Communications and Navigation--Kathleen Lachance

As of October 17th:           

            One of the most important design requirements for the communications system of Red Rover is that there can be no single point failure in the system. This is to say that individual failures in the system must not cause the entire communications system to cease proper operation. There must be back up designs for transmitting and receiving data, should one of the antennas malfunction. Additionally, the design must be able to incorporate receiving commands (from Earth or base station) at all times, even during the transmission of data from the rover. Furthermore, the best design for the communications system will be that which is able to minimize time delay in transmission between Mars and Earth, which can be as great as forty minutes round-trip[1].

            In addition, the communications from the rover to the base station, earth, and/or orbiter are limited to the maximum data transfer rate of which the rover’s technology is capable.  This maximum data transfer rate is, in turn, limited mainly by bandwidth allocation for the transmissions. Red Rover will be transmitting data using the X-band frequencies (8450-8500 MHz), which are designated for deep space research. Bandwidth allocation and frequency licensing is determined by the Office of Space Communications (NASA Headquarters) and the Federal Communications Commission (FCC).

            Finally, the overall mass and power limitations for the rover will play a role in the design of the communications system. Larger, heavier antennas are most likely not suitable for this application. The surface area of the antennas on the rover ought to be minimized so as to make the rover lighter and easier to store. Additionally, the power requirement of different types of antennas will impact the design, as the rover has a limited power budget.

There are several frequency bands used for Space Research that the Tigernauts could utilize in the communications system of the rover: Ka, S, X, and Ku frequency bands. Determining the band of frequency to use will directly affect the selection of antennas in the rover design. Also, it will directly affect the power budget of the rover, since certain frequency bands place higher power demands on the communications technology than others.

            The path of communications for the rover will also determine the type of hardware in the final design. There are multiple options for the path of communications. First, the rover could transmit data through the base station, which would in turn forward the transmission to Earth. Second, the rover could store information and could transmit to an orbiter in small bursts. The orbiter would then forward the transmission to the base station and to Earth. This path is shown in Fig. 1. Third, the rover could transmit directly to Earth, and the control station on Earth could relay the information to the base station on Mars, if required. Or the rover could have multiple paths of communications and could transmit directly to Earth and to the base station.

Earth

Base

Orbiter

Rover

1. Transmits to orbiter

2. Transmits to base station and Earth

Figure 1: Example Path of Communication

            After the frequency band and path of communications are determined, the hardware may be selected in the design of the rover’s communications system.  There are four main options for antennas in the rover design. These are omni-directional antennas, parabolic dish antennas, phased array antennas, and optical communication antennas. These options differ greatly in their operation, power consumption, and applicability to the rover design.

Omni-directional antennas have been used in several NASA spacecraft designs, such as the unmanned rover Sojourner used on the Mars Pathfinder Mission in 1997[2]. However, they have very high power requirements to transmit data, therefore using these as the primary link for the Mars Rover is in all likelihood not a feasible option. The next option is a parabolic dish antenna. Parabolic dish antennas are widely used in space communications. The technology is mature. They provide high gains, which enables high bandwidth communications without consuming too much power. However, they perform best on stationary or very slow moving platforms. They must also be mechanically pointed with great precision to function properly.

Another option is the phased array antenna. These have several advantages. They are “electronically” pointed by adjusting the phases of the transmitters, which allows for fast and precise steering. This would be a great advantage on the rover. They are also more efficient than parabolic dish antennas. The last option for communication design on the rover is optical communication. This is a relatively new technology, so there are many unsolved problems in the area. The hardware isn’t widely available, and would have to be custom designed and built. This makes optical communication an unfavorable option for use in the design of Red Rover. In the final selection of the design of the communication system, the disadvantages and advantages of option each must be considered and weighed. More information regarding selection is provided in the Concept Variants Evaluation and Selection section of this report.        

In order to determine the appropriate frequency band for use in the communications system of Red Rover, it is necessary to compare the features of each band. The Ka frequency band has extremely high power demand due to its very high attenuation[3]. Therefore, in order to minimize the power required to operate the communications system, the Ka frequency band will not be utilized in the Red Rover communications design. Next, bandwidth allocation is not guaranteed for the Ku band, therefore it is not the best option. The S and X frequency bands are comparable, as both have low attenuation. The communication system for the Mars rover will use the X frequency band, as this is the most commonly used band for space research.

            Next, after evaluation of the possible paths of communications for the rover, there are two paths selected. The first is to transmit data from the rover to a Mars orbiter in small bursts. The orbiter will then utilize its own power to forward the transmission on to Earth and the base station. Second, there will be an alternate path of communication from the rover directly to the base station. This will be incorporated into the design primarily as a back-up path of communications, and will not typically in use during normal operation of the rover. This helps to avoid any single-point failure in the design of the communications system should a problem arise with the rover-to-orbiter transmissions. The base station could utilize its power resources to transmit to Earth, should this become necessary.

Finally, the antennas selected in the design of the communications system of Red Rover are phased array antennas and omni-directional antennas. The Tigernauts hope to utilize phased array antennas to transmit to the orbiter. The phased array antenna does not need to be mechanically pointed, and this is an overwhelming advantage as the rover be traveling and will not necessarily be an agile vehicle capable of fine-tuning and strategic positioning. Omni-directional antennas (UHF/VHF) will be used for intercommunication between the rover and the base station on Mars. As stated previously, this is primarily a back-up path of communication, and will not be in continuous use do to its high power demands. Further details as to the precise size, number, and location of antennas in the communications system design will be described as the Red Rover project progresses.

As of November 14th:

Communications & Navigation

I. Design Requirements

One of the most important design requirements for the communications system of Red Rover is that there can be no single point failure in the system. This is to say that individual failures in the system must not cause the entire communications system to cease proper operation. There must be back up designs for transmitting and receiving data should one of the antennas malfunction. Additionally, the design must be able to incorporate receiving commands (from Earth or base station) at all times, even during the transmission of data from the rover.

            In addition, the communications from the rover to the base station, earth, and/or orbiter are limited to the maximum data transfer rate of which the rover’s technology is capable.  This maximum data transfer rate is, in turn, limited mainly by bandwidth allocation for the transmissions. Red Rover will be transmitting data using the X-band frequencies (8450-8500 MHz), which are designated for deep space research.

            Requirements for the navigation system are that the location of the rover at any given time is known by the scientists on Earth and the astronauts in the rover and at the base station. Also, the astronauts operating the rover need a way of determining which direction they are heading with respect to some fixed point.

 II. Design Options

            The path of communications for the rover will determine the type of hardware used in the design. There are multiple options for the path of communications. First, the rover could transmit data to the base station, which could then forward the transmission to Earth. Second, the rover could store information and transmit to a Mars orbiter in small bursts.

Earth

Base

Orbiter

Rover

1. Transmits to orbiter

2. Transmits to base station and Earth

Figure 1: Example Path of Communication

The orbiter would then forward the transmission to the base station and to Earth. This path is shown in Fig. 1. Third, the rover could transmit directly to Earth, and the control station on Earth could relay the information to the base station on Mars, if required.

There are four main options for antennas in the rover design. These are omni-directional antennas, parabolic dish antennas, phased array antennas, and optical communication antennas. These options differ greatly in their operation, power consumption, and applicability to the rover design.

Omni-directional antennas have been used in several NASA spacecraft designs, such as the unmanned rover Sojourner used on the Mars Pathfinder Mission in 1997[4]. However, they have very high power requirements to transmit data, therefore using these as the primary link for the Mars Rover is in all likelihood not a feasible option. The next option is a parabolic dish antenna. Parabolic dish antennas are widely used in space communications. The technology is mature. They provide high gains, which enables high bandwidth communications without consuming too much power. However, they perform best on stationary or very slow moving platforms. They must also be mechanically pointed with great precision to function properly.

Another option is the phased array antenna. These have several advantages. They are “electronically” pointed by adjusting the phases of the transmitters, which allows for fast and precise steering. This would be a great advantage as the rover will not necessarily be an agile vehicle capable of fine-tuning and strategic positioning.  Phased array antennas are also more efficient than parabolic dish antennas.

The last option for communication design on the rover is optical communication. This is a relatively new technology, so there are many unsolved problems in the area. The hardware isn’t widely available, and would have to be custom designed and built. This makes optical communication an unfavorable option for use in the design of Red Rover.

III. Final Design

Communications

            The primary path of communication will relay all video and voice data to a Mars -geosynchronous orbiter. This will be accomplished using a phased-array antenna, which will operate in the X –band (Space Research frequency band). The phased array antenna will consist of 683 transmitter elements, shown in Figure 4.0. The data requirements are estimated to be 7.5 Mbps, and all communications will be made in a bandwidth of 10 MHz.

As mentioned previously, the phase of the transmitters can be shifted to electronically “point” the antenna. This feature of the antenna will compensate for instability in rover vehicle motion during transmission. The phased array will have scan capability of +/- 60 degrees to account for this motion. Details of the link budget are provided in Table 4.0.

           

Figure 4.0 Phased array antenna[5]

 

 

Table 4.0 Link budget for phased array antenna[6]

The orbiter traveling around Mars will be equipped with Electra, a UHF telecommunications package that relays data to and from the rover[7]. By using the forwarding and return-link relay services of the orbiter, the mass and radio power requirements for the communications system of the rover are significantly reduced. The orbiter will use its own power resources to forward the transmission to Earth and the base station. By transmitting data to a Mars-geosynchronous orbiter, Red Rover is ensured a constant communication link to the base station and Earth.

For direct communications between the rover and the base station, Red Rover will utilize an omni-directional, low gain, UHF monopole whip antenna. This link will serve as a back-up path of communications for use in emergencies. Low gain antennas do no require accurate pointing because they transmit data with a very broad beam width[8]. Therefore, should an emergency arise in which the rover is inoperable, the antenna would not need to be mechanically pointed in order to transmit successfully. However, this feature comes with a price: low gain antennas require high power levels to transmit. In order for the rover to use a low gain antenna, the data transfer rate must be quite low. The maximum data rate using the low gain antenna for communications is 600 bps, so transmission of video data is not possible. However, since this link will be utilized in emergencies, a simple “SOS” signal is the only necessary transmission.

Figure 4.1 Pattern of the monopole whip antenna mounted close to the Martian surface[9].

 

The UHF monopole whip antenna is compact with a height of 25 cm and a mass of 0.25 kg. When the antenna is vertically positioned, it will radiate a vertically polarized field with a broad null in the axial direction. By mounting the monopole close to the Martian surface, the null is formed in the horizontal direction. This is due to its field reaction with the particles on the Martian surface at a small incident angle, as seen in Figure 4.1. This null behavior can be ignored for short distance communication between the rover and the base station.

Red Rover will have an omni-directional antenna to receive commands from Earth and the base station. Furthermore, this antenna can be used to transmit in the event that both the phased array and the monopole whip antenna cease to function. Should this happen, the transmission would go directly to the Mars-geosynchronous orbiter, which would forward the signal to Earth and the base station. This antenna does not need to be pointed mechanically, so the rover will not need to alter its position in any way to be able to receive/send transmissions from/to Earth[10].

Additionally, the rover will carry two identical transponders; one will be used as back-up in case the first malfunctions. The purpose of the transponder is to translate digital signals into radio signals that can be sent to the orbiter. Similarly, the transponder translates the received radio signals into digital signals that the rover computer can interpret[11]

 

Table 4.1 Mass and Power summary for communications system

 

 

Mass

Average Power

Component

#

 (Kg)

(W)

UHF Monopole Whip Antenna

1

0.25

12

Phased Array Antenna

1

12

90

Transpondera

2

0.75 each

5.0 each

Omnidirectional receiver

1

1

10b

 

 

 

 

 

TOTAL :

14 kg

107 W

 

a Only one in use

 

 

 

b No power required unless phased array fails

 

A summary of the mass and power requirements for the communications system is presented in Table 4.1. Different failure modes for the communication system and their responses are presented in Table 4.2. As shown in the table, there is no single point failure of the communication system.

 

Table 4.2 Failure modes of the communications system

Component

Failure Mode

Effect

Response

Criticality

 

Single / Multiple Transmitter

Insignificant

none

Minor

Phased Array

All transmitters

Inability to transmit

Can still communicate

Moderate

Antenna

 

video data

via monopole or

 

 

 

 

omnidirectional receiver

 

 

 

 

 

 

UHF Monopole

Single failure

None, but loss of

Use omnidirectional

Minor

Whip Antenna

 

first back-up if phased

receiver as second

 

 

 

array fails

back-up link

 

 

 

 

 

 

 

 

 

 

 

Omnidirectional

Transponder failure

None

Use back up

Minor

receiver

 

 

transponder

 

 

 

 

 

 

 

 

Navigation

Since the rover will be traveling out of sight of the astronauts inside the base station, it is important to know the relative position of the rover at all times. This is accomplished by using the Electra package on the Mars-geosynchronous orbiter. Besides functioning as a communications relay, Electra can provide specific information regarding the position of the rover.  To achieve this, there must be a second orbiter circling Mars. By combining the second orbiter’s position information and the Mars-geosynchronous orbiter’s information, Electra can provide precise Doppler data which determines the location of Red Rover on the surface of Mars[12].  This information is relayed the base station as well as to Earth, so that the position of Red Rover is known. Furthermore, it is important that the astronauts operating Red Rover have a way to know which direction they are heading. This is achieved by a directional gyro unit which would provide a stable reference for Red Rover’s heading with respect to Martian north.

[1] Bapna, D. et al. Earth-Moon Communications from a Moving Lunar Rover. The Robotics Institute, Carnegie Mellon University. Proceedings of the 42nd International Instrumentation Symposium, May, 1996, pp. 613-622.  

[2] Williams, Dr. David R. Mars Pathfinder Rover. National Space Science Data Center. 1997. < http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=MESURPR>.

[3]LeMaster, Edward A. et al., Mars Navigation System Utilizes GPS, IEEE AESS Systems Magazine, April 2003, vol. 14, pp.3-8.

[4] Williams, Dr. David R. Mars Pathfinder Rover. National Space Science Data Center. 1997. < http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=MESURPR>.

[5] http://www.ri.cmu.edu/pub_files/pub1/bapna_deepak_1996_1/bapna_deepak_1996_1.pdf

[6] http://www.ri.cmu.edu/pub_files/pub1/bapna_deepak_1996_1/bapna_deepak_1996_1.pdf

[7] http://mars.jpl.nasa.gov/mro/mission/sc_instru_electra.html

[9] http://tmo.jpl.nasa.gov/tmo/progress_report/42-136/136C.pdf

[10] http://www.ri.cmu.edu/pub_files/pub1/bapna_deepak_1996_1/bapna_deepak_1996_1.pdf

[11] http://mars.jpl.nasa.gov/mro/mission/sc_antennas.html

[12] http://mars.jpl.nasa.gov/mro/mission/sc_instru_electra.html