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Instructions for Installing Dual GPS Antennas

Trimble has provided these instructions for mounting GPS antennas when using Trimble SPS361 and SPS461 heading receivers and SPSx51+SPS551H, which are dual-antenna systems.

Accuracy and separation distance
Always try to maximize the horizontal distance between the antennas, and minimize the vertical separation distance. Trimble’s dual frequency heading system has no constraint on separation distance, unlike single frequency systems where the baseline length has to be input as a ‘seed’. The dual frequency solution is robust and fast to get operating.

Orientation of antenna relative to vessel
It does not matter if the antennas are port/starboard or forward/aft – try to locate them with the greatest separation and least sky obstruction. A calibration angle can be input to the receiver so that it displays and outputs true vessel heading.

If they are mounted port/starboard, the receiver’s attitude angle output will equate to roll. If mounted bow/stern, the attitude is pitch.

Quick mobilizations
When installing the antennas onto a vessel for a short duration project, consider mounting them onto an aluminium pole. This pole can be mounted on the top of the cabin or over an echo sounder sensor.

Calculating pitch and roll
To get the ‘other’ attitude value, add an SPS551H receiver to a dual-antenna SPSx61 system. The vector antenna from the SPSx61 can go to the port or starboard of the position antenna to provide heading and roll. The SPS551H antenna can go directly forward or aft of the SPSx61 position antenna to provide pitch. Configure the SPSx61 to output Moving Base CMR messages to the SPS551H. A single axis vector is available from the SPS551H in the NMEA AVR message.

If heave is needed, you require an SPS461 with RTK option. Trimble’s HYDROpro or other survey software is then used to compute water level (tide) and heave.

The Trimble GA530 antennas can typically be used with the 30-meter/100-foot antenna cable.

Trimble provides a Support Note entitled “Long Antenna Cable runs with SPS GPS” for very long antenna cable runs. This note is available from Trimble Marine dealers.

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