[Resolvido] Fatal error: Using $this when not in object cont

Aew galera podem me dar uma ajuda esta retornando o seguinte erro:

Fatal error: Using $this when not in object context in C:\xampp\htdocs\guia3\usuarios\estabelecimento\modules\generalModule.php on line 14

O codigo:

<?php

 Class Data{

 private $doc;
 private $tDoc;

 function __construct(){
 $this->doc = $_SESSION['doc'];
 $this->tDoc = $_SESSION['tDoc'];
 }

 static function getDados($table){
 $SQL = "SELECT * FROM `".$table."` WHERE `".$this->tDoc."`='".$this->doc."'";
 $q = mysql_query($SQL);
 $t = mysql_fetch_assoc($q);
 return $t;
 }

 function getMatrix($table){
 $SQL = "SELECT * FROM `".$table."` WHERE `".$this->tDoc."`='".$this->doc."'";
 $q = mysql_query($SQL);
 return $q;
 }

 function updateField($table,$fieldName,$newValue){
 $SQL = "UPDATE `".$table."` SET `".$fieldName."`='".$newValue."' WHERE `".$this->tDoc."`='".$this->doc."'";
 $q = mysql_query($SQL);
 if($q){
 return true;
 } else {
 return false;
 }
 }

 function causeError($pageReturn){
 $line = '<meta http-equiv="refresh" content="'.$pageReturn.'">';
 echo '<script language="JavaScript">alert("Ocorreu um erro durante a alteração!");</script>';
 echo $line;
 }

 function causeSuccess($pageReturn){
 $line = '<meta http-equiv="refresh" content="'.$pageReturn.'">';
 echo '<script language="JavaScript">alert("Alterado com sucesso!");</script>';
 echo $line;
 }

 }

?>