Posted on 02/02/2003 3:02:23 AM PST by kattracks
A timeline of events in the last flight of space shuttle Columbia. All times EST.
-- Jan. 16, 10:39 a.m.: Columbia rockets into orbit from Kennedy Space Center.
-- Feb. 1, 8:15 a.m.: Columbia fires braking rockets, streaks toward touchdown.
-- 8:53 a.m.: NASA loses temperature measurements for shuttle's left hydraulic system.
-- 8:58 a.m.: NASA loses measurements from three temperature sensors on shuttle's left side.
-- 8:59 a.m.: NASA loses eight more temperature measures and pressure measures for left inboard and outboard tires. One of the measurements remains visible to crew on a display panel; which crew acknowledges.
-- 8:59 a.m.: Final transmission. Mission Control radios: "Columbia, Houston, we see your tire pressure messages and we did not copy your last." Columbia replies: "Roger, uh, ..."
-- 9 a.m.: NASA loses all data and contact with Columbia at 207,135 feet.
-- 9 a.m.: Residents of Texas, Arkansas and Louisiana report hearing "a big bang" and seeing flames in the sky.
-- 9:16 a.m.: Columbia's scheduled landing time.
-- 9:29 a.m.: NASA declares an emergency.
-- 9:44 a.m.: NASA warns residents to stay away from possibly hazardous debris.
-- 11 a.m.: Kennedy Space Center lowers flag to half-staff.
-- 2:05 p.m.: President Bush announces: "Columbia is lost; there are no survivors."
-- 8:58 a.m.: NASA loses measurements from three temperature sensors on shuttle's left side.
-- 8:59 a.m.: NASA loses eight more temperature measures and pressure measures for left inboard
Anyone have pics of the lift-off?
The final voyage of Columbia
First what is telemetry?: Telemetry is the process of conveying information by transmitting information from one point to another. The Merriam-Webster dictionary term for telemeter is transmit (as the measurement of a quantity) by telemeter. In other words it is a process of conveying information, such as voltages and temperatures, from a remote location to another for subsequent data analysis or interpretation.
On the average satellite there are hundreds, possibly even thousands of individual items (including mission data) that need to be transmitted from the vehicle to a ground station for analysis such as a voltage, temperature, attitude and the like.
It is impractical if not downright impossible to simultaneously transmit all of this data to a ground station. So the data is injected into a serial telemetry stream that only requires one (or possibly a few) links between the vehicle and the ground. This process of inserting all of this data in a specific pre-determined pattern is called commutation.
To do this, the data must be digitized (i.e. converted to ones (1s) and zeros (0s). Due to fidelity reasons, link margin, and communications bandwidth, many of these parameters are sent down as an 8 bit (8 serial 1s and 0s) "word". Without going into the math here, this converts into 0 thru 255 decimal or the number 0 thru FF in hex. Usually these values are called Engineering Units (EUs).
If all data is lost, the number either reverts to 0 or 255, which usually is an out-of-limit condition.
AT THIS POINT I AM SPECULATING: If for some reason the connection to those sensors was severed, the data on the ground may just have shown an out-of-limit maxed one way or the other depending on how the bit stream was designed for that particular parameter.
A more basic word is "multiplexing." If a bunch of separate data channels share a physical link, each coming across as if it had a link unto itself, you're typically either carving up the one physical link's frequency spectrum into channels (frequency-division multiplexing, FDM) or thinly time-slicing it (time-division multiplexing, TDM). I would suspect it's FDM in this case.
We can further sub-divide this telemetry stream into sub frames with a counter or a sub-frame sync to allow greater flexibility in how the parameters will be downlinked. Lets say we are doing something special, like running the robotic arm on the Shuttle. The telemetry stream has a finite space, so using a different set of sub-frames, we can downlink more data at that moment on the arm than our normal State-of-Health (SOH) telemetry monitors.
Often this link is QPSK modulated allowing for a large data rate. I have seen data rates over 300 million (yes three hundred) bits per second.
Note: before the radio frequency (RF) link is modulated with this digital bit stream, the commutation has already occurred. So all the RF link "sees" (and is modulated with) is a stream of continuous ones and zeros.
Anyway, it might be worth mentioning to the lurkers that modern communications protocols nest inside each other rather neatly. The way the Internet is put together is an example. Application programs on different machines (say, the browser on your computer and the server software on FR) talk to each other through "network sockets," which are a software analog of the old phone company socket connector panel from the days when operators physically plugged you into the line of the person you're calling. Your browser program doesn't know or care what kind of modem you have, or whether your connection is broadband or dialup. Lower levels handle that. Your traffic stream goes down to levels which talk to each other, each not knowing or caring much about how other layers work. A lower layer breaks your data stream into packets. Another layer manages merging different packets coming from different programs running on your machine at the same time. (That is, you can be checking your email while your browser downloads a new page. The routing info in the packets keeps them from getting confused.) A lower-yet layer layer handles talking back and forth with your modem. This layer, and only this one, has to know the dirty details of what kind of modem you have.
On FR's server, it's the same kind of thing in reverse. A layer handles the physical link. A layer decodes and sorts the incoming packet stream (all kinds of stuff coming in from various people all over, in jumbled order). A layer breaks the packets open and translates them into data records that the network server applications recognize. The server applications recognize things like, "Here's another goddam 'My comments' request from VadeRetro."
But the shuttle is older than the Internet, so I'm not sure if there's as much commo layering there as I've been assuming. It sounds almost as if they wrote a primitive multiplexed network in the late 70s just to do telemetry.
Didn't see this before I posted my last. Clearly, there's some nesting of protocols.
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