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PyMaemo/Using Location API

Revision as of 15:46, 3 December 2009

Using Location API

Location framework provides a library called liblocation which is used for developing location aware applications in Fremantle. Liblocation supports internal GPS, network based methods and external bluetooth GPS.

Using liblocation from Python

To start implementing applications using liblocation API on Python, you need to install python-location:

[sbox-FREMANTLE_X86: ~] > apt-get install python-location

Then, on your Python scripts, import the location module:

import location

Liblocation has two public GObjects. GPSDControl is used for starting and stopping of location services, setting location method and interval, and listening for errors. GPSDevice has information about device status and contains the actual fix when one exists. The two GObjects are initiated as follows.

control = location.GPSDControl.get_default()
device = location.GPSDevice()

Location methods

Liblocation supports the following location methods which are defined in the location module:

• location.METHOD_USER_SELECTED: Liblocation will choose the best possible location method based on location settings in control panel. You can think of it as asking all the methods CWP+ACWP+GNSS+AGNSS. Choose this method if you don't have any special needs.
• location.METHOD_CWP - Complementary Wireless Positioning: This method provides either coordinates for center of current country with horizontal accuracy equalling radius of the country (MCC fix), or coordinates based on currently used GSM base station. Latter is used if such information exists in device's cache, which is updated when ACWP method is used. SIM card is needed for CWP method.
• location.METHOD_ACWP - Assisted Complementary Wireless Positioning: A method where device is located based on cellular base station to which device is registered to. SIM card and a network connection is needed for ACWP method. If no network connection is available, this equals to CWP. Application might receive MCC fixes before base station information from external location server is fetched and as a fallback if e.g. network is temporary unavailable.
• location.METHOD_GNSS - Global Navigation Satellite System: A method for using GPS receiver. Typically time for the first fix is significantly longer than with AGNSS. Neither SIM card nor network connection is needed for GNSS method, and GNSS can even be used in offline mode.
• location.METHOD_AGNSS - Assisted Global Navigation Satellite System A method for using GPS receiver with assistance data from external location server. A SIM card and a network connection is needed for AGNSS method. If no network connection or SIM card is available, this equals to GNSS.

Location resources are shared between applications, and applications can request different location methods. Fixes for all requested methods are sent for all applications listening to GPSDevice's "changed" signal, therefore application should judge whether fix it is receiving, is one that it needs. See GPSDeviceFix section for discussion.

If device is set for bluetooth GPS from control panel, it can used for locationing via USER_SELECTED, AGNSS and GNSS methods. In this case AGNSS and GNSS do not differ, because assistance server cannot be utilized.

Device caches cell information for ACWP and satellite information for AGNSS. Hence if a non-assisted location method is used immediately after it's assisted counterpart, it will probably work as the assisted one.

Location method is set as GPSDControl's "preferred-method" property. Several methods can be given by bitwise or'ing the method identifiers:

control.set_properties(preferred_method=location.METHOD_GNSS|location.METHOD_AGNSS)

ACWP and AGNSS methods require network positioning and GNSS and AGNSS require GPS to be enabled in location settings. If requirements for asked methods are not enabled when starting location, user is prompted a dialog to enable them. After that all the possible preferred-methods are started. If there is any, an error to application is signalled.

USER_SELECTED method makes expection to dialog rule. Albeit it uses all of the methods: CWP+ACWP+GNSS+AGNSS, dialog is shown only if both networking and gps are disabled.

If user changes location settings during location session, then used location method is modified to best available accordingly, and error signalled if none is available.

Here is table that summarizes differences between the methods. Accuracy refers to horizontal accuracy of the fix.

Location intervals

Liblocation supports a default interval (equals to one second) and intervals of 1, 2, 5, 10, 20, 30, 60 and 120 seconds between fixes. Due to performance and power consumption reasons, all the intervals provide fixes only if position of the device has changed. Therefore the interval reflects more how often device checks for coordinate changes, than the actual interval between fixes. Nature of GPS fixes being constantly changing guarantees that GPS fixes are provided with requested interval. However, if there are several applications using different intervals, then the used interval is the minimum of intervals requested by all applications.

Location interval is set as GPSDControl's "preferred-interval" property:

control.set_properties(preferred_interval=location.INTERVAL_60S)

GPSDevice and GPSDeviceFix

GPSDevice object has the following attributes (FIXME: currently only "fix" attribute is supported in Python):

• online: Whether there is a connection to the hardware
• status: Status of the device
• fix: Tuple containing actual fix data (latitude, longitude, etc)
• satellites_in_view: Number of satellites in view
• satellites_in_use: Number of satellites in use
• satellites: Array of satellites
• cell_info: Information about cell the device is connected to

The most useful attribute is naturally the "fix" tuple which contains position and movement of the device and accuracies for them. The available information is listed below (in the order they appear on the tuple). In parenthesis there is a identifier which can be bitwisely anded with the "fields" value, to see whether corresponding tuple value is set.

• mode: The mode of the fix
• fields: A bitfield representing which items of this tuple contain valid data
• time: The timestamp of the update (location.GPS_DEVICE_TIME_SET)
• ept: Time accuracy
• latitude: Fix latitude (location.GPS_DEVICE_LATLONG_SET)
• longitude: Fix longitude (location.GPS_DEVICE_LATLONG_SET)
• eph: Horizontal position accuracy
• altitude: Fix altitude in meters (location.GPS_DEVICE_ALTITUDE_SET)
• double epv: Vertical position accuracy
• track: Direction of motion in degrees (location.GPS_DEVICE_TRACK_SET)
• epd: Track accuracy
• speed: Current speed in km/h (location.GPS_DEVICE_SPEED_SET)
• eps: Speed accuracy
• climb: Current rate of climb in m/s (location.GPS_DEVICE_CLIMB_SET)
• epc: Climb accuracy

An application receiving a fix cannot know if the fix is a result from location method it requested. Therefore application should study whether fix is accurate enough to satisfy application's needs. This can be done by inspecting "eph" field, which is fix's horizontal accuracy in centimeters. Typical values for horizontal accuracies can be seen in the location methods table. If accuracy is not known, it has a value of NaN.

Liblocation signals and callbacks

The most useful signal in liblocation is GPSDevice's "changed" signal, which is emitted everytime a new fix is received. GPSDControl has an "error-verbose" signal which is emitted in case of an error. There is also a legacy signal "error", which doesn't give a reason for error.

You can connect to these signals in the usual way:

control.connect("error-verbose", on_error, user_data)
device.connect("changed", on_changed, user_data)

Below are examples for these signals' callbacks.

def on_error(control, error, user_data):
    if error == location.ERROR_USER_REJECTED_DIALOG:
        print "User didn't enable requested methods"
    elif error == location.ERROR_USER_REJECTED_SETTINGS:
        print "User changed settings, which disabled location"
    elif error == location.ERROR_BT_GPS_NOT_AVAILABLE:
        print "Problems with BT GPS"
    elif error == location.ERROR_METHOD_NOT_ALLOWED_IN_OFFLINE_MODE:
        print "Requested method is not allowed in offline mode"
    elif error == location.ERROR_SYSTEM:
        print "System error"

def on_changed(device, user_data):
    if not device:
        return
    if device.fix:
        if device.fix[1] & location.GPS_DEVICE_LATLONG_SET:
            print "lat = %f, long = %f" % device.fix[4:6]
        if device.fix[1] & location.GPS_DEVICE_ALTITUDE_SET:
            print "alt = %f" % device.fix[7]
        print "horizontal accuracy: %f meters" % (device.fix[6] / 100)
    # FIXME: not supported yet in Python
    #if device.cell_info:
    #    if device.cell_info[0] & location.GSM_CELL_INFO_SET:
    #        print "Mobile Country Code GSM: %d" % device.cell_info[1][0]
    #    if device.cell_info[0] & location.WCDMA_CELL_INFO_SET:
    #        print "Mobile Country Code WCDMA: %d" % device.cell_info[2][0]

    print "Satellites in view: %d, in use: %d" % (device.satellites_in_view, device.satellites_in_use)

Liblocation sends a "changed" signal also after locationing is started or stopped, in which case a last known fix is sent if such exists. Application can differentiate these fixes from real ones by inspecting device.status attribute which equals location.GPS_DEVICE_STATUS_NO_FIX if the fix is not real.

Starting and stopping locationing

Finally after everything above has been done, locationing can be started.

control.start()

If the chosen location method violates control panel location settings, then a dialog is shown to user. Dialogs ask user to enable necessary services. On user's refusal an error to application is sent. If no error is seen, fixes should be coming after a while. When locationing is no longer needed, it can be stopped.

control.stop()

Complete example

Here is a complete standalone example using liblocation. It starts location services after program is started, then when first fix arrives, prints it, stops services, and shutdowns.

import location
import gobject

def on_error(control, error, data):
    print "location error: %d... quitting" % error
    data.quit()

def on_changed(device, data):
    if not device:
        return
    if device.fix:
        if device.fix[1] & location.GPS_DEVICE_LATLONG_SET:
            print "lat = %f, long = %f" % device.fix[4:6]
            data.stop()

def on_stop(control, data):
    print "quitting"
    data.quit()

def start_location(data):
    data.start()
    return False

loop = gobject.MainLoop()
control = location.GPSDControl.get_default()
device = location.GPSDevice()
control.set_properties(preferred_method=location.METHOD_USER_SELECTED,
                       preferred_interval=location.INTERVAL_DEFAULT)

control.connect("error-verbose", on_error, loop)
device.connect("changed", on_changed, control)
control.connect("gpsd-stopped", on_stop, loop)

gobject.idle_add(start_location, control)

loop.run()

You can run this example with the following command:

run-standalone.sh python2.5 test.py