Remote Sensing in the 21th Century - Completely Remote Sensing, GPS, and GPS Tutorial
Remote Sensing in the 21th Century

At the outset of this Tutorial, at the top of the Overview2 page, a graphic prepared by the author of this Section showed likely earth-observing remote sensing satellites expected to be operational between 1995 and 2005. That illustration is reproduced here:

Launched and approved or proposed satellites used in land-centered earth-observing satellites.

While it is probable that the reality will not be as bountiful as the graphic indicates, it is certain that the number of high resolution satellites in operation will be significantly greater than before 1995. This Section builds upon that graphic by concentrating on the high resolution systems currently planned to be operating after the year 2000.

The systems can be usefully classified into four groups, (a) Broad area coverage, 5 to 30 meters resolution and multiple color bands, (b) Narrow swaths, 1 meter or less panchromatic resolution, and VNIR color only, (c) Hyperspectral sensors with 30 meter resolution and (d) Radar with 5 to 10 meter resolution. Their capabilities are described and compared in detail, including their spectral bands and resolutions, and their coverage capacity.

Purpose

Thus, this Section provides a brief overview of the explosion in land observing satellites planned for the first decade of the 21st Century and is intended as a wake-up call and planning tool for all who are interested in knowing and keeping track of the details of what is going on with the surface of our planet and in particular for those who are developing the skills to measure and understand the breadth and detail of the information that analysis of the satellite data could make available to us for the first time. The amount and quality of the land information data which the land observing satellite fleet in 2000 will be capable of providing could revolutionize both our scientific knowledge and our practical management of our Earth's resources. The satellites are however only the first step. Their value can only be realized through the ingenuity and efforts of the users.

The Satellites

Among the many predictions for the new millennium are the orbiting of at least 31 satellites by the year 2000 in polar orbit providing land cover data at resolutions of one to thirty meters. These satellites are summarized in the first chart.

Satellite names of the principal vehicles actually in or planned for orbit and launch dates from 2000 through 2006 are summarized by the next graph.

A similar chart shows the historical span of actual satellites that are primarily land observers which are now in orbit at the end of 2002, or once were operational (failed or decommissioned):

Chart showing land observing satellite history between 1972 and 2004.

This next chart, added by the principal writer (NMS), extends the time line to the year 2011:

Land observing satellite planned through 2011.

Landsat-like: The Landsat-like satellites have the middle resolution, broad area and multispectral coverage characteristic of the current satellites, Landsat, SPOT and IRS. These current programs are being extended and expanded. As can be seen in the Indian program, plans for the flight of four satellites through this period are the most operationally robust of the government group. The group will be joined by two satellites created by a cooperative program between China and Brazil and one, four satellite, private system.

High Resolution: The twelve high resolution systems will provide an order of magnitude improvement in ground resolution, at the expense of less area and multispectral capability. With the exception of one Indian and one Russian satellite, these satellites are all funded and operated by private corporations. The almost exclusive interest of the private sector investors in the high resolution systems indicates their belief that this is the space capability required to create commercially valuable information products.

Experimental (Hyperspectral): The three government funded hyperspectral satellites and the proposed private system will explore the potential for the development of new multispectral analysis based applications by providing near continuous radiometry over the visible, near IR and short wave IR spectrum.

Radar:The current Canadian and ESA radar programs will be continued into this period as well. Radar's all weather capability makes it the instrument of necessity for many observational problems and it will become increasingly valuable for general problems as better techniques for analysis are developed, including the integration of radar and optical data.

Technical Overview

The best way to understand the scope and variety of the data which will be available from the new millennium fleet is to look at the three principal observational dimensions of its data, ground resolution, land coverage frequency and spectral coverage. They are tied together in sometimes unfortunate ways, (from the user's point of view), by the laws of optics, orbital mechanics and the ultimate decision maker, economics. No one system can provide all the measurement features needed by the user community.

This review presents three summary maps of the data scope and variation which will be provided by the 31 satellites; land coverage and ground resolution, the spectral position of measured bands and the ground resolution of each band.

Land coverage and ground resolution: All but two of the satellites will cover the total land mass since they are in polar sun synchronous orbit. The two exceptions are SPIN-2 which is in a 65 degree orbit and QuickBird for which a 52 degree orbit inclination has been chosen.

Land coverage frequency must be considered in two ways: the frequency with which the system can provide images of the total globe, and the time it takes to revisit a given site. Because global coverage frequency is inversely proportional to the sensor's ground field of view or swath width, this parameter will be presented as one measure of coverage capability in the following graph which presents the ground resolution and the ground swath width for all of the satellites noted above.

The above plot provides a graphic illustration of the difference in coverage and resolution between the four classes of satellites. Besides the radar satellites, two satellites fall outside the boxes: The IRS C,D Pan sensor flies on a satellite that is in the Landsat-like box, but lies away from that category because it sacrifices swath width for its higher resolution. However, it can be pointed off the orbit path which allows 2 to 4 day revisits to specific sites. SPIN-2's escape from its box is described below.

The next illustration triplet shows with color bars the repeat frequency of some of the major satellites. Now refers to 1997. These show the number of the Landsat-like satellites that you could see (or more precisely the number of satellites that could see you at an equatorial [1; refers to notes at bottom of this page] site) on any day over a randomly selected 100 day period for the three satellites now in orbit, for the 8 government satellites in orbit in the year 2000, and for the 12 satellite fleet [2] resulting from adding the four Resource21 birds. The aperiodic nature of the second plot cries out for effective international cooperation to optimize the spacing of the coverage opportunities. (There is no indication that this is likely to happen).

The Landsat-type satellites are designed to provide fairly frequent global coverage by choosing the sensor ground swath and orbit parameters so that they will cover the complete equatorial surface each orbital repeat cycle. The current and planned satellites achieve this by having ground swaths between 120 and 200 kilometers. Their orbital periods and thus global coverage times, vary from 16 days for Landsat to 22, 24, and 26 days for the Indian, French and China/Brazilian satellites. Taken singly, even these repeat cycles are too long for many applications. However, the similarity of the sensor data from the 12 satellites in this group can, for many applications, make it possible to use the data from all of the satellites interchangeably and thus have available the one to seven day coverage rates illustrated in the figure above.

The High Resolution Group: In contrast to the Landsat-like group, half of this group has limited multispectral coverage, while the other half has none at all. It is obvious that as a group the critical measurement is the ground resolution which is essential for identifying man-made objects and for updating maps and GIS data bases. Whereas in the Landsat-like group the pan bands are used to sharpen the color bands, in this group the color bands will probably be used to add additional information to the pan band data. The much narrower ground swaths of the high resolution sensors, 4 to 36 kilometers, can only achieve total global coverage in periods ranging from 4 months to 2 years. Since the high resolution sensors being planned generate communication rates between 20 to 100 times that of Landsat this design limitation is caused by the practical and economic limits of the data collection systems. SPIN 2 avoids this problem since its data collection system is film return which places it in its unique position on the chart. However, for many users the good news is that the satellites are designed to be capable of quickly pointing off nadir and thus can see any given site in 2 to 4 days. Thus, even two high resolution satellites properly synchronized could provide daily repeat coverage nearly anywhere.

WARNING-Clouds severely effect the above quoted repeat times:

The above discussion of the repeat times should not be used without at least doubling the numbers quoted to provide some sense of the effect of clouds on the actual ability to get cloud free images, i.e. to actually see the desired targets. The above illustration presents the results of a simulation which recorded the best cloud free percentage images for each Landsat WRS site [3] collected over a 16 day period in early spring by using one, two, three and four satellites orbiting over a WRS grid containing the % of cloud cover in 5% increments for every day of the year [4].

The fact is obvious, our planet is habitually cloudy and the clouds obstruct satellite land views more than we would like for many of our time critical applications. The message is equally plain, multiple satellites (or radar) are required if we need assured land coverage in short time periods of weeks to months. (Note the four maps also represent very closely the collection capabilities of one satellite for 16, 32, 48 and 64 days.) As the maps make evident, the problem is geographically focused and as would be expected the agricultural belts, where frequent data are most required, are the cloudiest. It remains to be seen whether the small target areas and pointability of the high resolution systems will provide higher cloud-free data returns than those calculated for the large area non-pointing systems illustrated above.

Radar: The current and proposed radar satellites can provide data in a variety of resolution, swath combinations. The values on the figure represent their high resolution capabilities. Again, the practical limits on data rate have been an important factor in their resolution/swath tradeoffs.

Spectral Coverage and Ground Resolution: The illustration below provides the resolution of each band on a number of satellites (not all included in the previous illustration). (The bands are listed under their Landsat 7 band counterparts).

Band distribution on satellites with better than 30 m resolution.

Many of the bands on the above satellites are close to those used by Landsat. This is of course because the Landsat bands were placed in nearly all the wavelength windows free of severe atmospheric absorption. The Landsat-like satellites emphasize multispectral coverage, all of them having at least the lower SWIR band and three including both the upper SWIR and TIR bands. ASTER will provide even greater spectral definition in the upper SWIR and TIR regions.

It is important to note that all multispectral data may not be equally usable for all applications even when the same bands are available. For analysis that are critically dependent on measuring the absolute reflected radiation over years to decades, sensor calibration becomes a critical parameter. Landsat 7 and Resource21 systems will have Sun and Moon based calibration capabilities while the other systems will rely on internal lamps and ground targets for their calibration. Equally important to such applications is the ability to adjust the measured radiation for the varying atmospheric conditions. NASA is planning to operate Landsat 7 and AM-1 (Terra) in very close proximity to measure the atmospheric input using an AM-1 sensor (MODIS).

As shown in second figure above the multispectral resolutions range from 10 to 30 meters with the exception of the 6 meter sensor on IRS-P5 and 2A which achieve their higher resolutions by reductions in swath width. The panchromatic sensors of interest in this group range from 6 to 20 meters. Experience with integrating the 10 meter panchromatic data and the 20 meter multispectral data from SPOT has shown the value for many applications of the use of the pan band in sharpening the color bands.

The Hyperspectral Group: The Hyperspectral Group: The US government is launching several satellites to test the full potential of multispectral analysis for the identification of both man-made and natural surface elements. Because of the very high data rates required by the hyperspectral sensors, the resolution of these systems has been restricted to 30 meters. There is also a sense that 30 meters may well be more than sufficient to characterize the majority of at least the natural targets, i.e. mineral and vegetative cover. The Australian government is stimulating interest in the private sector for the commercial development and operation of a near hyperspectral system, since the sensor uses two groups of 32 bands instead of the spectrometers of the other systems. The hyperspectral satellites are being flown to explore the potential of using the full spectral response over the VNIR and SWIR spectrum. Note on the last figure above that hyperspectral is being defined as sensors with 32 to 256 bands per VNIR or SWIR range.

Radar: While the current and planned radar satellites will have only one frequency, they do have several polarization options and thus have a multidimensional analysis possibility analogous to the optical system's multispectral analysis.

Data Availability: The good news is that there are plans for more than 30 satellites capable of providing a wide range of land data information products. The amount, sophistication and variety of the land data that could be available for analysis is staggering. It is probably equally good news that all the data, government and private, except Landsat 7 data, will be available commercially at market determined prices. It's even better news that current US law requires that Landsat 7 data be made available to all at the " cost of furnishing user requests". However, to be available the data must be first acquired. Landsat 7 is the only system which plans to acquire and archive multiple total land cover data sets each year. The other government systems will be collecting for their own purposes and for orders acquired by their commercial sales outlets. The private systems' acquisition plans will be totally market driven.

Why So Many Satellites?: The large number of planned satellites may seem to be more than a few too many for needs of the Earth observing community. Before making that judgment however, it may be useful to consider the following points.

As noted above, none of the planned satellites will provide all of the data characteristics needed by the broad range of user requirements. Thus at least four systems would be needed to provide the different data types the fleet is currently planning. The day of the Battlestar Galactica - single satellites with suites of many instruments - appears to be over [5].

The need for multiple satellites was also discussed in the subsection on coverage frequency, which emphasized the negative effects of the world's (on average) 50% cloud cover. Resource21 is planning a four satellite system to meet their customers need for weekly observation of crop conditions. The Global Change Science goal of global of seasonal coverage requires a minimum of three to four satellites. The use of satellite data for disaster analysis and relief planning can be very effective but only if the satellite can acquire imagery almost immediately after the event, a possibility only if two or more pointable sensors are in orbit. For weather related disasters, radar is often the only system which can see the ground. Again multiple radar satellites would be required for sufficiently rapid coverage.

There is also the need to assure operational stability. Several land observation satellites have failed to make orbit - for example Landsat 6 and SPIN-2 - and two failed prematurely after attaining orbit - SPOT 3 and ADEOS. Obviously more than one system must be available to provide the operational assurance required if users are to be able to make the data a requirement for their activities. India, EarthWatch and Resource21 are planning operationally robust systems of four satellites each. CBERS and all of the high resolution satellite providers are planning two systems each.

As of late 2010, Landsat-5, 22 years in operation, is still functioning but is expected to cease usability at any time. Landsat-7 also is working but its images are somewhat compromised by scan line problems. In 2006, NASA announced an agreement with the U.S. Geological Survey to make certain U.S. scenes made by Landsat-7 available for download at no cost to the user public. Some information pertinent to this is to be found at this USGS website.

In retrospect, the Landsat series has proved to be and still remains the "creme de la creme" of earth-surface observing satellites. Despite the ever-growing competition, the Landsats are the prime systems for filling a vital niche: wide area coverage with multispectral sensors that provide data well suited to a variety of image processing procedures, including classification. The remote sensing community has repeatedly called for a policy that guarantees continuation of Landsat-style coverage. Commercial satellites are offering some appropriate data systems that provide this. An unsuccessful attempt spanning several years to privatize Landsat forced a reversion to a joint operation by NASA and the USGS, as mandated by a 1992 congressional law. The archived data from all the Landsats, going back to 1973, constitutes a valuable data source for environmentalists, planners, scientists and others. The user community has called for the continuation of the Landsat program so that there will be an unbroken data set for this format. This has resulted in what is called the Landsat Data Continuity Mission (LDCM) which would involve some Landsat type system. Contracts have been let for Landsat-8, whose launch will be no earlier than December of 2012. Here is a picture of this satellite; below is a table that specifies its sensor capabilities:

Landsat-8
Landsat-8's sensor specifications.

Some Further Thoughts

The goal of this Section is convince the land information user community and especially the so-called "value added" experts in industry and academia, that their cup is about to runneth over. The satellites are really coming, though probably not in the numbers presented in this Section. Roughly half are government funded and most of these are in or on the path toward construction. If only half the proposed commercial satellites make orbit there will be 20+ satellites in orbit by the mid-2000s.

However, checking the above launch schedule diagram with actual history will reveal that most satellites, whether government-supported or privately funded, tend to slip their launch dates, a few fail to reach orbit or otherwise function, some may be renamed, some are abandoned or modified, and new satellites are proposed. It is hard to keep such charts current and accurate, as programs fluctuate or are revised. One way to stay updated is to visit the site maintained by SpaceFlightNow; this Tracking Schedule includes satellites of all kinds and purposes.

The primary writer (NMS) has attempted to keep track of current and planned missions throughout the Tutorial. This has proved a daunting task, largely because the sheer number of satellites being considered. In July 2007, the writer found this chart which, though comprehensive, is a good example of how obsolete and uncertain these overview summaries can be:

One of several schedule charts summarizing earth-observing missions in the 2000s.

This table includes some of the satellites used in meteorological applications. To highlight this group, here are two additional charts that specify operating and planned missions in this category:

Polar Metsat schedule
Geostationary Satellite schedule.

The really big bucks, literally billions, required to create the earth-observing satellite systems are being spent by both the public and private sectors. It is now up to the users, public and private to invest in the development of the analysis technologies, the information products and the applications that will generate the dollars that will keep the new millennium satellites flying. The question is are you, they, anybody ready for the deluge?

Since the opening of the exploration of space by satellites, orbiting telescopes, and human-occupying spacecraft, hundreds upon hundreds of individual missions have been launched - most successfully. This will continue into the future. Elsewhere in the Tutorial, we have cited websites that describe past and current missions. One that summarizes many, but not all, NASA-related past, current and future missions, some pertinent to earth-observing satellites, can be found at this JPL website. The RST creator (NMS) has not found a single Internet site that gives a complete and comprehensive listing of all future planned (but not necessarily funded) missions. But here are three websites that each consists of a partial list: Welcome to the Future; RedOrbit (Space Missions in Development); and The Planetary Society.

Now, with this background move on to the next page which treats concisely what you have already been familiarized throughout the preceding Sections of the Tutorial: namely, the start of the Era of Commercialization of Space.


NOTES

  1. Since polar orbits cross near the polls and have constant width ground swaths, the ground overlap between orbits increases with latitude. At 60 degrees the overlap is 100% and thus the equatorial coverage rate doubles.
  2. I have taken the liberty in the third plot of assuming that the two Indian satellite series will eventually be planned to have the same orbital period in order to create the advantage of a total periodic period of 6 days in place of the aperiodic 11 and 12 day periods currently quoted (which also assumes that the two satellites of each series are placed in orbits halving their 22 and 24 day return periods.)
  3. The maps are the world as seen by the constant swath Landsat image and thus are greatly distorted at the higher latitudes. The Landsat World Reference System (WRS) maps the world in 30,107 185x170 kilometer squares.
  4. The WRS cloud data were created by the Air Force from a global data set for the year 1977 and represent the cloud coverage at the 9:30 AM local crossing time of the Landsat satellite.
  5. It is however worth noting that four of the Landsat-like systems, SPOT, IRS, CBERS and AM-1, do carry one or two other wide field of view sensors to provide daily to weekly coverage to supplement their main sensor data.

Source: http://rst.gsfc.nasa.gov/