MobiNav

• GGA – Global positioning system fix data
• GLL – Geographic position – latitude/longitude
• GSA – GNSS DOP and active satellites
• GSV – GNSS satellites in view
• RMC – Recommended minimum specific GNSS data
• VTG – Course over ground and ground speed
• GRS – GNSS range residuals
• GST – GNSS pseudo range error statistics
• TXT – Text messages
• ZDA – Time and Date

GPS works with weak signals. The signal strength on earth is approximately 15dB below the thermal noise floor. In order to design a reliable GPS enabled system, the following parameters have to be considered carefully during the design phase as they may significantly degrade the GPS performance.

1. Antenna shortcommings
• Poor Gain fo the GPS antenna
• Poor directivity(radiatin pattern) of the GPS antenna
• Improper orientatin of the antenna to the sky
• Poor matching between antenna and cable impedance
• Poor noise performance of the receivers input statge or the antenna amplifier
2. Electrical Environment
• Jamming form external signals
• Jamming form signals generated by re receiver itself
3. GPS related effects
• Signal path obstruction by building, foliage, covers, snow,etc
• Multi-path effects
• Satellite constallation and geomatry

Hot start is performed whenever the GPS receiver stil has access to valid ephemeris data and precise time. This is typically the case if the receiver has been shut off for less that 2 hours and the RTC has been running during that time. Furthermore, during the previous session, the receiver must have been navigating (to allow it todecode and store ephemeris data).

In hot start, the receiver can predict the currently visible SVs, and it there fore able to quickly acquire and track the signal. Because ephemeris is already known, there is no need to wait for the ephemeris broadcast to complete.

If external aiding information like ephemeris and/or precise position and time are provided to the receiver, the TTFF may significantly be improved.

Warm start is performed whenever the GPS receiver has only access to valid almanac data, and has not significantly moved since the last valid position calculation. This is typically th ecase, if the receiver has been shut off for more than 2 hours, but still has knowledge oflast position, time and almanac. This allows it to predict the current visible SCs. However, since ephemeris data is not available oroutdated, the receiver needs to wait for the ephemeris broadcast to complet.

In cold start, the GPS receiver has no knowledge of its last position or time. This is the case whenthe RTC has not been running; no valid ephemeris data or almanac data is available.

WAAS(Wide Area Augmentation System), 即广域增强系统。WAAS是美国联邦航空局（FAA）及美国交通部为提升飞行精确度而发展出来的，因为目前单独使用 GPS 并无法达到联邦航空局针对精确飞行导航所设定的要求。WAAS 包含了约25个地面参考站台，位置散布于美国境内，负责监控 GPS 卫星的资料。其中两个分别位于美国东西岸的主站台搜集其它站台传来的资料，并据此计算出 GPS 卫星的轨道偏移量、电子钟误差，以及由大气层及电离层所造成的讯息延迟时间，汇整后经由两颗位在赤道上空之同步卫星的其中之一传播出去。此 WAAS 讯号的发送频率与 GPS 讯号的频率相同，因此任何具备 WAAS 功能的 GPS 机台都可接收此讯号，并藉此修正定位信息。WAAS 可以校正由电离层干扰、时序控制不正确以及卫星轨道错误等因素所造成的 GPS 讯号误差，也能提供各卫星是否正常运转之信息。虽然 WAAS 目前尚未正式通过美国航空局的飞行使用认证，但此系统已开放给一般民众使用，例如从事航海或其它休闲活动的人们。

目前无WAAS功能的普通GPS接收机的正常精确度是15米，而一台具备WAAS功能的GPS接收机能在95%的情况下提供您误差小于三公尺的精准定位，而且您不必为了使用WAAS功能而支付任何使用费。

目前，WAAS 卫星讯号的适用范围只有北美洲。在其他地方并无任何地面参考站台，所以这些地区的 GPS 使用者就算能接收到 WAAS 讯号，也会因为讯号没经过适当的校正而无法提升其机台的精确度。即使在美国，由于卫星位于赤道上空的缘故，若所在位置之南方地平线有树林或山脉阻挡，也会不容易收到 WAAS 讯号 

EGNOS(European Geostationary Navigation Overlay Service), 即“欧洲静止轨道导航重叠系统”是欧洲全球卫星导航系统的开端。它是由欧空局、欧盟和欧洲空间安全组织共同发起的，是目前正在欧洲建立的伽利略全球卫星导航定位系统的先驱。

EGNOS的地面设施和静止卫星也没有覆盖到亚、非、南美和远东等地区。所以，中国国内还无法使用EGNOS的精确定位服务。

摘自《电脑报》 

随着技术的进步、应用需求的增加，GPS从当初的“养在深闺人未识”到今天“一朝选在君王侧”的蓬勃发展，GPS以全天候、高精度、自动化、高效率等显著特点及其所独具的定位导航、授时校频、精密测量等多方面的强大功能，已涉足众多的应用领域，使GPS成为继蜂窝移动通信（GSM）、互联网之后的全球第三个IT经济新增长点。　　一、民用领域

　　与上述产品相比，各种个人消费类GPS电子产品则更加接近人们的生活。有集成了GPS芯片和GIS数字地图的移动通信手机、GPS手持机、GPS手表，甚至GPS相机等，也有基于掌上电脑、笔记本电脑等移动设备的插卡（CF卡式GPS接收机）式、外接（GPS接收机）式等集成产品。目前国内市场上多见的是Garmin（高明）、Magellan（麦哲伦）、Navman等外国公司品牌的GPS手持机、汽车导航仪等产品。

倚天   — ETEN 

鼎天  — RoyalTek (台湾倚天资讯股份有限公司下属公司)

中天 — ETENCHINA

华硕 — ASUSTek

技嘉 –GIGABYTE

宇达电通  — MIO

宏达 — HTC

明基 — BenQ

环天 — Globalsat

長天科技 – Holux

凯宣科技– Emtac

Source E-TEN 

The G500 is the first Pocket PC Phone from E-TEN to have fully integrated GPS capabilities. The G500′s embedded RoyalTek GPS module, featuring the SiRF Star III high sensitivity chipset and an innovative internal antenna design, means that PDA or phone owners who also regularly need to access GPS services, no longer have to carry additional accessories or even a separate GPS device when they travel. No other product makes it as easy or as convenient as the G500 to have the latest satellite navigation technology always on hand. The G500 is designed to be the perfect all-in-one handheld device. At only 119 x 62 x 23mm the G500 is comparable in size to traditional mobile phone handsets. In addition to GSM phone and GPS functions, the device also serves as a high-specification PDA running Windows Mobile 5.0. The device uses a Samsung S3C2440 processor operating at 400 MHz and includes 128MB of Flash ROM and 64MB SDRAM memory, which can be used to save applications and files. The device uses a replicable 1,440mAh Li-Ion battery, which offers on average 3.5-4 hours of talk-time or 10-15 hours of Pocket PC usage. Finally, the G500 also features a 1.3M pixel CMOS camera and built in microphone allowing users to capture digital images, voice recordings, and even movies.