(wow) Words Of Wonders Level 1416 Answers – During the Apollo missions to the Moon, an important task for NASA was determining the position of the spacecraft. To achieve this, they developed a digital measurement system capable of determining the distance of an aircraft hundreds of thousands of kilometers with an accuracy of up to 1 meter.
The basic idea is to send a radio signal to the aircraft and determine how long it will take to return. Since the signal travels at the speed of light, time delay gives distance. The main problem is that the large distance from the aircraft will make the radar-like return pulse weak.2 The research system solves this problem in two ways. First, a complex transponder on the plane sends a signal. Second, instead of sending a pulse, the system emitted a long pseudo-random sequence of bits. By repeating this method for several seconds, it is possible to remove the weak signal from the noise.1
(wow) Words Of Wonders Level 1416 Answers
In this blog, I explain this surprisingly complex system of scoping. Creating and connecting random repeater systems was difficult with transistor circuits in the 1960s. Distance codes were to be integrated with Apollo's “Integrated S-Band” communication system, which used high frequency microwave signals. In aircraft, a special multi-frequency transponder supports Doppler velocity measurements. Finally, communication with aircraft requires a complex network of ground stations around the world.
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Apollo Service Module transponder with cover off showing internal circuitry. The transponder returns the distance signal to Earth and multiplies the frequency by the equivalent of 240/221.
A range system measures the time it takes for a signal to travel to the plane and back.3 As shown below, a range system is designed to fit an individual's body type. -a body sent to the plane. The aircraft's transponder returns the signal to the ground receiver. The returned signal is correlated with the original signal to determine the delay and thus the distance. Meanwhile, Doppler measurement (described later) provides the speed of the spacecraft.4
Block diagram of the Apollo measurement subsystem. Adapted from JPL Mark I Raging Subsystem Study
The key step is to correct the signals and allow to determine the time delay. When two signals are matched with the appropriate delay, the bits match. Since the week is pseudo-ID, the prizes won't match (except for uncertainty) if the two tokens don't match. . Because the signals can be correlated at long time intervals (several seconds), it is possible to determine the location even when the signal is very weak and noisy.
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The code structure has to be very long to define the different range; the system uses a pseudorandom sequence 5456682 slightly longer. Since the code is sent at 1 megahertz, it is repeated every 5.46 seconds. Radio signals can travel 800,000 kilometers and back in that amount of time, while the Moon is about 384,000 kilometers, so the system can measure more than twice the distance to the Moon. Note that in one megahertz, a signal is about 150 meters. The system achieves greater accuracy by comparing the levels of the signals.
In theory, the paths sent and received are changed until they match, and the change value gives the delay and thus the distance. However, trying 5 million different transmissions to match a sequence is not practical, especially with 1960's technology. To solve the matching problem, the sequence has many short codes ranging in length from 2 to 127 bits. These “subcodes” were short enough to be compared to brute force. Total delay can be determined by subcode delays. The system uses four subcodes: a subcode is 31 bits long, B 63 bits, C 127 bits, and X 11 bits, including a two-value CL clock. Since these lengths are primitive, the combined sequence has a total length equal to the product: 5, 456, 682. The important point is that a long code can be generated and matched by the application because small codes are short.
I have created an interactive page that shows how to create internal code monitors. Sub-sequences of lengths 31, 63 and 127 are generated using a well-known technique called linear feedback shift register (LFSR). In this process, a shift register of length N consists of bits. In each phase, a new bit is generated from one or two exclusive bits in the shift register. The new bit is moved in and the old bit out, giving the code bit. This procedure can generate a pseudorandom sequence of length 2
1 with good statistical properties. To generate a subcode of length 11, the X code is generated from the Legendre.7 sequence
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Subcode C is generated from the 7-bit (black) delay line. The last two bits are put into an XOR gate to produce a new (red) bit. Part of the pseudo-ID sequence is shown below the threshold.
Combining subcodes into a common code is not as easy as one might think, because each subcode must be uniquely identified in the output. Minor codes A, B and C are combined with maximum function
, which returns the common bit of the three inputs.6 The complete formula for the sequence of bits is (X·maj(A, B, C))⊕CL.
The measurement process is performed by the “Mark I Raging Subsystem” below. Although this is called a “special purpose binary digital computer”, it is not really a computer in the modern sense, but rather a state of the machine that has completed the necessary steps. First, the ground station sends a series of codes to the aircraft and synchronizes it with the received signal. Next, the classification system tries different offsets for subcode X and finds the offset with the best fit. It repeats the instructions for subcodes A, B, and C. Circuits do some complex math moduli8 to calculate ranges from odd subcodes. Finally, the Doppler subsystem determines the speed of the aircraft and is constantly added to the scale to keep the measurement updated as the aircraft moves. I have created an interactive page showing the following steps.
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A physical system made from “T-Pac modules,” boards that use transistors and other discrete components to build simple digital devices such as logic gates and flip-flops. -flops.9 T-Pac modules were produced by the Computer Control Company in 1958 and were designed to implement a digital system quickly and efficiently operating at 1 MHz. Assemblies of 32 T-Pac cards are placed in a “T-BLOC” box placed in a cabinet and 10 T-BLOCs are installed in two chambers. The cards are connected by inserting threads and tapered pins into a large grid.
One module is the T-Pac LE-10 “logic” module, below, implementing four IN ports. It was $98 in 1961 (about $700 in today's dollars), which shows how expensive digital intelligence was at the time. The intelligence system used about 300 digital modules, so a digital intelligence circuit would cost several hundred thousand (current) dollars, a recurring cost in each ground station.
Tasks that are considered mundane today are difficult with the technology of the time. For example, a range is stored as a 31-bit binary value. But instead of a register, the value is stored in a magneto-spring delay line in the form of pulses in a long nickel wire. To add values to the range, the circuit uses a one-bit serial adder that adds bits one at a time as they leave the nickel wire and then feeds the bits back into the wire.
An interesting circuit is a compatibility phase detector, which checks the compatibility between a small code and the received signal. The correlation starts as an analog voltage that is converted to a digital value by the “Voldicon”. The correlation is finally integrated by concatenating the binary values using another 31-bit register/adder. The number of samples considered can vary from 1 to 2
The Digital Ranging System That Measured The Distance To The Apollo Spacecraft
Models that can be set by the user with “Digiswitch” wheel switches. By integrating correlation over a long time interval, it is possible to detect a weak signal in the presence of noise. The system follows the optimal offset and stores the value in the second stop line. Total range time ranges from 1.6 seconds for a strong signal to 30 seconds in eclipse range.
The range system can also measure the speed of the aircraft by measuring the Doppler shift of the return signal. If the plane leaves the Earth, the waves will be longer, reducing the frequency. Conversely, the frequency will increase if the plane moves toward Earth.10 By measuring the frequency shift, the speed of the plane can be determined accurately.12
A moving source causes the wavelength to decrease as the source approaches or increase as the source moves away. Tkarcher diagram.
The Doppler effect affects radio system design in two ways. First, the aircraft can't simply receive and retransmit the signal, because the Doppler shift from upward travel will be lost. Instead, the frequency complex multiplies
