What is satellite laser ranging

The accuracy of the orbits was, however, at the 10—15 centimeter level. These validation results supported several model improvements for GPS satellite orbits including, in particular, the handling of solar and Earth albedo radiation pressure and antenna thrust, reducing the observed SLR bias with respect to the IGS orbits to 1. Solar radiation pressure — the force due to the impact of the photons — is tiny, but its continuing presence has a strong perturbing effect on satellite orbits.

Antenna thrust is also a small force. The transmission of GPS navigation signals results in a continuously acting reactive force in the radial direction acting on the satellite. Retroreflector array on Galileo satellites at bottom of satellite, below antenna array.

SLR also plays an essential role for calibrating improved radiation pressure models for the new satellite systems. The origin of this behavior is the elongated shape of the Galileo satellites compared to the more-or-less cubic shape of GPS satellites, causing much larger variations of the satellite cross-section exposed to the sun while orbiting the Earth. The observed SLR residuals triggered the development of improved radiation pressure models for Galileo satellites. As the estimated longitude of geostationary GNSS satellites such as those in the BeiDou constellation is highly susceptible to biases due to the small motion of the satellites with respect to the tracking stations, SLR may play an important role for precise orbit determination of this category of satellite.

Retroreflector array on BeiDou satellites. However, little publicly available microwave tracking data yet exists. Therefore, up to now, precise orbit determination heavily relies on SLR observations. Retroreflector array on NavIC satellites. Because GNSS is a one-way measurement technique, only pseudoranges and carrier phases can be measured, and clock synchronization is indispensable for positioning and orbit determination.

Radial orbit errors can therefore be absorbed to a large degree by satellite clock corrections. For the very stable clocks on board Galileo satellites, the SLR residuals show the same behavior as the microwave-derived clock corrections indicating that the clock corrections are, in fact, caused by radial orbit errors. SLR therefore provides a way to break this correlation and to separate radial orbit errors and satellite clock corrections.

This makes it possible to study and to characterize the physical behavior of onboard clocks including temperature-induced clock variations. Separation of orbit errors and satellite clock variations is crucial when using the first two Full Operational Capability Galileo satellites, which were released into wrong orbits, for relativistic experiments.

In a dual launch on Aug. With a sequence of maneuvers, the satellite orbit heights could be increased to 22, kilometers compared to the planned height of 23, kilometers and the eccentricity was decreased to 0. Regular SLR tracking of the two satellites plays an essential role in this experiment to separate clock variations due to orbit errors from those caused by the gravitational redshift.

In the near future, more than GNSS satellites carrying retroreflectors will be operational. Optimized tracking scenarios and session planning strategies will be indispensable. Normal points are compressed full-rate data obtained by averaging individual range measurements typically over five-minute intervals. To assess the capability of SLR for GNSS precise orbit determination based on the number of tracking stations and the distribution of observations, we performed a simple simulation.

The covariance analysis included observations of a single SLR station compared to networks of 6 and 17 globally distributed stations. No unfavorable weather conditions were considered and observations of different stations were assumed to be uncoordinated. If observations from three days are used for orbit determination, the errors on the middle day reduce to about meters right, first row.

The situation significantly improves if a global network of six stations is considered. Even for a single day of observations, an orbit precision of a few decimeters is reached left, second row while the orbit uncertainty further decreases to a few centimeters if observations from three days are used right, second row.

If, however, in an effort to reduce the number of observations per pass, only measurements at satellite culmination are acquired, the orbit precision is in the kilometer range for a six-station network and observations from one day left, third row. If observations from three days are used, the orbit precision is at the meter level right, third row.

Using three normal points per pass for a station network, the orbit precision reaches a few centimeters even within one day left, last row and about 1 centimeter for observations from three days right, last row. It should be noted that the covariance analysis does not consider any systematic observation or orbit modeling error. Formal errors of Galileo orbits in radial red , along-track green and cross-track blue directions.

First row: one SLR station, 1-day arc left , middle of 3-day arc right ; second row: six stations, 1-day arc left , 3-day arc right ; third row: six stations with tracking only at culmination, 1-day arc left , 3-day arc right ; fourth row: 17 stations, 1-day arc left , 3-day arc right.

Note the different scaling for the various plots. This simulation is very simple and not very realistic, but nevertheless indicates the capability of precise orbit determination for GNSS satellites using a limited number of observations per station.

The simulations demonstrate two facts. Firstly, even with just two or three normal points per satellite of a GNSS constellation, a significant fraction of the observation time of a station is required. Typically, a mid-latitude station can acquire about 60 normal points per day for a satellite constellation, amounting to several hours of observation time per day.

Secondly, the improvement in formal orbit accuracy only increases with the square root of the number of stations. More important than the number of normal points is their distribution along the orbit requiring SLR observations from several stations distributed over the globe.

These two findings make it obvious that coordination among SLR stations is indispensable for making economic use of the observing time of SLR stations while providing good coverage of normal points along all satellite orbits. For new GNSS constellations and new orbit types, SLR proves to be essential for calibrating radiation pressure models and allows us to separate orbit- and temperature-induced variations of onboard clocks. Given the large number of GNSS satellites from several constellations equipped with retroreflectors, coordination of observation scheduling among SLR stations will be crucial for optimizing the benefit-to-cost ratio.

Data from a global network of SLR stations are used to estimate the orbital parameters of satellites which revolve around the Earth's centre of mass. Therefore the position of the Earth's geocentre, the origin of the global reference frame, can be monitored through time. SLR has become an important geodetic instrument to be used for the establishment of an accurate global geodetic infrastructure and Earth monitoring science.

During the period LLR was the primary activity at the Orroral, which closed on 31 October and was replaced by a new facility located on Mount Stromlo. Most of the present SLR ground systems legacy systems operate at nm in the 5 — 10 Hz regime. Newer technology systems are now operating in the kilohertz region using photon-counting techniques that have greatly enhanced data productivity and pass interleaving capability.

Automation of operations is also playing a larger role now as the candidate targets proliferate with the launch of several new navigation constellations that require orbit calibration with SLR tracking. Satellite laser ranging retroreflector array The space segment is an array of retroreflectors that significantly increase the optical cross-section of the satellites.

The properties of the array depend on satellite altitude and application. Low Earth active satellites use arrays with symmetrically mounted cubes on a hemispherical assembly to provide return signal over a wide angular aspect. Geodetic satellites have high mass-to-area ratio, are passive spherical satellites, covered with retroreflectors.