| DEMS
Qualcomm Uses Russain SPIN-2 DEMs
Cellular antenna site is enhanced using high-resolution imagery and proprietary software
By Kevin P. Corbley As one of the world's leading providers of cellular telephone services, Qualcomm Inc. is continually employing new technologies to improve the quality of its calls and to gain an edge in an incredibly competitive international market.
The San Diego-based company recently joined a growing list of telecommunications firms utilizing a new source of geospatial information for cell site design. For a project in India, it is using highly accurate digital elevation models (DEMs) created from SPIN-2 satellite images. These DEMs boast 10 meter postings and are derived from true stereo-image pairs with 80% overlap.
Referring to SPIN-2 as a new source of geographic data is a somewhat misleading statement due to the existence of archive dates from 1981 to the present and the commercial availability of the data for more than a decade. The emergence of SPIN-2 as a major player in the commercial market, however, has occurred only in the past two years.
"The Russians first began selling the imagery in 1986, but what is new about SPIN-2 is that a major marketing initiative is underway with a substantial capital investment from private industry which has allowed for a significant improvement in quality and delivery time," said Peter M. P. Norris, president of SPIN-2 Inc. in Washington, D.C.
In 1995, Aerial Images Inc. of Raleigh, North Carolina, and Central Trading Systems, Inc. of New York finalized an exclusive data distribution arrangement with Sovinformsputnik, an organization designated by the Russian Space Agency to commercialize its programs. The deal enabled Aerial Images to reserve acquisition time on SPIN-2 missions and to sell its high-resolution images and derived DEM products. A year later the North Carolina firm formed SPIN-2 Inc., a wholly owned subsidiary, to manage worldwide marketing and distribution of the products.
Since then, several major high-tech organizations have become involved in the SPIN-2 marketing initiative. Most recently, Microsoft Corp., Digital Equipment Corp., and Kodak have joined in developing TerraServer, a huge on-line atlas of SPIN-2 images, USGS maps, and other GIS data. The primary selling points of the SPIN-2 data are its spatial resolution and cartographic accuracy. The dual-camera system acquires photographic images at 2 and 10 meter (m) resolution, which telecommunications companies have been tapping into for extremely detailed land-cover information and highly accurate DEMs for cellular network design.
"Telecommunications is the most rapidly growing segment of our client base right now," confirmed Norris. "Sales in this industry account for about 10% of our total revenues." Designing Cell Sites in India
In the past five years, satellite imagery has become a major source of map information used in the layout of cellular antenna sites and service areas. GIS software specifically designed for this purpose determines the best locations, positions, and power outputs of transmission antennas based on calculations of radio frequency (RF) propagation in a given project area.
The RF signals which carry a digital or analog cellular phone transmission are predictably attenuated over distance depending on the type of land cover over which they travel. Hills and valleys also play roles in the transmission path by blocking and deflecting signals. Cell site design software takes these land-use and topographic features into account in complex mathematical computations that model signal paths.
"We typically rely on satellite image data for international projects where mapping information isn't current, available or reliable," said Bob Davidson, a technical support specialist for Qualcomm's QEDesign software. "Imagery is preferred as a visualization tool by network planners because it shows what is really happening on the ground."
Qualcomm uses the QEDesign package to design new cellular service areas and to convert existing sites to a new digital cellular technology called CDMA. The software is now being used to build a new system in Jaipur, India, a city where the most recent maps are twenty to thirty years old.
Davidson called upon Horizons Technology Inc. of San Diego, California, to obtain map data for the India project. HTI had worked with Qualcomm in the past acquiring a variety of geospatial data products and was experienced in selecting remote sensing data. It suggested the SPIN-2 data for the Jaipur project.
"The SPIN-2 DEM is superior to others on the market because it is photogrammetrically produced," said Andrew M. Nickerson, director of business development at HTI. "And it is very accurate due to the currency of the image data."
HTI had run a scene search of the SPIN-2 archive and found a recent 10m resolution image of Jaipur. DEMs with 10m accuracy are offered by SPIN-2 at a cost of $24 to $30 per square kilometer (km), depending on the severity of the terrain. They are made at a Sovinformsputnik production facility in Moscow.
"We decided to try the SPIN-2 data because it was available quickly and it was inexpensive compared to others on the market," said Davidson.
The imagery arrived within two weeks, and the DEMs were delivered about two weeks after that. Qualcomm extracted 8 land-cover layers from the rectified images so its design software could calculate ground-signal interaction and user density statistics based on land use.
The DEM data came formatted for GIS use. Qualcomm engineers loaded the data sets into the software where it modeled potential RF signal degradation and interruption over distance in the hilly terrain in the Jaipur area.
The Jaipur cellular system is now under construction, and Qualcomm engineers are impressed with the quality and accuracy of the DEMs (see sidebar for more on DEMs).
"The SPIN-2 DEMs are composed of 10m postings and are superior to the DEMs available from SPOT Image, which have postings ranging from 20-100m," said Ken Ferguson, vice president of SPIN-2.
Posting refers to the density of the elevation points in the DEM. One with 10m postings, for example, has an elevation point measured every 10m. This is considered a dense posting pattern, which is attractive to cell site designers because it yields a higher degree of vertical accuracy.
"Accurate DEM data is important in cell site design because it translates directly into cost savings in terms of placing infrastructure," said Davidson. "If the computer-generated model is precise, it means less time in the field fine tuning the design, which reduces labor and equipment costs."
An exact computer model can potentially allow a telecommunications company to build fewer antenna sites or base stations, which can each cost between $2 and $8 million.
"That's the difference between winning and losing a contract," added Davidson. A New Spin on Acquisition
The accuracy of the SPIN-2 DEMs and other map products relates directly to the unique design of its imaging system. Unlike most other commercial remote sensing platforms, the SPIN-2 satellites carry photographic cameras instead of electro-optical sensors, which offer advantages in many cartographic applications.
Based on a Comet Class spacecraft equipped with both a TK-350 and KVR-1000 camera, the satellites are launched periodically from the Baikonur Cosmodrome in Kazakhstan and orbit the Earth for about 45 days collecting up to 10-million square km of images. The entire satellite returns to Earth for film processing.
Photogram-metric experts designed the two cameras to function either independently or in unison as a single mapping system. The KVR-1000 acquires extremely precise panchromatic photos with 2m spatial resolution, a 1.56m pixel size, and a 40 x 160km scene. These are used primarily for feature extraction applications in urban planning, parcel mapping, and agricultural monitoring.
TK-350 is a topographic camera developed specifically to collect stereographic images for determination of ground relief. Resolution is 10m and the scene size is 200 x 300km. In another departure from traditional imaging systems, the TK-350 acquires fore-and-aft stereo pairs with an 80% overlap.
"True stereo imaging is one of the main advantages of this camera," said Peter Norris. "This means it acquires true stereo images simultaneously instead of days apart."
He explained that this greatly enhances the quality and accuracy of the stereo pairs and the DEMs created from them because the overlapping images are acquired under identical atmospheric and ground conditions. There are no differences in haze, sunlight, vegetative growth or soil moisture to distort terrain measurements extracted from the two overlapping frames.
From a photogrammetric perspective, additional design features also contribute to the accuracy of the TK-350 camera.
"The accuracy of [DEM] ground points significantly depends on the ratio of the photo base to the shooting height at the moment the image is acquired," said Victor Lavrov, Deputy Director General of Sovinformsputnik. "Since [the satellites must orbit at 220km], the only way to increase the base/height ratio is to increase the frame format."
This is exactly what the Russians did with the TK-350 camera. Its frame size is a relatively large 300 x 450mm with a 350mm focal length. With the stereo overlap of 80%, the camera achieves a base/height ratio of 1:1, which results in extremely accurate ground points being extracted from the stereopairs.
Sovinformsputnik produces 1:50,000 topographic maps from the TK-350 photos. When no external ground control is used, the planimetric accuracy of these maps is typically 20-25m, root mean square, and the vertical accuracy is 10m RMS. If GPS derived control points are available, the accuracy of the 1:50,000-scale products increases to 15-20m horizontally and 5-7m vertically.
"TK-350 stereo pairs are available for about 60% of the Earth surface," said Ferguson, "and we can make DEMs for any area in the stereo-pair archive."
The most recent SPIN-2 mission was launched on February 17, 1998, and completed on April 3. Most of the photos acquired on that mission were ordered in advance by commercial customers. A Future in Telecommunications
A few years ago, the remote sensing industry feared the use of satellite images in cell site design would run its course due to the fact that eventually most major urban areas would be mapped and have cellular communication systems installed.
For several reasons, however, just the opposite is happening. The interest in satellite imagery has increased because wireless communication systems are no longer limited to urban areas. They are being installed instead of traditional telephone service in many rural and undeveloped areas around the world.
"Another thing we are finding is that every time a more accurate mapping product like SPIN-2 comes along, telecommunications companies want to buy it so they can improve the quality of service in proposed and existing wireless service areas," said Ken Ferguson. "Fine tuning cell sites and overlaying new digital systems onto existing analog systems will continue for a long time."
To meet those growing needs, SPIN-2 has scheduled launches approximately every six months for the next several years. A new mission is slated for October of this year, and SPIN-2 is already accepting acquisition orders for that flight. About the Author:
Kevin P. Corbley is a consultant and freelance writer specializing in remote sensing, GIS and GPS. He is located in Denver, Colorado, and may be reached at 303-722-0312 or by e-mail at [email protected]. Back |
