Jacob Williams

1
Testing of the ICESat BlackJack GPS Receiver
Engineering Model
ION GPS 2002 September 25, 2002

• Jacob Williams
• University of Texas at Austin
• Center for Space Research

2
Presentation Overview

• Background
• Hardware and Testing Setup
• Data Analysis
• Results
• Receiver Performance
• Anomalies
• Conclusions

3
ICESat Mission

• Ice, Cloud, Land Elevation Satellite.
• Laser altimetry coupled with precise position
  information.
  
• Spacecraft will incorporate two BlackJack GPS
  receivers.
  
• Ice-sheet topography and associated temporal
  changes, as well as cloud and atmospheric
  properties.

4
Background and Motivation

• Goal was to investigate and analyze BlackJack
  receiver performance
  
• Characterize expected on-orbit behavior and
  measurement accuracy of the receiver.
  
• Testing conducted on the ICESat Engineering Model
  (EM) provided by GSFC.
  
• Receiver tested using GPS simulator, calibrated
  to match conditions of ICESat orbit.
  
• In all, over 400 hours of data was collected.

5
BlackJack GPS Receiver

• Dual frequency space-capable GPS receiver
  developed by JPL.
  
• Codeless receiver that does not use the
  classified encryption key.
  
• Highly versatile, reprogramable, customizable
  hardware and software.

• Can be used for precise orbit determination,
  which is needed for a variety of spacecraft
  science applications, including satellite laser
  altimetry (ICESat) and gravity field measurements
  (GRACE).

6
BlackJack GPS Receiver

• Receiver Observables
• C1, P1, P2, L1, L2, SNRs
• 10 second epochs
• Receiver Navigation Solution
• Position, Velocity, Clock Offset, ?2
• Clock Steering
• Receiver uses the clock solution to steer
  receiver clock to GPS time.
  
• Only performed if 0.01
• 0.1 PPS Timing pulse synced to receiver clock

7
GPS Simulator

• GSSI STR-4760
• Dual Frequency Capable
• Pseudo-Y Code
• Can specify receiver trajectory (orbital,
  static), atmospheric properties, antenna gain
  pattern, etc.
  
• Can extract modeled data for post processing.
• Also generates 1 PPS timing signal.

8
Detailed Hardware Setup
Compaq
AlphaStation XP900
GPS Constellation Simulator
(GSSI STR-4760)
1215
AC
1805
RF1
RF2
SMA
AC
J6
ICESat
BNC
1836
BlackJack GPS
Receiver
TIMER 1
SMA
RG 142
1 PPS
Engineering
Model
J5
J3
DC Power Supply
P1
BlackJack
(HP 6633A)
28 V
Port Harness

• Receiver
• Simulator
• DAQ System
• Power Source
• VMS Workstation
• PC
• RS422 Converters

0.75 Amp limit
P2
P3
P4
AC
RS 422
RS 422
0.1 PPS
Port 1
Port 2
Port 12
(Beep)
(Data)
(Timing)
AC
For Timing Tests Only
Black Box
LD485A-HS
Opto-Isolated
Gender Changer
Gender Changer
AC
Mini-Black Box
AI RS422
RS 232
AC
Opto-Isolated
J01
J02
25-9 Pin Converter
BNC
COM1
NI Data Acquisition System
PXI 6608 Timing Board
AC
J21
J67
J20
AC
HP Vectra PC
NI CB-68LPR
9
Software

• Goddard software for commands and data
  collection
  
• BJInterface. Commanding receiver and collecting
  data stream.
  
• BJReader. Extracting observation files from raw
  data file.
  
• BJrnx (JPL) used to generate RINEX files.
• LabView VI for timing test.
• Receiver, simulator, and timing data collected
  and post processed using MATLAB.

10
Data Analysis
Receiver Channel 1
Receiver Channel 2

• Simulated signals used as truth for determination
  of accuracy of receiver observations.
  
• Double difference used for analysis of raw
  measurement accuracy.
  
• Position and velocity direct difference from
  simulator.

Simulator Channel 1
Simulator Channel 2
Satellite clock and geometry errors eliminated.
Receiver clock error dominates.
Receiver clock error eliminated.
Receiver and channel specific errors dominate.

• Timing signals of receiver and simulator
  differenced, and compared to time offset computed
  by receiver.

11
Simulator Gain Calibration

• Simulator gain level calibrated to achieve
  closest match with on-orbit CHAMP observations.
  
• ICESat antenna gain pattern used.
• Calibration curves created by plotting C1 PR vs.
  C1 SNR.

12
Receiver Performance

• High accuracy of dual frequency GPS observations
  and receiver navigation solutions.
  
• Receiver able to track simulators pseudo-Y code,
  as well as the unencrypted P-code.
  
• Orbital and static scenarios.
• High accuracy of receiver clock steering and
  timing pulse during nominal performance.

13
Results Observable Accuracy

• Double differences between two receiver channels
  and simulator truth.
  
• Very high quality dual frequency GPS
  observations.
  
• Error standard deviations for this PRN pair
• C1 123 mm
• P1 247 mm
• P2 285 mm
• L1 0.11 mm
• L2 0.18 mm

14
Results Receiver Navigation Accuracy

• Orbital scenario

Ionosphere On, 3-axis s 1.95 m
Ionosphere Off, 3-axis s 0.67 m
Due to known 15m bias issue
15
Results Clock Steering

• Sub-microsecond level clock steering during
  regions of valid navigation solutions.
  
• However, there were long periods of clock drift
  due to poor navigation solutions.

16
Observed Anomalies

• Corruption of observations from satellites with
  high relative acceleration.
  
• Affects receiver navigation solution and clock
  steering.
  
• Linkage between memory usage and receiver
  resets.
  
• L2 ramps.
• 15 meter pseudorange bias.

17
SNR Drops During High Acceleration

• In one 18 hr simulation, 17 of data epochs had
  at least one satellite with this anomaly.
  
• Occurs at high relative acceleration between
  receiver and GPS satellite.

18
Effect on Clock Steering

• The observations from satellites with SNR drops
  are corrupted.
  
• The navigation solution is corrupted (3-axis
  position s 720 m), indicated by a large ?2.
  
• Without a valid navigation solution, clock
  steering cannot be performed.

19
Effect on Clock Steering

• Some of the anomalous navigation solutions fall
  below 10,000 threshold.
  
• These solutions are used for clock steering, and
  offset is not properly driven to zero.

?2 10,000 No clock steering
?2
20
Timing Pulse
Measured Offset

• Difference between receivers clock offset
  solution and measured offset.
  
• Large errors (up to 60 µs) seen during SNR
  drops.
  
• Nominal s 5.5 ns
• Poor s 3.7 µs
• This is simply another measurement of the
  receiver navigation solution errors during epochs
  with SNR drops.
  

Receiver Computed
21
Memory Usage and Resets

• Memory usage in 8 day orbital simulation.
• Fairly consistent 38 hour period for the three
  natural receiver resets.
  
• Significant change in memory usage was not
  observed in static (ground) simulation.
  
• This reset rate is within ICESat specs.

22
L2 Ramps

• L1-L2 phase difference plots.
• Previously reported L2 ramps observed.
• Manifestation of very low (initial track.
  
• Large errors in P1, P2, L2.

Here, values of 0 were recorded for P2 SNR
23
Conclusions

• High quality dual frequency measurements,
  navigation solutions and clock steering.
  
• Anomalies
• In the EM, the high acceleration anomaly is a
  significant issue for receivers real-time
  navigation and clock steering.
  
• Memory reset rate is within ICESat specifications
  (1 per day).
  
• Other anomalies are relatively minor, and can be
  filtered out in post processing.
  
• EM several years old, and many of these anomalies
  have been fixed in subsequent software versions.
  
• High acceleration anomaly was present on CHAMP,
  but has been fixed (observed by comparing PR vs
  SNR curves for early and recent data).
  
• Exceptional performance as a commercially
  available dual frequency GPS receiver for precise
  orbit determination.