diff --git a/src/gooey.cpp b/src/gooey.cpp
index 6d7d33d..6f4eb49 100644
--- a/src/gooey.cpp
+++ b/src/gooey.cpp
@@ -50,12 +50,23 @@ gooDraw(SDL_Window* window, orbital_elements& orbit)
 	ImGui::NewFrame();
 
 	ImGui::SetNextWindowPos(ImVec2(0,0));
-	ImGui::SetNextWindowSize(ImVec2(300,400));
+	ImGui::SetNextWindowSize(ImVec2(300,270));
 	ImGui::Begin("Orbital Elements");
 
+	ImGui::Text("iota, inclination:            %f", orbit.iota);
+	ImGui::Text("omega, argument of periapsis: %f", orbit.omega);
+	ImGui::Text("mu, gravitational parameter:  %f", orbit.mu);
+	ImGui::Text("nu, true anomoly:             %f", orbit.nu);
+	ImGui::Spacing();
+
+	ImGui::Separator();
+	ImGui::Spacing();
+	ImGui::Text("Ellipse Parameters:");
 	ImGui::Text("a: %f km", orbit.ep.a);
+	ImGui::Text("b: %f km", orbit.ep.b);
 	ImGui::Text("e: %f", orbit.ep.e);
-	ImGui::Text("mu: %f km^3/s^2", orbit.mu);
+	ImGui::Text("c: %f km", orbit.ep.c);
+
 
 	ImGui::Text("a:");
 	ImGui::SameLine();
@@ -63,7 +74,7 @@ gooDraw(SDL_Window* window, orbital_elements& orbit)
 
 	ImGui::Text("e:");
 	ImGui::SameLine();
-	ImGui::InputDouble("##orbit.ep.e", &orbit.ep.e, 0.1f);
+	ImGui::InputDouble("##orbit.ep.e", &orbit.ep.e, 0.01f);
 
 	ImGui::End();
 
diff --git a/src/main.cpp b/src/main.cpp
index be31a17..e0cfde3 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -17,7 +17,7 @@
 
 const double SCALING = 0.001;
 static orbital_elements g_orbit = {};
-static ellipse_3d g_ellipse;
+static ellipse_3d g_ellipse3d;
 
 
 simple_mesh*
@@ -76,7 +76,37 @@ initOrbit()
 }
 
 void
-doFrameCallbackPre(render_state* rs)
+updateSatellitePosition(entity& satellite)
+{
+	const static glm::mat4 xform =
+		glm::rotate(glm::mat4(1.0), (float) M_PI_2, glm::vec3(1, 0, 0));
+
+	// TODO: decouple framerate from time_step
+	unsigned int time_step = 100; // NOTE: seconds
+
+	g_orbit.nu = getPropagatedTrueAnomaly(g_orbit, g_orbit.nu, time_step);
+	double r2 = getRadialPosition(g_orbit.ep, g_orbit.nu);
+	glm::vec2 coords = polarToRect(g_orbit.nu, r2);
+	glm::vec3 v = glm::vec3(coords, 0);
+	entSetWorldPosition(satellite, xform * glm::vec4(v.x, v.y, v.z, 1));
+}
+
+// NOTE: use validateEllipse(orbit.ep) before calling to avoid failing
+//	assertions
+void
+updateOrbit(orbital_elements& orbit, ellipse_3d& e3d, entity& ellipse_entity)
+{
+	orbit.ep = constructEllipseAE(orbit.ep.a, orbit.ep.e);
+	ellipse3DUpdate(orbit.ep, e3d);
+
+	for (uint i = 0; i < e3d.vert_count; i++)
+		ellipse_entity.mesh->vertices[i] = e3d.vertices[i];
+
+	entUpdateSimpleMesh(ellipse_entity, ellipse_entity.mesh, GL_LINE_LOOP);
+}
+
+void
+preFrameCallback(render_state* rs)
 {
 	// handle input
 	static input_state is = {};
@@ -91,50 +121,40 @@ doFrameCallbackPre(render_state* rs)
 	if (is.window_closed || is.escape)
 		rs->running = false;
 
-	// update orbital elements/ellipse_3d
-	g_orbit.ep = constructEllipseAE(g_orbit.ep.a, g_orbit.ep.e);
-	entity& ellipse_entity = rs->render_groups[0].entities[0];
-
-	// TODO: fix this
-	double angle = 2 * M_PI / g_ellipse.vert_count;
-	for (uint i = 0; i < g_ellipse.vert_count; i++) {
-		double a = angle * i;
-		double r = g_orbit.ep.a *
-			(1 - pow(g_orbit.ep.e, 2)) / (1 + g_orbit.ep.e * cos(a));
-		g_ellipse.vertices[i] = glm::vec3(polarToRect(a, r), 0);
-		// NOTE: update mesh
-		ellipse_entity.mesh->vertices[i] = g_ellipse.vertices[i];
-		// NOTE: update entity.render_object
-		entUpdateSimpleMesh(ellipse_entity, ellipse_entity.mesh, GL_LINE_LOOP);
-	}
-
-
-	// update satellite position
-	const static glm::mat4 xform =
-		glm::rotate(glm::mat4(1.0), (float) M_PI_2, glm::vec3(1, 0, 0));
-	entity& satellite = rs->render_groups[0].entities[2];
-
-	// TODO: decouple framerate from time_step
-	unsigned int time_step = 100; // NOTE: seconds
-
-	g_orbit.nu = getPropagatedTrueAnomaly(g_orbit, g_orbit.nu, time_step);
-	double r2 = getRadialPosition(g_orbit.ep, g_orbit.nu);
-	glm::vec2 coords = polarToRect(g_orbit.nu, r2);
-	glm::vec3 v = glm::vec3(coords, 0);
-	entSetWorldPosition(satellite, xform * glm::vec4(v.x, v.y, v.z, 1));
+	updateSatellitePosition(rs->render_groups[0].entities[2]);
 }
 
 void
-doFrameCallbackPost(render_state* rs)
+postFrameCallback(render_state* rs)
 {
-	gooDraw(rs->handles->window, g_orbit);
+	static orbital_elements c_orb = {};
+	orbitCopy(g_orbit, c_orb);
+	gooDraw(rs->handles->window, c_orb);
+
+	// TODO: will need more validation from GUI as added
+	if (!ellipsesEqual(c_orb.ep, g_orbit.ep)) {
+		// NOTE: handle the case where semimajor axis lowered with low
+		//	eccentricity would fail validation
+		if (c_orb.ep.a < g_orbit.ep.a && c_orb.ep.a > 0
+			&& c_orb.ep.e == g_orbit.ep.e)
+		{
+			c_orb.ep.b = c_orb.ep.a * sqrt(1 - pow(c_orb.ep.e, 2.0));
+		}
+
+		if (validateEllipse(c_orb.ep)) {
+			c_orb.ep = constructEllipseAE(c_orb.ep.a, c_orb.ep.e);
+			ellipseCopy(c_orb.ep, g_orbit.ep);
+			entity& ellipse_entity = rs->render_groups[0].entities[0];
+			updateOrbit(g_orbit, g_ellipse3d, ellipse_entity);
+		}
+	}
 }
 
 int
 main()
 {
 	render_state* rs = renInit("orbital shipping",
-		glm::vec2(1920, 1080),
+		glm::vec2(1280, 720),
 		SDL_INIT_TIMER);
 
 	if (rs == nullptr) {
@@ -163,8 +183,8 @@ main()
 	rs->render_group_count = 1;
 
 	entity& ellipse_entity = rs->render_groups[0].entities[0];
-	g_ellipse = constructEllipse3D(ep, 256);
-	simple_mesh* sm = constructEllipseMesh(g_ellipse, glm::vec3(255, 0, 255));
+	g_ellipse3d = constructEllipse3D(ep, 256);
+	simple_mesh* sm = constructEllipseMesh(g_ellipse3d, glm::vec3(255, 0, 255));
 	entInitMesh(ellipse_entity, sm, GL_LINE_LOOP);
 	entRotate(ellipse_entity, (float) M_PI_2, glm::vec3(1, 0, 0));
 
@@ -177,9 +197,9 @@ main()
 	sm = createSatelliteMesh();
 	entInitMesh(satellite_entity, sm, GL_TRIANGLES);
 	entRotate(satellite_entity, (float) M_PI_2, glm::vec3(1, 0, 0));
-	entSetWorldPosition(satellite_entity, g_ellipse.vertices[0]);
+	entSetWorldPosition(satellite_entity, g_ellipse3d.vertices[0]);
 
-	renDoRenderLoop(rs, 60 , doFrameCallbackPre, doFrameCallbackPost);
+	renDoRenderLoop(rs, 60 , preFrameCallback, postFrameCallback);
 
 	// TODO: clean up mesh pointers? don't remember if renderer does that
 	// 	automatically
diff --git a/src/orbits.cpp b/src/orbits.cpp
index dc05ee0..53274e8 100644
--- a/src/orbits.cpp
+++ b/src/orbits.cpp
@@ -24,9 +24,43 @@ constructEllipseAE(double a, double e)
 }
 
 bool
-validateEllipse(ellipse_parameters)
+validateEllipse(const ellipse_parameters ep)
 {
-	return false;
+	return (ep.a > 0 &&
+		ep.b > 0 &&
+		ep.a >= ep.b &&
+		ep.e >= 0 &&
+		ep.e < 1);
+}
+
+bool
+ellipsesEqual(ellipse_parameters& e1, ellipse_parameters& e2)
+{
+	return (e1.a == e2.a &&
+		e1.b == e2.b &&
+		e1.e == e2.e);
+}
+
+void
+ellipseCopy(const ellipse_parameters& e1, ellipse_parameters& e2)
+{
+	e2.a = e1.a;
+	e2.b = e1.b;
+	e2.e = e1.e;
+	e2.c = e1.c;
+	e2.p = e1.p;
+	e2.f1 = e1.f1;
+	e2.f2 = e1.f2;
+}
+
+void
+orbitCopy(const orbital_elements& o1, orbital_elements& o2)
+{
+	ellipseCopy(o1.ep, o2.ep);
+	o2.iota = o1.iota;
+	o2.omega = o1.omega;
+	o2.mu = o1.mu;
+	o2.nu = o1.nu;
 }
 
 ellipse_3d
@@ -37,17 +71,23 @@ constructEllipse3D(ellipse_parameters ep, uint vert_count)
 		vert_count > 0);
 
 	ellipse_3d e3d = { ep, nullptr, vert_count};
+	// TODO: need to free this allocation at some point
 	e3d.vertices = UTIL_ALLOC(vert_count, glm::vec3);
-	double angle = 2 * M_PI / vert_count;
+	ellipse3DUpdate(ep, e3d);
+	return e3d;
+}
 
-	for (uint i = 0; i < vert_count; i++) {
+void
+ellipse3DUpdate(ellipse_parameters ep, ellipse_3d& e3d)
+{
+	double angle = 2 * M_PI / e3d.vert_count;
+
+	for (uint i = 0; i < e3d.vert_count; i++) {
 		double a = angle * i;
 		// NOTE: solving for distance in polar coordinates relative to focus
 		double r = ep.a * (1 - pow(ep.e, 2)) / (1 + ep.e * cos(a));
 		e3d.vertices[i] = glm::vec3(polarToRect(a, r), 0);
 	}
-
-	return e3d;
 }
 
 double
diff --git a/src/orbits.h b/src/orbits.h
index c185d52..9a363bc 100644
--- a/src/orbits.h
+++ b/src/orbits.h
@@ -24,7 +24,7 @@ struct orbital_elements
 	ellipse_parameters ep;
 	double iota;	// NOTE: (ι) inclination
 	double omega;	// NOTE: (ω) argument of periapsis
-	double mu;		// NOTE: (μ) gravittional parameter
+	double mu;		// NOTE: (μ) gravitational parameter
 	double nu;		// NOTE: (ν) true anomaly
 };
 
@@ -43,15 +43,24 @@ ellipse_parameters
 constructEllipseAE(double a, double e);
 
 bool
-validateEllipse(ellipse_parameters);
+validateEllipse(const ellipse_parameters);
+
+bool
+ellipsesEqual(ellipse_parameters& e1, ellipse_parameters& e2);
+
+void
+ellipseCopy(const ellipse_parameters& e1, ellipse_parameters& e2);
+
+void
+orbitCopy(const orbital_elements& o1, orbital_elements& o2);
 
 // NOTE: create vertices for a 3d ellipse
 // NOTE: all vertices are in the x/y plane with z = 0
 ellipse_3d
 constructEllipse3D(ellipse_parameters ep, uint vert_count);
 
-bool
-ellipsesEqual(ellipse_parameters e1, ellipse_parameters e2);
+void
+ellipse3DUpdate(ellipse_parameters ep, ellipse_3d& e3d);
 
 /* NOTE: how-to propagate orbit position given initial true anomaly, semimajor
  *	axis, mean motion, and eccentricity: ref) section 4.4, Kepler's Problem,