public function startExportTask(ExportTask $et)
    {
        $url = $this->service_url . '/export';
        $json_data = $et->toJSON();
        $data = $this->service->prepAndSend($url, array(200), 'POST', $json_data, true, 'application/json', 'application/json');
        return TaskState::createFromJSON(json_decode($data));
    }