Real time gps (sample and Callback)

I would like to move a model continously with real time tracking between two points.

/Set bounds of our simulation time
var start = Cesium.JulianDate.fromDate(new Date(2015, 2, 25, 16));
var stop = Cesium.JulianDate.addSeconds(start, 120, new Cesium.JulianDate());

//Make sure viewer is at the desired time.
viewer.clock.startTime = start.clone();
viewer.clock.stopTime = stop.clone();
viewer.clock.currentTime = start.clone();
viewer.clock.clockRange = Cesium.ClockRange.LOOP_STOP; //Loop at the end
viewer.clock.clockRange = Cesium.ClockRange.CLAMPED;
viewer.clock.multiplier = 1;
viewer.clock.shouldAnimate = true;

var constant;
var position3;

function p1() {
var position1 = Cesium.Cartesian3.fromDegrees(lon1, lat1, alt1);
var position2 = Cesium.Cartesian3.fromDegrees(lon2, lat2, alt2);
position3 = new Cesium.SampledPositionProperty();

position3.addSample(start, position1);
position3.addSample(stop, position2);

var heading1 = Cesium.Math.toRadians(cou1);
var pitch1 = 0;
var roll1 = 0;
var hpr1 = new Cesium.HeadingPitchRoll(heading1, pitch1, roll1);
var orientation1 = Cesium.Transforms.headingPitchRollQuaternion(position3, hpr1);

var entity1;
function k1() {
constant = true;
var promise = Cesium.IonResource.fromAssetId($f1).then(function (resource) {
entity1 = viewer.entities.add({
availability : new Cesium.TimeIntervalCollection([new Cesium.TimeInterval({
start : start,
stop : stop
name : $name1,
position: new Cesium.CallbackProperty(getPos, constant),
orientation : new Cesium.VelocityOrientationProperty(position3),
model: {
uri: resource,
heightReference : Cesium.HeightReference.CLAMP_TO_GROUND,
minimumPixelSize : 128,
maximumScale : 2
viewer.trackedEntity = entity1;

function getPos() {
//return new Cesium.Cartesian3.fromDegrees(lon1, lat1); ???

function setPosCallback() {
constant = !constant;
entity1.position.setCallback(getPos.bind(entity1), true);
setInterval(setPosCallback, 1000);

Have you seen the time dynamic car code example?

This shows a car moving with points defined in time, but you can apply the same logic to take in points in real-time as they come from an external server.

If that doesn’t help, can you recreate the issue you’re having in Sandcastle using one of the 3D models from there so I can run it? This one has a few models you can use: