GPS-Base

The GPS-Base is a small, portable GPS Base Station suitable for transmitting Differential corrections (DGPS) to the RT3000, RT4000 and other products that use GPS. The position accuracy of Differential and RTK GPS receivers is improved when using the GPS-Base. The GPS-Base is a lower cost version of the RT-Base but provides the same accuracy as the RT-Base.

Two models of the GPS-Base exist; the GPS-Base-20 can supply L1 corrections suitable for 20cm positioning and the GPS-Base-2 can supply L1/L2 corrections suitable for 2cm positioning. Both models are identical in their operation.

Fast To Install

The GPS-Base has been designed with installation speed in mind. Simply connect the GPS Antenna and the Radio Modem Aerial; then turn on. Simple software on the PC is used to configure the GPS-Base. The current location can be saved and restored or a new location can be set by averaging the GPS measurements.

Overview

The figure below gives an overview of how Differential GPS works. The information from each satellite is measured by both the GPS-Base and by the GPS in the car. The GPS-Base works out the error in the satellite’s information and transmits it to the car using a radio link. The GPS in the car then applies the correction to each satellite’s measurement before it computes position and time.

For RTK (Real-Time Kinematic) Carrier-Phase measurements the principle is the same, but the GPS in the car also has to figure out the difference in the number of carrier-phase cycles between the GPS-Base and the car. The GPS-Base measures the carrier-phase of the signals from each satellite and transmits it to the car.

Differential GPS works in real-time because the corrections from each satellite vary slowly and predictably. The GPS in the car uses a model to predict the error from each satellite. It can update its model when the radio link transmits new data. It is not necessary for the GPS in the car to wait until the radio has transmitted the correction before it outputs its latest value.

For Pseudo-Range (not carrier-phase) Differential GPS the corrections can be up to 60s old (depending on the GPS in the car). For RTK corrections (20cm and 2cm) the corrections can be up to 30s old (again, depending on the GPS in the car).

Multipath Rejection

The GPS-Base uses Pulse-Aperture Correlator Technology to minimise the effects of multipath. The GPS-700 Pin-Wheel Technology Antenna includes a ground-plane to minimise ground surface multipath and reflections.

Radio Modems

Different countries have different licensing laws and different frequencies for radio transmissions. Many countries have license-free frequencies that can be used for Differential GPS signals. We can supply a range of different radios that can be used, license-free, in many countries.

DGPS Correction Type(s)

By default the GPS-Base uses the RTCA standard for transmitting its differential corrections. Special versions that use RTCM or CMR corrections are available on request.