Novatel OM-20000077 Positioning Modes of Operation, Single-Point or Autonomous, GPS System Errors

Chapter 6

Positioning Modes of Operation

The following single frequency modes of operation are described further in this chapter:

•Single Point or Autonomous

•Satellite-Based Augmentation System (SBAS)

See Appendix G, GPS Overview on Page 67 for an overview of GPS positioning.

6.1Single-Point or Autonomous

The NovAtel SUPERSTAR II receiver is capable of absolute single-point positioning accuracies of < 5 meters CEP (GDOP < 2; no multipath).

The general level of accuracy available from single-point operation may be suitable for many types of applications that do not require position accuracies of better than 5 m CEP. However, increasingly more and more applications desire and require a much higher degree of accuracy and position confidence than is possible with single-point pseudorange positioning. This is where differential GPS (DGPS) plays a dominant role in higher accuracy real-time positioning systems, see also Section G.3, GPS Positioning on Page 69.

By averaging many GPS measurement epochs over several hours, it is possible to achieve a more accurate absolute position.

The next section deals with the type of GPS system errors that can affect accuracy in single-point operation.

6.1.1GPS System Errors

In general, GPS SPS C/A code single-point pseudorange positioning systems are capable of absolute position accuracies of about 5 meters or less. This level of accuracy is really only an estimation, and may vary widely depending on numerous GPS system biases, environmental conditions, as well as the GPS receiver design and engineering quality.

There are numerous factors which influence the single-point position accuracies of any GPS C/A code receiving system. As the following list will show, a receiver’s performance can vary widely when under the influences of these combined system and environmental biases.

•Ionospheric Group Delays – The earth’s ionospheric layers cause varying degrees of GPS signal propagation delay. Ionization levels tend to be highest during daylight hours causing propagation delay errors of up to 30 meters, whereas night time levels are much lower and may be as low as 6 meters.

•Tropospheric Refraction Delays – The earth’s tropospheric layer causes GPS signal propagation delays. The amount of delay is at the minimum (about three metres) for satellite signals arriving from 90 degrees above the horizon (overhead), and progressively increases as the angle above the horizon is reduced to zero where delay errors may be as much as 50 metres at the horizon.

•Ephemeris Errors – Some degree of error always exists between the broadcast ephemeris’ predicted satellite position and the actual orbit position of the satellites. These errors will directly affect the accuracy of the range measurement.

•Satellite Clock Errors – Some degree of error also exists between the actual satellite clock time and the clock time predicted by the broadcast data. This broadcast time error will cause some bias to the pseudorange measurements.

•Receiver Clock Errors – Receiver clock error is the time difference between GPS receiver time and true GPS time. All GPS receivers have differing clock offsets from GPS time that vary from receiver to receiver by an unknown amount depending on the oscillator type and quality (TCXO verses

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