# Sepura SDS LIP

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#### Maciek_PE

##### Member
Hi,

I am trying to understand how GPS location is decoded in SDS LIP message, but I can't

Message received from my radio: 0A 00B8C31A53EB496D6200 (ShortLocationReport)
In bin: 0000101000000000101110001100001100011010010100111110101101001001011011010110001000000000

According to ETSI TS: Long. is encoded on 25 bits (1010000000001011100011000) and Lat. under 24 bits (011000110100101001111101)

LIP translator shows this location: Longitude:-134° 56' 11.6492'' Latitude:69° 48' 50.2734''
and AVL log file: 52.37570 16.23886

I tried to convert bin to dec and multiply 360/2^25 - but nothing fits to the results.

Could You help me, please?

#### slicerwizard

##### Member
To reverse engineer a protocol, you should gather more than one sample...

Also, you have two wildly different interpretations of the data, but you don't indicate which one has a chance of being correct.

#### DSheirer

##### Member
Premium Subscriber
Convert your latitude value to decimal degrees.

Code:
``````Latitude:69° 48' 50.2734'' = 69.813964833

69° = 69.00
48' / 60 =  0.80
50.2734'' / 3600 = 0.01396483

Latitude:69° 48' 50.2734'' = 69.813964833``````
Divide by 0.0000107288360595703125 (180 degrees divided by 2 ^ 24)

Truncate the result to the integer value
6507133.1545846518518518518518519 = 6507133

Convert to Binary:

6507133 = 011000110100101001111101

Here's the latitude value in your message:
Code:
``````00001010000000001011100011000 011000110100101001111101 01101001001011011010110001000000000
011000110100101001111101``````
You can find the format of the message and the formulas here:
http://www.etsi.org/deliver/etsi_ts/100300_100399/1003921801/01.03.01_60/ts_1003921801v010301p.pdf

Paragraph 6.3.30 has the formula for latitude
Paragraph 6.3.31 has the formula for longitude

cheers,
Denny

#### Maciek_PE

##### Member
slicerwizard said:
To reverse engineer a protocol, you should gather more than one sample...
Some other samples, with the results that I am trying to achieve:

0A 00BE2CA253A239006200 - 00 00101000000000101111100010110010100010010100111010001000111001000000000110001000000000
SeekFor_PDU_Type
Time Elapsed:< 5 Sec
Longitude:-134° 56' 4.9475''
Latitude:-75° 43' 58.8062''
Position Error:< 200 m
Horizontal Velocity: : 28.0 km/h
Direction Of Travel:S
Type Of Additional Data:Reason Of Sending
Reason Of Sending:Transmit inhibit mode OFF

0A 00B72A4A53BB5A000200 - 0000101000000000101101110010101001001010010100111011101101011010000000000000001000000000
SeekFor_PDU_Type
Time Elapsed:< 5 Sec
Longitude:-134° 56' 13.6267''
Latitude:51° 31' 57.5400''
Position Error:< 2 km
Position Error:< 20 km
Horizontal Velocity: : 52.8 km/h
Direction Of Travel:N
Type Of Additional Data:Reason Of Sending
Reason Of Sending:Suscriber Unit is Powered ON

0A 30CD050A51FF02000200 - 0000101000110000110011010000010100001010010100011111111100000010000000000000001000000000
SeekFor_PDU_Type
Time Elapsed:< 5 Sec
Longitude:-130° 42' 39.1003''
Latitude:-66° 35' 34.6161''
Position Error: Not Known.
Horizontal Velocity: : 1.0 km/h
Direction Of Travel:N
Type Of Additional Data:Reason Of Sending
Reason Of Sending:Suscriber Unit is Powered ON

0A 00CB4DC2529789000200 - 0000101000000000110010110100110111000010010100101001011110001001000000000000001000000000
SeekFor_PDU_Type
Time Elapsed:< 5 Sec
Longitude:-134° 55' 48.7427''
Latitude:-50° 25' 15.1108''
Position Error: Not Known.
Horizontal Velocity: : 124.4 km/h
Direction Of Travel:S
Type Of Additional Data:Reason Of Sending
Reason Of Sending:Suscriber Unit is Powered ON​

DSheirer said:
Convert your latitude value to decimal degrees...
Have you tried to convert Longitude like this?
In dec. it will be: ~225.

For this reason I created this topic

#### DSheirer

##### Member
Premium Subscriber
The latitude and longitude values are in two's complement format. If the leading bit of either the latitude or longitude value is a 1, then you know you have a negative hemispherical value (latitude = south or longitude = west) and you have to convert from two's complement form.

Original Message
Code:
``````0000 1010000000001011100011000 01100011010010100111110101101001001011011010110001000000000
longitude value``````
Convert from two's complement form

1010000000001011100011000 longitude in two's complement form
0101111111110100011100111 flip the bits
0101111111110100011101000 add one
0101111111110100011101000 = -12577000

Multiply by 180 degrees / 2 ^ 24
-12577000 * 0.0000107288360595703125 = -134.936571121 degrees

#### Maciek_PE

##### Member
Problem solved.

My LIP translator took wrong bits to decode lat. and long. from SDS (from Protocol identifier - rather than reject them).

Thank you guys for all your help.

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