NASA/TM--1999-209456

In-Space Internet-Based Communications

for Space Science Platforms Using

Commercial Satellite Networks

Robert J. Kerczewski, Kul B. Bhasin, Theodore P. Fabian, and James H. Griner

Glenn Research Center, Cleveland, Ohio

Brian A. Kachmar and Alan M. Richard

Analex Corporation, Cleveland, Ohio

November 1999

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NASA/TM--1999-209456

In-Space Internet-Based Communications

for Space Science Platforms Using

Commercial Satellite Networks

Robert J. Kerczewski, Kul B. Bhasin, Theodore P. Fabian, and James H. Griner

Glenn Research Center, Cleveland, Ohio

Brian A. Kachmar and Alan M. Richard

Analex Corporation, Cleveland, Ohio

Prepared for the

Fifth Ka Band Utilization Conference

sponsored by the Istituto Internazionale delle Comunicazioni

Taormina, Sicily Island, Italy, October 18-20, 1999

National Aeronautics and

Space Administration

Glenn Research Center

November 1999
NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Price Code: A03

Available from

National Technical Information Service

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Price Code: A03
IN-SPACE INTERNET-BASED COMMUNICATIONS FOR SPACE

SCIENCE PLATFORMS USING COMMERCIAL

SATELLITE NETWORKS

Robert J. Kerczewski *, Kul B. Bhasin, Theodore P. Fabian, and James H. Griner

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Brian A. Kachmar" and Alan M. Richard

Analex Corporation

3001 Aerospace Parkway

Cleveland, Ohio 44142

ABSTRACT

The continuing technological advances in satellite communications and global networking have

resulted in commercial systems that now can potentially provide capabilities for communications

with space-based science platforms. This reduces the need for expensive government owned

communications infrastructures to support space science missions while simultaneously making

available better service to the end users. An interactive, high data rate Internet type connection

through commercial space communications networks would enable authorized researchers anywhere to control space-based experiments in near real time and obtain experimental results immediately. A space based communications network architecture consisting of satellite constellations connecting orbiting space science platforms to ground users can be developed to provide

this service. The unresolved technical issues presented by this scenario are the subject of

research at NASA's Glenn Research Center in Cleveland, Ohio. Assessment of network architectures, identification of required new or improved technologies, and investigation of data

communications protocols are being performed through testbed and satellite experiments and

laboratory simulations.

INTRODUCTION

In satisfying the need for lower cost and higher performance for space-based research missions,

the National Aeronautics and Space Administration (NASA) is moving away from proprietary

special purpose, government-owned and operated communications systems. The current and anticipated future growth in commercial satellite communications networks is well timed to provide lower cost alternatives. Many of the planned new systems in low, medium and high earth

orbits can provide a range of coverages and data downlink capacities that can potentially meet

the needs of a variety of future space missions. Ka-band systems in particular have the potential

of providing much higher bandwidth and availability for space science applications without a

major government infrastructure investment.

* Phone: (216)433-3434, Fax: (216)433-8705, E-mail: rkerczewski@grc.nasa.gov

t

Phone: _216)433-8655, Fax: (216)433-8705, E-mail: bkachmar_grc.nasa.gov

NASA/TM-- 1999-209456 1
In addition to the anticipated cost reductions obtained using commercially available communications assets, greatly improved performance of future space science missions is also a major goal.

A key to improving accessibility, data integrity, and response times to the end user will be the

implementation of Internet connectivity between the space-based experimental platform and the

experimenter on the ground. An interactive Internet connection would enable authorized

researchers in virtually any location to control experimental parameters and retrieve the resulting

experimental data.

The feasibility and potential performance of such an Internet connectivity scenario is being explored through laboratory and satellite based research in Internet protocol performance at NASA

Glenn, including tests with NASA's Ka-Band Advanced Communications Technology Satellite

(ACTS). Key architecture, protocol performance, and technology issues are being identified and

active research efforts are underway to enable future space science missions to become orbiting

Internet nodes. NASA Glenn is developing the In-Space Internet Node Testbed Facility to enable this research.

As an example of a space-based research platform for which Internet connectivity would provide

a large improvement in performance, the Fluid Combustion Facility (FCF) will house multiple

experiments on the International Space Station (ISS). Through a network router located on the

spacecraft, individual experiments could be interactively addressed by different ground-based

experimenters, with experimental data immediately available to the experimenter. Other ISS

experiment modules, Space Shuttle experiments, and scientific spacecraft such as the Hubble

Space Telescope are additional examples of in-space Internet connectivity applications. A number of research issues concerning interconnection of orbiting platforms with ground facilities

through satellite networks also apply to aeronautical communications networks which in the future will include mobile aircraft acting as internet nodes, an area in which Glenn Research Center

also has a significant research program.

The following sections will describe the operational concepts of an Internet-based communications capability for in-space experiment platforms using commercial satellite infrastructure. The

key research and technology issues will be identified, and planned approaches for addressing

these issues will be discussed.

ARCHITECTURES FOR IN-SPACE INTERNET CONNECTIVITY

A study of the potential application of six current or proposed commercial satellite networks to

provide data relay services for low earth orbit (LEO) spacecraft and other space missions showed

that a large range of maximum daily data throughput, from 1.5 Mbits to 288 Gbits, could be

obtained, depending on the particular satellite network and the space science platform's orbital

characteristics t_] . The corresponding average service time per orbit, maximum null time, and

user terminal requirements also varied considerably. Hence, fulfilling a particular space

science mission need for Internet connectivity to ground users would require consideration of a

number of possible commercial satellite solutions, including a combination of two or more

satellite networks.

NASA/TM-- 1999-209456 2
Figure1is adepictionof potentialconnectivitybetweena user/experimenteron the ground and

an orbiting space science platform. Depending on its orbital characteristics, the science platform

has the potential to connect to commercial communications satellites in low, medium or geostationary earth orbits (LEO, MEO, or GEO, respectively). LEO or MEO satellites can relay

through GEO satellites to ground stations or can connect directly to gateway earth stations.

LEO, MEO and GEO constellations may include intersatellite links allowing access to specific

earth station locations. Experimenters can access the space science platforms through the gateways using standard terrestrial Internet connections, or more directly using their own dedicated

earth station.

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GEO ............................................................

;\ ME_y _."'_ ''_'_ _ ...'""_'"'" _,

\\\ S'_tence Platform _,, ''/ _ ,_/,/,, _'_¢

Experimenter

'; Gateways /'"""

4 ............. "_

Figure I - In-Space Science Platform Intemet connections to a Ground Based Experimenter.

The complexity of such architectures ranges from a relatively simple connection of an experimenter to a space science platform through a single GEO spacecraft to a complex connection

including a combination of LEO or MEO constellations with intersatellite linking, relays through

GEO satellites, and terrestrial Internet connection to a commercial gateway. The simplest architectures will present the fewest implementation problems, but more complex architectures

may be required to meet space science mission goals for coverage, connectivity time, and

data throughput.

NASA/TM--1999-209456 3
KA BAND IMPLEMENTATIONS

The architectures described above are generic in that they do not include specific existing or

planned commercial satellite networks. Many existing or planned satellite networks may be

amenable to inclusion in these architectures, regardless of their operating frequency band. However, Ka-Band satellite networks have several potential advantages over lower frequency band

satellites for the application being discussed.

A primary advantage of Ka-band is the availability of frequency spectrum adequate for high

data rate links. Lower frequency band satellites generally have less available bandwidth and are

currently fairly congested with other applications. A second advantage of Ka-band is that

hardware is smaller and lighter than for lower bands, reducing the cost of placing science

spacecraft in orbit.

Perhaps the most important potential advantage of Ka-band satellite networks is that none have

yet been built or deployed. More advanced technology, especially satellite payload designs

which are intended for Internet connectivity, is available for these next-generation systems. The

potential therefore exists to make great use of commercial satellites that are being designed for

Internet usage in creating Internet type connectivity to space science platforms.

RESEARCH AND TECHNOLOGY ISSUES

Operation of an Internet-like network in a rapidly changing physical environment presents major

technical challenges. Depending upon the complexity of the network architecture, significant

variations in the total path length between the space science platform and the ground-based experimenter will occur. The relative motion between the orbiting space science platform and the

network of commercial communications satellites (LEO, MEO, GEO or combination) combined

with the motion of the communications satellites relative to the ground stations can result in potentially complex path length profiles. In addition, hand-offs between communicating spacecraft

occur as the commercial satellites come into and move out of view of the space science platform.

The frequency of hand-offs could range from several per orbit to several per minute depending

on the commercial communications network and network architecture being used.

The path length variations result in a corresponding variation of RF propagation parameters that,

in turn, affect the signal quality, bit-error rate, and protocol performance. The two key RF

propagation parameters are the signal delay and the signal strength. The variation of signal delay

primarily affects the performance of Internet protocols as discussed in the following section.

The variation in signal strength, where longer path lengths result in reduced signal strength at

both space and ground receivers, creates a corresponding variation in the bit-error rate of the

transmitted data. Hence, the longer path lengths induce more errors, requiring more frequent retransmission of data packets and reducing the efficiency of the transmission. Overall network

performance will vary in a complex fashion, and understanding these variations and designing

the network architecture to accommodate them is important in meeting the goals of the space

science mission.

NASA/TM--1999-209456 4
Advanced hardware technology required for an in-space Internet network includes primarily

space-based tracking antennas and routers. Multi-element phased array antennas which can

electronically steer to acquire and track moving spacecraft, and rapidly re-aquire new spacecraft

during hand-offs must be made as small, lightweight and affordable as possible. Active phased

arrays based on monolithic microwave integrated circuit (MMIC) and other technologies are

being developed and tested for these applications under programs sponsored by NASA Glenn.

The tracking accuracy of antennas can affect the RF link performance, inducing additional errors

and gaps in connectivity and must be accurately understood in order to evaluate the potential performance of the network.

Network routers, performing functions similar to high-speed routers that currently provide the

switching functions for the Internet, must be developed for use in space in order to achieve a true

Internet type network in space. The routers must be small, lightweight, low power and radiation

tolerant to enable an efficient and cost effective space network. NASA Glenn Research Center is

beginning a program to develop touters for space applications that are compatible with the commercial Internet.

PROTOCOL ISSUES

Today's terrestrially based Internet network components generally remain fixed, relative to each

other. However, in moving from a terrestrially based network toward including space-based

nodes, touters, etc., one starts exercising the limits of the protocols developed for a terrestrially

based network. Two distinct protocol areas that are the focus of NASA Glenn's research are

transport protocols and routing protocols.

In the transport protocols area, the main focus is on TCP (Transmission Control Protocol) I21,

since the reliability and congestion control portions of this protocol are linked to the delay and

bit-error rate of the network. The effects of TCP over a ground-to-GEO satellite-to-ground link

have been well documented r31. However, when the delay is not fixed, for instance, in a groundto-GEO satellite-to-LEO satellite link, the varying delay may have an effect on the performance

of TCP. If the round-trip-time (RTT - time for data to be sent from the source computer to the

destination computer plus the time for the acknowledgment of the received data to be returned to

the source computer) changes too rapidly, TCP may assume a piece of data has been lost, when

in fact the acknowledgment for the data is on its way back. Such actions would greatly decrease

the efficiency of transmission and reduce network performance. Research is being conducted to

determine protocol adaptations that can accommodate the delay variations expected in a complex

space-based network and improve overall network performance.

The second area of study involves Internet routing protocols. Due to the movement of satellites,

relative to each other, there are times when a line-of-sight connection cannot be maintained between the two objects. Consider the simple case of a LEO communicating through a GEO to the

ground. In this scenario, when line-of-sight communication is lost to the GEO, the satellite

either must establish a connection to another GEO satellite, on the other side of the earth, or wait

until it passes around the earth to reacquire the previously lost GEO. In this case, changes in the

routing of packets may need to be made, in order to allow the LEO to start transferring data over

this new link. If the scenario is changed to communications between two LEO satellites, the

routing must be changed more often to keep relatively constant communication between the

NASA/TM-- 1999-209456 5
objects. Thismayincludethe two LEO satellites communicating over the same GEO satellite

or grow to a LEO-to-GEO-to-ground station-to-terrestrial Internet-to-ground station-to-GEOto-LEO connection. In this case there is a potential of multiple routing changes taking place

in a single orbit, with periods of outages or data lost due to packet corruption, re-ordering,

duplication, etc., during hand-offs, which decrease the performance of the transport protocol and

degrade the performance of the network.

Figure 2 displays the round trip times for communications in a typical LEO-GEO-LEO scenario.

Round-trip time (RTT) delays are graphed versus one orbit of time (approximately 101 minutes)

between two hypothetical LEO satellites, over one or more GEO satellites. Lower RTTs occur

when the two LEO satellites are within line-of-sight of the same GEO satellite. Higher RTTs

occur when the LEO satellites are communicating over different GEO satellites, and must be

patched together using a link between terrestrial ground stations. There are two loss of signal

(LOS) periods in this particular orbit, when either one or both of the LEO satellites are not within

line-of-sight of a GEO satellite. These routing problems grow with the size of the constellation.

NASA Glenn is researching new routing protocols, based on predictions of satellite movements,

to combat this routing problem.

1100

1000

"_ 900

_m 800

# 700

C

o 600 I:l:

L A

i

i

i

5o0 T

400

0 20 40 60 80 100 120

Tim e (M in)

Figure 2 - Hypothetical RTT Delays Between Two LEOs Communicating Over One or More GEOs

THE IN-SPACE INTERNET NODE TESTBED FACILITY

The In-Space Internet Node Testbed (ISINT) is a modular emulator of a RF satellite system, combined with data terminals, routers and Internet network elements to create a simulator/testbed for investigation and research of space-based Intemet networks. ISINT also contains interfaces to actual

satellite links, such as NASA's Advanced Communications Technology Satellite (ACTS). Figure 3

is a functional block diagram of the testbed. It is a part of the satellite communications research facilities at NASA Glenn Research Center. The facility is configurable to enable the emulation of a

variety of satellites, constellations, and combinations of space network elements.

NASA/TM-- 1999-209456 6
J

FltlI_l ar M_I:IIII

G _ou r'_ mlllt Qn

RF _ace Lirl.k T_,_tb_d

IR tl_' N_<3m

- . . :- - . . -

J

Figure 3 - ISINT Functional Block Diagram

The main component of the ISINT facility is the ACTS proof of concept model, which mirrors

the functionality of the ACTS satellite I41. Dynamic range link delay, the enhancement that allows emulation of moving satellites is done via a modified version of the Ohio Network Emulator software (ONE) [%. The ONE software monitors the link, and all packets on it are brought

into a buffer. By holding those packets for a fixed length of time, the delay function is achieved.

When the fixed time expires, the packet is released from the buffer and sent out the other network interface. By reading values from a predefined data set with orbital characteristics of a satellite constellation, the desired delay is infused into the emulation. Effectively, this allows for a

variable delay. The data set contains information on how long each delay should be, and at what

rate each delay should be changed.

Typical signal variations for a LEO spacecraft communicating via a GEO satellite are shown in

Figure 4, which is based on a simulation of a Space Shuttle communication link through a

Tracking and Data Relay Satellite (TDRS). The signal variation depicted can result in a bit-error

rate variation of up to six orders of magnitude, hence accurate simulation of this parameter is essential. Signal variation is modeled by varying the power received at the modems. Configurable

computer software facilitates automated signal power variation within the RF links of the ISINT

testbed. User defined orbit data sets create a representation of the signal variation experienced

by a spacecraft to communication satellite link. By changing the data sets, the complexion of the

emulation can be varied to implement rain fade, antenna acquisition and tracking models, satellite constellations, and architectural scenarios.

NASA/TM--1999-209456 7
TDRS_5 to Space Shuttle over Arbitrary 24-hour Period

46000.00

Time (hh:mm:ss)

Figure 4 - Typical Signal Variation

As use of the Internet increases, there is an ever-increasing demand for more bandwidth over

which to communicate. As previously discussed, Ka-band satellites can provide the needed

bandwidth, but standard Internet protocols have not been sufficiently tested over high speed,

long and variable latency links. Testing over the ISINT facility has begun using a single T 1 link

over both the RF testbed and over an actual ACTS link, with future experiments expanding to

higher data rates. 30 transfers of each file size ( 50 kbytes, 750 kbytes, 30 Mbytes, and

50 Mbytes) were sent over ACTS and over the Ka-band RF testbed from a simulated "space

node" to a "terrestrial node." Although this type of point to point communication over a GEO

transfer link is well understood, it is useful to validate the configuration of the testbed before

more complex architectures are pursued. Analysis of the file transfer times found that there was

less than 1% difference between files transferred over ACTS versus over the ISINT facility for

file sizes greater than 1 Mbyte [4]

Verification of the testbed's throughput performance compared to ACTS throughput was the first

step toward testing the more complex architectures and link paths discussed previously. Testing

using variable range delay has just begun. Future tests will validate the use of Internet protocols

over OC-1 and OC-3 satellite links. Although the need of 52 Mbps and 155 Mbps channels for

Internet traffic seems inconceivable today, a quick review of the growth of the Internet over the

past 10 years indicates a likely need for more bandwidth and faster devices. Undoubtedly,

Space Shuttle, ISS, and eventually other science spacecraft will need this kind of speed and

bandwidth also.

Additional long-term plans for the ISINT facility involve investigations into the feasibility of

using Point-To-Point Protocols (PPP) over variable delay satellite networks and into the effects

of the delay on remote control of instruments and devices.

NASA/TM-- 1999-209456 8
SUMMARY

The rapid advance of space communications and internet technologies can be harnessed to provide significantly better performance and lower costs for future space-based science experiments

and other space missions. In particular, Internet methods can allow ground based experimenters

direct control of orbiting experiments, greatly improving the value of space science platforms,

using commercial and especially Ka Band satellite networks. However, meeting mission needs

and optimizing experimental value may require complex space network architectures. To enable

efficient performance of such networks, the NASA Glenn Research Center is performing

research on critical protocol and technology issues. Using testbed and satellite experiments and

laboratory simulations such as the ISINT facility, NASA is developing approaches and technologies that will allow standard Internet techniques to be applied to control of future space science

mission experiments and collection of resulting data.

REFERENCES

[1] Younes, B., Flaherty, R. Chang, S., Berman, T., Chang, R., Lease, R., "Assessment

of Emerging Networks To Support Future NASA Space Operations", Fourth

Ka-Band Utilization Conference, November, 1998.

[2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981.

[3] Allman, M., Glover, D., and Sanchez, L., "Enhancing TCP Over Satellite Channels using

Standard Mechanisms", RFC 2488, BCP 28, January 1999.

[4] Richard, A. M. "ISINT Performance Validation Test Report", Technical Report, NASA

Glenn Research Center, June 1999 (publication pending).

[5] Allman, M., Caldwell, A., and Ostermann, S., "ONE: The Ohio Network Emulator" Technical Report TR-19972, Ohio University Computer Science, August 1997.

NASA/TM--1999-209456 9
REPORT DOCUMENTATION PAGE Fon_Approve_

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1. AGENCY USE ONLY (Leave b/ank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

November 1999 Technical Memorandum

5. FUNDING NUMBERS 4. TITLE AND SUBTITLE

In-Space Internet-Based Communications for Space Science Platforms Using

Commercial Satellite Networks

6. AUTHOR(S)

Robert J. Kerczewski, Kul B. Bhasin, Theodore P. Fabian, James H. Griner

Brian A. Kachmar and Alan M. Richard

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-632-50-5A-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-11954

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--1999-209456

11. SUPPLEMENTARY NOTES

Prepared for the Fifth Ka Band Utilization Conference sponsored by the Istituto Internazionale delle Comunicazioni,

Taormina, Sicily Island, Italy, October 18-20, 1999. Robert J. Kerczewski, Kul B. Bhasin, Theodore E Fabian, and James

H. Griner, NASA Glenn Research Center; Brian A. Kachmar and Alan M. Richard Analex Corporation, 3001 Aerospace

Parkway, Cleveland, Ohio 44142. Responsible person, Robert Kerczewski, organization code 5610, (216) 433-3434.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 17 Distribution: Nonstandard

This publication is available from the NASA Center for AeroSpace Information, (301) 621-0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The continuing technological advances in satellite communications and global networking have resulted in commercial

systems that now can potentially provide capabilities for communications with space-based science platforms. This

reduces the need for expensive government owned communications infrastructures to support space science missions

while simultaneously making available better service to the end users. An interactive, high data rate Internet type

connection through commercial space communications networks would enable authorized researchers anywhere to

control space-based experiments in near real time and obtain experimental results immediately. A space based communications network architecture consisting of satellite constellations connecting orbiting space science platforms to ground

users can be developed to provide this service. The unresolved technical issues presented by this scenario are the subject

of research at NASA's Glenn Research Center in Cleveland, Ohio. Assessment of network architectures, identification of

required new or improved technologies, and investigation of data communications protocols are being performed through

testbed and satellite experiments and laboratory simulations.

14. SUBJECT TERMS

Space communications: Satellite networks; lnternet

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A03

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