From: traurig@ncavax.decnet.lockheed.com
Subject: Re: GPS
Organization: Sanders Associates
Date: Wed, 26 Aug 92 14:16:43 GMT

	Enough of this drivel!!!  As a daily user of GPS and LORAN I can
uncategorically state that, even with selective availability turned on (see
below), either of those positioning services have more than enough accuracy to
spot jumpers, either illegally over clouds or as a convenience item on sunny
days.  However, because the satellite constellation is not complete, 3D GPS
service is not available all the time.  2D GPS fixes are wildly inaccurate
without good altitude info, so right now all pilots should be using the
"altitude aided" mode on their GPS receivers (altitude is manually entered, or
better yet, automatically taken from your encoder when required). 2D service is
available almost 100% of the time.  Also, differential GPS is available to
nautical users on an unofficial basis courtesy of the USCG, from Bangor, ME to
Washington, DC.  Accuracy is 2 meters regardless of selective availability. 
But I digress...

	Enough of my drivel!!!  Here is the official stuff, straight off the USCG BBS
(sorry, forget the number) and available via anonymous ftp from
tycho.usno.navy.mil (how I get it):

File  GPSSY  --  last updated  <920724.1302>

The following  Global Positioning System (GPS) policy and system segment
information was  obtained from the  1990  Federal  Radionavigation  Plan
(FRP),  prepared jointly by the  Department  of  Defense  (DOD)  and the
Department of Transportation (DOT).


GPS POLICY
GPS  is a  DOD  developed,  worldwide,  satellite-based  radionavigation
system that will be the DOD's primary radionavigation  system well  into
the next century.   The  constellation will  ultimately  consist  of  24
operational satellites (21 plus three operating  spares).   The DOD will
declare the  GPS  constellation fully  operational when  21  operational
(Block II) satellites are functioning in their assigned orbits.  This is
expected to occur in 1993.  Users are cautioned that the system is under
development, and signal availability and  accuracy are subject to change
without warning  at the discretion  of the  DOD.   Therefore,  until the
system is  declared operational, any use of the  system is at the user's
own risk.

GPS  will provide two levels of service - a Standard Positioning Service
(SPS) and a Precise Positioning Service (PPS).

  SPS POLICY:   SPS is  a positioning  and timing  service which will be
  available to  all GPS  users on  a continuous, worldwide basis with no
  direct charge.   SPS will  be provided  on the  GPS L1 frequency which
  contains  a  coarse  acquisition  (C/A) code  and  a  navigation  data
  message. SPS is planned to provide the capability to obtain horizontal
  positioning accuracy within 100 meters 2 drms (95 percent probability)
  and 300 meters  99.99 probability.   GPS L1  frequency also contains a
  precision  (P)  code that  is not  a part  of the  SPS.   During   GPS
  constellation  build-up,  the  P code  may be  periodically available.
  However,  when GPS  is declared  fully operational, the  P code is not
  planned to be available to the general public.

  PPS POLICY:   PPS is a highly accurate military positioning, velocity,
  and timing  service which will be available on a continuous, worldwide
  basis to users authorized by the DOD. PPS will be the data transmitted
  on GPS  L1 and L2  frequencies.   PPS was designed primarily for U. S.
  military use.   It will be denied to  unauthorized users by the use of
  cryptography.   PPS will be made available to U. S. Federal and Allied
  Government  (civil and military)  users through special agreement with
  the  DOD.   Limited,  non-Federal  Government, civil use of  PPS, both
  domestic and foreign,  will be considered  upon request and authorized
  on a case-by-case basis, provided:

   * It is in the U.S. national interest to do so.
   * Specific GPS security requirements can be met by the applicant.
   * A reasonable alternative to the use of PPS is not available.

For questions regarding GPS policy,  the user is advised to refer to the
regularly appearing  FRP.   The FRP is published every  two years and is
available from  the National Technical Information Service, Springfield,
VA 22161.   The lastest report number is DOD 4650.4 DOT-VNTSC-RSPA-90-3/
DOD-4650.4 for report date 1990.


GPS SYSTEM SEGMENTS
The GPS consists of three  major segments:   SPACE,  CONTROL  and  USER.

The  SPACE  segment,  when  fully  operational,  will be  composed of 21
satellites (plus three operational spares) in six orbital planes,  (four
satellites in each plane).  The satellites operate in circular 20,200 km
(10,900 nm) orbits  at an  inclination  angle of  55  degrees and with a
12-hour period.  The position is therefore the same at the same sidereal
time each day, i.e., the satellites appear four minutes earlier each day.

The CONTROL segment consists of five Monitor Stations (Hawaii, Kwajalein,
Ascension Island, Diego Garcia, Colorado Springs), three Ground Antennas,
(Ascension Island, Diego Garcia, Kwajalein), and a Master Control Station
(MCS) located at Falcon AFB in Colorado.   The monitor stations passively
track all  satellites in view,  accumulating ranging data.  This informa-
tion is processed at the  MCS to determine satellite orbits and to update
each satellite's navigation message.   Updated information is transmitted
to each satellite via the Ground Antennas.

The  USER  segment  consists of  antennas  and  receiver-processors that
provide positioning, velocity and precise timing to the user.


GPS SIGNAL CHARACTERISTICS
The satellites transmit on two L-band frequencies:  L1 = 1575.42 MHz and
L2 = 1227.6 MHz.   Two pseudo-random noise  (PRN)  codes are  in use:  a
coarse/acquisition (C/A) code with a 1.023 MHz chip rate and a period of
one millisecond (MS); and a precision (P) code with a 10.23 MHz rate and
a period of seven days.    The C/A code is available on the L1 frequency
and the P code is available on both L1 and L2.   The various satellites,
therefore,  all transmit  on the same frequencies,  L1 and L2,  but with
individual code assignments.   Due to the spread spectrum characteristic
of the  signals, the system provides a  large  margin  of  resistance to
interference.   Each satellite transmits a navigation message containing
its orbital elements,  clock behavior,  system time and status messages.
In addition,  an almanac  is also  provided which  gives the approximate
data for  each active satellite.   This allows  the user set to find all
satellites once the first has been acquired.

The system is referenced to its Monitor Stations, whose coordinates have
been precisely surveyed with respect  to the World Geodetic System 1972,
otherwise referred to as WGS-72.  The new standard is the WGS-84 and the
transition took place on January 10, 1987.


SELECTIVE AVAILABILITY, ANTI-SPOOFING
Selective  Availability  (SA),  the  denial   of   full   accuracy,   is
accomplished  by manipulating  navigation  message  orbit data (epsilon)
and  by  manipulating satellite clock frequency (dither).  Anti-spoofing
(A-S)  guards against fake transmissions of satellite data by encrypting
the P-code to form the Y-code.

SA  will not be  implemented on  the Block I and  will be implemented on
Block II  at the  SPS  levels, as  soon  as each  Block II  satellite is
operational. A-S will be exercised on Block II satellites intermittently
through 1992 and will be on after the  Full  GPS  Operational Capability
(FOC).   A-S does not apply to the Block I satellites.   Requests for an
"SA/A-S free vehicle" have been denied.

Policy reminder from the NAVSTAR GPS Program Office was issued in Notice
Advisory to NAVSTAR Users (NANU) No. 152-89254 on September 12, 1989:
  "Selective  Availability  (SA)  and  Anti-Spoof (A-S)  are  system
   capabilities  which  may  be  exercised  at any time, potentially
   causing degradation or loss of capability for unauthorized users."

IMPLEMENTATION OF SPS LEVELS
1. The following  unclassified information  was provided  in a 2SOPS
   bulletin:  Selective Availablity was implemented 15 Nov 91 to the
   Standard Positioning Service level.
2. The following unclassified information was provided in NANU 121-92282
   DTG 011354Z JUL 91:
   "SUBJ: REACTIVATION OF SELECTIVE AVAILABILITY DAY 182/0400-UFN
    1. CONDITION: SELECTIVE AVAILABILITY WAS REACTIVATED ON DAY 182
    (1 JUL 91) BEGINNING 0400 UTC UNTIL FURTHER NOTICE. CIVIL USERS
    WILL RECEIVE STANDARD POSITIONING SERVICE (SPS) ACCURACY AS
    SPECIFIED IN THE 1990 FEDERAL RADIONAVIGATION PLAN. CIVIL USERS
    ARE ALSO ADVISED THAT VARIATIONS IN GPS ACCURACY MAY OCCUR TO
    SUPPORT SPECIAL TEST REQUIREMENTS THROUGH THE REMAINDER OF THE
    SATELLITE CONSTELLATION BUILD-UP PERIOD. SUCH VARIATIONS MAY OCCUR
    WITHOUT PRIOR WARNING BUT WILL BE WITHIN LEVELS SPECIFIED FOR GPS
    SPS IN THE FEDERAL RADIONAVIGATION PLAN.
    2. POINT OF CONTACT FOR POLICY: Lt. Col. ROB WORLEY, USSPACECOM/SPJ3,
    (719)554-5770 OR DSN 692-5770."


CURRENT GPS CONSTELLATION
The current GPS constellation consists of 18 satellites.  Eleven Block I
satellites  have  been  launched  and  four  remain in service.  Block I
reflects various stages of system development and are not identical with
the operational (Block II) satellites.  The first Block II satellite was
launched in February 1989; fourteen Block IIs have been launched to date.
The remaining Block I satellites that are in service will be incorporat-
ed into the growing constellation.


File GPSTD -- last updated <920817.1950>


Information is retained for approximately seven days or completion of
event; be aware that the information provided below may change.

************************************************************************

A. GPS SYSTEM
1. GPS Automatic Time Steering
     In effect 22 Jan 92 at 2052 UT and will remain in effect until
       further notice at rate +/-2.0E-19 seconds per second squared.


B. CONSTELLATION STATUS
1. ANTI-SPOOF (A-S) TESTING
    Anti-spoof testing occurred during the periods 1 Aug 92 0100 UT
     to 3 Aug 92 1000 UT and 8 Aug 0100 UT to 10 Aug 1000 UT.
     Testing involved the following vehicles:  PRN02/SVN13, PRN16/
     SVN16, PRN17/SVN17, PRN20/SVN20, PRN21/SVN21, PRN23/SVN23,
     PRN24/SVN24, PRN25/SVN25, PRN26/SVN26, PRN28/SVN28.  Future
     tests are anticipated.  (NANUs 181-92216 & 187-92224/12 AUG)
2. BLOCK II LAUNCH
   a. An effort is underway to launch PRN27/SVN27 by 2 SEPTEMBER 92.
       The launch dates of September 9 and 14 will remain valid if
       the September 2 launch does not occur.
   b. PRN30/SVN30 is scheduled to be launched no earlier than
       30 OCTOBER 92.
3. IMPLEMENTATION OF STANDARD POSITIONING SERVICE (SPS) LEVELS
   a. According to the Second Satellite Operations Squadron (2SOPS)
       Operational Advisory Bulletin:
        Selective Availability (SA) was implemented 15 Nov 91 to the
         Standard Positioning Service (SPS) level.
   b. The following unclassified information was provided in Notice
      Advisory to Navstar Users (NANU) 121-91182, DTG 011354Z JUL 91:
      SUBJ: REACTIVATION OF SELECTIVE AVAILABILITY DAY 182/0400-UFN
      1. CONDITION: SELECTIVE AVAILABILITY WAS REACTIVATED ON DAY 182
      (1 JUL 91) BEGINNING 0400 UTC UNTIL FURTHER NOTICE. CIVIL USERS
      WILL RECEIVE STANDARD POSITIONING SERVICE (SPS) ACCURACY AS
      SPECIFIED IN THE 1990 FEDERAL RADIONAVIGATION PLAN. CIVIL USERS
      ARE ALSO ADVISED THAT VARIATIONS IN GPS ACCURACY MAY OCCUR TO
      SUPPORT SPECIAL TEST REQUIREMENTS THROUGH THE REMAINDER OF THE
      SATELLITE CONSTELLATION BUILD-UP PERIOD. SUCH VARIATIONS MAY OCCUR
      WITHOUT PRIOR WARNING BUT WILL BE WITHIN LEVELS SPECIFIED FOR GPS
      SPS IN THE FEDERAL RADIONAVIGATION PLAN.
      2. POINT OF CONTACT FOR POLICY: Lt. Col. ROB WORLEY, USSPACECOM/SPJ3,
      (719)554-5770 OR DSN 692-5770."


C. BLOCK I INDIVIDUAL SATELLITE STATUS
SVN  PRN
 1    4  Launched 22 FEB 78; turned off 17 JUL 85
 2    7  Launched 13 MAY 78; turned off week 8-12 FEB 88
 3    6  Launched 06 OCT 78; turned off 18 MAY 92
 4    8  Launched 10 DEC 78; turned off 14 OCT 89
 5    5  Launched 09 FEB 80; turned off 11 MAY 84
 6    9  Launched 26 APR 80; turned off 06 MAR 91
 7       Unsuccessful launch 18 DEC 81
 8   11  Launched 14 JUL 83; operating on RUBIDIUM STD
         Unusable  9 Aug 1000 to 1014 UT  (NANU 184-92223/11 AUG)
         Unusable 14 Aug 0945 to 1015 UT  (NANU 190-92227/14 AUG)
         Scheduled unusable 18 Aug beginning 2100 UT for up to
          7 days for maintenance.  The maintenance will include
          switching to the vehicle's last Rb frequency standard.
          (NANU 182-92238/4 AUG)
 9   13  Launched 13 JUN 84; operating on CESIUM STD
         Unusable 13 Aug 2334 UT to 14 Aug 0000 UT
          (NANU 192-92227/14 AUG)
         Unusable 17 Aug 0813 to 0825 UT  (NANU 194-92230/17 AUG)
10   12  Launched 08 SEP 84; operating on RUBIDIUM STD
         Unusable  9 Aug 1000 to 1029 UT  (NANU 185-92223/11 AUG)
         Unusable 12 Aug 1600 to 1620 UT  (NANU 189-92225/13 AUG)
         Unusable 15 Aug 2017 to 2018 UT  (NANU 193-92228/17 AUG)
11    3  Launched 09 OCT 85; operating on RUBIDIUM STD
         The repositioning maintenance scheduled for 17 Aug has
          been CANCELLED.  (NANU 191-92227/14 AUG)


D. BLOCK II INDIVIDUAL SATELLITE STATUS
SVN  PRN
13    2  Launched 10 JUN 89; usable 10 AUG 89; operating on Cs std
14   14  Launched 14 FEB 89; usable 15 APR 89; operating on Cs std
15   15  Launched 01 OCT 90; usable 15 OCT 90; operating on Cs std
16   16  Launched 18 AUG 89; usable 14 OCT 89; operating on Cs std
17   17  Launched 11 DEC 89; usable 06 JAN 90; operating on Cs std
18   18  Launched 24 JAN 90; usable 14 FEB 90; operating on Cs std
19   19  Launched 21 OCT 89; usable 23 NOV 89; operating on Cs std
20   20  Launched 26 MAR 90; usable 18 APR 90; operating on Cs std
21   21  Launched 02 AUG 90; usable 22 AUG 90; operating on Cs std
23   23  Launched 26 NOV 90; usable 10 DEC 90; operating on Cs std
          Unusable 10 Aug 2108 UT to 11 Aug 0404 UT for scheduled
           maintenance  (NANUs 179-92223 & 188-92223/12 AUG)
24   24  Launched 04 JUL 91; usable 30 AUG 91; operating on Cs std
25   25  Launched 23 FEB 92; usable 24 MAR 92; operating on Rb std
26   26  Launched 07 JUL 92; usable 23 JUL 92; operating on Cs std
28   28  Launched 10 APR 92; usable 25 APR 92; operating on Cs std