|
ESA's Product Control Service
USO Frequency Measurement
One of the critical elements of the Radar Altimeter system is the
Ultra Stable Oscillator (USO) as it clocks the range measurement.
The USO frequency is measured on ground during the passes
over the Kiruna ground station and is used at 15 MHz to drive the bit clock during the replay of
the tape recorder.
The frequency is thus directly recoverable on-ground from
the real-time bit-synchronizer at the ground station, comparing it with a standard atomic
frequency.
The USO information is received in ESRIN PCS on a weekly
basis (on Tuesdays) for the ERS operational satellites (on a daily basis after a RA sensor
special event). The USO value is then processed and a correction file is made available to the
user community to improve the accuracy of the Radar Altimeter products with respect to this
systematic effect, which is not taken into account in the ESA ground processors.
This ERS USO correction is to be added to the altimetric range.
USO Correction File Description
File Maintenance: Weekly basis
File naming convention:
ERSX_RA_USO_YYMMDD.TXT
X : 1 for ERS-1, 2 for ERS-2
YY : YEAR [0,99]
MM : MONTH [1,12]
DD : DAY [1,31]
The ERS USO correction file contains the following fields:
Date : date of the USO measurement
Time : time of the USO measurement (HH:MM:SS.SSS)
(Default value is 99:99:99.999)
Day : days number since the satellite launch.
F_15 (Hz) : USO measured frequency
[ Delta_F (Hz), Delta_R (mm) ] for the 3 types of products, URA/QLOPR,
OPR and WAP.
Delta_F (Hz) is the difference in frequency between F_15 and
F0_15.
Delta_R (mm) is the USO range correction and is computed
according to:
Delta_R = - (795000 * Delta_F / F_15) * 1000
F0_15 is the nominal 15 MHz USO frequency but different
values are used within the ESA ground processors. These values are given in the Table 1 here below:
|
|
URA / QLOPR products |
OPR products |
WAP products |
|
ERS-1 F0_15 (Hz) |
15000000.00 |
15000000.20 |
15000000.05 |
|
ERS-2 F0_15 (Hz) |
15000000.00 |
14999999.96 |
15000000.05 |
Table 1: The F0_15 characterization values used to process the ERS RA data
products.
|
Date |
Time |
Day |
F_15 (Hz) |
Delta_F (Hz) |
Delta_R (mm) |
Delta_F (Hz) |
Delta_R (mm) |
Delta_F (Hz) |
Delta_R (mm) |
|
|
|
|
URA/QLOPR |
URA/QLOPR |
OPR |
OPR |
WAP |
WAP |
|
17-Jul-1991 |
99:99:99.999 |
0 |
15000000.040 |
0.040 |
-2.120 |
-0.160 |
8.480 |
-0.010 |
0.530 |
|
04-Aug-1991 |
99:99:99.999 |
18 |
15000000.132 |
0.132 |
-6.996 |
-0.068 |
3.604 |
0.082 |
-4.346 |
Table 2: Example of two extracted records from the ERS-1
USO correction file and the corresponding fields labels.
Algorithm Description
The time delay formula presented here below is extracted
from the ESA document ER-IS-ESA-GS-0001, issue 2 b, page 4.2-63 to 4.2-66.
The corrected time delay T is given by:
T = (Ttm - Eft) * Cuso - Egt + 5/Fprf
Ttm = telemetered time delay (ns)
Eft = calibration time (ns)
(From the GS-0001 document in the section 4.2.9)
Cuso = F0_15/F_15
F0_15 = the nominal USO frequency (in Hz)
F_15 = the USO frequency measured on the ground at Kiruna (Hz)
Egt = ground calibration time of the order of 30 ns for ERS
Fprf = F_80/78432
F_80 = 16 * F_15/3
The corrected range R in meter is equivalent to:
R = T * c/2
Considering that [ 5 * 78432 * (3/(16 * F0_15)) * c/2 ] = 735300 meters, we get:
R = ( (Ttm - Eft) * c/2 + 735300 ) * F0_15/F_15 - Egt* c/2
where (Ttm - Eft) * c/2 + 735300 represents the mean altitude.
The ERS representative mean altitude for a 501day cycle is 795000 m.
R = 795000 * F0_15/F_15 - Egt* c/2
Delta_F = (F_15 - F0_15)
R0 = nominal range derived from the F0_15 USO frequency
R = range derived from the F_15 USO frequency
Delta_R = R - R0 = - 795000 * Delta_F/F_15 - Egt* c/2 + Egt* c/2
Delta_R = R - R0 = - 795000 * Delta_F/F_15
To correct the altimetric range, Delta_R shall be added to R0 (R = Delta_R + R0)
| 
