2009 東 南 正 財 位

北 方 係 瘟 神 ， 千 祈 咪 動 土 ， 要 放 安 穩 水 ， 可 減 去 煩 惱 。
西 方 係 病 神 ， 放 個 銅 葫 蘆 。
東 方 叫 三 煞 ， 要 用 水 養 草 。
東 北 名 戰 神 ， 紅 紫 係 法 寶 。
講 完 當 然 要 講 好 。
東 南 正 財 位 ， 放 把 風 扇 度 。
西 北 名 官 貴 ， 要 明 潔 方 好 。
西 南 係 偏 方 ， 見 水 局 最 高 。
南 方 文 昌 位 ， 富 貴 竹 可 補 。
最 緊 要 勤 力 ， 每 日 搵 工 做 。
國 家 要 團 結 ， 社 會 要 和 諧 。
家 庭 要 和 睦 ， 咁 就 大 家 好 。
祝 願 此 牛 年 ， 經 濟 早 復 甦 。 」

Network RTK, a technology behind CORS

I suddently want to know more about RTK to refresh my GPS knowledge. Surprise me is that this technology has been gone. The new technology is called Network RTK. Let me spend little time of mind to write down what is my understanding and I can read it later when I forget it.

To understand network RTK, we need to know what is RTK. RTK stands for Real Time Kinematic, which is a GPS positioning technology better than DGPS in term of accuracy and its capability to keep the accuracy when moving. RTK provides cm grade accuracy while DGPS can provide sub-meter accuracy. To enjoy RTK, the GPS receiver shall support dual frequency L1 and L2. DGPS requires single frequency only. In addition, you need a reference station with known position and a RTCM antenna to boardcast correction to rover (your RTK capable GPS reciever).

Why RTK requires dual frequency? Frankly, I am not understand the details. The reason should be to enable On-The-Fly Ambigurity Resolution. **I am asking my wife but she forgot it. Hopefully, she will give me some notes later.)

What I know is that phase of carrier and other correction information (such as orbital correction, ionospheric correction, tropospheric correction and geometric correction) would be transfered from the base station to the rover. At rover, it will compare its phase recieved with the phase sent by the base station to attain a relative position at millimeter level, and this is a accurate baseline to provide good measurement. Of course, accuracy of the rover cannot exceeds that of base station. I think the rover should recieve phase from satellites as well. Maybe it uses different frequencies between satellite and the rover? According to some references, RTK requires at least five satellites (only 4 after initialization) and 1 minute for initialization. In contrast, DGPS requires 3 satelliates only with no time delay for initialization.

Network RTK is not a latest technology in fact. It was developed in 90s and it has been established in many countries in Europe and States, even China and Hong Kong have established reference station network, which is termed as Continously Operating Reference Stations (CORS). CORS is an extension of RTK technology, it is composed of a network of reference stations and a central data processing center that is a services running 7x24 non-stop. Corrections would be transfered to rover using GSM or wireless network with a standard protocol called NTRIP, i.e. RCTM over IP, Network Transport of RTCM via Internet Protocol.

There are three Network RTK technologies being available in the market including VRS, FKP and IMAX.

1. Virtual Reference Station (VRS) is developed by Trimble, it requires a bi-directional communication between a data processing center and rover. To start measurement, rover needs to provide an approximate position to the center and it will select relevent reference stations within the proximity to calculate corrections (satellite orbit and clock errors, reference station receiver clock errors, multipath and particularly ionospheric and tropospheric effects.) for the rover. A virtual reference station would be created a few meters near the rover for on-going measurements.
2. The SAPOS (Satellite Positioning) community in Germany introduced a method for broadcasting network correction streams called “FKP.” FKP stands for the German word “Flächenkorrekturparameter,” which means area correction parameters. Within SAPOS, the providers standardize RTCM 2.3 with a proprietary extension (a type 59 message). This additional message describes the linear ionospheric and geometric correction around a physical reference station. The linear corrections are derived from a data processing center using multiple reference stations. This is running in a data boardcasting model different from VRS. However, I think, initial position could be sent from rover to center for selection of reference station to yield a better FKP.
3. The last one is iMax from Leica. iMax is a variant of Max. Max stands for Master-Auxiliary corrections while iMax stands for individualized Master-Auxillary corrections. The difference between Max and iMax is that Max supports RTCM3.0 that would be boardcasted to rover, while iMax supports RTCM2.3 and 3.0 that require bi-directional communication. However, the mechanism is that same that servers at the center will select closest master reference station and auxillary reference stations to the rover, estimation to correct ambigurity would be provided. Difference from VRS that Max uses real reference station but not virtual reference station. There is Auto-Max in which server would provide optimum master and auxillary configuration.
   

This is really amazing and technology is going very fast and more accurate. Then what is the new GPS trend in 2009?

Reference:
Lecia GPS SpiderNet-Take it to the Max
SUPPORT OF NETWORK FORMATS BY TRIMBLE GPSNET NETWORK RTK SOLUTION from Trimble