Normally, the bit rate of the telemetry transmitter is set to a value equal to the peak bit rate of the active telemetry sources. Depending on the design of the spacecraft, some sources may be constant bit rate and some sources may be variable bit rate. In traditional spacecraft designs, everything is constant bit rate and there is no need for fill data. In many newer spacecraft designs, various spacecraft subsystems produce telemetry packets which are multiplexed into a single stream that modulates the transmitter. Some science experiments are designed to vary their bit rate depending on external conditions. If you are looking for transient phenomena, like x-ray bursts, you may want to generate telemetry at a low rate when nothing interesting is happening. If a x-ray burst is detected, you can shift to a higher rate to capture the event in greater detail. Some spacecraft have many independent experiments, generating telemetry in a mix of constant and variable rates. If the sum total of the inputs is less than the output telemetry rate, the telemetry multiplexer must insert fill data to keep its output at a constant rate. The telemetry system's output bit rate can be set to a number of values so that it can adapt to current RF link conditions. Most spacecraft have a wide selection of telemetry modes to allow for varying requirements and conditions. These may range from very low bit rate engineering data via the omnidirectional antenna if the spacecraft is in trouble, to high bit rates via the high-gain antenna if everything is working properly and important science data is being transmitted. The engineers on the ground send commands to the spacecraft to set the appropriate mode for current conditions. The spacecraft itself may change modes if it detects a problem or if it hasn't received any commands in a specified period of time.
How much of the price of a ton of steel is attributable to labor costs? Some industrialists like to blame organized labor for all of their problems when they themselves have refused to invest in improved production techniques for decades.
The Space Shuttle does not use 486s. It uses IBM AP-101s, which are architecturally similar to the IBM 360/370 series of computers. See the Second Generation Computers FAQ.
If there is a COTS (commercial off-the-shelf) real-time operating system available that meets the system requirements, why go to the risk and expense of writing your own from scratch? Do you expect NASA to fabricate every component in the spacecraft?
Fill data is typically transmitted when the telemetry multiplexer does not have any engineering or science data to send. Due to the way synchronous communications links work, something is always being transmitted, even if there is no "real data" available.
This is a generic answer, as I am not familiar with the command and data handling systems on Spirit.
Most spacecraft commands are sent as fixed or variable length frames of synchronous data. A frame usually begins with a sync pattern, some header information, the actual command, and a trailer containing a checksum or CRC. There is no protocol, in the sense that most people use. Think of the commands as UDP datagrams. The engineers in the control center monitor the telemetry downlink to verify that the commands sent were received and decode properly. The spacecraft command decoder has parameters in the telemetry downlink such as "# of good commands received" , "# of commands rejected", and "last command received". For simple commands, like "heater #5 ON", you can check the downlink telemetry for the status of heater #5 and verify that it is in the correct state. For complicated operations, like uploading software or large data tables, the spacecraft can be commanded to do a memory dump to the downlink telemetry, allowing the upload to be verified.
As a general rule, spacecraft command processing is kept as simple as possible. You want the command decoder to be extremely reliable, so that even if half the hardware on the spacecraft is broken, you can still send it commands and have them executed. Rather than rely on an automatic protocol, it is usually better to take advantage of the cleverness of the engineers and computers on the ground. Since there is a human in the loop, take advantage of it. The human knows things that a protocol doesn't, like the spacecraft is about to go "over the hill" due to the motion of the Earth and the spacecraft. The engineer may also want to do things like uplink a sequence of commands into temporary storage, verify them with a dump, and execute them only after he is sure that all of the commands were successfully received and decoded. Some commands are designed to require multiple actions before they do something dangerous and/or irreversible to the spacecraft. For example, "arm pyro #36" and "fire pyro #36", which might deploy a solar panel.
I can't believe the number of people who are saying that the Republicans did something illegal or unethical. Sounds a lot like situational ethics.
If you put world readable documents in a public shared folder on a shared computer system, you have no right to complain when other system users read or copy them. You might as well post them on a bulletin board in the committee meeting room. Your intent is irrelevant, your actions are what count.
I'd be interested in knowing whether there is a cheap and simple modification for a standard camera flash unit that would reduce the duration of the flash. The high-speed flash units used regular xenon tubes, just like the ones in regular flash units. I used to own an automatic flash unit that used a thyristor to dump the charge on the capacitor when a photocell detected sufficient light having been reflected from the subject. The circuitry was fairly simple.
I thought that one of the advantages of the Edgerton setup was that he used a fast strobe, something on the order of 1/200,000 of a second. This produced much better detail than a generic camera flash unit.
If I remember correctly, he used a camera with an open shutter in a darkened room, triggering the strobe with the sound of the muzzle blast and an electronic delay.
My reasons for disliking case-insensitive software:
Ugliness -- Instead of a nice set of unique symbols, the system has to deal with the language and location dependent concept of case, which is an ugly mess.
Inefficiency -- Every string comparison is slower.
Imprecision -- If I write "abTable", I mean "abTable", not "abtable", "Abtable" or "ABTABLE". If I wanted it in another form, I would have written it that way in the first place.
Inconsistency -- Each file name and variable name should have a unique and distinct value. Allowing multiple names for a single object just creates confusion, inconsistency and illegibility.
I've worked overseas, both in and out of the military. In both cases, I had to pay state income tax to my legal state of residence, not to mention federal income tax and social security tax. If they are going to tax me, even when I am living on the other side of the world, they damn well better allow me to vote.
Initially, I think what is needed is a cheap home IPv6 router that will automatically tunnel over the cable/DSL IPv4 network to a IPv6 gateway, run by another party. The cable/DSL operators may not like it, as many prohibit VPNs on their current networks. A ban may not be very enforceable.
This echos the early days of the Internet, where IPv4 was layered on top of DECnet, SNA, X.25 and other protocols.
I wouldn't expect to see IPv6 in a firmware update. You will probably have to buy a new box to get IPv6 support.
The interesting thing will be the reaction of the mass-market ISPs, especially cable operators, who tend to view their residential customers as peons down on the farm.
Add another $90 million per year for operations and data analysis.
Every NASA mission, assuming the hardware still works, follows a curve of diminishing science returns. At some point, someone has to make a decision to pull the plug. HST may not be at that point yet, but they have to compete for NASA dollars and resources that can be used to fund new missions.
Scientific American has its own set of biases and prejudices, especially when you leave the realm of hard science and look at political and social issues, where they usually take a leftist line.
It reminds me of the old joke about a mental patient that believed that he was Napoleon Bonaparte. After many years of treatment, he was ready to be released. As a final test, they gave him a polygraph examination. When asked whether he was Napoleon Bonaparte, he said no. The polygraph examiner concluded that he was lying.
It costs a lot of money to keep the HST running. There is a small army of scientists, engineers and technicians that keep it healthy, schedule observations, point it at the right places, and download and distribute the data.
NASA has a severely constrained budget. Every dollar that is spent on HST is a dollar taken away from some other project. From a scientific point of view, you have to ask whether it makes more sense to terminate HST and spend the money on new spacecraft and missions. There are always many more proposals for new spacecraft than there is money to fund them.
If the police want to be assholes, they can hold on to your stuff for a very long time. If it is evidence, they can hold it while criminal proceedings are pending or in progress. If you have the money for a lawyer, he can make motions to the judge to force the government to return your stuff or justify why they need to hold it.
These sorts of problems are common in cases involving guns, where even if you are innocent, you may have to spend many thousands of dollars to get a court order forcing the police to return your property. The BATF is notorious for cases like these, where they seize someone's gun collection on questionable charges, intentionally damage the collection, and even if they drop the charges or lose, refuse to return the collection.
The problem is that many government officials believe that they can ignore the law when it is convenient or advances their agenda. They know that most people don't have the time, money and bloody-mindedness to get a court order and have it enforced.
The problem is that as a rough approximation, any wire a quarter wavelength or longer can be an effective antenna. In the HF band (3-30 MHz), that's anywhere from 25 meters down to 2.5 meters. It doesn't take a very long wire to be an effective radiator, plus you also have to consider the length of the electrical wiring inside the structure.
"Let's put something that looks like high-power broadband RF noise on long, unshielded, untwisted power lines, suspended in the air, otherwise known as antennas."
One idea that's been bouncing around in my head for years is to make an autonomous computer. The idea is to reserve all low-level and security sensitive functions, root access if you will, to the system software. Security policies would be enforced by the system software. There would be no Administrator or root accounts for users. There would be no backdoors for maintenance.
I remember reading about an old computer system, I believe it was a Burroughs computer, that used software to enforce security policy. Executable programs would only be loaded and run if they had a magic attribute set. Users could not set the attribute. Only a limited number of trusted programs, like the system's compiler, could set the attribute. The compiler contained and enforced security policy. It would not allow the user to compile a program that violated the system's security policy. This allowed the system to have enforceable security checks that were implemented in software instead of special purpose hardware.
I believe that current popular operating systems are fatally flawed at the architectural level. Fixing the thousands of implementation bugs will not solve the architectural problems.
If they were really serious about noise, they could use RF construction techniques and put the analog components in a shielded can on the motherboard, with bypass capacitors on the power/ground connections. You can shield anything if you are willing to spend some money.
The government is not that coordinated and efficient. An NSA officer is not going to wander in to a NASA office and ask for a copy of the CDs. Bureaucratic protocol must be followed. If it happened, the Director of the NSA would send a written memo to the NASA Administrator asking for a copy of the CDs. Many bureaucrats and lawyers in both agencies would be involved.
No matter what the law says, it isn't going to stop a lawyer with a subpoena or a government officer with a search warrant. Not to mention other situations where the law requires that a business provide information to the government.
Slashdot Mirror
Normally, the bit rate of the telemetry transmitter is set to a value equal to the peak bit rate of the active telemetry sources. Depending on the design of the spacecraft, some sources may be constant bit rate and some sources may be variable bit rate. In traditional spacecraft designs, everything is constant bit rate and there is no need for fill data. In many newer spacecraft designs, various spacecraft subsystems produce telemetry packets which are multiplexed into a single stream that modulates the transmitter. Some science experiments are designed to vary their bit rate depending on external conditions. If you are looking for transient phenomena, like x-ray bursts, you may want to generate telemetry at a low rate when nothing interesting is happening. If a x-ray burst is detected, you can shift to a higher rate to capture the event in greater detail. Some spacecraft have many independent experiments, generating telemetry in a mix of constant and variable rates. If the sum total of the inputs is less than the output telemetry rate, the telemetry multiplexer must insert fill data to keep its output at a constant rate. The telemetry system's output bit rate can be set to a number of values so that it can adapt to current RF link conditions. Most spacecraft have a wide selection of telemetry modes to allow for varying requirements and conditions. These may range from very low bit rate engineering data via the omnidirectional antenna if the spacecraft is in trouble, to high bit rates via the high-gain antenna if everything is working properly and important science data is being transmitted. The engineers on the ground send commands to the spacecraft to set the appropriate mode for current conditions. The spacecraft itself may change modes if it detects a problem or if it hasn't received any commands in a specified period of time.
How much of the price of a ton of steel is attributable to labor costs? Some industrialists like to blame organized labor for all of their problems when they themselves have refused to invest in improved production techniques for decades.
The Space Shuttle does not use 486s. It uses IBM AP-101s, which are architecturally similar to the IBM 360/370 series of computers. See the Second Generation Computers FAQ.
If there is a COTS (commercial off-the-shelf) real-time operating system available that meets the system requirements, why go to the risk and expense of writing your own from scratch? Do you expect NASA to fabricate every component in the spacecraft?
Fill data is typically transmitted when the telemetry multiplexer does not have any engineering or science data to send. Due to the way synchronous communications links work, something is always being transmitted, even if there is no "real data" available.
Most spacecraft commands are sent as fixed or variable length frames of synchronous data. A frame usually begins with a sync pattern, some header information, the actual command, and a trailer containing a checksum or CRC. There is no protocol, in the sense that most people use. Think of the commands as UDP datagrams. The engineers in the control center monitor the telemetry downlink to verify that the commands sent were received and decode properly. The spacecraft command decoder has parameters in the telemetry downlink such as "# of good commands received" , "# of commands rejected", and "last command received". For simple commands, like "heater #5 ON", you can check the downlink telemetry for the status of heater #5 and verify that it is in the correct state. For complicated operations, like uploading software or large data tables, the spacecraft can be commanded to do a memory dump to the downlink telemetry, allowing the upload to be verified.
As a general rule, spacecraft command processing is kept as simple as possible. You want the command decoder to be extremely reliable, so that even if half the hardware on the spacecraft is broken, you can still send it commands and have them executed. Rather than rely on an automatic protocol, it is usually better to take advantage of the cleverness of the engineers and computers on the ground. Since there is a human in the loop, take advantage of it. The human knows things that a protocol doesn't, like the spacecraft is about to go "over the hill" due to the motion of the Earth and the spacecraft. The engineer may also want to do things like uplink a sequence of commands into temporary storage, verify them with a dump, and execute them only after he is sure that all of the commands were successfully received and decoded. Some commands are designed to require multiple actions before they do something dangerous and/or irreversible to the spacecraft. For example, "arm pyro #36" and "fire pyro #36", which might deploy a solar panel.
If you put world readable documents in a public shared folder on a shared computer system, you have no right to complain when other system users read or copy them. You might as well post them on a bulletin board in the committee meeting room. Your intent is irrelevant, your actions are what count.
I'd be interested in knowing whether there is a cheap and simple modification for a standard camera flash unit that would reduce the duration of the flash. The high-speed flash units used regular xenon tubes, just like the ones in regular flash units. I used to own an automatic flash unit that used a thyristor to dump the charge on the capacitor when a photocell detected sufficient light having been reflected from the subject. The circuitry was fairly simple.
If I remember correctly, he used a camera with an open shutter in a darkened room, triggering the strobe with the sound of the muzzle blast and an electronic delay.
I've worked overseas, both in and out of the military. In both cases, I had to pay state income tax to my legal state of residence, not to mention federal income tax and social security tax. If they are going to tax me, even when I am living on the other side of the world, they damn well better allow me to vote.
Men get it too, it's just more common in women.
This echos the early days of the Internet, where IPv4 was layered on top of DECnet, SNA, X.25 and other protocols.
I wouldn't expect to see IPv6 in a firmware update. You will probably have to buy a new box to get IPv6 support.
The interesting thing will be the reaction of the mass-market ISPs, especially cable operators, who tend to view their residential customers as peons down on the farm.
Every NASA mission, assuming the hardware still works, follows a curve of diminishing science returns. At some point, someone has to make a decision to pull the plug. HST may not be at that point yet, but they have to compete for NASA dollars and resources that can be used to fund new missions.
Scientific American has its own set of biases and prejudices, especially when you leave the realm of hard science and look at political and social issues, where they usually take a leftist line.
It reminds me of the old joke about a mental patient that believed that he was Napoleon Bonaparte. After many years of treatment, he was ready to be released. As a final test, they gave him a polygraph examination. When asked whether he was Napoleon Bonaparte, he said no. The polygraph examiner concluded that he was lying.
It costs a lot of money to keep the HST running. There is a small army of scientists, engineers and technicians that keep it healthy, schedule observations, point it at the right places, and download and distribute the data.
NASA has a severely constrained budget. Every dollar that is spent on HST is a dollar taken away from some other project. From a scientific point of view, you have to ask whether it makes more sense to terminate HST and spend the money on new spacecraft and missions. There are always many more proposals for new spacecraft than there is money to fund them.
These sorts of problems are common in cases involving guns, where even if you are innocent, you may have to spend many thousands of dollars to get a court order forcing the police to return your property. The BATF is notorious for cases like these, where they seize someone's gun collection on questionable charges, intentionally damage the collection, and even if they drop the charges or lose, refuse to return the collection.
The problem is that many government officials believe that they can ignore the law when it is convenient or advances their agenda. They know that most people don't have the time, money and bloody-mindedness to get a court order and have it enforced.
The problem is that as a rough approximation, any wire a quarter wavelength or longer can be an effective antenna. In the HF band (3-30 MHz), that's anywhere from 25 meters down to 2.5 meters. It doesn't take a very long wire to be an effective radiator, plus you also have to consider the length of the electrical wiring inside the structure.
Dumping my sewage and garbage on your front lawn works fine for me too.
"Let's put something that looks like high-power broadband RF noise on long, unshielded, untwisted power lines, suspended in the air, otherwise known as antennas."
I remember reading about an old computer system, I believe it was a Burroughs computer, that used software to enforce security policy. Executable programs would only be loaded and run if they had a magic attribute set. Users could not set the attribute. Only a limited number of trusted programs, like the system's compiler, could set the attribute. The compiler contained and enforced security policy. It would not allow the user to compile a program that violated the system's security policy. This allowed the system to have enforceable security checks that were implemented in software instead of special purpose hardware.
I believe that current popular operating systems are fatally flawed at the architectural level. Fixing the thousands of implementation bugs will not solve the architectural problems.
If they were really serious about noise, they could use RF construction techniques and put the analog components in a shielded can on the motherboard, with bypass capacitors on the power/ground connections. You can shield anything if you are willing to spend some money.
The government is not that coordinated and efficient. An NSA officer is not going to wander in to a NASA office and ask for a copy of the CDs. Bureaucratic protocol must be followed. If it happened, the Director of the NSA would send a written memo to the NASA Administrator asking for a copy of the CDs. Many bureaucrats and lawyers in both agencies would be involved.
No matter what the law says, it isn't going to stop a lawyer with a subpoena or a government officer with a search warrant. Not to mention other situations where the law requires that a business provide information to the government.