ANDROID GPS MIDDLEWARE

GPS Android Runtime

LocationManagerService

* The system service that manages LocationProvider and issues location updates and alerts.
*  Allows applications to obtain periodic updates of the device's geographical location

  //location providers implementing the Geocoder services.

  - GeocoderProxy mGeocodeProvider;

 

  // This provider determines location based on availability of cell tower and WiFi access points

  - LocationProviderProxy mNetworkLocationProvider;

  // This provider determines location using satellites

  - GpsLocationProvider mGpsLocationProvider;

GpsLocationProvider

•A GPS implementation of LocationProvider used by LocationManagerService.

•This provider determines location using satellites (GPS/AGPS Devices).

•Interact with GPS HAL lib through JNI Layer .

•Get’s location information latitude, longitude, altitude etc.. forms Location object and pass it to LocationManagerService.

 

JNI Method table

•List of JNI method used by GPSLocationProvider to initialize gps hal lib and retrieve location information from gps hardware.

static JNINativeMethod sMethods[] = {
     /* name, signature, funcPtr */
    {"class_init_native", "()V", (void *)android_location_GpsLocationProvider_class_init_native},
    {"native_is_supported", "()Z", (void*)android_location_GpsLocationProvider_is_supported},
    {"native_init", "()Z", (void*)android_location_GpsLocationProvider_init},
    {"native_cleanup", "()V", (void*)android_location_GpsLocationProvider_cleanup},
    {"native_set_position_mode", "(IIIII)Z", (void*)android_location_GpsLocationProvider_set_position_mode},
    {"native_start", "()Z", (void*)android_location_GpsLocationProvider_start},
    {"native_stop", "()Z", (void*)android_location_GpsLocationProvider_stop},
    {"native_delete_aiding_data", "(I)V", (void*)android_location_GpsLocationProvider_delete_aiding_data},
    {"native_read_sv_status", "([I[F[F[F[I)I", (void*)android_location_GpsLocationProvider_read_sv_status},
    {"native_read_nmea", "([BI)I", (void*)android_location_GpsLocationProvider_read_nmea},
    {"native_inject_time", "(JJI)V", (void*)android_location_GpsLocationProvider_inject_time},
    {"native_inject_location", "(DDF)V", (void*)android_location_GpsLocationProvider_inject_location},
    {"native_supports_xtra", "()Z", (void*)android_location_GpsLocationProvider_supports_xtra},
    {"native_inject_xtra_data", "([BI)V", (void*)android_location_GpsLocationProvider_inject_xtra_data},
    {"native_agps_data_conn_open", "(Ljava/lang/String;)V", (void*)android_location_GpsLocationProvider_agps_data_conn_open},
    {"native_agps_data_conn_closed", "()V", (void*)android_location_GpsLocationProvider_agps_data_conn_closed},
    {"native_agps_data_conn_failed", "()V", (void*)android_location_GpsLocationProvider_agps_data_conn_failed},
    {"native_agps_set_id","(ILjava/lang/String;)V",(void*)android_location_GpsLocationProvider_agps_set_id},
    {"native_agps_set_ref_location_cellid","(IIIII)V",(void*)android_location_GpsLocationProvider_agps_set_reference_location_cellid},
    {"native_set_agps_server", "(ILjava/lang/String;I)V", (void*)android_location_GpsLocationProvider_set_agps_server},
    {"native_send_ni_response", "(II)V", (void*)android_location_GpsLocationProvider_send_ni_response},
    {"native_agps_ni_message", "([BI)V", (void *)android_location_GpsLocationProvider_agps_send_ni_message},
    {"native_get_internal_state", "()Ljava/lang/String;", (void*)android_location_GpsLocationProvider_get_internal_state},
    {"native_update_network_state", "(ZIZLjava/lang/String;)V", (void*)android_location_GpsLocationProvider_update_network_state },
};

At   frameworks\base\services\jni\com_android_server_location_GpsLocationProvider.cpp

  

Building a GPS Library

•Android defines a user space C abstraction interface for GPS hardware. The interface header is defined in include/hardware/gps.h.

•To implement a GPS driver, create a shared library that implements the interface defined in gps.h. You must name your shared library libgps.so.

•android does not rely on anything in the Linux kernel for GPS. just need to implement the interface in gps.h.

HAL Stub  

GPS-HAL Lib Initialization

•During boot up init process launches the System Server. System Server is the first java component to run in the system. It starts all android system services including LocationManagerService.

•LocationManagerService initializes GpsLocationProvider and gps device hardware.

Device data structure

  - gps.h includes device data structure gps_device_t .

  - gps lib hardware should define the device structure gps_device_t  and the public method   get_gps_interface.

struct gps_device_t {
  /**
  * Every device data structure must begin with hw_device_t
  */
  struct hw_device_t common;
  /**
  * return GpsInterface structure - to interact with gps lib hardware.
  */
  const GpsInterface* (*get_gps_interface)(struct gps_device_t* dev);
};

at   (hardware\libhardware\include\hardware\ Gps.h)

libgps

GpsInterface data structure

- gps.h includes device data structure GpsInterface.

- Gps device hardware should return initialized GpsInterface structure on function call get_gps_interface.

- GpsInterface structure includes number interface methods for location related input.

/** Represents the standard GPS interface. */
typedef struct {
size_t          size;
 
//opens the interface and provides the callback routines to the implmentation  of this interface
int   (*init)( GpsCallbacks* callbacks ); 
int   (*start)( void );  /** Starts the location tracking session. */
int   (*stop)( void );  /** Stop the location tracking session. */
void  (*cleanup)( void );
 
/** This is used by Java native function to do time injection. */
int   (*inject_time)(GpsUtcTime time, int64_t timeReference,
                         int uncertainty);
  
//This is used by Java native function to do location aiding data injection.
int  (*inject_location)(double latitude, double longitude, float accuracy);
void  (*delete_aiding_data)(GpsAidingData flags);
int   (*set_position_mode)(GpsPositionMode mode, GpsPositionRecurrence recurrence,
            uint32_t min_interval, uint32_t preferred_accuracy, uint32_t preferred_time);
const void* (*get_extension)(const char* name); /** Get a pointer to extension information. */
} GpsInterface;

at   (hardware\libhardware\include\hardware\ Gps.h)

GpsCallbacks data structure

-gps.h includes another device data structure GpsCallbacks.

-  GpsCallbacks structure includes number interface methods for gps device hardware to send the location data to GpsLocationProvider.

-  GpsLocationProvider  initialize GpsCallbacks structure and pass it to the gps device hardware through int   (*init)( GpsCallbacks* callbacks ) function call.

-After init function call connection between GpsLocationProvider and gps device hardware been completed established

/** GPS callback structure. */
typedef struct {
    /** set to sizeof(GpsCallbacks) */
    size_t      size;
    gps_location_callback location_cb;
    gps_status_callback status_cb;
    gps_sv_status_callback sv_status_cb;
    gps_nmea_callback nmea_cb;
} GpsCallbacks;
typedef void (* gps_location_callback)(GpsLocation* location); /** Callback with location information*/
typedef void (* gps_status_callback)(GpsStatus* status); /** Callback with status information */
typedef void (* gps_sv_status_callback)(GpsSvStatus* sv_info); /** Callback with SV status information */
typedef void (* gps_nmea_callback)(GpsUtcTime timestamp, const char* nmea, int length); /** Callback for reporting NMEA */

at   (hardware\libhardware\include\hardware\ Gps.h)

Reading data from GPS harware

• add property for GPS device like below

•  ro.kernel.android.gps=ttyS1 ( in system.prop)

• After recognizing GPS device, you can check your device in ‘/dev/ttyS1’.

• Gps library reads the gps location data from the device file path and form structure GpsLocation provided in gps.h and pass it to the JNI layer.

Structure – represents location

typedef struct {
   size_t          size;  /** set to sizeof(GpsLocation) */
   uint16_t        flags;  /** Contains GpsLocationFlags bits. */
   double          latitude;  /** Represents latitude in degrees. */  
   double          longitude;  /** Represents longitude in degrees. */       
    double          altitude;  /** Represents altitude in meters above the WGS 84 reference  ellipsoid. */
    float           speed;  /** Represents speed in meters per second. */
    float           bearing;  /** Represents heading in degrees. */
    float           accuracy;  /** Represents expected accuracy in meters. */  
    GpsUtcTime      timestamp;  /** Timestamp for the location fix. */
} GpsLocation;

GPS Runtime

Gps extended services

-  gps.h included some more interface to support AGPS, GPS XTRA etc..

-AGpsInterface is an extended interface for AGPS support.

-GpsXtraInterface is an extended interface for XTRA support.

-GpsLocationProvide gets the extended service interface through function call const void* (*get_extension)(const char* name) in GpsInterface.

typedef struct {
  size_t          size;
//Opens the AGPS interface and provides the callback routines
//to the implemenation of this interface.
void  (*init)( AGpsCallbacks* callbacks );
//Notifies that a data connection is available and sets
//the name of the APN to be used for SUPL.
int  (*data_conn_open)( const char* apn );
//Notifies that the AGPS data connection has been closed.
int  (*data_conn_closed)();
//Notifies that a data connection is not available for AGPS.
int  (*data_conn_failed)();
//Sets the hostname and port for the AGPS server.
int  (*set_server)( AGpsType type, const char* hostname, int port );
} AGpsInterface;

-gps.h includes different names for gps extended interface

   #define GPS_XTRA_INTERFACE      "gps-xtra“

  #define AGPS_INTERFACE      "agps“

-Gps device hardware implementation should return initialized extended interface on function call

   const void* (*get_extension)(const char* name).

typedef struct {
    /** set to sizeof(GpsXtraInterface) */
    size_t          size;
    /**
     * Opens the XTRA interface and provides the callback routines
     * to the implemenation of this interface.
     */
    int  (*init)( GpsXtraCallbacks* callbacks );
    /** Injects XTRA data into the GPS. */
    int  (*inject_xtra_data)( char* data, int length );
} GpsXtraInterface;

at   (hardware\libhardware\include\hardware\ Gps.h)