Katech AIS Test and Analysis System

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Connecting a GPS receiver to KATAS AIS receivers

Types of GPS Receivers

In broad terms, GPS receivers fall into two distinct categories. There are those that can be regarded as general-purpose instruments and are often portable units with internal batteries. These units normally have an RS232 serial interface which can output NMEA sentences at a fixed or variable baud rate, sometimes only via an optional data cable. These units often have non-isolated 12V cigar-lighter type power leads or else bare wires for connection to a suitable 12V power source.

On the other hand, specially designed maritime units designed for permanent fitment will, almost without fail, have a current-loop NMEA interface at a fixed 4800 baud rate and a galvanically isolated power supply. An RS232 facility is usually present also. More recent models within this category may be 'wheel-marked' which means that they conform to a set of minimum performance standards for available NMEA output sentences. GPS receivers within this category will usually have a high degree of programmability in regard to the selection and sequencing of output NMEA sentences.

The AIS Receiver 1 unit has no means of inputting serial data and so a GPS receiver must be connected to the computer's serial port. If the computer has no serial port, then it is recommended that this serial to USB adapter should be used.

The AIS Receiver/Decoder 1 (not USB-A), AIS Ship Monitor 1 & 2 and AIS Sea Scope 1 & 2 units have a basic GPS NMEA interface whereby only the GPRMC sentence is interpreted. If the chosen GPS receiver can be programmed to select certain sentences for transmission, then only the GPRMC sentence should be selected for greatest decoding efficiency. If the GPS receiver cannot output the GPRMC sentence, then it is incompatible with the basic NMEA interpreter on these models of KATAS AIS receiver. If a high-level charting program is in use, it may be possible to feed the AIS NMEA data into serial port 1 and the GPS NMEA data into serial port 2. This method is likely to result in the highest efficiency. If the computer is lacking in serial ports then the EasySync US232B and US232B/LC serial to USB converters are recommended which have been tested by Katech development staff and found to perform reliably. If it is not possible or applicable to use the computer to receive both NMEA data feeds, then the GPS NMEA data will need to be fed into the AIS receiver by means of an optionally available adapter lead. Within the AIS receiver, there are two ways in which the GPS data can be integrated into the outgoing AIS sentences and the method chosen will depend upon the requirements of the particular high-level charting software that is to be used. The GPS NMEA data can either be decoded and integrated within the outgoing AIS NMEA data as AIVDO sentences or else the GPRMC sentence can simply be re-sent unaltered amongst the outgoing AIS AIVDM sentences. The AIS receiver is unable to operate both modes together as this may lead to confusion within the charting program but one or the other method can be selected with the AIS receiver's software commands. If the GPS receiver can output NMEA data faster than the receiver's default rate of 4800 baud, then a small further increase in decoding efficiency may be achieved. The KATAS AIS receivers described here can accept GPS NMEA data at 4800, 9600, 19200 or 38400 baud and metal box units have a system of bleeper codes signifying errors within the incoming NMEA data stream. The GPRMC string is regarded as the minimum necessary to convey position and speed/course over ground. When the AIS receiver is set to encode AIVDO sentences, the heading information mirrors the course-over-ground data and the rate-of-turn parameter is set to zero.

The AIS Sea Scope Plus and AIS Sea Scope 1900 models have a separate optically-isolated NMEA interface which is not associated with the RS232 port. These models have a more advanced NMEA interpreter within their software that can take data from several different sentences and then to integrate it more efficiently into the outgoing AIS NMEA VDO sentences. These models cannot repeat the GPS sentence unaltered as the other models can. These AIS receivers require to be connected to a fixed maritime GPS receiver such as those manufactured by Furuno, Magnavox, Leica, MLR and others. These units often have local control as to the selection and sequencing of outgoing NMEA sentences.

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