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Publication Number: N/A
Date: November 1996

Field Strength Measurements of DGPS and FAA Beacons in the 285 to 325 kHz Band

Appendix B

Raw Data Structure - Type 2 and Type 3 Acquisition

The processed data are contained in binary files that are named 00000xxx.RT3 where xxx is the file number from 001 to 999. Each binary processed data file consists of a file header and one or more records as shown in Figure B1. Each record has its own header and a group of 50 data acquisitions at a specific frequency band (one of 41 bands between 285 kHz and 325 kHz). Each data acquisition consists of a data block containing four pieces of data.

Header information consists of various data types which can be accessed by knowing byte location within the particular header. The various data types are described by the following:

integer: 2 byte signed integer
unsigned integer: 2 byte unsigned integer
float: 4 byte IEEE floating point value
double: 8 byte IEEE floating point value
string: character string with the number of characters described in the following explanations.
Each character is one byte long.
Each string is terminated with a null character (\0).

The file header contains 25 elements, with total size of 300 bytes, and consists of the following:

byte
location
description data type size (bytes)
<0> records long int 4
<4> bands int 2
<6> band location int array 82 (41 integers)
<88> date string 10
<98> description string 80
<178> site int 2
<180> acquisitions per band int 2
<182> min sweep spacing float 4
<186> min repeat spacing float 4
<190> resolution bandwidth for DGPS float 4
<194> video bandwidth for DGPS float 4
<198> span for DGPS float 4
<202> system calibration factor float 4
<206> reference level for noise float 4
<210> spectrum analyzer attenuation float 4
<214> resolution bandwidth for noise float 4
<218> span for noise float 4
<222> external attenuation float 4
<226> reserved double #1 double 8
<232> reserved double #2 double 8
<242> reserved int #1 double 8
<244> reserved int #2 int 2
<246> reserved int #3 int 2
<248> reserved int #4 int 2
<250> reserved string string 50
<300> Start of 1st Record    

Byte location describes the number of bytes from the beginning of the header at which the particular element is located.

1) Records is the number of data records contained in the file.

2) Bands represents the number of different DGPS frequency bands measured.

3) Band Location is an integer array representing the particular frequency bands actually measured. The values are either a 1 or 0 depending on whether the particular frequency band was measured or not, 1 being an affirmative. The actual center frequency of the band can be determined by 285e3 + (1e3 * index), where index = 0.1,2,....40.

4) Date gives the date (UTC time) of the acquisition and is represented in the following format 00/00/00 (month/day/yr).

5) Description is a character string containing a description of the measurement site.

6) Site is a site number that should be correlated with the measurement notes.

7) Acquisitions Per Band is the number of data acquisitions or data blocks within each record (representing a specified frequency band). It also represents the total number of noise data points as described in the record header.

8) Minimum Sweep Spacing represents the minimum possible time in seconds between each data acquisition or data block within a frequency band. In actuality, the time between acquisitions may be longer than this.

9) Minimum Repeat Spacing represents the minimum possible time in minutes between repeat acquisitions within a frequency band as the data acquisition cycles through all the selected frequency bands. In actuality, the recycling time for the frequency band may be longer.

10) Resolution Bandwidth for DGPS is the resolution bandwidth setting in Hz of the HP8563A spectrum analyzer during the measurement of the actual DGPS signal.

11) Video Bandwidth for DGPS is the video bandwidth setting in Hz of the HP8563A spectrum analyzer during the measurement of the actual DGPS signal.

12) Span for DGPS is the frequency span setting in Hz of the HP8563A spectrum analyzer during the measurement of the actual DGPS signal.

13) System Calibration Factor is a calibration correction factor in dB that is added to the measured signal power as determined by the HP8563A spectrum analyzer. The result is that the power level recorded in the data block is the signal power as seen by the front end of the measurement system as it comes out of the antenna.

14) Reference Level for Noise is the reference level setting in dB of the HP8563A spectrum analyzer during the measurement of the noise floor (500 Hz offset from the DGPS signal).

15) Spectrum Analyzer Attenuation is the attenuation setting in dB of the HP8563A spectrum analyzer during the measurement of the actual DGPS signal and the noise floor.

16) Resolution Bandwidth for Noise is the resolution bandwidth setting in Hz of the HP8563A spectrum analyzer during the measurement of the noise floor (500 Hz offset from the DGPS signal). For the type 2 acquisition, the resolution bandwidth is set to 300 Hz.

17) Span for Noise is the frequency span setting in Hz of the HP8563A spectrum analyzer during the measurement of the noise floor (500 Hz offset from the DGPS signal). For the type 2 acquisition, the span is set to zero.

18) External Attenuation is any additional external attenuation in dB in the system not accounted for by the system calibration factor. This is added to the measured power.

19) Reserved Double #1 is a reserved area for a double floating point variable.

20) Reserved Double #2 is a reserved area for a double floating point variable.

21) Reserved Int #1 is a reserved area for an integer variable.

22) Reserved Int #2 is a reserved area for an integer variable.

23) Reserved Int #3 is a reserved area for an integer variable.

24) Reserved Int #4 is a reserved area for an integer variable.

25) Reserved String is a reserved character string.

The record header is located at the beginning of each data record. It has 6 elements, with a total size of 20 bytes, and consists of the following:

byte
location
description data type size (bytes)
<0> frequency band float 4
<4> average noise float 4
<8> standard deviation of noise float 4
<12> peak value of noise float 4
<16> reserved int #1 int 2
<18> reserved int #2 int 2
<20> noise data array of float (4 * acquisition-
per-band)
<20 + (4 * acquisition-
per-band)>
     

Byte location describes the number of bytes from the beginning of the header at which the particular element is located.

1) Frequency Band is the center frequency in Hz of the band for the data blocks within this record.

2) Average Noise is the average noise power in dB within the resolution bandwidth setting as determined by a single sweep across the frequency span located 500 Hz offset from the center frequency of the DGPS signal in this band.

3) Standard Deviation of Noise is the standard deviation of the noise power in dB within the resolution bandwidth setting as determined by a single sweep across the frequency span located 500 Hz offset from the center frequency of the DGPS signal in this band.

4) Peak Value of Noise is the peak noise power in dB within the resolution bandwidth setting as determined by a single sweep across the frequency span located 500 Hz offset from the center frequency of the DGPS signal in this band.

5) Reserved Int #1 is a reserved area for an integer variable.

6) Reserved Int #2 is a reserved area for an integer variable.

7) Noise Data is an array of floating point real numbers containing noise data. Out of a possible 601 data points, the data is sampled such that the total number of points stored in the array is equal to the "acquisitions-per-band" as described in the file header (generally 50). These data are acquired by setting the spectrum analyzer to zero span with a bandwidth the same as the resolution bandwidth set during DGPS signal acquisition (generally 300 Hz). The data is acquired in 700 ms with the samples evenly spaced across the 601 possible data point as acquired by the spectrum analyzer. For instance, if the "acquisitions-per-band" is 50, then every twelfth point out of the 601 possible data points acquired by the spectrum analyzer is stored in the noise data array. Each data point represents the power during a period of 1.16 ms.

Each data block comprises 4 elements with a total of 62 bytes. There are multiple data blocks within each record, each representing an acquisition within the frequency band. Generally, each acquisition is spaced approximately 1.3 seconds apart. The individual elements of the data block consist of the following:

byte
location
description data type size (bytes)
<0> GPS string char 50
<50> frequency float 4
<54> peak power float 4
<58> field strength float 4
<62> Start of next data block    

Byte location describes the number of bytes from the beginning of the data block at which the particular element is located.

1) GPS string is a character string containing the Trimble ASCII Interface Protocol (TAIP) output from the Trimble Navigation Placer GPS/DR receiver and has the following format:

>RPVAAAAABBBCCCCCDDDDEEEEEFFFGGGHI<

Field Significance
A UTC of fix (seconds)
B.C Latitude (in degrees, with decimal minutes: + = N, - = S)
D.E Longitude (in degrees, with decimal minutes: - = W, + = E)
F Speed (mph)
G Heading (0 = N)
H Source (0=2D GPS, 1=3D GPS, 2=2D DGPS, 3=3D DGPS, 6=DR, 8=Degraded DR, 9=Unknown)
I Age of Indicator (2=Fresh, <10 sec; 1=Old, <10 sec; 0=Not available)

If a particular field is not determined, only >RPV< would appear.

2) Frequency is the frequency in Hz at which the power measurement is determined. In this case, it is the frequency of the DGPS signal being measured (as opposed to the peak power across the span as determined in the type 1 acquisition - see Appendix A).

3) Peak Power is the power in dBm of the frequency having the greatest power across the frequency span of the acquisition. This value represents the power seen by the front end of the receiver as it comes from the antenna.

4) Field Strength is the field strength in dB microvolts / meter as seen by the receiving antenna. This is calculated from the "Peak Power" value and the antenna correction factor using the following equations.

Equation B1

where GdBi is the gain of the antenna in dBi, fMHz is the frequency in MHz, and ACF is the antenna correction factor in dB;

Equation B2

where A is the aperture of the antenna in units of m2, and is the wavelength of the carrier frequency in meters;

Equation B3

where c is the speed of light in meters per second (3e8 meters per second), and fHz is the carrier frequency in Hz;

Equation B4

where Pd is the power density in watts per m2, and Pm(watts) is the power in watts measured at the output of the antenna;

Equation B5

where E field in dB microvolts per meter measured at the antenna is the E field in dB microvolts per meter measured at the antenna, and 377 is the impedance of free space measured in ohms.

To determine the byte location for the header associated with a specific record, use the following formula:

Byte Location = 300 + (r - 1)(20 + (4 * (a * 66))

where:

r = the record number within the data file: 1, 2, 3, ...

a = the number of data blocks acquired within a record (typically 50).

To determine the byte location for a particular data block, use the following formula:

Byte Location = 300 + (r - 1)(20 + (a * 66)) + 20 + (a * 4) + ((d - 1) * 62)

where:

r = the record number within the data file: 1, 2, 3, ...

a = the number of data blocks acquired within a record (typically 50).

d = the number of the particular data block within the record: 1,2,3,....50

Figure B1. Data File Structure.

Figure B1. Data File Structure.

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