810-005, Rev. E
DSMS Telecommunications Link
Design Handbook
303
Media Calibration
Effective November 30, 2000
Document Owner: Approved by:
----------------------- -----------------------
P.H. Richter Date A. Kwok Date
Tracking and Navigation Service
Systems Development Engineer
Released by:
[Signature on file in TMOD Library]
------------------------
DSMS Document Release Date
Change Log
Rev Issue Date Affected Paragraphs Change Summary
Initial 1/15/2001 All All
Note to Readers
There are two sets of document histories in the 810-005 document, and these
histories are reflected in the header at the top of the page. First, the entire document is
periodically released as a revision when major changes affect a majority of the modules. For
example, this module is part of 810-005, Revision E. Second, the individual modules also
change, starting as an initial issue that has no revision letter. When a module is changed, a
change letter is appended to the module number on the second line of the header and a summary
of the changes is entered in the module's change log.
This module supersedes module MED-10 in 810-005, Rev. D.
Contents
Paragraph Page
1 Introduction.......................................................................................... 4
1.1 Purpose ............................................................................................ 4
1.2 Scope............................................................................................... 4
2 General Information .................................................................................. 4
2.1 Global Positioning System Data ..................................................................... 4
2.1.1 GPS Signal Structure.............................................................................. 5
2.1.2 GPS Receiver/Processor Assembly (GRA)............................................................. 6
2.1.3 Relation of Phase and Group Delay to Atmospheric Properties....................................... 7
2.2 Ground Weather Data ............................................................................... 14
Tables
Table Page
1. GPS Metric Data, Code Mode............................................................................ 7
2. GPS Metric Data, Non-Code Mode ....................................................................... 9
3. GPS Ephemeris Data................................................................................... 10
4. GPS Almanac Data..................................................................................... 12
5 Weather Data Transmitted from the SCA................................................................. 14
1 Introduction
1.1 Purpose
This module describes the capabilities of the equipment used by the
Deep Space Network (DSN) to obtain data from which correction factors can be
determined for media effects that limit navigational accuracy. The data are
forwarded from each Deep Space Communications Complex (DSCC) to the Net
work Operations Control Center (NOCC) where they are processed and archived.
1.2 Scope
The functional performance and data characteristics of the Deep
Space Station (DSS) Media Calibration Subsystem (DMD) are described. The DMD
is responsible for obtaining Global Positioning System (GPS) and ground
weather data for the NOCC Tracking Subsystem (NTK) and Navigation Subsystem
(NAV).
2 General Information
The DMD provides two types of data:
-GPS data consisting of L-band carrier phase and group delay of GPS
satellite signals, in addition to ephemeris and almanac data for the GPS
satellites.
-Weather data, consisting of temperature, barometric pressure, relative
humidity, precipitation rate, total precipitation, wind speed, and wind
direction.
2.1 Global Positioning System Data
The Global Positioning System GPS Operational Constellation consists
of at least 24 satellites that orbit the earth with a 12 sidereal-hour period.
There are often more than 24 as new satellites are launched to replace the older
ones. The orbit is such that the satellites repeat the same track and
configuration over any point approximately each 24 hours (4 minutes earlier each
day). There are six orbital planes (with nominally four satellites in each),
equally spaced (60 degrees apart), and inclined at about fifty-five degrees with
respect to the equatorial plane. This constellation provides the user with
between five and eight satellites visible from any point on the Earth. A minimum
of four satellite signals must be received to estimate the four unknowns of
position in three dimensions and time.
The DSCC GPS Receiver/Processor Assembly (GRA), which is part of the
DMD, makes use of the GPS data to provide carrier phase and group delay for the
GPS signals.
These data may then be used to characterize the Earth's ionosphere and
troposphere along the line of sight from a given satellite to the DSCC.
2.1.1 GPS Signal Structure
The GPS satellite signals are complex in structure, with each L-
band frequency being binary biphase-modulated with two pseudo-random noise
codes, the Coarse Acquisition (C/A) and Precision (P) codes, and a navigation
message.
The complete signal broadcast by a satellite may be represented as:
s(t) = [A_C * C(t)D(t)sin(2(pi)f_1*t) + A_P * P(t)D(t)cos(2(pi)f_1*t)] (1)
+ [A_p * P(t)D(t)cos(2(pi)f_2*t)]
where the first square bracket is the L1 signal at frequency f1, and the second
square bracket is the L2 signal at frequency f2. The terms appearing above have the
following definitions:
A_c and A_p = the constant amplitudes of the Coarse Acquisition (C/A) and Precision (P) codes
C(t) = the C/A-code modulation (= +/-1)
P(t) = the P-code modulation (= +/-1)
D(t) = the navigation message modulation (= +/-1)
f_1 = 154 f_0 = 1575.42 MHz
f_2 = 120 f_0 = 1227.60 MHz.
The C(t), P(t), and D(t) modulations are all synchronized to the
fundamental clock frequency, f_0, such that they have the following frequencies:
f_0 = 10.23 MHz (Note 1)
C(t)= f_0/10 = 1.023 Mbps
P(t)= f_0 = 10.23 Mbps
D(t)= f_0/204600 = 50 bps.
Note (1): To partially compensate for general and special relativistic effects
on the satellite clock (gravitational red shift and time dilation),
the actual value of f_0 is 10.23 MHz - 4.55 mHz.
The complete C/A code contains 1023 cycles (or "chips"), has a total
period of 1.0 ms, and is different for each satellite.
The P-code is more complicated and consists of two code segments (X1
and X2), which differ in length by 37 chips. These are added modulo 2 and timed
in such a way that exactly 403,200 X1 code segments correspond to exactly one
week, the period of the P-code. (The P-code actually has a total period of 37
weeks, with each satellite using only a single one-week segment of the total.)
The duration of the X1 code segment is thus 1.5 seconds and contains exactly
15,345,000 chips at 10.23 Mbps. As is the case with the C/A code, the P-code is
different for each satellite.
The navigation message also has a complex structure, with a total
period of 12.5 minutes (one master frame) and is divided into frames,
subframes, words, and bits. The first three subframes (lasting 6 seconds each)
repeat every 30 seconds, while the last two subframes are different in each of
25 consecutive frames (pages), after which the entire message repeats.
2.1.2 GPS Receiver/Processor Assembly (GRA)
The GRA provides the following functional capabilities:
1) Automatically acquire and track the L1 and L2 GPS signals for
specified satellites, usually all of those transiting
2) Extract and store GPS almanac and ephemeris data from the
navigation message
3) Measure the differential P-code group delay between the L1 and
L2 GPS signals
4) Measure the differential carrier phase between the L1 and L2 GPS
signals.
The almanac data, contained in subframe 5 of the GPS navigation
message, consist of approximate ephemeris data for all satellites and are used
by the GRA for signal acquisition.
The ephemeris data for a specified satellite (subframes 2 and 3)
provide a complete description of the orbit. When the data are combined with
measured signal delays, the local position and atmospheric path that the signal
has traversed can be determined.
Since the Department of Defense, which controls the GPS signal
content, may elect at any time to encrypt the P-code (resulting in what is
termed an anti-spoofing (A/S) mode of operation in which the encrypted, or Y-
code, is unavailable to civilian users of the system) the GRA operates in two
distinct modes to determine the differential group and phase delays of the
satellite signals.
In the normal, coded mode, the known P-code is used to determine the
carrier phase and group delay of each signal (L1 and L2) separately. The
computed differences may then be used to characterize the propagation medium
over the path of the signals. This provides the most precise determination due
to the length of the P-code.
In the codeless mode, advantage is taken of the fact that the same
unknown Y-code is transmitted on both the L1 and L2 channels with an unknown
delay. The product of the two signals is formed and the differential group and
phase delays are determined by cross-correlation. This method results in a
somewhat reduced accuracy.
The GRA simultaneously receives and processes the signals from up to
eight satellites selected to provide the longest unbroken tracks at any given
time. In addition to the data described above, the system provides various
status and health data on the signals being processed. Tables 1 through 4 list
the GPS parameters measured, their ranges and accuracy, and the sample
intervals provided.
2.1.3 Relation of Phase and Group Delay to Atmospheric Properties
The Earth's atmosphere may conveniently be divided into three regions
according to the effects produced on the propagation of electromagnetic
radiation:
(1) troposphere, stratosphere, and lower part of mesophere - region
between the Earth's surface and about 60 km altitude consisting of neutral
(unionized) gases
(2) ionosphere - region from about 60 km to between ~500 and 2000 km,
depending on the extent of extraterrestrial ionizing radiation, consisting of
partially ionized gases
. (3) plasmasphere - ionized region extending from ~2000 km to about four
Earth radii (26,000 km), where it blends into the solar wind of the Earth's
magnetosphere
At the frequencies in which the DSN operates, tropospheric
dispersion may be neglected and the refractivity represented by a dry and a
wet component whose approximate total zenith phase and group delays are:
delta t_D ~ 7.6 ns,
delta t_W ~ 0.3 ns - 1.4 ns.
The first varies linearly with pressure at the Earth's surface; the
second increases as the tropospheric moisture content increases.
Since tropospheric dispersion is negligible at L-band, these delays
cancel when differential delays are computed or measured between f_1 and f_2.
In the ionized portion of the Earth's atmosphere, the medium displays
anomalous dispersion at microwave frequencies. This causes the phase velocity to
exceed, and the group velocity to be less than, the speed of light in a vacuum,
c. Specifically, to a good approximation at L-band:
v/c = 1 + x/2 (2)
v_g/c = 1 - x/2 (3)
Table 1. GPS Metric Data, Code Mode
Parameter Units (1) Approximate Decimal Range
Delay Calibration 2^-7 ns +/-255 ns
Output Interval sec 1-300 s
L1-C/A Doppler Phase 2^-16 cycles +/-2.1 x 10^9 cycles
L1-C/A Doppler Phase Noise 2^-16 cycles 0-1 cycle
L1-P Doppler Phase 2^-16 cycles +/-2.1 x 10^9 cycles
L1-P Doppler Phase Noise 2^-16 cycles 0-1 cycle
L2-P Doppler Phase 2^-16 cycles +/-2.1 x 10^9 cycles
L2-P Doppler Phase Noise 2^-16 cycles 0-1 cycle
L1-C/A Group Delay 2^-11 ns +/-0.27 sec
L1-C/A Group Delay Noise 2^-11 ns 0-32 ns
L1-P Group Delay 2^-11 ns +/-0.27 sec
L1-P Group Delay Noise 2^-11 ns 0-32 ns
L2-P Group Delay 2^-11 ns +/-0.27 sec
L2-P Group Delay Noise 2^-11 ns 0-32 ns
C/A SNR (1 sec) 2^-4 volt/volt 0-4096
P1 SNR (1 sec) 2^-4 volt/volt 0-4096
P2 SNR (1 sec) 2^-4 volt/volt 0-4096
Receiver Clock Error 2^-32 sec +/-0.5 sec
L1-C/A Residual Phase 2^-10 cycles 0-0.25 cycle
Note (1): Least significant bit transmitted by the GRA. Table 2. GPS Metric
Data, Non-Code Mode
Parameter Units (1) Approximate Decimal Range
Delay Calibration 2^-7 ns +/-255 ns
Output Interval sec 1-300 sec
L1-C/A Doppler Phase 2^-16 cycles +/-2.1 x 10^9 cycles
L1-C/A Doppler Phase Noise 2^-16 cycles 0-1 cycle
L1-L2 Doppler Phase 2^-16 cycles +/-2.1 x 10^9 cycles
L1-L2 Doppler Phase Noise 2^-16 cycles 0-1 cycle
L1-C/A Group Delay 2^-11 ns +/-2.1 x 10^9 cycles
L1-C/A Group Delay Noise 2^-11 ns 0-32 ns
P2-P1 Group Delay 2^-9 ns +/-1.1 s
P2-P1 Group Delay Noise 2^-9 ns 0-128 ns
C/A SNR (1 s) 2^-4 volt/volt 0-4096
P2-P1 SNR (1 s) 2^-6 volt/volt 0-1024
Receiver Clock Error 2^-32 s +/-0.5 s
L1-C/A Residual Phase 2^-10 cycles 0-0.25 cycle
Note (1): Least significant bit transmitted by the GRA. Table 3. GPS Ephemeris
Data
Parameter Units (1) Approximate Decimal Range
Sample Year (Modulo 100) Year 0-99 yrs
Sample Day-of-Year Days 0-366 days
Sample Hours Hours 0-24 hrs
Sample Minutes Minutes 0-60 minutes
Sample Seconds seconds 0-60 s
GPS Week Number N/A
Satellite Number N/A
L2 Code Type/L2 Code On N/A
User Range Accuracy N/A
Issue of Data (Clock) N/A
Clock Data Reference Time (toc) 24 s 0-6.0 x 10^5 s
Time Correction Coefficient (af2) 2^-55 s/s2 +/-3.6 x 10^-15 s/s2
Time Correction Coefficient (af1) 2^-43s/s +/-3.7 x 10^-9 s/s
Time Correction Coefficient (af0) 2^-31s +/-3.9 ms
Issue of Data (Ephemeris) N/A
Amplitude of Sine Harmonic 2^-5m +/-1.0 km
Correction to the Orbit Radius (Crs)
Mean Motion Difference From Computed 2^-43 semicir/s +/-1.2 x 10^-5 mrad/s
Values (Delta N)
Mean Anomaly at Reference Time (Mo) 2^-31 semicir +/-180 deg
Amplitude of Cosine Harmonic 2^-29 radians +/-6.1 x 10^-2 mrad
Correction to the Argument of
Latitude (Cuc)
Eccentricity (e) 2^-33 0-0.03
Amplitude of Sine Harmonic 2^-29 radians +/-6.1 x 10^-2 mrad
Correction to the Argument of
Latitude (Cus)
Square Root of Semi-Major Axis 2^-19 m^(1/2) 0-8200 m1/2
(A1/2)
Ephemeris Reference Time (t0E) 24 s 0-6.0 x 105 s
Note (1): Least significant bit transmitted by the GRA. 10
Table 3. GPS Ephemeris Data (Continued)
Parameter Units (1) Approximate Decimal Range
Amplitude of Cosine Harmonic Correction 2^-29 radians +/-6.1 x 10^-2 mrad
to Inclination (Cic)
Right Ascension at Reference Time 2^-31 semicir +/-180 deg
(Omega0)
Amplitude of Sine Harmonic Correction 2^-29 radians +/-6.1 x 10-2 mrad
to Inclination (Cis)
Inclination at Reference Time (i0) 2^-31 semicir +/-180 deg
Amplitude of Cosine Harmonic Correction 2^-5 m +/-1.0 km
to the Orbit Radius (Crc)
Argument of Perigee (Omega) 2^-31 semicir +/-180 deg
Right Ascension Rate (Omega DOT) 2^-43 semicir/s +/-3.0 x 10^-3 mrad/s
Issue of Data (Ephemeris) N/A
Inclination Angle Rate (IDOT) 2^-43 semicir/s +/-1.2 x 10^-5 mrad/s
Note (1): Least significant bit transmitted by the GRA. Table 4. GPS Almanac
Data
Parameter Units (1) Approximate Decimal Range
Sample Year (Modulo 100) Year 0-99 yr.
Sample Day-of-Year Days 0-366 days
Sample Hours Hours 0-24 hrs
Sample Minutes Minutes 0-60 minutes
Sample Seconds s 0-60 s
GPS Week Number N/A
Satellite Number N/A
Data and Space Vehicle ID N/A
Eccentricity (e) 2^-21 0-0.03
Reference Time (tOA) 2^+12 s 0-6.0 x 10^5 s
Delta Inclination (di) 2^-19 semicir +/-11 deg
Right Ascension Rate (Omega DOT) 2^-38 semicir/sec +/-3.7 x 10^-4 mrad/s
Square Root of Semi-Major Axis 2^-11 m^(1/2) 0-8200 m1/2
(A1/2)
Right Ascension at Reference Time 2^-23 semicir +/-180 deg
(Omega0)
Argument of Perigee (Omega) 2^-23 semicir +/-180 deg
Mean Anomaly (M0) 2^-23 semicir +/-180 deg
Correction Term (af0) 2^-20 s +/-0.03 deg
Correction Term (af1) 2^-38 s +/-1.2 x 10^-7 s/s
Note (1): Least significant bit transmitted by the GRA.
where:
v = phase velocity = omega/k
v_g = group velocity = d(omega)/dk
k = wave vector (lambda/2(pi))
x = (f_p/f)^2 << 1
f = frequency of interest
f_p = plasma frequency = (N*e^2/m(epsilon_0))^(1/2)/2(pi)
N = electron density (electrons/m^3)
e = electronic charge
m = electronic mass
epsilon_0 = permittivity of free space.
In terms of the above, the phase (delta t) and group (delta t_g) delays at
frequency f may be written:
delta t_g = -delta t = (1.345x10^-7/f^2) x TEC, s (4)
where TEC = integral(N dl) is the total electron content (TEC) along the propagation
path (electrons/m^2).
The corresponding differential delays are given by:
DELTA t_g = -DELTA t = 1.345x10^-7 (1/f_2^2 - 1/f_1^2) x TEC, s (5)
where DELTA t_g = delta t_g(f_2) - delta t_g(f_1).
Since the TEC along the satellite line of sight may vary between
~10^16 and 4x10^18 m^-2, the group and phase delays typically range between ~0.5 ns
and 90 ns, and the differential delays between ~0.35 ns and 35 ns, although
larger values are often observed during periods of high solar activity.
2.2 Ground Weather Data
The ground weather data are generated by instruments located near the
Signal Processing Centers (SPC) at each DSCC. In particular, the wind speed and
direction sensors are adjacent to the 34m HEF antennas.
All data are asampled once per second by the instruments, and the
resulting data stream is transmitted to the Subsystem Control and Monitor
Assembly (SCA) of the DMD. Here the data are packaged and transmitted to the NTK
and NAV at regular intervals and stored for up to five days for later recall.
Table 5 lists the weather parameters measured, their ranges and accuracy, and
the interval of transmission to the NTK, NAV, and DMC.
Table 5 Weather Data Transmitted from the SCA
Parameter Range Accuracy Transmission Interval
Default Range
Temperature -50 to +50deg C +/-0.1deg C 60 s 10 s to 1 hr
Barometric Pressure 600 to 1100 mbar 1.0 mb 60 s 10 s to 1 hr
Relative Humidity(1) 0 to 100% 2% 60 s 10 s to 1 hr
Dew Point -40 to 50deg C +/-0.5deg C 60 s 10 s to 1 hr
Temperature
Precipitation Rate 0 to 250 mm/hr 5% 60 s 10 s to 1 hr
Total Precipitation >0 mm 5% 60 s 10 s to 1 hr
Wind Speed(2) 0 to 100 km/hr +/-0.6 km/hr 60 s 10 s to 1 hr
Wind Direction(2) 0 to 360 deg +/-3.6 deg 60 s 10 s to 1 hr
See notes on following page.
(1) Relative humidity is calculated from the measured weather parameters
according to the formula:
RH =10^x percent, (6)
where:
x = 2 + 2300 (1/T - 1/T_d),
T = temperature in Kelvins
T_d = dew point temperature in Kelvins.
(2) Wind data are averaged over 10-s intervals by converting the polar velocity vector:
v_w =S_w * (e-hat)(theta) (7)
where:
S_w = wind speed
(e-hat)(theta) = wind direction unit vector
to rectangular form,
v_w = S_x * (i-hat)(theta) + S_y * (j-hat)(theta) (8)
and computing and , where
^2 = ^2 + ^2,
= arctan(/),
S_x = S_w * cos(theta),
S_y = S_w * sin(theta).