GPS

What is GPS?(3 major segments)

The Global positioning system (GPS) is a space-based satellite navigation system that provides location and time information, where there is an unobstructed line of sight to 4 or more GPS satellites.

components :

Space segment: hardware in orbit around the earth,
Control segment: a system of ground stations,
User segment: handheld receivers

Basic Steps

Positioning
Timing
SV Ranging
Trilateration
Correction of errors

Two basic steps of localization technologies

Measuring
Calculation

How GPS works?

Range from each satellite calculated
range = time delay * speed of light
Determining Range: Receiver and satellite use same code, compare incoming code with receiver generated code.
Trilateration is used to determine you position or “fix”
At least 3 satellites required for 2D fix. However, 4 satellites should always be used (receiver clock error)(SVs have highly accurate atomic clocks!)

Disadvantages of GPS

Not apply for indoor environments.
The largest problem is multi-path effect!

ERROR

(Most): due to the atmosphere
Satellite clock errors
SV position errors
Quality of GPS receiver
Multi-path errors

Multi-path

Multi-path occurs when signals are reflected off of objects such as trees or buildings.

Other positioning technologies

cellular based
WiFi positioning
Bluetooth positioning
RFID chips embedded in the environment

Three typical methods of localization

ToA: Time of Arrival
TDoA: Time difference of Arrival
(The biggest advantage is that it doesn’t need time synchronization.)
signal feature based localization
(Not rely on special hardwares, but leverage the signal feature directly 直接利用信号特性)

随着人类对外部世界的改造，对未知领域与空间的拓展，人类需要的信息来源、种类、数量、精度不断增加，对信息获取的手段也提出了更高的要求。传感器因此而生。在现实生活中，传感器无处不在。

Sensor:

A sensor is a converter that measures physical quantity and converts it into a signal.

由敏感元件和转换元件和测量电路（有时还会加上辅助电源）组成，能感受到并被测量，并能将检测和感受到的信息按一定规律转化为电信号并输出。

别名：transducer, probe, gauge, detector

stimulus = physical quantity = measurand

Types

Active Sensor: generate electric current
Passive sensor: require additional electrical energy.
contact sensor: a sensor that requires physical contact with the stimulus.
non-contact sensor: require no physical contact.

智能传感器有一定的信息处理能力（含有微处理器）。

传感器的常用参数

Input : stimulus or measurand
output: electrical signal
Transfer Function:
Relation between input and output

Range, span, IFS, OFS of sensors

Range：lowest and highest values of the stimulus (两个值）
Span: the arithmetic differences （上⾯两个值的差）between the highest and lowest values of the stimulus (that can be sensed within acceptable errors.)
Input full scale (IFS) = span
output full scale (OFS) : difference between the highest and lowest values of the output of the sensor corresponding to the span of the sensor.

ERROR

Errors : deviation from “ideal”
static errors: not time dependent
dynamic errors : time dependent
system errors: constant at all times and conditions. can be corrected by calibration.
random errors : due to uncontrolled variables. can be reduced by averaging.

Some terms about sensors

Resolution 分辨率: the minimum increment of stimulus to which it can be responded 能感知到的最小变化.
Accuracy 准确性: tells you how close a measured value to the true value 注意和分辨率区分.
precision: how close the measured values to each other 更集中.
repeatability or reproducibility ：failure of the sensor to represent the same value under identical conditions when measured at different times. 不同时间，统一条件，不同结果
sensitivity 敏感度: change in output for a given unit change in input. represents the slope of the transfer function. 代表可变换函数的斜率
response time：(delay time)
indicates the time needed for the output to reach steady state(达到稳定所需要的时间）
Reliability (MTBF)：
the ability of the device to perform its stated function, under normal operating conditions without failure.

Wireless Sensing

Contact sensing 如手环检测心率；手机触摸屏
Contact-free sensing (Light, sound, electricity, magnetism)

Acoustic Sensing

Audio (<20kHz)
Ultrasound (>20kHz, unaudible)

RF sensing

Basic principle: Targets such as humans in the transmission paths will reflect, refract, diffract 反射，折射，衍射， and scatter the RF signals following physical laws. By capturing these signals and processing the patterns, events in the environment can be sensed.

3 Types of commonly used wireless media: WiFi, Millimeter Wave, UWB (Ultra Wide Band)

Advantages of RF sensing:

contactless
pervasiveness 广泛性
low cost (both money and energy)
privacy preservation 隐私保护.

Application of RF Sensing :

Localization
Human activity recognition
Gesture recognition
Vital signs

3 Types of wireless sensing :

Acoustic Sensing
RF Sensing
RFID

Crowd Sensing 群体感知

New Sensing paradigm: Mobile Crowd Sensing

It treat mobile devices of ordinary users as basic sensing units, and leverage them to distribute sensing tasks and collecting sensing data.

Basic Feature

Crowd-based information sensing
Cooperative data transmission
Crowd intelligence for complex tasks

People/Human-centric:"Prosumer(Producer+Consumer)

Advantages

Low cost
easy maintenance
Good scalability 扩展性强（只需要招募更多用户）

Challenges

Sensing quality
Privacy protection
Incentive 激励

智能传感器(not mentioned in exam)

自学习、自诊断与自补偿能力
符合感知能力
灵活的通信能力

MEMS

Micro-Elextro-Mechanical Systems 微机电系统，泛指特征尺度在亚微米毫米范围的装置。

无线传感器与执行器网络的研究(not mentioned in exam)

随着WSN的不断深入，人们认识到在某些应用环境中必须要将执行器和传感器结合起来，无线传感器与执行器网络(wireless sensor and actor network，WSAN)

传感器节点很多且静态不移动，执行器节点移动，不需要很多。

各个节点之间都要能够协同通信。

自主机制：各个传感器节点选择合适的执行器节点执行。
半自主机制：传感器节点传到汇聚节点（相当于中控中心）再选择执行器节点执行。
协同机制：控制者在控制中心根据汇聚节点收集到的传感器节点信息，向汇聚节点发送指令，再由执行器节点执行。

无线多媒体传感网络的研究(not mentioned in exam)

Wireless multimedia sensor network(WMSN)需要对视频、音频、图像等多媒体信息感知、传输、处理，需要比传统WSN更加直观、清晰的信息。

其他：

Undeerwater Wireless Sensor Network, UWSN 水下无线传感网络 主要利用声波进行通信和组网。

Wireless Underground Sensor Network, WUSN 地下无线传感器网络

Integrate NanoSensor System, INS 集成纳米传感器系统

原文链接：https://blog.csdn.net/jtwqwq/article/details/121913804