Navigating to that new café in your town might occur to you as a simple task. Open your Maps App, let it determine your position and let it guide you. This service is handy for private users and also for commercial users such as the aviation sector, agriculture, and of course road traffic.
In our previous Navigation highlight, we gave insight into determining a navigation satellite’s position – a mandatory step before the data can be used to determine someone’s position on Earth. And quite exciting!
Lasers! Phew phew phew!
Lasers play an important role when it comes down to verifying a satellite’s position. We spoke to one of our colleagues Henno Boomkamp to understand how this works in more detail. Henno works as consultant for Satellite Navigation and a part of his job is to perform precise calculations to determine a satellite’s position.
Henno, which data can we use in order to get a first idea where a satellite is?
Henno: “There are several sources of data. In the case of a newly launched satellite, we know for example the details of the launch and more or less its orbit after separation. This is when we receive the first telemetry radio signals on high frequency via S-band. Through the measurements of the Doppler effect we can improve the estimate where the satellite is, at around 20-50 metres precision."
Telespazio VEGA staff are also experts on determining satellite orbits up to a centimetre level. - Photo Telespazio VEGA Deutschland / J. Mai
Is this already precise enough?
Henno: “For some satellite missions, perhaps, but not for most Earth observation satellites or navigation satellites. For precise orbit determination dedicated tracking measurements are used, which nowadays come primarily from Global Navigation Satellite Systems (GNSS) such as Galileo, GPS or GLONASS. GNSS data is used to compute the orbits of low Earth orbiting satellites, but also the orbits of the GNSS satellites themselves."
And how do you know that your orbits are correct?
Henno: “Well, this is where lasers come in! An orbit prediction is provided to the International Laser Ranging Service, ILRS, for them to track satellites with a laser. To make it very simple, they send laser pulses in the direction where we believe the satellite to be at a certain time – if it comes back we can compute how much time it took to travel to the satellite and back. Since we know the speed of light, we can determine the distance of the satellite, and have an independent range measurement to the orbit. We have to be very precise in the prediction because the satellite is moving, Earth is moving and the satellite can be thousands of km away, and it is not very big.”
||Wettzell Laser Ranging System (WLRS), the satellite and lunar laser ranging system of the geodetic observatory in Wettzell, Bavaria.|
But doesn’t this mean that there has to be a mirror on-board the spacecraft? How else would the laser pulse be reflected?
Henno: “This is exactly right. Many satellites are equipped with mirrors, so-called Laser Retro Reflectors (LRR). They have the property that they reflect a wave, in this case laser light, back to its source with minimal scattering. A good example is the LAGEOS satellite: It looks like a golf ball and its payload consists only of LRR’s. It’s a perfect, passive measurement system for example to study the gravity field of the Earth. Its data improves our models of the Earth, which in turn helps in the orbit determination of many other satellites.”
||Diagram showing how a corner reflector works.|
|Image of LAGEOS satellite, courtesy of NASA
Is the integration of LRR’s a speciality of navigation satellites?
Henno: “Not necessarily. The LRR’s enable us to determine very precise orbits of satellites through laser ranging. The laser data is in most cases used as an independent validation for the satellite orbits that are computed from other tracking data, such as GNSS, and is fundamental for precise orbit determination of many Earth observation satellites."
So let’s imagine you have collected a set of laser measurements from the ILRS. What happens next?
Henno: “It is essential to keep doing precise orbit determinations throughout a satellite mission, and not just once after launch. The orbit of a satellite changes constantly, by at least a few hundred meters per day, due to all sorts of small perturbations such as gravity of the sun, moon, planets, or radiation pressure from the sun and earth, or effects of asymmetric gravity and tides of the Earth. The laser data forms an important independent verification of the orbit accuracy that is being achieved."
And how often would you determine a satellite’s orbit?
Henno: “The work is a constant circle of specific tasks: The primary tracking data, such as GNSS data is collected continuously. At regular intervals, like once per day, a dataset from the recent past is processed to compute the precise orbit of the satellite over this period. This orbit can be extended a few hours or days into the future to get a prediction, and these predictions are for instance needed by the ILRS to track the satellite with lasers.”
And how many laser measurements are available?
Henno: “There are around 50 Laser Ranging Stations in the world. All of them can track the low satellites up to 1000 km height, but only around 20 of them can track the much higher GNSS satellites. A few of them can even track the Moon, where some laser reflectors were left behind by the Apollo astronauts. The best stations manage to produce two or three passes per day for a GNSS satellite. There are about 35 GNSS satellites with LRRs and ideally each of these 35 satellites has a handful of passes per day. So you could say that it is possible to validate orbits several times per day.”
Further Links about Satellite Navigation
Where is Galileo? We know who knows!
ESA: Navigation Facility - Galileo
DLR: Galileo Control Centre Oberpfaffenhofen
Telespazio: The Group's involvement - spaceopal - Galileo Control Centre Fucino