Simrad HS80/MX575C, User Manual

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Operation | 

HS80/MX575C User Manual

Operation

GPS overview

For your convenience, both the GPS and SBAS operation of the HS80/MX575C features 
automatic operational algorithms. When powered for the first time, the HS80/MX575C 
performs a ‘cold start’ that involves acquiring the available GPS satellites in view and the SBAS 
differential service for HS80 and beacon DGPS service for the MX575C.
If SBAS is not available in your area, an external source of RTCM SC-104 differential corrections 
may be used. If you use an external source of correction data, it must support  an eight data 
bit, no parity, one stop bit configuration (8-N-1).

GPS operation

The GPS receiver is always operating, regardless of the DGPS operation mode. The following 
sections describe general operation of the HS80/MX575C’s internal GPS receiver.

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Note:  

Differential source and status have no impact on heading, pitch, or roll. They only have 

an impact on positioning and heave.

Automatic tracking

The HS80/MX575C’s internal GPS receiver automatically searches for GPS satellites, acquires 
the signals, and manages the navigation information required for positioning and tracking.

Receiver performance

The HS80/MX575C works by finding four or more GPS satellites in the visible sky. It uses 
information from these satellites to compute  a position within 4.0 m. Since there is some 
error in the GPS data calculations, the HS80/MX575C also tracks a differential correction. The 
HS80/MX575C uses these corrections to improve its position accuracy to better than 1.0 m.
The two main aspects of GPS receiver performance are 1) satellite acquisition, and 2) 
positioning and heading calculation.
When the HS80/MX575C is properly positioned, the satellites transmit coded information to 
the antennas on a specific frequency. This allows the receiver to calculate a range to each 
satellite from both antennas. GPS is essentially  a timing system. The ranges are calculated 
by timing how long it takes for the signal to reach the GPS antenna. The GPS receiver uses a 
complex algorithm incorporating satellite locations and ranges to each satellite to calculate 
the geographic location  and heading. Reception of any four or more GPS signals allows the 
receiver to compute  three-dimensional coordinates and a valid heading.

Differential operation

The purpose of differential GPS (DGPS) is to remove the effects of selective availability (SA), 
atmospheric errors, timing errors, and satellite orbit errors, while enhancing system integrity. 
Autonomous positioning capabilities of the HS80/MX575C will result in positioning accuracies 
of 4.0 m 95% of the time. In order to improve positioning quality to better than 1.0 m 95%, 
the HS80/MX575C is able to use differential corrections received through the internal SBAS 
demodulator or beacon receiver, or through externally-supplied RTCM corrections.

Automatic SBAS tracking

The HS80 automatically scans and tracks SBAS signals without the need to tune the receiver. 
The HS80 features two-channel tracking that provides an enhanced ability to maintain a lock 
on an SBAS satellite when more than one satellite is in view. This redundant tracking approach 
results in more consistent tracking of an SBAS signal in areas where signal blockage of a 
satellite is possible.  The MX575C is configured to receive beacon DGPS corrections. However, 
it can also be configured to receive SBAS or external RTCM corrections from the SIMRAD-MX 
CDU.

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