GPS Q&A

Q.I want to use real-time differential GPS, but am confused by the options and am unsure whether I really need real-time.
D.L. Memphis, Tenn.

A.There are a number of real-time differential solutions available for code phase differential GPS. You can either obtain these services from service providers or set up your own real-time solution. Firstly, you will need to consider if your data collection needs require real-time or whether post-processing is adequate. Secondly, it is important to know if your GPS/GIS data collection system caters for the use of real-time differential GPS. If you establish that real-time is the best option for your application, then there are several factors to consider about each real-time solution - these are discussed below.

Real-time vs. Post-processing
It is understood that the GPS signal is subjected to a number of errors that degrade the accuracy of the GPS signal. These errors include Selective Availability (S/A), which is an intentional degradation of the signal and other system errors. Differential processing is required to remove S/A and reduce other correctable errors. Multi-path is an example of an error that is not correctable. Differential processing can be applied in real-time or in post-processed mode. Real-time differential GPS refers to the use of a telemetry link between the base station and the roving GPS receiver so that the differential correction information can be applied in the field in real-time. Post-processing occurs after the data has been collected. A file is logged at the base station that is used to process the rover file later in the office using post-processing software.
     Fundamentally, real-time GPS gives you the benefit of knowing where you are now! Post-processing tells you where you were. Knowing where you are now is important when you are navigating to a site or feature. Using real-time differential, you can navigate to a feature or site with accuracies that range from 5 meters to sub-meter depending on the equipment and techniques used.
     If you are working in real-time it is important to understand the system's capability when the real-time link is lost. How is the data handled? Does the system allow you to continue to collect GPS positions, allowing you to correct the data later in the office using post-processing software? Some systems will continue to log uncorrected data with no interruptions to your data collection, while other systems will require you to close the data file and start a new data file with the real-time option turned off. Starting a new file may be disruptive to your work. A system that can smoothly transition between logging corrected and uncorrected data will offer you the most flexibility.

Considerations About the Different Real-Time Solutions
Once you have established that you want to work in real-time, the next important step is to establish the most appropriate telemetry link for your application. The four more common real-time options are:     ¥ Medium Frequency (MF) Beacon
    ¥ L-Band satellite differential
    ¥ FM sub-carrier
    ¥ setting up your own real-time solution
     There are advantages and disadvantages with regard to each service, as well as geographical limitations. Ultimately your location may define what real-time option and service provider you use. Below is an explanation of each real-time option, and issues that should be considered with each specific option.

Using MF Beacon
The MF Beacon real-time differential service is a free service set up for maritime navigation purposes and coverage exists in many coastal areas around the world. GPS/GIS data collection systems combined with a beacon receiver can use the real-time signal for differential correction. The range of the MF Beacon varies from site to site, and ranges can vary from 50 to 300 miles. The range is also defined by the beacon receiver. Good beacon receivers will pick up the MF Beacon transmission at greater than the range specified for the MF Beacon site, while low end beacon receivers tend to not have as good a range. Each MF Beacon station uses a fixed base receiver which degrades the accuracy of the corrected GPS position with distance from the base station.
     Today several manufacturers are integrating GPS and beacon technology. Some configurations combine both the GPS and beacon technology into a single antenna and a single receiver, while other configurations combine either the antenna or the receiver. Integration of the technology reduces the amount of equipment you need to carry in the field, especially if less batteries are required to power the integrated system compared to that required to power individual GPS and beacon receivers.
     The greatest advantage of the MF Beacon service is that in almost all countries it is a free service. The cost of using the service is limited to the purchase price of the MF Beacon receiver. For people working within range of an MSK Beacon this solution offers many advantages. It is reliable, free and the technology for receiving the signal is being integrated with a GPS/GIS data collection systems - reducing the equipment you need to carry. The disadvantage of using the MF Beacon is that the signal is susceptible to man-made noise, for example noise from power lines, vehicles and even some GPS receivers and data collectors.

Using Satellite Differential
Satellite differential is a service that uses communications satellites to transmit the information. These satellites have a wide area of coverage. In order to provide accurate differential correction information across the area of coverage, several base stations compute corrections over the entire region. The correction data from all these base stations are then available to the user's receiver so that a more accurate, customized, correction can be computed anywhere within the coverage area. This differs from the fixed base station correction information which degrades with distance from the base station site.
     There are several satellite service providers. Each service tends to make use of several communications satellites in order to cover most of the world. A fee is charged for the use of the satellite differential service. The cost varies between providers and countries. While it may seem more desirable to utilize the free MSK Beacon differential service, the coverage of the satellite differential service makes this a viable option.
     In order to make use of the satellite differential service it is necessary to have a satellite differential receiver and antenna. Typically this can be used with your mapping system. The ergonomics of the satellite differential receiver vary between manufacturers. Like with the MF Beacon receivers the satellite differential technology is being integrated with GPS, reducing the amount of equipment you need to carry. The disadvantage of the satellite differential system is that it can be difficult to receive the signal in a canopy environment especially as you get closer to either the North or South Pole.

Using FM Sub-carrier
The FM sub-carrier differential service makes use of the sub-carrier modulation on commercial broadcast FM radio stations to transmit digital data for differential correction. Service providers are able to take advantage of the existing FM radio infrastructure to transmit differential correction information. The coverage of the FM sub-carrier differential service differs between sites and countries. The FM sub-carrier system uses a fixed base station and so like the MSK Beacon option, the accuracy of the corrected GPS position will degrade with distance from the base station.
     In order to utilize the information it is necessary to have FM sub-carrier receiver. The receivers designed for this are small, lightweight and low cost. If you are not working within range of an MSK Beacon site or are using a GPS/GIS data collection system that does not integrate real-time technology then the size and cost make this option very attractive. As with the satellite service there is a fee for using the FM sub-carrier solution. Rates vary between providers and countries.
     The disadvantage of the FM sub-carrier solution is the range an individual site has, both the MF Beacon and satellite differential systems have greater coverage.

Setting Up Your Own Solution
The final option to be discussed is setting up your own real-time solution. When setting up your own solution you have a number of choices including VHF, UHF and even cell phones for the telemetry link. This option gives you more control over the real-time solution in terms of availability, coverage and cost, however it can add extra complications. By taking control of the real-time solution you are now responsible for your own equipment and ensuring it is operational when you need it to be. Beware of spending a lot of time dealing with poor or failing radio links, licensing requirements and hidden costs. Once you are looking at a range of greater than 1-2 miles you will usually be required to get a license for the radio - most GPS/GIS projects cover areas greater than this. In many countries there are strict regulations governing the use of radio and telemetry links. As real-time GPS requires a radio or telemetry link it may be necessary to have a license for the solution you want to use. Often the greater the range you need for your radio or telemetry link the more stringent the regulations are. Obtaining a license may add to the cost of your solution. The cost of the setup will also increase as the range required for the radios becomes greater. This is because a more powerful radio transmitter is usually required.
     You can purchase many different radios that can be used for the setup of your own real-time solution and some GPS manufacturers offer radios for this purpose. In some cases the radios may have been approved for use in specific countries, so it worth investigating the options offered by a GPS manufacturer.

Summary
When considering the use of real-time differential GPS there are issues to consider about both the service and the GPS/GIS data collection system. There are a number of real-time differential correction sources for you to consider. These solutions do differ in terms of availability, cost and setup requirements. It is important to select the most appropriate option for your project area. While there are many advantages to using real-time DGPS, the selection of the wrong real-time solution and the incorrect GPS may outweigh the advantages.

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