ICESat-2 ATLAS Level1B数据产品处理算法理论基础

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"ICESat-2_ATL02_ATBD_r004.pdf" 是一份关于ICESat-2（冰、云和陆地卫星-2）ATLAS（先进地形激光高度计系统）产品的算法理论基础文档，特别是针对ATL02（Level 1B）数据产品的处理。该文档由NASA GSFC（戈达德太空飞行中心）和合作机构的专家共同编写，详细阐述了从Level 1A数据到Level 1B数据的转换过程，包括科学单位转换、系统校准和环境效应修正。 ICESat-2是一项空间任务，旨在通过其搭载的ATLAS仪器精确测量全球冰层、海洋表面和陆地高度变化，以了解气候变化的影响。ATLAS是一种先进的激光测高系统，能发射激光脉冲并接收反射回的信号，从而计算地面高度。它生成的数据分为多个级别，其中Level 1B数据产品是初步处理的结果，包含了经过科学单位转换的时间有序遥测数据，并且已经考虑了系统校准和环境因素的修正。在ICESat-2科学研究者领导的处理系统（SIPS）上，实施了这些复杂的算法。SIPS包含了两个关键组件：科学算法软件（ASAS）和调度与数据管理系统（SDMS）。ASAS负责将原始数据转化为标准的科学数据产品，从Level 0到Level 4，每个级别都包含了不同级别的处理和信息。Level 0产品是最原始的遥测数据，而Level 4产品则是经过多步骤处理的最终科学数据。文档中详述了Level 1B处理的具体步骤，包括如何解析和部分重新格式化Level 1A数据，以及如何应用校准参数来修正系统误差。此外，还讨论了如何考虑环境效应，如大气折射和地球曲率，以提高高度测量的准确性。这份ATBD文档对于理解ICESat-2数据产品的生成过程至关重要，为科学家和研究人员提供了数据处理的理论依据和详细说明。通过这个处理流程，ICESat-2能够提供高精度的地形信息，这对于监测极地冰盖消融、森林覆盖变化、海洋表面高度以及地球表面其他地形特征的变化具有重要意义。这些数据对于气候建模和环境研究有着不可估量的价值，同时也为决策者提供了关键的科学信息，帮助他们应对全球气候变化的挑战。

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ICESat-2-SIPS-SPEC-0150 Software Version 004

Table 1: Photon counting electronics terminology

Term

Description

Background photons

Photons from any source other than the laser that strike the detector.

Background photons

per second

Background power divided by energy per photon.

Counting Efficiency

Combined effect of the detector’s efficiency and the throughput of any

optics between the input and the detector.

Dark electrons

All electrons emitted from the detector cathode not due to incident

photons.

Energy per photon

Planck’s constant times the speed of light divided by wavelength.

Events (in a time

interval)

Nonlinear function of total electrons in that time interval and earlier;

electrons that are emitted within the detector dead time after a previous

electron do not result in events.

Photoelectrons

Electrons emitted from the detector cathode in response to photons striking

the detector

Receive Events (RX)

Digital edges coming from the DEM, assumed to be the result of electrons

emitted from the detector cathode. May also be referred to as a Return

Event.

Return photons

Photons emitted by the ATLAS laser that scatter off an object downrange,

are collected by the ATLAS receiver, and strike the detector.

Time tags

Each time tag corresponds to a particular start or return event and gives the

time at which events undergo a time-to-digital conversion (TDC) by a

PCE.

Transmit Events (TX)

Digital edges coming from the SPD at threshold crossings. May also be

referred to as a Start Event.

Table 2: Primary measurements

Purpose

Measurement

Calculation

Resolution (Units)

Elevation

Time of Flight

Elapsed Time from

Transmit Pulse to Return

Pulse

•

USO: Coarse Time (10 ns)

•

TDC: Fine Time (200 ps)

Geolocation

Time of Day

GPS Time Elapsed from 1

PPS to Transmit Pulse

•

GPS (100 ns)

•

IMET (40 ns)

•

AMET (10 ns)

Geolocation

Pointing

Responsibility of POD/PPD

ICESat-2-SIPS-SPEC-0150 Software Version 004

Table 3: Key IceSat-2 / ATLAS Parameters

Parameter Name

Value

Units

Notes

Orbital Altitude

481 to 511

Laser Firing Rate (nominal)

kHz

Transmitted Energy (typical strong beam)

48 to 172

µJ

Best-estimate actual energies for all beams

are in ATL02. See Section 5.2.

Transmitted Energy (typical weak beam)

12 to 43

µJ

Transmitted Wavelength

532.272

Transmitted Beam divergence, 85% enclosed

energy (typical)

µrad

Measured profiles of all beams are in CAL

Transmitted Beam Eccentricity

0.4

none

Transmitted Beam diameter, 85% enclosed

energy (typical)

10.5

At 500 km altitude

Transmitted Beam Angular Spacing (long

dimension)

6.6

mrad

approximately across track

Transmitted Beam Angular Spacing (short

dimension)

mrad

approximately along track

Transmitted Beam Spacing (long dimension)

3.3

approximately across track, at 500 km

altitude

Transmitted Beam Spacing (short dimension)

2.5

approximately along track, at 500 km

altitude

Pointing Direction (nominal)

N/A

Nadir is along the centerline of cross-track

direction of the beam pattern. Position along

the centerline varies with roll.

Track Pair Spacing (nominal)

spacing between weak and strong track in

the same pair, at 500 km altitude

Receiver Aperture Diameter

0.802

Receiver Aperture Effective Area

0.41

Less than aperture circular area due to

obstruction in telescope

Receiver Field of View Angular Diameter

83.5

µrad

Receiver Field of View Diameter

41.75

At 500 km altitude

Receiver Optical Throughput (typical at peak

wavelength; does not include obstruction in

telescope)

.41

none

Best-estimate actual products of receiver

throughput and efficiency are in ATL02.

See Section 5.3.

Receiver Counting Efficiency (typical)

0.15

none

Receiver Effective Optical Bandwidth

Rectangular band at peak wavelength

throughput

Event timing precision (typical)

200

Width of delay-line cell

Single-photon time-of-flight uncertainty (typical

standard deviation)

800

Best-estimate time-of-flight uncertainties

are in ICESat-2-ATSYS-TN-0660.

1.3 Description of Relevant Hardware Interactions

The ATLAS system-level components shown in Figure 3 are relevant to the scope of the detailed

computations presented in this ATBD. This section summarizes how these components are used in photon time

tagging and within the timing apparatus presented in the next section and throughout the remainder of this

document.

ICESat-2-SIPS-SPEC-0150 Software Version 004

The ATLAS Serial Card (ASC) is housed within the Main Electronics Box (MEB). The ASC has four

primary functions:

1) house two SpaceWire routers to interface the Single Board Computer (SBC) with the three PCEs and the

Spacecraft;

2) provide a serial interface to the laser electronics responsible for providing the 10-kHz laser fire

command to the laser;

3) provide the necessary time indexing to correlate the Spacecraft GPS time reference from the active GPS

Receiver (A or B) to an Ultra-Stable Oscillator (USO) used for precise timing; and

4) distribute an internal 1 pulse per second (1 PPS) signal to the various boards in the MEB that need it.

All timing events recorded by ATLAS can be related to the internal USO clock, referred to hereafter as the

“ruler clock.” The ruler clock and other timers have various resolutions that are hierarchically nested. This

multi-resolution timing apparatus is portrayed graphically in Figure 4.

Figure 3: ATLAS System-Level Components Relevant to the Computations Described in the ATL02

ATBD

At the lowest resolution, 1-second intervals are counted off both internally in the ASC and externally via

the Spacecraft GPS Receivers (GPSR). There are two GPSRs, referred to as GPSR A and GPSR B, according to

which receiver is in use. The GPSR provides about 100-ns resolution time reference to the instrument via a one

pulse per second (1 PPS) signal and a corresponding Time-at-the-Tone-was (TAT) message to the MEB via the

ASC. The SBC receives the TAT message and distributes it to the PCE LEON3FT flight software over the

SpaceWire link. Note that the PCE LEON3FT may be referenced, interchangeably, as the PROC or LEON3

card.

As Figure 4 shows, these nested time-measurement resolutions are used from coarse scale to fine scale

for measuring the time of events. At the top, and coarsest resolution, there is the 1 PPS signal. This is the first

step in aligning relative time measurements with the GPS time reference via linear interpolation using the

ICESat-2-SIPS-SPEC-0150 Software Version 004

ATLAS Mission Elapsed Time (AMET) counter. The source of the 1 PPS can be either ATLAS internal or

external. For the internal 1 PPS the ASC takes its 100-MHz USO clock and divides it down to 1 Hz. This is the

default source of the 1 PPS signal and is referred to as the internal 1 PPS or the ASC 1 PPS. Additionally, the

external 1 PPS is taken directly from either GPSR A or B time signal received by the ASC. We refer to that

source as the external 1 PPS, Spacecraft (SC) 1 PPS, or simply GPS 1 PPS. Whatever the source, the ASC

distributes a 1 PPS signal to each PCE card, which then correlates it to its internal time measurements, which

includes the Instrument Mission Elapsed Time (IMET), major frames, and minor frames.

Figure 4: Timing apparatus uses hierarchically nested, multi-resolution time scales

At the next level of resolution is the 50-Hz data processing frame, otherwise known as the major frame.

Nominally, each major frame is comprised of 200 minor frames consisting of a single laser shot and its

associated return event(s). The purpose of the major frames is to granulize the data processing by the flight

software for scheduling data processing, data management, operational parameter settings, and housekeeping.

The Dataflow Controller (DFC) on each PCE maintains a major frame counter for both the strong and weak

beam. These counters are used as the major frame ID. The major frame counters start counting after PCE

power-on (or reset) and can count up for about 2.7 years before rolling over. The major frame ID is used to

identify individual major-frame dependent data carried by both housekeeping and science telemetry packets to,

in part, identify the start/return events and associated time tags.

USO$

Clock

𝐷

USO

Latest$T0

USO$

Clock

1pps

1 second

AMET$

Counter

IMET

Counter

Major$

Frames

1 2 3

...

5049

1$Hz

10$kHz

Minor$

Frames

1 ... 200

100$MHz

...

50$Hz

25$MHz

Zoom

10$kHz

100$MHz

Major Frame

Minor Frame

Time

Low

Resolution

High

Resolution

Tx/Rx$measurements$are$relative$to$the$latest$T0.

The$USO$provides$the$“ruler$clock”$for$the$measurements.

ICESat-2-SIPS-SPEC-0150 Software Version 004

Because it stems from the internal USO clock we can enjoy clock synchronization across the three

Photon Counting Electronics (PCE) cards that also receive the USO clock for internal timing alongside the 1

PPS reference. So, when using internal 1 PPS to define every 1-second period, 10,000 T0 events will always be

observed within said period. However, when using the asynchronous external 1 PPS, there will be some clock

phase skew and, hence, the number of T0 events can differ from 10,000 by either plus or minus one count. It is

for that reason that the internal 1 PPS source is favored as the default source to distribute to each PCE and fall

back to the external 1 PPS sources (from either GPSR A or B) only for fault tolerance. This is shown

graphically in Figure 5.

Nevertheless, the use of the external 1 PPS source for internal timing is not expected to adversely affect

the data processing algorithms beyond what can be corrected by further ground processing. However, testing

with the redundant GPSR 1 PPS signals as external sources of 1 PPS for the PCEs will not be tested

operationally. Regardless of the 1 PPS source distributed to the PCEs, both the internal and external 1 PPS

signals are used concurrently by the ASC to latch the current AMET value every 1 PPS pulse. This latch

mechanism is also shown in Figure 5.

The ASC divides down the USO clock to a 10 kHz frequency and uses this to drive the laser fire

commands. Each command results in the emission of laser light from one of the two ATLAS lasers. The lasers

are redundant units so only one laser is operational at a time. Part of the outgoing laser pulse is picked off and

routed to the Start Pulse Detector (SPD). After filtering, the smoothed laser pulse is timed by reporting the time

of the leading and trailing edges of the voltage pulse across two adjustable thresholds. Details of this are

discussed in future sections. For now, it is sufficient to say that each T0 clock advance (clock tick) is associated

(by its trigger) with precisely one laser transmit pulse. The subsequent return events captured by the Detector

Array Assembly (DAA) and communicated to the PCEs RX input channels via the Detector Electronics Module

(DEM) as electrical pulses. These return events are referred to as RX events. The RX event data is related to the

TX event also communicated to the PCEs TX input channels via the SPD. The TX and associated RX events are

all collected within the same major frame. The major frame is processed by the on-board signal processing on

each PCE card, which is also responsible for start/return time tagging, RX event histogramming, and

transmitting of the resulting science data via downlink telemetry.

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