by February 6, 2003 0 comments



Earlier satellites used customized systems for communication, with each mission or set of missions being more or less self-contained, and no one thought of interpretability with other systems. Now, as the cooperation among agencies and nations grow, interpretability becomes important.

Recognizing the fact, NASA, the US Defense department and National Security Agency of the US have jointly designed, specified, implemented and are testing a set of protocols called Space Communi- cations Protocols Standards (SCPS).

The SCPS suite currently has four modules: file handling, transport, security and network. Instead of being an entirely new system, it is a new version of the existing standards (TCP/IP, File Transfer Protocol) optimized for wireless networks and satellite links. So, to the end user, the new standard functions the same as TCP/IP.

 
Different
Rules

Consider the default response to packet loss. In the wired terrestrial environment, packet loss can be attributed to network congestion. So, the right response of TCP is for the sending entity to reduce its transmission rate to lighten the congestion. But, in space packet loss can likewise come from congestion, but it can also be caused by the satellite going over the horizon, or by ratty (high-bit error rate) links caused by rain, solar flares, or interference from other signals or satellites. The recovery times from errors caused by such conditions are usually long, therefore, reducing the transmission in such a situation results in a lower throughput on the noisy channel, which is exactly the wrong response.

The SCPS addresses layers three through seven of the OSI model. Its major part covers layers three and four of the OSI model and the IP and TCP layers of the TCP/IP protocol suite.

SCPS-File Protocol (SCPS-FP): File transfer protocols coordinate the movement of files between systems. SCPS-FP is derived from FTP with extensions to support the movement of file records, addition of integrity checking, and resume support after interruption.

SCPS-Security Protocol (SCPS-SP): It operates between the Transport Protocol (SCPS-TP) and the Network Protocol (SCPS-NP) and provides Authenti- cation, Access Control, Integrity and Confidentiality. It uses a variety of existing security protocols such as SP3/NLSP, IETF IP Security.

SCPS-Network Protocol (SCPS-NP): Network protocols route data through the intermediate systems to the destination. It functions as the IP (Internet Protocol) with the additional capability of supporting both fixed routing and connectionless routing of packets through space and wireless data links. SCPS-NP also offers multiple routing options and supports packet lifetime control. 

SCPS-Transport Protocol (SCPS-TP): Transport Protocols support end-to-end communications between systems, ie, systems active only at end points and not intermediate routers. SCPS-TP is the most prominent part of the protocol suite since it gives major performance improvement in the space environment. SCPS-TP is a modified version of TCP (Transmission Control Protocol) but the terrestrial environment for which TCP was designed is markedly different from that of space. As an example, 

 
Costliest
Software mistake

June 4, 1996. The first launch of Ariane 5.
40 secs after liftoff, the rocket broke up, taking down with it $500 million worth of launch vehicle and
satellite(s). The enquiry commission appointed to look into the incident concluded that it was a software error. 
The Inertial Reference System requires some computations done to align it till about 9 secs before lift-off. To avoid complications if a lift-off is aborted, the computations continue till 50 secs after liftoff, but are useless. This system was used in the Ariane 4 series and was reused without rechecking in Ariane 5. But Ariane 5 had a completely different set of flight parameters, leading to an error being fed to the computations for the IRS, well into flight! This error, in turn, made the IRS change the flight path of the vehicle drastically, leading to it disintegrating.Talk about reusing software!

 
Satellite
to Net

On October 22, 2001, PSLV —C3 took off form
SriHarikota, successfully launching TES, PROBA and BIRD into space. PROBA (Project for Onboard Autonomy) weighs in under a hundred kilos, but it is unique in many ways. One of the unique factors about PROBA is that it is hooked on to the Internet. Images taken by the cameras on PROBA will directly go to a Web server at its control center in
Redu, Belgium. The images become directly available to users from this Web server, as 
soon as it comes in from 
the satellite. 

 
Webserver
in
space

UoSat-12, launched on April 21, 1999 has the world’s first Web server in space. On January 25, 2001, HTTP was used to transfer data from the satellite to the ground.
AISAT-1, launched by the same company (Surrey Satellite Technology) in December 2002, is the first satellite to use the Internet Protocol for routine in-orbit operations. AISAT-1 implements the CCSDS (Consultative Committee for Space Data Systems) CFDP (File Delivery Protocol), operating over an IP link at 8 Mbps to deliver payload data to the ground.

 
Traffic 
monitoring

If you think that satellites are only for cutting edge work, or for military uses, it is time for you to think again. Computers can combine with satellites to perform some fairly mundane tasks also. Like monitoring traffic congestion.
This was tried out by the European Space Agency between March 1999 and November 2002, in Rotterdam and Den Haag districts of Netherlands. A computer each was installed on to 15 postal service vehicles. Using GPS and a mobile communication system designed to communicate directly with satellites, data from these computers was transmitted to a central monitoring facility. This helped them in understanding traffic congestion patterns better than with traditional systems which measure traffic only at specific points along selected routes. There is no need for interaction between the driver and the computer in the vehicle.
This method can be extended for fleet management, breakdown support, theft protection etc, based on a network of satellites.  

 
Internet2
and the search for ET

The world’s largest radio telescope at
Arecibo, Puerto Rico sifts through 800 terabytes (819 GB or 840 million MB) of data from deep inside the universe each day. It can gather 40 megabytes of data per second. To handle the enormous data-transmission requirements, it has been using a super high-speed Internet2 connection since late 2001. 
Internet2 is an initiative to build Quality of Service into the Internet by providing dedicated backbone connectivity to academic and other research-intensive Internet usage. The connection is 155 Mbps over a T3 carrier. One famous application of the Arecibo radio telescope is the SETI project, which uses it to monitor radio signals coming from space to search of extra-terrestrial life.

Performance tests of SCPS–TP vs TCP–show that SCPS-TP is well suited to the long delay, high bit-error rate environments of satellites with a performance improvement by a factor of 10 and more.

Anoop Mangla

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