Post on 12-Jan-2016
description
Deep Occultations
With GRAS
C. Marquardt, A.von Engeln and Y. Andres
Slide: 2
Outline
Motivation
GRAS Data Characteristics
Deep Occultation Examples
Attenuation and Maximum Bending Angle Gradients
Conclusions
Slide: 3
Deep Occultations
During the last (October 2009) FORMOSAT-3/COSMIC Workshop, Sergey (see Sokolovskiy et al., 2010, Radio Science) made us aware that...
− …signals from strong bending in the lower troposphere may be found at SLTAs below -150 km
− …neglecting deep occultation signals may cause systematic negative biases in the lower tropospheric retrievals
− …including noise from very low SLTAs may cause systematic positive biases
A number of questions remained:
- Can GRAS get deep occultation signals at all, given that it requires code tracking for raw sampling (and had a lower SLTA limit @ -145 km at the time)?
- How far down does atmospheric information spread / do we need to track RO signals?
- How weak can these signals become - what sensitivity must (future) RO receivers have?
- Are there (fundamental?) limits to our ability to observe strong bending angle (gradient)s due to the measurement principle (which is correlation against imperfectly orthogonal codes)?
- Is signal amplitude a reliable indicator for atmospheric information?
Slide: 4
Deep Occultations with GRAS
In December 2009 (10th - 16th Dec.), we modified GRAS to do raw sampling measurements down to – 300km SLTA.
This presentation provides an analysis of the information contained in deep occultation measurements obtained with GRAS
- statistics from 5 days (11th -15th December 2009)
- selected examples from 14th December 2009
In October 2010, we permanently changed the GRAS configuration to track down to -250 km SLTA.
Slide: 5
GRAS Measurement Modes (and Consequences)
Dual Frequency Carrier Tracking: code and carrier for L1 and L2 are tracked; both
(+ C/A) are reported @ 50 Hz
Single Frequency Carrier Tracking: C/A code and carrier phase are tracked; C/A
code and carrier are reported @ 50 Hz
Single Frequency Raw Sampling: C/A code tracked, 1 kHz sampling of carrier
Either L2 or RS due to hardware constraints
− CL data gaps in rising occultations which are currently not covered by RS data
SF carrier tracking and raw sampling can occur simultaneously
GRAS requires the C/A code being tracked even in raw sampling mode
− Loss of C/A code tracking during raw sampling causes data gaps in most
occultations
− Range model is used internally to aid C/A code acquisition, but the receiver
doesn’t rely on it for measurements (which might be overly pessimistic)
− Re-acquisition latency is in the order of 1 sec
Slide: 6
Meridional Penetration (10/2007)
Meridional density distribution of lowest SLTA for RS data segments longer than 0.2 sec
Occultations reach deeper in the tropics…
…and data is clearly cut-off prematurely at -145 km SLTA at low latitudes
There’s an interesting feature for rising occultations around the cut-off altitude (-145 km SLTA)
Slide: 7
Meridional Penetration (Dec 2009)
Meridional density distribution of lowest SLTA for RS data segments longer than 0.2 sec
Same pattern for setting and rising occultations ending/beginning above -200 km SLTA…
…an additional feature at very low SLTAs for rising occultations.
Slide: 8
Penetration down to -200 km SLTA (setting, RS@1kHz)
Slide: 9
Penetration down to -200 km SLTA (setting, RS@50Hz)
Slide: 10
Penetration down to -200 km SLTA (setting, spectra)
There is information in GRAS data for SLTAs below the then current (-145 km SLTA) cut off
Is the low SLTA information always related to the atmosphere?
Slide: 11
Penetration down to -200 km SLTA (setting, cont’d)
Diagonal lines indicate cross-PRN C/A code correlations in the correlations…
…which so far we mainly used to advertise GRAS’s high sensitivity
Slide: 12
PRN Cross-Correlations
GPS (and other GNSS systems) distinguish signals from different satellites by
correlating a satellite specific replica code with the observed signal (which has this
code modulated on top of the carrier frequency)
For raw sampling / open loop data, this is the L1 C/A code
Nominal maximum cross-correlations between C/A codes from different PRNs are
in the order of -24 dB, varying with doppler offset (can be as large as -21 dB)
This is no problem if the measured atmospheric signal is stronger than any PRN
cross-correlations
Slide: 13
Penetration down to -200 km SLTA (setting, cont’d)
< -30 dB for signal
Slide: 14
Penetration down to -200 km SLTA (setting, spectra)
There is information in GRAS data for SLTAs below the then current (-145 km SLTA) cut off
Is the low SLTA information always related to the atmosphere?
Slide: 15
Penetration down to -200 km SLTA (setting, cont’d)
Is the low SLTA information always related to the atmosphere?
Well – probably both no (left) and yes (right)
Slide: 16
Penetration down to -200 km SLTA (setting, cont’d)
-30 dB for cross tracking
-30 dB for signal
Slide: 17
Penetration below -200 km SLTA
setting rising rising
Slide: 18
Penetration below -200 km SLTA
setting rising rising
Slide: 19
Penetration below -200 km SLTA (setting)
Doubtful if “signal” below -200 km SLTA is related to atmosphere, even in this case...
Signal attenuation in deep occultations quickly becomes as low as cross-PRN tracking events; is this a fundamental limit for the observation of deep occultations?
Slide: 20
Penetration below -200 km SLTA (rising)
Data below -200 km in rising occultations was related to cross-PRN tracking in all other cases analysed...
...strongly suggesting that there might not be much information on atmospheric occultations below -200 km SLTA
Slide: 21
Meridional Penetration (Dec 2009, once more)
Atmospheric signal above –200 km SLTA
PRN cross tracking below
EUMETSAT SWG 29September 2010Slide: 22
Attenuation Statistics
Boxplot…
…for rising…
…and setting occultations
Not sure if I completely understand this yet…
Slide: 23
Attenuation and Bending Angle Gradients
Assuming defocussing as main mechanism for signal attenuation, attenuation is (approximately)
Strongest bending occurs on top of bending angle spikes; typical values for GRAS (L = 3300 km):
Smoothing in retrievals further reduces resolvable gradients (and thus max bending angle values)
dad
LA
AM
1
120
2
M [dB] dα/dh [rad/m] Comments
-20 -3.0 x 10-5
-21 -3.8 x 10-5 Nominal GRAS requirement
-24 -7.6 x 10-5 EPS/GRAS-SG requirement; nominal PRN cross correlation
-30 -3.0 x 10-4 Observed PRN cross correlations
Observed atmospheric signal
“Observed” ECMWF bending angle gradients (thanks Sean!)
Slide: 24
Conclusions
Can GRAS get deep occultation signals at all, given that it requires code tracking for raw sampling?
- Yes.
How far down does atmospheric information spread / do we need to track RO signals?
- GRAS: down to -200 km SLTA; everything below are very likely PRN cross-correlations (or so weak that they cannot be observed by GRAS)
- Signals can become weaker than PRN cross-correlation already at -150 km SLTA
How weak can these signals become - what sensitivity must (future) RO receivers have?
- GRAS data: - 30 dB attenuation (or more), i.e. in the order of or below PRN cross correlation level
- Some evidence for GRAS noise level around - 40 dB
Are there fundamental limits to our ability to observe strong bending angle (gradient)s?
- Receivers w/o showing PRN cross correlations: may miss lowest part of deep occultations, limiting the ability to measure steep bending angle gradients on top of large bending angle ‘spikes’. 50 Hz limitation?
- Receivers w/ PRN cross correlations: require separation of atmospheric and cross PRN signal contributions
Is signal amplitude a reliable indicator for atmospheric information?
- No, because cross-correlation events cannot be distinguished by amplitude data alone
EUMETSAT SWG 29September 2010Slide: 25
Attenuation Statistics
Boxplot…
…for rising…
…and setting occultations
Not sure if I completely understand this yet…