[IGSMAIL-1599] GIAC Response to Capstone Requirements

[Ruth]

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IGS Electronic Mail      Tue May 13 11:18:00 PDT 1997      Message Number 1599
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Author: Ruth Neilan
Subject: GIAC Response to Capstone Requirements


Below is an important and interesting message that I am posting for CDR.
David Minkel, NOAA, concerning the US Air Force Capstone Requirements
Document (CRD), a planning document to address the future of Position,
Navigation and Timing Systems intended to include civilian use
considerations and input.  For reference see IGSMail # 1591, "FGCS/GIAC
News Flash, Capstone Requirement", dated May 1, 1997.

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Ruth,
Attached below is the GIAC's summary of user requirements as provided to
Hank Skalski.  The summary is based on the user comments we received and
personal conversations with GPS users.  We did include all written
submissions (not attached here) along with this summary to insure all
requirements where accurately passed on to the working group.

Please post this on your list server for the information of your
subscribers.  If anyone wishes to provide input to the CRD I will be happy
to accept comments (my signature file at the bottom has the pertinent info.
for making contact).

Thanks,
Dave Minkel

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GPS Interagency Advisory Council
response to the
U.S. Air Force Capstone Requirements Document


The GPS Interagency Advisory Council (GIAC) polled its constituents for
input to the U.S. Air Force's Capstone Requirements Document (CRD).  As of
8 May 1997, 20 responses had been received from users of U.S.
Position/Navigation/Timing (PNT) systems.  Note that all suggestions for
enhanced PNT capability deal specifically with GPS or enhancements to GPS,
and no other systems were discussed.  In addition, all but one response
addressed positioning requirements; one user discussed time transfer
requirements.  There were no responses discussing the requirements of
atmospheric researchers.  However, the National Geodetic Survey has
incorporated the needs of the National Weather Service=EDs Forecast Systems
Laboratory; their requirements will be satisfied by the positional
requirements reported by other user groups.

A brief summary of the user groups that the respondents represent is as
follows: geodesy/crustal dynamics/geomatics (5),
GIS/mapping/surveying/facilities management (7), time transfer (1),
navigation (2), natural resource management (4), other uses (1).

The questions posed to the users were:

1) What is your main application for a PNT system?

2)  What are the short comings of GPS in your application?

3)  What improvements could be made to GPS or future PNT system that would
better support your application?


Based on our survey, the user requirements for PNT systems (GPS) can easily
be sorted into three categories based on required accuracy and application;
the positional accuracy requirements shown for each group of users depicts
the most stringent requirements for that group.  The requirements reported
are based on the responses received by GIAC, personal conversations with
users, and the requirements of the GIAC member agencies.

The three GPS application groups are as follows:

Near real-time positioning with an accuracy of 2 centimeters horizontal
(2s) and 4 centimeters vertical (2s) with augmentation of the PNT service.
This group uses dual frequency carrier phase observations to achieve
real-time, high-accuracy positioning primarily for the purposes of land,
air, and marine navigation.  Current implementation of this technology
requires a static reference station(s) collecting carrier phase data from
GPS.  These data are transmitted to the user and used for near real-time
positioning at the few centimeter level.  The major difficulty in using
this technique is resolution of the integer biases of the carrier phase;
atmospheric refraction, signal multipath, poor satellite coverage, and poor
satellite geometry impede resolution of the integer biases.  Currently this
technique only works over short distance and lacks sufficient integrity.

Real-time positioning with an accuracy of 1 meter horizontal (2s) and 5
meters vertical (2s)  with no augmentation of the PNT service.
Requirements for this group of users can be satisfied with code
observations, possible with an improved PNT system.  Some applications, or
geographic locations of operations, preclude use of even a well integrated
augmentation system.

Post mission positioning with an accuracy  of 1-2 millimeters (2s).  These
users require the maximum achievable accuracy, but do not require
positional information at the time of the observations.  Both code and
carrier phase are used in this technique.


Recommended enhancements to GPS:
Addition of at least one more coded signal for a total of three
frequencies.  Three frequencies allow complete resolution of ionospheric
refraction and can greatly improve wide-laning.  While a full code
implementation is desirable, a code adequate to insure continuous carrier
lock and avoidance of Doppler source confusion is acceptable.  The
frequency should be selected to maximize its use for wide-laning.  Other
considerations during frequency selection should include water vapor
determination and minimizing the effect of ionospheric refraction.

Increase the number of available satellites and types of orbits.  At a
minimum, there must be six satellites in view at anytime at any location in
the world.  Near polar and near equatorial orbits should be added to
improve geometry and availability.  Utilize LEO, MEO, and GEO orbital
planes.  The stronger Doppler signal of the LEOs (range rate higher by
approximately a factor of 10 when compared to GPS) isolates the user
position, and this in turn could provide positions of sufficient accuracy
that integer fixing might not be required.  Orbits for the LEOs could be
determined from GPS or by geometric means.  The additional satellites and
improved geometry also yields positional accuracies with the errors more
evenly distributed in the position components.  In addition to the obvious
advantage(s) for the navigator, GEOs provide the timing community with a
continuous reference during satellite swapping and allow use of directional
antennas for improved SNR.

Increase the chipping rate above the P-code rate.  The higher the chip
rate, the smaller the code wavelength  and, consequently, the lower the
code multipath.  Reduced code multipath can lead to improvements in
ambiguity resolution and can significantly improve code DGPS to the point
that many applications might not require carrier phase based positioning.

Provide sub-meter ephemerides, improved clock correctors, and atmospheric
parameters (ionospheric and tropospheric refraction) in the navigation
message(s).  While this will obviously aid the point position user, these
improvements will also aid the differential code and carrier phase users.

=46uture denial of accuracy techniques should use a method(s) that does not
adversely affect satellite tracking.  While the effects of SA can be
adequately mitigated for many users by DGPS techniques, application of AS
can not be easily mitigated.  Current technology incorporates specialized
equipment and techniques to allow the user to track carrier phase during
AS, however, tracking performance is degraded and the expense of the
equipment is higher.

Publicly owned (or Federally funded) augmentation systems should utilize
the same frequency bands with the intent of minimizing the cost of user
equipment and providing the greatest area of interoperability.

Improve (reduce) acquisition time.  Many possible applications, such as
tracking marine mammals, cannot use GPS due to the time required to obtain
positions.  Many marine mammals are on the surface of the water for only a
few seconds; reduction of acquisition time to less than 5 seconds would
allow these researchers to utilize GPS.  Reduced acquisition times would
also aid the user working under dense overhead canopies.

If precise orbits and clock correctors can not be provided in real time,
provide them after the fact (24-hour delay) to allow users of carrier phase
techniques, working in remote areas (Antarctic, etc.) to perform data
quality checking.  While the GPS satellites would be the best source of
these data, any source that does not require additional specialized
equipment would be acceptable.

Utilize "satellites of opportunity" to improve the availability of GPS
signals.  All Federally owned satellites launched in the future should
include a navigation transponder.  In addition, private sector entities
with satellite constellations, such as Iridium, Teledisc, Globalstar, etc.
should be approached as providers of navigation signals.  This might be a
very economical way to reach implementation of Receiver Autonomous
Integrity Monitoring (RAIM).

Provide more satellite specific information to the user community, such as:
consistency of radio frequency amongst the constellation, transmit antenna
phase center to center of mass relationship for each SV, phase pattern of
XMIT antenna, etc.

The handheld PPS equipment available to the civil sector is limited in its
ability to support agency missions since it is designed primarily for DoD
missions.  Most civil agencies using PPS equipment are involved in some
form of Geographic Information System (GIS) based mapping activities.  The
PLGR and PLGR+96, for example, have limited utility for civil agencies due
to the limited data input and storage capability and limited ability to
input external sensor data (such as range and azimuth from laser range
finders).  Coordinated (civil and military) PPS equipment purchases could
result in more capable PPS equipment, at lower cost, for both groups.


Satellite Shadowing:
Many users need improved signal penetration in dense canopy environments -
increased signal strength, additional satellites, and additional orbits are
possible answers to this problem.  Ideally, the PNT of the future would
work in buildings, parking garages, tunnels, etc. as well as in the clear.

Satellite shadowing, and the resultant loss in position, is the next major
hurdle that a satellite-based PNT must address and resolve.  Many land
navigation applications can not effectively utilize GPS due to the
shadowing problem: this is especially true for applications based on
carrier phase observations.  Many possible applications, such as guidance
systems for the blind, can not tolerate any interruption of positional data.

Possible solutions, using current technology and GPS, are utilization of
inertial systems in an integrated system and improved frequency references
(clocks) for user equipment.  Whatever the solution(s), the design of
future satellite-based PNT systems should consider integration of other
techniques or technologies to solve this short fall.


System Management Issues:
Assurance of uninterrupted civil access to the GPS signal, for both the
domestic and international civil sectors, is of paramount importance.  As
emergency services and critical navigation services become more reliant on
GPS, an interruption of service will become less tolerable.  Furthermore,
use of GPS for these services will not occur as long as there is a
reasonable doubt in the availability of the signal.

This issue is of extreme importance to non-domestic civil users and may
preclude their acceptance of any U.S.-provided posnav system for critical
services.  As an example, consider a non-domestic civil defense agency
whose responsibility is the evacuation of citizenry during emergencies.  If
their city/country might be in a theater of conflict, use of GPS as their
primary posnav system can not be considered.  Obviously, this would also be
of concern to domestic civil defense agencies since they would be forced to
rely on units (such as the National Guard) that have PPS equipment in hand.
 Denial of accuracy/service techniques must address this issue.

Lastly, as long as there is incomplete coverage, or inadequate performance,
of well integrated augmentation systems, the user community will continue
to use the presence of SA as rationale for the development of independent
PNT systems.


Closing Comments:
The GIAC appreciates the opportunity to present a forecast of the future
needs, with regard to PNT systems, for the civil sector.  As the civilian
sector becomes the majority user of a posnav system like GPS, the
requirements of the civil sector must become fully integrated into future
systems.  If the civil sector perceives (whether correctly or not) that
their needs are not being seriously considered it is quite likely that they
will attempt to pool their resources and establish a system designed with
civil use as the only consideration.

The Capstone Requirements Document effort is an excellent start at
involving the civil sector in the planning for future PNT systems.  The
GIAC hopes the CRD is the first step in a continuing dialog between the
military and civilian users of U.S. PNT systems.  In future dialogs with
the private sector, we request a longer lead time than was possible with
the CRD.  Polling the non-federal, civilian users of PNT systems has
legislative constraints, such as the Federal Paperwork Reduction Act, that
must be considered.  Efforts like the CRD are not prohibited, but do
require more time in the civil sector than in the military sector.  To
reach the goal of increased public involvement in PNT planning, the GIAC
volunteers its services, in whatever manner that is mutually acceptable, to
assist USAF and Department of Defense in future efforts.

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CDR Dave Minkel, NOAA		   Office:    301-713-3169  ext.141
NOAA, National Geodetic Survey     Fax:       301-713-4315
1315 East-West Highway, Room 8609
Silver Spring MD 20910
ATTN: N/NGS                        Internet:   dminkel at ngs.noaa.gov
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[Mailed From: "Ruth E. Neilan" <ren at cobra.jpl.nasa.gov>]