# Presentation: 605534

## Section 22

_***Section 22***_
***Global Positioning System***
***(GPS)***

*** ******. . . Dave Quinn***
*EO-1 GPS Receiver Lead Engineer *

## GPS Subsystem

***GPS Subsystem***

***Off the shelf GPS receiver (Tensor), preamp/splitter and antennas from Space Systems / Loral.***
***Power Conditioning Unit designed and built in-house.***
***Provides orbital navigation and time information for C&DH and ACS/EFF applications.***

**Receiver Processor Unit **
**(RPU)**

**Preamp / Splitter**
**Assembly**
**(P/SA)**** **

**Hemispherical Antennas**

**Power Conditioning Unit **
**(PCU)**

**Power Input**

**Telemetry / Data IO**

## GPS -- Performance Testing

***GPS -- Performance Testing***

**Position Requirement:****	**** 450m - 3s, in-track and cross track**
**	****1 Hz SPS Solution Capability: ****	****m = 95.35 m, s = 62 m**
**	****1 Hz Filter Solution Capability :****	****m = 43.28 m, s = 33 m**
**Velocity Requirement:  1.0 m/s - 3s, in-track and cross track**
**	****1 Hz SPS Solution Capability : ****	****m = 0.74 m/s, s = 0.44 m/s**
**	****1 Hz Filter Solution Capability : ****	****m = 0.13 m/s, s = 0.14 m/s**

## GPS -- Box-Level Testing

***GPS -- Box-Level Testing***

***Contract Acceptance Testing ****
***Random vibration, thermal vacuum, EMI/EMC and acceptance tests on receiver, preamp/splitter assembly and antennas performed by Space Systems / Loral.***
***PCU Environmental testing ****
***Random vibration,EMI/EMC and temperature cycling on in-house built power conditioning unit.***
***Detailed Functional Characterization at GSFC ****
***Performed three month characterization of entire GPS subsystem using GPS constellation simulator to understand operation and optimize operational configuration.***
****  Provided results of all above in detailed acceptance package to SWALES before handover of unit  ***

## GPS -- Spacecraft Level Testing

***GPS -- Spacecraft Level Testing***

***Discussed in more detail as part of S/C presentation  (ACS section)***
***Provided timecode for S/C during I&T, CPT’s  and Thermal Vacuum Testing via rooftop antenna***
***Constellation Test #1 (5/99) - verified ability to cold start system and  GPS subsystems ability  to provide navigation information to ACS system. Could not test “luke warm start” capability.***
***Constellation Test #2 (TBD 12/99) - reverify cold start and end-to-end capability of GPS to provide navigation information to ACS/C&DH system, retest loading of almanac and “luke warm” start capability***
***Thermal Vacuum/Thermal Balance Testing ***
***GPS provided S/C time code during TB/TV via rooftop antenna***
***GPS timecode dropped out on numerous occasions (see Special Topic to follow)***

## GPS -- Spacecraft Testing

***GPS -- Spacecraft Testing***

***Operating hours (as of 11/18/99)***
***21 days, 6hrs,44 mins since start of I&T (511 hours)***
***18 days, 4hr, 20 mins since S/C environmental testing (436 hours)***
*** ******No configuration changes, non-conformances (except dropouts), no action items, no RFA’s***
***Problem report written on GPS dropouts during TV/TB***
***No outstanding waivers***
***Ready to fly assuming understand cause and correction for dropouts during TV/TB***

## Special Topic:

***GPS Signal Level***

## Loss of GPS Signal During TV/TB

***Loss of GPS Signal During TV/TB***

***During Thermobalance/Thermovac testing significant GPS dropouts occurred during both hot and cold soaks***
***ASIST I&T data indicates that 1/4 of time GPS not providing timing information to C&DH***
***Most likely cause:***
***Cable Losses and insufficient signal strength between GPS rooftop antenna and antenna’s may have resulted in inability to lock onto 4 SV’s even though they were available***

## Diagnostics

***Diagnostics***

***Three sets of GPS  Receiver housekeeping data (directly out of receiver) have been collected and analyzed to date:***
***GPS receiver data during TB/TV - indicates dropouts when receiver cannot see 4 SV’s, occurred 20 percent of time during TB/TV***
***Spare GPS receiver data collected over 24 hours (Nov 15-16) with presumed similar RF amps and cabling from TB/TV, indicates higher signal strength and fewer dropouts, and was not identical reproduction of cabling from TB/TV (which could not be reproduced)***
***Spare GPS receiver data collected over 24 hours (Nov 16-17) with improved cabling - reduced dropout rate over TB/TV but was similar to previous test. Conclusion, either the difference in receivers or GSE cabling was responsible for the observed difference***

## Diagnostics

***Diagnostics***

***Additionally, will collect and analyze GPS receiver housekeeping during four day CPT (Dec 6-9) to see if any additional drop outs occur***
***S/C utilizing fully characterized cables and amps***
***Performance has been nominal during CPT***
***Will run additional test on both flight and spare units to confirm observed differences were due to GSE cables, not due to receiver difference***

## Impact to Mission

***Impact to Mission***

***IF GPS does NOT properly operate on orbit:***
***Will need to load EO-1 ephemeris daily and will need daily tracking support from ground stations***
***EFF validation will be severely impacted, as it requires on-board GPS for navigation information to maintain position relative to Landsat 7***
***Will cause degradation in navigation accuracy expected and ACS target attitude generation***

## Summary

***Summary***

***GPS is ready to fly assuming that most likely suspect of GSE - RF amps and cabling - is the cause for dropouts during TB/TV***