Android, CyanogenMod and the GPS fix

There are a lot of people around providing gps fixes for Android. Most of them don’t really help at all. I want to explain here what variables the code recognizes and what they do. The other thing is SuplCert which is the certificate to verify the ssl connection for the supl server. Most of the time poeple provide the wrong certificate.

gps.conf

There are two locations in the code which read the gps.conf file. GpsLocationProvider.java [1] GpsXtraDownloader.java [2] and loc_eng_cfg.cpp [3]. The GpsLocationProvider reads the following variables from the config:

    SUPL_HOST
    SUPL_PORT

    C2K_HOST
    C2K_PORT

GpsXtraDownloader.java

    XTRA_SERVER_1
    XTRA_SERVER_2
    XTRA_SERVER_3

This is pretty obvious. It then calls a function from libloc_api called loc_eng_set_server() to set the agps server. libloc_api itself reads also the gps.conf to get the values for the following variables:

    INTERMEDIATE_POS (Turn on intermediate position)
    ACCURACY_THRES   (Turn-off  intermediate positions outside required accuracy)
    ENABLE_WIPER     (The code only reads this variable but it doesn't do anything)
    DEBUG_LEVEL      (0-5, default: 4)

SuplCert

In most of the flashable gps fixes you find a SuplCert. Most of them I looked at have the wrong certificate. They include a Verisign certificate but supl.google.com is signed by Thawte.

This is a SSL encrypted supl request to supl.google.com on port 7275.

However as you can see above. There is no support for a TLS supl request in the code. It doesn’t make sense to add the host, port and certificate in CyanogenMod.

Supl Protocol

For supl you need to have a cell connection. You ask the radio for the cell parameters and send it to the supl server. The supl server will return the gps coordinates of the tower or an estimation of your location.

You can get the cell id information with the following AT commands:

       $ AT+COP?
       +COPS: 0,2,"24405",2
       OK
       $ AT+CREG=2
       OK
       $ AT+CREG?
       +CREG: 2,1,"59E2","31B0"
       OK
       $ AT+CREG=0
       OK

       This results in:

       MCC = 244     (Mobile Country Code)
       MCN = 05      (Mobile Country Network)
       LAC = 0x59e2  (Local Area Code)
       CI  = 0x31b0  (Cell Identifier)

Conclusion

The default gps.conf by CyanogenMod provides all needed variables to have supl support for a faster GPS fix.

With DEBUG_LEVEL=5 you normally see the following lines:

    D/libloc  (  169): loc_eng_init created client, id = 1
    D/libloc  (  169): loc_eng_ni_init: entered.
    D/libloc  (  169): loc_eng_set_server, type = 1, hostname = supl.google.com, port = 7276
    D/libloc  (  169): loc_eng_set_server, addr = supl.google.com:7276
    D/libloc  (  169): loc_eng_ioctl called: client = 1, ioctl_type = RPC_LOC_IOCTL_SET_UMTS_SLP_SERVER_ADDR
    D/RPC     (  169): 3000008c:00020002 call success.
    D/RPC     (  169): 3000008c:00020002 sending call (XID 2).
    D/RPC     (  169): 3000008c:00020002 received REPLY (XID 2), grabbing mutex to wake up client.

[1] frameworks/base/services/java/com/android/server/location/GpsLocationProvider.java
[2] frameworks/base/services/java/com/android/server/location/GpsXtraDownloader.java
[2] hardware/qcom/gps/loc_api/libloc_api/loc_eng_cfg.cpp