Ntia special Publication 94-30 a technical Report to the Secretary of Transportation on a National Approach to Augmented gps services

  Commercial FM Subcarrier System

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3.3.3  Commercial FM Subcarrier System
This system uses the existing infrastructure of FM radio station subcarriers to broadcast localized pseudorange correction factors to the user community. The system consists of a GPS reference station and one or more FM stations which broadcast the GPS corrections. A block diagram of a typical FM subcarrier system is shown in Figure 3-5. Companies providing FM subcarrier DGPS service currently contract with 300 to 400 FM stations across the Nation.
Accuracy.  The accuracy provided by these types of services depends, in part, upon the level of service required and paid for by the user. Service providers offer a variety of accuracy options for the user ranging from 1 to 3 meters to several tens of meters.
Integrity (Time to Alarm). FM subcarrier systems can include integrity monitors to continuously validate that corrections remain within specified tolerances. Although current systems do not necessarily provide real-time integrity monitoring, this function is technically achievable.
Availability. Availability is a function of the maintenance schedule or the failure of independently-managed broadcast stations, the GPS reference stations, and the various data links involved. The broadcast stations used are those that operate 24 hours a day and have redundant equipment including backup power and alternate transmitters to ensure that scheduled maintenance will not generally result in an outage. The National Association of Broadcasters does not have statistics regarding FM station availability; however, it asserts that most stations do better than one hour outage per year (99.988%). FM stations have a strong financial incentive to stay operational and many stations have been on the air continuously for years. A conservative estimate predicts one day of down time every 10 years resulting in an availability of 99.97%.
Coverage Area.  FM Broadcast stations cover 96% of the population and 80% of the land area of North America [7]. The coverage area of an entire subcarrier system depends upon the contractual arrangements made by the operator of the system, percent injection of the subcarrier and receiver capabilities. An example of subcarrier coverage using many Public Broadcasting System (PBS) stations for CONUS is shown in Figure 3-6.
Security.  FM subcarrier systems are encrypted in order to provide a means of controlling user access to system services.
Infrastructure Cost.  The infrastructure costs are borne by the service provider.
User Cost.  User costs include the purchase of receiver equipment and the user service fee. User equipment for this architecture is expected to cost on the order of $500 or less and may have service fees on the order of a few dollars per day.

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