interplanetary missions and astronomical radio observations. Currently, there are three sites based approximately 120° apart in the Mojave Desert, California (USA), Madrid (Spain) and Canberra (Australia). The dispersion of antennas allow mission controllers to keep constant watch on all interplanetary missions regardless on Earth's rotation.
This network may be used for an interplanetary internet link.
Use for Communication
Typically, one or more of the 34 meter diameter antennas is used to communicate with spacecraft at Mars. The data rate of the downlink from Mars can be anywhere from a few bits/second to on the order of 1 Mpbs. Most missions use the DSN at X-band, but a small number of missions are using S-band or Ka-band.
The DSN can provide ranging and doppler information for use in orbit determination of spacecraft. Long two-way doppler tracking is commonly used.
Delta-DOR is a navigation technique that is similar to Very Long Baseline Interferometry. Delta-DOR requires two base stations separated by a mile or two, a quasar, and a target spacecraft which must emit several tones spanning a few MHz. From this, angular measurement is obtained. 
The DSN requires a line of sight between the antenna and the vehicle, which imposes obvious constraints on the system. For rovers and landers, in order to use a Direct to Earth (DTE) connection, the vehicle must be on the Earth-facing side of the planet, which means that DTE communication is only possible for about half of the Martian day. DTE connections for rovers and landers are uncommon, and instead the surface asset will use a UHF proximity link to transmit data to an orbiter, which relays the signal to DSN over X-band or Ka-band.
There are six orbiters around Mars. Orbiters are occulted when their orbit takes them behind Mars from the viewpoint of the Earth station.
Interference from the solar radiation can cause loss of signal when the angle between the Earth, Sun, and Mars is less than 5°. This loss of signal occurs during a superior conjunction, which occurs once per 26 month synodic period, and can last from a day to over a week depending on the relative geometry due to the non-co-planar orbits of Earth and Mars.
The Deep Space Network supports approximately 35 missions as of 2018, and significant planning is required to schedule time on the DSN for all space assets that need it. Mars vehicles represent only a fraction of the total users, but when non-Mars missions appear close in the sky to Mars there can be a significant bottleneck. One technique that is used to relieve strain on the system imposed by the many Mars missions is Multiple Spacecraft per Antenna (MSPA). With this technique, one 34 meter antenna can be used to simultaneously downlink data from up to four spacecraft, and uplink to one spacecraft.
The Sardinia Radio Telescope, a 64 meter aperture steerable antenna in Italy, can be used for deep space communications. NASA may potentially be able to use this if needed.
The former Soviet Union constructed a number of large aperture antennas for deep-space communication to support their Mars missions. More recently, this network was to be used to support the 2011 Fobos-Grunt mission, but the mission failed before reaching deep space.
China has several large antennas. Most are located in mainland China, but a new groundstation was recently constructed in Argentina. The network was used to track Chang'e 2, which had a mission profile similar to NASA's Clementine mission, with operations in cis-lunar space followed by operations around an asteroid.
The American company Atlas Space Operations is building a commercial capability which they claim will be equivalent to one of the DSN's 70 meter dishes. Atlas LINKS (TM) will be an array of small antennas, not a large aperture antenna like those the DSN utilizes.
- How long is the gap and how often does it occur?