GPS

Trimble trimble.com

Trimble Knowledge Network trimble.com

Trimble Learning Center trimblelms.com

GPS.gov gps.gov

US Coast Guard Navigation Center navcen.uscg.gov 

GNSS status

Up to date information about the status of developing global satellite networks.

GPS status at the US Coast Guard Navigation Center website: http://www.navcen.uscg.gov/?pageName=GPSmain

GLONASS status at the Russian space agency website: http://www.glonass-ianc.rsa.ru/pls/htmldb/f?p=202:1:15353442977007243169 

Galileo status at the European space agency website: http://www.esa.int/esaNA/galileo.html

Satellite launch schedules: http://www.satelliteonthenet.co.uk/launch.html

NMEA data gpsinformation.org

The National Marine Electronics Association (NMEA) has developed a specification that defines the interface between various pieces of marine electronic equipment. The standard permits marine electronics to send information to computers and to other marine equipment. 

GPS receiver communication is defined within this specification. Most computer programs that provide real time position information understand and expect data to be in NMEA format. This data includes the complete PVT (position, velocity, time) solution computed by the GPS receiver. The idea of NMEA is to send a line of data called a sentence that is totally self contained and independent from other sentences. There are standard sentences for each device category and there is also the ability to define proprietary sentences for use by the individual company. All of the standard sentences have a two letter prefix that defines the device that uses that sentence type. (For gps receivers the prefix is GP.) which is followed by a three letter sequence that defines the sentence contents. In addition NMEA permits hardware manufactures to define their own proprietary sentences for whatever purpose they see fit. All proprietary sentences begin with the letter P and are followed with 3 letters that identifies the manufacturer controlling that sentence. For example a Garmin sentence would start with PGRM and Magellan would begin with PMGN.

Each sentence begins with a ' and ends with a carriage return/line feed sequence and can be no longer than 80 characters of visible text (plus the line terminators). The data is contained within this single line with data items separated by commas. The data itself is just ascii text and may extend over multiple sentences in certain specialized instances but is normally fully contained in one variable length sentence. The data may vary in the amount of precision contained in the message. For example time might be indicated to decimal parts of a second or location may be show with 3 or even 4 digits after the decimal point. Programs that read the data should only use the commas to determine the field boundaries and not depend on column positions. There is a provision for a checksum at the end of each sentence which may or may not be checked by the unit that reads the data. The checksum field consists of a '*' and two hex digits representing an 8 bit exclusive OR of all characters between, but not including, the ' and '*'. A checksum is required on some sentences.

There have been several changes to the standard but for gps use the only ones that are likely to be encountered are 1.5 and 2.0 through 2.3. These just specify some different sentence configurations which may be peculiar to the needs of a particular device thus the gps may need to be changed to match the devices being interfaced to. Some gps's provide the ability configure a custom set the sentences while other may offer a set of fixed choices. Many gps receivers simply output a fixed set of sentences that cannot be changed by the user. The current version of the standard is 3.01. 

gpsinformation.net gpsinformation.net

GPS - NMEA sentence information aprs.gids.nl, 2001

RTN

Real-Time Network (RTN) Surveying water.usgs.gov

USGS Global Positioning Application and Practice water.usgs.gov

SXBlue Series sxbluegps.com

SXBlue RTN User's Guide sxbluegps.com

SXBlue RTN is an NTrip / Direct IP (DIP) client that allows the SXBlue GPS receivers to use RTK/DGNSS corrections from a network or a server. 

It was designed to allow field data collection software that do not feature a built-in NTrip client to receive RTK corrected positions from a single connection to your SXBlue GPS. SXBlue RTN connects to the Internet to receive differential corrections either via the standard NTrip protocol 

or to a Direct IP address server. It uses only one communication port with the SXBlue GPS (usually Bluetooth) to both send corrections and receive position data. A virtual COM port is created on your computer/PDA to allow your data collection software to receive differentially corrected positions. 

RTK Networks 

An RTK Network (or RTN, Real-Time Network) consists of three segments: the Server, the Ntrip Broadcaster (Caster) and the Client. 

The server commonly hosts the GNSS base station receiver that is computing the differential corrections with an antenna set at known coordinates. A client can connect directly to the server using a Direct IP address and a TCP port number. This is what is referred to as a DIP in SXBlue 

RTN. This is a direct link from the client to the server.

In an RTN, the servers are connected to one or more Casters. The Caster follows a standard protocol called NTrip (Networked Transport of RTCM via Internet Protocol) to stream differential GNSS data over the Internet. Connection to an NTrip Caster requires an address, a port number, a user ID and a password. The Caster holds the Source Table which contains the list of all the Mountpoints (Servers) in the Network. From this table, the Client can choose a single baseline solution (a specific server) or, if the caster features it, a virtual reference solution (a correction specific to the client’s location; in this case the client sends its position to the network via a standard NMEA GGA string. The SXBlue RTN client handles this automatically). 

SXBlue RTN 

The diagram below illustrates the principle of operation of the SXBlue RTN utility for both Windows Mobile and Desktop computers. An Internet connection is required on the PC or PDA (Wi-Fi, ActiveSync, Cellular, etc) to access the corrections from the Server or NTrip Caster. 

- SXBlue RTN establishes the link to the Server/Caster and receives the differential corrections via the Internet 

- It initiates a bi-directional link with the SXBlue GPS receiver (usually via Bluetooth). It sends the corrections and receives the GPS positions (usually via standard NMEA messages) 

- If requesting a virtual solution from the Caster, SXBlue RTN activates/receives the GGA sentence from the SXBlue GPS receiver and sends it to the Network so the corrections can be specific to the user’s location 

- It makes the corrected GPS positions available to the Data collection/Mapping Software via a Virtual COM port internal to the computer/PDA.

On a PDA, SXBlue RTN uses the Windows Mobile “External GPS” utility that comes standard with Windows Mobile 5.0 and above. In the case of a desktop PC, it uses the VSPE (Virtual Serial Port Emulator) that is deployed during the installation process (see section 2.1 above)