GNSS Radio Occultation
The GeoOptics constellation contains a number of spacecraft in low-earth orbit equipped with a GNSS / GPS Radio Occultation sensor. This sensor, a modified GNSS / GPS receiver, measures properties of the received signal and from it provides measurements of refractivity, and therefore temperature and pressure, of the the atmosphere.
The arrival of the Global Positioning System (GPS) in the 1980s first made the idea both practical and (potentially) useful. GPS system designers in the early 1970s had the vision to extend the transmission beamwidths of the satellites comfortably beyond the Earth’s limb, allowing them to be refracted by the Earth’s atmosphere. With a suitably programmed GPS receiver on a low Earth satellite, pointed at the Earth’s limb, those signals can be harnessed to produce tremendously accurate atmospheric data.
The rapid motion of the low orbiting receiver dominates the changing geometry and largely determines the duration of the occultation, typically a few minutes. The much higher GNSS satellites move relatively slowly and can be regarded as almost stationary during the occultation. For that reason, most occultations occur in the forward or backward direction with respect to the receiver’s line of motion, with relatively few occurring off to the sides.
HOW DOES GNSS-RO / GPS-RO WORK?
We can think of the transmitted GNSS signals as sinusoidal electromagnetic waves. Each satellite broadcasts at two radio frequencies, about 1.2 and 1.6 billion cycles per second (gigahertz). Traveling at the speed of light, these signals have wavelengths of about 24 and 19 cm, or roughly 4 and 5 wavelengths per meter, respectively. Thus over a distance of 100 m, the two signals complete a little more than 400 and 500 cycles. In principle one can measure distance by counting the number of cycles between a transmitter and a receiver. As an orbiting receiver watches a GPS signal set (or rise from) behind the earth, the signal must pass through the atmosphere in a limb-grazing geometry. The atmosphere bends and delays the signal, increasing its effective path length and thus the number of cycles along the raypath. At the surface the increased path length caused by the atmosphere can reach 2.5 km, or more than 12,000 cycles at the shorter wavelength. By counting the extra cycles we can measure this excess delay with a precision of about 0.005 cycle, or ~1 mm, better than one part in a million. This accounts for the extraordinary performance of GNSS-RO in recovering atmospheric profiles.
From these profiles we can compute climate averages, contour maps, and wind fields and watch the atmosphere expand as it warms. In the lowest few km where moisture can become prominent the contributions of temperature and moisture to the signal delay become ambiguous. However, since we can model (project downward from ~5 km altitude) the temperature based on climatological patterns, we can then recover moisture in the lower atmosphere with high accuracy. In fact, operational weather forecasting systems have now been developed to ingest the pure refractivity or bending angle measurement, which is not ambiguous, and let the model, combined with information from other sensors, sort out the relative contributions of temperature and moisture at different altitudes, a strategy that has proven highly effective in extracting the most information from all sensors involved.
RADIO OCCULTATION ADVANTAGES
On the left is one of the first GNSS-RO temperature profiles ever recorded, acquired by GPS/MET on April 5, 1995 just one day after launch. The RO profile is compared against a nearby weather balloon (radiosonde) profile and a weather model estimate from the European Center for Medium-range Weather Forecasts (ECMWF). Note that the model profile cannot capture the detail of the other two. (GNSS retrieval by JPL, 1995)
The red dots in the image on the right are the locations of the world’s weather balloon launch sites. Each site lofts a balloon every 12 hours at 0:00 and 12:00 universal time. The green dots show the 1-day GNSS-RO coverage with six COSMIC satellites, demonstrating how even a small constellation provides considerably better worldwide coverage. (Illustration by COSMIC Project, UCAR)
In addition to atmospheric data, GNSS-RO provides highly accurate data on the Earth’s ionosphere, the site of space weather. Electron density profiles can be combined tomographically to generate accurate 3D maps of the global electron distribution updated every few minutes. This can be invaluable in monitoring and eventually predicting hazardous geomagnetic storms caused by solar activity. (Fig. from Mannucci et al., 2nd GPSRO Users Workshop, Aug 2005.)
HISTORY
Mariner IV approaching Mars – July 15, 1965
July 15, 1965. Mariner IV was transmitting data as it was coming in for the closest approach around Mars. In the moments as it disappeared behind the planet, the transmitted signal, as viewed from Earth, passed through the thin Martian atmosphere, from its upper reaches down to the surface, executing history’s first planetary atmospheric and ionospheric radio occultation experiments. Minutes later as the signal reappeared on the other side of the planet, the first rising occultation was observed from Earth.
Subsequent occultation experiments were performed at Venus and the outer planets, Jupiter and Saturn Uranus and Neptune, and many of their moons. In fact, most major bodies of the solar system were studied by radio occultation before we began to look homeward. For radio occultation to be of use in studying the environs of Earth, which we can observe at close hand by other means, we need many signal sources in space, a tall order.
The first proof-of-concept experiment, called GPS/MET (for GPS Meteorological experiment) was launched in April 1995 and was a breakthrough success for the GPS-RO technique. NASA followed by sponsoring five JPL-led GPS-RO experiments on five different international missions from 1998-2005. These missions included Ørsted (Danish), Sunsat (South African), CHAMP (German), SAC-C (Argentine), and GRACE (US-German).
In April 2006, the pioneering six-satellite COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) mission, sponsored primarily by Taiwan’s National Space Organization and managed in the U.S. by UCAR, was launched on a Minotaur rocket from Vandenberg AFB, becoming the first dedicated constellation for demonstrating the operational use of GNSS-RO data in weather forecasting.
Due to the successful yet aging COSMIC program, U.S. agencies and Taiwan developed a follow-on GNSS-RO mission called COSMIC-2. Six COSMIC-2 satellites were launched successfully on June 25, 2019 into low-inclination orbits.