diff --git a/src/RealtimeQC.adoc b/src/RealtimeQC.adoc
index 95c576c..1608b70 100644
--- a/src/RealtimeQC.adoc
+++ b/src/RealtimeQC.adoc
@@ -96,4 +96,5 @@ List of all contributors an their contributions.
 [bibliography]
 == References
 
+* [[GTSPP:1990]] Intergovernmental Oceanographic Commission (1990) GTSPP Real-Time Quality Control Manual. Paris, France, UNESCO, 128pp. (Intergovernmental Oceanographic Commission Manuals and Guides; 22).
 * [[BGC_nitrate]] Johnson Kenneth, Maurer Tanya, Plant Joshua, Bittig Henry, Schallenberg Christina, Schmechtig Catherine (2021). BGC-Argo quality control manual for nitrate concentration. https://doi.org/10.13155/84370
diff --git a/src/realtime_qc.bib b/src/realtime_qc.bib
index abef43b..b9c6060 100644
--- a/src/realtime_qc.bib
+++ b/src/realtime_qc.bib
@@ -1,4 +1,27 @@
 
+@manual{GTSPP:1990,
+    title = "GTSPP Real-Time Quality Control Manual",
+    organization = {UNESCO--IOC},
+    address ="United Nations Educational, Scientific and Cultural Organization 7, Place de Fontenoy,
+75352, {P}aris 07 SP",
+    year = "1990",
+    note = "SC-90/WS-74",
+    series = "Intergovernmental Oceanographic Commission Manuals and Guides; 22",
+    key = "{UNESCO--IOC}, 1990",
+}
+
+@manual{GTSPP:2010,
+    title = "GTSPP Real-Time Quality Control Manual",
+    organization = {UNESCO--IOC},
+    address ="United Nations Educational, Scientific and Cultural Organization 7, Place de Fontenoy,
+75352, {P}aris 07 SP",
+    edition = "First Revised Edition",
+    year = "2010",
+    note = "IOC/2010/MG/22Rev.",
+    key = "{UNESCO--IOC}, 2010",
+    doi = "10.25607/OBP-1425",
+}
+
 @manual{johnson2021bgc,
   title={BGC-Argo quality control manual for nitrate concentration},
   author={Johnson, Kenneth and Maurer, Tanya and Plant, Joshua and Bittig, Henry and Schallenberg, Christina and Schmechtig, Catherine},
diff --git a/src/tests.adoc b/src/tests.adoc
index 3f49ce3..590434a 100644
--- a/src/tests.adoc
+++ b/src/tests.adoc
@@ -11,11 +11,16 @@ X_i = \left| V_i - \frac{V_{i+1} + V_{i-1}}{2} \right|
 
 == Spike
 
+The spike check was first proposed in GTSPP:1990 and defined as:
+
 [stem]
 ++++
-X_i = \left| V_i - \frac{V_{i+1} + V_{i-1}}{2} \right| - \left| \frac{V_{i+1} - V_{i-1}}{2} \right|
+X_i = \left| V_i - \frac{V_{i+1} + V_{i-1}}{2} \right| - \left| \frac{V_{i-1} - V_{i+1}}{2} \right|,
 ++++
 
+where $i$ is the time index, i.e. it assumed the data $V$ was sorted by time.
+This check is largely used without any modification.
+
 == Digit Rollover
 
 [stem]