iOS issue in background location updates. It stops after around 24 hours - ios

Question

In my app I want to fetch and upload location on server in regular intervals. for foreground it works fine but for background it has issue as described below. One thing to mention here I use different class for foreground and separate code for background.
To fetch location in background I do the following. It does upload location for about 24 - 40 hours but get stopped after 24-40 hours. I did not get "willTerminate" event. I have also written code to capture global execption so that I could log SigKill if user kill my app but it too does not get logged reason could be we can't catch SigKill.
Now I am stuck don't know what to do to keep my app alive in background to upload location in regular intervals.
//////////////////////////////// didfinish lauch ///////////////////////////////
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
self.locationManager = [[CLLocationManager alloc] init];
self.locationManager.delegate = self;
return YES;
}
////////// **did enter background**
- (void) applicationDidEnterBackground:(UIApplication *)application
{
self.bgTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
[[UIApplication sharedApplication] endBackgroundTask:self.bgTask];
self.bgTask = UIBackgroundTaskInvalid;
}];
timer = [[NSTimer scheduledTimerWithTimeInterval:8*60
target:self
selector:#selector(changeAccuracy)
userInfo:nil
repeats:YES] retain];
[self.locationManager startUpdatingLocation];
}
- (void) changeAccuracy
{
self.bCallbackStarted = NO;
[self.locationManager setDesiredAccuracy:kCLLocationAccuracyBest];
[self.locationManager setDistanceFilter:100];
}
///////// **did enter fore ground**
- (void)applicationWillEnterForeground:(UIApplication *)application
{
[self.locationManager stopUpdatingLocation];
}
////////// **did update location**
-(void)locationManager:(CLLocationManager *)lm didUpdateLocations:(NSArray *)locations
{
if (self.bCallbackStarted == YES) {
NSString* logStr = [NSString stringWithFormat:#"URLFilteringAppDelegate::didUpdateLocations not uploading location because self.bCallbackStarted == YES"];
LOGI(logStr)
return;
}
self.bCallbackStarted = YES;
[[AppUtility sharedInstance] setLocationStatus:YES];
CLLocation *location = [locations lastObject];
if(location.coordinate.latitude == 0 && location.coordinate.longitude == 0)
{
[UserSettings sharedInstance].Latitude = [NSString stringWithFormat:#"%#", #"Unavailable"];
[UserSettings sharedInstance].Longitude = [NSString stringWithFormat:#"%#", #"Unavailable"];
}
else
{
[UserSettings sharedInstance].Latitude = [NSString stringWithFormat:#"%.8f", location.coordinate.latitude];
[UserSettings sharedInstance].Longitude = [NSString stringWithFormat:#"%.8f", location.coordinate.longitude];
}
[[LocationManager sharedInstance] sendLocation];
/// this is set to save battery
[lm setDesiredAccuracy:kCLLocationAccuracyThreeKilometers];
[lm setDistanceFilter:99999];
}