Linux Out of the Real World
The data produced by the payload is sent over a serial connection to the Rack Interface Computer. It bounces around on the orbiter for a little while, then is beamed to ground side via a satellite in NASA's Tracking and Date Relay Satellite System (TDRSS, everyone calls them Tetris satellites). The data goes through some other NASA systems (including communications-relay vessels in the Pacific Ocean) and, finally, makes it to MSFC. At MSFC, it enters a machine named (in good NASA style) the “Virtual Remote Users Gateway”, VRUG for short. The VRUG is connected via a dedicated NASCOM line to our Remote Payload Operations Command Center (Remote POCC) in Boulder. The data then goes into a pile of ISDN-to-Ethernet routing hardware and into a network card in our ground side support computer (another Linux machine, used for development of the experiment's software and the analysis of returned data). On its screen, the ground side support computer displays squiggly lines (which the biologists like to look at) and pictures of plants. Another channel going up to orbit from ground side exists using the same hardware interfaces (RIC and VRUG).
The data from the payload describes the conditions in the orbiter. From Boulder, it is sent over the Net to the ground control experiment in Florida. The ground control is similar to the payload in orbit, but it has an Ethernet card instead of the third serial port and a fragile (but cheap) and spacious magnetic hard disk of the garden variety. The ground control produces data of the same form as the orbiting experiment, but with (hopefully) different content. This data is sent back over the Net to the support machine in Boulder for analysis.
An unusual instance of a common problem affects communications with the orbiter. Each TDRSS satellite can see a small portion of the sky: when the limb of the Earth passes between the orbiter and the satellite, line of sight is lost and no data can be sent between them. Several TDRSS satellites are in orbit, and large portions of the orbiter's possible locations are covered, but not all. When no satellites are visible from the orbiter, no communication is possible. This situation is called a Loss Of Signal (LOS). NASA announces the LOSs with high accuracy and long precognition, but they still cause headaches for experimenters. (E-mail your politicians and ask for more Tetris satellites.)
NASA does not guarantee the delivery or correctness of data sent through their pipes. I once asked a member of their technical staff how reliable the channel is, and he replied “Oh, I think probably no more than one corrupt or dropped character in a hundred.” Observations made during last year's experiment indicate that the error rate is significantly lower than that estimate.
When you rent volume on the orbiter for your experiment, you can also rent bandwidth to ground side. You must specify the number of bits per second to reserve for your payload, and you are guaranteed no less. You then get a connection to the Rack Interface Computer. The RIC presents a three-wire RS-232 connection: transmit and receive only, no handshaking.
Data generated by the experiment must be encapsulated in little packets in accordance with a specification from NASA. The fields in the header and footer of these packets are used for routing within the orbiter's communication equipment and include a checksum. If the RIC accepts your packet, NASA will do their best to deliver it to your machine at ground side, but no guarantees are made. Data sent back to the payload from ground side is encapsulated in the same packets and go over the same wires. All packets traded between the payload and the RIC contain data that is from or to the ground side support computer, wrapped in RIC packets.
There is no change at the RIC interface, no automated notification from NASA to the payload that a LOS is imminent or occurring. The obvious way to do this notification would be to use the regular RS-232 handshaking lines.
Our Remote POCC is in Boulder, Colorado. At this end of the line, NASA presents a twisted pair Ethernet interface. You connect using TCP/IP to two specified ports on two specified hosts on this network. These computers are collectively called the Virtual Remote Users Gateway (VRUG). The VRUG interface is more complex than the RIC interface.
All communication with the VRUG (in both directions) is encrypted. The computer people at MSFC asked us to identify our operating system, and then supplied us with two object files, containing compiled C-callable functions to encrypt and decrypt data. The data sent over the TCP stream between our computer and theirs is packetized, but using packets different from those used by the RIC. These packets can contain data to and from the orbiter, commands to configure the VRUG interface and “telemetry” data from the orbiter (a standard data set provided by NASA at no extra charge, describing ambient conditions within the orbiter). Checksums are dutifully computed and checked on data going over the TCP link.
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