GPS timestamp help. Android device synchronization fails.

Hello everyone! I've encountered a problem, and need help to figure out what's happening. The idea is to synchronize 2 or more android devices with respect to a GPS location timestamp. I've heard that GPS time is very accurate, unlike system time, which may vary across few seconds. However the results I get are not what I expected.

 void Start() {
     Input.location.Start ();
     double UTC_timestamp = Input.location.lastData.timestamp;
     Input.location.Stop ();  
 }

So in this case, UTC_timestamp represents the total number of seconds since January 1st 1970, 00:00:00.

But if I request a timestamp on 2 different devices, I am getting quite a sure difference, and my attempts to synchronize them via satellite's timestamp fails.

Here is how I tested that:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class GPSTime: MonoBehaviour {
     double UTC_timestamp;
     public Text txt_UTC;
     IEnumerator Start() {
         UTC_timestamp = getGPSTime ();
         
         while (Application.isPlaying) {
             yield return new WaitForSeconds (1);
             UTC_timestamp ++;
         }
     }
     
     void Update() {
         print("UTC: "+ UTC_timestamp);
         txt_UTC.text = "UTC: " + UTC_timestamp;
     }
     
     private double getGPSTime() {
         Input.location.Start ();
         UTC_timestamp = Input.location.lastData.timestamp;
         Input.location.Stop ();
         return UTC_timestamp;
     }
 }

I run this on two different devices, and the timestamp I am getting differs quite significantly:

So out of three app launches I get the following:

         DEVICE 1                                          

1) 1438782375.605

2) 1438782610.260

3) 1438782681.926

4) 1438782960.266

         DEVICE 2                                          

1) 1438782505.306

2) 1438782680.011

3) 1438782675.226

4) 1438782967.400

So first launch differs by ~130 seconds!!! Second trial differs by ~70 seconds, and third trial differs by 6-7 seconds. Fourth trial differs by around 7 seconds.

Why such strange differences? What can I do to get the best results possible?