David T. Chelmins

Glenn Research Center, Cleveland, Ohio

Glenn Goddard TDRSS Waveform 1.1.3

On-Orbit Performance Report 

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NASA/TM—2014-218292 1 

Glenn Goddard TDRSS Waveform 1.1.3 

On-Orbit Performance Report 

David T. Chelmins 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Background and Introduction 

The objective of the Space Communications and Navigation (SCaN) Testbed is to study the 

development, testing, and operation of software defined radios and their associated applications in the 

operational space environment to reduce cost and risk for future space missions. The SCaN Testbed was 

launched on July 20, 2012 to the International Space Station (ISS) on a Japanese H-II Transfer Vehicle 

(JAXA HTV3), and transferred and installed via Extravehicular Robotics (EVR) on the ExPRESS 

Logistics Carrier-3 (ELC3) in the inboard, Ram-facing, Zenith-facing payload location on an exterior 

truss of the ISS (Figure 1). 

SCaN Testbed contains three software-defined radios, which provide data communications 

capabilities that can be reconfigured on-demand by software changes. The software that defines the 

communications behavior is referred to as a “waveform”. The simplest SDR consists of an analog-todigital converter (ADC), a digital-to-analog converter (DAC), a diplexer, and an antenna. The waveform 

defines the digital signals that are sent to and received from the ADC and DAC, respectively. 

The Jet Propulsion Laboratory (JPL) software-defined radio (SDR), shown in Figure 2, is one of the 

SDRs in the testbed that supports full-duplex communications at S-band and simplex receive operation at 

L-band frequencies. The S-band capabilities of the radio have been tested with NASA’s Tracking and 

Data Relay Satellite System (TDRSS) network of geosynchronous satellites. An engineering model (EM) 

of the JPL SDR remains at the NASA Glenn Research Center (GRC) on the ground, installed into the 

SCaN Testbed Ground Integration Unit (GIU) for development and testing. 

The Glenn Goddard TDRSS (GGT) waveform, version 1.1.3, was installed on the JPL SDR prior to 

launch to ISS. This waveform is compliant with the capabilities of TDRSS and allows the radio to support 

full-duplex communication with a Tracking and Data Relay Satellite (TDRS). A block diagram of the 

waveform functionality is shown in Figure 3. The waveform operates using binary phase shift keying 

(BPSK), and it is compliant with the Space Telecommunications Radio System (STRS) architecture 

standard. 

An extensive set of communications testing was performed with the GGT waveform prior to launch 

of SCaN Testbed. The GGT Waveform Performance Databook (SCaN Testbed project document GRCCONN-DBK-0924) documents the waveform performance on the ground. This set of performance data 

will be used as a baseline for evaluating on-orbit performance. 

This report covers the results of on-orbit performance testing completed using the waveform 

configurations in Table 1 over TDRSS. Testing was completed between January 2013 and April 2013, 

with operations coordinated from the Telescience Center (TSC) at the GRC, shown in Figure 4. Some of 

the tests helped debug problems with the end-to-end data paths and ground support systems that are part 

of the SCaN Testbed. 
NASA/TM—2014-218292 2 

Figure 1.—SCaN Testbed Installed on ISS 

Figure 2.—JPL Software Defined Radio 

Figure 3.—GGT Waveform Functional Diagram 
NASA/TM—2014-218292 3 

Figure 4.—SCaN Testbed Operations at the GRC TSC (NASA C-2012-650) 

TABLE 1.—GGT MODES OF OPERATION 

Mode Description TDRSS 

service 

Data rate 

Tx/Rx, 

kbps 

Symbol rate 

Tx/Rx, 

kSps 

PN 

modulation 

Coded and 

scrambled 

A VLDR Test Mode 

(Data Group 1, Mode 2) 

MA 24 / 18 24 / 18 On Off 

B VLDR Operational Mode 

(Data Group 1, Mode 2) 

MA 24 / 18 48 / 36 On On 

C VLDR Test Mode 

(Data Group 1, Mode 2) 

SA 24 / 18 24 / 18 On Off 

D VLDR Operational Mode 

(Data Group 1, Mode 2) 

SA 24 / 18 48 / 36 On On 

E LDR Test Mode 

(Data Group 2) 

SA 192 / 155 192 / 155 Off Off 

F LDR Operational Mode 

(Data Group 2) 

SA 192 / 155 384 / 310 Off On 

G MDR Test Mode 

(Data Group 2) 

SA 769 / 769 769 / 769 Off Off 

H MDR Operational Mode 

(Data Group 2) 

SA 769 / 769 1539 / 1539 Off On 

Although the GGT waveform is completely configurable, it also provides several standard 

configurations, known as “modes”. Each mode, referenced by a letter, defines the full-duplex behavior of 

the waveform (e.g., frequency, data rate, error correction). A full listing of the waveform modes tested onorbit is shown in Table 1. Each mode is classified as very low data rate (VLDR), low data rate (LDR), or 

medium data rate (MDR). TDRSS provides either single access (SA) or multiple access (MA) services 

(refer to the Space Network User’s Guide, 450-SNUG, published by the Goddard Space Flight Center). 

The waveform is capable of spread-spectrum operation by modulating with a pseudo-noise (PN) code. 

Forward error correction (coding) is implemented using a rate-1/2 convolutional code. 
NASA/TM—2014-218292 4 

The SA and MA services have assigned frequencies. SCaN Testbed has a 2216.5 MHz return link and 

a 2041.027 MHz forward link when using the SA service. The MA service uses a 2287.5 MHz return link 

and a 2106.5 MHz forward link. These frequencies are negotiated through the NASA spectrum 

management office. 

The next several sections will outline the testing conducted with GGT 1.1.3 and make relevant 

comparisons to testing completed on the ground using the waveform. The objective of this report is to 

provide a performance baseline for future users of the JPL SDR. The report will also provide a gauge for 

the radio frequency (RF) link prediction accuracy. 

GGT Waveform Tests Executed On-Orbit 

Table 2 shows a list of the test events that involved the GGT waveform version 1.1.3. 

TABLE 2.—GGT 1.1.3 ON-ORBIT TESTS 

Date Day of 

year 

SN event Waveform 

mode 

RF link 

direction 

Forward link 

result 

Return link 

result 

01/29/2013 29 2 D Receive Only (1) N/A 

01/29/2013 29 3-1 A Receive Only (1) N/A 

01/29/2013 29 3-2 A Receive Only N/A 

01/29/2013 29 4-1 A Receive Only N/A 

01/29/2013 29 4-2 B Receive Only N/A 

02/05/2013 36 2 A Full Duplex (2) 

02/05/2013 36 4 C Full Duplex (1) 

02/05/2013 36 5 C Full Duplex (2) 

02/06/2013 37 1 E Full Duplex (2) 

02/06/2013 37 2 G Full Duplex (3) 

02/06/2013 37 3 A Full Duplex 

02/06/2013 37 4 C Full Duplex 

02/06/2013 37 5 C Full Duplex 

02/10/2013 41 1 F Full Duplex (1) 

02/10/2013 41 2 A Full Duplex 

02/10/2013 41 3 A Full Duplex 

02/10/2013 41 4-1 E Full Duplex 

02/10/2013 41 4-2 F Full Duplex (1) (1) 

02/10/2013 41 5 C Full Duplex 

02/12/2013 43 1 H Full Duplex (1) 

02/13/2013 44 1 B Receive Only N/A 

02/13/2013 44 2 D Full Duplex 

02/13/2013 44 3 D Full Duplex (2) 

04/01/2013 91 1 D Full Duplex 

04/01/2013 91 2 D Full Duplex 

04/01/2013 91 3 D Full Duplex 

04/02/2013 92 1 F Full Duplex 

04/02/2013 92 2 F Full Duplex 

04/04/2013 94 1 G Full Duplex 

04/04/2013 94 2 G Full Duplex 

04/16/2013 106 1 H Full Duplex 

04/16/2013 106 2 H Full Duplex 

– Test passed: the link behavior was consistent with expectations 

(1) – Test failed: no data was received on the link 

(2) – Test failed: bit errors were introduced by the ground system 

(3) – Test failed: the waveform was unable to lock on the RF signal 
NASA/TM—2014-218292 5 

Several of the early events in Table 2 experienced problems with the data path. Most of these issues 

were resolved between Julian day of year (DOY) 44 and DOY 91. Nevertheless, even events affected by 

data issues provide useful information about the RF link. 

Bit Error Rate Performance 

The bit error rate (BER) is the primary metric used to evaluate the performance of the GGT 

waveform. BER traditionally is evaluated in terms of a corresponding energy-per-bit to noise density 

(Eb/N0) ratio. The BER of a test, or the number of incorrect bits received divided by the total number of 

bits received, is actually an indicator of an underlying binomial statistical distribution. This distribution 

can be estimated by a Poisson distribution with the equation: 

   = ln(1 ) ln 	


()

! 








where n is the total number of bits received, p is the bit error rate, CL is the confidence level, and N is the 

total number of error bits. In order to establish the true BER of the system, an infinite amount of bits 

would need to be received. The equation above provides a statistical glimpse at the true BER. A common 

interpretation is that for a given test where N errors were received out of n bits, the underlying bit error 

rate is p or better with CL% confidence. 

In this report, to save computation time, the BER performance is calculated to a 95 percent 

confidence level using a Monte Carlo simulation. This is performed using the “berconfint” function in the 

MATLAB communications toolbox. The result is a range of BERs, with 95% confidence that the true 

BER is within that range. 

Just as there is uncertainty in the BER statistical distribution, there is also uncertainty in the Eb/N0. 

On-orbit tests are conducted over real links, which have changing path lengths, noise floors, estimated 

gains and power levels, and other losses. There is a tradeoff between time to collect sufficient bits for 

BER certainty and time to evaluate a single Eb/N0 with high certainty. 

In this report, Eb/N0 is determined from the link prediction. Since Eb/N0 varies over time, a 

corresponding Eb/N0 range is also provided for each BER measurement. In general, the total variance 

will be approximately 0.5 dB. The ground testing Eb/N0 will not vary, since the power level and noise 

floor were held relatively steady during BER testing. 

BER is also temperature-dependent. The noise figure of the radio’s RF components will vary over 

temperature. In the JPL radio, the Radio Frequency Module (RFM) contains a temperature sensor which 

is used to plot noise figure (Figure 5). For more information on the data behind Figure 5, refer to SCaN 

Testbed document GRC-CONN-ANA-0854. 

Figure 5.—JPL SDR Noise Figure 
NASA/TM—2014-218292 6 

Forward Link Bit Error Rate Performance for Uncoded Modes 

The GGT 1.1.3 waveform release has a software bug which causes a “flaring” effect in the BER 

curve. The bug is due to a FPGA register that overflows at high power levels, causing the waveform bit 

synchronization to see spurious bit flips. On a positive note, the flare makes BER characterization much 

easier because a characteristic error rate can be established over a wide range of Eb/N0 values. If the 

waveform performed closer to theoretical BPSK, many times there would be 0 bit errors over the limited 

duration of the test RF links. 

All BER curves for the GGT uncoded modes (i.e., A, C, E, G) are shown in Figure 6. The theory line 

shows the ideal BPSK modulation performance, the baseline line shows the waveform performance 

measured during ground testing prior to launch, and the observed boxes show the waveform performance 

measured on-orbit. The baseline contains vertical lines which show the BER confidence at the measured 

Eb/N0 points. 

The observed boxes each represent a BER point; however they also show the uncertainty in the 

measurement Eb/N0 (since the Eb/N0 was changing while the measurement was taken) and the 

uncertainty in the BER (since the pass time limits the number of bits that can be collected). The horizontal 

size of the box represents Eb/N0 uncertainty, and the vertical size of the box represents BER uncertainty. 

Note that as the BER becomes worse, the BER becomes more certain since enough bits can be collected 

to establish that BER. 

Figure 6.—GGT 1.1.3 Uncoded Forward Link BER Performance 
NASA/TM—2014-218292 7 

In some cases, the observed box extends downwards, outside of the graph. This means that there were 

0 errors observed during the duration of the RF link. Statistically, the bit error rate has an upper limit (i.e., 

the BER is known to be better than some threshold based on the number of bits received). However, the 

lower limit is unknown. The link could have a BER of 10-10 or a BER of 10-100 and it would have 

performed the same during the duration of the test. 

Considering the performance in Figure 6, modes A and C generally match the ground baseline while 

modes E and G perform better than baseline by about 2 dB. The performance improvement could be 

attributable to at least two factors: 1) noise floor reductions due to lower operating temperatures, and/or 2) 

incorrect Eb/N0 estimation. 

It is unlikely that the waveform performance has improved significantly on-orbit. The RFM 

temperature during on-orbit testing is generally around 15 to 20 °C, versus 35 to 40 °C during ground 

testing. Referencing GRC-CONN-ANA-0854, this temperature change could lower the radio noise figure 

by 0.5 to 1.0 dB at the automatic gain control (AGC) setting of 85, which is the operating point where the 

noise power saturates the AGC algorithm. 

Incorrect Eb/N0 estimation could be due to a number of sources. The Eb/N0 values used for the plots 

are based on a prediction which takes into account the transmitter power, receiver gain and RF path loss, 

and antenna patterns and pointing. The TDRS could be transmitting higher power than expected; in many 

cases, the received power has been 2 to 3 dB above the advertised capability. The Space Network User’s 

Guide (SNUG), which specifies TDRS characteristics, generally contains worst-case numbers that are 

intended for operational capability instead of calibrated measurements. Additionally, the SCaN Testbed 

antenna patterns are not precisely known in the International Space Station (ISS) RF environment. The 

patterns have been characterized on-orbit using the General Dynamics radio (also part of SCaN Testbed), 

but ultimately this leads to a catch-22 condition where the system under test is being used to establish a 

reference baseline. 

In general, however, the uncoded forward link performance shows that the link can be predicted 

within 2 to 3 dB accuracy. 

Forward Link Bit Error Rate Performance for Coded Modes 

The GGT 1.1.3 waveform implements Viterbi decoding for modes B, D, F, and H. In general, a coded 

waveform mode allows a better BER than an uncoded waveform mode for all Eb/N0s. However, the BER 

curve becomes steep and more difficult to test. In many cases, the waveform will have a very fine line 

between not being able to achieve bit synchronization and operating without observed errors. This makes 

both ground testing and on-orbit testing more challenging. 

Coded forward link performance is shown in Figure 7. Testing for modes B, D, and F was completed 

without any observed errors during the (~40-min) duration of the test. Given the steep BER curves, it 

would be difficult to manipulate the on-orbit power to match a point on any of the ground/baseline curves. 

Both tests conducted in mode H produced a BER that was consistent with ground test results, within 1 to 

2 dB. This loosely supports the claim that the forward link Eb/N0 is estimated correctly to within 2 dB. 

Using GGT 1.1.3, it is possible to receive data without bit errors during a 40 min pass at rates up to 

155346 bits per second (bps). Even in mode H (769450 bps), the BER is a low10-7 for tests conducted 

with TDRSS. 
NASA/TM—2014-218292 8 

Figure 7.—GGT 1.1.3 Coded Forward Link BER Performance 

Return Link Bit Error Rate Performance for Uncoded Modes 

Unlike with the forward link measurements that use a predicted Eb/N0, the return link Eb/N0 is 

measured by the integrated receiver (IR) at the White Sands Complex, which is the primary ground 

station for TDRSS. This value is translated to a signal-to-noise-density ratio (C/N0) in data automatically 

returned by the Space Network Access System (SNAS) during an event. Empirical testing has shown that 

the SNAS C/N0 is artificially higher than the equivalent IR Eb/N0; unfortunately, the IR data is not 

returned routinely during or after a test. 

To establish a usable Eb/N0, the SNAS C/N0 is adjusted based on prior observed deviations between 

the IR Eb/N0 and the calculated SNAS Eb/N0. The initial results show that the adjustment is based on 

data rate and coding, although there may be other factors that are not yet known. As a result of this 

uncertainty, the return link Eb/N0 could have 2 to 3 dB error. 

The uncoded return link performance is shown in Figure 8. The baseline performance was found 

during ground testing using a RT Logic TDRS Simulator (TSIM), which supports the various S-band 

TDRSS modes of operation. The implementation loss of the TSIM may be different than the 

implementation loss of the integrated receiver, meaning that the ground test results may be better or worse 

than the IR performance. 

Testing in modes A, E, and G was completed using the medium-gain antenna (MGA), leading to a 

smaller variation in Eb/N0 over each pass. The mode A tests had no observed errors, and the mode E tests 

generally were consistent with TSIM performance. The mode G tests show performance that is better than 

theory, which indicates that the Eb/N0 for this particular mode is being estimated too low in the C/N0 to 

Eb/N0 conversion process by approximately 2 to 3 dB. Further calibration of the SNAS data is required. 
NASA/TM—2014-218292 9 

Figure 8.—GGT 1.1.3 Uncoded Return Link BER Performance 

Testing in mode C was completed using the Space Network low-gain antenna (SN-LGA), which is 

less directional and results in a much wider variation in Eb/N0 over a given pass. However, there is also 

less power received, which makes the BER simultaneously worse and much more certain. The tradeoff 

with using the MGA for this testing is that mode C likely would not have any observed errors. In the 

future, testing could be completed by reducing the radio transmit power and using the MGA to establish a 

tighter BER point. 

Return Link Bit Error Rate Performance for Coded Modes 

The GGT 1.1.3 waveform supports 4 coded transmit modes: B, D, F, and H. Each mode applies rate1/2 convolutional coding to the transmit symbols. The ground receiver uses a Viterbi decoder to recover 

the data bits. 

Waveform performance for the coded return link is shown in Figure 9. 

As with the coded forward link modes, the coded return link performance is difficult to measure since 

it is unusual to have bit errors with such a short RF event. Mode B was not tested (performance would be 

similar to mode D). Both modes D and F ran a number of tests that completed without any observed 

errors. Mode H, due to its high data rate, was able to establish a characteristic BER. The performance here 

is within the realm of possibility: worse than theory but better than the TSIM baseline. Assuming the 

TSIM performance is very close to IR performance, the mode H Eb/N0 estimate is low by 2 to 3 dB, 

similar to mode G above. 
NASA/TM—2014-218292 10 

Figure 9.—GGT 1.1.3 Coded Return Link BER Performance 

Forward Link Frequency Prediction 

GGT provides a telemetry value for the forward link Costas loop frequency offset. The JPL radio 

contains a temperature-compensated crystal oscillator (TCXO) that varies over temperature. Figure 10 

shows the oscillator drift at the SA transmit frequency, 2216.5 MHz, measured during thermal-vacuum 

(TVAC) testing. The frequency varies from about +200 Hz to +1200 Hz. During TVAC, one 

compensation table was developed and verified to offset the baseband frequency. Following additional 

data analysis, the table was revised to provide a more accurate correction shown by the lowest curve in 

green. 

Since the same TCXO is used for both transmit and receive operations, the frequency offset in 

Figure 10 provides insight into the accuracy of the received frequency measurement. The TCXO cannot 

be compensated perfectly because it displays some hysteresis (for example, around 35 °C in the figure). 

Fortunately, the SNUG allows 700 Hz variation on the transmit frequency, which corresponds to a 

similar 700 Hz variation in the receive frequency accuracy. 
NASA/TM—2014-218292 11 

Figure 10.—JPL SDR TCXO Frequency Drift Over Temperature 

For more information on the radio temperature compensation, please refer to SCaN Testbed project 

document GRC-CONN-ANA-0855 (“Analysis of JPL FM SDR Temperature Compensation”). 

The GGT waveform carrier recovery process determines the frequency offset of the received signal 

relative to the local oscillator expected frequency. That is, the local oscillator is programmed to 

2041.027083 MHz for a SA signal and the oscillator offset required to match the center frequency of the 

received signal is provided in telemetry. 

It would be incomplete to discuss the performance of the GGT waveform frequency accuracy without 

first mentioning the performance of the Space Network. In general, TDRSS applies Doppler 

compensation on the forward link so that the received frequency at the radio should be close to the 

intended center frequency. The TDRSS broadcast frequency is reported to the nearest 10 Hz and the 

expected broadcast frequency can be found using a STK model. For the tests in this report, TDRSS tends 

to transmit on average 150 Hz lower than expected. 

The maximum, minimum, and average observed forward frequency offset applied by the GGT 

waveform is shown in Figure 11. These values are corrected for any frequency error introduced by the SN 

Doppler compensation. 

On average the radio provides about 360 Hz more compensation than expected, which indicates that 

the JPL SDR oscillator is tuned about 360 Hz lower than the oscillator in TDRS. A negative value in the 

chart indicates that the waveform is increasing the frequency above expected, and a positive value 

indicates a lower frequency. What this means is that the oscillator compensation table may need adjusting 

on orbit to better reflect the TCXO performance over temperature. However, it is notable that all averages 

are within the 700 Hz acceptable range, which indicates the TCXO is in general performing similar to 

ground testing. 

The maximum and minimum frequency offset is also shown for reference; however the values are 

less meaningful. The Costas loop bandwidth increases as the waveform symbol rate increases, so the 

frequency becomes harder to estimate in a high rate mode. An extreme frequency offset more likely 

implies inaccuracy in the Costas loop estimate and not TCXO error. 
NASA/TM—2014-218292 12 

Figure 11.—GGT Forward Frequency Offset by Mode 

The forward frequency offset is compared to temperature in Figure 12. A negative offset value 

indicates that the waveform frequency was too high—the frequency was increased too much. Therefore, 

the transmit frequency will also be too high and can be reduced by approximately 360 Hz on average. 

Figure 12 does not show any trend over temperature. This is not surprising since the TCXO tends to 

be operated over a very narrow temperature band on-orbit, and the underlying TCXO does not have a 

significant trend over this range. The oscillator frequency varies more significantly (and exceeds 

+700 Hz) from 35 to 60 °C, which has not been seen outside of TVAC testing. 

The figure shows that the temperature compensation tables could be updated with a DC offset. 

However, there is no significant trend in the data that would allow fine-tuning of the TCXO for the onorbit conditions. 

Forward Link Automatic Gain Control 

The JPL SDR contains a series of gain stages that increase the received signal level (see JPL 

document D-49240, “CoNNeCT JPL-SDR FM HDD” for more information). The gain level is 

programmable in software and GGT implements a control algorithm in the FPGA. The algorithm adjusts 

the AGC gain until the wideband power at the analog-to-digital converter (ADC) is approximately 

25 percent full scale. 

The AGC level is programmable by digital number (DN). A lower DN provides higher gain, and a 

higher DN provides less gain. For example, DN 60 would amplify more than 120. In practice, the AGC 

level is limited by noise. During ground testing, the AGC reached a floor at digital number (DN) 85 for 

signals lower than approximately –110 dBm. 
NASA/TM—2014-218292 13 

Figure 12.—GGT Forward Frequency Offset Versus TCXO Temperature 

Roughly speaking, each DN represents about 0.5 dB of gain. For example, a change in the AGC value 

from 85 to 105 implies that the signal has become 10 dB stronger and therefore needs 10 dB less 

amplification. The exact AGC performance measured during TVAC is documented thoroughly in SCaN 

Testbed project document GRC-CONN-ANA-0854, “Analysis of JPL FM SDR Receiver Noise Figure 

and Gain”. 

Figure 13 shows the AGC ranges seen on-orbit by waveform mode. Note that modes G and H have 

been excluded because the AGC set point is slightly different to compensate for the BER flare. The chart 

is interesting because it shows that the lowest AGC value of 85 on the ground is in some cases the highest 

observed AGC value on orbit! The average change of 10 DN suggests that the noise floor on-orbit of the 

space-facing antennas is approximately 5 dB lower than the noise floor seen during ground testing. 

Another way to look at the AGC measurements is to compare the AGC to input power. This 

comparison is shown in Figure 14, with a couple interesting observations. First, the on-orbit AGC range is 

significantly different than the one seen on the GIU for similar input power. This makes it more difficult 

to predict on-orbit performance by simply running a test on the GIU. Second, there is a large span 

(~4 DN) to each cluster of AGC points at a given power level on orbit. This implies that the radio could 

see a power variation of 2 dB, independent of the signal level transmitted by TDRSS. The variation could 

be due to a change in the noise floor or an unknown interferer. 

One conclusion of this analysis is that it may be difficult to predict waveform received power on-orbit 

with better than 2 dB accuracy using the AGC. Assuming changes in the noise floor are causing the AGC 

variation—which is a safe assumption given the AGC=75 data point around –106 dBm—then the Eb/N0 

could vary unpredictably. 

Another more positive conclusion is that the AGC appears to be a slightly better indicator of input 

power on orbit than on the GIU. During ground testing, the AGC changed significantly only for input 

power greater than –100 dBm. On orbit, the AGC range extends to approximately –105 dBm or lower. 
NASA/TM—2014-218292 14 

Figure 13.—AGC Range by Mode 

Figure 14.—Average AGC Setting vs Predicted Average Input Power 
NASA/TM—2014-218292 15 

Characteristic Input Power Levels 

The testing completed on GGT 1.1.3 tended to occur over certain specific combinations of payload 

antenna and TDRSS power level, as shown in Table 3. These combinations resulted in specific bins of 

input powers (such as in Figure 14) and a correspondingly limited range of Eb/N0 values. Testing was 

limited to the TDRS satellites F8 and F10 to simplify link predictions. Grayed cells indicate that the test 

combination was not performed with GGT 1.1.3. 

For each antenna, there is also a range of input powers that can be seen during a 40-min pass. These 

ranges are shown in Table 4. Note that the MGA antenna typically has a 1 dB variation throughout the 

pass, since it is steered by a gimbal assembly, while the SN-LGA antenna can vary by 5 dB. 

TABLE 3.—AVERAGE POWER FOR TDRS TEST COMBINATIONS 

 TDRS MA, F8-F10, 

dBm 

TDRS SA, F8-F10 

normal power, 

dBm 

TDRS SA, F8-F10 

high power, 

dBm 

SN-MGA –106.4 –103.7 -99.5 dBm 

SN-LGA –115.5 

NEN-LGA 

TABLE 4.—TYPICAL POWER RANGE FOR TDRS TEST COMBINATIONS 

 TDRS MA, F8-F10, 

dBm 

TDRS SA, F8-F10 

normal power, 

dBm 

TDRS SA, F8-F10 

high power, 

dBm 

SN-MGA –106 ... –107 –103 ... –104 –99 ... –100 

SN-LGA –114 ... –119 

NEN-LGA 

The choice of antenna depends on the test objective. The MGA antenna is optimal for delivering 

stable power, but the SN-LGA antenna has far more varying link characteristics and can be used for 

acquisition threshold testing. There are some options for setting received power levels outside of the 

standard ranges: 

1. The NEN-LGA can be used to communicate with a TDRS as it comes over the horizon. The 

initial power level is similar to the SN-LGA level, but the received power will decrease by about 

20 dB over a 40-min pass. 

2. The SN-MGA can be mispointed to reduce the received power by an arbitrary amount. This 

requires sufficient knowledge of the antenna pattern, but it has been demonstrated using the other 

radios in SCaN Testbed. 

For BER testing, the Eb/N0 is determined by the data rate as well as the input power. Simply 

choosing a wider variation of data rates provides a substantial amount of Eb/N0 possibilities. 

Transmit Power Levels 

TxRF Sensor 

The JPL SDR contains a “TxRF” sensor, which is similar to a square law voltage detector on the 

transmit power out of the solid state power amplifier (SSPA). In general, the TxRF value increases as the 

RF output power of the radio increases. A detailed analysis of the sensor was conducted during TVAC, 
NASA/TM—2014-218292 16 

and this is available in the SCaN Testbed project document GRC-CONN-ANA-0892, “Analysis of JPL 

SDR Flight Model Transmitter Characteristics”. 

The GGT waveform includes an Automatic Level Control (ALC) algorithm that boosts the baseband 

power level by adjusting the I and Q drive levels to the digital-to-analog converter (DAC). The ALC uses 

an 8-point moving average on the TxRF value and then operates using closed-loop feedback to 

increase/decrease the baseband drive levels until the TxRF value enters a certain range. 

The ALC target levels are programmed ahead of time and are specific to each mode. In general, these 

points are about 0.5 dB into compression on the SSPA. The ALC adjustment threshold is set to 15, 

meaning that the baseband power is modified until the TxRF value is within 15 DN of the target level. 

The settings for each waveform mode are shown in Table 5. 

The observed on-orbit TxRF values are shown in Figure 15. Although the set points are not shown on 

the chart, all of the average values are within 3 DN of the desired target. The change in output power over 

the ranges is only about 0.3 dB on average despite the 100+ DN variation. 

The TxRF values themselves do not indicate that the hardware is performing as expected. Since the 

ALC is a closed-loop algorithm, the drive level will increase until the TxRF sensor displays the correct 

value. The JPL SDR does not contain a redundant transmit power sensor, so the baseband drive level 

would simply be increased if the SSPA began to transmit at reduced capacity. Fortunately, this would 

eventually cause the DAC to clip, which would significantly increase the error vector magnitude (EVM) 

and result in a very poor BER. 

TABLE 5 – ALC TARGETS AND THRESHOLDS 

GGT 

Mode

TxRF Target, 

DN

Approx. Tx power, 

dBm

Adjustment threshold, 

DN

A 409 +37.5 15 

B 409 +37.5 15 

C 392 +37.6 15 

D 392 +37.6 15 

E 401 +37.5 15 

F 401 +37.5 15 

G 400 +37.4 15 

H 400 +37.4 15 

Figure 15.—TxRF Ranges 
NASA/TM—2014-218292 17 

Return Link Carrier-to-Noise Density 

Another check of the radio transmitter characteristics is the return link receiver C/N0 value. However, 

this is an indirect method. The S-band transmission of the JPL SDR is received by a TDRS transponder 

where it is shifted to Ku-band for the space-to-ground link. The signal is received by a dish at the White 

Sands Complex and routed to the integrated receiver (IR). The IR performs Eb/N0 estimation using a 

matched filter technique. 

The Eb/N0 estimator is converted to a C/N0 value that is returned through SNAS. As previously 

mentioned, the conversion process appears to add a bias to the C/N0, which makes the C/N0 value higher 

than expected by 3 to 8 dB. The offset depends on data rate and coding. 

SCaN Testbed is able to remove the SNAS C/N0 bias in post-processing. The IR Eb/N0s can be 

requested from White Sands after an event and compared to the SNAS C/N0s. After several coded and 

uncoded events at each data rate, a static offset was determined and removed from the SNAS values. 

However, it may take more events before the bias is better understood. 

The return link C/N0 performance is shown in Figure 16 using the corrected SNAS C/N0. 

Figure 16 shows that the return link performance for the GGT 1.1.3 events is generally close to the 

predicted performance. In the ideal case, all points would be on the 1:1 line through the center of the plot 

(i.e., the predicted C/N0 matches the measured C/N0). Modes A, D, E, and G match the ideal line. Mode 

C was tested using the SN-LGA antenna, so some variation can be expected due to the lower C/N0 and 5 

dB power range over the pass. Mode F is consistently 1 to 2 dB better than expected and mode H is 

consistently 2 to 3 dB worse than expected. Since there is a minimal dataset for both of these modes, the 

performance can be attributed to an incorrect adjustment of the SNAS C/N0 value. 

Another way of viewing C/N0 accuracy is to consider the return link BER in Figure 8 and Figure 9. 

The mode E BER matched TSIM performance, and here mode E is shown within 1 dB of the expected 

C/N0. Both mode G and mode H had a BER that seemed to indicate the Eb/N0 was low by 2 to 3 dB. The 

C/N0 results show that mode H appears to be lower than predicted by the same amount, which indicates 

the issue is C/N0 calibration and not a lower-performing link. However, mode G is very close to 

predicted, so more testing would be needed to improve prediction accuracy. 

Figure 16.—Return Link C/N0 Results 
NASA/TM—2014-218292 18 

Since the measured C/N0 values overall are close to predicted, it is reasonable to say that the SSPA is 

performing nominally. The return link C/N0, despite the bias, is a safer indicator of transmit power than 

the TxRF sensor since it is an independent measurement. 

Future Work 

Testing of GGT 1.1.3 is complete. GGT version 1.1.4 was released, and the new version includes a 

number of bug fixes and upgrades including: 

 BER flare is resolved 

 Soft-decision Viterbi is added 

 Forward link power estimator is added 

 Forward link signal-to-noise ratio estimator is added 

Future testing will focus on verifying the GGT 1.1.4 bug fixes and new signal estimators. 

Conclusions 

This report has considered a number of the performance aspects of GGT 1.1.3, including bit error rate 

(summary in Table 6), frequency estimation, automatic gain control values, the TxRF sensor, and return 

link C/N0. In general, the performance of GGT meets expectations to within 2 to 3 dB of the predicted 

power levels across all modes of operation. 

Future testing must focus on reducing measurement uncertainties. In particular, the variation in noise 

floor identified in this report points to the potential for in-band interference. The uncertainty in converting 

the SNAS C/N0 to a receiver Eb/N0 also must be reduced before more consistent performance can be 

attained. 

TABLE 6.—SUMMARY OF TYPICAL BER PERFORMANCE FOR GGT 1.1.3 

GGT mode TDRS Tx 

power 

SN-LGA SN-MGA 

Forward Return Forward Return 

A (MA) Normal ------ ------ 110–7 110–7

B (MA) Normal ------ ------ 110–7 ------ 

C (SA) High ------ 110–4 ------ ------ Normal 110–4 ------ 

D (SA) High ------ ------ ------ 110–7

Normal ------ 110–7

E (SA) High ------ ------ 210–7

110–8

Normal ------ ------ 

F (SA) High ------ ------ 110–8

110–8

Normal ------ ------ 

G (SA) High ------ ------ 110–5

110–5

Normal ------ ------ 

H (SA) High ------ ------ 110–8

110–7

Normal ------ ------