Flight Real-Time Simulation System and Technology

Author: Wang Xingren / Edited by: Wang Xingren
Publisher:
Publish Date: 2000-04-01
Features: Brief Introduction Flight simulators are typical human-in-the-loop real-time simulation systems and are examples of virtual reality technology. This book takes flight simulators as the background to discuss modeling theory and methods, simulation software, simulation computers and interfaces, and environmental simulation (including visual system, motion system, control load system, and audio system). The book is divided into twelve chapters. The writing emphasizes the combination of theory and practice, with a strong engineering background and the characteristics of interdisciplinary integration. This book is suitable for engineering technicians engaged in the design, development, and application of flight real-time simulation systems, as well as faculty and students of relevant majors in higher education institutions. It can also serve as a textbook and reference book for postgraduate students in related fields.
Excerpt: `putc()` is a macro similar to a function that writes a character to a user-specified stream. `putc()` requires two parameters: the first parameter tells `putc()` what character to output, and the second parameter tells `putc()` where to write the character. When using the `putc()` function, use the following syntax:
```c
int putc(int c, FILE stream);
```
The first parameter in the parameter list is the integer value of the output character, and the second parameter is a pointer to the address for writing the character. `stream` is simply a pointer connected to a special file or device. The `putc()` function returns an integer value. If `putc()` successfully writes the character, it returns the value of that character; if `putc()` fails, it returns `EOF`. The following example shows how to use `putc()`:
```c
int c;
for (c = 65; c <= 90; c++) {
putc(c, stdout);
}
```
The `putc()` function is called with two arguments, `c` and `stdout`. The parameter `c` is used as the loop counter in the `for()` statement. In each iteration of the `for()` statement, `putc()` outputs (writes) the character value of `c`, where `c` is the loop counter. `putc()` writes the character to the device pointed to by `stdout`, which is a pointer to the standard output device, typically the display screen.
Sometimes, writing a single character is not enough, and you need to write a complete string at once. In this case, you can use the `puts()` function. The following example shows how `puts()` outputs a string to the screen:
```c
char my_string[] = "This is a test!";
puts(my_string);
```
You can see that the `puts()` function requires a parameter that points to a string. Only then does `puts()` output the string pointed to by `my_string`. In the first line of this example, `my_string` is declared as a character array, which is a string and has been initialized. The `puts()` function sends the string to `stdout` and appends a newline character at the end of the string.
### 1.5.2.2 `printf()` Function
When you only need to output character-based information, character and string output functions do a good job. However, sometimes you may need to output various types of data, not just character data. To achieve this, you can use the `printf()` function. `printf()` is just one function in the `printf()` family, and all functions in this family output formatted data, but they differ in parameters and output objects. With `printf()`, you can generate formatted output, and the function allows you to control the appearance of the data, or its format. The syntax for the `printf()` function is:
```c
int printf(const char format, argument, ...);
```
You can see that the parameter list of `printf()` consists of two parts: the first part is the format string, which controls the format of the output data, and the second part consists of the data that `printf()` is to output. The format string controls how the `printf()` function displays the content of the parameters in a formatted manner. This string contains text and conversion specifiers, which determine the type and format of the output data. The parameters must have enough variables to match each conversion specifier. If there are not enough variables, unexpected results may occur. The following example shows a `printf()` function with a format string that contains standard text and conversion specifiers:
```c
printf("The answer is: %d", sum);
```
### 3.6 Modeling and Simulation VVA
#### 3.6.1 VVA Concept
The effectiveness of modeling and simulation lies in the correctness and confidence of the modeling and simulation. Only when the correctness and confidence of modeling and simulation are guaranteed can the simulation results have practical significance. Adopting Verification, Validation, and Accreditation (VVA) technology for modeling and simulation is an effective approach. Verification (V) is the process of determining whether the model and simulation accurately reproduce the developer's conceptual description and technical requirements. Validation (V) is the process of determining the extent to which the model and simulation accurately reproduce the real world from the perspective of intended use. Accreditation (A) is the determination by an authoritative body of whether a model and simulation are acceptable for a specific purpose. VVA is an important part of the development, application, and improvement of modeling and simulation. In VVA technology, "Verification" and "Validation" improve and enhance the correctness of modeling and simulation, while "Accreditation" determines the confidence level of modeling and simulation. The accuracy of a large simulation system includes both static (steady-state) and dynamic aspects. The accuracy of the entire simulation system is composed of the accuracies of various subsystems. In other words, the required accuracy of the entire flight simulation system will be allocated to the accuracy requirements of each system. Therefore, the verification, validation, and accreditation of the model should be throughout the entire process of development, application, and improvement of the modeling and simulation system.
VVA for modeling and simulation involves multiple technologies and testing methods. The key technology lies in the standardization of VVA work modes and the automation and visualization of VVA. VVA work modes are multi-level, including the verification of models and the validation of simulation results, the latter involving data acquisition, data tracking, comparison and display of simulation results with validation data.
#### 3.6.2 VVA Process
The VVA work mode and process for modeling and simulation are shown in Figure 3.11. The VVA process first involves the analysis of the modeling object, proposing modeling requirements based on the simulation objectives, and abstracting a conceptual model. The conceptual model includes relevant assumptions, algorithms, the structure of modeling and simulation, and a description of the expected application. The conceptual model must also propose the feasibility of inputting data for modeling and simulation. The process of verifying the conceptual model must demonstrate whether the conceptual model accurately describes the simulation object and whether the simulation structure meets the requirements of the expected application. A verification and validation plan must be designed during the modeling and simulation process. The "technical specifications" of modeling and simulation include requirements and standards for software and hardware, including network and protocol standards for distributed interactive simulation. Based on these, further design verification is conducted, verifying the connection between the structural design and the conceptual model, ensuring that the preliminary design requirements are met.
In "Software Development/Modification," the simulation software is continuously refined. During this process, standards related to software structure, documentation, testing, and quality assurance should be applied to enhance the management of modeling and simulation documentation and configuration. Additionally, through a series of simulation experiments, verification and validation are conducted to ensure whether the simulation software accurately reflects the modeling and simulation plan. Simulation results are compared with theoretical results or other experimental results to validate the effectiveness of the mathematical simulation model and software.
Subsequently, connection and debugging of software, hardware, and their networks are performed, including human participation. During system operation, the realism of the simulation system's reproduction of entity performance is verified. The correctness and accuracy of the simulation system are analyzed, as well as the impact of the accuracy of each component on the accuracy of the entire simulation system.
Throughout the modeling and simulation process, in addition to the emphasis on integrating verification and validation into the entire lifecycle of modeling and simulation development, it is also important to emphasize that documentation must be throughout the entire development process to ensure effective verification and validation. The documentation includes descriptions of modeling and simulation indicators, performance, and data requirements. Without these documents, modeling and simulation would be difficult to proceed correctly. Therefore, all VVA processes must be formally and comprehensively documented.