Click to read the full Innovation article, “ESA’s multi-modal space mission to improve geodetic applications”

Innovation Insights with Richard Langley

In the beginning of the space age, there was only one space-based positioning technique: satellite Doppler. Shortly after the launch of the first satellite, Sputnik 1, on Oct. 4, 1957, it was realized that by using a receiver to measure the Doppler frequency shift of a satellite’s transmitted signals combined with knowledge of the satellite’s orbit, the position of the receiver could be determined.

The United States Navy used this concept to develop the Navy Navigation Satellite System, commonly known as Transit. Although its initial use was for positioning Polaris submarines, it was released for commercial use in July 1967. Transit was used worldwide for positioning and navigation until it was decommissioned at the end of 1996. We talked about Transit in the introduction to the article “Easy Peasy, Lemon Squeezy: Satellite Navigation Using Doppler and Partial Pseudorange Measurements” in this column’s October 2012 edition.

Next on the scene was very long baseline interferometry (VLBI). This was, and still is, a technique for high-resolution mapping of galactic and extragalactic radio sources such as quasars. It was invented by Canadian and American radio astronomers with the Canadians getting the first interference “fringes” on a transcontinental baseline on May 21, 1967. VLBI uses radio telescopes, separated by 100s or 1,000s of kilometers, to record signals on storage media (previously magnetic tape and subsequently disk-based systems) synchronized by atomic clocks, typically hydrogen masers. The recordings are played back and cross-correlated at a central facility to produce the observation data – essentially the difference in arrival times of the radio signals at the radio telescopes. It was apparent that VLBI measurements could also be used to precisely determine the vector baselines between pairs of radio telescopes eventually down to a few millimeters, so VLBI became an important geodetic technique, even measuring the drift of the continents in essentially real time. We featured an article on VLBI in this column in February 1996, “The Synergy of VLBI and GPS.”

Around the same time that VLBI was being developed, satellite laser ranging (SLR) made its debut. SLR works by precisely measuring the two-way travel time of laser pulses sent from telescopes on Earth to arrays of corner-cube reflectors on specially equipped satellites. The first experiments were conducted with Beacon Explorer A in 1964. Initial results had a range accuracy of about three meters. Since then, more than 100 satellites have been launched with SLR reflectors, including the GLONASS, Galileo, BeiDou and Quasi-Zenith navigation satellites, the Indian regional satellites and a couple of GPS satellites with more to come. Ranging precisions are now as good as a few millimeters. Laser ranging is also conducted using reflector arrays on the surface of the moon. Back in September 1994, we had an SLR article in this column, “Laser Ranging to GPS Satellites with Centimetre Accuracy.”

Skipping over GNSS, with which most of us are very familiar, then came Doppler Orbitography and Radio Positioning Integrated by Satellite (DORIS). DORIS was developed in France by a group of institutions led by the Centre National d’Études Spatiales. Rather than transmitting signals from satellites and measuring the Doppler shift at receivers on the ground, the system transmits signals from a global network of ground-based beacons, which are picked up by receivers on specially equipped satellites and the data is subsequently downloaded to Earth. The first such equipped satellite was SPOT-2, launched in January 1990. Since then, 18 more satellites with DORIS receivers on board have been launched to date. DORIS, along with the other techniques, was discussed in the online GPS World article, “NASA Helps Maintain International Terrestrial Frame with GNSS,” published in February 2016.

Like the global navigation satellite systems with the International GNSS Service, the other techniques have their coordinated services, too: the International VLBI Service for Geodesy and Astrometry (IVS), the International Laser Ranging Service (ILRS), and the International DORIS Service (IDS).

All of these techniques and services contribute to the refinement of the International Terrestrial Reference Frame (ITRF), on which all positioning activities on Earth eventually depend. Tying the contributions from the different services together involves accounting for any systematic differences, which are reduced in part by using positional data at collocated sites where two or more techniques are sited with the vector ties between the instruments carefully measured. The September 1996 edition of “Innovation” was on the IERS and was aptly titled “International Terrestrial Reference Frame.”

The…