Calling another activity from a broadcast receiver

Question

I am building an app which monitors GPS location at 10 second intervals in a background thread and tests it against some criteria, and if the test is successful it then sends a message to a broadcast receiver. The following code for the receiver gets the the latitude and longitude from the background thread, and works fine just displaying the result in a toast.

package com.barney.gpstracking;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.widget.Toast;

public class PosReceiver extends BroadcastReceiver {
    double lat, lon;
    String slat, slon;

    @Override
    public void onReceive(Context context, Intent intent) {
        Bundle params = intent.getExtras();
        if (params != null) {
            lat = params.getDouble("lati");
            slat = String.valueOf(lat);
            lon = params.getDouble("long");
            slon = String.valueOf(lon);
        }
        Toast.makeText(context, "Position is:" + lat + "  " + lon, Toast.LENGTH_LONG).show();
    }
}

However, if I replace the toast with a call to start another activity as follows:

            lon = params.getDouble("long");
            slon = String.valueOf(lon);
        }
        Intent goPop=new Intent(context,PopUpActivity.class);
        context.startActivity(goPop);
    }
}

then it compiles and installs on my device ok but I get a run-time error as soon as it tries to receive a broadcast. The PopUpActivity that I am trying to call works perfectly if I try calling it from elsewhere (e.g. from the main activity). I guess therefore that the goPop intent above is incorrect. I have looked at advice in response to similar questions, but I can't figure out the problem. Can anyone help me please?