diff --git a/app/qtapp/rtknavi_qt/mondlg.cpp b/app/qtapp/rtknavi_qt/mondlg.cpp
index 158d53547..277809a6c 100644
--- a/app/qtapp/rtknavi_qt/mondlg.cpp
+++ b/app/qtapp/rtknavi_qt/mondlg.cpp
@@ -1065,7 +1065,7 @@ void MonitorDialog::setObservations()
 //---------------------------------------------------------------------------
 void MonitorDialog::showObservations()
 {
-    char tstr[40], id[8], *code;
+    char tstr[40], id[8];
     int i, k, n = 0, nex = ui->cBSelectObservation->currentIndex() ? NEXOBS : 0;
     int sys = sys_tbl[ui->cBSelectNavigationSystems->currentIndex()];
     int std = ui->cBSelectObservation->currentIndex();
@@ -1107,7 +1107,7 @@ void MonitorDialog::showObservations()
         ui->tWConsole->item(i, j++)->setText(id);
         ui->tWConsole->item(i, j++)->setText(QString("(%1)").arg(obs[i].rcv));
         for (k = 0; k < NFREQ + nex; k++) {
-            code = code2obs(obs[i].code[k]);
+            const char *code = code2obs(obs[i].code[k]);
             if (*code) ui->tWConsole->item(i, j++)->setText(code);
             else ui->tWConsole->item(i, j++)->setText("-");
         }
diff --git a/app/qtapp/rtkplot_qt/plotinfo.cpp b/app/qtapp/rtkplot_qt/plotinfo.cpp
index 52071029b..8c3acd8bf 100644
--- a/app/qtapp/rtkplot_qt/plotinfo.cpp
+++ b/app/qtapp/rtkplot_qt/plotinfo.cpp
@@ -382,7 +382,6 @@ static int _strcmp(const void *str1, const void *str2)
 // update observation type pull-down menu --------------------------------------
 void Plot::updateObservationType()
 {
-    char *codes[MAXCODE + 1], *obs;
     int i, j, n = 0, codeMask[MAXCODE + 1] = { 0 };
 
     trace(3, "updateObservationType\n");
@@ -393,9 +392,11 @@ void Plot::updateObservationType()
             codeMask[observation.data[i].code[j]] = 1;
 
     // count codes
+    const char *codes[MAXCODE + 1];
     for (uint8_t c = 1; c <= MAXCODE; c++) {
         if (!codeMask[c]) continue;
-        if (!*(obs = code2obs(c))) continue;
+        const char *obs = code2obs(c);
+        if (!*obs) continue;
         codes[n++] = obs;
     }
     qsort(codes, n, sizeof(char *), _strcmp);
diff --git a/app/winapp/rtknavi/mondlg.cpp b/app/winapp/rtknavi/mondlg.cpp
index 51897b931..e7998d1dc 100644
--- a/app/winapp/rtknavi/mondlg.cpp
+++ b/app/winapp/rtknavi/mondlg.cpp
@@ -991,7 +991,7 @@ void __fastcall TMonitorDialog::SetObs(void)
 void __fastcall TMonitorDialog::ShowObs(void)
 {
 	AnsiString s;
-	char tstr[40],id[8],*code;
+	char tstr[40],id[8];
 	int i,j,k,n=0,nex=ObsMode?NEXOBS:0,sys=sys_tbl[SelSys->ItemIndex];
 	
         obsd_t *obs = static_cast<obsd_t *>(calloc(MAXOBS * 2, sizeof(obsd_t)));
@@ -1023,7 +1023,7 @@ void __fastcall TMonitorDialog::ShowObs(void)
 		Tbl->Cells[j++][i+1]=id;
 		Tbl->Cells[j++][i+1]=s.sprintf("(%d)",obs[i].rcv);
 		for (k=0;k<NFREQ+nex;k++) {
-			code=code2obs(obs[i].code[k]);
+                        const char *code=code2obs(obs[i].code[k]);
 			if (*code) Tbl->Cells[j++][i+1]=s.sprintf("%s",code);
 			else       Tbl->Cells[j++][i+1]="-";
 		}
diff --git a/app/winapp/rtkplot/plotinfo.cpp b/app/winapp/rtkplot/plotinfo.cpp
index 54255fde0..28be1e407 100644
--- a/app/winapp/rtkplot/plotinfo.cpp
+++ b/app/winapp/rtkplot/plotinfo.cpp
@@ -333,7 +333,6 @@ static int _strcmp(const void *str1, const void *str2)
 void __fastcall TPlot::UpdateObsType(void)
 {
     UTF8String s;
-    char *obs,*codes[MAXCODE+1];
     int i,j,n=0,cmask[MAXCODE+1]={0},fmask[10]={0};
     const char *obstypes[] =  {"L1/LC","L2/E5b","L5/E5a","L6","L7","L8"};
 
@@ -342,9 +341,11 @@ void __fastcall TPlot::UpdateObsType(void)
     for (i=0;i<Obs.n;i++) for (j=0;j<NFREQ+NEXOBS;j++) {
         cmask[Obs.data[i].code[j]]=1;
     }
+    const char *codes[MAXCODE+1];
     for (i=1;i<=MAXCODE;i++) {
         if (!cmask[i]) continue;
-        if (!*(obs=code2obs((uint8_t)i))) continue;
+        const char *obs = code2obs((uint8_t)i);
+        if (!*obs) continue;
         codes[n++]=obs;
     }
     qsort(codes,n,sizeof(char *),_strcmp);
diff --git a/src/convrnx.c b/src/convrnx.c
index 958821a7c..696666e69 100644
--- a/src/convrnx.c
+++ b/src/convrnx.c
@@ -395,7 +395,8 @@ static void setopt_obstype(const uint8_t *codes, const uint8_t *types, int sys,
                            rnxopt_t *opt)
 {
     const char type_str[]="CLDS";
-    char type[16],*id,ver;
+    const char *id;
+    char type[16],ver;
     int i,j,k,idx;
     
     trace(3,"setopt_obstype: sys=%d\n",sys);
diff --git a/src/rinex.c b/src/rinex.c
index 6dc53ad5b..4b3beea7f 100644
--- a/src/rinex.c
+++ b/src/rinex.c
@@ -2374,7 +2374,7 @@ static void outrnxobsf(FILE *fp, double obs, int lli, int std)
 static int obsindex(int rnxver, int sys, const uint8_t *code, const char *tobs,
                     const char *mask)
 {
-    char *id;
+    const char *id;
     int i;
 
     for (i=0;i<NFREQ+NEXOBS;i++) {
diff --git a/src/rtcm3e.c b/src/rtcm3e.c
index 7a57f7899..d5beb3a4b 100644
--- a/src/rtcm3e.c
+++ b/src/rtcm3e.c
@@ -1955,7 +1955,7 @@ static int to_satid(int sys, int sat)
 static int to_sigid(int sys, uint8_t code)
 {
     const char **msm_sig;
-    char *sig;
+    const char *sig;
     int i;
     
     /* signal conversion for undefined signal by rtcm */
diff --git a/src/rtkcmn.c b/src/rtkcmn.c
index 4443e9b0f..c7ca7bc8d 100644
--- a/src/rtkcmn.c
+++ b/src/rtkcmn.c
@@ -255,8 +255,8 @@ const char *formatstrs[32]={    /* stream format strings */
     NULL
 };
 
-static char *obscodes[MAXCODE + 1]={       /* observation code strings */
-
+// Observation code strings.
+static const char *obscodes[MAXCODE + 1]={
     ""  ,"1C","1P","1W","1Y", "1M","1N","1S","1L","1E", /*  0- 9 */
     "1A","1B","1X","1Z","2C", "2D","2S","2L","2X","2P", /* 10-19 */
     "2W","2Y","2M","2N","5I", "5Q","5X","7I","7Q","7X", /* 20-29 */
@@ -602,7 +602,7 @@ extern uint8_t obs2code(const char *obs)
 * return : obs code string ("1C","1P","1P",...)
 * notes  : obs codes are based on RINEX 3.04
 *-----------------------------------------------------------------------------*/
-extern char *code2obs(uint8_t code)
+extern const char *code2obs(uint8_t code)
 {
     if (code<=CODE_NONE||MAXCODE<code) return "";
     return obscodes[code];
@@ -610,7 +610,7 @@ extern char *code2obs(uint8_t code)
 /* GPS obs code to frequency -------------------------------------------------*/
 static int code2freq_GPS(uint8_t code, double *freq)
 {
-    char *obs=code2obs(code);
+    const char *obs=code2obs(code);
 
     switch (obs[0]) {
         case '1': *freq=FREQL1; return 0; /* L1 */
@@ -622,7 +622,7 @@ static int code2freq_GPS(uint8_t code, double *freq)
 /* GLONASS obs code to frequency ---------------------------------------------*/
 static int code2freq_GLO(uint8_t code, int fcn, double *freq)
 {
-    char *obs=code2obs(code);
+    const char *obs=code2obs(code);
 
     switch (obs[0]) {
         case '1':  /* G1 */
@@ -642,7 +642,7 @@ static int code2freq_GLO(uint8_t code, int fcn, double *freq)
 /* Galileo obs code to frequency ---------------------------------------------*/
 static int code2freq_GAL(uint8_t code, double *freq)
 {
-    char *obs=code2obs(code);
+    const char *obs=code2obs(code);
 
     switch (obs[0]) {
         case '1': *freq=FREQL1; return 0; /* E1 */
@@ -656,7 +656,7 @@ static int code2freq_GAL(uint8_t code, double *freq)
 /* QZSS obs code to frequency ------------------------------------------------*/
 static int code2freq_QZS(uint8_t code, double *freq)
 {
-    char *obs=code2obs(code);
+    const char *obs=code2obs(code);
 
     switch (obs[0]) {
         case '1': *freq=FREQL1; return 0; /* L1 */
@@ -669,7 +669,7 @@ static int code2freq_QZS(uint8_t code, double *freq)
 /* SBAS obs code to frequency ------------------------------------------------*/
 static int code2freq_SBS(uint8_t code, double *freq)
 {
-    char *obs=code2obs(code);
+    const char *obs=code2obs(code);
 
     switch (obs[0]) {
         case '1': *freq=FREQL1; return 0; /* L1 */
@@ -680,7 +680,7 @@ static int code2freq_SBS(uint8_t code, double *freq)
 /* BDS obs code to frequency -------------------------------------------------*/
 static int code2freq_BDS(uint8_t code, double *freq)
 {
-    char *obs=code2obs(code);
+    const char *obs=code2obs(code);
 
     switch (obs[0]) {
         case '2': *freq=FREQ1_CMP; return 0; /* B1I */
@@ -695,7 +695,7 @@ static int code2freq_BDS(uint8_t code, double *freq)
 /* NavIC obs code to frequency -----------------------------------------------*/
 static int code2freq_IRN(uint8_t code, double *freq)
 {
-    char *obs=code2obs(code);
+    const char *obs=code2obs(code);
 
     switch (obs[0]) {
         case '5': *freq=FREQL5; return 0; /* L5 */
@@ -813,8 +813,8 @@ extern void setcodepri(int sys, int idx, const char *pri)
 *-----------------------------------------------------------------------------*/
 extern int getcodepri(int sys, uint8_t code, const char *opt)
 {
-    const char *p,*optstr;
-    char *obs,str[8]="";
+    const char *p,*optstr, *obs;
+    char str[8]="";
     int i,j;
 
     switch (sys) {
diff --git a/src/rtklib.h b/src/rtklib.h
index d54a362ca..f4b8d5ad7 100644
--- a/src/rtklib.h
+++ b/src/rtklib.h
@@ -1392,7 +1392,7 @@ EXPORT int  satsys  (int sat, int *prn);
 EXPORT int  satid2no(const char *id);
 EXPORT void satno2id(int sat, char id[8]);
 EXPORT uint8_t obs2code(const char *obs);
-EXPORT char *code2obs(uint8_t code);
+EXPORT const char *code2obs(uint8_t code);
 EXPORT double code2freq(int sys, uint8_t code, int fcn);
 EXPORT double sat2freq(int sat, uint8_t code, const nav_t *nav);
 EXPORT int  code2idx(int sys, uint8_t code);