diff --git a/src/lat_lon2n_E.ts b/src/lat_lon2n_E.ts
index c90a980..40965a5 100644
--- a/src/lat_lon2n_E.ts
+++ b/src/lat_lon2n_E.ts
@@ -1,7 +1,6 @@
-export function lat_lon2n_E(
-  latitude: number,
-  longitude: number,
-): [x: number, y: number, z: number] {
+import type { Vector3 } from "./vector.js";
+
+export function lat_lon2n_E(latitude: number, longitude: number): Vector3 {
   const sinLat = Math.sin(latitude);
   const cosLat = Math.cos(latitude);
   const sinLon = Math.sin(longitude);
diff --git a/src/matrix.ts b/src/matrix.ts
new file mode 100644
index 0000000..42df31a
--- /dev/null
+++ b/src/matrix.ts
@@ -0,0 +1,5 @@
+export type Matrix3x3 = [
+  [n00: number, n01: number, n02: number],
+  [n10: number, n11: number, n12: number],
+  [n20: number, n21: number, n22: number],
+];
diff --git a/src/n_E2lat_lon.ts b/src/n_E2lat_lon.ts
index bf8614a..eaee04a 100644
--- a/src/n_E2lat_lon.ts
+++ b/src/n_E2lat_lon.ts
@@ -1,8 +1,10 @@
+import type { Vector3 } from "./vector.js";
+
 export function n_E2lat_lon(
-  x: number,
-  y: number,
-  z: number,
+  n_E: Vector3,
 ): [latitude: number, longitude: number] {
+  const [x, y, z] = n_E;
+
   const sinLat = z;
   const cosLat = Math.sqrt(y ** 2 + x ** 2);
   const cosLatSinLon = y;
diff --git a/src/vector.ts b/src/vector.ts
new file mode 100644
index 0000000..c46de37
--- /dev/null
+++ b/src/vector.ts
@@ -0,0 +1,2 @@
+export type Vector3 = [x: number, y: number, z: number];
+export type Vector4 = [x: number, y: number, z: number, w: number];
diff --git a/test/jest/n_E2lat_lon.spec.ts b/test/jest/n_E2lat_lon.spec.ts
index dddd724..7b8cad6 100644
--- a/test/jest/n_E2lat_lon.spec.ts
+++ b/test/jest/n_E2lat_lon.spec.ts
@@ -22,11 +22,11 @@ describe("n_E2lat_lon()", () => {
   })("matches the Python implementation", async ([latitude, longitude]) => {
     const [x, y, z] = lat_lon2n_E(latitude, longitude);
 
-    const expected = await nvectorTestClient.n_E2lat_lon(x, y, z);
+    const expected = await nvectorTestClient.n_E2lat_lon([x, y, z]);
 
     expect(expected).toMatchObject([expect.any(Number), expect.any(Number)]);
 
-    const actual = n_E2lat_lon(x, y, z);
+    const actual = n_E2lat_lon([x, y, z]);
 
     expect(actual).toMatchObject([expect.any(Number), expect.any(Number)]);
     expect(actual[0]).toBeCloseTo(expected[0], 10);
diff --git a/test/nvector-test-api.ts b/test/nvector-test-api.ts
index fcbe1f6..9274d98 100644
--- a/test/nvector-test-api.ts
+++ b/test/nvector-test-api.ts
@@ -1,4 +1,5 @@
 import { WebSocket } from "ws";
+import type { Vector3 } from "../src/vector.js";
 
 export type NvectorTestClient = {
   lat_lon2n_E: (
@@ -6,11 +7,7 @@ export type NvectorTestClient = {
     longitude: number,
   ) => Promise<[x: number, y: number, z: number]>;
 
-  n_E2lat_lon: (
-    x: number,
-    y: number,
-    z: number,
-  ) => Promise<[latitude: number, longitude: number]>;
+  n_E2lat_lon: (n_E: Vector3) => Promise<[latitude: number, longitude: number]>;
 
   close: () => void;
 };
@@ -34,7 +31,7 @@ export async function createNvectorTestClient(): Promise<NvectorTestClient> {
       return [x, y, z];
     },
 
-    async n_E2lat_lon(x, y, z) {
+    async n_E2lat_lon([x, y, z]) {
       const { latitude, longitude } = await call<{
         latitude: number;
         longitude: number;