我正在開發一個C++應用程序,它應該使用gpsd檢索收到的類型爲$ GPGGA的NMEA語句。這個想法是每秒從gpsd讀取一次,並解析最後一個$ GPGGA收到的句子,提取我感興趣的兩個字段:質量指標和參考站點ID。我使用C++ libgpsmm庫,定期調用gpsmm::read()
和gpsmm::data()
,直接訪問客戶端數據緩衝區。首先,我使用gpsfake和僞造的GPS日誌(指定gpsfake選項「-c 0.5」,以便每秒鐘有兩個句子)做了幾次測試。當兩個gpsd請求之間的時間小於或等於400ms時,結果是OK的。如果我用更多的時間嘗試,結果是意想不到的,在每次閱讀中都有一段NMEA句子,其中包含大量重複數據以及一些截短的句子。當我嘗試使用每秒寫入40個句子的真實GPS時,情況會更糟糕:在這種情況下,讀取之間的時間應該爲〜10ms或更少,以便獲得正確的結果。gpsd客戶端數據緩衝區
以下是打印收到的NMEA句子的一個更簡單的程序。它運行良好,模擬甚至是真正的GPS。但是,如果我取消註釋usleep()調用,這會使程序每秒檢查一次緩衝區,但客戶端數據緩衝區不會給出合理的結果。
#include <iostream>
#include "libgpsmm.h"
using namespace std;
#define WAITING_TIME 5000000
#define RETRY_TIME 5
#define ONE_SECOND 1000000
int main(void)
{
for(;;){
//For version 3.7
gpsmm gps_rec("localhost", DEFAULT_GPSD_PORT);
if (gps_rec.stream(WATCH_ENABLE|WATCH_NMEA) == NULL) {
cout << "No GPSD running. Retry to connect in " << RETRY_TIME << " seconds." << endl;
usleep(RETRY_TIME * ONE_SECOND);
continue; // It will try to connect to gpsd again
}
const char* buffer = NULL;
for (;;) {
struct gps_data_t* newdata;
if (!gps_rec.waiting(WAITING_TIME))
continue;
if ((newdata = gps_rec.read()) == NULL) {
cerr << "Read error.\n";
break;
} else {
buffer = gps_rec.data();
// We print the NMEA sentences!
cout << "***********" << endl;
cout << buffer << endl;
//usleep(1000000);
}
}
}
}
這裏是具有usleep()函式輸出呼叫評論(即連續地讀取數據。):
$ ./GPSTest1
***********
{"class":"VERSION","release":"3.7","rev":"3.7","proto_major":3,"proto_minor":7}
***********
{"class":"WATCH","enable":true,"json":false,"nmea":true,"raw":0,"scaled":false,"timing":false}
***********
$GPGGA,202010.00,3313.9555651,S,06019.3785868,W,4,09,1.0,39.384,M,16.110,M,10.0,*46<CR><LF>
***********
$GPGGA,202011.00,3313.9555664,S,06019.3785876,W,4,09,1.0,39.386,M,16.110,M,11.0,*4D<CR><LF>
***********
$GPGGA,202012.00,3313.9555668,S,06019.3785882,W,4,09,1.0,39.394,M,16.110,M,12.0,*49<CR><LF>
***********
$GPGGA,202013.00,3313.9555673,S,06019.3785911,W,4,09,1.0,39.395,M,16.110,M,13.0,*49<CR><LF>
***********
$GPGGA,202014.00,3313.9555670,S,06019.3785907,W,4,09,1.0,39.409,M,16.110,M,14.0,*4F<CR><LF>
***********
$GPGGA,202015.00,3313.9555657,S,06019.3785905,W,4,09,1.0,39.395,M,16.110,M,15.0,*4A<CR><LF>
這是當線被註釋的輸出(即,緩衝檢查每秒一次):
$ ./GPSTest2
***********
{"class":"VERSION","release":"3.7","rev":"3.7","proto_major":3,"proto_minor":7}
***********
{"class":"DEVICE","path":"/dev/pts/0","activated":"2012-11-05T23:48:38.110Z","driver":"Generic NMEA","native":0,"bps":4800,"parity":"N","stopbits":1,"cycle":1.00}
$GPGGA,202013.00,3313.9555673,S,06019.3785911,W,1,09,1.0,39.395,M,16.110,M,13.0,*49<CR><LF>
0}
$GPGGA,202013.00,3313.9555673,S,06019.3785911,W,1,09,1.0,39.395,M,16.110,M,13.0,*49<CR><LF>
":"Generic NMEA","native":0,"bps":4800,"parity":"N","stopbits":1,"cycle":1.00}
$GPGGA,202013.00,3313.9555673,S,06019.3785911,W,1,09,1.0,39.395,M,16.110,M,13.0,*49<CR><LF>
***********
$GPGGA,202013.00,3313.9555673,S,06019.3785911,W,1,09,1.0,39.395,M,16.110,M,13.0,*49<CR><LF>
***********
$GPGGA,202016.00,3313.9555642,S,06019.3785894,W,1,09,1.0,39.402,M,16.110,M,16.0,*4E<CR><LF>
$GPGGA,202017.00,3313.9555643,S,06019.3785925,W,1,09,1.0,39.404,M,16.110,M,17.0,*42<CR><LF>
$GPGGA,202017.00,3313.9555643,S,06019.3785925,W,1,09,1.0,39.404,M,16.110,M,17.0,*42<CR><LF>
$GPGGA,202017.00,3313.9555643,S,06019.3785925,W,1,09,1.0,39.404,M,16.110,M,17.0,*42<CR><LF>
***********
任何建議?起初,我試圖直接分析gps_data_t
結構,但在結構的所有領域中,與NMEA句子中的搜索相比,似乎很難確定質量指標和參考站ID。
感謝您的回答,timrorr! 一些評論: 1)我想過增加緩衝區大小之前......但我無法在libgps所做的所有定義中識別它,我認爲必須有一種通常的方法來獲得我想要的。 2)喚醒後清除緩衝區並不能解決我的問題,因爲我的真實應用程序無法阻止等待消息,因爲它必須同時處理其他消息。 3)我正在使用20Hz的GPS接收機:我每秒接收10個GGA,10個TVG和1個ZDA。 – pabloderosario
當然,解決我的問題的最好方法是將GPS數據採集和消耗這些信息分爲兩個不同的線程。這樣,緩衝區不會被破壞,而且也不需要更多的讀取。 – pabloderosario