This is a pretty basic treatment of the subject of differential GPS. Try Starlink for a more exhaustive discussion of the subject.
Differential GPS (DGPS) is a method of eliminating errors in a GPS receiver to make the output more accurate. This process is based on the principal that most of the errors seen by GPS receivers in a local area will be common errors. These common errors are caused by factors such as clock deviation, selective availabilty and changing radio propagation conditions in the ionosphere. If a GPS receiver is placed at location for which the coordinates are known and accepted, the difference between the known coordinates and the GPS-calculated coordinates is the error. This receiver is often called a "base station".
The error, which the base station has determined, can be applied to other GPS recievers (called "rovers"). Since the sources of the error are continuously changing, it is necessary to match the error correction data from the base station very closely in time to the rover data. One way of doing this is to record the data at the base station and at the rover. The data sets can be processed together at a later time. This is called post processing and is very common for surveying applications. The other way is to transmit the data from the base station to the rover. The error calculation is made in the rover in real time. This process is called real-time dgps.
Circular Error Probable (CEP)
The CEP is the radius of a circle in which the true horizontal coordinates will be located 50% of the time.
Spherical Error Probable (SEP)
The SEP is the sphere in which true position fixes will be located 50% of the time.
300 meters - 100 meters
This is the accuracy range that the Department of Defense guarantees from the Standard Positioning Service (SPS), the only service commonly available to civilian users.
25 meters - 10 meters
Cheap handheld receivers in the $300 to $3000 range with basic DGPS can usually achieve accuracies in this range.
5 meters - 1 meter
Better handheld receivers and mapping grade receivers can get down to this level of accuracy. They will cost from $500 to $5000.
1 meter - 10 cm
Better quality mapping receivers and low-end surveying equipment can get this accurate. Such receivers will generally use carrier phase measurement techniques instead of code-based solutions and will cost more than $3000 per unit. Users requiring better accuracy can get it by taking long observations (between 20 seconds and 2 hours) and by surveying multiple points as a network and using network adjustment routines. Sub-centimeter accuracies are possible using these techniques.
10 cm - sub-centimeter
High end surveying receivers and geodetic receivers are used to reach this level of accuracy. Receivers of this class, costing more than $15,000 per unit, will always (correct me if I'm wrong) use carrier phase measurement techniques and will usually use both of the GPS frequencies. A relatively new technique known as ambiguity resolution on the fly (AROF) allows receivers to start producing high-quality solutions very quickly (within 40 seconds) without complicated initialization procedures.
In the post processing method, both the base station and the rover must record data simultaneously. How this is done is dependent on the situation. One way is to record the data right in the GPS receiver. This is common in surveying applications where the base and rover are being used to measure a specific baseline.
Sometimes it is physically inconvenient to record the data in the roving GPS receiver. In such cases the roving user may record the data in a laptop PC, a palmtop PC such as an Apple "Newton" or in a specialized data collector. The usual reason for doing this is that the data collecting device gives the user more flexibility in tagging the GPS data with desired information.
In a situation where the base station will be permanently fixed in one spot and will be used for several rovers, a community base station may be installed. A community base station is nothing more than a GPS receiver permanently connected to a PC. The GPS data is stored in the PC. It is possible to install a BBS system on the base station PC so that roving users can download data sets for remote processing. The PC collects base station GPS data and saves it to files in time-block increments.
Having collected GPS data for post processing, it remains to get the two data sets together. This is not always a straightforward process. Data formats are often proprietary or dependent on the technology used. Users will usually have the same brand of equipment for base stations and rovers so this is not a problem. Sometimes manufacturers will have conversions for different data formats. However, this is not always the case. A data format called RINEX (Receiver INdependent EXchange) may sometimes be used as an alternative. A discussion of the RINEX format is available at the USCG NAVCEN.
If the user desires improved accuracy at the time the equipment is being used, real-time processing must be employed. For real-time processing, special formats are employed. There are two predominent formats currently being employed.NMEA-0183 is a data format commonly employed for communications between ship-borne navigation electronics. This format is a voluntary industry standard originated by the National Marine Electronics Association. GPS receivers output this format but do not accept it (corrections? - ed). A discussion of the NMEA-0183 format is available at the USCG NAVCEN.
The second format, RTCM-104 is an attempt by the Radio Technical Commission for Maritime Services to standardize DGPS operation. The standard is the result of a request by the Institute of Navigation to the RTCM to develop recommendations for DGPS transmission. The RTCM formed Special Committee 104 (SC-104, get it?) titled "Differential NAVSTAR GPS Service". Version 2 of this service is used by many beacon systems (including the US Coast Guard system). Version 2.1 includes additional information for the transfer of real-time kinematic data. Copies of the RTCM SC-104 Standards document may be purchased from the RTCM or from Navtech Seminars. Addresses for both are listed here. A discussion of the RTCM-104 format is available at the USCG NAVCEN.
Do-it-yourself
Sometimes the only way to get what you want when you want it. Not necessarily all that expensive, either depending on what you are trying to accomplish.Community Base Stations
Some organizations maintain base stations for post-processing and are willing to share the data. Check around state Department of Transportation offices and civil engineering and surveying departments at universities.
Government Radio Beacons
The U.S. Coast Guard has tested a system of high frequency DGPS radiobeacons positioned around the coast and along certain waterways. Starlink Incorporated maintains a list of sites and system status for the Coast Guard system and for systems for other countries. The most up-to-date information on the US and Canadian DGPS systems is available from the USCG NAVCENDownloaded from the USGS GPSIC BBS, 3/95:
The FAA is considering three types of differential GPS service for aviation use: (1) local area DGPS (LADGPS), which would be located at each airport or closely grouped airports to support instrument approaches to current CAT I weather minimums; (2) wide area DGPS (WADGPS), which would provide GPS integrity broadcast (GIB) and accuracy improvements for all of North America; and (3) use of kinematic carrier phase positioning for instrument approach and landing.
All three types of DGPS service are still under develoment; however, WADGPS/GIB is in the FAA budget for procurement and installation. The basic concept for WADGPS/GIB is to have several GPS ground monitoring stations (about 20 for North America) with two master control stations where differential corrections and integrity for each satellite are determined. This information will be sent to two communications satellite earth stations and relayed to the aircraft via a satellite signal that is similar to a GPS signal with unique codes. This signal may also be suitable for ranging providing improved navigation availability.
Commercial Services
There are commercial services for getting real time differential data. Commercial services are great where you cannot afford the expense of maintaining or operating your own base stations. If you are a business person trying to map the location of your clients, for example, commercial service is probably the way to go. The ones I know about are listed in the DGPS Services page. For the most part, they all require you to purchase a radio receiver of some sort and sign a service contract. All the receivers produce RTCM-104 output. However, GPS receivers do not have a standard connector and you may have to make your own.
The technology you pick for a real-time dgps data link depends on what you are trying to accomplish and how much money you have.
HF/VHF/UHF/Microwave Radio
HF radios are best for transmitting over long distances. This is the technology used by the U.S. Coast Guard's DGPS system. The drawback to using HF is that the receive antennas are relatively large (rather like a CB antenna) which can be a hassle if you are trying to survey with it. Transmit antennas can be even larger, a problem if you have space limitations.VHF and UHF Radios are very light weight and use small, convenient antennas. The drawback here is that VHF radio signals are largely limited to line-of-sight. UHF is even worse. If you are mapping or surveying with two units you will probably only get a couple of miles range maximum. To go further you will have to get the transmit antenna up higher, using a mast or tower. Hilly terrain and dense trees can limit where you can hear the signal.
Microwave Radios - I haven't seen any manufacturers of low data rate microwave equipment. There's no reason it wouldn't work, though. Some amateur radio equipment is available in the 1.2 GHz range. There are a few handie-talkies and mobile radios available in this band.
It doesn't take a lot of equipment to set up your own radio link. What you need is a digital radio-modem. This is basically a radio transceiver that has a modem built in it. Many of the VHF/UHF radio modems I've seen have RS-232 ports and can transmit 9600 baud which is more than enough for most DGPS. You do nothing more than plug the GPS receiver being used for a base station into the digital radio and tell the GPS to output RTCM-104 on that RS-232 port. The other radio is plugged into the rover which is told to receive RTCM-104 on that RS-232 port. It's really pretty simple.
Some manufacturers of digital radios include:
Northwest Technical Services and Gateway Communications are the only two companies I've found on the internet that deal in digital radio equipment and I don't see them using the equipment I'm aware of.
- Navstar Navigation Systems
- Pacific Crest
- Racal Positioning Systems
Radio/Television Subcarrier
There is an international standard for sending data over radio and television subcarriers called the Radio Data System (RDS). Both DCI and Accqpoint services deliver DGPS signals this way.
Wireless Data Service
RAM and ARDIS are commercial networks that allow users to transmit digital data. I don't know about ARDIS but RAM can connect to a user's TCP/IP network, sending IP packets to the remote user. Cellular Digital Packet Data (CDPD) is a similar digital service that is just now being supplied by cellular telephone carriers. McCaw Communications has a web site describing it's CDPD services.
Cellular Telephone
It is certainly feasible to use cell phones for DGPS links; however, there are some limitations. First, cellular phones are not designed for data transmission. My experience indicated that the highest data rate you could reliably (as such) expect is 2400 bps which is OK for most DGPS I'm aware of. However, the quality of the circuit can be pretty bad, causing data errors and lost connections. Also, if you are moving, you can experience disruptions while moving from cell to cell.Most cell phones do not include modems and most cell phones do not have the interface that allows you to plug a modem into them. Many cell phones will not accept data at all - those that are are marked with an MC^2 logo or otherwise noted as data/fax ready. Generally, it takes a modem with a special interface and connector to plug into the phone. All of these I have seen are PC Card modems that are meant to be used in a laptop PC. There are adapters that will allow a regular modem to be plugged into a cell phone. They cost on the order of $250.
One additional thing to note - GPS receivers do not include facilities to dial a telephone. There is a way to do it - you would have to program the dial string in the modem's power-up configuration cmos. That way, when you turn the modem on, it should dial. Simlarly, you would have to configure the receiving modem to autoanswer.
Plain Old Telephone Service (POTS)
This should work quite well but you have that pesky telephone line. However, as above, GPS receivers aren't designed to dial or answer modems. You'd have to connect the RS-232 output of the GPS base station to a modem that had been programmed so that auto-answer mode was part of it's power-up configuration. To get the DGPS signal to a rover, the rover would either have to be connected to an external modem that had an automatic dial string in it's power up configuration or be connected to a PC with a modem that could dial for it.
Internet
I've not heard of anyone successfully doing this but there has been some talk about it on the sci.geo.satellite-nav newsgroup recently. I don't see why it wouldn't work but you still have to get the data from the internet to the GPS receiver - possibly a wireline or radio link. If anybody does it let me know.
Langley, Richard B. "Communications Links for DGPS"
GPS World 4, no. 5 (May 1993) : 47-51
Langley, Richard B. "The Mathematics of GPS"
GPS World 2, no. 7 (July/August 1991) : 45-50