SCIAMACHY Orbit Analysis

The definition of mission scenarios, states and timelines is only possible after a thorough SCIAMACHY orbit analysis has been performed. This analysis has to model the viewing conditions from SCIAMACHY to Sun and moon over the mission lifetime, based on the ENVISAT reference orbit and the instrument TCFOV constraints. Particularly, the orbit analysis has to address

1.) ENVISAT orbit
2.) Yaw-steering
3.) Refraction
4.) Orbit events
5.) Sun occultation (SO&C)
6.) Sub-solar measurement
7.) Moon occultation (MO&C)
8.) Eclipse
9.) Ascending Node Crossing (ANX)
10.) Kiruna coverage


A summary of the ENVISAT orbit, how it relates to solar and lunar events such as rising and setting together with illustrations is given in chapter four of the SCIAMACHY book: Instrument operations, including mission scenarios, states and timelines.

1.) ENVISAT orbit

The orbit of ENVISAT is specified as listed in the table below. The associated local coordinate system is the right handed system FLO* (Local Relative Reference System) with X (pitch), Y (roll) and Z (yaw). The -Y direction points towards the velocity vector, i.e. flight direction and Z defines the zenith direction.

Parameter

 

semi-major axis a
inclination i
eccentricity e
mean local time at descending node
argument of perigee
orbital period
mean altitude
orbits per day
repeat cycle
7159.4927 ± 0.068 km
98.549387 ± 0.009
0.001165 (-0.001165/+0.005)
10:00 a.m. ± 5 min
90.0 ± 3
100.6 min
799.790 km
14 11/35
35 days (501 orbits)

Line of sight definitions use the angle azimuth and elevation. Azimuth is counted clockwise from -Y to the projection of the target line of sight onto the -Y/X plane. Elevation angle is the angle between this projection and the target line of sight. The nadir point has an elevation of 90, the zenith is equivalent to an elevation of -90.


2.) Yaw-steering

The nominal attitude control mode of ENVISAT is the Stellar Yaw-Steering Mode. This mode transforms the Local Relative Reference System FLO* into the system FLO1 (Local Relative Yaw Steering Orbital Reference System). It compensates the impact of Earth rotation in the sub-satellite nadir point. The main effect of the yaw steering mode is a sinusoidal oscillation of the local -Y axis around flight direction. The maximum amplitude of ± 3.92 occurs at ascending (positive values point left of flight direction) and descending (negative values point right of flight direction) node.
One of SCIAMACHY's mission goals is the quasi-simultaneous measurement of the same volume of air both in limb and nadir mode. This is achieved by executing the limb measurement with an offset relative to flight direction. The size of the offset must be such that Earth rotation moves the volume of air into the sub-satellite track just at that moment when ENVISAT passes over it and SCIAMACHY operates a nadir state. Taking the orbital parameters and Earth's angular velocity into account, matching limb and nadir observations are separated in time by 450 sec.
In limb mode, SCIAMACHY scans an area lying 3280 km ahead at the horizon in flight direction. The location of the area is determined by the actual amplitude of ENVISAT's yaw steering. As this is a function of the Earth velocity in the sub-satellite nadir point, an orbital phase shift of about 27 occurs between required offset for limb/nadir matching and actual offset as defined by ENVISAT yaw-steering. Thus a yaw-steering correction is implemented on-board SCIAMACHY. The correction compensates for this phase shift in limb measurements.


3.) Refraction

The scheduling of observations close to the horizon is affected by refraction. Therefore state and timeline definitions for Sun and moon occultation purposes must take this fact into account. As Sun and moon have about identical apparent diameters of about 31.5 arcmin, refraction impacts Sun and moon in the same way.
Using an analytical model, the refraction angle as a function of time during rise was determined. At the horizon, the refraction angle amounts to approx. 70 arcmin (entry and exit of line of sight into atmosphere must both be considered). The Sun/moon is still well below the horizon, when the refracted image can be observed above the Earth's limb. When the Sun/moon centre has reached an altitude of about 17 km, refracted image and the true position of the solar/lunar disk overlap and refraction can be neglected. Below the critical altitude of 17 km, the apparent angular rate of the rising true Sun/moon is significantly higher than that of the rising refracted image.


4.) Orbit events

The Orbit Analysis is based on an orbit propagation and target, i.e. Sun and moon, line of sight calculations using the ENVISAT provided Customer Furnished Items (CFI) s/w package and the ENVISAT reference orbit. The results of these calculations are further analysed with the SOST Orbit Analysis tool. Of particular importance for SCIAMACHY are the events

Around these events the orbit analysis investigates event time, azimuth/elevation angle of Sun or moon or azimuth rate/elevation rate of Sun or moon. Additionally, parameters like e.g. geographic latitude of sub-satellite point at event, geographic latitude of tangent point at sunrise/moonrise or lunar phase in MO&C window are also determined. The time resolution amounts to 1 sec.
For routine operations the analysed time interval spans usually one year. As each timeline corresponds to one orbit interval, a major task of the orbit analysis is to determine the time elapsed between events. This is a pre-requisite in the definition of timelines. The orbit analysis eliminates potential conflicts between timelines already in the timeline definition process, i.e. the scheduling of SCIAMACHY timelines by the ENVISAT mission planning system is expected to detect no errors.
The results of the orbit analysis are presented on our web-site as graphical displays of parameter versus day number. Each year, starting from 2002, is treated separatly. The monthly moon visibilty periods are indicated in each display as a grey shaded box.


5.) Sun occultation & calibration (SO&C)

The mean local time at descending node of 10 a.m. causes the Sun to appear always on the left side of ENVISAT. SCIAMACHY observes sunrise above the northern hemisphere. In the SO&C window the events

define the start/stop events of associated SO&C orbit intervals. In addition, the time between

is also analysed. The SO&C graphical displays provide

These displays show the evolution of a particular parameter with time over the year. Additional SO&C information is provided as

The SO&C related parameters referring only to the Sun vary over a year but can be considered constant on a yearly basis as long as the reference orbit remains unchanged. SO&C parameters also addressing the moon exhibit also a change with years.


6.) Sub-solar measurement

The sub-solar event is defined with solar azimuth = 270 assuming no SCIAMACHY misalignment around the yaw axis. During the sub-solar measurement the Sun moves horizontally through the sub-solar TCFOV with a rate which is determined by the angular rate of ENVISAT. Variation of the solar elevation over a year causes the Sun to move vertically through the sub-solar TCFOV. Highest elevation, i.e. Sun is at upper part of TCFOV, is reached beginning of June, lowest elevation beginning of February.

The sub-solar graphical displays provide

Sub-solar related parameters show variability over a year but are constant on a yearly basis.


7.) Moon occultation & calibration (MO&C)

Lunar occultations follow the same concept as solar occultations. The moon is used to probe the Earth atmosphere up to an altitude of 100 km. Above the atmosphere the moon is a target for calibration & monitoring measurements. While sunrise is mainly defined by the relatively stable position of the Sun w.r.t. ENVISAT's orbital plane, the properties of moonrise in SCIAMACHY's limb TCFOV are determined by the orientation of the lunar orbital plane w.r.t. ENVISAT's orbital plane and the ecliptic. In this plane the moon completes one orbit within one synodic period of 29.53 days.
SCIAMACHY moon visibility is depicted in the figure below. Between points 1 and 2 the moon can be observed above the southern hemisphere. The moon visibility above the northern hemisphere between points 3 and 4 coincides with sunrise and is not used operationally. Caused by the lunar orbital motion, the moon moves through the limb TCFOV from left to right with a rate of about 1 per orbit, starting at a lunar phase of 0.6-0.7 and ending shortly after full moon.
The orbit analysis shows that the orbit parameters derived in the MO&C window may exhibit a large variability within a single monthly visibility period. This has to be compensated by timeline definition and mission planning. The variability causes moonrise to occur over a wide range in geographic latitudes. The azimuth range of the limb TCFOV yields an approximate monthly moon visibility period of

Relevant MO&C window events are

They define the start/stop events of associated MO&C orbit intervals. In addition, the time between end of MO&C window to eclipse start is also analysed. The MO&C graphical displays present

These displays show the evolution of a particular parameter with time over the year. As in the case of the SO&C window, additional MO&C information is provided as

All moon related parameters change within one year and over the mission lifetime. In particular the monthly moon visibilty periods are a function of year. They can be found in the lunar monthly visibility files.

Note: 'Lessons learned' from the Commissioning Phase have shown that moon occultations can be successfully executed only when they occur on the nightside. Due to the large field of view (2.1° x 2.1°), the Sun Follower (SF) always detects in dayside occultations, additionally to the moonlight, a strong signal from the bright Earth atmosphere at low lunar altitudes. This prevents SF control from acquiring and tracking the moon. Only when the moon has reached altitudes > 100 km, the SF can be used in both cases (dayside and nightside) for lunar monitoring measurements above the atmosphere. The actually useful moon occultations can be derived from this specific MO&C display, which outlines only those occultations, where the moon rises on the nightside for a line of sight (altitude = 20 km) to the center of the lunar disk.


8.) Eclipse

Eclipse is defined for SCIAMACHY to start when the refracted solar disk moves below the horizon. From this event on eclipse and calibration measurements may start. The ENVISAT ANX time falls always into the eclipse phase.

The eclipse graphical displays provide

Eclipse related parameters show variability over a year but are constant on a yearly basis.


9.) Ascending Node Crossing (ANX)

ENVISAT provided scheduling information often refers to the ANX time. Therefore the SCIAMACHY Orbit Analysis also determines the duration of the time intervals between ANX and

The corresponding yearly curves can be found on the ANX graphical displays.


10.) Kiruna Coverage

For various MPS and procedure driven activities, in particular during the Commissioning Phase, it is important to know the times of Kiruna passes. This type of information can be related to

Usually in both cases the coverage refers to a pass with a spacecraft elevation of 5. Kiruna coverage parameters are provided in the Kiruna coverage files.



Page generated 20 February 2002; last update: 22 July 2015