Example of using observable from within a Service on Ionic 2. Full blog post here: http://christianvarga.com/how-to-create-and-monitor-observables-in-ionic-2-service/

api-service.ts

import { Injectable } from '@angular/core';
import { Http, Headers, RequestOptions } from '@angular/http';
import { Observable } from 'rxjs/Observable';
import 'rxjs/Rx';

@Injectable()
export class ApiService {
  private errorObserver: any;
  public error: any;

  constructor(public http: Http) {
    this.errorObserver = null;
    this.error = Observable.create(observer => {
      this.errorObserver = observer;
    });
  }

  get(url) {
    let headers = new Headers({
      'X-API-KEY' : 'my-api-key',
      'Content-Type': 'application/json; charset=utf-8'
    });
    let options = new RequestOptions({ headers: headers });
    return this.http.get(url, options)
      .map(res => res.json())
      .catch(error => this.handleError(error));
  }

  private handleError(error) {
    this.errorObserver.next(error);
    return Observable.throw(error.json().error || 'Server error');
  }
}

app.component.ts

import { Component } from '@angular/core';
import { Platform } from 'ionic-angular';

import { ApiService } from '../providers/api-service';

@Component({
  template: `<ion-nav [root]="rootPage"></ion-nav>`
})
export class MyApp {
  ...

  constructor(platform: Platform, private apiService: ApiService) {
    platform.ready().then(() => {
      ...

      this.apiService.error.subscribe((error) => {
        if (error.status == 403) {
          // unauthorised, redirect to login
        }

        // error, alert message
      });
    });
  }
}

Simple