Technology in Crop Production

The Low Latency positioning mode delivers 20 Hz position fixes with around 20 millisecond latency with a precision that is only slightly less accurate than Synchronized RTK positioning. Based upon the predictability of the reference station phase data.

http://www.scribd.com/doc/91055018/19/Fast-Update-Synchronized-RTK-5-or-10-Hz

A large part of the solution latency in Synchronized RTK processing isdue to the data formatting and transmission of the base station data tothe rover (seeFigure2.1onpage10). The BD950 receiver includes a Low Latency positioning mode for applications that demandcentimeter-level accuracy almost instantaneously. The Low Latencypositioning mode delivers 20Hz position fixes with around 20mseclatency with a precision that is only slightly less accurate thanSynchronized RTK positioning.The Low Latency positioning scheme relies on the predictability of the base station phase data. Phase measurements observed at a fixedbase receiver generally exhibit a smooth trend. Variations in the carrierphase are caused by:

• Cycle slips
• Satellite motion
• Receiver and satellite clock variations
• Atmospheric delay

Given a brief history of base station phase measurements, the BD950receiver is able to accurately predict what they will be in the next fewseconds. Instead of waiting for base station carrier phasemeasurements to arrive, the BD950 rover predicts or projects what thebase carrier phase measurements will be for the current epoch. Abaseline solution is then generated using the projected base stationcarrier phase measurements and the observed rover receiver carrierphase. The latency of the position solution derived from projectedcarrier phase is around 20 milliseconds for the BD950 receiver.With the Low Latency positioning scheme, accuracy is traded fortimeliness. An increase in the data link delay relates to an increase inthe projection time of the base station phase data. This leads to anincrease in the uncertainty of the RTK solution.Figure2.3presents anempirically derived model for the base receiver phase projection errorsas a function of data link delay.

The base phase prediction errors are governed by:

• Unmodeled Selective Availability errors
• Short term instabilities in the receiver and satellite clocks
• Unmodeled satellite orbit variations

A data link latency of 1 second would result in phase projection errorsapproaching 0.02 cycles (0.004m). Multiplying the phase projectionerrors by a PDOP of 3.0 would yield an increase in noise for the LowLatency RTK solution of 3.0 × 0.004 = 0.012m over theSynchronized RTK solution. In many applications the slight noiseincrease in the Low Latency Solution is tolerable.