diff --git a/src/streamsvr.c b/src/streamsvr.c
index 850abe7b2..524d99ad6 100644
--- a/src/streamsvr.c
+++ b/src/streamsvr.c
@@ -33,6 +33,155 @@
 *                           use integer types in stdint.h
 *-----------------------------------------------------------------------------*/
 #include "rtklib.h"
+#include <ctype.h>
+
+/* GGA filter constants and types */
+#define MAX_NMEA_LEN 1024
+
+/* NMEA sentence parser state machine */
+typedef enum {
+    GGA_STATE_SEARCH_START,     /* Looking for '$' */
+    GGA_STATE_COLLECT_SENTENCE, /* Collecting sentence until \r\n */
+    GGA_STATE_VALIDATE         /* Validating complete sentence */
+} gga_parser_state_t;
+
+/* GGA filter state for input stream */
+typedef struct {
+    gga_parser_state_t state;
+    char sentence[MAX_NMEA_LEN];
+    int sentence_len;
+} gga_filter_t;
+
+/* Calculate NMEA checksum */
+static unsigned char calc_nmea_checksum(const char *sentence, int len)
+{
+    unsigned char checksum=0;
+    int i;
+    
+    /* Start after '$', stop before '*' */
+    for (i=1; i<len && sentence[i]!='*'; i++) {
+        checksum^=(unsigned char)sentence[i];
+    }
+    return checksum;
+}
+
+/* Validate NMEA sentence format and checksum */
+static int validate_nmea_sentence(const char *sentence, int len)
+{
+    int i,star_pos=-1;
+    unsigned char calc_cs;
+    int recv_cs_val;
+    
+    /* Must start with '$' */
+    if (len<7 || sentence[0]!='$') return 0;
+    
+    /* Find '*' for checksum */
+    for (i=1; i<len-2; i++) {
+        if (sentence[i]=='*') {
+            star_pos=i;
+            break;
+        }
+    }
+    
+    /* Must have checksum */
+    if (star_pos==-1 || star_pos+3>len) return 0;
+    
+    /* Validate checksum format (two hex digits) */
+    if (!isxdigit((unsigned char)sentence[star_pos+1])|| 
+        !isxdigit((unsigned char)sentence[star_pos+2])) {
+        return 0;
+    }
+    
+    /* Calculate and compare checksum */
+    calc_cs=calc_nmea_checksum(sentence,star_pos);
+    if (sscanf(&sentence[star_pos+1],"%2x",&recv_cs_val)!=1) {
+        return 0;
+    }
+    
+    return (calc_cs==(unsigned char)recv_cs_val);
+}
+
+/* Check if sentence contains GGA */
+static int is_gga_sentence(const char *sentence, int len)
+{
+    /* Look for "GGA" after talker ID (positions 3-5 typically) */
+    if (len>=6) {
+        return (strncmp(&sentence[3],"GGA",3)==0|| 
+                strncmp(&sentence[2],"GGA",3)==0);  /* Handle 2-char talker IDs */
+    }
+    return 0;
+}
+
+/* Initialize GGA filter */
+static void init_gga_filter(gga_filter_t *filter)
+{
+    filter->state=GGA_STATE_SEARCH_START;
+    filter->sentence_len=0;
+}
+
+/* Process data through GGA filter, return filtered data length */
+static int process_gga_filter(gga_filter_t *filter, const uint8_t *input, int input_len, 
+                             uint8_t *output, int output_max)
+{
+    int output_len=0;
+    int i;
+    
+    for (i=0;i<input_len;i++) {
+        unsigned char byte=input[i];
+        
+        switch (filter->state) {
+            case GGA_STATE_SEARCH_START:
+                if (byte=='$') {
+                    filter->sentence[0]='$';
+                    filter->sentence_len=1;
+                    filter->state=GGA_STATE_COLLECT_SENTENCE;
+                }
+                break;
+                
+            case GGA_STATE_COLLECT_SENTENCE:
+                if (filter->sentence_len<MAX_NMEA_LEN-1) {
+                    filter->sentence[filter->sentence_len++]=byte;
+                }
+                
+                /* Check for end of sentence */
+                if (byte=='\n' && filter->sentence_len>=2&& 
+                    filter->sentence[filter->sentence_len-2]=='\r') {
+                    filter->sentence[filter->sentence_len]='\0';
+                    filter->state=GGA_STATE_VALIDATE;
+                }
+                /* Handle sentences that are too long */
+                else if (filter->sentence_len>=MAX_NMEA_LEN-1) {
+                    filter->state=GGA_STATE_SEARCH_START;
+                    filter->sentence_len=0;
+                }
+                break;
+                
+            case GGA_STATE_VALIDATE:
+                /* We have a complete sentence, validate and check for GGA */
+                if (validate_nmea_sentence(filter->sentence,filter->sentence_len)&& 
+                    is_gga_sentence(filter->sentence,filter->sentence_len)) {
+                    /* Copy GGA sentence to output if there's space */
+                    if (output_len+filter->sentence_len<=output_max) {
+                        memcpy(output+output_len,filter->sentence,filter->sentence_len);
+                        output_len+=filter->sentence_len;
+                    }
+                }
+                
+                /* Reset for next sentence - but check if current byte starts a new one */
+                filter->state=GGA_STATE_SEARCH_START;
+                filter->sentence_len=0;
+                
+                /* Don't lose the current byte - process it again */
+                i--; /* Will be incremented by for loop */
+                break;
+        }
+    }
+    
+    return output_len;
+}
+
+/* Global GGA filter state for input stream */
+static gga_filter_t gga_filter;
 
 /* test observation data message ---------------------------------------------*/
 static int is_obsmsg(int msg)
@@ -526,7 +675,18 @@ static void *strsvrthread(void *arg)
                 
                 /* relay back message from output stream to input stream */
                 if (i==svr->relayback) {
-                    strwrite(svr->stream,buff,n);
+                    /* Apply GGA filtering if relaying back to NTRIP client */
+                    if (svr->stream[0].type==STR_NTRIPCLI) {
+                        uint8_t filtered_buff[4096];
+                        int filtered_len=process_gga_filter(&gga_filter,buff,n, 
+                                                          filtered_buff,sizeof(filtered_buff));
+                        if (filtered_len>0) {
+                            strwrite(svr->stream,filtered_buff,filtered_len);
+                        }
+                    }
+                    else {
+                        strwrite(svr->stream,buff,n);
+                    }
                 }
                 /* write data to log stream */
                 strwrite(svr->strlog+i,buff,n);
@@ -580,6 +740,8 @@ extern void strsvrinit(strsvr_t *svr, int nout)
     for (i=0;i<nout+1&&i<16;i++) strinit(svr->strlog+i);
     svr->nstr=i;
     for (i=0;i<16;i++) svr->conv[i]=NULL;
+    /* Initialize GGA filter for input stream */
+    init_gga_filter(&gga_filter);
     svr->thread=0;
     initlock(&svr->lock);
 }