MobiNav

All Things Mobility

décembre 21st, 2005

Map-Aided GPS Navigation

[Source : GPS World]

Installed GPS navigation systems are becoming popular options for new car buyers. and many aftermarket, portable, and PDA- or cell phone–based systems are available in the marketplace.

A basic GPS navigation system can provide continuous, accurate navigation, except when the GPS signals are blocked by buildings, tunnels, or other obstructions or when multipath or interference reduces position accuracy. A factory-installed system might include additional sensors such as an odometer or gyroscope to provide dead-reckoning navigation when GPS signals are lacking.

Another aid to accurate navigation is map-matching in which the computed position fix is snapped onto the nearest road. However, depending on the fix error and the density of the road network, the system may or may not snap the fix onto the correct road. In this month’s column, guest authors Syed and Cannon examine a novel technique that tightly integrates information from accurate maps with raw GPS and gyro data to determine a vehicle’s position. Using classic statistical theory and fuzzy logic algorithms, the technique improved vehicle navigation accuracy in an urban canyon setting by more than 30 percent. — R.B.L.

The confluence of wireless technology and GPS has led to the development of a new set of applications to serve the location-based needs of users. These applications are popularly known as location-based services, and a major portion of the location-based services market deals with applications using in-car navigation systems. These systems supply vehicle-position information for applications to provide guidance to nearby points of interest, advise on current traffic or weather conditions, and even generate location-based advertising.

Progressive industry leaders are building solid foundations to support well-conceived solutions for new location applications and value-added services. The other strong impetus for the development of location-based services comes from the United States Federal Communications Commission’s Enhanced 911 mandate, which directs that all mobile phones be located for 67 percent of 911 calls with an accuracy of 100 meters for network-based technologies and 50 meters for handset-based technologies.

A key to the success of location-based services applications is accurate georeferencing of the position output obtained from a navigation system. Georeferencing involves relating the position to a map database to identify the road on which a vehicle is traveling. This procedure is called map matching. Systems that combine navigation and map data need the following characteristics to be effective and broadly used: affordable cost, accuracy of approximately 20 meters (at a 95-percent confidence level), automatic initialization without any input from users, effective display of position and location-based information, and a reliable map database.

In this article, we outline some of the challenges of positioning in urban areas and the problems that can occur when attempting to overlay inaccurate positions on a map. We introduce a new map-aided navigation technique, which integrates raw GPS measurements and map data, and provide some results from field tests to show the increased level of reliability when these two data types are integrated tightly.

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décembre 10th, 2005

Glossaire de GPS

Glossaire de GPS (source: http://www.gissky.net/)

A
Ambiguity不明确值
经由对卫星的连续观测量中,以计算载波相位的未知周期值,其一定为整数。
Ambiguity Function method 不明确值函数法
一种决定不明确值的方法,它使一对接收仪间基线向量解答中的变方因子(Variance factor)为最小。
Argument of latitude 纬度自变量
真近点离角与近地点自变量之和。

Azimuth and Elevation
用方位角和仰角描述物体在天空的位置,这种位置是相对位置,相对于观察点而言。观测者的默认位置为地面,但也可以改变位置。方位角指的是罗盘方位,相对于物理的北而言,方位角在地面以下的一个角度。地平线被定义为一个巨大的,围绕观察者的一个虚拟的圆球。从观察者的角度看来,罗盘方位按照顺时针角度从北开始。也就是说,0度方位角表示正北,90度方位角表示正东,180度方位角表示正南,270度方位角表示正西,360度方位角表示角度回归,依然是正北。
仰角,也叫做海拔高度。当方位角测量完毕之后,需要用仰角来描述被观察物体相对于观察者的高度。如果观察者在地面上,那么仰角范围就在0度到90度之间,有时仰角范围还会在-90度到90度之间,这是因为被观察物体在观察者下方。如果观察者在飞机中观察地面物体,那么仰角就应该为负了。
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B  
Bandwidth波段宽
讯号频谱的宽度,以赫兹(Heartz ,频率单位,周/秒)表示。
Baseline基线
由同时接GPS资料之两测站所组成之一基线。
Beat frequency成拍频率
当两不同频率讯号混合时,会产生另两种不同频率,分别为原两频率的和与差。
例如在一式中: cosAcosB=cos(A+B)+cos(A-B)/2,原讯号为 A与B,生成成拍讯号为 A+B与 A-B。
Binary Pulse code moduIation二元脉冲电码调制
利用一列二元电码的脉冲调制。此电码常被加上明确意义的
O和1来示电波相位或方向的改变。
Binary biphase modulotion二元双相位调制
0 度或 180 度固定频率载波的相位改变。其模式为y=Acos(wt+P),其振幅函数A为一系列 +I与 -I的值(分别表示在 O度与180度之相位改变)。
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octobre 16th, 2005

Accuracy requirement for air user

Current air user requirements in meters

Phase

Catagory

Position

Height

En route

>= 100

>= 100

Approach

I

17.1

4.1

And

II

5.2

1.7

landing

III

4.0

0.6

Referrences:
[1] B. Hofmann-Wellenhof, H. Lichtenegger, and J. Collins « GPS – Theory and Practice« , Third, revised edition, 1994, Spring-Verlag Wien New York