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…