Space shuttle wireless, part 1, by David Geer

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Wireless Business & Technology

Space shuttle wireless

By: David Geer

Which "wireless that works" application wins the award for long-standing, highly critical, and longest distance? That's the wireless at work at NASA; in particular, the wireless technologies that are crucial in shuttle missions.

Powerful wireless technologies and applications can be unique from one mission to the next. This was the case with the recent payload experiments of the NASA OMNI project (Operating Missions as Nodes on the Internet). This project was in fact an example of successful wireless IP research on the STS-107 (Space Transportation System) mission. There are certain wireless systems and procedures however that are crucial to all missions.

Let's identify three primary stages of the space shuttle missions for which wireless is very important: launch, orbit, and re-entry.

The Launch

During the launch, wireless data is critical to vehicle safety, range safety, and public safety. "Range safety is the set of processes to protect people (public and participating) from injury/death, property (public and private) from damage, and the reputation of the U.S. government from embarrassment when conducting range-related activities (i.e., launching rockets, testing flight vehicles, etc.)," says Michael Patterson of the Safety Office of NASA GSFC's Wallops Flight Facility. Those are huge responsibilities for wireless data and communications.

Constant contact is maintained between the vehicle and the ground over NASA's S-band System. Initially, this connection is directly with the ground, and later it's through the TDRSS (Tracking and Data Relay Satellite System). The S-band frequency is close to that of microwave ovens, though lower, at 2287MHz coming from the orbiter to the relay satellites and 2106MHz going from the relay satellites to the orbiter. The latency for data over the S-band System from the shuttle through the TDRSS and to the ground station is about 250ms. That is a propagation delay based on the speed of light and the distance the signal must travel.

The S-band link must be used from launch through early orbit, as it is the only one that can be used when the payload doors are closed. The S-band is also the prime link during landing. There are three ground stations that may be communicated with; two are air force stations near the Kennedy Space Center. These centers communicate with the shuttle until about six minutes into its flight when the signal then turns toward the TDRSS link. Under normal circumstances the shuttle stays on this link throughout the mission using the S-band or the Ku-band. Once into orbit the shuttle releases a dish of about three feet. This is used to communicate over the Ku-band to the TDRSS.

Communication with the crew is vital in case of an anomaly, or if there should be any reason to abort the mission. This helps to ensure the health and safety of the crew and of the general public on the ground below. As a backup, there is also an emergency UHF analog link to communicate with crew members should the need present itself. This link is routinely tested. The S-band link works so well that the UHF link is maintained and reserved only for use as a backup.

From launch until the engines shut down and the vehicle takes its initial coast into orbit, wireless communication is critical. Wireless technology helps to determine whether there are any problems during this phase that would be reason to abort the mission, and to notify the crew and determine an abort destination for the vehicle.

Depending on the nature of the problem that creates an abort situation, the crew may be instructed to return to the launch site, to land in the water, or to do a transatlantic landing - in Europe at one of their airports. Agreements have been worked out with several of these sites, which have the technology to support a shuttle landing. You can see how the crew's safety and the safety of the general public are dependent on wireless during this stage.

The Orbit

The shuttle's orbit must be circularized through an elemental maneuvering system. A series of rocket engines on the shuttle fire to help establish orbit by circularizing or boosting the vehicle, which depends on what the particular mission is. Communications during this process ensure that the vehicle is performing properly and, so, these communications critically support the circularizing of the vehicle.

During orbit, the crew may leave the inner security of the shuttle to conduct EVAs (Extravehicular Activities) where they are outside the orbiter. During EVAs, crew members have radio communications from their space suits back to the orbiter and then on through the TDRSS and down to earth.

Ground control can support any task or issue encountered every step of the way via these communications. "One of the things that's very important is that TDRSS provides continuous connectivity from the orbiter to the ground and vice versa. You can do these critical kinds of things like eight-hour space walks, repairing the Hubble Space Telescope or building the space station, and the crew can maintain continuous connectivity," says Phil Liebrecht, associate director and program manager for Mission Services, NASA Goddard Space Flight Center.

Through this communication the entire ground crew and the shuttle crew operate as a team to support each careful step and answer every question the crew might have. The TDRSS provides this continuous communication so that critical operations can be conducted. This is how wireless expands the capabilities of the entire mission. Though just a few astronauts are in space, the minds of NASA are there with them providing support, expertise, and resources.

In addition to the S-band that provides constant voice, health and safety, and engineering data, there is a second and very important frequency: the Ku-band. The Ku-band frequency is 13775MHz from the TDRS (Tracking and Data Relay Satellite) out to the orbiter and about 15GHz from the orbiter back to the TDRS. The Ku-band link is available during orbit when the cargo bay doors can be opened.

Once in orbit, the vehicle deploys a dish-sized antenna through the cargo bay doors. This antenna allows the shuttle to communicate up to the TDRS at about 50Mbps. This multipurpose connection can be used for science data for science payloads and high-resolution digital TV (most any type of video from and to the orbiter on a two-way link) as well as the usual e-mail transmissions, including both text and graphics.

Typically, the high-rate, high-resolution digital TV signal is sent from the orbiter via TDRSS to the ground. However, Johnson Space Center has the capability to send low-rate digital videos to the orbiter as needed. This may be for use in flight operations, medical conferences, and private family video (two-way video communications). JSC can upload documentation, instructions, and procedures - whatever is needed.

The shuttle is equipped for closed-circuit TV (analog). Though cameras can be mobile with the crew, mounted on a number of ports inside the shuttle, or deployed on the remote manipulator system (the arm) outside, the number and locations of cameras are different depending upon the needs of the specific mission.

Re-Entry

All the communications about the shuttle temperatures during re-entry, all the information about the wheel wells and tires and all that data about the plasma buildup, comes across the S-band through the TDRSS. "Besides the TDRSS being the primary system that provides the global coverage for the orbiter, it also allows the vehicle to communicate up. When it's re-entering there is a plasma (formed of ionized gas particles) that builds up underneath the vehicle due to extreme heat. That plasma prevents the orbiter from communicating down to the ground. It essentially reflects the energy," says Liebrecht, of NASA's Goddard Space Flight Center.

With missions that predate the space shuttle program, such as the Apollo, the Gemini, and the Mercury, there was a 10-15 minute communications blackout during re-entry, specifically because of this plasma buildup. The shuttle has the same blackout and can't connect directly with the ground stations. The signal won't penetrate the plasma buildup under the orbiter. The signal that goes up to the TDRS is seldom affected however, so there is no communications blackout with the shuttle as with earlier missions. Because the orbiter is always in motion however, there are some disruptions in the link, which lead to disruptions in communications.

"The plasma also kind of licks around the edge of the orbiter and occasionally that impacts even the links going up to the TDRS," says Liebrecht. "All of those parameters - until the point that they lost communications with the [Columbia] crew - all of those parameters and the crew's voices were coming back through TDRSS."

Landing

Contrary to some reports, once the shuttle has re-entered the atmosphere it is capable of landing without wireless communication from the ground. About 10 minutes before landing, after the plasma has cleared, the orbiter returns to communications with the ground stations. The shuttle comes within line of sight of the communications facility at Kennedy. This facility, at Merritt Island, is often referred to as MILA, an acronym, which stands for Merritt Island Launch Area. MILA is a ground station located at Kennedy Space Flight Center in Florida. This acronym was adopted decades ago in the Mercury days. MILA supports shuttle launch and landing operations including communications to the vehicle during launch and during the landing approach.

Data sent to the shuttle for adjusting and leveling the vehicle at the correct angle relative to the timing and duration of the burn to re-enter is calculated on the shuttle and on the ground and is crucial to safe re-entry. This information is critical to entering the atmosphere with optimal safety. The tracking data off the vehicle, the TDRSS, the ground stations, and the ground radars, is used to provide additional navigation data for the vehicle. "Periodically they update state factors, telling the onboard systems where the orbiter is; that is critical," says Leibrecht.

Conclusion

A launch that is safe for the shuttle and the rest of us here on the planet below, as well as a successful orbit, a profitable mission, and a successful re-entry are dependent on these S-band and Ku-band wireless communications. Coming in Part 2 of this series, more technical detail of the S-band and Ku-band systems as used in NASA's Space Shuttle missions.

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