英语翻译

英语翻译
The hardware interface for GPS units is designed to meet the NMEA requirements.They are also compatible with most computer serial ports using RS232 protocols,however strictly speaking the NMEA standard is not RS232.They recommend conformance to EIA-422.The interface speed can be adjusted on some models but the NMEA standard is 4800 b/s (bit per second rate) with 8 bits of data,no parity,and one stop bit.All units that support NMEA should support this speed.Note that,at a b/s rate of 4800,you can easily send enough data to more than fill a full second of time.For this reason some units only send updates every two seconds or may send some data every second while reserving other data to be sent less often.In addition some units may send data a couple of seconds old while other units may send data that is collected within the second it is sent.Generally time is sent in some field within each second so it is pretty easy to figure out what a particular gps is doing.Some sentences may be sent only during a particular action of the receiver such as while following a route while other receivers may always send the sentence and just null out the values.Other difference will be noted in the specific data descriptions defined later in the text.NMEA consists of sentences,the first word of which,called a data type,defines the interpretation of the rest of the sentence.Each Data type would have its own unique interpretation and is defined in the NMEA standard.The GGA sentence (shown below) shows an example that provides essential fix data.Other sentences may repeat some of the same information but will also supply new data.Whatever device or program that reads the data can watch for the data sentence that it is interested in and simply ignore other sentences that is doesn't care about.In the NMEA standard there are no commands to indicate that the gps should do something different.Instead each receiver just sends all of the data and expects much of it to be ignored.Some receivers have commands inside the unit that can select a subset of all the sentences or,in some cases,even the individual sentences to send.There is no way to indicate anything back to the unit as to whether the sentence is being read correctly or to request a re-send of some data you didn't get.Instead the receiving unit just checks the checksum and ignores the data if the checksum is bad figuring the data will be sent again sometime later.