Flight Dynamics System External Interface Control Document

Draft September 7, 1999

# 1 Purpose and Scope

This interface control document describes all the interfaces involving
the ICESat Flight Dynamics System (FDS). Interfaces exist between the
FDS and the Mission Operations Center (MOC), Multi-Mission Flight
Dynamics (MMFD) at Goddard Space Flight Center, the Earth Polar Ground
Stations (EPGS) through the Wallops Orbital Tracking Information System
(WOTIS), the ICESat Science Team (ST), and JPL.

# 2 Related Documents

TBD

# 3 Interface Requirements

Figure 1 contains the Flight Dynamics System block diagram, showing all
major interfaces. These interfaces will be discussed in subsequent
paragraphs.

![](media/image1.emf){width="5.990972222222222in"
height="4.409722222222222in"}

Figure 1. Flight Dynamics Block Diagram

## 3.1 Mission Description

The ICESat mission is part of the Earth Observing System (EOS)
multi-mission program to acquire the data necessary for a long-term
study and understanding of Earth's global processes and systems. Nominal
mission duration is five years (three year design lifetime for
spacecraft and instruments) and includes near-continuous nadir-pointing
data collection. The ICESat mission is designed to retrieve globally
distributed cryosphere, atmosphere and land topography altimetry data
sets. The laser altimetry portion of the GLAS instrument includes a 1.0
m telescope aperture and a cluster of three solid state lasers. Only one
laser is operational at a time, with the other two included for
redundancy. The instrument measures the round trip transit
time-of-flight of a laser pulse (1064nm wavelength) as it exits from the
instrument boresight, reflects (and is scattered) from the Earth's
surface (ice sheets, land, etc.), and is collected by the telescope
receiver. GLAS uses this data to provide elevation measurements to
within 10 cm accuracy. The spacecraft bus supports the GLAS instrument
by providing power, data services, thermal control, spacecraft pointing
control, orbit maintenance propulsion, and space-to-ground
communications.

ICESat will be launched from the Vandenberg AFB Launch Facility in July
2001, aboard a Delta II (TBR) launch vehicle. The satellite is a
three-axis stabilized, self-contained free-flyer, designed to operate in
a near-circular orbit at an altitude of 600 km and an inclination of
94°.

## 3.2 Interfaces to the Mission Operations Center

The ICESat Mission Operations Center shall be located at and operated by
the University of Colorado's Laboratory for Atmospheric and Space
Physics (LASP). Real-time operations of the ICESat spacecraft shall be
performed by the MOC. The MOC shall also perform off-line operations
functions, including planning and scheduling, orbit and attitude
analysis, and housekeeping telemetry data processing, checking,
archiving, and analysis.

The MOC is host to the FDS. The many interfaces between the MOC and FDS
are described in the paragraphs below. Interfaces between the MOC and
FDS are greatly simplified due to co-location.

### 3.2.1 Planning/Scheduling Products

#### 3.2.1.1 Station Contact Predictions

FDS shall provide estimates of station contact times for use by the MOC
each day for the entire mission. The spacecraft operators use these data
for day to day operations. These products shall be transferred to the
MOC via the LASP computer network, using the same formats and data paths
as used for QuikSCAT. These items are produced using the STK software.

#### 3.2.1.2 Apogee/Perigee Times

FDS shall provide estimates of apogee and perigee times for use by the
MOC each day for the entire mission. The spacecraft operators use these
data for day to day operations. These products shall be transferred to
the MOC via the LASP computer network, using the same formats and data
paths as used for QuikSCAT. These items are produced using the STK
software.

#### 3.2.1.3 Ascending Node Times

FDS shall provide estimates of ascending node times for use by the MOC
each day for the entire mission. The spacecraft operators use these data
for day to day operations. These products shall be transferred to the
MOC via the LASP computer network, using the same formats and data paths
as used for QuikSCAT. These items are produced using the STK software.

#### 3.2.1.4 Eclipse Entry/Exit Times

FDS shall provide estimates of eclipse entry and exit times for use by
the MOC each day for the entire mission. The spacecraft operators use
these data for day to day operations. These products shall be
transferred to the MOC via the LASP computer network, using the same
formats and data paths as used for QuikSCAT. These items are produced
using the STK software.

#### 3.2.1.5 Orbit Estimates and Orbit Propagations

FDS shall provide both Operational Orbit Determination (OOD) and
propagated state vectors for use by the MOC each day for the entire
mission. The spacecraft operators use these data for day to day
operations. These products shall be transferred to the MOC via the LASP
computer network, using the same formats and data paths as used for
QuikSCAT. These items are produced using the MicroCosm and STK software
packages.

#### 3.2.1.6 Initial Condition Vectors (ICVs) 

The FDS shall generate initial condition vectors for the SCC and/or GPS,
as needed, for use by the MOC at any time during the entire mission.
These data shall be requested by the MOC when needed. These are uploaded
to initialize the spacecraft computer and/or GPS receiver after power on
or a reset.

#### 3.2.1.7 Base Time Load

The FDS shall generate the base time load to correlate the spacecraft
clock (clock correlation code) for use by the MOC at any time during the
entire mission. These data shall be requested by the MOC when needed.
This is uploaded to allow the spacecraft to compute and use UTC.

### 3.2.2 GPS Data

The MOC shall provide tracking data from the flight GPS receiver to the
FDS after each X-band downlink. The MOC shall receive, format, and
archive the GPS data from the X-band downlink. The FDS shall use these
data to estimate the spacecraft trajectory. Data transfer shall be via
the LASP computer network. Data format is TBD.

### 3.2.3 Propulsion Data

The MOC shall make propellant tank pressure and temperature available to
the FDS when requested. The FDS shall use these data for propulsion
system modeling. The MOC shall receive, format, and archive these data
after downlink from the spacecraft. Data transfer shall be via the LASP
computer network. Data format is TBD.

### 3.2.4 Maneuver Plan

The FDS shall provide maneuver plans to the MOC after the design of each
maneuver. These plans shall consist of thruster on and off times and
spacecraft attitude, for all orbit change and maintenance maneuvers. The
MOC shall convert these maneuver plans into commands and tables for
upload. Data transfer shall be via the LASP computer network. Data
format is TBD.

### 3.2.5 Groundtrack Monitoring

The FDS shall provide data on groundtrack monitoring and prediction to
the MOC daily. The spacecraft operators shall use these data to
determine or predict when maintenance maneuvers are necessary. These
data shall consist of both graphical and tabular output. Data transfer
shall be via the LASP computer network. Data format is TBD.

### 3.2.6 Frozen Orbit Monitoring

The FDS shall provide data for monitoring and prediction of the frozen
orbit to the MOC daily. The spacecraft operators shall use these data to
monitor the frozen orbit, and plan maneuvers to maintain the frozen
orbit, if necessary. These data shall consist of both graphical and
tabular output. Data transfer shall be via the LASP computer network.
Data format is TBD.

### 3.2.7 Quaternions

The FDS shall provide pointing quaternions to the MOC daily. These data
will allow the laser to point at the ideal groundtrack. From these data
the MOC shall generate tables for upload to the spacecraft. These
quaternions shall be calculated using software developed by CCAR,
installed on the LASP computer network, and maintained and operated by
FDS. Data transfer shall be via the LASP computer network. Data format
is TBD.

### 3.2.8 S-Band Data

The MOC shall make all available to the FDS all data on the S-band
downlink. The MOC shall receive, format, and archive these data after
downlink from the spacecraft. Data transfer shall be via the LASP
computer network. Data format is TBD.

### 3.2.9 Operational Orbit Determination Solution

The FDS shall provide all Operational Orbit Determination (OOD)
solutions to the MOC. The FDS shall estimate and propagate the
spacecraft state daily during routine operations, processing GPS
tracking data with the COTS software MicroCosm. Data transfer shall be
via the LASP computer network. Data format is TBD.

### 3.2.10 Orbit and Attitude Verification

The FDS shall provide support for orbit and attitude verification.

### 3.2.11 Solar Array Encoder Data

The MOC shall provide all solar array encoder conversions to the FDS.
The MOC shall obtain these data from the downlinked telemetry. These
data shall be used to produce more accurate atmospheric drag estimates
for orbit determination. Data format is TBD.

### 3.2.12 Archive

The FDS shall provide data to the MOC for archiving. The MOC shall
archive important data (TBD) from all the main FDS functions. Data to be
archived includes:

3.  Planning and scheduling products from STK

4.  OOD solutions from MicroCosm

5.  Maneuver designs from the maneuver design software

6.  Groundtrack monitoring and prediction data

7.  Off-nadir pointing quaternion data.

Data transfer shall be via the LASP computer network. Data format is
TBD.

## 3.3 Interfaces to Multi-Mission Flight Dynamics

MMFD, located at Goddard Space Flight Center, shall support pre-launch
and early orbit operations for ICESat. This support shall be similar to
that provided by MMFD to QuikSCAT. This support will nominally decrease
gradually and terminate three weeks after launch.

### 3.3.1 Tracking System Accuracy

MMFD shall work with FDS and the NASA ground stations to analyze and
verify tracking system accuracy.

### 3.3.2 Post --Separation Trajectory Predictions

MMFD shall interface between the launch vehicle contractor and the FDS
to acquire and process the launch and post-separation trajectory
predictions. Six weeks prior to launch, the launch vehicle contractor
will provide a spacecraft separation vector based on the nominal
spacecraft trajectory. The MMFD shall use this vector to provide the
FDS, and the EPGS with pre-launch nominal acquisition data in the form
of IIRVs. If a later, updated post-separation state vector is received
prior to launch, MMFD shall also provide these data. In addition, Best
Estimated Trajectory (BET) data will be provided to the MMFD by the
launch vehicle contractor 7 to 10 days prior to launch. The MMFD shall
update the FDS, DSN, and the EPGN with a separation state extracted from
this trajectory when available.

### 3.3.3 Launch Data

MMFD shall interface with the launch site to acquire and process Launch
Trajectory Ascend System (LTAS) data and Redundant Inertial Flight
Control Assembly (RIFCA) data to process the initial orbit data. MMFD
shall receive these data from Vandenburg during launch vehicle ascent
and evaluate the quality of these data. MMFD shall extract a post
spacecraft separation state from the best available source and provided
to supporting elements (see Section 3.3.6).

### 3.3.4 NORAD Data

MMFD shall interface between the NORAD and the FDS by acquiring and
processing NORAD B3 tracking data to generate an orbit prediction. These
data shall be available a few hours after launch.

### 3.3.5 UTDF Data

MMFD shall interface between the NASA ground stations and the FDS to
acquire and process Universal Tracking Data Format (UTDF) angles
tracking data after passes to generate the orbit predicts in the form of
IIRVs. These data will consist of autotrack angle data from the Svalbard
11-meter antenna, the Alaska 11-meter antenna, and the Wallops 11- or
8-meter antenna.

### 3.3.6 Post Separation Trajectory

MMFD shall generate an initial orbit prediction after spacecraft
separation and distribute it as an IIRV to the DSN, MOC (FDS), and WOTIS
for use by the ground stations. This vector shall be based on MMFD's
judgement of the best available data from the sources available. MMFD
shall provide this state vector within 15 minutes of separation. The
product to EPGS and FDS shall be an IIRV, while that to the DSN shall be
an Intercenter Vector (IVC).

### 3.3.7 Update Orbit Predictions -- First 24 Hours

MMFD shall update initial orbit predictions as needed for the first 24
hours after launch, and distribute them as IIRVs to the DSN (TBD), MOC
(FDS), and WOTIS for use by the ground stations. The first orbit
solution shall be delivered between L+4 and L+6 hours, and shall consist
of a series of IIRVs on four-hour centers.

### 3.3.8 Update Orbit Predictions -- First Week

MMFD shall update initial orbit predictions every 24 hours for the first
week after launch, and distribute them as IIRVs to the DSN (TBD), MOC
(FDS), and WOTIS for use by the ground stations. The resulting
acquisition data shall be transmitted to the stations weekdays by 1900Z,
and shall consist of a series of IIRVs on four-hour centers.

### 3.3.9 Orbit Determination

If GPS data is not available for processing by the FDS, MMFD shall
perform orbit determination based on angle data from the ground tracking
stations and distribute the solutions to the FDS. This shall be
performed for the first three weeks of the mission only.

### 3.3.10 Maneuver Consultation

MMFD shall provide consultation for maneuver and trajectory planning
during the first few weeks after launch. This will serve as a
verification of the maneuver and trajectory planning performed by FDS.

## 3.4 Interfaces to Tracking Stations 

Components of the EOS Polar Ground Station network (EPGS) are the Alaska
Ground Station (AGS) located at the Poker Flat Research Range in Alaska,
the Svalbard Ground Station (SGS) located on Spitzbergen Island in
Norway, and the Wallops Ground Station (WGS) located at Wallops Island,
Virginia (use of McMurdo is TBD). The Wallops Orbital Tracking
Information System (WOTIS) will also provide scheduling support.

### 3.4.1 IIRVs

The FDS shall provide Improved Inter-Range Vectors (IIRVs) data to the
tracking stations. These data are used to point the communication
antennas at the spacecraft. Data transfer shall be (same as QuikSCAT).
Data format shall be the standard IIRV format.

Prior to launch, the FDS shall use the predicted post-separation
spacecraft trajectory to provide EDOS and the EPGS with pre-launch
nominal acquisition data in the form of IIRVs. The pre-launch nominal
acquisition data shall be provided no earlier than L-7 days and no later
than L-3 days. These acquisition data shall be updated if a later,
updated post-separation state vector is received by the FDS prior to
launch.

The FDS shall update the EDOS and EPGS with acquisition data within 15
minutes after spacecraft separation. Acquisition data shall be updated
throughout the LEOP (Launch and Early Orbit Operations) when necessary,
based on the latest orbit solution.

## 3.5 Interfaces to JPL

### 3.5.1 GPS Data

The FDS shall obtain GPS tracking data from the IGS network of ground
tracking sites from JPL (TBD). JPL routinely obtains and archives
tracking data from the IGS network. These data shall be combined with
GPS data from the flight receiver to produce the OOD solutions. This
data transfer shall occur daily during routine operations, and by
operator request during early orbit ops or orbit changing. Data transfer
shall be via the internet or ftp. Data format is as provided by JPL.

## 3.6 Interfaces to the Science Team

The ICESat Science Team (ST) is distributed at various academic and
government institutions around the country and world. Science Team
activities will be directed from the Center for Space Research at the
University of Texas at Austin. The ST will interact with the ICESat MOC
and FDS through the Science Computing Facility (SCF), located within the
EOSDIS at Goddard.

Data transfer from the FDS to the ST shall be via the MOC archive. All
important FDS data (TBD) shall be placed in the mission archive at the
MOC. The archive shall be accessible to authorized users via the
internet or ftp.

### 3.6.1 Operational Orbit Determination Solution

The FDS shall provide the Operational Orbit Determination (OOD) solution
to the ST daily. The FDS shall place these data in the MOC archive. The
ST shall be capable of retrieving these data from the archive. Data
format is TBD.

### 3.6.2 Maneuver Plan

The FDS shall provide maneuver plans to the ST. These plans shall
consist of thruster on and off times, spacecraft attitude, and expected
orbit changes, for all orbit change or maintenance maneuvers. The FDS
shall place these data in the MOC archive. The ST shall be capable of
retrieving these data from the archive. Data format is TBD.

### 3.6.3 Precision Orbit Determination Solutions

The ST shall periodically provide Precision Orbit Determination (POD)
solutions to the FDS. These solutions shall be used to verify and
evaluate accuracy of the OOD solutions. Data transfer shall be via the
internet or ftp (TBD). The FDS shall be capable of using the POD
solutions to verify OOD accuracy. Data format is TBD.

### 3.6.4 Spacecraft Mass Properties

The FDS shall provide to the ST spacecraft mass and inertia properties,
including their variation with time. These data shall be used for POD by
the ST. The FDS shall place these data in the MOC archive. The ST shall
be capable of retrieving these data from the archive. Data format is
TBD.

### 3.6.5 Star Tracker Data

The FDS shall make CT-602 star tracker data available to the ST. These
data shall be used by the ST for precision attitude determination. The
FDS shall place these data in the MOC archive. The ST shall be capable
of retrieving these data from the archive. Data format is TBD.

### 3.6.6 Targets of Opportunity Data

The ST shall identify targets of opportunity to the FDS. The FDS shall
calculate and transfer to the MOC quaternions that allow the laser to
point at the targets of opportunity. These quaternions shall be
calculated using software developed by CCAR, installed on the LASP
computer network, and maintained and operated by FDS. The MOC shall
generate tables for upload to the spacecraft. Data identifying targets
of opportunity shall be transferred from the ST to the FDS via TBD. Data
format is TBD.