Subject: [IGSMAIL-2824]: IGS orbit/clock navigation & sat. clock interpolations with no SA
From: Jan Kouba <[email-masked]>
To: [email-masked]
Date: Fri, 05 May 2000 20:43:05 GMT

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IGS Electronic Mail      05 May 13:43:09 PDT 2000      Message Number 2824
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Author: J. Kouba

Subject: IGS Orbit/clock navigation & sat. clock interpolations with no SA

Dear Colleagues,

The removal of SA (as announced on May 2 (Wk 1060/day 2) at about 04:00 UTC 
is expected to have a significant impact on precise point navigation with the 
IGS orbit/clocks held fixed,as well as satellite clock and data interpolations. 
With no SA it should now be possible to interpolate the IGS satellite clocks, 
currently sampled at 5  min intervals, at or below the 20 cm precision level. 
Consequently,it should now be possible to do precise navigation at any interval 
(instant) at this (20 cm) precision level with IGS orbits/clocks fixed. The 
navigation precision of better than 10 cm with IGS orbits/clocks fixed is being 
demonstrated daily and weekly in the summaries of the IGS Rapid(IGR) and Final  
combinations, respectively (at the 15 min sampling intervals of sp3 
orbit files).

For your information, the first table below contains the expected clock errors 
based on individual frequency standard specifications (Cs, Rb and HM). The 
second table contains the actual Allan std's (for 100 sec interval), computed 
for the second day (Wk 1060/d3) of the SA-free IGR combined orbit/clock 
solutions.The first day (1060/d2) gave almost identical results. All but one Cs 
(PRN 6) are well below 5.e-12, i.e. they are closer to the high performance Cs 
tube specifications  (1.e-12/100). Only one Rb (PRN8) has exceeded the usual Rb 
specs (1.e-12/100sec).An interesting observation is that most Rb performed more 
like Cs (a random walk,i.e. showing the sqrt(t) dependency at least for up to 4 
hour intervals!) The only exception is the already mentioned PRN 8 Rb which has 
shown a time dependency about  half the way between the sqrt(t) and t- 
dependency. 

The Table 1 should give an idea what to expect for clock (station/sat) 
interpolation and/or extrapolation (due to frequency standard errors only). For 
example for a 5 min clock interpolation, a conservative clock error estimate 
caused by a straight line interpolation is less or equal to the half of the 
interval, I.e. the 2.5 min (150 sec) value. This should be quadratically 
increased by up to a clock noise sigma (about .1 ns (3 cm) or less for IGS 
combined clocks). In practice one'd do a polynomial (at least 3 point 
interpolation) so the interpolation errors should be even less. As mentioned 
above, the Rb satellites should be treated as Cs in Table 1, so that 
interpolations within a 5 min interval the frequency standard errors should be 
within 20 cm for all but one Cs' and below 3.6 cm for all but one Rb's 
standards. For extrapolation use the full interval, eg. for a 60 min  
extrapolation use 60 min (3600 sec) etc. HM's specs are included here for 
comparisons and due to the fact that a number of IGS stations are equipped 
with receivers that use unaltered (so far) external HM frequency standard 
signals with  the HM's  of much better specs than the one shown in the Table 1.

Hope this helps 

J. Kouba

PS. I've (re)used the following (old) references related to freq. standards:

1. Fell. P.J., 1980, "Geodetic Positioning using GPS Satellites, OSU- DGS
Report #299, June.

2. Kouba, J. and J.A. Orosz, 1982, Ecaluation of ELECTRAC receivers and
Oscillator effects on Doppler Data Quality .., Proc. 3rd Inter. Symp. on
Satell. Pos, Las Cruces, NM, pp. 937-952.

3. Vessot, R.F.C, 1976, Frequency and Standards, Method of Experimental 
Physics, Vol. 12, Part C, Radio Observations (ed. M.L. Meeks), Academic Press
pp 198-227.
p

----------
Table 1
Inter/Fq Std.  Primary Cs   Super Tube Cs  Standard Cs       Rb       HM
     Specs/100s   1.E-13         1.E-12      1.E-11        1.E-12    1.E-14

    1 sec          .3 mm            3 mm        30 mm        3 mm     .003 mm
   30             1.6              16          160          16        .003
  150             3.6              36          360          45         .2
  300  	          5.2              52          520          90         .6
  600             7.3              73          730         180        1.5
  900             9.0              90          900         270        2.5
 1800            12.7             127         1270         540        5.1
 3600            18.0             180         1800        1080       10.5
 7200            25.4             254         2546        2160       21.3
 14400           36.0             360         3600        4320       42.9

Table 2:		CURRENT	BLOCK II/IIA/IIR SATELLITES			
         		==================================		

LAUNCH			LAUNCH	      FREQ		US SPACE	
ORDER	PRN	SVN	DATE	       STD	PLANE	COMMAND	Allan/100s
-----------------------------------------	------------
*II-1	14	14	14-Feb-89			19802	
II-2	2	13	10-Jun-89	Cs	B3	20061	4.0E-12
II-3	16	16	18-Aug-89	Rb	E5	20185	9.6E-13
II-4	19	19	21-Oct-89	Cs	A5	20302	3.7E-12
II-5	17	17	11-Dec-89	Cs	D3	20361	
II-6	18	18	24-Jan-90	Cs	F3	20452	3.5E-12
*II-7	20	20	26-Mar-90			20533	
II-8	21	21	2-Aug-90	Cs	E2	20724	2.7E-12
II-9	15	15	1-Oct-90	Cs	D5	20830	3.6E-12
IIA-10	23	23	26-Nov-90	Cs	E4	20959	4.1E-12
IIA-11	24	24	4-Jul-91	Rb	D1	21552	6.6E-13
IIA-12	25	25	23-Feb-92	Cs	A2	21890	2.8E-12
*IIA-13	28	28	10-Apr-92			21930	8.3E-13
IIA-14	26	26	7-Jul-92	Rb	F2	22014	7.8E-13
IIA-15	27	27	9-Sep-92	Cs	A4	22108	4.9E-12
IIA-16	1	32	22-Nov-92	Cs	F4	22231	2.8E-12
IIA-17	29	29	18-Dec-92	Rb	F1	22275	8.3E-13
IIA-18	22	22	3-Feb-93	Rb	B1	22446	4.7E-13
IIA-19	31	31	30-Mar-93	Cs	C3	22581	4.2E-12
IIA-20	7	37	13-May-93	Rb	C4	22657	7.0E-13
IIA-21	9	39	26-Jun-93	Cs	A1	22700	2.9E-12
IIA-22	5	35	30-Aug-93	Cs	B4	22779	3.8E-12
IIA-23	4	34	26-Oct-93	Rb	D4	22877	4.9E-13
IIA-24	6	36	10-Mar-94	Cs	C1	23027	8.1E-12
IIA-25	3	33	28-Mar-96	Cs	C2	23833	3.9E-12
IIA-26	10	40	16-Jul-96	Cs	E3	23953	2.9E-12
IIA-27	30	30	12-Sep-96	Cs	B2	24320	1.6E-12
IIA-28	8	38	6-Nov-97	Rb	A3	25030	3.0E-12
***IIR-1		42	17-Jan-97				
IIR-2	13	43	23-Jul-97	Rb	F5	24876	9.3E-13
IIR-3	11	46	7-Oct-99	Rb	D2	25933	3.7E-13

*  Satellite is no longer in service.			
**  US SPACE COMMAND, previously known as the NORAD object number;	
also referred to as the NASA Catalog number.  Assigned at successful
launch.					
***  Unsuccessful launch.