POD Flow Chart by Bernese 5'0

1
Investigation of POD Instrumentation Factors for
FORMOSAT-3/COSMIC Mission
Investigation of POD Instrumentation Factors for
FORMOSAT-3/COSMIC Mission
C. L. Fu, B. H. Wu, and Y. A. Liou National
Space Organization, Hsin-Chu, Taiwan Also
affiliated with National Central University

• Setup of antenna phase center offset and orbit
  RMS
• Bernese 5.0 inputs antenna normal and antenna
  phase offset.
• One cm phase offset is induced to POD antennas
  Upward and East/North (horizontal) directions to
  study its impact on orbits
• ?X, ?Y, ?Z, and RMS

• Antenna Radiation Pattern
• NSPO
• Hybrid near-field antenna test system is
  recently set up to measure antenna radiation
  pattern.
• Eleven antennas aboard each FORMOSAT-3 are
  tested.
• UCAR/Ball Aerospace (Data from UCAR)

• Antenna Alignment

POD Flow Chart by Bernese 5.0

• Only Flight Model 3 data are shown here
• Locations for GOX antenna centers (cm)
• Angle between normal to antenna and X-axis
  (deg.) tan?1(z/x)

• GOX Antenna Measurements
• In contrast to CHAMP, SAC-C and GRACE satellites
  having one POD antenna, there are two POD
  antennas on each FORMOSAT-3 satellite for orbit
  determination and ionospheric sounding.
• Fore and aft POD antennas are installed on the
  spacecrafts front and back sides, respectively.
• The angle between the line linking antenna and
  spacecraft physical centers and /- X-axis is
  30, while the angle between the normal to
  antenna and /- X-axis is 15.

• Listed L1/L2 phase centers are in S/C coordinate
  system.

• Cable Delay

• Within one diurnal cycle, each POD antenna can
  receive signals from 2 to 9 GPS satellites at any
  moment based on simulation.
• At least four GPS-transmitted signals are needed
  to determine LEO orbit so that signals received
  by two POD antennas must be combined if the
  number of GPS satellites is less than 4.
• Time and phase delays between POD antennas and
  GOX receiver must be corrected.

• We show that 1cm phase offset may result in 6
  mm orbit deviation, while the POD requirement for
  geodetic mission is mm - cm. Proper correction
  to the signal propagation delays through antenna
  cables is needed. With the results, the accuracy
  of retrievals of ionospheric and atmospheric
  variables from radio occultation will be
  improved.
• The authors are grateful to the FORMOSAT3 Program
  Office for the instrumentation measurements. They
  also thank Prof. C.W. Huang and Dr. Svehla for
  technical assistance in the POD calculations by
  using Bernese 5.0.

• The delay is measured by using the Network
  Analyzer 8510 whose accuracy is 0.01 ns.
• Lengths of four GOX cable lines range from 50 to
  100 cm. They cause signal delays by 2 to 4 ns.
• A length difference of 47 cm between two POD /-
  X cables causes 2 ns delay in data transmission.

• Geometric alignment of GOX antennas, cable
  propagation delays, and antenna phase center
  offset/variation are measured in this work.