A typical vertical single-frequency ionospheric correction profile over the 24-hour day.
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A typical vertical single-frequency ionospheric correction profile over the 24-hour day. |
The ionosphere, the layer of the atmosphere between about 60 and 1000 kilometers high, causes a frequency-dependent signal delay that can be a major source of GPS navigation error. The magnitude of this delay depends on user location (particularly latitude), time of day, time of year, and solar activity. For a satellite directly overhead, errors occurring during daytime hours typically range between 5 and 10 meters but can exhibit significant spatial and temporal variation. For a satellite located close to the horizon, the delay can be up to three times as great.
The dual-frequency GPS user can factor out ionospheric delays by using the time delays from the L1 and L2 pseudorange measurements. Single-frequency receivers (primarily civilian) must use a model broadcast by the satellites to partially compensate for ionospheric effects. The vertical single-frequency correction is a cosine function with peak amplitude at 14:00 local time and a constant nighttime offset of 5 nanoseconds (which translates to roughly 1.5 meters).
The elevation-dependent obliquity factor increases from 1.0 at vertical to about 3.0 at a 5-degree elevation. |
The amplitude and period of the model are computed from coefficients uploaded to the satellite daily by the control segment. An elevation-dependent obliquity factor converts the vertical correction to a slant correction. The single-frequency correction model typically reduces statistical ionospheric error by about 50–60 percent.
With the discontinuation of selective availability, ionospheric error has become the dominant error source for the single-frequency user. Modernized Block IIR satellites, to be launched beginning in 2003, will have a second civilian signal on L2, providing civilian users the opportunity to implement dual-frequency corrections, resulting in navigation and timing performance that approaches the level of the authorized user.