bill feess, aerospace karl kovach, arinc
DESCRIPTION
Proposed Satellite Mini-Almanac for L2C Message Type 6. Bill Feess, Aerospace Karl Kovach, ARINC. 2 May 2001. Problem. Long time to transmit a full set of constellation almanacs ICD-GPS-200 design takes 12.5 min to transmit full set of almanacs - PowerPoint PPT PresentationTRANSCRIPT
1
Bill Feess, Aerospace
Karl Kovach, ARINC
2 May 2001
Proposed Satellite Mini-Almanac for L2C Message Type 6
2
Problem
• Long time to transmit a full set of constellation almanacs– ICD-GPS-200 design takes 12.5 min to transmit full set of almanacs
• L2C design requires 12 sec to transmit 1 message (= 1 subframe)– Half as fast as ICD-GPS-200 design due to ½-rate FEC encoding
• Baseline L2C design uses 1 message to transmit 1 SV almanac– Same as ICD-GPS-200 design (1 message = 1 subframe), 300 bits total
• When 5 different message types are being transmitted, it would take 60 sec between each transmission of an SV almanac
• To transmit a full set of almanacs, up to 24 - 28 minutes would be required, depending on constellation size (24 to 28 SVs)
• This long time poses an operational problem to some users
3
ICD-GPS-200 Design
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
Master Frame (12.5 minutes)
Frame 1 (0.5 min)
Frame 2 (0.5 min)
Frame 3 (0.5 min)
Frame 4 (0.5 min)
Frame 5 (0.5 min)
Frame 6 (0.5 min)
Frame 7 (0.5 min)
Frame 8 (0.5 min)
Frame 9 (0.5 min)
Frame 10 (0.5 min)
Frame 11 (0.5 min)
Frame 12 (0.5 min)
Frame 24 (0.5 min)
Frame 25 (0.5 min)
4
Baseline L2C Schemes
Message Type 1Long Frame (1.0 min)
Message Type 2 Message Type 3 Message Type 4 Message Type 5
Message Type 1 Message Type 2 Message Type 3 Message Type 4 Message Type 5
Message Type 1 Message Type 2 Message Type 3 Message Type 4 Message Type 5
Long Frame (1.0 min)
Long Frame (1.0 min)
Message Type 1Medium Frame (0.8 min)
Message Type 2 Message Type 3
Message Type 1 Message Type 2 Message Type 4 Message Type 5
Message Type 1 Message Type 2 Message Type 3
Medium Frame (0.8 min)
Medium Frame (0.8 min)
Message Type 4
Message Type 4
Message Type 1Short Frame (0.6 min) Message Type 2 Message Type 3
Message Type 1 Message Type 2 Message Type 4
Message Type 1 Message Type 2 Message Type 5
Short Frame (0.6 min)
Short Frame (0.6 min)
Message Type 1 Message Type 2 Message Type 4Short Frame (0.6 min)
Message Types:1 = Clock/Eph Part 1
2 = Clock/Eph Part 2
3 = Iono, Bias, Health
4 = Almanac
5 = Text (NANUs)
Or Some Other Flexible Mix of Long, Medium, and/or Short Frames
5
Solutions to Problem
• Do Nothing (simply tell users "tough luck")– Half-hour to collect a full set of almanacs isn't that long
• Not for many types of receivers that are ON for hours at a time
• However, handheld receivers are a much different story
• Find a Way to Transmit the Almanacs Quicker– Limited on maximum available data rate -- not feasible– Stagger almanac transmissions between SVs -- feasible
• e.g., A/C/E-plane SVs transmit almanacs for B/D/F-plane SVs
– Compress the almanac data to make it smaller -- feasible• But how much compression is possible?
6
Compressing Almanacs - I
• Almanacs are used by a GPS receiver for:– SV visibility determination
• What SVs are visible or will soon become visible
– Geometry-based SV selection• GDOP, PDOP, HDOP, etc.
– SV signal acquisition aid• Approximate Doppler offset and code delay
• Almanacs are NOT used by a GPS receiver for:– Positioning, timing, or navigation
• Not enough precision (this is what clock/ephemeris data is for)
7
Compressing Almanacs - II
• Driver on almanac precision is code delay accuracy– Needed for direct P(Y)-code signal acquisition
• Acquire P(Y)-code signal without help from C/A-code
– Direct P(Y) acquisition needs fairly good accuracy• Roughly about the GPS receiver's time uncertainty
• 10 sec 3,000 m 2-week old almanac
• L2C almanacs not used for direct P(Y) acquisition– L2C receivers don't need to do direct P(Y) acquisition
• They'll do the normal C/A-code or Moderate-code acquisition
• Therefore, no major accuracy driver on L2C almanac precision
– Direct P(Y) receivers will still use ICD-GPS-200 almanacs• Collected from the P(Y)-code signal on either L1 or L2
8
Compressing Almanacs - III
• Really 2 main drivers on L2C almanac precision– Angular accuracy for visibility and geometry
• Wag a sensitivity threshold of a "couple of degrees"
• Receivers that do visibility/geometry computations every 5 min
– Doppler accuracy for signal acquisition• Wag a sensitivity threshold of a "couple of hundred Hertz"
• Receivers that have 32 Doppler search bins
• See what can do by editing ICD-GPS-200 almanacs– Using the above wags as guidelines
9
8 BITS
MESSAGE TYPE ID
6BITS
PREAMBLE
PRN
6BITS
MESSAGETOW COUNT*
17 BITS
"ALERT" FLAG - 1 BIT
9 3315 41
PRNa
6BITS
47
8 BITS
57 65
M0
24 BITS
e
16 BITS
105
- 4 LSBs
DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 4 SECONDS
DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 4 SECONDS
i
16 BITS
145
24 BITS
161
1
0
16 MSBs
185
16 BITS
209
DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 4 SECONDS
CRC
24 BITS
RESERVED
26 BITS
277229225
AS/SV CONFIG - 4 BITS
3932
DATA ID - 2 BITS
WNa
10 BITS
toa
8 BITS 20 MSBs
8173
L1/L2 SV HEALTH
A
A
129
0
8 LSBs
af0
11 BITS
af1
11 BITS
251240
* MESSAGE TOW COUNT = 17 MSBs OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE
RESERVED FOR L5
8 BITS
Figure 30-4. Message Type 4 Format
Compressing Almanacs - IVa
10
Compressing Almanacs - IVb
Parameter No. ofBits
ScaleFactor
EffectiveRange
Units
e 16 2-21 dimensionlesstoa 8 2-12 602,112 secondsi 16 2-19 semi-circlesOMEGA_DOT 16 2-38 semi-circles/secHealth 8 N/A discretes(A)½ 24 2-11 meters½
OMEGA_0 24 2-23 semi-circles 24 2-23 semi-circlesM_0 24 2-23 semi-circlesaf0 11 2-20 secondsaf1 11 2-38 sec/sec
11
Compressing Almanacs - IVc
Parameter No. ofBits
ScaleFactor
NumericalRange
OperationalRange
Units
e 16 2-21 0 - 0.03125 0 - 0.02 dimensionlesstoa 8 2+12 0 - 1044480 0 - 602,112 secondsi 16 2-19 0.0625 -0.006 - +0.0167 semi-circlesOMEGA_DOT 16 2-38 1.19x10-7 -3x10-9 -1x10-9 semi-circles/secHealth 8 N/A N/A N/A discretes(A)½ 24 2-11 0 - 8192 5149 - 5159 meters½
OMEGA_0 24 2-23 1 1 semi-circles 24 2-23 1 1 semi-circlesM_0 24 2-23 1 1 semi-circlesaf0 11 2-20 9.77x10-4 in range secondsaf1 11 2-38 3.72x10-9 in range sec/sec
12
Compressing Almanacs - IVd
Parameter No. ofBits
ScaleFactor
NumericalRange
OperationalRange
Units
e 16 2-21 0 - 0.03125 0 - 0.02 dimensionlesstoa 8 2+12 0 - 1044480 0 - 602,112 secondsi 16 2-19 0.0625 -0.006 - +0.0167 semi-circlesOMEGA_DOT 16 2-38 1.19x10-7 -3x10-9 - 1x10-9 semi-circles/secHealth 8 N/A N/A N/A discretes(A)½ 24 2-11 0 - 8192 5149 - 5159 meters½
OMEGA_0 24 2-23 1 1 semi-circles 24 2-23 1 1 semi-circlesM_0 24 2-23 1 1 semi-circlesaf0 11 2-20 9.77x10-4 in range secondsaf1 11 2-38 3.72x10-9 in range sec/sec
Very SmallDon't Care
Assume Circular Orbit
Combine if a Circular Orbit
Very Small
Simplify
Small, Only ±2 DegreesRelative to the Nominal
Small Relative to a NominalA Scale Factor of 2-6 Will Give a Resolution
of ±1.4 Degrees
13
Compressing Almanacs - IVe
Parameter No. ofBits
ScaleFactor
NumericalRange
OperationalRange
Units
e 0 Assume 0.00 dimensionlesstoa 8 2+12 0 - 1044480 0 - 602,112 secondsi 0 Assume +0.0056 semi-circlesOMEGA_DOT 0 Assume -2.5x10-9 semi-circles/secHealth 3 N/A N/A L1, L2, L5 discretesA
* 8 2+9 65,024 50,000 metersOMEGA_0 7 2-6 1 1 semi-circlesM_0
Argumentof Latitude 7 2-6 1 1 semi-circles
af0 0 Assume 0.00 secondsaf1 0 Assume 0.00 sec/sec
* Relative to a nominal value for A of 26,559,710 m
14
Compressing Almanacs - V
• Have 3+8+7+7 unique bits per Almanac (= 25 bits)– Two ways to pack the almanac data bits
• Can pack into a number of fixed message types– Type X always has almanacs for PRNs 1 to n– Type Y always has almanacs for PRNs n+1 to 2n– Etc.
• Can pack into a single message type– Must include PRN number with each almanac
– Trade-off considering 236 usable bits per message type• Less 10 bits (WNa) + 8 bits (toa) common across all almanacs
• Turns out to be a wash for up to 28 SVs (always 4 messages)
15
Compressed Almanac Proposal
• Proposal here is to compress to 7 SVs per almanac message– 31 bits total per almanac (6 bits for PRN + 25 bits for
orbit/health)
• With this compression, a complete set of almanacs for a 28 SV constellation could be sent in 4 min or less– Factor of 7 savings
• Result is thus 7 times shorter almanac collect time– Less drain on handheld receiver battery– Eliminate any need for periodic "almanac download" actions
16
Proposed "Mini-Almanac" Packet
31 BITS
PRNa
6 BITSDELTA_A
8 BITSOMEGA_0
7 BITS
ARGUMENTOF LATITUDE
7 BITS
L1 HEALTHL2 HEALTHL5 HEALTH
1 7 15 22 29 30 31
Reference Values:e = 0
i = +0.0056 SC (i = 55 deg)
OMEGA_DOT = -2.5x10-9 SC/sec Aref = 26,559,710 m M0+ = Argument of Latitude Satellite clock terms not transmitted
17
Proposed Message Type 6 Format with Mini-Almanacs
8 BITS
MESSAGE TYPE ID
6BITS
PREAMBLE
PRNtx
6BITS
MESSAGETOW COUNT*
17 BITS
"ALERT" FLAG - 1 BIT
9 3315
WNa
10BITS
PACKET 1
31 BITS
52
PACKET 2
21 LSB
PACKET 3
31 BITS
DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 4 SECONDS
DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 4 SECONDS
31 BITS
1
17 MSB
184
31 BITS
215
DIRECTION OF DATA FLOW FROM SV MSB FIRST100 BITS 4 SECONDS
CRC
24 BITS
PACKET 7
31 BITS
39
32
DATA ID - 2 BITS
toa PACKET 2
10 MSB
91
246
* MESSAGE TOW COUNT = 17 MSBs OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE
PACKET 4 PACKET 5
153
PACKET 5 PACKET 6
277
14 LSB
122
604142
SPARE - 1 BIT
BITS8
18
Example L2C Schemes
Message Type 1Medium Frame (0.8 min)
Message Type 2 Message Type 3
Message Type 1 Message Type 2 Message Type 5
Message Type 1 Message Type 2 Message Type 3
Medium Frame (0.8 min)
Medium Frame (0.8 min)
Message Type 6
Message Type 1Short Frame (0.6 min) Message Type 2 Message Type 3
Message Type 1 Message Type 2
Message Type 1 Message Type 2 Message Type 5
Short Frame (0.6 min)
Short Frame (0.6 min)
Message Type 1 Message Type 2Short Frame (0.6 min)
Message Types:1 = Clock/Eph Part 1
2 = Clock/Eph Part 2
3 = Iono, Bias, Health
4 = Long Almanac
5 = Text (NANUs)
6 = Mini-Almanacs
Or Some Other Flexible Mix of Long, Medium, and/or Short Frames
Message Type 6
Message Type 6
Message Type 1 Message Type 2 Message Type 5Medium Frame (0.8 min) Message Type 6Elapsed Time:3.2 min (192 sec)
Message Type 6
Message Type 6
Message Type 1Short Frame (0.6 min) Message Type 2 Message Type 3
Message Type 1 Message Type 2
Message Type 1 Message Type 2 Message Type 5
Short Frame (0.6 min)
Short Frame (0.6 min)
Message Type 1 Message Type 2Short Frame (0.6 min)
Message Type 6
Message Type 6Elapsed Time:4.8 min (288 sec)
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• Canvass GPS receiver manufacturers on concept– e.g., Brief concept at the L2C Industry Day
• Verify requirements for mini-almanac accuracy– R_Dot (i.e., Doppler offset 350 Hz OK?)
– Elevation Angle (i.e., visibility computation 2 degrees OK?)
– Others (e.g., consensus on no direct L2C acquisition?)
• Validate accuracies with proposed message structure and bits
• Modify the draft L2C signal PIRN to document new design
• Consider same change for the L5 signal data design
Follow-Up Work
20
Back-up Slides
21
Type 4 vs Type 6 Messages
• No conflict between Type 4 and Type 6 messages– Type 6 doesn't necessarily replace Type 4
• L2C "flexible protocol" allows either or both (or even neither)
• The L2C PIRN leaves the decisions up to the operator/users
– Some benefit to having both Type 6 and Type 4• Supports all conceivable receiver designs and user needs
– Some benefit to having just Type 6 (or just Type 4)• Minimize throughput load on a very low data rate channel
• Don't repeat redundant information unless benefit gained
22
Staggered Almanac Messages
• L2C "flexible protocol" is really very flexible– Any message type in any 12-sec slot from any SV
• Within reason due to SV memory and operator workload
• Enables many schemes to transmit common data– Message Type 1 and Type 2 contain unique data
• Data which is unique to the transmitting SV
– Message Types 6, 5, and 4 contain common data• Data which is the same no matter which SV transmits it
• Staggered almanac messages is one such scheme
23
Previous Almanac Schemes
Message Type 1Medium Frame (0.8 min)
Message Type 2 Message Type 3
Message Type 1 Message Type 2 Message Type 5
Message Type 1 Message Type 2 Message Type 3
Medium Frame (0.8 min)
Medium Frame (0.8 min)
Message Type 6-1
Message Type 1Short Frame (0.6 min) Message Type 2 Message Type 3
Message Type 1 Message Type 2
Message Type 1 Message Type 2 Message Type 5
Short Frame (0.6 min)
Short Frame (0.6 min)
Message Type 1 Message Type 2Short Frame (0.6 min)
Message Types:1 = Clock/Eph Part 1
2 = Clock/Eph Part 2
3 = Iono, Bias, Health
4 = Long Almanac
5 = Text (NANUs)
6 = Mini-Almanacs
Or Some Other Flexible Mix of Long, Medium, and/or Short Frames
Message Type 6-2
Message Type 6-3
Message Type 1 Message Type 2 Message Type 5Medium Frame (0.8 min) Message Type 6-4Elapsed Time:3.2 min (192 sec)
Message Type 1Short Frame (0.6 min) Message Type 2 Message Type 3
Message Type 1 Message Type 2
Message Type 1 Message Type 2 Message Type 5
Short Frame (0.6 min)
Short Frame (0.6 min)
Message Type 1 Message Type 2Short Frame (0.6 min)Elapsed Time:4.8 min (288 sec)
Message Type 6-1
Message Type 6-2
Message Type 6-3
Message Type 6-4
24
Message Type 6-3
Staggered Almanac Scheme
Message Type 1Long Frame (1.0 min)
Message Type 2 Message Type 3
Message Type 1 Message Type 2 Message Type 5Long Frame (1.0 min)
Message Type 6-1
Compare this 1.0 minute elapsed time against the "traditional scheme" 24 to 28 minute elapsed time
Message Type 6-4
Total Elapsed Time:1.0 min (60 sec)
Message Type 6-2
Message Type 6-3
A/C/E-Plane SVs
Message Type 1Long Frame (1.0 min)
Message Type 2 Message Type 5
Message Type 1 Message Type 2 Message Type 3Long Frame (1.0 min) Message Type 6-2
Message Type 6-4
Message Type 6-1
B/F/D-Plane SVs
Message Type 1Long Frame (1.0 min) Message Type 2 Message Type 3
Message Type 1 Message Type 2 Message Type 5Long Frame (1.0 min)
Message Type 6-1
Message Type 6-4
Message Type 6-2
Message Type 6-3
Message Type 6-3Message Type 1Long Frame (1.0 min) Message Type 2 Message Type 5
Message Type 1 Message Type 2 Message Type 3Long Frame (1.0 min) Message Type 6-2
Message Type 6-4
Message Type 6-1