This document has benn adapted from N5_DT2_TAPES.DOC which was
produced when the original magnetic tapes were copied to optical disk.
OVERALL STRUCTURE
-----------------
* Files corresponding to one magnetic tape's worth of data are
copies of our original compressed N5 DT2 tapes.
* Data was originally stored in 12 bit 'words' and now occupies the 12
least significant bits of a 16 bit word. - Data range is 0 - 4095.
* The byte ordering within 16 bit integers follows the dec convention of
least significant byte first on the "tape".
"FILE" STRUCTURE
----------------
* Each "file" has a basic structure of the following type:
CAL DATA RECORD (IF AVAILABLE)
SCR ORBIT HEAD RECORD
SCR BLOCKED DATA
SCR ORBIT END RECORD
EOF (1 1)
* Records/Blocks will have the format:
( 3654 Sync Code (7106 Octal) (2 bytes indicating start of record)
( 3654 " " " " (" " " " " " )
( Record length in words (2 bytes)
( Block # (2 bytes)
BLOCK( Identifier (2 bytes)
( Data
( End mark (2 bytes)
( Checksum (2 bytes)
* End marks signify
End of block (EOB) - 2321 (4421 Octal)
End of file (EOF) - 2730 (5252 Octal) Used at the end of each orbit
End of data (EOD) - 3371 (6453 Octal)
* Checksum calculated as least significant 12 bits of sum of words 1 to
(n-1) where n is the last word of the block. This is a 1's complement
computation.
RECORD IDENTIFIERS
------------------
577 (1101 Octal) Cal data used
192 (300 Octal) SCR orbit head
193 (301 Octal) SCR raw ) These two blocks for each
194 (302 Octal) SCR formatted ) major frame
195 (303 Octal) SCR orbit end
CAL DATA BLOCK (88 16 BIT INTEGERS) See note (1)
--------------
WORD FUNCTION
0 [SPARE]
1-4 B1
5-8 B2
9-12 B3
13-16 B4
17-20 A1
21-24 A2
25-28 A3
29-32 A4
33-36 C1
37-40 C2
41-44 C3
45-48 C4
49-52 D1 Low gain setting
53-56 D2 " " "
57-60 D3 " " "
61-64 D4 " " "
65-68 D1 High gain setting
69-72 D2 " " "
73-76 D3 " " "
77-80 D4 " " "
For scaling factors see note (2).
ORBIT HEAD BLOCK (21 16 BIT INTEGERS) See note (3)
----------------
WORD FUNCTION
0,1 Orbit #
2 Source
3 Day #
4,5 Time of MF1
6 # of MFS in orbit
7 Accession #
8,9 Flags
10,11 Equator crossings
12,13 Day/night crossings
SCR BLOCKED DATA See note (4)
----------------
RAW DATA BLOCK (472 16 BIT INTEGERS)
--------------
The raw data is exactly as transmitted, including the original control words.
WORD FUNCTION
0 Accession #
RAW HEADER BLOCK (52 WORDS)
----------------
1 3654 (7106 Octal) Sync Code repeated twice every block
2 3654 " " " " " " " "
3 Block length (Total number of words in block, all inclusive)
4 Block #
5 Playback orbit # (15 bits) See note (5)
6 " " "
GMT Day # (9 bits) See note (5)
7 Start time of major frame (17 bits) See note (6)
8 " " " " "
Data source (2 bits)
Major frame # (10 bits) See note (6)
9 " " "
10 Data content flags (6 bits) See note (7)
Sync summary bits (2 bits) " " "
P or E bit (1 bit )
Time flag (1 bit )
11,12 Latitude (24 bits) See note (8)
13,14 Longitude (24 bits) " " "
15 Altitude (12 bits) " " "
16-29 Sync flags (160 bits) See note (9)
29-35 ESMR nadir data (80 bits)
36-38 Pitch fine error (32 bits) See note (10)
38-41 Roll fine error (32 bits) " " "
41-43 RMP indicated rate HI (32 bits) " " "
44-46 Thir scan hsg temp (32 bits)
46-48 Subsystem status bits (28 bits) See note (11)
49,50 [Spare]
51 EOB Last frame flag
52 Checksum
SCR1 OR SCR2 BLOCK (412 WORDS) See note (12)
------------------
53 3654 (7106 Octal) Sync Code repeated twice every block
54 3654 " " " " " " " "
55 Block Length (total number of words in block, all inclusive)
56 Block #
57-390 Digital A and analog data (4000 bits) See note (13)
391-437 Analog housekeeping functions (560 bits) See note (14)
438-461 Digital B functions (286 bits) See note (15)
Block identification (1 bit ) " " "
462 [Spare]
463 EOB Last frame flag
464 Checksum
FORMATTED DATA BLOCK (205 16 BIT INTEGERS)
--------------------
WORD FUNCTION
0 Accession #
OBSERVATION IDENTIFICATION (14 WORDS)
--------------------------
1 Day
2,3 Time (secs)
4 Lat(deg N)*8
5 Long(deg E)*8
6 THIR temp
7 ESMR max
8 ESMR min
9
10-14 Major frame flags See note (16)
CALIBRATED RADIANCES (49 WORDS)
--------------------
15 B1
16 B2
17 B3
18 B4
19 A1
20-23 A2
24-27 A3
28-31 A4
32-35 C1
36-39 C2
40-43 C3
44-47 C4
48-51 D1
52-55 D2
56-59 D3
60-63 D4
This block contains 4 and 16 second radiances: 5 X 16-second averages
for the top channels, and 4 X 11 samples for the lower channels.
Zero indicates bad or missing data.
For scaling factors see note (2).
FORMATTED RAW DATA (105 WORDS)
------------------
64 B1 16 Sec ramps
65 B2 "
66 B3 "
67 B4 "
68 A1 "
69-72 A2 4 * 4 Sec ramps
73-76 A3 "
77-80 A4 "
81-84 C1 "
85-88 C2 "
89-92 C3 "
93-96 C4 "
97-100 D1 "
101-104 D2 "
105-108 D3 "
109-112 D4 "
113-117 Digital A housekeeping
118-156 Analog housekeeping (1/16)
157 FOVC ramp
158-165 ESMR raw
166 Pitch
167 Roll
168 Yaw
16 SEC RADIANCES (29 WORDS)
----------------
169 B1
170 B2
171 B3
172 B4
173 A1
174 A2
175 A3
176 A4
177 C1
178 C2
179 C3
180 C4
181 D1
182 D2
183 D3
184 D4
185 A2 Decloud
186 A3 "
187 A4 "
188 C4 "
189 B1-B2 Smoothed
190 B2-B3 "
191 B3-B4 "
192 C3 Decloud
193 SST See note (17)
194-196 [Spare]
ORBIT END BLOCK (9 16-BIT INTEGERS)
---------------
WORD FUNCTION
0 Accession #
1 = 0, If orbit accepted
-1, If orbit erased
+1, If end of data
NOTES
-----
SCR data on the "tapes" consists of one "file" per orbit.
Users should correlate data using major frame time, not major frame numbers.
(1) Calibration Block.
-----------------
An exact copy of the block at the start of the input tape from which the
orbit was extracted. This is repeated before each relevant orbit and is a copy
of the calibration tape used for the day's data.
The calibration parameters used for each day's data consist of a total of
80 numbers, arranged in 20 groups of 4 numbers each. The 20 groups correspond
to the 20 channels of the instrument (- D channels on low and high gain have
different calibration parameters).
The ordering of the 20 groups is as follows:
B1 to B4
A1 to A4
C1 to C4
D1 to D4 low gain setting
D1 to D4 high gain setting
The state of the D channels gain can be determined from the information
given in the flag words - see Note (16) below - or from the Digital B Function
information (Note (15) below).
The 4 numbers within each group are:
(i) EZ = electrical zero (counts)
(ii) S-EZO = space - electrical zero offset (counts)
(iii) r = stray radiation always set to zero in practice
(iv) G = g x (160 x 2^11)/S
where g = gain (volts/unit radiance)
S = scaling factor used to represent radiances, See note (2) below.
(2) Radiance Scale Factors.
----------------------
The contents of the appropriate locations should be divided by the
following scale factors in order to obtain a result in radiance units
(mW/m^2/ster/cm^-1).
CHANNEL SCALE FACTOR
------- ------------
A & B 16
C1 400
C2 40
C3 & C4 20
D1 2 x 10^4 )
D2 5 x 10^3 ) D channels on low gain
D3 750 )
D4 1 x 10^3 )
D1 & D2 5 x 10^5 )
D3 6 x 10^6 ) D channel on high gain
D4 1 x 10^4 )
(3) SCR Orbit Head Block.
--------------------
Copy of the block on seven track magnetic tape, except where the reading
program discovered a data block before reading an orbit head block. In these
cases, the day, orbit, etc., are all set to zero. - Data values are set to
zero but identification words are not changed hence checksum comparisons give
rise to errors and so these orbit head blocks should not appear on the "tapes"
in the archive.
(4) SCR Blocked Data.
----------------
Formed by stacking six pairs of corresponding raw and formatted data
blocks, to form a superblock. Missing formatted blocks are replaced by zeros.
However, the filler blocks are only 176 words long, NOT 205 words long -
the 16 second radiances were not allowed for in these filler blocks on the
magnetic tapes. Other SCR formatted blocks exist which are also 176 words
long but do actually contain data.
(5) PLAYBACK ORBIT NUMBER
---------------------
This can take a value up to 32,768 ( = 15 bits).
WORD: 5 BITS: 0-11 least significant bit (LSB)
6 0-2 most significant bit (MSB)
GMT DAY NUMBER
--------------
Runs from 1 through to 365.
BITS: 3-11
(6) START TIME OF MAJOR FRAME
-------------------------
The start time of the current major frame, in integer seconds of day.
WORD: 7 BITS: 0-11 MSB
8 0-4 LSB
MAJOR FRAME NUMBER
------------------
BITS: 7-11 LSB
9 0-4 MSB
(7) DATA CONTENTS FLAG
------------------
There are six bits allocated to this item. A "0" bit indicates no data
and "1" indicates the type of data that will follow.
BIT: 0 1 2 3 4 5
ITPR NEMS SCR1 SCR2 ESMR EXPH
EXPH = THIR, SCMR, ESMR Houskeeping Data
SYNC SYMMARY BITS
-----------------
WORD: 10 BIT: 6 - Sync Loss Flag; set to 1 if one or more of the 80
minor frame sync loss flags is set to 1, otherwise 0.
7 - Sync Quality Flag; set to 1 if one or more of the 80
minor frame sync quality flags is 1, otherwise 0.
(8) ORBITAL EPHEMERIS DATA
----------------------
Data will contain an implied decimal point:
Latitude and Longitude -
BIT: 23 18 15 12 9 6 3 0
S X X X X X X X X X. X X X X X X X X X X X X X X
Altitude -
BIT: 11 6 3 0
S X X X X X X X X X X X
The S (Sign) Bit will be set to "+" for Northern and "-" for Southern
Latitudes. "1" is negative, "0" is positive. The longitude and altitude will
always be positive. Longitude will be 0 to 360 degrees East.
(9) MINOR FRAME SYNC LOSS AND SYNC QUALITY FLAGS
--------------------------------------------
There are 80 bits allocated to the sync quality flags and 80 bits to the
sync loss flags, in pairs from minor frame 1 to 80. Sync loss flag is
followed by sync quality flag.
Two summary bits will indicate that at least one minor frame sync quality
flag is set or one sync loss flag is set within this major frame. - See Note
(7) above.
Major frames with the summary sync loss flag set should be handled as
follows:
Take the time difference between the last major frame before the major frame
with the sync loss flag set and the first major frame after without sync loss
flag set. Determine the number of major frames which should have been
received. Then,
a) if this equals the number of major frames received, data not flagged sync
loss can be assumed to be usable,
b) if it does not equal the number of received major frames, data is missing.
Major frame times are not necessarily correct except for the following case:
(i) The first major frame with sync loss flagged data after a
major frame without the flag set - the data from minor
frame 79 back to the first minor frame in which the sync
loss flag is set is valid. Remaining data to be processed
only at the discretion of the User.
If all 80 individual minor frame sync loss flags are set
the frame contains data from two or more major frames.
(- This data should only be used at the discretion of
the User.)
(10) CONTROLS INFORMATION
--------------------
In three blocks: pitch - 4 'words', 8 bits each
4 roll 'words'
4 RMP Indicate Rate 'words'
All three control functions are 1/1 samples.
The pitch bias commanded in will be indicated on the Pitch Fine Error
Telemetry. Status Bit 25 will indicate if Ephemeris data is being compensated
for Pitch Bias.
(11) SUBSYSTEM STATUS BITS
---------------------
The status of the functions listed below will be that of the first sample
only regardless of whether the function has a normal sampling rate of 3/16 or
1/1.
------------------------------------------------------------------------------
BIT STATUS INDICATORS
------------------------------------------------------------------------------
1 Primary COMSTOR verify yes/no
2 Redundant COMSTOR verify yes/no
3 ITPR electronics on/off
4 NEMS data unit on/off
5 NEMS data chan 1 on/off
6 NEMS data chan 2 on/off
7 NEMS data chan 3 on/off
8 NEMS data chan 4 on/off
9 NEMS data chan 5 on/ogg
10 ESMR radiometer pwr on/off
11 SCMR electronics on/off
12 THIR electronics on/off
13 S-band A on/off
14 S-band B on/off
15 SCMR S-Band on/off
16 SCR pwr on/off
17 SCR chopper mtr on/off
18 ITPR chopper mtr on/off
19 ESMR antenna scan on/off
20 TDRE xmtr on/off
21 TDRE xmtr output - 8W/4W - 2W
22 TDRE xmtr output 4W/2W
23 SCMR motor on/off
24 Satellite day-night indicator, 1=day, 0=night
25 Ephemeris data compensation bit, 1=uncompensated, 0=compensated*
26-28
------------------------------------------------------------------------------
* Ephemeris data will normally be compensated for pitch bias
(12) SCR1 or SCR2 Block.
------------------
SCR1 blocks contain Digital A data. SCR2 blocks contain Analog data,
plus SCR Housekeeping Digital A data. All other functions are identical in
the two blocks.
Functions are ordered as listed in the tables below and each function is
in time order.
(13) DIGITAL A AND ANALOG DATA
-------------------------
Data will be output as 10 bit 'words'.
In SCR1 blocks:
------------------------------------------------------------------------------
FUNCTION NAME FORM RATE NO. OF SAMPLES
------------------------------------------------------------------------------
SCR exp sensory data ch A DIG A 5/1 80
SCR exp sensory data ch B DIG A 5/1 80
SCR exp sensory data ch C DIG A 5/1 80
SCR exp sensory data ch D DIG A 5/1 80
SCR HSK Dig A DIG A 5/1 80
------------------------------------------------------------------------------
In SCR2 blocks:
------------------------------------------------------------------------------
FUNCTION NAME FORM RATE NO. OF SAMPLES
------------------------------------------------------------------------------
Sig channel ramp o/p A ALOG 5/1 80
Sig channel ramp o/p B ALOG 5/1 80
Sig channel ramp o/p C ALOG 5/1 80
Sig channel ramp o/p D ALOG 5/1 80
SCR HSK Dig A DIG A 5/1 80
------------------------------------------------------------------------------
(14) ANALOG HOUSEKEEPING FUNCTIONS
-----------------------------
Data is output as 10 bit 'words'.
------------------------------------------------------------------------------
Analog Housekeeping Function Listing.
FUNCTION NAME FORM RATE
------------------------------------------------------------------------------
S FT F T ANALOG 1/16
SCR MOD PCU ANALOG 1/16
F/T THML ANALOG 1/16
SW THML ANALOG 1/16
CHOP I 1 ANALOG 1/16
CHOP I 2 ANALOG 1/16
S HSG T1 ANALOG 1/16
S HSG T2 ANALOG 1/16
S HSG T3 ANALOG 1/16
S HSG T4 ANALOG 1/16
S HSG T5 ANALOG 1/16
S HSG T6 ANALOG 1/16
BLACKBODY T ANALOG 1/16
CAL MIR T ANALOG 1/16
CHP MTR T ANALOG 1/16
S FT T A ANALOG 1/16
S FT T B ANALOG 1/16
S FT T C ANALOG 1/16
SCR MOD CP T ANALOG 1/16
SCR CAL MD T ANALOG 1/16
SCR CHP REST ANALOG 1/16
SCR PCU STBT ANALOG 1/16
SCR PCU SW T ANALOG 1/16
-6V BBTH ANALOG 1/16
CLAMP A ANALOG 1/16
S FT T D ANALOG 1/16
FWA SHDT ANALOG 1/16
FWA MTRT ANALOG 1/16
CLAMP B ANALOG 1/16
FWB HTR P ANALOG 1/16
FWB SHDT ANALOG 1/16
FWB MTRT ANALOG 1/16
CLAMP C ANALOG 1/16
FWC HTR P ANALOG 1/16
FWC SHDT ANALOG 1/16
FWC MTRT ANALOG 1/16
CLAMP D ANALOG 1/16
S FT T E ANALOG 1/16
FWD SHDT ANALOG 1/16
FWD MTRT ANALOG 1/16
FOV DRIVE ANALOG 1/1
------------------------------------------------------------------------------
(15) DIGITAL B FUNCTIONS
-------------------
------------------------------------------------------------------------------
Digital B Function Listing.
FUNCTION NAME FORM RATE
------------------------------------------------------------------------------
POWER ON/OFF DIG B 1/16
CHOPPER PWR ON/OFF DIG B 1/16
PRE-CAL ON/OFF DIG B 1/16
D CHAN GAIN HI/NORM DIG B 1/16
CAL ENA/INH DIG B 1/16
FOVC ENA/INH DIG B 1/16
EA CAL YES/NO DIG B 1/16
BB CAL YES/NO DIG B 1/16
HSG CAL YES/NO DIG B 1/16
SP CAL YES/NO DIG B 1/16
FW POS 1 YES/NO DIG B 1/16
FW POS 2 YES/NO DIG B 1/16
FW POS 3 YES/NO DIG B 1/16
FW POS 4 YES/NO DIG B 1/16
FOVC POS CNTR/NO DIG B 1/1
CHP SYNC 1/0 DIG B 1/1
FWA P B1 1/0 DIG B 1/1
FWA P B2 1/0 DIG B 1/1
FWA P FIN YES/NO DIG B 1/1
F/WHL B ENA/INH DIG B 1/1
FWB P B1 1/0 DIG B 1/1
FWB P B2 1/0 DIG B 1/1
FWB P FIN YES/NO DIG B 1/1
F/WHL C ENA/INH DIG B 1/1
FWC P B1 1/0 DIG B 1/1
FWC P B2 1/0 DIG B 1/1
FWC PFIN YES/NO DIG B 1/1
F/WHL D ENA/INH DIG B 1/1
FWD P B1 1/0 DIG B 1/1
FWD P B2 1/0 DIG B 1/1
FWD PFIN YES/NO DIG B 1/1
------------------------------------------------------------------------------
BLOCK IDENTIFICATION
--------------------
WORD: 461 BIT: 10 = 0, for SCR1 block
1, for SCR2 block
(16) MAJOR FRAME FLAGS
-----------------
Bits are set to 1 when the following conditions are satisfied at any time
during the orbit:
WORD: 10 BIT: 0 SCR power on
1 Chopper power on
2 Calibration imminent
3 D channels on high gain (else low gain)
4 Calibration enabled
5 FOVC enabled
6 Earth view
7 Black body view
8 Housing view
9 Space view
10 Filter wheel select position 1
11 " " " " 2
11 0 " " " " 3
1 " " " " 4
2 SCR2 format being used (Analog not Digital A data)
3 Satellite day=1, night=0
4 THIR on
5 ESMR on and scanning
6 S-band A on
7 " B on
8 " SMCR on
9 Beacon on
10 Latitude, longitude corrected for pitch bias
11 End of orbit detected
12 0 Checksum error in header block
1 " " " " "
2 Minor frame SYNC error
3 Bad filter position detected
4 " chopper SYNC
5 FOVC motion bad
6 Bad end of block
11 Discontinuity in time code from previous major frame
13 [SPARE]
14 0 = 1, slots for calibrated data do contain radiances
0, slots for calibrated data contain raw ramps
(this occurs where instrument is not viewing
earth, i.e. during calibration sequences)
(17) SEA SURFACE TEMPERATURE/GEOGRAPHY
---------------------------------
Over land areas, this word represents the mean surface height tabulated
for a 1 degree latitude/longitude grid. Over ocean areas, this word contains
the mean climatological sea-surface temperature tabulated for a 2.5 degree
latitude/longitude grid for monthly intervals.
The two cases are distinguished as follows :
(i) If positive, we have land and mean surface height is given in units of
100 feet.
(ii) If negative, we have ocean and mean sea-surface temperature is the
absolute magnitude in units of tenths of a degree Celsius.
(18) SCR Orbit End Block.
-------------------
These blocks were copied directly to nine track magnetic tape, except when
an erased orbit occured. If an erased orbit was indicated, then the orbit was
erased from the stack nine track tape. Hence all orbit end blocks on the
"tapes" in the archive should flag accepted orbits.
The calibrated radiances and related scaling factors information, major
frame flag, and sea surface temperature information has been taken from 'The
Selective Chopper Radiometer on Nimbus V Archived Data.', Atmospheric Physics
Memorandum No. 77.1. Most of the information for the raw header and SCR1/SCR2
block formats has been selected from the 'Data Formats For Nimbus E' document.
See these documents for more information if required.
SPECIAL NOTES (made when magnetic tapes were copied to optical disks)
-------------
1) Checksum Errors -
A vast number of checksum errors have occurred whilst copying the series
as a whole that are due to the data on the magnetic tape actually being
corrupt (and not in the given format). - There are cases where it appears
that the odd extra bit(s) being set, either in writing to the magnetic tape
or at an earlier stage in data handling, lead to incorrect checksum
comparisons. Also, data values greater than 4095 have been found and these
are clearly not possible for data originally stored in 12-bit 'words'.
Some data records are short (- end-of-block markers appear at the wrong
position in the records). There is no definite way of knowing which data
values are missing.
A number of data records have no end-of-block markers at all.
NB - All data records with checksum errors have not been written to the
virtual tape files.
2) Orbit Numbers -
These have been extracted from the data and written out to the log files.
Although they should be consecutive (give or take a couple of numbers),
this is not always the case. Even though some of the numbers are
improbable-looking no checksum errors occurred during the copying process
(impling that the data is as on magnetic tape).
3) Day Numbers -
Again, extracted from the data and output to the log files. On the whole
these are sensible but there are values that are not in sequence or clearly
incorrect. Since no copying errors have occurred we can only conclude that
the data is an exact copy of the magnetic tape information.
4) Problems occured towards the end of tapes N00121 and N00124 as a result of
parity errors; we were unable to record these tapes beyond the points
where the errors occured - 45465 blocks (N00121) and 25881 blocks (N00124).