AndroidBLE--Measuring distance（测量距离）

iBeacon介绍

iBeacon是苹果在2013年WWDC上推出一项基于蓝牙4.0（Bluetooth LE | BLE | Bluetooth Smart）的精准微定位技术，当你的手持设备靠近一个Beacon基站时，设备就能够感应到Beacon信号，范围可以从几毫米到50米。

Measuring distance（测量距离）

TX power ，用于确定你和beacon之间距离有多近。根据这个值不但可以获得粗略的信息（比如靠近/远离/不在范围内等），也可以获取精确到米的距离，TX power是距离设备1米测得的信号强度值（RSSI- Received Signal Strength Indication，接收到的信号强弱指标）。假如接收到的信号强度减弱了，那么我们可能在远离。只要知道1米距离的RSSI，以及当前的RSSI（我们可以从接收到的信号中一块获取到这些信息），就可计算出当前距离beacon基站的距离

Android 平台

1 Ibeacon扫描SDK库

Android最常用的是android-beacon-library

beacongithub地址

具体使用方法在项目的README.md 描述的比较详细

2 altbeacon内置设备模型以及用于beacon测距的算法中的系数

{
"models":
[
{
"coefficient1": 0.42093,
"coefficient2": 6.9476,
"coefficient3": 0.54992,
"version":"4.4.2",
"build_number":"KOT49H",
"model":"Nexus 4",
"manufacturer":"LGE"
},
{
"coefficient1": 0.42093,
"coefficient2": 6.9476,
"coefficient3": 0.54992,
"version":"4.4.2",
"build_number":"LPV79",
"model":"Nexus 5",
"manufacturer":"LGE",
"default": true
},
{
"coefficient1": 0.9401940951,
"coefficient2": 6.170094565,
"coefficient3": 0.0,
"version":"5.0.2",
"build_number":"LXG22.67-7.1",
"model":"Moto X Pro",
"manufacturer":"XT1115",
"default": false
},
{
"coefficient1": 0.1862616782,
"coefficient2": 8.235367435,
"coefficient3": -0.45324519,
"version":"6.0",
"build_number":"MPE24.49-18",
"model":"XT1092",
"manufacturer":"Motorola",
"default": false
}
]
}

3 ALTBeacon中的测距方法

@Override
public double calculateDistance(int txPower, double rssi) {
if (rssi == 0) {
return -1.0; // if we cannot determine accuracy, return -1.
}

LogManager.d(TAG, "calculating distance based on mRssi of %s and txPower of %s", rssi, txPower);

double ratio = rssi*1.0/txPower;
double distance;
if (ratio < 1.0) {
distance =  Math.pow(ratio,10);
}
else {
distance =  (mCoefficient1)*Math.pow(ratio,mCoefficient2) + mCoefficient3;
}
LogManager.d(TAG, "avg mRssi: %s distance: %s", rssi, distance);
return distance;
}

方法中的公式用到两个值，一个txpower 一个rssi ，用到两个值就可以通过公式求出距离

4 移动端距离计算

如果当前的设备没有在altbeacon内置设备模型列表中，beacon SDK 则选择Nexus 5系数进行距离计算，如下

public static double calculateAccuracy(int txPower, double rssi) {
if (rssi == 0) {
return -1.0; // if we cannot determine accuracy, return -1.
}
double ratio = rssi * 1.0 / txPower;
if (ratio < 1.0) {
return Math.pow(ratio, 10);
} else {
double accuracy = (0.42093) * Math.pow(ratio, 6.9476) + 0.54992;
return accuracy;
}
}