Angular js Http promises in typescript

I have a problem with my code which I hope someone here could help me understand and point me in the right direction. The issue is in nesting promises. A specific method needs to make two Http requests to the server - the second one after the first one accomplishes successfully, and the result of the second one should be resolved in the calling class.

Whenever using the pattern I show in my example in methods that contain a single Http call it worked as expected. Here is the code:

Service1:

public class Service1 extends IService1 {
    public PostData(url : string, data: any) : ng.IHttpPromise<any> { 
        return this.$http.Post(url, data);
    }

    public GetData (url : string) : ng.IHttpPromise<any> {
        return this.$http.Get(url);
    }
}

Service2:

public class Service2 extends IService2 {
    private srv1 : IService1;

    static inject = ["Service1"];

    public constructor(service1){
        this.srv1 = service1;
    }

    public GetLetters(ids : number[]) : ng.IHttpPromise<any>{
        var scope = this;
        return this.srv1.PostData("api/letters", ids).success(function (data){
            return scope.srv1.GetData("api/lettters/" + data);
        })
    }
}

Controller

public class Controller1 {

    private service2 : IService2;
    private array : Letter[];

    static inject = ['Service2']

    public constructor (Service2 : IService2){
        this.service2 = Service2;
    }

    public someFunc() : void 
        var scope = this;
        // I have problems here when I try to retrieve the letters 
        this.service2.GetLetters(someIds).success((data) => {
            array = data;
        });
    }
}

Is there a different approach I should take here? The data inside the controller is the resolved data from the outer promise of the GetLetters method. I want to get the resolved data of the inner promise