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 -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 (10 th - 16 th 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 (11 th -15 th December 2009) -selected examples from 14 th 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 50 Hz  Single Frequency Carrier Tracking: C/A code and carrier phase are tracked; C/A code and carrier are 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,

Slide: 9 Penetration down to -200 km SLTA (setting,

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

Slide: 18 Penetration below -200 km SLTA setting 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 29 September 2010 Slide: 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) M [dB]dα/dh [rad/m]Comments x x Nominal GRAS requirement x EPS/GRAS-SG requirement; nominal PRN cross correlation x 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 29 September 2010 Slide: 25 Attenuation Statistics  Boxplot…  …for rising…  …and setting occultations  Not sure if I completely understand this yet…