GPS Receivers

R Nave

Handheld GPS Positioners

The most popular type of GPS receiver is a handheld positioning and navigation unit which monitors the C/A code on the L1 carrier of the GPS signal structure. Such units must be used outdoors with a clear view of the sky, and are capable of locking onto the signals from four or more GPS satellites. The signals allow them to calculate the distances to four satellites, and with that data they can calculate position on the earth's surface in latitude and longitude within +/- 100 meters 95% of the time. They can measure altitude to an accuracy of +/- 180 meters. Since the four signals received are stabilized by atomic clocks, the timing accuracy of an ordinary digital watch is sufficient for the calculations done in the handheld unit. Waypoints may be set and stored in the units, and the received signal used to calculate range and direction to those waypoints. Speed and direction of travel can also be displayed. This makes it clear why they have become extremely popular with hikers and hunters.

R Nave

Differential GPS

Position accuracies to about +/- 2 meters can be obtained by comparison of a field GPS receiver with a fixed receiver at a known position. Receivers for this kind of positioning differ from the popular hand-held units in that they have memories for storage of data in both the moving and stationary units and are calibrated to take data at the same time. The field data is then compared to the base station data in postprocessing to obtained the increased accuracy.

This accuracy is obtained using just the C/A code on carrier L1 of the GPS signal structure.

To obtain position uncertainties under a meter in DGPS, receivers must measure both the C/A code and the phase of the L1 carrier. A sequence of phase measurements must be made to achieve the increase in accuracy. For such measurements to be made in real time, a radio link between the base and field stations must be used. A more accurate version of DGPS is called relative positioning GPS.

R Nave

Relative Positioning GPS

A more sophisticated version of differential GPS is used by surveyors to obtain accuracies on the order of a centimeter for the difference in two positions. The GPS receivers for this process must record the C/A code and the phase of carrier L1 of the GPS signal structure at every epoch of the Navigation Message, on the order of every 12 to 15 seconds. The data from two receivers is postprocessed to obtain the sub-centimeter accuracy.

Maintaining this accuracy for distances over 30 km requires dual frequency receivers that measure the C/A code plus the phases of both carriers, L1 and L2. For both long lines and rapid measurement of shorter distances, receivers make use of all five of the measurable signals of the signal structure.

R Nave

Go Back

Kinematic GPS

Relative measurements "on the fly" may be made with kinematic packages on GPS receivers, so that accuracies comparable to the relative positioning GPS can be made while moving. Real time kinematic GPS requires a radio link between the field receiver and the base station.

R Nave

GPS Signal Structure

The two carriers and the two signal codes of the GPS system are derived directly from the onboard atomic clock oscillators. By decoding the navigation message from three GPS satellites, the position on the earth can be calculated.

R Nave