Another limitation in the accuracy obtained by precision GPS users is caused by errors in the dynamic model used to predict the behavior of a GPS satellite as it expands and contracts due to the space environment. The current model could be improved if better information was available on the thermal emissivity of the exterior of the spacecraft surfaces, including its solar panels. In addition, telemetry information on temperatures of spacecraft surfaces is needed.

For long baseline applications, the limiting error source is usually the GPS orbit error, even when high-accuracy post-processed orbits are utilized. To reach accuracies of 10^-9 times baseline length, the orbits must be known to about 10^-9 times the distance to the satellites (approximately 20,000 km), or about 0.02 meters. In other terms, an unmodeled acceleration of 10^-12 g would accumulate to 0.02 meters after a 12-hour orbit. Unequal radiation of heat from sides of the GPS satellite causes accelerations much larger than 10^-12 g.

With a minimal amount of effort, the thermal properties of the materials on the exterior surfaces of the Block IIR satellite could be determined. To accomplish this, instruments to measure temperature could be added to the GPS satellites prior to launch, and the data received from these instruments could be transmitted to the ground. This would allow accelerations of the spacecraft surface, which result from uneven heating in space, to be calculated. These accelerations could then be included in models to improve accuracy. In the absence of deterministic models developed through actual measurements, these radiation pressure parameters can only be estimated.

In order to support the Block IIR crosslink capability, the specifications related to the UHF antenna on the Block IIR satellites are different from the Block IIA specifications. As a result of these UHF antenna changes, the L-band antenna on the Block IIR satellites will be less symmetrical and will have a narrower pattern. For angles beyond the limb of the Earth as viewed from GPS satellites, this change will probably result in a reduction in the L₁ power currently observed with Block IIA spacecraft by approximately 3 dB.¹⁴ For spacecraft applications of GPS, this reduction in received L₁ power over that currently observed with Block IIA satellites and the narrower antenna pattern could decrease the ability of low-Earth orbit and geosynchronous satellites to receive GPS signals and make GPS-based positioning more difficult from orbit. By increasing the L₁ signal power or improving the symmetry of the L-band antenna, spacecraft applications using GPS could be greatly enhanced.

ACCQPOINT Communications Corp.

Art Humble

Aerospace Corporation

Mohan Ananda

Harold Bernstein

Chia-Chun (George) Chao

John Clark

Bill Feess

Paul Massatt

Prem Munjal

David Nelson

Bryant Winn, Jr.

Air Force Space Command

John Anton

Harrison Freer

Richard Koons

Earl Pilloud

Christopher Shank

Aircraft Owners and Pilots Association

Stephen Brown

Air Force Space and Missiles Center

Lynn Anderson

Kim Cirillo

Steve Decou

Charlie Golden

Bernard Gruber

Brian Knitt

Donald Latterman

Mark MacDonald

Ricardo Martinez

Al Mason

Charles Meadows

John Nauseef

Donna Shipton

Stephen Steiner

Earl Vaughn

Michael Wiedemer

Air Transport Association of America

William Russell

Allen Osborne Associates

Robert Snow

Alpha Instrumentation/Information Management (AI²M)

Daniel Alves

ANSER

David Martin

Association of American Railroads

Howard Moody

Ashtech Incorporated

Jonathan Ladd

Aviation Management Associates, Inc.

Larry Barnett

Booz·-Allen & Hamilton

Michael Dyment

Central Intelligence Agency

Terry McGurn

Crown Communications

Harry Hodges

Deere & Company

Wayne Smith

Defense Intelligence Agency

Barry Joseph

Albert Glassman

Defense Mapping Agency

Mike Full

Steven Malys

Scott True

William Wooden

Department of Transportation

Peter Serini

George Wiggers

Differential Corrections Inc.

Ron Haley

Bruce Noel

Eagle/Lowrance Electronics

Steve Schneider

Rafi Kedar

E-Systems

Anton Gecan

Federal Aviation Administration

Paul Drouilhet

Joseph Dorfler

Dave Peterson

Martin Pozesky

Federal Highway Administration

Frank Mammano

Lee Simmons

Federal Highway Administration, Turner Fairbanks Highway Research Center

Jim Arnold

Garmin International

Gary Kelley

General Railway Signal

Gordon Quigley

Global Telecommunications & Information Systems

Eric Bobinsky

IVHS America

Jim Costantino

Jerry Marsh

INMARSAT

Jim Nagle

Interstate Electronics

Peter Canepa

Jim Grace

ITT Aerospace/Communications Division

Peter Brodie

Laurence Doyle

Thomas Ernst

Jon Schnabel

Jansky/Barmat Telecommunications, Inc.

Melvin Barmat

Jet Propulsion Laboratory

William Melbourne

George Purcell

John E. Chance & Associates, Inc.

Andy Bogle

Philip Stutes

Johns Hopkins University,

Applied Physics Laboratory

Lee Pryor

Joint Chiefs of Staff

Jim Burton

Pat Carlile

Joe Lortie

Tim Meyers

William Owens

Leica, Inc.

Paul Gaylean

Litton Aero Products

Abdul Tahir

Loral Federal Systems

Brian Hemley

Magellan Systems Corporation

Randy Hoffman

Jim White

Magnavox Electronic Systems Company

Walter Airth

Vito Calbi

Kenneth Lindenfelser

Martin Marietta Astro Space Division of Lockheed-Martin

Robert Bebee

Aniruddha Das

Jim Graf

John Hrinkevich

John Mergen

Robbin Shultz

Massachusetts Institute of Technology, Lincoln Laboratory

William Delaney

Jay Sklar

MITRE Corporation

Robert Berkowitz

Robert Bales

Bakry ElArini

Thomas Hsiao

Young Lee

Kan Sandoo

Andrew Zeitlin

Motorola

Robert Denaro

National Aeronautics and Space Administration, Wallops Island

Bill Krabill

National Air Intelligence Center

Scott Feairheller

Jay Purvis

Frank Scenna

National Institute of Standards and Technology

Marc Weiss

National Oceanic and Atmospheric Association

Don Pryor

Benjamin Remondi

Odetics Precision Time Division

Don Greenspan

Office of the Assistant Secretary of Defense for Command, Control Communications and Intelligence

Noel Longuemare

Jules McNeff

Office of the Assistant Secretary of the Air Force for Acquisition

Matthew Brennen

Lee Carrick

Chad Pillsbury

PlanGraphics, Inc.

Mike Kevanney

Rand Corporation

Gerald Frost

Irving Lachow

Scott Pace

Riverside County Flood Control and Water Conservation District

William Young

Rockwell-Collins

Tyler Trickey

Mike Yakos

Rockwell International Corporation

Rich Arris

Denny Galvin

Steve Fisher

Steven Scott

RTCA, Inc.

David Watrous

Sea River Maritime

Robert Freeman

Scripps Institute of Oceanography

Jean-Bernard Minster

Stanford University

Bradford Parkinson

TASC

Iris Roberts

Tampa Bay Vessel Information and Positioning System, Inc.

John Timmel

Mike Shiro

3S Navigation

James Danaher

Trifed Corporation

Robert Ballew

Louis Decker

Ray Helmering

Trimble Navigation

Ann Ciganer

Walt Melton

Charles Trimble

True Time

Bruce M. Penrod

University Navstar Consortium

Randolph Ware

U.S. Coast Guard

George Privon

U.S. Department of Agriculture

Galen Hart

U.S. GPS Industry Council

Michael Swiek