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11 January 1999
Source:
http://www.usace.army.mil/usace-docs/eng-circulars/ec1110-1-90/
(30K, PDF file)
DEPARTMENT OF THE ARMY EC 1110-1-90
U.S. Army Corps of Engineers
CECW-EP Washington, D.C. 20314-1000
Circular
No. 1110-1-90 1 July 1998
EXPIRES 30 JUNE 2000
Engineering and Design
USE, ACQUISITION, AND SECURITY OF PRECISE POSITIONING
SERVICE GPS RECEIVERS FOR CIVIL APPLICATIONS
Distribution Restriction Statement
Approved for public release; distribution is unlimited.
DEPARTMENT OF THE ARMY EC 1110-1-90
U.S. Army Corps of Engineers
CECW-EP Washington, D.C. 20314-1000
Circular
No. 1110-1-90 1 July 1998
EXPIRES 30 JUNE 2000
Engineering and Design
USE, ACQUISITION, AND SECURITY OF PRECISE POSITIONING
SERVICE GPS RECEIVERS FOR CIVIL APPLICATIONS
1. Purpose. This circular disseminates policy and procedures on
the use, acquisition, decryption, and security of tactical Global
Positioning System (GPS) receivers for civil applications, such
as the Precision Lightweight GPS Receiver (PLGR). Procedures are
defined for obtaining these military grade receivers containing
secure Precise Positioning Service (PPS) capabilities. This
circular effects the transfer of proven military technology to
civil users.
2. Applicability. This circular is applicable to USACE commands
having civil works, military construction, and environmental
restoration responsibilities. Personnel of the U.S. Army Corps
of Engineers (USACE) are authorized to use tactical GPS receivers
in the conduct of Corps civil and military construction programs.
3. References.
a. Rules for Obtaining Navstar GPS Security Devices, DOD
GPS Joint Program Office, Revision A, 12 June 1997.
b. Navstar Global Positioning System Cryptographic Key
Ordering Instructions, U.S. Space Command, Revision 3, June 1997.
c. EM 1110-1-1003, NAVSTAR Global Positioning System
Surveying, 1 August 1996.
4. Distribution. Approved for public release. Distribution is
unlimited.
5. Background. The Navigation Satellite Time and Ranging
(NAVSTAR) Global Positioning System is a space-based satellite
radio navigation-system developed by the U.S. Department of
Defense. GPS receivers provide land, marine, and airborne users
with continuous three-dimensional position, velocity, and time
data (PVT). This information is available free of charge to an
unlimited number of users. The system operates under all weather
conditions, 24 hours a day, anywhere on Earth. GPS provides two
levels of positional accuracy: Standard Positioning Service (SPS)
and Precise Positioning Service (PPS). For security reasons, DOD
degrades/encrypts the GPS signals, using selective availability
(SA) and anti-spoofing (AS) techniques. Any civil user can
access the SPS signal with a $100 to $500 commercial grade receiver
and obtain an accuracy of approximately 100 m (95%).
Authorized PPS users can access and decode the encrypted P(Y)-code
signal and thus obtain an approximately 10 m positional accuracy
unaffected by AS and SA. Access to the PPS signal is
controlled through the use of cryptographic techniques, and is
limited to U.S. and allied military forces. DOD authorizes PPS
access to other government and selected private sector users
provided appropriate security requirements and other selection
criteria are met. As a DOD component, USACE is authorized access
to the tactical PPS signal for its civil works, military
construction, or environmental restoration missions. The small,
hand-held, PLGR receivers (AN/PSN-11R) can provide real-time, 10-
meter absolute positioning or navigation accuracy, and have wide
tactical use in military air/land/sea navigation, and related
tactical mapping, surveying, and positioning uses. These same
receivers can be used to support a variety of USACE civil
functions and applications, including GIS development, natural
resource management, surveying, land/air/sea navigation, and
emergency management.
6. PLGR Project Applications. Stand-alone GPS receivers can
compute and display "absolute" geographic positions in real-time
throughout most of the world. Absolute GPS positioning is
distinguished from differential GPS (DGPS) positioning which
requires a simultaneous comparison of positions between two
nearby GPS receivers; typically using a communications data link
between the two receivers. Accuracies of code-phase DGPS range
between 0.5 and 10 m. DGPS carrier-phase differencing techniques
can provide relative positional accuracies at the millimeter
level. Project functional accuracy requirements and economics
will determine whether absolute (either SPS or PPS) or differential
GPS techniques are required. PPS accuracies at the 10 meter
level will yield sufficient accuracy for many environmental, GIS,
and project management applications. PLGR receivers represent an
economical navigation and positioning tool compared to
traditional surveying or differential GPS methods. In general,
PLGR accuracies are suitable for GIS mapping scales of 1:12,000
(1" = 1000 ft) or smaller. PLGR accuracies may also have
application in dredge/scow positioning or monitoring,
environmental mapping, vehicle navigation, and emergency
management operations. Where only 100 meter accuracy is
required, less-expensive SPS receivers should be used. For
accuracies exceeding 10 meters, differential GPS techniques are
necessary. All GPS receivers can be operated with minimal
training.
a. Current USACE Applications. The use of a PLGR receiver
could be applicable whenever the mission requires positioning
accuracies that are better than the 100 meters obtained by an SPS
receiver, but do not require the specialized equipment needed to
acquire differential GPS accuracies. Some of the applications
where the use of a PLGR receiver has proven to be effective
include emergency management, real estate, OCONUS control
surveys, archeological surveys, GIS data collection, wetland
delineation, and Corps regulatory activities.
b. Training Options. In order to obtain maximum efficiency
from the operation of a GPS receiver, either PPS or SPS, the user
must be trained in the operation of the receiver and the receiver
interface with ancillary equipment. Training options exist
within USACE (U.S. Army Topographic Engineering Center), other
government agencies, and the private sector.
7. Acquisition of PPS Receivers. Outlined below are two
procurement options available to USACE commands to obtain PPS
receivers.
a. PLGR/SOLGR PPS Receivers. USACE users can purchase the
PLGR receivers using a Department of Agriculture (USDA) multi-agency
contract. The cost in 1998 for a PLGR with a complete
complement of accessories (PLGR Kit) is $2,091.00. Although the
USDA contract will expire in 1998, the USDA is currently working
toward establishing an amendment to the existing contract or a
new contract for delivery of PPS receivers starting in January
1999. The receiver that will probably be delivered in 1999 and
beyond will be termed the Special Operations Lightweight GPS
Receiver, or SOLGR. This receiver will offer several
improvements over the PLGR such as 12 channels, dual frequency
operation, waterproof down to 24 meters, programmable function
keys.
(1) PLGR/SOLGR Receiver Description. These hand-held PPS
receivers are manufactured by Rockwell Incorporated, Collins
Avionics Division. The PLGR and SOLGR will determine positions
to an accuracy of 16 meters SEP when operating in the PPS mode
and 100 meters when operating in the SPS mode. Additionally,
both instruments contain Wide Area GPS Enhancement (WAGE) for
autonomous positioning accuracy to 4 meters CEP and Secure (Y-code)
Differential GPS (SDGPS) for positioning accuracy to less
than 2 meters CEP. Many vendors interface with the PLGR and
SOLGR for GIS applications.
(2) Acquisition Procedures. To procure the PLGR or SOLGR
from the USDA contract, first contact the U.S. Army Topographic
Engineering Center (USATEC), Geospatial Engineering Branch, ATTN:
CETEC-TD-G, 7701 Telegraph Road, Alexandria, VA 22315-3864,
telephone (703) 428-6798, e-mail: <pcervari@tec.army.mil>, and
request an order form. Complete the form, prepare the paperwork
(MIPR, credit card, etc.) to cover cost of required equipment,
and fax to USATEC (703-428-6135). USATEC will place the order
with the vendor, receive and load the cryptographic keys, check
the equipment to insure proper operation, and Fed-Ex the hardware
to the ordering office.
b. Non-PLGR/SOLGR PPS Receivers. USACE users can also
purchase any other PPS receivers directly from the manufacturer.
These receivers can be purchased using standard competitive
procurement practices.
(1) Procedure. Approval to purchase a PPS receiver directly
from the manufacturer must first be obtained by submitting
correspondence to the GPS Joint Program Office (JPO),
Headquarters Space and Missile Systems Center, ATTN: CZU, Los
Angeles AFB, 2435 Vela Way, El Segundo, CA 90245-5500, defining
the project that requires more precise positioning than can be
obtained using SPS receivers. The correspondence received from
GPS JPO will authorize direct negotiations with the vendor(s).
8. Cryptographic Key Control and Use.
a. General Discussion. PPS requires receivers be loaded
with a cryptographic code so that the effects of Selective-
Availability (SA) are negated, and to provide for Anti-Spoofing
(A-S) capability. To receive the cryptographic keys requires a
COMSEC custodian and to load the key into the GPS receiver
requires a COMSEC fill device.
b. Acquiring COMSEC Keys. To obtain the cryptographic
keys, the COMSEC custodian must process a request for keying
material through the Validating Authority for the U.S Army, the
Communications Security Logistics Agency (CSLA), ATTN: SELCL-KP-
KEY, Ft. Huachuca, AZ 85613-7090, who validates the operational
need for PPS accuracies and forwards the request to the GPS
Controlling Authority. The Deputy Undersecretary of Defense
Space (ODUSD/C3I) has designated HQ U.S. Space Command as the
Controlling Authority for PPS cryptographic keying material, who,
in turn, will notify the appropriate Distribution Center to make
distribution to the identified COMSEC Custodian. The actual
cryptographic keys are classified CONFIDENTIAL and must be
handled accordingly. A keyed receiver is not classified but must
be safeguarded like any valuable piece of equipment. If an
annual key is not compromised and there are no accidental
“zeroizations” of the keyed receiver, the receiver need only be
keyed once per key-year.
c. Loading the PPS Receiver. There are several ways the
user can have his PPS receiver loaded with the necessary COMSEC
key.
(1) District/Division COMSEC Custodian. If the District or
Division Office has a COMSEC Custodian, and the COMSEC Custodian
has processed and obtained the yearly COMSEC key and the fill
devices, each year the PPS receiver will need to be taken to the
COMSEC Custodian to have the new key loaded into the receiver.
Note: If the receiver is not re-keyed, it will continue to
operate, but will operate only as an SPS receiver until re-keyed.
(2) U.S. Army Topographic Engineering Center. Those
Districts and Divisions that do not have a COMSEC Custodian may
Fed-Ex the receiver(s) overnight to the U.S. Army Topographic
Engineering Center, 7701 Telegraph Road, ATTN: CETEC-TD-G,
Alexandria, VA 22315-3864. Enclosed with the receiver(s) to be
re-keyed should be a completed return Fed-Ex shipping document.
CETEC will replace the memory battery, check PPS operation and
return the receiver(s).
(3) Other Military Offices. For those USACE commands who do
not have a COMSEC Custodian but have a military installation
close to their office, the military installation may have a
COMSEC Custodian, the COMSEC keys, and the fill devices. The
military office may be willing to re-key the PPS receivers. The
USACE command should contact the military installation to
determine if this approach is feasible.
9. Proponency and Technical Support. The HQUSACE proponent for
this circular is Engineering Division, Directorate of Civil
Works, ATTN: CECW-EP. Technical assistance on security
requirements or the acquisition of PPS receivers may be obtained
from the Geospatial Engineering Branch, U.S. Army Topographic
Engineering Center, ATTN: CETEC-TD-G, 7701 Telegraph Road,
Alexandria, VA 22315-3864, (703) 428-6798, e-mail:
<pcervari@tec.army.mil>.
FOR THE COMMANDER:
[Signature]
1 Appendix ROBERT W. BURKHARDT
APP A - Additional Colonel, Corps of Engineers
Information on GPS Executive Director of Civil Works
EC 1110-1-90
1 Jul 98
APPENDIX A
Additional Information on GPS
A-1. Components of GPS. The GPS system consists of three major
segments: the space segment, the control segment and the user
segment.
a. Space Segment. The Space Segment consists of a nominal
constellation of 24 operational satellites (including 3 spares)
which have been placed in 6 orbital planes 10,900 miles (20,200
km) above the Earth's surface. The satellites are in circular
orbits with a 12-hour orbital period and inclination angle of 55
degrees. This orientation nearly ensures a minimum of five
satellites in view at any given time, anywhere on Earth. Each
satellite continuously broadcasts two low-power spread-spectrum
RE Link signals (L1 and L2). The L1 signal is centered at
1575.42 MHZ and the L2 signal is centered at 1227.6 MHZ.
b. Control Segment. The Control Segment consists of a
Master Control Station (in Colorado Springs), and a number of
monitor stations at various locations around the world. Each
monitor station tracks all the GPS satellites in view and passes
the signal measurement data back to the Master Control Station,
where the computations are performed to determine precise
satellite ephemeris and satellite clock errors. This data is
then up linked to the individual satellites, and subsequently
rebroadcast by the satellite as part of its navigation data
message.
c. User Segment. The User Segment is the collection of all
GPS receivers and their application support equipment such as
antennas and processors. This equipment allows users to receive,
decode, and process the information necessary to obtain accurate
position, velocity, and timing measurements. This data is used by
the receiver's support equipment for specific application
requirements.
A-2. Characteristics of GPS Signals. The satellites transmit
their signals using spread spectrum techniques employing two
different spreading functions: a 1.023 MHZ coarse/acquisition
(C/A) code on L1 only and a 10.23 MHZ precision (P) code on both
L1 and L2. The two spreading techniques provide two levels of
GPS service: Precise Positioning Service (PPS) and Standard
Positioning Service (SPS). SPS uses C/A code to derive position,
while PPS uses the more precise P(Y)-code.
(1) The P-code has a number of advantages over C/A code.
First, the chipping rate of the P-code is 10 times faster,
therefore the wavelength is 1/10th as long, giving the P-code a
much higher resolution. Second, the higher chipping rate spreads
the signal over a wider frequency range that makes the P-code
much more difficult to jam. Third, by encrypting the P-code
(creating the Y-code), the receiver is not susceptible to
spoofing, or false GPS signals intended to deceive the receiver.
(2) The drawback of P-code is that it is relatively
difficult to acquire because of its length and high speed. For
this reason, many PPS receivers first acquire C/A code, then
switch over to the P(Y)-code.
(3) Y code is an encrypted version of P code, used for anti-
spoofing (A-S). Due to the similarity of these two codes, they
are referred to collectively as P(Y)-code.
(4) Superimposed on both the P-code and the C/A code is a
navigation message (NAV-msg) containing satellite ephemeris data,
atmospheric propagation correction data, satellite clock-bias
information, and almanac information for all satellites in the
constellation.
(5) The GPS satellites use Bi-Phase Shift Keyed (BPSK)
modulation to transmit the C/A and P(Y)-codes. The BPSK
technique involves reversal of the carrier phase whenever the C/A
or P(Y)-code transitions from 0 to 1 or from 1 to 0.
(6) To the casual observer, the very long sequence of ones
and zeros that make up the C/A and P-codes would appear to occur
in a random fashion and blend into the background noise. For
this reason, they are known as pseudo-random noise (PRN). In
actuality, the C/A and P-codes generated are precisely
predictable to the start time of the code sequence and can be
duplicated by the GPS receiver. The amount the receiver must
offset its code generator to match the incoming code from the
satellite is directly proportional to the range between the GPS
receiver antenna and the satellite.
(7) By the time the spread spectrum signal arrives at the
GPS receiver, its signal power is well below the thermal noise
level. To recover the signal, the receiver uses a correlation
method to compare the incoming signals with its own generated C/A
or P(Y) codes. The receiver shifts its generated code until the
two codes are correlated.
A-3. Determining Positions.
a. The receiver continuously determines its geographic
position by measuring the ranges (the distance between a
satellite with known coordinates in space and the receiver*s
antenna) of several satellites and computes the geometric
intersection of these ranges.
b. To determine a range, the receiver measures the time
required for the GPS signal to travel from the satellite to the
receiver antenna. The resulting time shift is multiplied by the
speed of light, arriving at the range measurement.
c. Since the resulting range measurement contains
propagation delays due to atmospheric effects, as well as
satellite and receiver clock errors, it is referred to as a
“pseudorange.” A minimum of four pseudorange measurements is
required by the receiver to mathematically determine time and the
three components of position (latitude, longitude, and
elevation). The solution of these equations may be visualized as
the geometric intersection of four ranges from four known
satellite locations.
d. If one of the variables is known, such as elevation,
only three satellite pseudorange measurements are required for a
PVT solution, and only three satellites would need to be tracked.
A-4. GPS Error Budgets and Accuracies.
a. GPS accuracy has a statistical distribution that is
dependent on a number of important factors, including: dilution
of precision (DOP) satellite position and clock errors,
atmospheric delay of satellite signals, selective availability,
signal obstruction, and multipath errors.
b. Each satellite follows a known orbit around the earth
and contains a precise atomic clock. The monitor stations
closely track each satellite to detect any errors in its orbits
or clock. Corrections for errors are sent to each satellite as
ephemeris and almanac data. The ephemeris data contains specific
position and clock correction data for each satellite while the
almanac contains satellite position data for all satellites. The
NAV set receives the ephemeris and almanac data from the
satellites and uses this data to compensate for the position and
clock errors when calculating the NAV data.
c. There are two ways to compensate for the atmospheric
delays: modeling and direct measurement. The ionospheric and
tropospheric delays are inversely proportional to the square of
the frequency. If a receiver can receive L1 and L2 frequencies,
it can measure the difference between the two signals and
calculate the exact atmospheric delay.
d. Currently, most receivers use mathematical models to
approximate the atmospheric delay. The tropospheric effects are
fairly static and predictable and a model has been developed that
effectively removes 92-95 percent of the error.
e. The ionosphere is more difficult to model due to its
unusual shape and the number of factors that affect it.
Therefore, a model has been developed that requires eight
variable coefficients. Every day, the Control Segment calculates
the coefficients for the ionospheric model and uplinks them to
the satellites. The data is then rebroadcast in the NAV messages
of the C/A- and P(Y)-codes. This model can effectively remove 55
percent of the ionospheric delay.
f. Multipath errors result from the combination of data
from more than one propagation path. This distorts the signal
characteristics from which the range measurements are made,
resulting in pseudorange errors. These errors are dependent on
the nature and location of a reflective surface peculiar to each
user location. The effects are less detrimental for a moving
user since small antenna movement can completely change the
multipath characteristics.
g. The receiver is designed to reject multipath signals.
First, the active patch antennas are designed to have a sharp
gain roll-off near the horizon while providing nominal gain for
the primary satellite signal. Since most multipath signals are
reflected from ground structures, they tend to be attenuated.
Second, the antenna is right-hand polarized. When a right-hand
polarized GPS signal is reflected off a conductive surface, it
becomes left-hand polarized, and rejected by the antenna. The
receiver also has hardware and software designed to reduce the
effects of any multipath interference errors.
A-5. GPS Positioning Services. GPS satellites provide two
levels of navigation service: Standard Position Service (SPS) and
Precise Position Service (PPS).
(1) SPS receivers use GPS information broadcast in the clear
and is available to anyone in the world. This information
contains built-in errors that limit the accuracy of the receiver.
This is a security technique called Selective Availability (SA).
These SA errors are variable. In normal conditions, the U.S.
government guarantees that these errors do not exceed 100 meters
horizontal, 140 meters vertical, and time accuracy of 340
nanoseconds 95 percent of the time. Thus, there are times when
an SPS receiver error exceeds these limits. SA is always on.
SPS receivers are for civil use and a PLGR without crypto keys
will act like an SPS receiver.
(2) PPS receivers use the same information as SPS receivers.
They also read encoded information that contains the corrections
to remove the intentional SA errors. Only users who have crypto
keys to decode this information get the PPS accuracy. U.S.
government agencies and some Allies are authorized to have these
crypto keys. A PLGR with valid crypto keys loaded and verified
is a PPS receiver.
(3) To protect authorized users from hostile attempts to
imitate the GPS signals, a security technique called Anti-spoofing
(A-S) is also used. This is an encrypted signal from
the satellites that can only be read by PPS receivers. A
receiver with valid crypto keys loaded and verified, reads this
encrypted signal and operates in a spoofing environment.
(4) Normal operation of the GPS receiver requires
undisturbed reception of signals from as few as four satellites
(in normal 3-D mode) or three satellites in fixed-elevation mode.
The signals propagating from the satellites cannot penetrate
water, soil, walls, or other similar obstacles. The antenna and
the satellites are required to be in a "line-of-sight" with each
other. Therefore, GPS cannot be used for underground positioning
in tunnels, mines, or subsurface marine navigation. In surface
navigation, the signal can be obscured by buildings, bridges, and
other matter that might block an antenna*s line-of-sight from the
GPS satellites in view. In airborne applications, the signal can
be shaded by the aircraft*s body during high banking angles.
A-6. Differential GPS (DGPS). Differential GPS (DGPS) may be
used to eliminate the effects of SA and correct certain bias-like
errors in the GPS signals. A Reference Station receiver measures
ranges from all visible satellites to its surveyed position.
Differences between the measured and known ranges are computed
and applied to differential equipped receivers in a local area.
These differences (or Pseudo-Range Corrections) can be transmitted
by radio and applied in real-time, or can be downloaded into
computer software and applied during postprocessing.
[End]