Bill Feess, Aerospace Karl Kovach, ARINC

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














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)


Message Type 4

Message Type 4

Message Type 1Short Frame (0.6 min) Message Type 2 Message Type 3



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?

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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

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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

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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


i

16 BITS

145

24 BITS

161

1

0

16 MSBs

185

16 BITS

209


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

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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

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Compressing Almanacs - IVc


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

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Compressing Almanacs - IVd


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


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

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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)

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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

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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

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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



31 BITS

1

17 MSB

184

31 BITS

215


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

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Example L2C Schemes







Message Type 6






Message Type 1 Message Type 2Short Frame (0.6 min)




4 = Long Almanac

5 = Text (NANUs)

6 = Mini-Almanacs


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 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

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Back-up Slides

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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

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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

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Previous Almanac Schemes







Message Type 6-1










4 = Long Almanac

5 = Text (NANUs)

6 = Mini-Almanacs


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 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

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Message Type 6-3

Staggered Almanac Scheme



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 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

Bill Feess, Aerospace Karl Kovach, ARINC

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