Gnss Internet Radio 1.4.11 48 [PORTABLE]
Eufemia Graybill
Now that cellular coverage is available in much of the world, this is less of an issue. However, some applications still require a local radio link. Many people use WiFi or WiMax or this, others use 900MHz, 2.4GHz, or 5.8GHz radio links in the unlicensed ISM bands. The precise regulations and rules vary depending on what part of the world you live in. The rapid growth in RTK applications for aerial drone use has changed the landscape in this respect. Short range radio transmitter circuit boards can be obtained by hobbyists for tens of dollars, allowing them to connect reliability over small coverage regions.
Now that cellular coverage is available in much of the world, this is less of an issue. However, some applications still require a local radio link. Many people use WiFi or WiMax or this, others use 900MHz, 2.4GHz, or 5.8GHz radio links in the unlicensed ISM bands. The precise regulations and rules vary depending on what part of the world you live in. The rapid growth in RTK applications for aerial drone use has changed the landscape in this respect. Short range radio transmitter circuit boards can be obtained by hobbyists for tens of dollars, allowing them to connect over small regions.
With RTN surveying, a permanent network of reference stations is required. Spacing of the reference stations can be 10-50 miles and can cover a local, regional, or statewide area. The reference station network continuously streams data (using LAN, Internet, or radio links) to a central location (server). The server then performs several functions including storage of RINEX data, performance of quality assurance checks on the raw data, network modeling and estimation of systematic errors, calculation of and conversion of correction data to a user format (RTCM format or CMR+), and communication of the data to the users. The user then receives the corrections (using LAN, Internet, radio links, or a cellular modem) in real time.
Whether your corrections are for a base you have mounted on a building or at a site or even for a short RTK session, they can be delivered over IP. Of course, this only works in areas where cellular or other wireless internet coverage is available. Some newer GNSS receivers come with IP and NTRIP (read on for details) capabilities as standard features, but with a little ingenuity and some free tools you can IP-enable nearly any RTK receiver to do this. The following article shows you how.
There are other radio-based options for transmitting corrections, and each option has certain advantages and disadvantages over UHF radios. Spread spectrum 900MHz radios and long-range Bluetooth or WiFi radios (2.4 GHz or 5GHz) are common options. These radios have the advantages of license-free use and better interference mitigation, but their major disadvantage is that they create a decreased coverage area and are often not a comparable alternative to UHF radios.
The dilemma that radio-based RTK operations face is that the geographic area where RTK corrections from a single reference station can be used to produce survey-grade positions is much larger than the geographic area covered by radios. Hence, a user is not taking full advantage of the potential of the RTK system when using radio-based transmissions for corrections. Using IP-based transmissions for corrections can help users get more functionality from their RTK systems.
These sophisticated RTN often offer many more services than some surveying companies may need (e.g., VRS- or MAC-style corrections). Therefore, a subscription to an RTN could be seen as overkill if a company wants simply to take advantage of the many benefits of internet-based transmission of corrections for the area surrounding their reference station(s). That is not to say that RTN does not have advantages over single reference station RTK systems. RTN provides more consistent correction quality and a means of connecting to a geodetic reference frame over a wide region. But if the range of a single reference station RTK system fits your needs and operations, you can reduce or eliminate radio-based communication legalities and challenges through IP-based communications.
Using free open-source software, any company that operates its own GNSS reference station, has a spare computer with a serial port kicking around, and has an active internet connection can easily transmit their own corrections using NTRIP: Networked Transport of RTCM via Internet Protocol. This article explains how internet-based transmission of differential GNSS corrections works and how to set up such a system within your company.
The NTRIP transmission technique is based on the popular Hypertext Transfer Protocol (HTTP) streaming standard (e.g., internet radio) and uses the client-server principle for transferring data. The server is known as the NTRIP broadcaster (a.k.a. NTRIP caster or simply caster). A caster creates the multiple- and simultaneous-user access to the corrections being transmitted by a single GNSS reference station. It also provides security (via username and password) and management of the connected clients, and it creates mountpoints (i.e., channels a user can connect to) to uniquely identify the different corrections being made available to clients.
The major advantage of the Lefebure caster is realized by companies that have only differential GNSS corrections from a single reference station that they want to make available via the internet. Built into the caster is a single NTRIP server that is designed to read input from an RS232 serial port on the computer hosting the Lefebure caster application. The standard serial port configuration options can be selected to match the serial output from an attached GNSS receiver. This allows for any Windows OS-based personal computer to host the caster application (hence reducing overhead costs). The caster will accept additional GNSS data streams from other external NTRIP servers via TCP/IP connections.
You now are ready to start having users connect to the caster and experience the advantages of internet-based transmission of differential GNSS corrections. In order for an NTRIP client or NTRIP server to connect to the caster, the public IP (v4) address and port number of the host computer needs to be made available to users. For NTRIP clients to connect, a registered username and password must be used. For external NTRIP servers to connect, a new or unused mountpoint must be available and the name of the mountpoint along with its password must be provided to the administrator of the external NTRIP server.
ORGN partners and subscribers with valid ORGN rover accounts have access to real-time kinematic correctors computed by Leica Spider software. These correctors are served over the internet and accessed by the user via a cell modem connected to a GNSS rover in the field.
Satellite Navigation is based on a global network of satellites that transmit radio signals from medium earth orbit. Users of Satellite Navigation are most familiar with the 31 Global Positioning System (GPS) satellites developed and operated by the United States. Three other constellations also provide similar services. Collectively, these constellations and their augmentations are called Global Navigation Satellite Systems (GNSS). The other constellations are GLONASS developed and operated by the Russian Federation, Galileo developed and operated by the European Union, and BeiDou, developed and operated by China. All providers have offered free use of their respective systems to the international community. All providers have developed International Civil Aviation Organization (ICAO) Standards and Recommended Practices to support use of these constellations for aviation.
The software-defined radio (SDR) has an infinite number of interpretations depending on the context for which it is designed and used. By way of a starting definition, we choose to use that of a reconfigurable radio system whose characteristics are partially or fully defined via software or firmware. In various forms, the SDR has permeated a wide range of user groups, from military and business to academia and the hobby radio community.
SDR technology has evolved steadily over the decades following its birth in the mid-1980s, with various surges of activity being generally aligned with new developments in related technologies (processor power, serial busses, signal processing techniques and SDR chipsets). At present, it appears that we are experiencing one such surge, and the GNSS SDR is expanding in many directions. The proliferation of collaboration and code-sharing sites such as GitHub has enabled communities to share and co-develop receiver technology; the rise in the maker-culture and crowdsourcing has led to the availability of high-performance radio-frequency (RF) front ends; and the adoption of SDRs by some major telecommunications companies has led to the availability of suitable integrated circuits.
Higher up the cost, power and complexity structure are radios designed explicitly to support SDR applications that happen to cover GNSS bands such as the Lime LMS6002d/LMS7002M and the Analog Devices AD9364. Notably, these provide receive and transmit channels and frequency coverage up to 6 GHz.
Simultaneously, the software-defined GNSS receiver architecture has morphed in multiple directions, enjoying virtually unlimited processing power of cloud computing, or availing itself of fully integrated RF and host-processor modules. As the use cases and host environments for GNSS receivers continue to diversify and the need for flexibility in the receiver continues to increase, it may be that the software-defined GNSS receiver emerges as a contender for the ASIC receiver for certain specialized use cases. Furthermore, as navigation is increasingly provided by an internet-connected device, the software-defined radio may even carve out its own niche, to become the go-to solution.
JAMES T. CURRAN received a Ph.D. in electrical engineering in 2010 from the Department of Electrical Engineering, University College Cork, Ireland. He is a radio-navigation engineer at the European Space Agency in the Netherlands.
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