This is the second of a three part series about parallax problems with satellite data. Part 1 presented an overview of parallax in this context and reviewed differences in orbital characteristics of
geostationary and polar satellites. In this section we’ll examine more
closely parallax issues associated with geostationary satellites. Part 3 will cover the topic as it relates to polar satellite data.
As discussed in the previous post, a satellite must be very high in order to orbit at a rate equivalent to the earth’s rotation, making it geosynchronous. The diagram above shows the scale of this orbital distance relative to the diameter of the earth. “Nadir” is the point on the globe where a satellite is looking directly downward. For geostationary satellites, the longitude may vary, but the latitude will always be over the equator. Since the orbital distance is very far from earth, the viewing angle “off-nadir” initially changes slowly with increasing latitude, but then increases significantly near the edge of the visible earth disk. As a result, above 50 degrees north (and south) parallax displacement error grows significantly.
The parallax offset problem is not restricted to the poles and can occur near any edge of the full disk view of the earth. However, it is mitigated somewhat in lower latitudes by an array of geostationary weather satellites positioned strategically around the equator. While this provides most non-polar locations with narrow viewing angle options, it does not completely eliminate it. Here is an interesting blog from CIMSS that presents an example of parallax displacement differences in the mid-latitudes between three geostationary satellites.
The United States has
two primary Geostationary Operational Environmental
Satellites (GOES) in operation: GOES-West over the eastern
pacific, and GOES-East over the eastern US and South America. Other
geostationary satellites are operated by Japan, the European Union
(EUMETSAT), China, Russia, and India.
In October 2016, the first in a series of new generation geostationary satellites, GOES-R,
is scheduled for launch. The
advanced technology on this satellite will significantly improve detection and
observation capabilities, directly affecting public safety and
economic prosperity. Sensor capabilities on GOES-R will
be very similar to Japan’s newest geostationary satellite,
Himawari-8, that is already in orbit around 140° east. The example
below is a “true-color” image from Himawari-8 in which the Australian continent is near the center of the southern hemisphere, while the Bering Sea and Aleutian Islands are near the far northern edge.
resolution with the new GOES-R satellite will be four times greater,
it is important to remember that parallax will continue to be a problem in polar regions. The graph below is an easy way to estimate the
effect of parallax cloud displacement in geostationary satellite
The “Normalized Cloud Offset” (NCO), plotted as a function of latitude, is the ratio of the northward cloud displacement to the height of the cloud (in equivalent units). For example at 60 degrees in the graph, the NCO is around 2.5, so a cloud top of 20,000 ft (or 3.8 mi) is offset 9.5 miles (3.8 mi x 2.5). The direction of the apparent offset, is to the north (away from the the satellite) along a great circle arc from satellite’s nadir point at the equator. From the graph you can see that displacement error grows exponentially north (or south) of 50 degrees in latitude.
In the northern latitudes, polar satellite data can add critical information about the location and structure of weather features, since the center of a pass has virtually no parallax displacement, and resolution is significantly improved. Below is a comparison of two Infrared (IR) satellite images of convective clouds near Chichagof Island in Southeast Alaska, one from GOES-West, and the other from a NOAA polar satellite passing overhead around the same time.
In addition to the obvious resolution differences, the polar data shows that GOES has a convective cell
incorrectly displaced 6 miles to the north. From the Normalized Cloud Offset curve, at a latitude of around 58° N the NCO is around 2.0, but displacement is dependent cloud top height as well as latitude. This height is not known but can be inferred from the cloud top temperature measured by the satellite, which is around -35° C. A nearby atmospheric radiosonde sounding at that time (below) shows temperatures in that range to be around 5000 m or 3.1 mi. Multiplying 3.1 mi by the NCO of 2.0 gets a displacement of 6.2 mi as verified by the two images.
Here is another example a little farther north over the Kluane and Wrangell-Saint Elias regions. In this case the displacement error is much greater, around 14-16 miles, because the clouds are located farther north and the tops are much higher. In the far northern regions of Alaska, the displacement error is so great that it is often difficult to quantify.
For northern latitudes, the high frequency of geostationary data is invaluable for tracking the movement and evolution of cloud features, however the displacement of these features due to parallax should always be taken into account. When following critical weather events, comparisons between geostationary and polar satellite data can be an important exercise in order to to correctly determine the location of affected areas.
- Carl Dierking
Parallax is the
apparent difference in the location of an object based on point of
view. With satellite data this is always something important to
consider, because the features you see may not actually be where they appear
to be. This is the first of a three part series focused on the parallax problem. This introduction will review orbital characteristics of the two most common types of meteorological satellites with very different parallax issues. The next two parts will describe those issues in greater detail.
As depicted in the illustration below, the apparent position of clouds can be a bit different than their actual location depending on the angle of view. Also, at large viewing angles high clouds are displaced a greater distance than low clouds, so the apparent size of a feature can be exaggerated. In a similar way, an increase in the viewing angle reduces resolution due to stretching of the sensor footprint.
In order to account for parallax, it’s necessary to understand the orbital characteristics of the two basic types of satellites for meteorological data, geostationary
and polar, since that is what determines their viewing angle over Alaska.
Geostationary satellites (named GOES in the US) are centered over the
equator and set in orbit at the same speed as the earth’s rotation
(geosynchronous) so they can continuously view the same portion of
the globe. To do that they must be at a very high altitude,
roughly 22,000 miles. The advantage of this is the ability to precisely monitor cloud
motion at very high temporal frequencies. For example the new GOES-R
satellite being launched in late 2016 will be capable of routinely
providing imagery over the same location at 1 min intervals. However, one disadvantage over Alaska is the large viewing angle.
The full disk Infrared (IR) image above is an example of the earth
view as seen by the current GOES satellite with Alaska very close to
the northern edge.
Mapping that GOES IR imagery to a polar stereographic view focused on Alaska (above) doesn’t actually look that bad from a distance, despite the large viewing angle.
But zooming in closer (above), you can see how cloud features get very distorted over northern Alaska and the Arctic. This is also where cloud displacement due to parallax becomes very large. With the new GOES-R satellite, resolution will improve by four times, however, parallax over the state will still be a problem.
Polar satellites orbit over the poles at a substantially lower altitude, around 850 miles, and at a much faster speed. It takes around 90 minutes for a polar satellite to complete one full orbit around the earth. As the earth turns below it, the swath of each pass covers an area just to the west of the previous pass. There is usually some overlap with successive passes that is dependent on latitude and the width of the swath. Polar regions have more frequent overlapping coverage since the orbital tracks always pass near the poles.
The example above is an SNPP VIIRS IR image from around the same time as the previous GOES IR image. This is the first of a new generation of satellites from the JPSS satellite program with
significant improvements over the previous polar fleet. The next of
these new satellites, JPSS-1, is scheduled for launch in early 2017.
Polar satellite imagery instruments typically scan around 50-60 degrees to the left and right as they pass over the earth. The center of the
scan along the track of the satellite (nadir) looks straight down, so
this portion of the imagery has virtually no parallax displacement.
Also, because of the lower orbit, these satellites will have much
higher resolution in the polar regions.
This close up of the SNPP VIIRS IR image (above) is at the same scale as the previous GOES image, but it reveals much greater detail over northern Alaska and the Arctic. Since the satellite is looking more directly downward, we can expect little displacement of clouds and other features due to parallax. However, that would not be the case near the edge of the scan where parallax increasingly becomes an issue.
So, both types of satellites have some amount of parallax displacement in locations where their viewing angles are large, which is a function of their orbit. The next two parts will focus on each of these types in greater detail.
GINA has developed a series of “Quick Guides” to inform users about how the various kinds of satellite imagery and other satellite products GINA produces can be used in analyzing and forecasting Alaska’s weather. While the intended audience of these Quick Guides is forecasters with the National Weather Service in Alaska, the information is interesting for a broader audience, and the examples of imagery over Alaska are great to look at.
Above is the Quick Guide describing the use of imagery at the 1.61 micron wavelength. Conventional visible satellite imagery cannot tell the difference between sea ice and clouds, because both appear white. But with 1.61 micron imagery, identifying areas of sea ice and snow-covered ground that have clear skies above, and identifying areas covered by clouds is much easier. Such distinctions are very important for pilots and mariners.
Imagery from the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument aboard the Suomi National Polar Partnership (SNPP) satellite helps National Weather Service (NWS) forecasters in Alaska better analyze and forecast a variety of weather phenomena, including the behavior of sea ice, storms threatening Alaska’s coastline, and summertime wildfires over Alaska’s interior, just to name a few.
Above is an example of VIIRS imagery being used to distinguish areas of clouds from areas of sea ice back in July of 2014. Notice how up over the Arctic Ocean clouds appear pink while the sea ice appears blue. Areas of ice-free ocean are black.
A paper has just been published in the journal Remote Sensing, “User Validation of VIIRS Satellite Imagery,” with contributions from GINA and NWS Alaska. Check out pages 6 through 16 of the paper to see several of examples of VIIRS imagery (including the example above) helping meteorologists wrestle with Alaska’s complex and often awe-inspiring weather patterns and provide the best possible weather forecasts and warnings to the Alaska public.
Satellite-based sounder data from the Suomi National Polar Partnership (S-NPP) satellite are
available at National Weather Service (NWS) offices in Alaska. GINA receives the data from the satellite via a pair of antennas in Fairbanks, processes the data, and then delivers the resulting imagery and products to the NWS. All of this is done with minimal latency, so NWS meteorologists have the imagery and products as soon as possible for use in generating weather forecasts and warnings.
As the S-NPP satellite passes overhead, it’s Cross-track Infrared Sounder (CrIS) collects data at a series of points, and those points appear as green dots on the NWS display software, as per the screen capture above. When a forecaster clicks one of the dots, the display changes to show a profile of temperature and humidity for that specific location, as shown in the image below.
date, the emphasis in Alaska has been to use these sounding data to identify areas of extremely cold air aloft (colder than about -65 degrees Celsius, or -85 Fahrenheit) which can gel the fuel in jets transiting the arctic and thus poses a hazard to aviation.
For the future, there are plans also to use these soundings during Alaska’s summer wildfire season. The vast majority of wildfires in Alaska are started by convective storms, including “dry lightning” thunderstorms with bases high above the ground that produce significant numbers of lightning strikes but very little rainfall. Satellite-based soundings can assist meteorologists in identifying and forecasting the convective environment and serve as as a useful supplement to the conventional weather balloon network. Alaska’s great advantage is that, thanks to its high latitude, it receives more polar satellite passes per day than any other part of the country. Given that other observing systems (such as radar) are sparse in Alaska, there is a motivation and a need to make the most of the advantage polar-orbiting weather satellites provide.
More information about the S-NPP satellite and the follow-on series of Joint Polar Satellite System (JPSS) satellites can be found at http://www.jpss.noaa.gov/
Photo by Greg Wirth